Capacitors star banks

It is common practice to leave the star-connected capacitor banks ungrounded when used in the system or use delta-connected banks to prevent the flow of third harmonic currents into the power system through the grounded neutral. 

Figure 25.6 Configuration of a double-star capacitor bank making 14 400 kVAr...

It may be observed that when the number of units in parallel are fewer than six, the voltage across the healthy units will rise by more than 10%, even on a failure of only one unit. The units in parallel therefore must be a certain minimum number to ensure that in no case will the voltage across the healthy units in the event of a failure of a few units exceed 10%. Single-star capacitor banks (Figure 25.5(a))... 

The fault current on a double star can therefore be calculated on the same basis as for a single-star capacitor bank. 

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-della connected capacitor units In (his case, it will establish a line-to-line fault with a heavy fault current. Protection by HRC fuses would be more appropriate than for a grounded star or using an MCCB. For series-parallel connected units I his is applicable in HT capacitor banks comprising a number of small units arranged in series and parallel combinations. The fault current in such cases for an isolated neutral can be expressed by... 

This method is applicable to single-star or delta-connected capacitor banks. Unbalance can be detected through the use of an RVT (residual voltage transformer) (Section 15.4.3). See Figure 26.4. The theory of operation is that any unbalance, of the system or the capacitor bank, will shift the neutral and reflect as the residual voltage across the open delta and can be used for the protective scheme. The unbalance voltage across the open delta in the event of failure of a unit in any series group can be expressed by... 

For more than one capacitor bank one RVT for each bank will be necessary for the correct discharge of each bank, (Figure 26.5). An RVT, if provided with an additional star-connected tertiary winding rated for 110 V line to line, can aiso check the discharge condition and its duration and if it meets the design requirements. This can even be monitored remotely through a relay and control panel. 

Figure 26.6 Detecting an unbalance in doubie-star identical capacitor banks through a current unbalance method...

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