Ground Differential Element - GE L90 Instruction Manual

9.1 OVERVIEW
2
  
I I
=
REST_A LOC_PHASOR_RESTRAINT_A
The fault severity for each phase is determined by following equation:
where P is the pickup setting.
This equation is based on the adaptive strategy and yields an elliptical restraint characteristic. The elliptical area is the
restraint region. When the adaptive portion of the restraint current is small, the restraint region shrinks. When the adaptive
portion of the restraint current increases, the restraint region grows to reflect the uncertainty of the measurement. The com-
puted severity increases with the probability that the sum of the measured currents indicates a fault. With the exception of
"Restraint", all quantities are defined in previous sections. "Adaptive Restraint" is a restraint multiplier, analogous to the
slope setting of traditional differential approaches, for adjusting the sensitivity of the relay.
Raising the restraint multiplier corresponds to demanding a greater confidence interval, and has the effect of decreasing
sensitivity while lowering it is equivalent to relaxing the confidence interval and increases sensitivity. Thus, the restraint
multiplier is an application adjustment that is used to achieve the desired balance between sensitivity and security. The
computed severity is zero when the operate phasor is on the elliptical boundary, is negative inside the boundary, and posi-
tive outside the boundary. Outside of the restraint boundary, the computed severity grows as the square of the fault current.
The restraint area grows as the square of the error in the measurements.
The line ground differential function allows sensitive ground protection for single-line to-ground faults, allowing the phase
differential element to be set higher (above load) to provide protection for multi-phase faults. The L90 ground differential
function calculates ground differential current from all terminal phase currents. The maximum phase current is used for the
restraint. The L90 is applied in dual-breaker applications to cope with significant through current at remote terminals that
may cause CT errors or saturation.
The line ground differential function uses the same CT matched and time-aligned phasors as the phase-segregated current
differential function. The operate signal is calculated for both real and imaginary parts as follows:
I I
=
OP_87G_RE LOC_PHASOR_RE_A
I
+
REM1_PHASOR_RE_C
I I
=
OP_87G_IM LOC_PHASOR_IM_A
I
+
REM1_PHASOR_IM_C
The terms for the second remote terminal are omitted in two-terminal applications.
The maximum through current is available locally and re-constructed from the received remote restraint based on the max-
imum remote restraint current shown in the previous section and as indicated below.
For two-terminal applications:

If I
REM_REST_A
9
For three-terminal applications:
9-4
2
 
I
+
REM1_PHASOR_RESTRAINT_A
2
 
S I
= –
A DIFF_A
I I
+ +
LOC_PHASOR_RE_B LOC_PHASOR_RE_C
I
+
REM2_PHASOR_RE_A
I I
+ +
LOC_PHASOR_IM_B LOC_PHASOR_IM_C
I
+
REM2_PHASOR_IM_A
2 2 2
   
BP , then I
REM_REST_A
2
 
else I
REM_REST_A
L90 Line Current Differential System
2
 
I
+
REM2_PHASOR_RESTRAINT_A
2 2
   
2P I
+
REST_A

9.1.7 GROUND DIFFERENTIAL ELEMENT

I
+
REM1_PHASOR_RE_A
I I
+ +
REM2_PHASOR_RE_B REM2_PHASOR_RE_C
I
+
REM1_PHASOR_IM_A
I I
+ +
REM2_PHASOR_IM_B REM2_PHASOR_IM_C
2
 
I
REM_RESTRAINT_A
---------------------------------------------------- -
=
2
2S
1
2
  
I 2 S
–
REM_RESTRAINT_A
------------------------------------------------------------------------------------------ -
=
2
2S
2
9 THEORY OF OPERATION
2

(EQ 9.10)
(EQ 9.11)
I
+
REM1_PHASOR_RE_B
(EQ 9.12)
I
+
REM1_PHASOR_IM_B
(EQ 9.13)
(EQ 9.14)
2
 
BP
2
1
BP
+
GE Multilin