GE Multilin DTP-B, Instruction Manual

Page: 11 / 90

2. APPLICATION
2-4 DTP-B Digital Transformer Protection GEK-106243E
means of independent settings we will inform the relay about this connection, so that the DTP will perform
internally the magnitude corrections resulting from changes wye-delta and vice-versa.
•
Determine the burden on each CT using the following expressions:
a) For wye connected CTs:
N x e + 2.5 f
Z = B +
⎯⎯⎯⎯⎯⎯⎯
+ 2.27 R
Ω
1000
b) For delta connected CTs:
N x e + 2.5 f
Z = 2B +
⎯⎯⎯⎯⎯⎯⎯
+ 2.27
Ω
1000
where:
B = DTP relay total burden (0.04
Ω
approx.).
N = CT secondary turns
e = CT resistance per turn in milliohms.
f = CT resistance per lead in milliohms.
R = one-way control cable lead resistance (at 75ºC)
The multiplying factors associated with f and R account for two cable leads instead of one, resistance value
increase due to temperature rise, and the resistance of the longest CT leads.
•
Determine the CT secondary current for 8 times the tap value:
Is = 8 x relay tap
Note: For the assumed fault, all of the fault current is supplied by one CT. Consequently, the CT current and the
relay current are the same whether the CT is connected in wye or delta.
•
Determine the CT secondary voltage required at 8 times the tap value:
Esec = Is x Z
•
From the excitation curve of the CT that is being used, determine the excitation current (Ie) which
corresponds to the voltage Esec.
•
Determine the percent error in each CT through the following expression:
Ie
% error =
⎯⎯⎯⎯
x 100
Is