Inrush current limitation

Reduced-voltage starting. A reactor, resistor, or transformer is temporarily connected ahead of the motor during start to reduce the current inrush and limit voltage dip. This is accompanied by reduced starting torque. For reactor or resistor start, the torque decreases as the square of current for transformer start, the torque decreases directly with line current. The reactor, resistor, or transformer can be adjusted to give a proper balance between torque and current. 

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. 

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. 

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. 

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. 

When it is required to limit the inrush current a fixed reactance (linear) reactor is more suitable. A variable type reactor will be necessary when it is to be used for voltage regulation or load sharing. In circuits where harmonics may be present, saturated type reactors may be preferred. 

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. 

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. 

The key features of the current-ramp soft-start unit are that it limits the motor inrush current to a specific amount, it can provide physical soft start, and it limits motor torque. These units are used mainly for electrical soft start (reducing inrush current). The shutdown of production is a... 

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

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