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

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?
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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
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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
S
S
Source
Grounded wye
cap bank
S
S
SS
Grounded Wye High
side Transformer bank
S
S
Source
S
S
S
One cutout has blown and the capacitor neutral was not
grounded
The transformer bank
is still grounded
In this situation the high voltage can appear and so can phase
reversal
Grounded Wye High
side Transformer bank
S
S
Source
S
1.732VLL/6
VT
S
S
The circuit becomes for the
blown fuse:
-jXc=1/jwC
If I put a voltmeter between these two
points my voltage becomes from the
triangle geometry: 1.732VLL/6
jXL=jwL
1.732VLL/2
VLL
VT=jXL/(jXL-jXC)(1.732VLL/6)
1/3*1.732VLL/2
So with the equivalent circuit how can Phase
Reversal and High voltage occur
Source
A
B
1.732VLL/6
+1.732VLL/2
N
A’
C
Normal Rotation
ABC which is
Counterclockwise
from the source.
-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
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
-jXc=1/jwC
1.732VLL/6
VT
jXL=-2VLL/1.732
VT=jXL/(jXL-jXC)(1.732VLL/6)
-2VLL/1.732=(1.732VLL/6)(XL/(XL-XC))
or –4= XL/(XL-XC)
XC=(5/4)XL
When
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.
XC=(3/2)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.
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
SSS
S
S
Source
S
Source
S
S
S
S
S
One cutout has blown
The transformer bank
is still grounded
In this situation the high voltage can appear and so can phase
reversal
S
S
Source
S
S
S
The transformer bank
is still grounded
One cutout has blown
Again there is voltage here
This is the equivalent
circuit
Vb
Vn
Vc-Vn = Vn + Vn-Vb
-jXc jXL -jXc
Vc
Vc+Vb = Vn + 2Vn
-jXc jXL -jXc
Vc=V -60
Vb=V 60
V = (j2XL- jXc) Vn
-jXc
XcXL
Vn = XL V
2XL- Xc
Full phase reversal happens
at Xc=5/2XL
Vc+Vb= V 0
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
SS