Why is it necessary to have a well Grounded Capacitor Bank on our Distribution system

We ground the neutral of our Distribution capacitor banks- Why? Our distribution system is a multi-grounded system Our three phase transformer banks are grounded wye high side mostly. Most of our distribution protection uses single phase protection devices, such as cutouts. So that for three phase installation one cutout can open and the other 2 can remain closed. If we don’t ground the distribution cap bank and one fuse/cutout opens then we can get into phase reversal and/or high voltage across line to ground connected loads

This problem is documented in Engineering Manual section 4, but the actual explanation is not clear. Our normal installation- one fuse can blow and nothing abnormal happens as everything looks like 3 single phase systems Typical single line of a capacitor installation on our distribution circuit Grounded Wye High side Transformer bank Grounded wye cap bank S S Source S S S S

Grounded Wye High side Transformer bank Source S S S The transformer bank is still grounded One cutout has blown and the capacitor neutral was not grounded In this situation the high voltage can appear and so can phase reversal

S S S S S Source 1.732VLL/2 1/3*1.732VLL/2 VLL Grounded Wye High side Transformer bank S S Source S S S The circuit becomes for the blown fuse: If I put a voltmeter between these two points my voltage becomes from the triangle geometry: -jXc=1/jwC 1.732VLL/6 jXL=jwL VT 1.732VLL/2 1.732VLL/6 1/3*1.732VLL/2 VLL VT=jXL/(jXL-jXC)(1.732VLL/6)

So with the equivalent circuit how can Phase Reversal and High voltage occur Source B 1.732VLL/6 +1.732VLL/2 This is how much I need to shift A’N to have complete line to line phase reversal, it is negative as it is opposite to AN. A’N=-2VLL/1.732 N A A’ -4/3*1.732VLL/2 C Clockwise rotation A’BC can be achieved if A’ voltage can be shifted as shown and this would have to be the voltage developed across the A phase transformer with the blown line fuse Normal Rotation ABC which is Counterclockwise from the source.

When When -jXc=1/jwC jXL=-2VLL/1.732 VT 1.732VLL/6 VT=jXL/(jXL-jXC)(1.732VLL/6) -2VLL/1.732=(1.732VLL/6)(XL/(XL-XC)) or –4= XC=(5/4)XL XL/(XL-XC) When Normal line to line volt appears across all legs but has reversed rotation, also the transformer is experiencing 2*phase to ground voltage so this would not be good for single phase loads. XC=(5/4)XL Normal line to ground voltage appears across the transformer but the rotation is still reversed, in this case you don’t have full line to line voltage motors will try to run in the reverse direction, but single phase loads will not be over voltaged. When XC=(3/2)XL

What about if I hook the capacitors up in a delta configuration with a grounded Wye connected system will I still get the possibility of an over voltage and phase reversal The answer is yes, let see

S S S S S S S S S S S Source Source The transformer bank is still grounded One cutout has blown In this situation the high voltage can appear and so can phase reversal

S S S S S Source Again there is voltage here The transformer bank is still grounded One cutout has blown Again there is voltage here This is the equivalent circuit

Vb Vc-Vn = Vn + Vn-Vb Vn -jXc jXL -jXc Vc Vc+Vb = Vn + 2Vn Vc=V -60 Vc+Vb= V 0 -jXc jXL -jXc Vb=V 60 V = (j2XL- jXc) Vn -jXc XcXL Vn = XL V Full phase reversal happens at Xc=5/2XL 2XL- Xc

This can also happen for a grounded wye bank on a grounded wye system with a delta connected transformer as load. Typical of all our systems, but the saving grace is the fact that we have three phase protective devices Our typical system set up, this is a 3 phase breaker Grounded wye cap bank S S Source S S S S