GE 889 Instruction Manual page 213

CHAPTER 4: SETPOINTS
..\Transformer\Winding 1
Item Name
Rated MVA
Nominal Ph Ph Voltage
Connection
Grounding
W 1
889 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
Differential and Restraint Current Calculations
Two Winding Transformer
Differential currents are calculated as follows:
C C
Id = I [w1] + I [w2]
A A A
C C
Id = I [w1] + I [w2]
B B B
C C
Id = I [w1] + I [w2]
C C C
Restraint currents are calculated as follows:
C
Ir = max {|I [w1]| , |I
A A
C
Ir = max {|I [w1]| , |I
B B
C
Ir = max {|I [w1]| , |I
C C
where Id , Id and Id are the phase differential currents, and Ir
A B C
restraint currents.
EXAMPLE
Consider a typical Wye/Delta power transformer with the following data:
Value
5.000
13.800
Wye
Within Zone
• 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
Winding 2 load current: I
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 = 1 , - magnitude compensation factor for winding 1 - REFERENCE
W1
Winding 1 currents = (M
Magnitude compensation for winding 2 currents:
M = (CT .V )/ (CT
W2 W2 W2
factor for Winding 2 currents
Winding 2 currents = (M
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:
C
[w2]|}
A
C
[w2]|}
B
C
[w2]|}
C
..\Transformer\Winding 2
Unit Item Name
MVA Rated MVA
kV Nominal Ph Ph Voltage
Connection
Angle With Respect To W1
Grounding
W 2
(W1) = 2.5MVA/(√3 * 13.8kV) = 104.5 Amps
load
(W2) = 2.5MVA/(√3 * 4.16kV) = 347 Amps
load
*I (W ))/CT = 104.5/500 = 0.209 x CT
W1 load 1 W1
.V ) = (1500*4.16)/(500*13.8) = 0.9043, -magnitude comp.
W1 W1
* I (W ))/CT = (0.9043*347)/1500 = 0.209 x CT
W2 load 2 W2
SYSTEM
, Ir , and Ir are the phase
A B C
Value Unit
5.000 MVA
4.160 kV
Delta
30 ° lag
Not Within Zone
W1
W2
4–83