Figure 103: Condenser Bushing System Vectors - GE MiCOM P40 Agile Technical Manual

P14D
(a)
I
A
E00821

Figure 103: Condenser bushing system vectors

In the figure above:
(a) Shows three healthy voltages, three capacitor currents that lead their respective voltages by 90º and sum to
zero,
(b) Shows B phase earthed, A and C voltages are √3 times their healthy magnitude & at 60º to each other, giving
correspondingly altered capacitor currents Ia and Ic.
The vector sum of the A & C phase capacitor currents is:
I = √3 x Ia,
f
= √3 x √3 x I ,
A
= 3 x I
A
Therefore, the total fault current I
(c) Shows the vector sum of the fault condition I
Therefore, I is the current which will flow in the Neutral Displacement Relay under fault conditions (neglecting the
f
impedance of the relay itself).
For example, for a 60pF capacitor on a 33kV system, the single capacitor healthy condition current I
I = V / X
A A C
= V / (1/2pfC)
A
3
/ (1/(2 x p x 50 x 60 x 10
= 33 x 10
√3
= 0.359mA
Therefore, the total fault current which would flow in an NVD relay (neglecting the impedance of the relay itself):
I = 3 x 0.359 = 1.08mA
f
The table below shows the total fault current I
150pF capacitor.
C (pF) 60.00 90.00
Xc (MΩ) 53.08 35.39
VA (kV) 19.00 19.00
IA (mA) 0.359 0.539
If (mA) 1.08 1.62
P14D-TM-EN-8
(b)
Ia = √3I
V
A
equals three times a single capacitor healthy condition current I
f
.
f
-12
))
for a 60pF capacitor, and also I
f
150.00
21.23
19.00
0.898
2.69
Chapter 10 - Voltage Protection Functions
V
Cf
A V = √3V
Af A
(c)
I = 3I
f A
.
A
is given by:
A
for a 90pF capacitor, and for a
f
201