Capacitors surges

The higher Ihe value of C. the lower will be the voltage overshoots in the rectified voltage and the inverter circuit would be fed by an almost cotistant voltage soitrce. The capacitor iti the circuit also provides an indirect protection from the voltage surges. 

With the use of surge arresters and surge capacitors (Section 17.10)... 

All electrical equipment are designed for a specific BIL, as indicated in Tables 11.6, 13.2, 14.1, and 32.1(A) for motors, switchgears and bus systems respectively, and Tables 13.2 and 13.3 for the main power system (line clearances and insulators). If the actual severity of a prospective surge, i.e. its amplitude and/or rise time or both, is expected to be higher than these levels (higher amplitude and lower rise time) the same must be damped to a safe level, with the use of surge arresters, surge capacitors or both as discussed later. 

Above we have analysed ihe case of an induclion moior during a swiiching sequence for systems of 2.4 kV and above. The phenomenon of voltage surges in iransformers, capacitors, inierconneciing cables or overhead lines elc. is no differem, as Ihe circuit conditions and sequence of switching will remain the same for all. 

The same theory will apply in capacitor switching except that the interrupting current will now be leading the voltage by almost 90° instead of lagging. See also Section 23.5 to determine the amount of surge voltages and inrush currents. 

Curve-0b2 - Tamed and damped TRV with the use of a surge capacitor and a surge arrester... 

A surge capacitor offers a near-open circuit during normal operation (at or near the power frequencies) and a near-short-circuit to the arriving surges at surge frequencies, while the inductance of the motor windings (<> will... 

To illustrate the above, consider a surge capacitor of 0.25 // F being used in parallel with a 350 kW, 6.6 kV motor. If the likely FOW is presumed to have an amplitude up to 5 p.u., then the energy this capacitor can absorb... 

Total energy absorbed by the arrester and the surge capacitor = 1.81 -F 5,4 = 7,21 Joules... 

Figure 17.23 Use of a surge capacitor in association with a surge arrester to provide taming as well as dampening effects to a steep fronted TRV...

If the turn-to-turn voltage, ef, and the front time, so achieved falls within the design parameters, no additional measures would be necessary to further reduce the TRV. However, if It is felt that the dampened voltage, ef, may exceed the required value of r.r.r.v. during operation, a surge capacitor may also be introduced in the circuit as illustrated in Figure 17.23 to reduce the wave front a2 to b2, so that the front time is enhanced to a permissible... 

With this particular arrester, the motor has to be specially designed for the higher level of FOW impulse voltage withstand. But the manufacturer can always modify the protective characteristics of the arrester, depending upon the system s requirements. The matter may therefore be referred to the manufacturer for recommendations. The arrester may be fitted with a 0.25, uF surge capacitor in parallel, to reduce the steepness of the FOW (for the arrester of example 17.5, it is 14/0.2 kV/jUS) to a safer value. 

A surge capacitor loo, when used in parallel to supplement such an arrester, will not be able to make the arrester suitable for such a duly. This is effective only against very steep surges having a very high surge frequency of lO kHz and more. The frequency of a surge can be determined by... 

Suitable for all applications.. Switching of capacitor banks, however, may j)ose problem because of high TRVs. These may cause a restrike of the arc plasma and give rise to switching surges... 

This will be the surge frequency of the switching circuit when the sixth capacitor is switched on a circuit that has five capacitor units already switched. The improved value of this switching frequency, /s, af si the introduction of inductance 42.93 uH in each capacitor circuit will become. 

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