P.3.96
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
You can access the value of the tap coefficient for the charging current as a
read-only setting to aid testing and troubleshooting, if required.
87CTCCC
This setting specifies the transformer winding compensation matrix (0–12)
required for the calculated charging current given the location of the voltage
source (Y or Z relay voltage terminal as specified by the 87LINEV setting)
and the power line, in relation to the transformer windings.
To select this setting correctly, choose the compensation matrix that is
appropriate for the transformer winding (vector group) to which the voltage
source the 87LINEV setting specifies, connects.
EXAMPLE 3.4
Consider a sample system of Figure 3.44 and assume further the line
positive-sequence susceptance of 1.7 mS primary, and PT ratios of
2160 at Relays 1 and 2, and PT ratio of 1150 at Relay 3.
Note that the unit of the 87CCB1 setting is secondary mS, therefore:
At Relay 1, you must enter 1.7 mS •
2160/(max(1000/5,1000/5)) = 18.36 mS.
At Relay 2, you must enter 1.7 mS • 2160/(max(1200/5,1200/5)) =
15.30 mS.
At Relay 3, you must enter 1.7 mS • 1150/(1000/5) = 9.78 mS.
The 87VTCC setting specifies the voltage of the power line, therefore
87VTCC = 240 kV at all three relays.
The relays calculate (see Equation 3.3) and apply the following taps to
bring the calculated charging current to the 1 pu base quantity (1 pu is
the transformer nominal current):
Relay 1 uses 87TAPCC = 5.196.
Relay 2 uses 87TAPCC = 4.330.
Relay 3 uses 87TAPCC = 1.591.
Relays 1 and 2 have their 87LINEV voltage sources connected to the
wye-winding of the in-line transformer. As a result, the calculated
charging currents should undergo the same transformation as the
physical currents measured at this winding. Therefore, set 87CTCCC =
1 for this example (the same as for the W and X currents of the two
relays).
Relay 3 has its 87LINEV voltage source connected to the delta-
winding of the in-line transformer. As a result, the calculated charging
currents should undergo the same transformation (phase shift) as the
physical current measured at this winding. Having set 87CTCCC = 1 in
Relays 1 and 2, set 87CTCCC = 0 in Relay 3.
Assume now a similar application in which Relay 3 has the 87LINEV
source having a ratio of 2100 connected on the line side of the
transformer.
In this case you will enter 87CCB1 = 1.7 mS • 2100/(1000/5) = 17.85 mS
and the relay will apply a tap of 87TAPCC = 2.406 (see Equation 3.3).
The proper vector group compensation matrix will be 87CTCCC = 1.
87LINEV is the relay voltage terminal (Y or Z) selected for the 87L
voltage-related functionality,
CTR_LOC is the CT ratio of the relay (higher of the two ratios if both
the W and X relay current input terminals configured for
the 87L function).
where:
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