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Differential protection for generator-motor-short line (87G-M-L)
Preface
Definitions
Magnitude matching
FUNCTION CHARACTERISTICS
Stabilized Differential NG10 Relay for generator, motor or short line.
• i
instantaneous value of L1 phase current on side H
L1(H)
• i
instantaneous value of L2 phase current on side H
L2(H)
• i
instantaneous value of L3 phase current on side H
L3(H)
• i
instantaneous value of L1 phase current on side L
L1(L)
• i
instantaneous value of L2 phase current on side L
L2(L)
• i
instantaneous value of L3 phase current on side L
L3(L)
These currents are conventionally regarded as positive if incoming in their reference terminal phase
current input of the relay, negative if outgoing.
• i
instantaneous value of L1 compensated phase current on side H
L1C(H)
• i
instantaneous value of L2 compensated phase current on side H
L2C(H)
• i
) instantaneous value of L3 compensated phase current on side H
L3C(H
• i
instantaneous value of L1 compensated phase current on side L
L1C(L)
• i
instantaneous value of L2 compensated phase current on side L
L2C(L)
• i
instantaneous value of L3 compensated phase current on side L
L3C(L)
i
L1(H)
i
= i
vector of instantaneous value of phase currents on side H
L(H)
L2(H)
i
L3(H)
i
L1(L)
i
= i
vector of instantaneous value of phase currents on side L
L(L)
L2(L)
i
L3(L)
i
L1c(H)
i
= i
vector of instantaneous value of compensated phase currents on side H
Lc(H)
L2c(H)
i
L3c(H)
i
L1c(L)
i
= i
vector of instantaneous value of compensated phase currents on side L
Lc(L)
L2c(L)
i
L3c(L)
With compact notation the previous vectors are referred to the i
index of the two sides of the differential protection.
In order to correct any amperometric reversal polarity or phase cyclical sequence, and making equal
amplitude and phase currents on the sides of the differential protection and eventually eliminate the
element of homopolar sequence, the relay performs the compensation of amplitude, polarity, phase
and cyclic sequence and zero sequence currents as follows:
i
(w) = M(w) ∙ P(w) ∙ C(w) ∙ i
(w) whith w = H, L
LC
L
where:
M(w) is a compensating factor in the current amplitude of the side w.
P(w) is the polarity matrix of the of the amperometric polarities of the side w..
M(w) is a compensating factor in the current amplitude of the w side; it expresses the values that are
multiplied by the currents of the w side in order to obtain an amplitude equal to that of the currents
on one side chosen as a reference.
If inside the Set \ Base menu the MatchType=ESTERNAL parameter is set (external compensation
with adapter CTs), then:
M(w) = 1
However, if inside the Set \ Base menu the MatchType=INTERNAL parameter is set then M(w) is
calculated by the relay as follows:
A) Calculation of the difference (mismatching) between the CT primary rated current and rated cur-
rent of the sides of the protected device:
m
= I
/I
Mismatching coefficient on side H of the protected device
H
npH
ng
m
= I
/I
Mismatching coefficient on side L of the protected device
L
npL
ng
where I
and I
are the CTs rated current on sides H and L and I
npH
npL
protected device.
The relay checks that the values of the mismatching coefficients are between 1.0 and 2.5(1.0 ≤ m(x)
≤ 2.5); if the condition is not met, a warning message is issued and a correction of the I
parameters is required.
NG10 - Manual - 04 - 2022
(w) and i
(w), where w = H, L is the
L
LC
is the rated current of the
ng
, I
ng
npH
, I
npL
60
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