State-of-the-art solutions for controlling harmonics

Power & ElectricalsTECHNOLOGY
194

Introduction:
Harmonics are the multiples of fundamental frequencies. They are generated due to non-linear loads. Non-linear loads, by definition are the equipments which draw non-sinusoidal current even from a sinusoidal voltage source. The examples of non-linear loads are Rectifiers, Induction furnaces, UPS Systems, Variable frequency drives and so on . . .

The adverse effects of harmonics in industrial plant are well known:

Effects of harmonics:

The harmonics adversely affect almost all the components of any industrial plant :

  • Power factor improvement capacitors draw excessively high current if voltage is contaminated with harmonics.
  • The magnetic equipments like motors, generators and transformers are abnormally heated up due to harmonics. This is due to increased copper loss, hysterisis loss and eddy current loss.
  • Fuses, Circuit breakers, Protective relays malfunction due to harmonic currents.
  • Neutral cables get over heated due to addition of zero-phase sequence triplen harmonic current.
  • Due to the adverse effects of the harmonics, harmonics needs to be controlled.
  • There are two philosophies of harmonic control.
  • To eliminate or reduce the harmonics by taking care in the equipment design. This is essentially in the “Green Power Technology”.
  • Elimination or reduction of harmonics which are already generated by the non-linear equipment which was not designed to take care of harmonics.

In industries both these philosophies are prevalent for harmonics control.

Prevention of harmonics by design

1) Multipulse converter
3-phase rectifier consisting of 6-diodes having 6-pulse design is shown in Fig – 1. This is a basic building block of variable frequency drives, UPS systems, battery chargers and so on . . . This rectifier has typically 62% current distortion (THD).

Fig. 1

Please refer Fig – 2 showing the input current waveform. In order to reduce this current distortion by design multi-pulse converters are commonly used.

Fig. 2

Please refer to Fig – 3 showing schematic of 12-pulse rectifier. It consists of 12 diodes instead of 6 diodes. The two 6-pulse converters are connected to two secondaries of input transformer. One secondary is star connected and other secondary is delta connected to give 30º phase shift.

Fig. 3

The current distortion is reduced from 62% to about 8% in this configuration. The input current waveform of 12-pulse rectifier is shown in Fig – 4. This technology is further extended to 18-pulse or 24-pulse converters to further reduce the current distortion.

Fig. 4

This technique is used in high power rectifiers. For example in HVDC transmission (High Voltage Direct Current transmission) multiple converters are used. However, multi-pulse converters have disadvantage of using more number of devices leading to relatively poor efficiency. They also required intricate transformer design and balancing required for current sharing by multiple converters. Hence the state-of-art technology in converters is PWM converter.

2) PWM converter
The schematic of PWM converter is shown in Fig – 5. It uses 6-IGBTs in place of 6-diodes. PWM converter has following advantages.

  • It can work at unity power factor and it can also be made to operate at leading power factor to compensate for the poor power factor created by other lagging power factor loads.
  • It can work in both ways i.e. it can transfer the power from mains input to output as well as it can feed back power from regenerative loads to mains. Thus it can lead to energy conservation in case of some applications like Centrifuge.
  • It can stabilize DC link output voltage against fluctuations in mains input voltage.

All the above techniques of harmonics control are the examples of harmonics controlled by design. However, these techniques can not take care of existing harmonics in the plant. The equipments which controlled the existing level of harmonics are given in the following session.

1) Passive harmonic filter:
Passive harmonic filter consists of inductor and capacitor in series. This combination is tuned to the harmonics to be eliminated. The schematic of passive harmonic filter is shown in Fig – 6. The passive harmonic filter is simple and economical. It is very effective for applications where the load configuration is fixed and supply frequency is relatively constant. However, these filters have the following limitations.

These filters can be over loaded due to harmonic inrush current coming from some other load which can damage the filter.

  • This filter becomes less effective if the supply frequency varies.
  • If the load configuration changes this filter can not effectively filter the harmonics.
  • The inductor and capacitor used in the filter can resonate with power factor improvement capacitor used in the plant at some harmonic frequency.

To overcome these limitations Active Harmonic Filter is invented.

2) Active harmonic filter:

The schematic of active harmonic filter is shown in Fig – 7.

Fig. 5

Active harmonic filter has a current sensor connected in series with a non-linear load which is to be compensated to reduce the harmonics. The harmonic components of current in the non-linear load is sensed and equal and opposite current is generated by active harmonic filter. The current of active harmonic filter cancels the harmonic current of the non-linear load. As a result the source current is pure sinusoidal which does not contain harmonics.
Advantages of active harmonic filter

  • It reduces the harmonic current distortion by eliminating the harmonics.
    The harmonics can be selectively eliminated by configuring the active harmonic filter in user programmable manner.
  • Like passive harmonic filter this filter does not resonate with any components of industrial plant.
  • This filter is dynamic by design and can adapt to changes in load configuration.
  • This filter can compensate for lagging power factor and it can also take care of 3-phase current balancing.
    Thus active harmonic filter is a state of art solution for harmonic mitigation.

Conclusion :
Use of more and more non-linear loads is becoming common in today’s industrial plants. Therefore, harmonic elimination has become the necessity of the day. Depending upon the equipment used in the industrial plant different harmonic elimination techniques can be adopted. Whenever any new equipment is to be designed it should be designed to take care of harmonics by using multiple converter or PWM converter whereas, if the existing harmonics are to be taken care of either passive filter or active filter can be used.

About author

Mr. S. B. MAHAJANI
Mr. S. B. MAHAJANI
M.Tech from IIT, Bombay and MBA in Operations Research.

The author Mr. S.B.Mahajani is working with M/s.Amtech Electronics (India) Ltd, as Dy.General Manager.
AMTECH is the leading manufacturer of Variable Frequency Drives and Soft Starters. AMTECH also provides
Total solution for Power Quality and Energy Conservation.

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