Switching inrush

Excessive charging currents (switching inrush or making currents) 23/750... 

When a capacitor circuit is compensated through a series reaetor. either to suppress the system harmonics or to limit the switching inrush currents (Section 23.11) or both, it will require suitable adjustment in its voltage and capacitive ratings, fhe series reactor will dampen the switching currents but consume an inductively reactive power and offset an equivalent amount of capacitive kVAr. and require compensation. The following example w ill elucidate this. 

The figures of Li i tind may be substituted in equiitinn (22.13) for Lt and C respectively to derive a more accurate switching inrush current. 

In HT systems, where a series reactor is already being used, to suppress the system harmonics this would also serve to limit the switching inrush currents and no separate reactor would be necessary,... 

By providing 12 turns of 150 mm mean diameter of the 50 mm flexible copper cable connecting each 60 kVAr capacitor bank a self-inductance of roughly 42.93 x 10 H can be introduced into each switching circuit, which will limit the switching inrush current to almost the permissible value of the making current (/, ) of the switching device. 

The starting of an induction motor does not relate to simple switching alone. It also involves its switchgears to control its starting inrush current, starting torque, or both, and its overload and short-circuit protection. 

Check selection of fuses for the type of load and switching. Perhaps they are under-rated for the starting inrush current and its duration. For selection of fuse ratings see Section 12.10.4. 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

The electromagnetic unit, however, as used in a residual VT (Section 15.4.3) or a capacitor VT (Section 15.4.4) should be suitable for carrying the heavy discharge or inrush currents during a capacitor discharge or switching respectively. 

Maximum inrush occurs when five steps of the PFCR are ON and the sixth is switched ON... 

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. 

In capacitor switching, introduction of an inductor coil (Section 23.I1) can contain not only the inrush current but also tame the current phasor to shift closer to the voltage and thus limit the TRV on an interruption. 

For simplicity and to obtain a close approximation, the value of/ may be considered to be negligible and therefore ignored. It may give a higher value of inrush current, which is on the safe side to select the switching device. 

At the instant of switching the surge impedance, Z, (equation (23.12)), and the natural frequency of the switched circuit, i.e. the transient frequency, /i, (equation (2.2.14)). determines the amount of inrush current. 

Let us refer to equation (23.13) for the inrush current, / on parallel switching. If we can increase the impedance of the switching circuit by introducing a resistance R or inductance L, or both, we can easily control this current to a desired level. 

If switching is effected when the incoming capacitor is already charged, then the inrush current, will be nearly twice this, i.e. 

If we are able to provide an inductance of this value with each capacitor bank of 60 kVAr the problem of excessive inrush transient current can be overcome and the component ratings as chosen above will be sufficient to switch a parallel circuit. 

Parallel switching of capacitor banks (i) Inrush current... 

In SVCs Ihe number of switchings is of no relevance, as they arc free from inrush currents. Switching is performed at the instant when the cuiTcnt wave is passing through its natural zero. Static devices in various combinations and feedback control systems, which may be computer-aided, can tilmost instantaneously (< I cycle) generate or absorb reactive power, as may be demanded by the system. Correction... 

Suitable lor systems svhere fre( uenl switching is not likely and the system is I ree frotn harmonic generating sources. They are the sell-healing type and their output may become reduced with lime (bectiuse of railures of capacitor elements as a result of ssvitching inrush currents and. system harmonics). If the system conditions are not conducive, an inductor coil may be provided to limit the harmonic effects (Section 23.9). Some manufacturers, as standard practice, provide an inductor coil inside the shell to contain the inrush current and also dampen the harmonics. 

For limiting inrush currents Generally, it is not required for the LT systems. For larger banks, however, say, 500 k VAr and above, when they are installed close to the supply source, so that the line reactance up to the bank is too small to limit the switching currents, the use of reactor would be advisable. 

In LT capacitors controlled through automatic p.f. conection relay it i s recommended that the capacitors are not switched more than 1000 times during a year or, say, three or four switchings per day. More switchings will mean more frequent overvoltages and inrush currents which may reduce the life of a capacitor. In HT capacitor banks, how ever. such a situation may not be relevant as HT capacitors inay be switched only once or twice a day. 

Figure 27.9 Use of current limiting reactor, (1) to limit the fault current, or (2) to limit inrush current during a capacitor switching...

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