GE 845 Instruction Manual page 215

CHAPTER 4: SETPOINTS
845 TRANSFORMER 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.
Three-Winding Transformer
Differential currents are calculated as follows:
C C
Id = I [w1] + I [w2] + I
A A A
C C
Id = I [w1] + I [w2] + I
B B B
C C
Id = I [w1] + I [w2] + I
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
EXAMPLE
Consider a typical Wye/Delta power transformer with the following data:
• Winding 1: CT Bank 1 – J1 = 500/5 CT ratio
• Winding 2: CT Bank 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
C
[w2]|}
A
C
[w2]|}
B
C
[w2]|}
C
C
[w3]
A
C
[w3]
B
C
[w3]
C
C C
[w2]| , |I [w3]|}
A A
C C
[w2]| , |I [w3]|}
B B
C C
[w2]| , |I [w3]|}
C C
(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
SYSTEM
, Ir , and Ir are the phase
A B C
W1
4–87