Current Differential Relay
GARD 8000 SYS RFL Electronics
June 20, 2014 13-53 973.334.3100
In addition, a calculation for an external dual breaker fault is done. If this is detected, tripping on both
ends is blocked.
An external dual breaker fault is detected by subtracting the arithmetic sum of the two inputs from the
sum of the absolute value of the two inputs. If the absolute value of the difference is above a preset
bias, an External Dual Breaker Fault is declared. This function can be enabled or disabled.
13.5.1.2 CURRENT DIFFERENTIAL PROTECTION
Each relay measures the local current, stores the locally measured value, and transmits the
measurement to the remote relay.
When a relay receives a message regarding the current measured at the remote relay it compares it to
the current measured locally for the same phase at the same instant in time.
The differential current calculated for that half-cycle is compared to a calculated operate level. If the
differential current exceeds the operate level a trip is issued.
Figure 13-36. Charge Comparison Current Differential Relaying Principle
The GARD current differential relay is also available in a three terminal version. The same operating
principles as the two terminal relay are retained, with some additional supervision logic.
See the diagram on the following page.
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