Power Quality

Introduction

It is necessary to prevent the power line disturbances from disrupting the operation of critical loads such as computers used in process control, medical equipments, operation theatre, large computer center, R & D laboratories and the like.

1. The voltage supplied by the a.c. main should ideally be perfect sine wave without any harmonics and voltage fluctuations
2. The voltages in a three phase system should be balanced, with each phase displaced by 120 degrees with respect to the others.

However, this does not happen in real practice due to the presence of disturbances such as over-voltage, under-voltage, voltage spikes, chopped voltage waveform, harmonics and electromagnetic interference.

1. The over-voltages (Swell) may be caused by sudden decrease in load or unbalance caused due to faults in the supply system.
2. Under-voltage (Sag) may be caused by over-load conditions such as starting of big induction motors and compressors or due to power line faults.
3. Spikes of voltages may be caused by switching- in and switching-out of power factor correction condensers or big motors and welding sets working in the vicinity.
4. The chopped voltages may be due to a.c. to d.c. line commutated thyristor converters.
5. The voltage harmonics may be caused by variety of sources such as thyristor converters, magnetic saturation of power system transformers etc.
6. Electromagnetic interference (EMI) is produced due to rapid switching of voltages and currents in power converters. In case of critical applications, where shut down due to the above disturbances is not acceptable, the back-up of the a.c. main is provided by means of UPS. The power conditioners provide an effective way of suppressing some or all of the electrical disturbances other than the power outages and frequency deviations.

The wave shapes of typical disturbances are shown in Fig.

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