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
Re-Evaluating Electric Power System Harmonic
Distortion Limits for Shipboard Systems
Prepared by: M. Steurer, P. Ribeiro, Y. Liu
Center for Advanced Power Systems (CAPS)
Florida State University
June 25, 2004
ABSTRACT
The advent of high power, high voltage semiconductor technology is transforming the naval
electric power shipboard design. Navy ships will feature an integrated electric power system and
all-electric drives in order to accomplish goals like improved maneuverability, advance high
electric power weapon capability, reduced manpower and maintenance, higher flexibility, and
improved fight-through capabilities. The future electric naval ships will demand a much higher
power and energy density than commercial ships. Therefore, traditional electric system design
criteria are unlikely to be fully applicable for the design and operation of NAVY all-electric ships
with adequate power quality and reliability.
Issues associated with the time-varying / probabilistic nature, non-characteristics and
interharmonics, the real impact on system equipment (heating versus electronic interference), and
the limitation by voltage and / or current requires a detailed review of the design standard used for
harmonic distortion analysis on shipboard naval power systems.
Harmonic voltage distortion in shipboard power systems, although always present, do not seem to
be a major source of concern and common voltage harmonic distortion limits established by the
standards are not in general violated. This implies the need to review the present numerical
values and procedures related to harmonic voltage distortion design on shipboard systems.
Alternative harmonic current distortion, although restricted by some standards, are not easily
applied. Therefore, current limits should consider other aspects such a load factor and ultimately
the overall harmonic voltage distortion.
This report investigates key design electrical interface criteria subject to possible alterations and
proposes a novel approach to harmonic limits in military standards. It is expected that the flexible
limits will better meet all the complex system requirements associated with future of all-electric
NAVY ships.
The proposed approach reflects a simpler individual harmonic voltage table of values with three
stages of corresponding THDs (Caution, Possible Problems and Recommended Action). The 95%
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THD should be kept under 9% and short-term values should not to exceed 15% (for less than 5
minute bursts). The values for individual harmonic voltage and THD are an attempt to both
simplify and allow for flexibility within the typical ranges encountered isolated or weak systems.
Chapter 1 presents an introduction to the questions and issues associated with harmonic distortion
harmonic distortion levels, standards and impact on shipboard system design. In chapter 2 the
insufficiency of the individual harmonic distortion and total harmonic distortion to characterize
harmonic distortion impact are considered. Chapter 3 presents a summary of the alternative
indices which can be used to characterize different impacts on system equipment. In chapter 4 the
time-varying and probabilistic nature of harmonic distortion is described, while chapters 5 and 6
consider the impact on electrical and shipboard equipment, respectively. Chapter 7 presents a
summary of the several international harmonic standards and comments about their objectives and
differences. Chapter 8 shows the results of some initial verification / experiments on harmonic
immunity using a PC switched mode power supply and in chapter 9 a philosophy and harmonic
levels are proposed to deal with the new power quality environment of existing and future
shipboard power systems. Chapter 10 makes some final observations and suggests future possible
work.
ii
Contents
ABSTRACT ....................................................................................................................................... i
LIST OF FIGURES........................................................................................................................... v
LIST OF TABLES ........................................................................................................................... vi
CHAPTER 1 Introduction................................................................................................................. 1
CHAPTER 2 Insufficiency of Common Indices ............................................................................... 3
CHAPTER 3 Alternative Indices ...................................................................................................... 5
Total harmonic distortion ........................................................................................................................................ 6
K-factor ................................................................................................................................................................... 6
Telephone influence factor and I●T product ........................................................................................................... 7
Flicker related index ................................................................................................................................................ 7
CHAPTER 4 Time Varying and Probabilistic Aspects .................................................................. 10
CHAPTER 5 Impact on Equipment ................................................................................................ 12
CHAPTER 6 Impact on Shipboard Power Systems Design ........................................................... 13
CHAPTER 7 Different Harmonic Standards .................................................................................. 15
IEEE 519 ................................................................................................................................................................... 18
MIL-STD-1399 Section 300A .................................................................................................................................. 19
IEEE STD 45 ............................................................................................................................................................ 19
Other Standards and Normative Documents ............................................................................................................. 21
Other Harmonic Design Issues .................................................................................................................................. 29
CHAPTER 8 Some Initial Verification / Experiments on Harmonic Immunity ............................ 31
CHAPTER 9 Proposed Philosophy ................................................................................................ 35
CHAPTER 9 Proposed Philosophy ................................................................................................ 35
CHAPTER 10 Concluding Remarks .............................................................................................. 41
REFERENCES ............................................................................................................................... 42
APPENDIX I Harmonic Source...................................................................................................... 44
I.1 Introduction.......................................................................................................................................................... 44
I.2 Basic concepts ..................................................................................................................................................... 44
Fourier transform .................................................................................................................................................. 44
Direction of flow of harmonics ............................................................................................................................. 45
I.3 Harmonic sources ................................................................................................................................................ 45
Magnetizing currents ............................................................................................................................................. 46
Power conversion devices ..................................................................................................................................... 47
iii
I.4 Conclusions.......................................................................................................................................................... 51
APPENDIX II Interharmonics ........................................................................................................ 53
II.1 Introduction ........................................................................................................................................................ 53
II.2 Cycloconverters .................................................................................................................................................. 54
II.3 Measurements ..................................................................................................................................................... 55
II.4 Effects................................................................................................................................................................. 57
II.5 Analysis .............................................................................................................................................................. 59
II.6 Mitigation ........................................................................................................................................................... 59
II.7 Limits.................................................................................................................................................................. 60
APPENDIX III Power System Characteristics at Interface............................................................. 63
APPENDIX IV IEEE STD 519 Proposed Revisions June 08, 2004............................................... 65
iv
LIST OF FIGURES
Figure 1 Coverage of different distortion index in frequency domain .............................................. 5
Figure 2 A voltage waveform distorted by a 20% second harmonic, 90° phase difference ............. 8
Figure 3 Switched mode PC power supply test - RTDS test implementation ................................ 31
Figure 4 Damaged parts on PC ATX 300W computer power supply............................................. 33
Figure 5 Probability density function versus levels of disturbances and immunity........................ 36
Figure 6 Example of flexible limitation of harmonics as a function of equipment temperature .... 37
Figure 7 Proposed harmonic voltage (%) limits versus IEEE 519.................................................. 39
Figure 8 Proposed short term THD (%) .......................................................................................... 39
Figure I.1 Direction of harmonic flow ............................................................................................ 45
Figure I.2 Saturated DC-biased transformers [I.2] .......................................................................... 47
Figure I.3 Operation of a saturated DC-biased transformer [I.2] .................................................... 47
Figure I.4 The schematic of a three-phase 3-pulse half-wave cycloconverter ................................ 49
Figure I.5 A representative topology of a single-phase ICC and a sample switching strategy. To is
one cycle length ........................................................................................................................ 50
v
LIST OF TABLES
Table 1 Recommended limit for distortion indices ......................................................................................................... 9
Table 2 Summary of the impact of harmonics on electric equipment ........................................................................... 12
Table 3 Harmonic distortion limits on shipboard design............................................................................................... 14
Table 4 Comparison of harmonic standard limits.......................................................................................................... 16
Table 5 Harmonic voltage limits (%) ............................................................................................................................ 17
Table 6 Harmonic limits from IEEE 519 (PCC point of common coupling) ................................................................ 18
Table 7 Harmonic limits from MIL-Std-1399 Section A .............................................................................................. 19
Table 8 Values of individual harmonic voltages at the supply-terminals for LV and ................................................... 21
Table 9 Indicative values of planning levels for harmonic voltage (in % of the nominal voltage) in MV, HV and EHV
power systems ....................................................................................................................................................... 23
Table 10 Limits for harmonic voltages (in % of the declared voltage of LV and MV power systems) ........................ 25
Table 11 Indicative values of planning levels for harmonic voltages (in % of the declared voltage of HV and EHV
power systems) ...................................................................................................................................................... 25
Table 12 Target levels for harmonic voltages (in % of the supplied voltage of MV and HV power systems) .............. 26
Table 13 Planning levels for harmonic voltage distortion ............................................................................................. 27
Table 14 Test Cases ...................................................................................................................................................... 32
Table 15 Proposed voltage harmonic limits For shipboard power systems ................................................................... 38
Table III. 1 Power system characteristics at interface ................................................................................................... 63
Table III. 2 Power system characteristics at interface (continued) ................................................................................ 64
vi
CHAPTER 1 INTRODUCTION
In the last twenty years there has been an explosion in the number and variety of electric
propulsion ships being built around the world with everything from cruise liners to amphibious
assault ships. This revolution has occurred due to power electronic drive technology which is used
for the electric propulsion system. The drive adjusts the propeller speed by varying the motor
frequency. This has been accomplished by the development of high power solid state switching
devices.
It is expected that the level of harmonic distortion will increase as more electronically controlled
loads are added to shipboard power systems. This will certainly increase the levels of harmonic
distortions and this new situation needs to be properly evaluated from a standard of harmonic
limitation perspective.
Traditionally, harmonic standards for electric shipboard power systems have been adopted directly
from the power utility standards. Although approach may have worked in the past one needs to
realize that these two systems are different and shipboard applications require specific standards
which take into account its electrical characteristics. The characteristics of shipboard power
systems regarding harmonic distortion have been defined by both commercial and military
standards Those standards include IEEE Standard 519 [1], IEEE Standard 45 [2], and MIL-STD1399 Section 300 [3].
Standards specify the harmonic currents and voltages which can be tolerated. However,
unnecessarily strict limits can increase ownership cost and equipment weight and reduce system
effectiveness. A total voltage harmonic distortion of 5% has been traditionally used as the
maximum steady state (95% of time) limit for low voltage (LV) and medium voltage (MV)
systems (for utility systems voltages are less than 1 kV in LV systems and less than 69 kV in MV
systems). New standards, however, are moving up the limit to 6.5% (planning) and to 8%
(compatibility). Furthermore, the utility philosophy with harmonic standards was to limit
contribution of an individual customer who may connect loads of more or less arbitrary
characteristic, unpredictable at the time of system design, on a shared distribution system. In the
case of an isolated all-electric shipboard system this concept does no longer apply since only one
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“customer” exists and the type and characteristics of loads are well known at the design stage. It
may be more beneficial from the overall ship system perspective to allow higher and / or more
flexible limits in the future. As an example, distribution systems and off-shore platforms have
been reported to operate well with harmonic voltage distortion levels of over 8%. Considering that
very little harmonic distortion problems have been reported in shipboard power systems one may
wonder about the adequacy of the numerical values imposed by harmonic distortion limits.
Another aspect is that traditional harmonic limits were based primarily on thermal effects,
whereas today the sensitivity of electronic equipment (e.g. controllers for power electronic
converters) may actually be equally if not more important. Other considerations such as capacitor
aging and short-term distortion need to be addressed as well.
2
CHAPTER 2 INSUFFICIENCY OF COMMON INDICES
When limiting harmonics, several aspects need to be considered rather than the total harmonic
distortion (THD). The time dependence nature of the distortions and the type of equipment needs
to be taken into account in order to provide an effective way to control any damaging or disturbing
effect of harmonic pollution.
Like other engineering indices, the individual harmonic distortion and total harmonic distortion,
just to name the mostly common used indices, are a combination or part of the information which
attempts to describe a multidimensional and complex phenomenon into a single number. Thus, for
example, the individual and total harmonic distortion may not include information more than
clarifying its impact on certain possible power quality problems. Furthermore, the indices can also
be misused and / or applied to cases outside design specification. A case in point is the
IEEE Standard 519 which has often been used as a utility regulation or an equipment
specification.
Historically, these indices and associated limits have been established a long time ago to study
specific areas. With time, the numbers have expanded in terms of their area of application and
migrated into standard limits. Typically, the following voltage indices are used: instantaneous
waveform, spectrum, peak amplitude, RMS, harmonic RMS, THD , and crest factor
With the development of new power equipment, particularly power electronic technologies, these
indices have lost some of their meaning and usefulness. For example, individual components and
THD values, the most common indices, have limited applicability for determining the impact of
harmonic distortion on electrical and electronic equipment. The determination of life of the
electrical insulation and performance of electronic equipment require additional indices not
usually addressed by electrical standards.
One important parameter is the peak or maximum voltage, which depends on an infinite
combination of phase and amplitude values of the fundamental component and harmonics. It
influences partial discharges, intrinsic degradation, and space charge accumulation.
3
The individual harmonic currents, and voltages, and the THDs are directly related to thermal
losses and can serve as a possible measure of disturbances. But they have little significance in
terms of determining the actual impact on equipment life, and particularly, performance.
Therefore, the design of power systems under non-sinusoidal (harmonic distortion) conditions
must take into account other indices (e.g., crest factors), and even actual waveforms.
4
CHAPTER 3 ALTERNATIVE INDICES
Total harmonic distortion is a useful index to descript a distorted voltage or current waveform.
When Fourier transform is used to transform a distorted waveform from time domain to frequency
domain, there are two sorts of information, amplitude and phase angle. Traditional THD ignores
the phase angle information, and only considers a small range of frequency components, that is to
say, fundamental and its integer multiples (usually not higher than 64th harmonic). Figure 1 shows
the coverage of THD in frequency domain. Generally, to consider the impact of other frequency
components or phase angles, alternative indices are needed. Those alternative indices include
Crest Factor, K-factor, Telephone inference factor (TIF), and extended THD. In this section, the
alternative indices are briefly introduced, and a novel power quality index, asymmetry factor, is
proposed. A comprehensive distortion factor based on the weighted summation of all distortion
factors is proposed.
Amplitude
(p.u.)
Harmonics
Interharmonics
1.0
0.5
0
Extended THD and K-factor
Flicker
related index
TIF
Up to 64th
harmonic
Crest Factor and Asymmetry Factor
...
60
120
Phase angle
(degree)
180
240
300
3000 3060
Frequency
(Hz)
3120
Coverage of different
distortion index
180
Crest Factor and Asymmetry Factor
90
0
...
60
120
180
240
300
Frequency
(Hz)
3000 3060
3120
-90
-180
Figure 1 Coverage of different distortion index in frequency domain
5
Total harmonic distortion
The THD is used to define the effect of harmonics on the power system voltage or current. The
THD is expressed in percentage and is defined as,
THD 
I
2
k
k
(1)
I1
where k refers to the (integer) harmonic order, k = 2, 3, 4 …, Ik is the amplitude of k order
harmonic, and I1 is the power frequency component (e.g., 60 Hz component). In the Equation,
interharmonic are not considered. To consider interharmonics, the THD can be redefined as,
THD 
I
2
h
h
(2)
I1
where h refers to the index number of all harmonics and all non-standard frequency components,
Ih is the amplitude of harmonics or non-standard frequency components, and I1 is the power
frequency component.
Crest factor
The crest factor (CF) of an AC wave is
max( Vpp, | Vnp |)
Vrms
CF 
(3)
where Vrms refers to the root mean square of the signal under study.
K-factor
The K-factor of a load current refers to transformer losses. K-factor is based on the assumption
that the eddy current losses, due to each harmonic component of a nonsinusoidal current, are
proportional to the square of the harmonic order of the component. The K-factor under nonstandard conditions is defined as,
N
K
fh
( f
h 1
N
0
 (I
h 1
6
Ih )2
(4)
h
)
2
where N is the total number of harmonics or non-standard frequency components, h is an index
sequence number for all harmonics and all non-standard frequency components, and fh is the
frequency of the hth frequency components.
Telephone influence factor and I●T product
The TIF is a measure of the THD in which the root of the sum of the squares is weighted using
factors (or weights). The TIF reflects the response of the human ear and the way in which currents
in a power circuit induce voltages in an adjacent communications circuit. The TIF under nonstandard conditions is defined as,
N
 (w I
h 1
TIF 
h h
)2
(5)
N
 (I
h 1
h
)
2
where wh is the TIF weight of the hth frequency components. The TIF does not reflect current
amplitude information. The I●T product is an alterative index which incorporates the current
amplitude. The I●T product is defined as,
I T 
N
 w I 
h 1
2
h h
(6)
In Equation (6), it may be necessary to interpolate to obtain the wh weights because usual TIF
weights are tabulated for (integer) harmonics of 60 Hz.
Flicker related index
Flicker is defined as the human perception of unsteadiness of visual sensation induced by a light
stimulus whose luminance or spectral distribution fluctuates with time. Flicker can be described
by either flicker factor or unit of perceptibility, . Flicker factor may be defined as the ratio of the
rms amplitude of a voltage fluctuation and the rms value of the voltage. Note that there are
alternative definitions for flicker factor. Flicker is defined based on the response of the human eye
to voltage fluctuation from about 0.5 – 25 Hz. Different frequencies in this range can have
7
different impact on the human eye. Research show that human beings are most sensitive to about
8.8 Hz voltage fluctuation. One unit of perceptibility is the minimum flicker corresponding to the
condition that human beings are able to perceive that an incandescent lamp is flickering.
Asymmetry factor (AF)
Except for the same impact that odd harmonics can have, even harmonics and interharmonics can
cause waveform asymmetry as shown in Figure 2 Waveform asymmetry is a phenomena that the
positive peak of a distorted waveform is not equal to the negative peak. Asymmetric peaks of
voltages stress insulation, and asymmetric peaks in current cause excess heating. Waveform
asymmetry often occurs with the change of zero-crossing point, which can have an impact on
protection relays or PLL-based measurement devices.
Voltage (p.u.)
Positive peak
Fundamental
Zero crossing points
Distorted waveform
1
2nd harmonic
0
Previous zero
crossing points
-1
Negative peak
0
0.01
0.02
0.03
0.04
0.05
Time(s)
Figure 2 A voltage waveform distorted by a 20% second harmonic, 90° phase difference
To capture the waveform asymmetry, the term, asymmetry factor (AF), is proposed,
AF |
Vpp  | Vnp |
|
V1
(7)
where V1 is the rated circuit voltage (these expressions could also be applied to current). In (7),
Vpp refers to the positive peak of the wave, and Vnp to the negative peak.
Comprehensive Distortion Index (CDI):
8
Although Crest Factor and Asymmetry Factor covers both of the amplitude and phase angle
information as shown in Figure 1, they focus on the maximum peak amplitude and the difference
between the positive peak and the negative peak. Actually, no single distortion index can give a
complete information on how severe a distorted waveform is. Therefore, a comprehensive
distortion index based on the weight summation of all existing distortion indices is proposed. The
CDI can be expressed as,
N
CDI   ci Di
(8)
i 1
where Di is a distortion factor (e.g., THD, K-factor, and TIF), ci is the coefficient of the different
distortion factors.
The limits for alternative indices are presented in Table 1 below. The THD is used here as a
reference for the other indices.
Table 1 Recommended limit for distortion indices
Distortion Index
Recommended limit
THD
See Chapter 6 - 5 to 10%
Crest factor
No limit is given
K-factor
IEEE Standards
Flicker related
The limit for an individual frequency component are based on a
indices
frequency v.s. limit graph given in IEC STD 61000-4-15
TIF
TIF < 100 for 5 1-20 MVA synchronous machines
TIF < 75 for 20 MVA and above (ANSI STD C50.12-1982)
IT product
IT < 10,000 (unlikely), 10,000 < IT < 25,000 (possible),
25,000 <IT (probable), (ANSI STD 368)
AF
TBD
CDI
TBD
9
CHAPTER 4 TIME VARYING AND PROBABILISTIC ASPECTS
The assessment of harmonics is not exact or uniform, since there will be unpredictable variations
in either the non-linear sources and / or parameters of the system which affect the summation. The
combination of a number of harmonic time-varying sources will generally lead to less than the
arithmetic sum of the maximum values due to uncertainty of magnitude and phase angle. Hence
the resulting summation is extremely difficult to estimate accurately.
A number of methods have previously been developed to combine harmonics in addition to the
vectorial composition method. These methods were developed to combine a number of sources
connected at the same bus bar. The main conceptual objection with these methods is the fact that
the use of any method, apart from the vectorial (RMS), would imply the concept of wave
distortion is neglected. However, the harmonics cannot always be assumed to be constant and the
use of alternative summations may be helpful to approach the problem. Furthermore, there is no
apparent reason why alternative methods to combine harmonics cannot be used in a large network,
since the impedance between sources will not change significantly during certain periods of time.
Strictly speaking, harmonic analysis may be applied only when currents and voltages are perfectly
steady. This is because the Fourier transform of a perfectly steady distorted waveform is a series
of impulses suggesting that the signal energy is concentrated at a set of discrete frequencies. Thus,
the transfer relationship between current and voltage (impedance) is a single value at each
component of frequency (harmonic); although different impedances at different frequencies have
different values.
When there is variation of the distorted waveform the Fourier transform result of the waveform is
no longer concentrated but the energy associated with each harmonic component occupies a
particular region within the frequency band.
There is no definition of time varying harmonics, but it can be expected that the voltage and
current waveforms will vary from cycle to cycle to some extent.
10
This ever-present time-varying nature of harmonics in power systems requires a probabilistic
treatment that needs to be incorporated in harmonic analysis, studies, and eventually in the
standards. This time-varying behavior is due to continuous changes in system configurations and
operating modes of loads (linear and nonlinear).
In order to cope with such variations, harmonic standards need to specify the tolerable limits
during short term bursts of harmonics.
Reference [4] has suggested as a possible approach by proposing a curve which is based on
cumulative distribution and the maximum duration and maximum duration of individual burst for
a THD measurement. The actual limits need to be adjusted to each specific situation and
equipment involved. These limits must provide an economical compromise between operational
efficiency and equipment lifetimes. There will be no solution which avoids compromise. The
correspondence of duration limits and equipment thermal limits is apparent, and any duration limit
should be a small fraction of power equipment thermal time constants.
Probabilistic techniques may be applied to the analysis of harmonic current from several sources.
However to generalize the analysis, there is a need to measure the Probabilistic Density Functions
(PDF)describing harmonic current variation for a variety of loads. There is also a need to
understand the non stationary nature of the current variation in order to predict the compatibility
levels. It is probable that the harmonic audits currently in progress could yield the appropriate
information.
To determine the compatibility level by calculation it will be necessary to determine the non
stationary trends within the natural variation of power system harmonic distortion.
There are circumstances where harmonic analysis does not apply because the rates of change
associated with current variation are too fast. It is possible to determine the limits to which
harmonic distortion should be applied by considering information transferred into the frequency
domain. A formal approach to understanding this problem might present new insight into the
limits to which harmonic analysis is appropriate.
11
CHAPTER 5 IMPACT ON EQUIPMENT
Harmonics have a number of undesirable effects on power system components and loads. Table 2
shows a summary of types of impact, mechanisms and common practice disturbing / limiting
values. Other aspects like loss of life, and other interference and high frequency issues need to be
taken into account.
Recent investigations have revealed that variable speed-drive controllers may be less sensitive to
high THD than traditionally anticipated while they may be more sensitive to other power quality
effects. The board operated fine under THD values up to 14.5% whereas it malfunctioned for
single-phase voltage sags that caused a voltage phase shift in the faulted phase [6].
Table 2 Summary of the impact of harmonics on electric equipment
Type of
Impact
Mechanical
Equipment
Mechanism
Limiting values
I2R Pulsating
Torques
2
I R Pulsating
Torques
I2R
Limiting
parameters
VTHD (2nd
harmonic)
ITHD, VTHD (2nd
harmonic)
ITHD
Motors
Neutro conductor
overloading
Transformers
I2R
ITHD
TBD
I2R
Capacitors
Capacitor fuses
Power supplies
Ballasts for
fluorescent lamps
Metering
instruments
Protection, relays
and breakers
Zero crossing firing
circuits
I2R
I2R
I2R
I2R
Ih, ITHD
Vh, VTHD
VTHD
VTHD
Vh VTHD
Vh, VTHD
De-rating
standards >10%
>10%
>10%
15 to 20%
>10%
Ih, ITHD, Vh, VTHD,
Phage Angle
Ih, ITHD, Phase
Angle, Vh, VTHD
Waveform
distortion
TBD
>20%
TBD
>10%
10-15%
Voltage Sag /
Swell, THD
Waveform,
TBD
Generators
Excessive
Heating
Electronic
Interference
Cables
UPS, timers, peak
sensing
Distortion
phase-shift
Distortion
phase-shift
Waveform,
multiple Z
crossings
Insufficient
voltage
12
2 to 3%
8%
TBD
TBD
CHAPTER 6 IMPACT ON SHIPBOARD POWER SYSTEMS DESIGN
Considering the new power systems environment with higher power density, non-linear demands
and broad spectrum of sensitive electronic controlled devices, the traditional harmonic distortion
limits for shipboard design may need to be revised. The Navy has in the past adopted IEEE 519
which is currently under revision and intends to increase the voltage THD from 5% to 8% for
voltages below 1kV. This is a consequence of recognizing the need for compatibility limits rather
than fixed values which can impose unnecessary restrictions to the design. This becomes a more
critical issue for shipboard design due to the premium power paid for extra equipment for filtering
and footprint. Table 3 depicts the possible impacts associated with shipboard power systems
design in terms of impact, mitigation cost and benefits as distortion levels become more
pronounced.
Preliminary efforts to characterize these costs on the distribution network have been published by
the IEEE (see bibliography). These studies show that the most important cost component is likely
to be the costs associated with applying mitigation measures, such as harmonic filtering, to reduce
harmonic levels. The costs should include also the total active power loss and the capital invested
in design and construction of filtering stations needed to maintain THDV below an acceptable %
level.
Reference [7] indicate very little problems with harmonics on shipboard (only 10% of those
surveyed indicated potential problems. 90% did not know or never heard of any problems. Further
more reference [8] reported that values over 10% VTHD may create communication problems. It
remains to be determined the actual impact in terms of losses, mitigation cost and benefits of
increasing harmonic distortion on shipboard design power systems design.
13
Table 3 Harmonic distortion limits on shipboard design
Impact
Losses
Interference
Mitigation Approach
Filtering
Compatibility
Negligible
TBD
Negligible
Unlikely
NA
X% of
Equipment
NA
0%
TBD
Possible
Y% of
Equipment
0%
15%
TBD
(contingency)
Possible
Z% of
Equipment
Fraction of
filtering
Impact Issues
Distortion
Level
<< 5%
> 5%
10%
14
Benefit
If
Compatibility
NA
X% of
Equipment
+ footprint
Y% of
Equipment
+ footprint
z% of
Equipment
+ footprint
CHAPTER 7 DIFFERENT HARMONIC STANDARDS
Different conditions determined by new technologies and power environment will require a costbenefit tradeoff analysis which addresses the impact on the ship platform versus equipment impact
in terms of weight, space, power consumption, reliability, total ownership cost, and total ship
effectiveness, including the so called fight-through requirement particularly for Navy ships.
Table 4 and Table 5 bellow show a list of different international harmonic standards, their
characteristics, and numerical values for voltage distortion for both utility and isolated systems. It
becomes evident some significant differences among them which prompt us to consider a reevaluation of their validity and rationale.
15
Table 4 Comparison of harmonic standard limits
Standard
EN50160:
2000
IEEE-519
IEC 610004-30
IEC 610003-6 :1996
NRS048
-2 :96
EDF
Emerald
Contract
ER G5/4
HQ
Voltage
Charact
ONS Brazil
MIL Std
1399300A
Status
European
Standard
ANSI
Standard ;
Recommende
d Practice
Technical
Report
Technical
Report
National
Standard
Premium
Power
National
Standard
Voluntary
National
Standard
Military
Standard
Where
19
European
Countries
USA
and
other
countries
International
International
South
Africa
France
England
Quebec
Brazil
USA
Purpose
Supply
Voltage
Emissions
and Supply
Voltage
PQ
measurement
methods
Planning
Levels
Minimu
m
standard
used by
the
regulator
Supply
voltage
characteristi
cs
Planning
levels for
controlling
emissions
Supply voltage
characteristics
Transmis
sion
Supply
voltage
character
istics
Shipboard
95%
95%
Indices
95%
Indices
short-time
No definite
indices
Period
Measurem
ent
One week
At least one
week
At least one
week
Measurem
ent
Method
IEC
61000-4-7
IEC 610004-7
IEC 61000-47
At least
one
week
At least one
week
IEC 610004-7
16
One week
One Week
IEC 61000-4-7
One
Week
Table 5 Harmonic voltage limits (%)
95% Probability – Recommended New Limits (LV and MV Networks) – Design / Planning
Order
IEC 61000-4-30
EN50160
ER G5/4
NRS 048-2
IEC 610003-6
IEEE
519
MIL 1399
2
2
2
1.6
2
1.6
3
3
3
5
5
4
5
4
3
3
4
1
1
1
1
1
3
3
5
6
6
4
6
5
3
3
6
0.5
0.5
0.5
0.5
0.5
3
3
7
5
5
4
5
4
3
3
8
0.5
0.5
0.4
0.5
0.4
3
3
9
1.5
1.5
1.2
1.5
1.2
3
3
10
0.5
0.5
0.4
0.5
0.4
3
3
11
3.5
3.5
3
3.5
3
3
3
12
0.46
0.5
0.4
0.2
0.2
3
3
13
3
3
2.5
3
2.5
3
3
14
0.43
0.5
0.2
0.2
0.2
3
3
15
0.4
0.5
0.3
0.3
0.3
3
3
16
0.41
0.5
0.2
0.2
0.2
3
3
17
2
2
1.6
2
1.6
3
3
18
0.39
0.5
0.2
0.2
0.2
3
3
19
1.76
1.5
1.2
1.5
1.2
3
3
20
0.38
0.5
0.2
0.2
0.2
3
3
21
0.3
0.5
0.2
0.2
0.2
3
3
22
0.36
0.5
0.2
0.2
0.2
3
3
23
1.41
1.5
1.2
1.5
1.2
3
3
24
0.35
0.5
0.2
0.2
0.2
3
3
25
1.27
1.5
0.7
1.5
1.2
3
3
>25
2.27(17/h)-0.27
Na
0.2+0.5*(2
5/h)
0.2+1.3*(2
5/h)
0.2+0.5*(25/
h)
3
3
THD
8%
8%
4%
8%
6.5%
5%
5%
17
Following are the main characteristics of the most relevant standards for shipboard power system
design regarding harmonic distortions:
IEEE 519
The standard describes measurement techniques and equipment requirements, but does not impose
specific indices. A project application guide P519A however indicates that the 95% probability is
the appropriate level to compare with the IEEE 519 limits. No specific time integration for
assessing the index is given. The objective of the standard is to recommend engineering practices
for utilities and is given in the form of voltage distortion limits for different class of voltage
levels. These limits, given in Table 4, are to be used as system design values for the worst case for
normal operation (longer than 1 hour). For shorter periods, the limits may be exceeded by 50%.
The limits are intended to utility systems, but they have become very popular and have migrated
into design of many other systems. As a consequence its utilization needs to be questioned.
The IEEE 519 is under current review and an increase in allowable distortion values for the lower
voltage (1kV) range is likely to occur. Also measurement procedures and short-term harmonic
will follow IEC Standards. See Appendix IV for a summary of the reviews presently considered as
of June 08, 2004.
Table 6 Harmonic limits from IEEE 519 (PCC point of common coupling)
Bus voltage at PCC [kV]
Individual voltage Vh (%)
Total distortion VTHD (%)
69 and below
3
5
Between 69 and 161
1.5
2.5
Above 161
1
1.5
It is important to notice that IEEE Standard 519 is not a law, a utility rule or regulation, an ASD
specification; it is not intended as an isolated / shipboard power system standard. It does require
filters for utility customers, nor outlaw 6-pulse rectifiers, nor regulate customer generation.
The IEEE Standard 519 does provide a basis for rules, includes tutorial material and suggests
design improvements.
18
The standard also recognizes the role of the source (voltage) and the load (current) which means
that when applying the standard to an isolated system the resultant voltage should be the ultimate
parameter to limit the distortions.
MIL-STD-1399 Section 300A
This standard section establishes electrical interface characteristics for shipboard equipment
utilizing AC electric power to ensure compatibility between user equipment and the electric power
system. Characteristics of the electric power system are defined and tolerances are established.
User equipment shall operate from a power system having these characteristics and shall be
designed within these constraints in order to reduce adverse effects of the user equipment on the
electric power system. Test methods are included for verification of compatibility. The shipboard
electric power system serves a variety of user equipment such as aircraft elevators, air
conditioners, communication equipment, weapon systems, and computers. Electric power is
centrally generated and distributed throughout the ship from the switchboard to power panels and
finally to the user equipment served. Ship design requires that conversion equipment be
minimized and that most equipment served to be designed to operate from the type I, i.e. 60 Hz,
power system. This standard has limits very similar to the IEEE 519 as seen on Table 7 below.
Table 7 Harmonic limits from MIL-Std-1399 Section A
Voltage Type I
400 V
Individual voltage Vh (%)
3
Total distortion THDV (%)
5
IEEE STD 45
This standard also uses the IEEE STD 519 as the basis for its values and calculation procedures.
The power quality recommendations for ship service systems, as specified in Clause 4 of the
standard for voltage and frequency should be used also in integrated electrical propulsion plants
with the following exceptions:
- A dedicated propulsion bus should normally have a voltage total harmonic distortion of no
more than 8%. If this limit is exceeded in the dedicated propulsion bus, it should be verified by
documentation or testing that malfunction or overheating of components does not occur.
19
- A non-dedicated main generation/distribution bus should not exceed a voltage total harmonic
distortion of 5%, and no single voltage harmonic should exceed 3%
- A harmonic distortion calculation and measurements should normally be carried out in
accordance with a method equivalent to IEEE Std 519-1992.
In IEEE STD 45 it is stated that normally it will be desired to perform an initial calculation as
input to the final design of electrical equipment, such as generators and transformers, and a final
calculation after the equipment is designed using the final design parameters. Measurement of
harmonic distortion, if desired to verify analytic results, should be done in a variety of typical
operating conditions and voltage levels to establish a baseline for the system. In order to achieve
the THD limits, consideration should be given to using equipment such as transformers, passive
or active filters, or rotating converters. When passive harmonic filters or capacitors are used for
nonlinear current compensation, attention should be paid to any adverse effect of fluctuations on
the RMS and peak values of system voltage. Failure of fuses in harmonic filter circuits should be
detected.
Section 4.6 of IEEE STD 45 is transcribed below for the sake of clarity:
“Solid state devices such as motor controllers, computers, copiers, printers, and video display
(may list some specific equipment used in NAVY ships here if we know some) terminals produce
harmonic currents. These harmonic currents may cause additional heating in motors, transformers,
and cables. The sizing of protective devices should consider the harmonic current component.
Harmonic currents in nonsensically current waveforms may also cause EMI and RFI. EMI and
RFI may result in interference with sensitive electronics equipment throughout the vessel.
Isolation, both physical and electrical, should be provided between electronic systems and power
systems that supply large numbers of solid state devices, or significantly sized solid state motor
controllers. Active or passive filters and shielded input isolation transformers should be used to
minimize interference. Special care should be given to the application of isolation transformers or
filtering as the percentage of power consumed by solid state power devices compared with the
system power available increases. Small units connected to large power systems exhibit less
interference on the power source than do larger units connected to the same source. Solid state
20
power devices of vastly different sizes should not share a common power circuit. Where kilowatt
ratings differ by more than 5 to 1, the circuits should be isolated by a shielded distribution
system..Surge suppressors or filters should only be connected to power circuits on the secondary
side of the equipment power input isolation transformers.
To reduce the effects of radiated EMI, special considerations on filtering and shielding should be
exercised when main power switchboards and propulsion motor drives are installed in the same
shipboard compartment as ship service switchboards or control consoles. IEEE Std 519™-1992
provides additional recommendations.
Other Standards and Normative Documents
Cenelec EN 50160
Indices
During each period of one week, the percentile 95 of the 10 min mean rms value (Uh, Sh) of each
individual harmonic voltage is the quality index to be compared to the relevant Voltage
Characteristic.
Objectives
The document defines and describes the main characteristics of the voltage at the customer's
supply terminals in public low voltage and medium voltage electricity distribution systems under
normal operating conditions.
The standard gives the limits or values (see Table 8 below) within which any customer can expect
the voltage characteristics to remain, and does not describe the typical situation for a customer
connected to a public supply network. Resonances may cause higher voltages for an individual
harmonic.
Table 8 Values of individual harmonic voltages at the supply-terminals for LV and
21
IEC 61000-4-30 [9]
The basic measurement of voltage harmonics, for the purpose of this standard, is defined in
61000-4-7 class 1 [10]. This standard shall be used to determine a 10/12-cycle gapless harmonic
sub-group measurement.
Measurement interval: One week minimum assessment period for 10-min values, and daily
assessment of 150/180-cycle values for at least one week.
Evaluation techniques: 150/180-cycle time period and/or 10-min values might be considered.
Contractual values may be applied to individual harmonics, or range of harmonics, or other
groupings, e.g. even and odd harmonics, according to agreement between parties in the contract.
The following techniques are suggested for all values, but other evaluation techniques might be
agreed between the parties:
1. The number, or percent, of values during the interval that exceed contractual values might be
counted;
2. And/or the worst-case values might be compared to contractual values (the measurement
interval might be different for this possibility, for example one year);
22
3. And/or one or more 95 % (or other percentage) probability weekly values for 10-minute values,
and/or 95% (or other percentage) probability daily values for 150/180-cycle tie period values,
expressed in percent, might be compared to contractual values.
IEC 61000-3-6[9]
Indices
The basic standard used for harmonic and interharmonic measurements is IEC 61000-4-7. The
minimum measurement period should be one week. For harmonics, indices should be:
1. The greatest 95 % probability daily value of Uh (R.M.S. value of individual harmonic
components over "very short" 3 s periods) should not exceed the planning level.
2. The maximum weekly value of Uh (r.m.s. value of individual harmonics over "short" 10min
periods) should not exceed the planning level.
3. The maximum weekly value of Uh,vs should not exceed 1,5 to 2 times the planning level.
Planning levels are levels that can be used for planning purposes in evaluating the impact on the
supply system of all consumer loads. Planning levels are specified by the utility for all voltage
levels of the system and can be considered as internal quality objectives of the utility. Planning
levels are equal to or lower than compatibility levels. Only indicative values are given in IEC
61000-3-6 because planning levels may differ from case to, case, depending on network structure
and circumstances. As an example, see the planning levels for harmonic voltages presented in
Table 9 below.
Table 9 Indicative values of planning levels for harmonic voltage (in % of the nominal voltage) in
MV, HV and EHV power systems
23
NRS 048-2:1996 [11]
Indices
The assessment period shall be at least 7 continuous days. On each phase of the supply voltage,
the instrument samples and records each harmonic voltage at intervals of 3 s or less. These
samples are summed over each 10 min period, to obtain 10 min rms values, V10, h over each
period of 24 h.
For each harmonic and for the THD, for each 24h day, the highest 10 min rms values which are
not exceeded for 95% of the time are recorded for each phase.
For each harmonic order and for the THD, the highest of the values on each phase shall be
retained as the assessed daily values.
The assessed levels which are to be compared with the limits are the highest of the assessed daily
values over the full assessment period.
Objectives
The limits for harmonics on LV and MV networks are given in Table 10 below.
24
Table 10 Limits for harmonic voltages (in % of the declared voltage of LV and MV power
systems)
Indicative values of planning levels for harmonics on HV and EHV networks are given in Table
11 below.
Table 11 Indicative values of planning levels for harmonic voltages (in % of the declared voltage
of HV and EHV power systems)
25
Contrat Emeraude [12]
Indices
For each harmonic order, the rms voltage is summated over a 10 min period. Each of these values
are compared to the indicative compatibility levels.
Objectives
The indicative compatibility levels for MV and HV networks are given in Table 12below.
Table 12 Target levels for harmonic voltages (in % of the supplied voltage of MV and HV power
systems)
ER G5/4 [13]
Engineering Recommendation G5/4 “Planning Levels for Harmonic Voltage Distortion and the
Connection of Non-linear Equipment to Transmission Systems and Distribution Networks in the
United Kingdom” is published by the Electricity Association, the trade association for the UK
electricity industry. Engineering Recommendation G5/4 is a code of practice used throughout the
UK by all Network Operators; it describes three stages of requirements for the connection of nonlinear equipment to public distribution and transmission systems in the UK. The Stage used for
26
any particular connection is based on the voltage at the point of connection and the level of
harmonic distortion expected on the network when the new load is connected.
Indices
A set of samples is taken of the waveform, with an aggregate of between 1 and 3 seconds, in an
overall window of 10 seconds. These are processed by an FFT (with Hanning weightings to the
samples) and the harmonic levels found over this window. After 6 such processes (i.e., each
minute) the one is selected which has the highest THD, and the data for this is output as the
harmonic content for that minute. Over the duration of the survey each site is monitored each
minute for 7 days.
Objectives
The limit currently applied is that the 95 percentile should remain below the levels (to be
confirmed) shown in the Table 13 below
Table 13 Planning levels for harmonic voltage distortion
For shipboard General Requirements of Electrical and Electronic Systems see Summary on
Appendix III
27
COMPARATIVE ANALYSIS
Indices
The previous information indicates that the most common index for harmonic voltage is the socalled short time interval index or the 10-minute value as per IEC-61000-4-7. For the
characteristics of the supply voltage, the level of harmonics to be compared with the objectives is
usually the value corresponding to 95% cumulative probability of weekly statistics.
With regard to planning levels, more detailed indices are also proposed in IEC61000-3-6 (the socalled 3-sec. very short time interval (Vh3-sec: 95% daily and max. weekly) because the aim here
is to limit emissions which also require a closer control of higher disturbances allowed to very
short time emissions. It should however be questioned whether the maximum weekly Vh3sec is a
relevant index for controlling harmonic emissions considering that transients such as transformer
switching may cause high levels of harmonics for seconds. Indeed maximum values of Uh3-sec
are likely to be inflated by a number of transients.
Objectives
For medium voltage systems, the most common values of objectives for harmonic voltages
correspond to table 2 of standard EN50160. It should be noted that a new project IEC 61000-2-12
(77A/321/NP) currently at draft stage proposes new compatibility levels for medium voltage
public networks (see table 12) which slightly differ from levels presented in EN50160 especially
for high order harmonics. Harmonization between those two standards may eventually be needed.
By comparison, harmonic voltage levels allowed in ANSI/IEEE Std 519 are definitely more
severe for low order harmonics (up to 50% for harmonic order 5), but the allowed levels are the
same ones for all harmonic orders. It should also be noted that this standard is currently under
revision.
Compatibility levels may be seen as achieving an optimum co-ordination between disturbance
levels on the supply system and the immunity of the equipment connected to it. However it is
important to realize the different purpose of voltage quality objectives for transmission systems.
On the contrary of low voltage systems, quality objectives for harmonics (flicker, unbalance) in
28
HV-EHV transmission systems are not directly related to equipment immunity because the enduse equipment is not directly connected at HV-EHV. Indicative levels given in IEC-61000-3-6 for
HV-EHV systems are well below levels that could cause immediate disturbances on the
equipment. Limiting disturbances on transmission systems plays a role of coordination between
different parts of the system or different voltage levels. Therefore voltage quality levels at HVEHV may differ from case to case depending on the system configuration, the transfer
characteristics between the different voltage levels (attenuation or amplification), the actual
disturbance levels on the system, etc. Planning levels play this role. Voltage characteristics may be
equal or higher than planning levels depending on the margin required to ensure these
characteristics.
Standard such as EN50160 does not exist for defining voltage characteristics for transmission
systems at HV-EHV. National standards or regional guidelines however give indicatives values
for harmonic voltages. In these cases, the most common values at HV -EHV correspond to
planning levels published in report IEC 61000-3-6 with local adaptations to account for specific
system configuration or circumstances. The desired margin between planning levels and voltage
characteristics is however obtained by using less severe indices for the latter.
Standard ANSI/IEEE 519 recommends harmonic voltage limits at HV-EHV for system design
purpose. For voltage levels higher than 161 kV, this standard recommends harmonic voltage
levels that are also definitely more severe (up to 50% for harmonic order 5) than planning levels
proposed in IEC-61000-3-6, but the allowed levels are the same ones for all harmonic orders. This
standard is currently under revision.
Other Harmonic Design Issues
Power electronics can enhance ship efficiency and maneuverability and consequently reduce the
volume devoted to machinery within the ship. For future Navy ships, novel applications such as
electric rail guns, energy weapons, electromagnetic aircraft launchers, electric armor and high
power radar and sonar systems will increase their fight-through capability. However, these
applications will certainly increase the content of harmonic injection and proper design guidelines
need to be developed.
29
The inclusion of such loads will require detailed engineering studies related to stability,
electromagnetic compatibility and control of the power system. In addition, the impact of PWM
voltage waveforms on electrical machines needs to be investigated and proper design limits
established.
30
CHAPTER 8 SOME INITIAL VERIFICATION / EXPERIMENTS ON HARMONIC IMMUNITY
In order to verify the actual impact of harmonic distortion on sensitive equipment an initial test
was conducted on a computer switched-mode PC ATX 300W power supply. The results indicate
high tolerance to harmonic distortion. RTDS Test Implementation - Setup: See Figure 3 below.
Figure 3 Switched mode PC power supply test - RTDS test implementation
Steady state harmonic distortions (VTHD 5 to 18%) with individual components (varying from 3
to 12%). Harmonic phase angles kept the same (for 0, 90 and 180 degrees). Alternatively,
different combination of harmonic phase angles should be used to produce different waveforms
which could create performance degradation (multiple zero-crossings, etc.) Table 14 shows the
cases planned and/or carried out. After several tests the power supply failed for Case 6A (total
VTHD around 18%). Figure 4 shows the picture of the small resistor and inductor (from the
output dc filters) which failed due excessive heating. More detailed analysis will be necessary to
determine the final cause and ways to increase immunity.
31
Table 14 Test Cases
Table of Cases for Testing PC ATX Power Supply
Case
Number
Case Description
Observations
THD (%)
1A, 1B, 1C
All Harmonic voltages at 0.03
Phase Shift of Fundamental 0 (A), 90 (B),
180 (C)
(*) harmonics
3,5,7,9,11,13,15
7.9
2A, 2B, 2C
All Harmonic voltages at 0.04
Phase Shift of Fundamental 0 (A), 90 (B),
180 (C)
(*) harmonics
3,5,7,9,11,13,15
13.2
3A, 3B, 3C
5th and 7th Harmonic at 0.08 others at 0.03
Phase Shift of Fundamental 0 (A), 90 (B),
180 (C)
15.60
4A, 4B, 4C
11th and 13thHarmonic at 0.08 others at 0.03
Phase Shift of Fundamental 0 (A), 90 (B),
180 (C)
15.60
5A, 5B, 5C
5th and 7th Harmonic voltages at 0.12
Phase Shift of Fundamental 0 (A), 90 (B),
180 (C)
Time-Varying
Short term burst of
10 minutes
18.2
6A, 6B, 6C
11th and 13th Harmonic voltages at 0.12
Phase Shift of Fundamental 0 (A), 90 (B),
180 (C)
Time-Varying
Short term burst of
10 minutes
18.2
Computer power supplies are usually designed to operate over a range of AC input voltages. They
produce a DC voltage that is affected by the waveshape of the AC waveform. Harmonic distortion
has the effect of actually reducing the computer power supply's operating voltage. That variation
is compounded by the normal variation of 10%. The end result is that the computer supply may
malfunction, hiccup, or fail to provide the required output logic voltages, resulting in potential bit
errors.
A further negative side-effect of effectively lowering the computer's operating voltage range is the
reduction of "hold-up" time, essential for the ability of the power supply to "ride through" power
sags (caused by other equipments' start ups). The result is more potential bit errors.
32
Figure 4 Damaged parts on PC ATX 300W computer power supply
Future Work
From the initial tests with a computer power supply the following is concluded:
• Power supplies can tolerate highly distorted voltages (>15%), much higher than
expected and established by harmonic standards.
• More detailed tests with power supplies and other sensitive equipment should be
planned to be conducted with the presented setup. Future investigations should
concentrate on control cards for inverters, drivers and other power electronic applications.
33
• These experiments will help determine the optimum harmonic distortion levels for
shipboard electrical power system environment.
34
CHAPTER 9 PROPOSED PHILOSOPHY
Four conditions are necessary for the establishment of a good harmonic standard.

It must be comprehensive in order to adequately take the complexity and the dynamics of
the problem into account.

It must be flexible to accommodate the different conditions and networks (e.g.
reconfiguration of power systems).

It must be effective in the sense of guaranteeing the compliance with the compatibility
conditions necessary for the proper operation of the system (equipment and loads).

It must be simple enough for both design and operating engineers so they can understand
and apply it unequivocally.
One basic aspect that needs to be taken into consideration is that the objective of harmonic
standards and limits is to provide guidelines to establish compatibility rather than strict limits. [5]
illustrates this concept by graphing the probability density function versus the levels of
disturbances and immunity levels. The graph also points to the region where harmonic planning
and immunity tests limits should be set. In the power system interference is inevitable and
therefore there is some overlapping between the distributions of disturbance and immunity levels
which are specified by the standards. Harmonic distortion may be equal to or higher than the
compatibility level. Planning levels may be equal to or lower than the compatibility level and are
specified by the owner of the power system. Immunity test levels are specified by relevant
standards or agreed upon between manufacturers and users. In a shipboard system the owner of
the utility system and load user is the same and this makes the process different form the
traditional land based utility power systems.
35
Compatibility Level
Probability Density
Planning Levels
Voltage Characteristics
Immunity Test
TestsLevels
System Disturbance
Disturbances
Level
Equipment
Immunity Levels
Harmonic Distortion / Disturbance Level (%)
Figure 5 Probability density function versus levels of disturbances and immunity
Considering that the limits established have been traditionally based on thermal aspects, it is
recommended that a re-evaluation of the impact of the required numerical values on the
effectiveness of the design of the systems for harmonic distortion.
Following are the basic parameters that need to be considered and well defined for the
establishment of effective harmonic standards:

Voltage peak values.

Harmonic voltage waveforms.

Probabilistic nature of harmonic sources.

Distinction between the impacts at different network voltage levels.

Definition of individual harmonic levels based on previous knowledge of possible
excessive disturbing influence and use of total harmonic current distortion as a measure of
their aggregate effect.

Planning/expansion considerations to account for the evolution of the network state
regarding harmonic distortions.

True sensitivity of (particularly electronic) equipment to harmonics
36
As a consequence, the proposed approach / philosophy is intended to provide flexibility and
insight on diagnosing harmonic problems and promote the concept or compatibility, rather than
strict limits. The approach may provide information on individual harmonic distortion
compatibility and allow for reference values to be adjusted to individual equipment. Additional
rules could be incorporated to expand the concept and applications to other power quality
parameters and more realistic diagnostic evaluations.
It is proposed that limits may be evaluated based on actual impact of harmonic distortions on
different equipment and as a function of the severity of the problem. As an example, Figure 6
shows how a THD voltage limits could be specified in a way to relate to the impact of the
distortion. This approach may seem to be particularly helpful for isolated systems such as a
shipboard since compatibility levels may be easier to determine due to the limited dimensions of
Temperature (Celsius)
Harmonic Distortion (THDv)
Versus Equipment Heating
Color Code Harmonic Criteria
Below
Normal
Equipment Heating
the system.
No Problem
100
Below Normal
Normal
110
Over
Heating
120
Very
Hot
90
Caution
Possible Problems
Imminent Problems
150
1
Normal
Levels
0
3
Caution
4
5
Possible
Severe
Dangerous
Problems Distortions
Levels
6
7
8
9
THDv in %
Figure 6 Example of flexible limitation of harmonics as a function of equipment temperature
The method is then translated to Table 15 and illustrated in (Figure 7 Proposed harmonic voltage
(%) limits versus IEEE 519 for a value corresponding to 95 % non-exceeding probability levels)
and Figure 8 Proposed short term THD (%) which proposes limits for short-term harmonic bursts.
The values for individual harmonic voltage and THD specified in Table III are an attempt to both
37
simplify and allow for flexibility within the typical ranges encountered isolated or weak systems.
The short-term limits need to be further validated, but are also based on typical systems which can
be tolerated without damage to equipment.
Table 15 Proposed voltage harmonic limits For shipboard power systems
Order
Recommended Limits
Even
2
Odd
Triple
2
3
4
5
1
5
6
5
1
7
8
5
1
9
10
1
1
11
12
4
0.5
13
14
4
0.5
15
16
0.5
0.5
17
18
3
0.5
19
20
3
0.5
21
22
0.5
0.5
23
24
2
0.5
25
2
38
>25
1
Caution – 5 - 7%
Possible Problems – 7 - 9%
THD
Recommended Action - > 9%
7
Harmonic Order
6
IEEE 519 Vh Limits (odd components)
Limits
Veveni
Vtriplei
4
Voddi
IEEE519i
2
0
0
5
10
1
15
20
25
i
Harmonic Voltages
27
Figure 7 Proposed harmonic voltage (%) limits versus IEEE 519
Voltage THD (%)
20
THDProblems2i
20
Proposed Short-Term
Limits
15
THDCaution2i
THDShortTermi
10
Proposed Max 95% THD Limit
IEEE519THDi
5
IEEE 519 THD Limit
0
0
0
1
5
10
15
i
Time (M inutes)
Figure 8 Proposed short term THD (%)
39
20
25
25
In most existing standards the limit for even harmonics is stricter than that for odd harmonics. For
example the IEEE 519 limits even order current harmonics to 25% the level permitted for odd
order harmonics. This is because even harmonics can cause voltage asymmetry and DC
components. However, for higher frequencies (over the 14th), the impact is reduced and values
higher than 0.5% and up to 1% can be tolerated.
40
CHAPTER 10 CONCLUDING REMARKS
This report reviews some of the issues and difficulties regarding the harmonic design aspects on
shipboard power systems and proposes the consideration of higher and more flexible limits which
have the potential to effectively satisfy equipment requirements and improve the overall ship
performance. The report proposes a simpler individual harmonic voltage table of values with three
stages of corresponding THDs (Caution, Possible Problems and Recommended Action) and shortterm limits. The 95% THD should be kept under 9% and short-term values should not to exceed
15% (for less than 5 minute bursts). With regard to interharmonics (see Appendix II) we propose
to remain within the limits suggested by the IEC until further investigations demonstrate that
higher values can be accepted.
The design of shipboard power systems should ensure that the quality of the supply system is
adequate / compatible with all types of loads and system equipment. In order to achieve this
objective harmonic distortion standards and limits should be established in such a way that they
allow true optimization of the new class of integrated all-electric ship power systems. Furthermore,
advanced system analysis methods such as full transient simulations (off-line and in real-time) shall
be employed more in the future to fully investigate the sensitivity of systems and equipment to the
proposed harmonic limits.
41
REFERENCES
[1] IEEE Std 519-1992, “IEEE Recommended Practices and Requirements for Harmonic Control
in Electric Power Systems.”
[2] IEEE STD 45™-2002 - Revision of IEEE Std 45-1998, “IEEE Recommended Practice for
Electrical Installations on Shipboard.”
[3] MIL-STD-1399 (NAVY), Section 300A, 13 October 1987, “Interface Standard for Shipboard
Systems, Electric Power, Alternating Current.”
[4] W. Xu, C. Siggers, M.B. Hughes, and Y. Mansour, "Developing Utility Harmonic Regulations
Based on IEEE Std. 519", IEEE Transactions on Power Delivery, Vol. 10, No. 3, June 1995,
pp.1423-1431.
[5] Mack Grady, W.; Santoso, S.; Power Engineering Review, IEEE ,Volume: 21 , Issue: 11 ,
Nov. 2001, Pages:8 – 11.
[6] Y. Liu, M. Steurer, S. Woodruff, P. F. Ribeiro, “A Novel Power Quality Assessment Method
Using Real Time Hardware-in-the-Loop Simulation” to be presented at the ICHQP11, Sept 12-15,
2004, Lake Placid, NY, USA.
[7] I. Jonasson, L. Soder, “Power Quality on Ships. A Questionnaire Evaluation Concerning
Island Power System,” International Conference on Harmonics and Quality of Power, October
2000, Vol. 2, pp. 639-644.
[8] P. M. Nicolae, “Modeling the Influence of External Residual Harmonic over the Three-Phase
Short-Circuit Transient Electromagnetic Processes in a Cylindrical Rotor Synchronous
Generators,” Electric Machines and Drives, 1999. International Conference IEMD '99, 9-12 May
1999, Pages:311 – 313.
[9] IEC, “Electromagnetic compatibility (EMC) - Part 4-30: Testing and measurement techniques
- Power quality measurement methods,” IEC 61000-3-6.
[10] IEC, “Electromagnetic compatibility (EMC) - Part 4: Testing and measurement techniques,”
IEC 61000-4-7.
[11] NRS 048-2:1996 – Electricity, Quality of supply, part 2: minimum standards (South Africa,
September 2000)
[12] Electricity de France, “Power Quality Contract”, 1997.
[13] Engineering Recommendation, “G5/4 Engineering Technical Report 122,” Engineering
42
Council, UK.
43
APPENDIX I HARMONIC SOURCE
I.1 Introduction
Non-linear loads and system devices draw distorted current through the supply system. Thus
distorted voltages appear on the system and it is necessary to predict them, for they are
potentially harmful.
The prediction is almost always made by using harmonic components, because it then simplifies
to a calculation of sinusoidal currents and voltages in a passive network. Rigorous interactive
frequency domain and time domain solutions have been developed. An attempt to incorporate
similar analysis for transmission system studies leads to an unacceptable degree of complication.
On the other hand, there is reason to believe that the usual approach of using a constant current
source could lead to significant errors under certain system conditions.
A review of the basic characteristics in terms of the mechanism of harmonic generation is
presented in Section 5.3.
I.2 Basic concepts
Fourier transform
The Fourier transform is the theoretical basis for representing a non-sinusoidal waveform in the
frequency domain. It is important to remember that the harmonic components individually have
no physical significance. They are fictitious components which simplify the calculations allowing
the use of Ohm's and Kitchoff's laws and also are a way to quantify a distorted waveform. What
exists in reality is the distorted waveform.
When calculating the harmonics produced by a particular source, a number of aspects regarding
practical conditions have to be considered, such as unbalance in the supply system, non-linear
sources, and system resonances and variations with time. Consequently, only site measurements
can fully assess the harmonics produced.
44
Direction of flow of harmonics
Before describing different harmonic sources, a basic concept about the direction in which
harmonics flow is explained, and that is: a non-linear load (see Figure I.1) can be represented by
a generator injecting currents into the supply systems. The supply system can be seen as a passive
network for harmonic frequencies. Alternatively at the fundamental the non-linear element is a
linear impedance.
Generator
Distribution
transformer
Power flow
Non-linear load
YY
(a) Diagram of a distribution system
1
1
Amplitude (p.u.)
Amplitude (p.u.)
Harmonics
0.5
0
-0.5
0
0.5
0.5
0
1
1.5
2
0
2
4
6
Time (S)
Harmonics
(b) Time domain waveform
Line impedance
8
Line impedance
Generator
Generator
impedance
Linear
load
Harmonic
Fundamental
(c) Frequency domain
Figure I.1 Direction of harmonic flow
I.3 Harmonic sources
In general, the harmonic sources can be classified according to their physical cause as:
(a) Magnetizing currents, e.g. transformers and saturable reactors.
(b) Flux distribution, e.g. synchronous generators.
(c) Power conversion applications, e.g. power control and computers.
(d) Arc processes in arc furnaces.
45
The harmonics in generators are normally produced by non-sinusoidal flux distribution in
the air gap and by saturation. However, in practical terms, the content of harmonic voltage
produced by synchronous generators may be neglected in harmonic propagation studies.
Although the arc furnace constitutes one of the most common distorting loads, there is no
application of arc furnace in all-electric ships. Therefore, description of the mechanism of
generation of harmonics are focused on the magnetizing currents and power conversion devices.
Magnetizing currents
Transformers: Because of the curve of the hysteresis loop for iron and steel, transformers give
rise to harmonic magnetizing currents. These currents do not normally cause any wave distortion
of consequence. An exception would be when high voltages occur due to a system upset.
Exciting currents increase very rapidly with an increase in voltage. In fact transformer standards
do specify that a 110% name-plate voltage of the transformer should not overheat without load.
In other words, at 110% voltage, the exciting losses may equal the normal full load losses of the
transformer. At 130% of rated voltage the current contains over 50% third and higher harmonics.
Even under normal excitation condition, transformer core may have entered, slightly, the
saturation region and begin to generate some harmonics in the excitation current. The degree of
the saturation depends on the transformer design.
Overexcitation is basically caused by
overvoltage. This problem is particularly onerous in the case of transformers connected to large
rectifier plant following load rejection [I.1, [I.2, [I.3]. Overvoltage drives the peak operation
point of the transformer excitation characteristics up to saturation region so that more harmonics
are generated. In this case, the magnetizing current of overexcitation is often symmetrical.
Except odd order harmonics, a transformer can generate even harmonics when it is biased
by a DC. Figure I.2 shows two different types of DC-biased transformers. Figure I.2 (a) shows a
transformer biased by a DC current produced from a power electronic load (e.g., half wave
rectifier). Other examples that cause a DC bias on the load side of the transformer include
various types of electronically switched loads. Figure I.2 (b) shows that a transformer is biased by
a DC voltage (e.g., geomagnetically induced voltage) on the system side. In the presence of a DC
component, the transformer can be single-sided saturated. As a result, even harmonics
(especially, second harmonic) occur in the other side of the transformer. Figure I.3 shows how
the even harmonics are generated. A simplified B-H curve is used in Figure I.3, where slope is
46
the slope of the saturation region. Significant even harmonics (e.g., second and fourth harmonics)
are shown in Figure I.3.
i L (t )
iS (t )
Supply
voltage
Supply
voltage
Half wave
rectifier
DC
voltage
(a) DC current biased transformer
vS (t )
v L (t )
Linear
load
(b) DC voltage biased transformer
Figure I.2 Saturated DC-biased transformers [I.2]
B
Slope
sfo
rm
New reference
an
0
Fo
ur
ier
Tr
H
0.8
0.6
0.4
0.2
0
DC
Amplitude (p.u.)
1
0
2
4
6
8
10
12
Harmonics (order)
Figure I.3 Operation of a saturated DC-biased transformer [I.2]
Saturable reactor: Saturable reactors also produce harmonics by the same mechanism as in
transformers. These are iron-core inductances which have supplemented control windings. When
voltage decreases the extra winding inductance is reduced to permit more current flow. The
generation of harmonic currents however, under normal condition is very limited.
Power conversion devices
With the development of solid-state technology, more and more power electronic devices are
used in power grids to convert a.c. power to d.c. power or different amplitude, different
frequency a.c. power. In the electronic converting process, the converter breaks or chops the a.c.
current waveforms by allowing the current to flow during a portion of the cycle. The a.c current
is determined by the characteristics of the converter, e.g. type of connection, control,
47
characteristics of the supply network and nature of the load. Because the ‘chopping’ frequency
can be unsynchronized, inter-harmonics can be also generated from those power conversion
devices (see appendix B).
There are a large number of power conversion devices used in all-electric ships. They includes
variable speed drives (VSD), AC/DC converters, computer power supply, cycloconverters, and
superconducting magnetic energy storage (SMES). They can be classified according to the solidstate devices used:
(a) Thyristor-based power conversion devices, such as line commutated converters,
cycloconverters, integral cycle controllers, and statistic VAR compensator
(b) IGBT, GTO, CMOS-based power conversion devices, such as PWM converters.
Line commutated converters: The introduction of economic and reliable line commutated
converters has caused a significant increase in harmonic-generating loads, and they have
dispersed over the entire power system. In most cases, line commutated converters are the cause
of harmonic problems in power distribution systems. These devices are work horse circuits for
ac/dc power conversion. The common application of static power converters is in adjustable
speed drives for motor control. Another application is in HVDC terminals. The device can be
operated as a six-pulse converter or configured in parallel arrangements for higher pulse
operation. Theoretically, a static power converter load draws currents from the source system that
consist of positive and negative currents which are equally separated. The pulse number refers to
the number of "humps" on the dc output voltage that are produced during every ac cycle.
If a thyristor is triggered at zero firing angle, it acts exactly like a diode. The term line
commutated converter refers to the fact that the load actually turns thyristors off, rather than them
being turned off by external control circuits. The ideal ac current waveform for a six-pulse
converter is on for 120 degrees and off for another 60 degrees. During the on period, the dc load
current is assumed constant in the ideal case due to the assumed existence of a large series dc
inductor.
We see that the ac harmonic currents generated by a six-pulse converter include all odd
harmonics except triplens. Harmonics generated by converters of any pulse number can be
expressed as,
h  pn  1
48
(I.1)
where n is any integer and p is the pulse number of the converter. For the ideal case, converter
harmonic current magnitudes decrease according to 1/h rule [I.1, [I.3, [I.4].
Cycloconverters: The cycloconverter is a device that converts ac power at one frequency
into ac power at a lower frequency. Cycloconverters are usually used in low speed and large
horsepower applications, such as rolling mills and electric traction. It can also be used in ship
propellers. Figure I.4 shows the schematic of a three-phase 3-pulse half-wave cycloconverter.
The harmonic frequencies generated by a cycloconverter depend on the output frequency, which
is varied in operation to control motor speed. The output frequency of a cycloconverter can be
controlled by precisely timing the firing pulses at its thyristor gates through computer control.
The harmonic components generated from cycloconverter can be expressed as
f = (np  1) fi  2 k l fo
(I.2)
where fi is the system frequency (60 Hz or 50 Hz), fo is the cycloconverter output frequency, p is
the pulse number, l and n are any integers, and k is the number of phases [I.5]. Obviously, it can
generate inter-harmonics as well as harmonics.
Single phase cycloconverter
P Converter N Converter
A
B
C
Load
Figure I.4 The schematic of a three-phase 3-pulse half-wave cycloconverter
Integral cycle controllers (ICC): ICCs are extensively used in heaters, ovens, furnaces, and
spotwelders. ICCs are AC switched controllers that entail current blocking at an integral number
of cycles to effect control. Figure I.5 shows a topology of a single-phase ICC and a sample
switching strategy. An ICC is composed of a pair of back-to-back SCRs and a switching
controller. Significant inter-harmonics can be generated from ICCs [I.6].
49
i L (t )
vS (t )
v L (t )
RL
Switching
controller
iL(t)
6T0
Figure I.5 A representative topology of a single-phase ICC and a sample switching strategy. To is
one cycle length
Converter load: Converter loads [I.4, [I.7] may draw DC and low frequency currents from
supplying transformers. The transformer cores are biased by these load currents and driven to
saturation. For example, a cycloconverter with single phase load will draw DC currents from
source transformer when its output frequency fo and input frequency fi have the relationship of
fi=2nfo , here n is an integer [I.4].
Static VAR Compensator (SVC): SVC is used as a voltage controller in the power
system. This device controls network voltage by adjusting the amount of reactive power supplied
to or absorbed from the power system. The applications of the SVC are usually for local
compensation of reactive power to industrial loads and for regulation of utility network voltages
to improve transfer capabilities across the transmission system. Typical configuration of an SVC
consists of shunt capacitors with a thyristor-controlled reactor (TCR) connected in parallel.
Pulse-Width Modulated Converters: PWM converters use power electronic devices that
can be turned off and turned on. Therefore, voltage and current waveforms can be shaped more
desirably. The switching components can be thyristor that are forced off by external control
circuits, or they can be GTOs or power transistors. The latter devices are usually used because of
their fast switching characteristics needed for effective PWM.
In a PWM converter, the
switching devices are controlled to switch on and off to produce a series of pulses. These pulses
are to be varied in width to produce a pulsed three-phase voltage wave for the load. Due to their
50
low efficiencies, PWM converters are limited to low power applications in the several hundred
kW or hp ranges, such as computer power supply.
Other Power Electronic Devices
Other power electronic devices which may generate harmonics in the power system
include static phase shifters, isolation switches, load transfer switches, and energy storage and
instantaneous backup power systems as well as those devices covered under the subjects of
Flexible AC Transmission System (FACTS) and Custom Power Systems (CPS).
I.4 Conclusions
The distorting sources are analyzed and classified according to their characteristics. When
attempting to classify the harmonic distortion, the harmonic sources should be represented
whenever possible by an instantaneous spectrum. However, because of the harmonic variability
observed in practice in certain electronic loads or arc furnaces, an independent spectrum based on
average or maximum values can be used.
There is reason to believe that the usual assumption of a constant current source, by injecting
currents, could lead to significant error unless corrections are made for the assumptions. Under
certain conditions the harmonic currents generated may change appreciably if the system
impedance becomes unduly high or low due to resonances.
References
[I.1] J. Arrillaga, D. A. Bradley, P. S. Bodger, Power System Harmonics, pp94-98, John Wiley &
Sons, 1985.
[I.2] Y. Liu, G. Heydt, “Power System Even Harmonics and Power Quality Indices,” Journal of
Electric Power Components and Systems. (Submitted).
[I.3] R. P. Stratford, "Analysis and Control of Harmonic Current in Systems with Static Power
Converters," IEEE Trans. on Industry Applications, Vol. IA-17, No. 1, January/February 1981,
pp. 71-78.
51
[I.4] Brian R. Pelly, Thristor Phase-Controlled Converters and Cycloconverters. Operation,
Control and Performance, pp361, John Wiley & Sons, 1971.
[I.5] Y. Liu, G. Heydt, R. Chu, “The Power Quality Impact of Cycloconverter Control
Strategies,” IEEE Transactions on Power Delivery. (In press).
[I.6] Y. N. Chang, G. Heydt, Y. Liu, “The Impact of Switching Strategies on Power Quality for
Integral Cycle Controllers,” IEEE Transactions on Power Delivery, vol. 18, No. 3, pp. 1073 1078, July 2003.
[I.7] L. Bolduc, J. Aubin, "Effects of Direct Currents in Power Transformers, Part I. A General
Approach, Part II. Simplified Calculations for Large Transformers", Electric Power System
Research, 1, 1978
52
APPENDIX II INTERHARMONICS
II.1 Introduction
Cycloconverters used in shipboard propulsion systems can cause interharmonics which can affect
the shipboard prime power system.
IEEE 519 indirectly addresses interharmonics by discussing cycloconverters which are one of the
primary sources of interharmonics on power systems. IEEE 519 does not however, provide any
general technical description of the phenomena, methods of measurement, or guidelines for
limits. As the sophistication of power electronic interfaces to the power system increases, the
frequencies present in the supply current are less likely to be limited to harmonics of the
fundamental.
In this section the basic concepts and characteristics of interharmonics are described with the
intent to understand their impact on the shipboard power systems.
The IEC-1000-2-1 [II.1] defines interharmonics as follows:
“Between the harmonics of the power frequency voltage and current, further frequencies can be
observed which are not integers of the fundamental. They can appear as discrete frequencies or as
a wide-band spectrum.”
Harmonics and interharmonics of a waveform can be defined in terms of its spectral components
in the quasi-steady state over a range of frequencies. The following table provides a simple, yet
effective mathematical definition:
Harmonic
f = h * f1 where h is an integer > 0
DC
f = 0 Hz (f = h* f1 where h = 0)
Interharmonics
f ≠ h * f1 where h is an integer > 0
Sub-harmonic
f > 0 Hz and f < f1
53
where f1 is the fundamental power system frequency
The term sub-harmonic does not have any official definition but is simply a special case of
interharmonics for frequency components less than the power system frequency. The term has
appeared in several references and is in general use in the engineering community so it is
mentioned here for completeness. Use of the term sub-synchronous frequency component is
preferred, as it is more descriptive of the phenomena.
One may note that if two steady state signals of constant amplitude and different frequencies are
linearly superimposed, the resulting time domain waveform is not necessarily periodic even
though its components are. An example of such a case is two frequencies that differ in frequency
by a non-rational number – never periodic. A practical example of this situation is the ripple
control system used in some countries (e.g. f1 = 50, f2 = 175) – periodic over 7, 50 Hz cycles.
This situation presents interesting challenges when it comes to decomposing such a waveform
back into its original steady state components. This will be addressed later when measurement
techniques are discussed.
II.2 Cycloconverters
The currents injected into the power system by cycloconverters have a unique type of spectrum.
Cycloconverters used in shipboard propulsion systems have characteristic frequencies of
fi = (p1 • m ± 1) f1 ± p2 • n • fo
where:
p1 = pulse number of the rectifier section
p2 = pulse number of the output section
m = 0, 1, 2, 3, … (integers)
n = 0, 1, 2, 3, … (integers)
(m and n not simultaneously equal to 0)
54
fo = output frequency of the cycloconverter
Because of load unbalance and asymmetries between phase voltages and the firing angle, noncharacteristic frequencies may be present given by the formula: [II.2].
fi = (p1 • m ± 1) f1 ± 2 • n • fo
Cycloconverters can be thought of as a special case of a more general class of power electronic
device - the Static Frequency Converter. Static frequency converters transform the supply voltage
into ac voltage of frequency lower or higher that the supply frequency. They consist of two parts,
the ac-dc rectifier and a dc-ac inverter. The dc voltage is modulated by the output frequency of
the converter and as a result, interharmonic currents appear in the input current according to
equations 1 and 2 causing interharmonic voltages to be generated in the supply voltage.
The magnitude of these frequency components depends on the topology of the power electronics
and the degree of coupling and filtering between the rectifier and inverter sections. The
cycloconverter is generally the most severe of these devices due to the direct connection between
rectifier and inverter common in typical cycloconverter designs, but modern adjustable speed
drives may also be of concern.
II.3 Measurements
The measurement of interharmonics poses some problems for traditional power system
monitoring equipment.
As discussed earlier, a waveform consisting of just two frequency
components that are not harmonically related may not be periodic.
Most power system monitors that perform frequency domain measurements take advantage of the
usual situation where only harmonics are present. These instruments use phase locked loop
technology to lock on to the fundamental frequency and sample one or more cycles for analysis
using the Fast Fourier Transform (FFT). Due to the phase lock, single cycle samples can yield
accurate representation of the harmonic content of the waveform as long as there are no
interharmonic components. When frequencies other than those harmonically related to the
55
sampling period are present, and/or the sampled waveform is not periodic over the sampling
interval, errors are encountered due to end-effect.
The power industry has been able to extract a lot of information out of measurements made on
the power system for harmonic analysis due to the dominance of this special case of the general
situation - power system frequency components are dominated by frequency components
harmonically linked to the power system fundamental frequency. This special case simplifies
accurate measurement of magnitudes and phase angle of these components, determine power
flow at these frequencies easily as well as their direction of flow - all things that are much more
difficult to do in the general case in the frequency domain.
Identification of interharmonic components need not inherently involve any analysis or
association relative to the power supply frequency. This method of signal analysis is analogous to
signal analysis techniques used in the communications and broadcast industries. Concepts, limits,
standards, and measurement equipment have been successfully used in these industries for
decades to identify steady state signal levels without the need to phase lock or otherwise
reference some arbitrary frequency or dominant signal component.
The method normally used to minimize end-effects and obtain accurate magnitude and frequency
information in the general case of spectral analysis involves the use of windowing functions.
Windowing functions weight the waveform to be processed by the FFT in such a way as to taper
the ends of the sample to near zero. There is considerable art involved in the selection of
appropriate windowing functions for different types of analysis, but for the purpose of this
discussion, the popular Hanning window will be used.
A method for simplifying interharmonic measurements is being proposed by the IEC. This
method would fix the sampling interval of a waveform to result in a fixed set of spectra
forharmonic and interharmonic evaluation. The present proposal would fix the frequency
resolution at 5 Hz (10 or 12 cycle sample windows for 50 or 60 Hz systems respectively). Phase
locked loop or other line frequency synchronization technique would be used to minimize signals
being registered in frequency bins due to end-effect errors. The resulting frequency bin spacing
56
should result in harmonic components being resolved accurately with a minimum of
contamination of their frequency bins by interharmonic components.
Interharmonic components that are in between the 6 Hz or 10 Hz bins would spill over primarily
into adjacent interharmonic bins with a minimum of spill into harmonic bins. This approach is
attractive for compliance monitoring and compatibility testing since compatibility levels can be
defined based on the energy registered in the fixed interharmonic bins and the resulting total
interharmonic distortion figure rather than relying on precise measurement of specific
frequencies. The drawback is that this method may not be suited for the diagnostic mode of
monitoring in all cases.
A number of other methods have been reported in the literature and are applicable in a variety of
situations. Some of the more interesting methods include the interpolated FFT [II.3]
II.4 Effects
For interharmonic frequency components greater than the power frequency, heating effects are
observed in the same fashion as those caused by harmonic currents. In addition to heating effects,
a variety of system impacts have been reported. These effects include CRT flicker, torsional
oscillations, overload of conventional series tuned filters, overload of outlet strip filters,
communications interference, and CT saturation.
Interharmonics that excite torsional oscillations in turbogenerator shafts can be a significant
concern. One report [II.5] showed that one large (775 MW) turbine generator has been putat risk
by a current source converter of a type used for slip energy recovery in induction motors. The
reported result was torque amplitudes that reached twice the nominal value. With the number of
stress cycles greater than 106, there was an expectation of a serious reduction in the service life
expectancy of certain shaft sections.
57
One of the more important effects of interharmonics is the impact on light flicker. Since a
renewed look at flicker standards and measurements is currently underway within IEEE,
interharmonic flicker effects are described here as an example of possible system impacts.
Modulation of a steady state interharmonic voltage on the fundamental power system voltage
introduces variations in system voltage amplitude and rms value:
u(t) = sin (2 p f1 t) + a sin (2 p fi t)
The maximum voltage change in voltage amplitude is equal to the amplitude of the
interharmonic voltage, while the changes in voltage rms value is depending both on the
amplitude and the interharmonic frequency.
The addition of harmonics to the supply signal does not affect the fluctuation because these
harmonics are synchronized with the fundamental of the power system. However,
interharmonics, which are not synchronized, do affect the peak amplitude of the AC voltage
supply. Consequently, recharging of the capacitor varies from one cycle to another, resulting in
an increase in the fluctuation upstream of the regulator and, since this fluctuation is excessive, it
affects the operation of the equipment.
For interharmonic values of only 5%, the modulation amplitude is as high as 10% up to the
seventh harmonic then decreases exponentially.
Differentiation between peak and rms deviation impacts can be important since some loads are
affected more by peak variations than rms variations. For example, compact fluorescent lamps
have been shown to be more sensitive to peak variations than rms variations. Incandescent lamps
however, are more sensitive to rms variations. It is interesting to note that the IEC standard
flickermeter [II.6] is sensitive to rms variations as opposed to peak variations.
As interharmonics are a source of voltage fluctuation, the risk of light flicker exists if the level of
interharmonic voltages exceeds certain immunity levels.
58
II.5 Analysis
When it comes to defining compatibility levels and limits for interharmonics in power systems, a
set of appropriate indices must be defined to facilitate standards development. The use of the
proposed IEC method discussed in the measurement section has the benefit of enabling the
specification of limits for each of several sets of partial interharmonic groups (as described in the
CEA Guide to Performing Power Quality Surveys [II.7]). The magnitude of each interharmonic
group could therefore be an index.
The proposed IEC method defines interharmonic groups. These indices are the rms value of the
interharmonic components between adjacent harmonic components. The frequency bins directly
adjacent to the harmonic bins are omitted.
In harmonic analysis, engineers are used to simplifying indices such as total harmonic distortion
(THD) that provide a general indication of the condition of the waveform. Similar indices are
possible for waveforms with interharmonic components [ ].
II.6 Mitigation
There are a variety of techniques that can be used to mitigate interharmonics. The most common
technique is through the use of passive filters. This approach has been used successfully over
many years to control interharmonic and harmonic distortion from arcing loads and
cycloconverters.
Traditional filtering methods used for harmonic control are not sufficient when interharmonics
are present. This is due to the fact that a simple series tuned harmonic filter causes a new parallel
resonance at a frequency just below the tuned frequency.
When interharmonics are not present this is not a problem, but if they are, significant
magnification of the voltage distortion can occur at the parallel resonant frequency. For example,
a series tuned fifth harmonic filter has a sharp parallel resonance near the fourth harmonic.
59
To overcome this problem, filters must be designed with damping resistors to minimize the
magnitude of the parallel resonance. Unfortunately, this is expensive and results in additional real
power losses in the filter.
Another difficulty with an interharmonic filter design, is that interharmonic producing loads tend
to produce frequency components over a wide range of frequencies. This leads to the design of
multi-stage filters which adds to the complexity and cost.
In addition to the use of passive filter schemes described above, a new generation of devices is
now becoming available. These devices, commonly referred to as active or dynamic filters, use
advanced power electronic techniques to continuously control harmonic and interharmonic levels
in real-time. A guide to the application of active power conditioners is expected to be published
by Cigré in the near future.
II.7 Limits
Interharmonic voltage limits are not well established internationally. An interharmonic voltage
limit of 0.2% of the fundamental voltage is given in IEC publication 1000-2-2 [11] based on the
following:
• Risk of interference to low frequency power line carrier
systems (ripple control)
• Risk of light flicker
The CENELEC Standard EN 50160 gives no values pending more experience. Given this lack of
clear standards in this area, more work is needed to gather practical experience necessary to
suggest compatibility levels and emission limits to standards setting organizations.
In addition to setting limits, clear measurement protocols must be defined. The revision to IEC
1000-4-7 is expected to address this issue. Also IEEE 519 revision will address the limits for
interharmonics.
60
Interharmonic currents present the same problems with heating and inductive interference as do
harmonic currents. Therefore it is recommended that interharmonic currents be limited in the
same manner as harmonic currents in IEEE 519-1992 Table 10.1 for the third interharmonic
group (IH2-3) and above.
The end user must also limit interharmonic currents sufficient to ensure that the Pst due to flicker
is less than 1.0. This evaluation is done according to the guidelines given in 61000-3-7.
Limit individual interharmonic component voltage distortion to less than 1%, 2% or 3%
(depending on voltage level – same breakdown as 519 standard) above 90 Hz up to 3 kHz to
protect low frequency Power Line Carrier (PLC), address sensitivity to light flicker within 8 Hz
of harmonic frequencies, and account for resonances created by harmonic filters.
In systems where turbine generators are electrically nearby, additional calculations may need to
be made to define the acceptable voltage or current limit necessary to protect the generator. This
procedure to be defined or referenced at a later date by the Interharmonics Task Force.
For large, single frequency current injectors (e.g. 10 Hz from cycloconverter), a system study
should be done to examine the impact of subsynchronous current components on metering and
protection CT’s (possible CT saturation concern) and sub-synchronous resonance potential.
Add caveat about magnetic core saturation effects when low frequency IH are present.
Add caveat that using the flicker meter to limit IH below the fundamental addresses the flicker
impact issue, but lower Pst values may be necessary to address other concerns such as SSR.
Flicker meter 0-15 Hz and 85 – 100 (assuming 50 hz fundamental) additive flicker output is 0 so
other means of limiting IH components in that are necessary.
References
61
[II.1] CEI/IEC 1000-2-1:1990, “Electromagnetic Compatibility”, Part 2: Environment, Sect. 1:
Description of the environment – Electromagnetic environment for low-frequency conducted
disturbances and signalling in public power supply systems. First Edition, 1990-05
[II.2] [2] B. R. Pelly, “Thyristor Phase-Controlled Converters and Cycloconverters”, John Wiley
and Sons, New York, 1971.
[II.3] P. Langlois and R. Bergeron, “Interharmonic Analysis by a Frequency Interpolation
Method”, Proceedings of the Second International Conference on Power Quality, pp.E-26:1-7,
Atlanta, US, Sep 1992.
[II.4] X. Dai. et. al., “Quasi-synchronous Sampling Algorithm and its Application – 2. High
Accurate Spectrum Analysis of Periodic Signal”, Conference Record of IEEE IMTC/93, Los
Angeles, US, pp. 94-98, May 1993.
[II.5] Fick, H: "Excitation of subsynchronous torsional oscillations in turbine generator sets by a
current source inverter", Siemens Power Engineering, 1982, Vol 4, pp 83-86.
[II.6] CEI/IEC 868, “Flickermeter”, Functional Design Specification, First Edition, 1986.
[II.7] CEA Guide to Performing Power Quality Surveys
[II.8] R. F. Chu and J.J. Burns. “Impact of Cycloconverter Harmonics”, IEEE Transactions on
Industry Applications, Vol. 25, No. 3, May/June 1989, pp. 427-435.
[II.9] L. Tang, D. Mueller, “Analysis of DC Arc Furnace Operation and Flicker Caused by 187
Hz Voltage Distortion”, Presented at IEEE-PES 1993 Summer Meeting July 18-22, 1993,
Vancouver, BC.
[II.10] W. Shepard, “Thyristor Control of AC Circuits”, Bradford University Press, England,
1976.
[II.11] X. Dai, R. Gretsch, “A Fast and Accurate Method to Measure Interharmonics in Power
Systems”.
[II.12] CEI/IEC 1000-2-2:1990, “Electromagnetic Compatibility”, Part 2: Environment, Section.
2: Compatibility levels for low frequency conducted disturbances and signalling in low-voltage
power supply systems. First Edition, 1990-05
[II.13] Yacamini, R: "Power system harmonics. Part 4 Interharmonics", Power Engineering
Journal, August 1996, pp 185 - 193.
62
APPENDIX III POWER SYSTEM CHARACTERISTICS AT INTERFACE
Table III. 1 Power system characteristics at interface
CHARACTERISTICS
TYPE 1
TYPE 2
TYPE 3
460 or 115
volts rms
460 or 115
volts rms
460, 115 or
115/200 volts
rms
 5%
 5%
 2%
 7%
 7%
 3%
(c) Line Voltage Unbalance
3%
3%
2%
(d) Voltage Modulation
2%
2%
1%
 16%
 16%
 5%
2 seconds
2 seconds
0.25 second
2500 volts
2500 volts
2500 volts
(460 V sys)
(460 V sys)
(460 V sys)
1000 volts
1000 volts
1000 volts
(115 V sys)
(115 V sys)
(115 V sys)
 6%
 6%
 2½%
± 20%
± 20%
± 5½%
(i) Maximum Total Harmonic Distortion
5%
5%
3%
(j) Maximum Single Harmonic
3%
3%
2%
(k) Maximum Deviation Factor
5%
5%
5%
VOLTAGE
(a) Nominal User Voltage
(b) User Voltage Tolerance
(1) Average of the 3 line-to-line voltages
(2) Any one line-to-line voltage including
(b)(1) and line voltage unbalance tolerance
(e) Voltage Transient
(1) Voltage transient tolerance
(2) Voltage transient recovery time
(f) Voltage Spike (peak value, includes
fundamental)
(g) Maximum Departure Voltage resulting from
(b)(1) and (d) combined, except under
transient or emergency conditions
(h) Worst Case Voltage Excursion from
nominal user voltage resulting from (b)(1),
(d), and (e)(1) combined, except under
emergency conditions
WAVEFORM (VOLTAGE)
63
Table III. 2 Power system characteristics at interface (continued)
CHARACTERISTICS
TYPE 1
TYPE 2
TYPE 3
(l) Nominal Frequency
60 Hz
400 Hz
400 Hz or 60
Hz
(m) Frequency Tolerances
± 3%
± 5%
± ½%
(n) Frequency Modulation
½%
½%
½%
(o) Frequency Transient Tolerances
± 4%
± 4%
± 1%
2 seconds
2 seconds
0.25 second
5 ½%
6 ½%
1 ½%
-100 to + 12%
-100 to + 12%
-100 to + 12%
Up to 2
minutes
Up to 2
minutes
Up to 2
minutes
-100 to + 35%
-100 to + 35%
-100 to + 35%
(1) Lower limit (-100%)
Up to 2
minutes
Up to 2
minutes
Up to 2
minutes
(2) Upper limit (+35%)
2 minutes
0.17 second
0.17 second
FREQUENCY
(p) Frequency Transient Recovery Time
(q) Worst Case Frequency Excursion from
nominal frequency resulting from (m),
(n), and (o) combined, except under
emergency conditions
EMERGENCY CONDITIONS
(r) Frequency Excursion
(s) Duration of Frequency Excursion
(t) Voltage Excursion
(u) Duration of Voltage Excursion
64
APPENDIX IV IEEE STD 519 PROPOSED REVISIONS JUNE 08, 2004
•
Update sponsors
–
PES T&D Committee
–
IAS IPC Committee
–
•
Introduction
–
Establish applicability of 519
–
Demonstrate PCC via single-line diagram
–
•
Definitions
–
•
Update according to content modifications as appropriate
Chapter 4 “Harmonic Generation” eliminated
•
–
Include bullet list of “causes” in introduction
–
•
Chapter 5 “System Response Characteristics” eliminated
–
Add cautions in introduction regarding the possibility of resonances creating
compliance problems
–
Also caution that customer-side modifications that create undesirable resonances
are customer’s responsibility to fix
•
•
This is part of the “shared responsibility” flavor of 519
Chapter 6 “Effects of Harmonics” significantly reduced (to general information) and
moved to an annex
–
Update TIF based on Std. 776-1992 (R2003)
–
Include this material in 519.1 “next time” and then remove from future 519
revisions
–
•
Chapter 7 “Reactive Power Compensation and Harmonic Control” eliminated
–
•
Information is either well understood now, widely documented in other literature,
or not necessary for a “limits” document
Chapter 8 “Analysis Methods” mostly eliminated
–
Provide up-to-date bibliography
65
–
Move Section 8.5 “Notching” into annex
–
Use Figure 8.7(a) (or similar) in introduction to define PCC and related concepts
–
•
Chapter 9 “Measurements” completely re-written
–
Endorse IEC 61000-4-7 (2ed) and 4-30
–
Provide sufficient material to understand meter outputs and on-line statistics
–
Harmonize with 1159 as necessary
–
•
Chapter 10 “Recommended Practices for Individual Consumers”
–
Make sure PCC is clear in introduction and again here
•
–
Section 10.3 moved to annex
•
–
519 is an interface standard, not an equipment standard
Update with additional info from 519-1981
Section 10.4: remove “value added” for higher pulse order
•
If 5th, 7th (11th, 13th, etc.) are below 25%, then user gets credit (1.5x,
1.75x, 2.0x, etc) regardless of how the <25% levels were obtained
–
Section 10.4: Make sure that definition of load current (IL) is clear
–
Current limit tables
•
Justify limits for generation to avoid the “negative load” concept
•
Do not eliminate even-order limits
•
Make each table a full page with common and consistent footnotes,
explanations, and disclaimers…
•
Interharmonics should not create objectionable lamp flicker or other
undesirable or damaging effects
•
–
Interharmonic limit curve based on flicker in annex
Express limits using 4-7 & 4-30 measurement protocol
•
95th percentile of the 10 minute values less than the given limits over a
period of one week
•
99th percentile of the 10 minute values less than 1.5 times the given limits
(over one week)
•
99th percentile of the 3 second values less than 2.0 times the given limits
over one (each) day
–
Section 10.4.1 “Transformer Heating” reduced to a table footnote
–
Section 10.4.2 “Probabilistic aspects” eliminated
•
Captured using 4-7 & 4-30 protocols
66
–
Section 10.5 “flicker” eliminated
–
•
Chapter 11 “Applications”
–
Put chapter in front of “customer limits” chapter
•
Emphasize that voltage problems are what are trying to be avoided
•
Current limits depend on system and economics
–
Section 11.2 “addition of harmonics” eliminated
–
Section 11.3 “short duration” eliminated
•
Captured using IEC protocol
–
Section 11.4 eliminated
–
Section 11.5 “voltage limits” focused on compatibility concept
•
–
Refer to “effects” material that was moved to annex
Limit table (Table 11.1)
•
Add new row for <1kV (5% & 8%)
•
Remind about PCC, especially for <1 kV case
•
95th percentile of 10 minute values less than limit values over one week
•
99th percentile of 3 second values less than 1.5 times the limit over one
day
•
Add new limits for h>13 for <1kV and 1-69 kV rows
•
–
Possible additional modification
•
–
•
•
0.5 times existing limits for these higher orders
Changing rows from 69-161 kV and >161 kV to 69-230 kV and > 230
kV???
Section 11.6 “interference with communications” moved to annex with other TIF
material
Chapter 12 “Recommended Practices for Utilities”
–
Almost all 519.1 material
–
Capture philosophy in introduction
New Structure
–
Expanded introduction focusing on why 519 is needed (effects) and where it is to
be applied (PCC definition)
•
Very limited information on “what is required” and “how” to apply it
67
–
–
This is the domain of 519
Measurement protocol (should majority be in annex?)
•
IEC 61000-4-7 & 61000-4-30
–
Voltage distortion limits
–
Current distortion limits
–
Multiple annexes with “good, old” and “good, new” material
–
Expanded bibliography
68