Download advent of harmonic filters

POWER ELECTRONICS
GENESIS, TRENDS AND IMPROVEMENT.
ABSTRACT
The latest buzzword, in energy conscious industries, is the improvement of power quality.
Good quality of electric power can not only result in handsome savings in power bills but
also helps in avoiding unwarranted breakdowns. This paper aims at analysing the reasons
for poor quality of power and solutions for improving it. The change in magnitude and
power factor can be corrected without much of difficulty with only problem being that of
maintenance of shape, which is distorted by harmonics. We look at the factors causing in
the production of harmonics, what impact these harmonics have on other equipments in
the load serenity. We look at the types of harmonics, their effects and problems caused by
them. Active filters are the ultimate solution for reduction of harmonics which use the
IGBT technology in additions to the pulse width modulation technology (PWM) which
combines with control techniques to emerge successfully with improved power quality by
fighting with the problems of harmonics to an extent that the THD (total harmonic
distortion) is within acceptable limits i.e. <5%, these filters are the ultimate solution and
far win over their counter parts i.e. passive filters.
INTRODUCTION:
Electric power quality, in general, means how pure is the quality of power being used,
without any abnormalities, which cause disturbances in it.
he voltage, which is the primary driving force behind all electrical equipment in the system,
is an alternating sinusoidal and is characterized by its magnitude, frequency and wave
shape. Sags, Surges, spikes and power failures all these disturbances are created due to
modification of amplitude and can be measured and removed by using voltage stabilizing,
conditioning and UPS systems. To ensure good power quality apart from the requirement
of magnitude and frequency to be within acceptable limits, it is important that the wave
shape is maintained as a ‘sine wave’ throughout the electrical network.
HOW POWER QUALITY DETERIORATES:
Although power is generated in the pure form, electrical consumers or loads distort it
progressively due to the manner in which it is consumed. This problem of power pollution
was not heard of in the past but is taking a serious form due to extensive use of energy
eficient power electronic (non-linear) loads.
In the past major loading of supply system was made of large slowly changing electrical
loads like motor, transformer, which were linear in nature i.e. the current consumed was
directly proportional to the supply voltage. The current waveforms being sinusoidal with
only power quality problem being that of power factor.
In the last 10 to 15 years, the gradual and never ending prolification of computers, microprocessor system and power electronics in all kinds of industrial, commercial and domestic
facilities have completely changed the nature and profile of supply loading. These
nonlinear loads consume non-sinusoidal current when sinusoidal voltage is applied to them
by conducting only during a fraction of the fundamental period. The current drawn is
characterized by sharp rise and fall during only a fraction of the fundamental period .The
current waveform contains a considerable amount of odd harmonics, the magnitude of
which may be higher than the fundamental current component. In the past all these nonlinear loads were confined to industries and hence the power quality by and large was
satisfactory.
The number of such polluting electrical gadgets is increasing at an alarming rate thereby
polluting the power quality in modern electrical networks. Problems in computers due to
poor power quality are

Damage to computer chips, causes computer downtime, computer errors etc.,

UPS system do not address transient problems effects

Poor power quality can cut the lifetime of motor to half.
The distorted current is a result of several sine wave currents at multiples of fundamental
frequency. These high frequency components are called as ‘Harmonics’. When the
harmonic currents flow through the power distribution system, harmonic voltages are
developed.
These harmonic voltages are superimposed over the incoming pure supply voltage causing
distortion in the voltage. Since voltage is common to all loads (linear or non-linear),
harmonic distortion spreads in the electrical system causing varying degree of damage
depending upon the susceptibility and sensitivity of the electrical equipment.
SOURCES OF HARMONICS

Switched mode power supplies (SMPS)

Electronic fluorescent lighting ballets

Variable speed drive

Uninterrupted power supply (UPS)
HARMONICS
1) Types:
a) Less then fundamental (sub harmonics)
b) Greater than fundamental
a) Multiple of fundamental harmonics i.e. odd & even
b) Non-multiple of fundamental (Inter harmonics)
[Fundamental harmonics
50 Hz + 1 Hz (normally)
50 Hz + 3 Hz (exited)]
2) Nature of Occurrence
a) Steady
b) Intermittent
3) Effects
A) NUISANCE
i) Communication noise.
ii) Light flickers
iii) T.V. interference
iv) Computer mal-operation
v) Transformer noise
B) ECONOMICS
i) Meter error
ii) Reduction of equipment life
iii) Increased cost
C) DAMAGE
i) Overheating
ii) Over voltage
ii Capacitor failure
iv) Motor vibration
EFFECT OF HARMONICS ON CAPACITORS:
Electric Utilities i.e. Electricity boards in India are offering attractive incentives
for power factor (PF) improvement to industrial consumers installing power capacitors in
their electrical network to achieve power factor to unity [1.0] Power factor is displacement
of current waveform in time with reference to voltage wave forms. Capacitor banks are
even present in many industrial plants for reactive power compensation purposes.
The power factor correction i.e. improvement in power factor is basically done by
capacitors. These power factor correction capacitors react adversely in the presence of
harmonic distortion in the system. Although capacitors, being linear loads, do not generate
harmonics, they tend to amplify existing harmonics.
The amplification of harmonics by capacitors is caused by the electrical phenomenon of
‘Resonance’. Which is similar to symphonic vibration in a stringed musical instrument.
These ingredients cause electrical resonance viz., capacitors, inductors, non-linear loads.
Elimination of any one of these avoids occurrence of resonance. As majority of modern
electrical loads are both inductive and non-linear, their elimination is not possible.
Elimination of capacitance is impossible, as it would reduce PF below acceptable limits.
REASONS FOR LIMITING HARMONICS
Harmonic pollution causes a number of problems. A first effect is the increase of
RMS value and the peak value of distorted waveform. This is illustrated in the figure below
(next page), which shows the increase of these values as more harmonic components are
added to an initially undistorted waveform. The RMS-value and peak value of undistorted
waveform are defined as 100%. For example let us consider fundemental, 3rd, 5th
harmonics with the help of the fig 1. in the next page. The increase of RMS-value increases
heating of the electrical equipment. Furthermore, circuit breakers may trip due to higher
thermal or instantaneous levels. All fuses may blow and capacitors may get damaged.
KWH meters may get fault readings. The winding and iron losses of motors increase and
they experience perturbing torques on the shaft. Sensitive electronic equipment may be
damaged. Equipment, which uses supply voltage, as reference may not be able to
synchronize properly, and either apply wrong firing pulses to switching elements or switch
off. Interference with electronic communication equipment may occur. Excessive amount
of harmonics leads to premature ageing of the equipment.
FIGURE 1
Some more effects of harmonics are
* Reduced efficiency
* Insulation damage
* Malfunctioning of equipment
* Increased copper losses etc.
International Electromechanical Commission (IEC) has issued technical reports
which outline assessment procedures to determine whether distorting loads may
be connected to the supply system. In many Cases, the regulations impose a limit
for the total harmonic distortion (THD) of the voltage or current
Present at the point of common coupling (PCC). The PCC is the location at
which the plant is connected to the public power system (generally at the
primary of the main transformer(s)). The THD expresses the relative importance
of the harmonics with respect to the fundamental Component. It is expressed as a
ratio of total harmonic voltage to the fundamental voltage and is called total harmonic
distortion (THD). Generally 5% THD is considered an allowable limit for an electrically
clean supply.
ADVENT OF HARMONIC FILTERS
In order to improve power factor without causing harmonic amplification,
harmonic filters are to be used. Harmonic filters are suitably modified capacitors used
together with series reactors, which are specially designed to improve power factor and
suppress harmonic amplification. These types of harmonic filters are the passive type and
extensively used in place of conventional power factor correcting in harmonic rich
environments. A passive filter basically consists of a series circuit of reactors and
capacitors
The figure below illustrates schematically the description of passive filtering with a
harmonic generator; impedance representing all other loads a filter and a network.
e.g. Blocking Reactor: Blocking Reactor sometimes called as re-tuned filter or low pass
filter. Harmonic currents generated by, for example, a frequency converter are shunted by
this circuit designed to have low impedance at a given frequency compared to the rest of
the network. The main function of this is to protect the capacitor unit firm being exploited
by the excessive level of power harmonics in electric network. Although passive filters are
cost effective there are some disadvantages like they provide certain amount of reactive
power which is not desirable when the loads to be compensated are ac drives which already
have a good power factor they are susceptible to overload, sensitive to frequency changes
and network changes and each passive filter can be used to filter only one harmonic
component. These limitations can be overcome by the use of active filters.
ACTIVE FILTERS – THE ULTIMATE SOLUTION
Active filters are a relatively new technology and overcome the disadvantages of
passive filters. They make use of IGBT semiconductors with various control loops to
increase power factor and reduce harmonics, thereby reducing amperage, KVA,
circulating currents, harmonic resonance, line voltage imbalance and closed delta-winding
losses. The active filter measures the harmonic currents and generates actively a harmonic
current spectrum in opposite phase to the original distorting current that was measured.
The original harmonics are thereby cancelled. THE PRINCIPLE OF ACTIVE
HARMONIC FILTERING IS AS SHOWN BELOW
It allows for a concept that may not be overloaded. The compensating currents created by a
3 phase Insulated Gate Bipolar Transistor (IGBT) inverter bridge that is able to generate
any given voltage waveform with PWM (pulse width modulation) technology. The IGBT
Bridge uses a DC voltage source realized in the form of DC capacitor. The generated
voltage is coupled to the network via reactors and small filter circuit. We use IGBT because
they offer high switching frequencies that allow the generation of high frequency harmonic
currents, and relatively low on state losses when compared to MOSFET’s. The PWM
reactors transform voltage source inverter into current source. Finally the output filter
absorbs the high frequency components introduced by the PWM switching action. With the
help of Pulse width modulators (PWM) it is possible to control the dc voltage, but in case of
active filters , an AC voltage needs to be created and controlled from a DC source. This can
be achieved with a time varying duty cycle. Any duty cycle may be used providing that its
frequency is strictly inferior to half that of carrier frequency. The system is called as pulse
width modulation since, the ac output voltage , which has the same shape as duty cycle, is
generated by controlling the width of the voltage pulses. Now a days closed loop control
techniques are being implemented in which the filter directly its effect on the filtration by
comparing the resultant waveform with the pure one which is given as a reference, with the
help of feedback, furthermore the closed loop control system ensures that measurement
errors do not result in a higher distortion apart from improved efficiency and accuracy.
Open and closed loop current control as performed by active filters is shown in the figure
below.
An open loop control system needs higher-class current sensors thereby increasing the risk
of inaccuracy and reduced efficiency. Active filters reduce the energy cost by minimizing
harmonics (THD<5%), increase power factor up to 100%, correct voltage imbalances and
increase system reliability due to improvement in power quality. Active filters generate
counter harmonic currents in opposite direction so as to totally cancel the harmonic
currents generated by the non-linear loads. Active filters are the most efficient type
harmonic filtering solution and are being used in critical areas where harmonic control is
crucial to the reliability of the installation.
BENEFITS OF ACTIVE FILTERS

Reduced energy costs

Elimination of higher and lower order harmonics

Improve load power factor

Voltage improvement and stability

Surge and transient protection

Reduce voltage unbalance

Reduce power line losses.
CONCLUSION
Considering all the above facts, it may not be surprising if electrical utilities across the
country start penalizing industrial consumers for high harmonic levels like they do for
poor power factor. This is because high harmonics are similar to poor power factor in
terms of reducing the efficiency of power distribution system. These days when industrial
efficiency standards are being continuously improved, the concept of power quality
management is going to play a very significant role in the near future. Thankfully there
are qualified consulting engineering firms who should be able to help decide the right
solution for a particular situation.