Page 43 - Power Electronic Control in Electrical Systems

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Power electronic control in electrical systems 33

components). Such components can have undesirable effects, including additional
losses in motors and generating units, oscillating torque in AC machines, increased
ripple in rectifiers, malfunction of several types of equipment, saturation of trans-
formers, and excessive triplen harmonics and neutral currents. 3
The harmonic content in the voltage supply waveform is another important
measure in the quality of supply. Harmonics above the fundamental power frequency
are usually eliminated by filters. Nevertheless, harmonic problems often arise
together with compensation problems and some types of compensator even generate
harmonics which must be suppressed internally or filtered.
The ideal compensator would
(a) supply the exact reactive power requirement of the load;
(b) present a constant-voltage characteristic at its terminals; and
(c) be capable of operating independently in the three phases.

In practice, one of the most important factors in the choice of compensating equip-
ment is the underlying rate of change in the load current, power factor, or impedance.
For example, with an induction motor running 24 hours/day driving a constant
mechanical load (such as a pump), it will often suffice to have a fixed power-factor
correction capacitor. On the other hand, a drive such as a mine hoist has an
intermittent load which will vary according to the burden and direction of the car,
but will remain constant for periods of one or two minutes during the travel. In such
a case, power-factor correction capacitors could be switched in and out as required.
An example of a load with extremely rapid variation is an electric arc furnace, where
the reactive power requirement varies even within one cycle and, for a short time at
the beginning of the melt, it is erratic and unbalanced. In this case a dynamic
compensator is required, such as a TCR or a saturated-reactor compensator, to
provide sufficiently rapid dynamic response.
Steady-state power-factor correction equipment should be deployed according to
economic factors including the supply tariff, the size of the load, and its uncompen-
sated power factor. For loads which cause fluctuations in the supply voltage, the
degree of variation is assessed at the `point of common coupling' (PCC), which is
usually the point in the networkwhere the customer's and the supplier's areas of
responsibility meet: this might be, for example, the high-voltage side of the distribu-
tion transformer supplying a particular factory.
Loads that require compensation include arc furnaces, induction furnaces, arc
welders, induction welders, steel rolling mills, mine winders, large motors (particularly
those which start and stop frequently), excavators, chip mills, and several others.
Non-linear loads such as rectifiers also generate harmonics and may require har-
monic filters, most commonly for the 5th and 7th but sometimes for higher orders as
well. Triplen harmonics are usually not filtered but eliminated by balancing the load
and by trapping them in delta-connected transformer windings.
The power-factor and the voltage regulation can both be improved if some of the
drives in a plant are synchronous motors instead of induction motors, because the
synchronous motor can be controlled to supply (or absorb) an adjustable amount of
reactive power and therefore it can be used as a compensator. Voltage dips caused by

3
Triplen (literally triple-n) means harmonics of order 3n, where n is an integer. See x2:12.