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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.

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we ground the neutral of our distribution capacitor banks why
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
slide3
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

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slide5

Grounded Wye High side Transformer bank

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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

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Grounded Wye High side Transformer bank

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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
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.

slide8

-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

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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

slide10

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The transformer bank is still grounded

One cutout has blown

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

slide11

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Source

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The transformer bank is still grounded

One cutout has blown

Again there is voltage here

This is the equivalent circuit

slide12

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

slide13

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

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