Capacitors inrush

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. 

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. 

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. 

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. 

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. 

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

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 smaller units, they are u.sed to limit only the inrush currents during a parallel operation and may be rated for 0.2% of the capacitors kVAr rating. They must be connected on the neutral side of a star-connected capacitor bank. Also refer to the notes below step 4 of Example 23.4. 

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. 

To limit the inrush and harmonic currents through the capacitor banks. 

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

These are meant to limit the inrush currents occurring during a switching operation of a capacitor. They are connected in series with the capacitors and may be short-time rated for the values of the inrush currents and continuously rated for normal line currents. They are almost the same as the series reactors with fixed reactance. 

Many switcher ICs are in fact designed with a certain minimum on-time (especially the current mode control types). They also keep to the minimum pulse width until about 0.2 to 0.3V on the feedback pin. In such cases, with a reasonably large output bulk capacitor, you will see a huge inrush of current into the output capacitor, even before the latter starts to rise appreciably. You should also be aware that inrush current into the input capacitor of any topology is very high, and no switch action can even hope to prevent that. 

But as soon as we put in a capacitor, we now also need to limit the inrush current into it — all capacitors connected directly across a dc source, will exhibit this uncontrolled inrush — and that can t be good either for noise, EMI, or for efficiency. Of course we could simply opt for a resistor to subdue this inrush, and that in fact was the approach behind the early bucket regulators (Figure 1-2). 

But we still need to limit the capacitor charging current ( inrush current ). And as indicated, we could use a resistor. That was in fact the basic principle behind some early linear-to-switcher crossover products like the bucket regulator shown in Figure 1-2. 

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