• Electrical India
  • Apr 13, 2017

Power Harmonic Filters For Technological Excellence

In the earlier years, harmonics were not prevalent in most of the industries due to the balanced linear loads using three phase induction motors along with incandescent lighting, heating etc. but the rapid advancement of the power electronics in industrial application makes industrial loads non-linear type…

- Shaikh Shamser Ali


 Harmonics are AC voltages and currents with frequencies that are integer multiples of the fundamental frequency. In the earlier years, harmonics were not prevalent in most of the industries due to the balanced linear loads using three phase induction motors along with incandescent lighting, heating etc. but the rapid advancement of the power electronics in industrial application makes industrial loads non-linear type. These non-linear loads draw non-sinusoidal current from the sinusoidal voltage waveform. The distortions thus produced in the voltage and current waveform from the sinusoidal waveform are called harmonic disorders.

How the harmonics are generated

Harmonics are generated due to increasing number of non-linear loads as explained bellow:

  When the system voltage is linear but the load is non linear, the current will be distorted and become non-sinusoidal. The actual current will become higher than an ammeter or any other measuring instrument, at the fundamental frequency, could measure that. Following figure illustrates the difference between the apparent current measured by an instrument and the actual current

Here, Ia = Active Component of Current
Ir = Apparent current measured by an ammeter
Ih = Actual current due to harmonic distortions
Φ = Displacement angle between the system Voltage and apparent current defining the PF of the load
α = Actual Phase displacement due to harmonic distortions

  When the supply system itself contains harmonics and the voltage is already distorted, the linear loads will also respond to such voltage harmonics and draw harmonic currents against each harmonic present in the system and generate the same order of current harmonics.

  When the system voltage and load are both non-linear, (A condition which is more common) the voltage harmonics will magnify and additional harmonics will be generated, corresponding to the non-linearity of the load and hence will further distort an already distorted voltage waveform

  A load is “non-linear“ when the current drawn does not have the same waveform as the supply voltage. The harmonic spectrum depends on the type of load. i.e. switch-mode power supplies, motors during start-up, transformers during switch-on, frequency-controled motors....

• Non-linear loads cause voltage and current distortion.
• Voltage harmonics depends on grid / source stability, current harmonics and network impedance.
• Current harmonics are mainly load dependent.
• Shunt connected electrical equipment are sensitive to voltage harmonic and crest.
• Series connected devices like cable / transformer are affected by current harmonics thus increases distribution losses and demagnetic interference.
• Major harmonic related problems are due to current and voltage harmonics.

Harmonics will thus mean:

1. Higher voltage and current than apparent.
2. Adding to line loading and losses.
3. Reducing the actual load PF.

Sources of harmonics

Transformers:

  One common source of harmonics is iron core devices like transformers. The magnetic characteristics of iron are almost linear over a certain range of flux density, but quickly saturate as the flux density increases. This non-linear magnetic characteristic is described by a hysteresis curve. Because of the non-linear hysteresis curve, the excitation current waveform is not sinusoidal. A fourier analysis of the excitation current waveform reveals a significant third harmonic component.

Generators

  Generators produce some 5th harmonic voltages due to magnetic flux distortions that occur near the stator slots and non-sinusoidal flux distribution across the air gap.

  Other producers of harmonics include Rectifiers, Inverters, Variable Speed Drives, Welders, Arc furnaces, Voltage Controllers, Frequency Converters etc.

  Semiconductor switching devices produce significant harmonic voltages as they abruptly chop voltage waveforms during their transition between conducting and cut-off states.

  Inverter circuits are notorious for producing harmonics, and are in widespread use today in every spectrum of the industry. A variable speed drive is one application that makes use of inverter circuits, often using pulse width modulation (PWM) synthesis to produce the AC output voltage. Various synthesis methods produce different harmonic spectrum. Regardless of the method used to produce an AC output voltage from a DC input voltage, harmonics will be present on both sides of the inverter and must be mitigated.

Impact of harmonics on power factor

  Power Factor (PF) is a measure of the efficiency of utilisation of a power distribution system. The closer the PF to unity, the more will be the efficiency to do the useful work. With linear loads, the PF depends on the phase relationship between the current and voltage sine waves. When these two waves are in phase, the PF is unity and no system capacity is wasted.

  Linear loads, such as resistance heaters and incandescent lights are 100% efficient in converting real power to heat and therefore have a PF of unity. Induction motors require real power and reactive power, which is measured in KVAR. The reactive current that flows in the system creates a magnetic field that enables the motor to operate, but does not contribute to the work done by the motor. Reactive current also causes the current wave to lag behind the voltage wave. This process is called displacement.

  The apparent power for a motor can be calculated using the equation,

KVA=√ (KW2+KVAR2)

  Since the apparent power for a motor is larger than the active power, the PF is less than unity. The PF for a system powering only linear loads is called the displacement power factor. Unless the loads are pure resistance, the PF will be less than unity.

  Today however, many electrical systems also have harmonic currents on their lines. Harmonics are caused by non-linear or pulsed loads and their current causes the apparent power to exceed the active power by a substantial amount.
The apparent power for a non-linear load can be calculated using the equation,

KVA =√ (P2+Q2+DVA2)

  The presence of harmonics increases the apparent power that must be delivered to do a certain amount of work, therefore lowering the PF. In these situations, the form of power factor present is called distortion power factor. In a System consisting of both linear and non-linear loads the true power factor (TPF) is a sum of cosine of both displacement and distortion angles.

  If harmonic currents are introduced into a system, the true PF will always be lower than the displacement PF. For example, the displacement PF for a computer is close to unity (usually about 0.95) whereas the true PF, which includes harmonics, is around 0.7. For both linear and non-linear loads, the result of extra current that does NO real work (Whether it is reactive current or harmonic current) is a reduced capability for the system to support useful loads.

  For linear loads, measurements can be carried out to determine displacement power factor with a number of instruments. These instruments can measure Kilowatts (KW) and Kilo-Volt-Amperes (KVA) and some can directly read Power Factor (PF). When harmonics are present, meters with true RMS capability must be used to accurately account for the total current, which includes the current at the Fundamental 50/60 Hz and the harmonic currents to determine the true PF. Also, it is advisable to read the true RMS value of the voltage, since harmonic currents may cause voltage waveform distortion in some systems.

Impact of harmonics on capacitors

  Harmonic component affects the performance of a capacitor unit significantly due to the diminishing reactance at higher frequencies, which adds to its loading substantially and can be analysed as follows:

Xc = 1/(2πfc) i.e Xc α 1/f

  This means that the capacitor will offer a low reactance to the higher harmonics and will tend to magnify the harmonic effect due to higher harmonic currents. In fact, harmonic currents have a greater heating effect compared to fundamental current. The effective current caused by all the harmonics present in the system can be expressed as:

Where, Ic = Rated current of the Capacitor
Ich3, Ich5, Ich7 .. etc. = amplitude of the harmonic current components at different Harmonic orders

  To compensate for the harmonic effects, capacitor unit is designed for a minimum of 70 - 100 % continuous overload capacity. Summarising the above, the harmonic quantities when present in a system on which are connected a few capacitor banks affect the capacitors as follows:

1. Over current resulting in higher losses.
2. Over current resulting to an over voltage across the capacitor units, which would inflict greater dielectric stress on capacitor elements.
3. Since the harmonic disorders occur at higher frequencies than the fundamental, they cause higher dielectric losses

  Harmonic output of a capacitor unit:


(V in Volts and Ic in Amperes)

and

Therefore, 
(or)
Generalizing KVARh is proportional to Vh .at fh
(or) KVARh is proportional to 


  The rating of the capacitor unit will thus vary in a square proportion of the effective harmonic voltage and in direct proportion to the harmonic frequency. This rise in the KVAR, however will not contribute to the improvement of system PF, but only to the overloading of the capacitors themselves.

Impact of harmonics on varying loads

  Wherever variation in loading pattern is observed, Automatic Power Factor Correction systems are installed for maintenance of healthy power factor. However, one cannot indiscriminately add power factor correction capacitors to a system without understanding how their presence will affect the system, especially in the presence of harmonics.

Resonance

  The operation of non-linear loads in a power distribution system creates harmonic currents that flow throughout the power system. The inductive reactance of the power system increases and the capacitive reactance decreases as the frequency increases, as shown in this fig.



Parallel Resonance

  At a given harmonic frequency in any system where a capacitor exists, there will be a crossover point where the network impedance and capacitive reactances are equal. This crossover point, called the parallel resonant point, is where the power system has coincidental similarity of system impedances. Every system with a capacitor has a parallel resonant point. Parallel resonance causes problems only if a source of harmonics exists at the frequency where the impedances match. This is typically called harmonic resonance. Harmonic resonance results in very high harmonic currents and voltages at the resonant frequency.

Parallel Resonant Circuit and its equivalent

  At resonant frequency the resultant impedance of a parallel resonant circuit increases to a very high value. This leads to excitation of parallel resonance circuit between the power factor correction capacitor and the network inductance resulting in a high voltage across the inductors and very high circulating current inside the loop.

Series Resonance

  The increased use of non-linear loads distorts the current waveform thereby affecting the voltage profile. In case of voltage distortion the series resonant circuit formed by the capacitance of the capacitor and the short circuit inductance of the transformer draws high harmonic current through the capacitor. Series resonance can create high voltage distortion in the LV side of the transformer.

Series Resonant Circuit and its equivalent

  For example, consider a 1500 KVA transformer and a capacitor bank rating of 250 KVAR. Substitute, these values in the equation given below will yield the harmonic order at which this combination will form a parallel resonant circuit
[A 1500 KVA transformer with 5% impedance yields about 30 MVAsc (1.5 MVA ÷ 0.05)]. So the 250KVAR capacitor bank will be resonant with that source impedance at the 11th harmonic. If any amplitude of 11th harmonic current flows on the power system at that bus, the effect could be catastrophic.

  Due to the combination of inductance and capacitance in series the net impedance of the circuit reduces to a bare minimum level at the resonant frequency and this impedance is basically resistive in nature at resonant frequency, since at resonant frequency, the capacitive reactance and the inductive reactance are equal. This low impedance to the input power at resonant frequency will result in multiple increases in current.

Effect of harmonics on power system

Distribution side:

Tripping of circuit breakers and fuses

  Due to resonance effects, the current levels may rise to multifold levels, which results in tripping of the breakers and melting fuses. This situation results into serious problems in industries, which rely on the quality of power for the continuous operation of their sensitive processes.

Impact of Harmonics on Transformers

  Transformers are designed to deliver power at network frequency (50/60Hz). The iron losses are composed of the eddy current loss (which increase with the square of the frequency) and hysterics losses (which increase linearly with the frequency). Eddy current concentrations are higher at the ends of the transformer windings due to the crowding effect of the leakage magnetic fields at the coil extremities. Very often, the damage to the coils in a transformer is not known until a failure occurs. With increasing frequencies the losses also increase, causing an additional heating of the transformer.

Impact of Harmonics on Motors

  Hysteresis and eddy current losses are part of iron losses that are produced in the core due to the alternating magnetic field. Hysteresis losses are proportional to frequency, and eddy current losses vary as the square of the frequency. Therefore, higher frequency voltage components produce additional losses in the core of AC motors, which in turn, increase the operating temperature of the core and the windings surrounding the core. Application of non-sinusoidal voltages to motors results in harmonic current circulation in the windings of motors.

  Stray motor losses, which include winding eddy current losses, high frequency rotor and stator surface losses, and tooth pulsation losses, also increase due to harmonic voltages and currents.

  The interaction between the positive and negative sequence magnetic fields and currents produces torsional oscillations of the motor shaft. These oscillations result in shaft vibrations. If the frequency of oscillations coincides with the natural mechanical frequency of the shaft, the vibrations are amplified and severe damage to the motor shaft may occur.

Overloading of the capacitors

  The rated current through capacitor is calculated as follows:

  However due to harmonics, the capacitors are overloaded according to the following equation producing excessive heat.

  As the capacitive reactance decreases with the frequencies, even smaller amplitudes of the harmonic voltages result into higher currents, which are detrimental to the capacitors.

Losses in distribution equipment

  Harmonics in addition to the fundamental current cause additional losses in the cables, fuses and also the bus bars.

Excessive currents in the neutral conductor

  Under balanced load conditions without harmonics, the phase currents cancel each other in neutral, and resultant neutral current is zero. However, in a 4-wire system with single-phase non-linear loads, odd numbered multiples of the third harmonics (3rd, 9th, 15th) do not cancel, rather add together in the neutral conductor.

  In systems with substantial amount of the non-linear single-phase loads, the neutral currents may rise to a dangerously high level. There is a possibility of excessive heating of the neutral conductor since there are no circuit breakers in the neutral conductors like in the phase conductors.

Malfunctioning of the Electronic Controls and Computers

  Electronic controls and computers relay on power quality for their reliable operation. Harmonics result into distorted waveforms, neutral currents and over voltages, which affect the performance of these gadgets.

Measurement errors in the metering systems

  The accuracy of metering systems is affected by the presence of harmonics. Watt-hour meters accurately register the direction of power flow at harmonic frequencies, but they have amplitude errors, which increase with frequency.

Generation Side

  Generally power electronic devices cause line current to be non-sinusoidal. This harmonics increases the losses in the stator conductors and it affects the field and damper currents, thus distorting the voltage profile. The voltage profile distortion depends on the current harmonics and the alternator characteristics. The following points needs to be accounted:

Increased summated net current flows through the alternator winding.

  Reverse harmonic current flow in the windings: Harmonic current flows back into the alternator, thereby distorting the voltage waveform (voltage harmonics). Basically voltage harmonics affects all the loads either linear or non-linear and the voltage harmonics also induces current harmonics even for linear loads.

  Increased winding resistance for harmonic frequencies: Harmonic current produces large heating than the fundamental current due to the increase in resistance at different frequencies. 5th Harmonic current produces approximately five times more heat and similarly higher order of harmonics produces higher heating. This leads to higher temperature in the winding.

  Rotor Jerking: Due to the reverse flow of harmonics, there is blocking movement in the alternator speed i.e., jerking of the rotor takes place due to harmonic current flow.

  Rotor Retardation: Due to inherent distortion in the voltage waveform, induced EMF rotates at different frequencies. Depending on the phase angle of the 5th Harmonics, this may have retardation or acceleration effect on the rotor.

Remedies to overcome power harmonics

  Elimination of voltage and current harmonics by using harmonic filters is an easy option to any harmonics problem.

Types of Passive Filter Systems

  Passive harmonic filters are reactor-based systems basically used for the suppression of harmonics and maintenance of healthy power factor. These filters are broadly classified as:

1. Detuned Filters
2. Tuned Filters

  The classification of de-tuned filters and tuned filters basically depends on the tuning frequency of the filter reactor & capacitor circuit and the selection of harmonic filter type depends on the level & order of harmonics present in the distribution network.

De-Tuned Harmonic Filters

  The de-tuned filters are effective in circuits where variation of Q and different order of harmonics level are anticipated. Such filters are shunt connected with matching tuning frequencies below the predominant harmonic frequency, thus having most reliable life, but also achieving the required harmonic reduction. The selection of the tuning frequency depends on the system impedance behavior under varying loads or constant loads.

  The system impedance needs a detailed harmonic behavioural study to arrive at the correct tuning frequency. However, depending on the predominant level of harmonics present, following tuning frequencies are generally selected:

Blocking Filter

  Wherever the level of 3rd harmonics is predominant in the distribution network, it is necessary to select Blocking Filter systems of tuning frequency at 154.8 Hz. These filters are designed to block the effect of 3rd harmonics affecting the life of shunt connected capacitors and reduces the risk of harmonic resonance and amplification.

Suppression Filter

  Wherever the level of 5th harmonics is predominant, it is necessary to select Suppression Filter systems of tuning frequency at 279 Hz. These filters are also designed to block the effect of 5th harmonics affecting the life of the shunt connected capacitors & suppress the line current harmonics. In general following are the benefits of Blocking & Suppression Filters:

1. Avoids premature failure of capacitors due to the basic blocking nature of the filters
2. Harmonic amplification due to impedance matching is avoided
3. Possibility of harmonic resonance is avoided

Tuned Harmonic Filters

  The tuned filters are basically designed to match to the predominant harmonic frequency, but slightly tuned away from the harmonic frequency, since the supply frequency variations needs to be accounted. Even the tuned filters act like de-tuned filters when the frequency is at the normal level, but they operate as tuned filters only at the minimum defined supply frequency.

  These types of filters are tuned above the detuned filter frequencies, thus having the impact of blocking filter and harmonic absorption capability. In other words, these filters not only improve the power factor, but also absorb the harmonics. Here, the capacitors are fully protected due to the blocking nature of the filters, thus ensuring a long life for the filter circuit. Such filters are normally used for both harmonic suppression and power factor improvement.

Active Filter Technology

  Active filters are IGBT based power electronic devices installed in parallel to the harmonic generators. It analyses the harmonic current produced by the non-linear loads and supplies a 180° out-of-phase compensating current, either over the entire spectrum from the 2nd to the 25th harmonic or a specially selected harmonic. This technique is called as Active Injection Mode (AIM). This is not done by absorbing currents, but by injecting additional currents whenever required. A current transformer first measures the current being drawn momentarily by the load. The control unit in the harmonic filters then analyses this current for amplitude and harmonics. It consequently feeds a current into the supply system whose amplitude and individual harmonic numbers is exactly equal to the current drawn by the load but which is, however, 180° out of phase with it. The harmonic currents cancel each other out and the supply network only has to supply the fundamental frequency and is not contaminated with harmonics at the point of connection, provided that the system has been appropriately dimensioned.

  The combination of harmonic filter and harmonic load appears to the network as an overall linear load drawing a sinusoidal current. Installation is quite simple. A three-phase feeder with or without a neutral conductor needs to be available. The current transformer is then installed in the line to the non-linear load. One great advantage of the active filter compared to conventional techniques is its flexibility in adapting the corrective power. Depending on the requirements, the filter can supply more or less corrective current.

  Even on overload, the filter does not switch off, but assumes a current-limiting mode i.e., the filter supplies its maximum current and in doing so compensates for a large proportion of the harmonics. Interaction with other system components, such as UPS units is therefore reduced to a minimum that is not critical. There is no problem to extend the system or install a combination of several filters. If operating or network conditions change, the filter automatically adapts to the new conditions within the scope of its nominal rating.

Selection of harmonic filters

  Harmonics filter systems should be best based on harmonic study. Harmonic filter system design depends on the filter reactive power output, tuning frequency and impedance of the network at the point of connection. The followings should be considered while selecting the appropriate harmonic filter:

  Considering the amplitude of harmonic current measured, sizing of the filter circuit is arrived based on the distributed network harmonics consisting of different orders.

  Appropriate tuning frequency is selected based on the measurement and network analysis.

  Based on the tuning frequency of passive filter, the current harmonics of that particular tuning frequency will be absorbed by the filter. Such filters can be tuned for a particular order (Frequency) of harmonics.

Benefits of harmonic filters

  Harmonic filters offer superior harmonic filtration along with following incomparable benefits:

Improvement in Voltage Profile

  With the installation of harmonic filters, the RMS value of voltage waveform is improved, thereby avoiding motor winding burn-outs, repetitive fuse failures etc. due to low voltage and improves voltage regulation.

Improvement in current profile

  With the installation of harmonic filter the net current is brought down due to the elimination of harmonic current thereby reducing losses in the electrical distribution system and leads to enhance life of transformers, cables, switchgear etc.

Conclusion

  Power harmonics is an electrical phenomenon. The negative effects due to power harmonics is not necessarily be only electrical but can also be mechanical as explained on ‘Impact of harmonics’ earlier in this article. Awareness and acceptance of having better power quality is increasing and the industries are gradually realising the positive impact of having electrical power without harmonics.

  Primary aim of the end user to install a harmonics filter could be the elimination of harmonics but an economic analysis will prove that the expenditure to install the harmonic filter can generate an ROI of more than 30%, should it be looked at as an investment. Implementation of harmonic filters as Demand Side Management (DSM) project operating on ‘Performance Contracting’ basis will not only benefit the end user and the harmonic filters supplier but will also benefit the utility and funding organisation.


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