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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% i 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 1 “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