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FUNCTION CHARACTERISTICS
Differential protection 87G-87M-87T
Preface
Stabilized Differential for Generator, Motor and two-winding power transformers with dual-slope
percentage characteristic.
Based on the selection of the protected object type is activated (Transformer) or not (Motor-Genera-
tor) the second and fifth harmonic restraint for the differential protection element.
Definitions
• i
L1(H)
instantaneous value of L1 phase current on side H
• i
L2(H)
instantaneous value of L2 phase current on side H
• i
L3(H)
instantaneous value of L3 phase current on side H
• i
L1(L)
instantaneous value of L1 phase current on side L
• i
L2(L)
instantaneous value of L2 phase current on side L
• i
L3(L)
instantaneous value of L3 phase current on side L
These currents are conventionally regarded as positive if incoming in their reference terminal phase
current input of the relay, negative if outgoing.
• i
L1C(H)
instantaneous value of L1 compensated phase current on side H
• i
L2C(H)
instantaneous value of L2 compensated phase current on side H
• i
L3C(H
) instantaneous value of L3 compensated phase current on side H
• i
L1C(L)
instantaneous value of L1 compensated phase current on side L
• i
L2C(L)
instantaneous value of L2 compensated phase current on side L
• i
L3C(L)
instantaneous value of L3 compensated phase current on side L
With compact notation the previous vectors are referred to the i
L
(w) and i
LC
(w), where w = H, L is the
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 amplitude, polarity, phase and cyclic sequence and zero
sequence currents compensation is performed by relay, as follows:
i
LC
(w) = M(w) ∙ P(w) ∙ C(w) ∙ i
L
(w) with w = H, 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.
C(w) is the phase compensation matrix and cyclic sequence and zero sequence currents of the
homopolar sequence of the side w.
i
L(H)
= i
L2(H)
vector of instantaneous value of phase currents on side H
i
L1(H)
i
L3(H)
Lc(H)
= i
L2c(H)
vector of instantaneous value of compensated phase currents on side H
i
L1c(H)
i
L3c(H)
i
Lc(L)
= i
L2c(L)
vector of instantaneous value of compensated phase currents on side L
i
L1c(L)
i
L3c(L)
i
L(L)
= i
L2(L)
vector of instantaneous value of phase currents on side L
i
L1(L)
i
L3(L)
XMR-D EQUIPMENT MANUAL
Ed. 2.9 - 02/2021
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