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

2. 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

3. 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

5. 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

6. 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)

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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