GE TRANSFORMER MANAGEMENT RELAY 745 Instruction Manual page 114

5.6 S4 ELEMENTS
The issue of maloperation due to heavy external faults resulting in CT saturation is handled by a programmable timer. The
timer provides the necessary delay for the external fault to be cleared by the appropriate external protection with the added
benefit that if the RGF element remains picked up after the timer expires, the 745 operates and clears the fault. This
approach provides backup protection. Since the RGF element is targeted at detecting low magnitude internal winding fault
currents, the time delay for internal faults is of little consequence, since sensitivity and security are the critical parameters.
For example, consider a transformer with the following specifications:
10 MVA, 33 kV to 11 kV, 10% Impedance, Delta/Wye30
R = 6.3 ohms
g
Phase CT Ratio = 600 / 1 A
Rated Load Current = I
Maximum Phase-to-Ground Fault Current = I
For a winding fault point at 5% distance from the neutral:
From Figure 5–23: Fault Currents vs. Points from Neutral on page 5–63, we see that the I
ligible and therefore 3I
I 3I
=
gd
Slope
=
Time Delay: dependent on downstream protection coordination (100 ms typical)
The Winding 1 Restricted Ground Fault setpoints are described below:
5
• W1(3) RESTD GND FAULT PICKUP: Enter the minimum level of ground differential current (in units of Phase CT pri-
mary associated with the winding, where the RGF is set) for the Winding 1(3) Restricted Ground Fault element.
• W1(3) RESTD GND FAULT SLOPE: Enter a slope percentage (of ground differential current to maximum line current).
• W1(3) RESTD GND FAULT DELAY: Enter the time that the Winding 1(3) Restricted Ground Fault element must
remain picked up before the element operates.
5-64
Courtesy of NationalSwitchgear.com
= 10 MVA / ( 3 11 kV) = 525 A
×
rated
gf(max)
×
I 0.05 I
=
(
fault gf max
= 0 (approximately). Therefore, the maximum phase current = I
o
I
fault
I 0 ------------------------------------------------ -
– = –
0 g
Phase CT Primary
I
50 A
gd
---------- - --------------- - 9.5% (select Slope Setting = 9%)
= =
I 525 A
max
Figure 5–26: RESTRICTED GROUND FAULT SCHEME LOGIC
745 Transformer Management Relay
= 11 kV / ( 3 6.3) = 1000 A
×
×
0.05 1000 A 50 A
= =
)
50 A
×
0 ----------- - 0.08 CT
= – = =
600
5 SETPOINTS
(EQ 5.10)
increase due to the fault is neg-
p
= I = 525 A (approx.), and
max rated
Pickup Setting
(EQ 5.11)
GE Multilin