GE UR Series C70, Инструкция по эксплуатации

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9-2 C70 Capacitor Bank Protection and Control System GE Multilin
9.1 OVERVIEW 9 THEORY OF OPERATION
9
Figure 9–1: BANK OVERVOLTAGE PROTECTION FOR GROUNDED AND UNGROUNDED BANKS
For a capacitor bank that is ungrounded the voltage across the string is equal to the difference between the system phase-
to-ground voltage and the bank neutral voltage. For instance elements shown in the ungrounded banks in the above figure
will be overstressed when the quantities below exceed nine times the element safe operating voltage limit.
(EQ 9.1)
These quantities are what the bank phase overvoltage protection measures when the bus source is set for wye VTs, the
auxiliary channel of the neutral source is set for a neutral voltage, and the bank overvoltage protection ground setpoint is
set to ungrounded.
When the neutral voltage of an ungrounded wye capacitor bank is not measured or for some reason it is desired not to use
the neutral voltage measurement, the string voltage can still be determined using the assumption that the bank is balanced.
For an balanced ungrounded bank the neutral voltage
V
X
is:
(EQ 9.2)
As such, the A-string voltage is:
(EQ 9.3)
Similarly, the B-string and C-string voltages are:
(EQ 9.4)
These quantities are what the bank phase overvoltage protection measures when the bus source is set for wye VTs, and
the bank overvoltage protection ground setpoint is set to “Ungrd w/o Vn”.
In applications where the bus VTs are delta connected, the bus phase-to-ground voltages and therefore the bus zero
sequence voltage are unknown. However, as shown immediately above, string voltages can be determined for an
ungrounded balanced bank using only phase-to-phase bus voltages. In the case of a grounded bank, we can calculate that
the A-string voltage is:
834744A1.CDR
Grounded bank
V
A
A
B
C
V
B
V
C
V
X
V
A
A
B
C
V
B
V
C
Ungrounded bank
V
A
V
X
–
RMS
, V
B
V
X
–
RMS
, or V
C
V
X
–
RMS
V
X
1
3
--- V
A
V
B
V
C
+ +
( )
=
V
A
V
X
–
RMS
V
A
1
3
--- V
A
V
B
V
C
+ +
( )
–
RMS
=
1
3
--- 2 V
A
V
B
V
C
– –
RMS
=
1
3
--- V
AB
V
CA
–
RMS
=
1
3
--- V
BC
V
AB
–
RMS
and
1
3
--- V
CA
V
BC
–
RMS