Charging Current Compensation - Toshiba GRL100-701B Instruction Manual

between the remaining terminals to be performed.
• If the slave terminal is out-of-service, the master and another slave terminal maintain the
differential protection.
When two terminals are out-of-service in a three-terminal line, the remaining terminal continues
the current differential protection using the local current irrespective of whether it is a master
terminal or a slave terminal.
2.2.8

Charging Current Compensation

When differential protection is applied to underground cables or long overhead transmission
lines, the charging current which flows as a result of the capacitance of the line (see Figure
2.2.8.1) appears to the protection relay as an erroneous differential current.
The charging current can be compensated for in the setting of the relay's differential protection
sensitivity but only at the expense of reduced sensitivity to internal faults. In addition, the actual
charging current varies with the running voltage of the line and this must be taken into account in
the setting.
In order to suppress the effect of the charging current while maintaining the sensitivity of the
differential protection, GRL100 is equipped with a charging current compensation function,
which continuously re-calculates the charging current according to the running line voltage and
compensates for it in its differential current calculation. The running line voltage is measured by
VT inputs to GRL100.
The user enters values for line charging current and for the line voltage at which that charging
current was determined in the settings [DIFIC] and [Vn], and these values are used by the relay
to calculate the capacitance of the line. The relays at each line end share the line capacitance
between them, that is they divide by two for a two-terminal line, and by three for a three-terminal
line. In the case of a three-terminal line, if the relay at one terminal is out-of-service for testing
(see out-of-service terminal detection), the other two terminals are automatically re-configured
to divide the line capacitance by two.
Each terminal continuously calculates its share of the charging current at the running line voltage
on a sample by sample basis as follows:
I c = C dV / dt
where,
Ic = line charging current
C = line capacitance calculated from settings [DIFIC] and [Vn]
V = measured line voltage
The relay then calculates the line current compensated for the charging current on a sample by
sample basis as follows:
Terminal A
GRL100
Figure 2.2.8.1 Charging Current
 24 
Terminal B
GRL100
Ic
6 F 2 S 0 8 5 0