Unit 3 power quality esrmnotes.in, Lecture notes of Electrical and Electronics Engineering

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UNIT-3.OVER VOLTAGES

• SOURCES

– 2 Main sources of transient over

voltages on utility system

1. Capacitor Switching

2. Lightning

Sources of Sags and Interruptions

• Transient overvoltages can be

generated at

• High frequency - Load switching &

lightning

• Medium frequency - Capacitor

energizing

• Low frequency

Drawback

Use of capacitors yield oscillatory

transients when switched.

Fixed Capacitor Bank - energized all the

time

Others are switched according to load

levels

Various control means, including time,

temperature, voltage, current, and reactive

power, are used to determine when the

capacitors are switched. esrmnotes.in | Class notes made easy.

• Common symptoms of PQ problems related to utility capacitor switching overvoltages is that the problems appear at nearly the same time each day.
• On distribution feeders with industrial loads, capacitors are frequently switched by time clock in anticipation of an increase in load with the beginning of the working day.
• Common problems are ASDs trips and malfunctions of other electronically controlled load equipment that occur without a noticeable blinking of the lights or impact on other, more conventional loads.

While such brief transients up to 2.0 pu are not generally damaging to the system insulation, they can often cause misoperation of electronic power conversion devices.

Controllers may interpret the high voltage as a sign that there is an impending dangerous situation and subsequently disconnect the load to be safe.

The transient may also interfere with the gating of thyristors.

Switching of grounded-wye transformer banks may also result in unusual transient voltages in the local grounding system due to the current surge that accompanies the energization.

Figure 4.3 shows the phase current observed for

the capacitor-switching incident

At the customer location, high-energy surge arresters can be applied to limit the transient voltage magnitude at the customer bus.

Energy levels associated with the magnified transient will typically be about 1 kJ.

Another means of limiting the voltage magnification transient is to convert the end-user power factor correction banks to harmonic filters.

An inductance in series with the power factor correction bank will decrease the transient voltage at the customer bus to acceptable levels.

A 3 percent reactor is generally effective.

While offering only a small impedance to

power frequency current, it offers a

considerably larger impedance to the

transient. Many types of drives have this

protection inherently, either through an

isolation transformer or a dc bus reactance.

Lightning

Lightning is a potent source of impulsive

transients.

Figure 4.6 illustrates some of the places

where lightning can strike that results in

lightning currents being conducted from the

power system into loads. esrmnotes.in | Class notes made easy.

Many investigators in this field postulate that lightning surges enter loads from the utility system through the interwinding capacitance of the service transformer as shown in Fig. 4.7.

The concept is that the lightning impulse is so fast that the inductance of the transformer windings blocks the first part of the wave from passing through by the turns ratio.

However, the interwinding capacitance may offer a ready path for the high-frequency surge. This can permit the existence of a voltage on the secondary terminals that is much higher than what the turns ratio of the windings would suggest.