Why is it necessary to have a Why is it necessary to have a well Grounded Capacitor Bank well Grounded Capacitor Bank

on our Distribution systemon our Distribution system

We ground the neutral of our We ground the neutral of our Distribution capacitor banks- Why?Distribution capacitor banks- Why?

Our distribution system is a multi-grounded systemOur distribution system is a multi-grounded system Our three phase transformer banks are grounded Our three phase transformer banks are grounded

wye high side mostly.wye high side mostly. Most of our distribution protection uses single Most of our distribution protection uses single

phase protection devices, such as cutouts. So that phase protection devices, such as cutouts. So that for three phase installation one cutout can open and for three phase installation one cutout can open and the other 2 can remain closed.the other 2 can remain closed.

If we don’t ground the distribution cap bank and If we don’t ground the distribution cap bank and one fuse/cutout opens then we can get into phase one fuse/cutout opens then we can get into phase reversal and/or high voltage across line to ground reversal and/or high voltage across line to ground connected loadsconnected loads

This problem is documented in Engineering This problem is documented in Engineering

Manual section 4, but the actual explanation Manual section 4, but the actual explanation is not clear. is not clear.

Typical single line of a capacitor installation on our distribution circuit

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Grounded Wye High side Transformer bank

Grounded wye cap bank

Our normal installation- one fuse can blow and nothing abnormal happens as everything looks like 3 single phase systems

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Grounded Wye High side Transformer bank

The transformer bank is still groundedOne cutout has blown and the capacitor neutral was not

grounded

In this situation the high voltage can appear and so can phase reversal

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Grounded Wye High side Transformer bank

If I put a voltmeter between these two points my voltage becomes from the triangle geometry:

VLL

1.732VLL/2

1/3*1.732VLL/2

1.732VLL/6

The circuit becomes for the blown fuse:

jXL=jwL

-jXc=1/jwC

1.732VLL/6 VT

VT=jXL/(jXL-jXC)(1.732VLL/6)

So with the equivalent circuit how can Phase So with the equivalent circuit how can Phase

Reversal and High voltage occurReversal and High voltage occur

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C

B

A

Normal Rotation ABC which is Counterclockwise from the source.

A’

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

1.732VLL/6+1.732VLL/2

N-4/3*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

jXL=-2VLL/1.732

-jXc=1/jwC

1.732VLL/6 VT

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 XC=(5/4)XL 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.

When XC=(3/2)XLNormal 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.

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

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The transformer bank is still groundedOne cutout has blown

In this situation the high voltage can appear and so can phase reversal

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The transformer bank is still groundedOne cutout has blown

Again there is voltage here

This is the equivalent circuit

Vc

Vb

Vn Vc-Vn = Vn + Vn-Vb

-jXc -jXcjXL

Vc+Vb = Vn + 2Vn

-jXc -jXcjXL

Vc=V -60

Vb=V 60Vc+Vb= V 0

V = (j2XL- jXc) VnXcXL-jXc

Vn = XL

2XL- Xc

V Full phase reversal happens at Xc=5/2XL

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

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Grounded wye cap bank

Our typical system set up, this is a 3 phase breaker