CHAPTER 4: SETPOINTS SYSTEM
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL 4–83
Differential and Restraint Current Calculations
Two Winding Transformer
Differential currents are calculated as follows:
Id
A
= I
A
C
[w1] + I
A
C
[w2]
Id
B
= I
B
C
[w1] + I
B
C
[w2]
Id
C
= I
C
C
[w1] + I
C
C
[w2]
Restraint currents are calculated as follows:
Ir
A
= max {|I
A
C
[w1]| , |I
A
C
[w2]|}
Ir
B
= max {|I
B
C
[w1]| , |I
B
C
[w2]|}
Ir
C
= max {|I
C
C
[w1]| , |I
C
C
[w2]|}
where Id
A
, Id
B
and Id
C
are the phase differential currents, and Ir
A
, Ir
B
, and Ir
C
are the phase
restraint currents.
EXAMPLE
Consider a typical Wye/Delta power transformer with the following data:
• Winding 1: (K2) = 500/5 CT ratio
• Winding 2: (K1) = 1500/5 CT ratio
• Transformer is 50% loaded
Magnitude Compensation
Based on 50% loading the winding currents are calculated as follows:
Winding 1 load current: I
load
(W1) = 2.5MVA/(3 * 13.8kV) = 104.5 Amps
Winding 2 load current: I
load
(W2) = 2.5MVA/(3 * 4.16kV) = 347 Amps
Since Winding 1 is always magnitude compensation reference, the currents from winding
1 , are scaled only to be presented in times Winding 1 CT.
M
W1
= 1 , - magnitude compensation factor for winding 1 - REFERENCE
Winding 1 currents = (M
W1
*I
load
(W
1
))/CT
W1
= 104.5/500 = 0.209 x CT
W1
Magnitude compensation for winding 2 currents:
M
W2
= (CT
W2
.V
W2
)/ (CT
W1
.V
W1
) = (1500*4.16)/(500*13.8) = 0.9043, -magnitude comp.
factor for Winding 2 currents
Winding 2 currents = (M
W2
* I
load
(W
2
))/CT
W2
= (0.9043*347)/1500 = 0.209 x CT
W2
To check that the measured currents from both windings will sum-up to zero after
applying magnitude compensation, one can perform the following simple calculations:
..\Transformer\Winding1 ..\Transformer\Winding2
ItemName Value Unit ItemName Value Unit
RatedMVA 5.000 MVA RatedMVA 5.000 MVA
NominalPhͲPhVoltage 13.800 kV NominalPhͲPhVoltage 4.160 kV
Connection Wye Connection Delta
Grounding WithinZone AngleWithRespectToW1 30°lag
Grounding NotWithinZone
W1W2
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