CHAPTER 9: THEORY OF OPERATION OVERVIEW
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 9-11
9
Figure 9-4: Neutral overvoltage restraint
9.1.4.5 Auto-setting command
As a convenient alternative to manually determining the unbalance ratio settings, the relay can automatically calculate
these settings from its own measurements while the capacitor is in-service, as described in the Commands chapter. The
technique the relay uses is to set the operate signal variable in equation 9.25 to zero and solve for the unbalance ratio k-
values using the average of several successive measurements of the voltages.
Determination of the two k-values using only one equation is possible as the k-values are known to be real-valued, so the
complex-valued equation can be separated into real and imaginary parts, with the same two k-values in each. A solution
exists as long as the cross product of the B and C-phase voltages is non-zero; that is, the capacitor bank is in-service.
However, the assumption made here is that when the auto-set command is executed, the capacitor is in an acceptably
balanced state, wherein the operating signal ought to be zero. Following the auto-set command, the protection measures
changes from the state that existed at the time the auto-set command executed.
9.1.5 Phase current unbalance (ANSI 60P)
9.1.5.1 Operating principle
The phase current unbalance function is based on the balance between interconnected and nominally identical phase
strings, and is applicable to both grounded and ungrounded installations. A window CT measuring the vectorial difference
between the two phase currents is required for each phase as shown. While the two phase strings are actually identical,
the measured current is zero. The failure of an element in either string results in a difference current, which is sensed by the
relay. However, with the two strings slightly mismatched due to manufacturing tolerances, an inherent difference current
can be present. Compensation for this inherent unbalance current is available to increase the sensitivity of the function.
The phase current unbalance function uses the following operate signal:
I
OP(A)
= |I
DIF(A)
- k
A
I
A
| Eq. 9-43
In this equation, k
A
is the capacitor bank leg-A inherent unbalance factor setting. The currents are as defined in the
following figure and are in per-unit on the nominal current of the differential source. This equation involves phasors, not
magnitudes; that is, the vector sum of the currents is created by the protection function implementing the method.
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