Resistance-Earthed Star Windings; Solidly-Earthed Star Windings; Figure 72: Ref Protection For Resistance-Earthed Systems; Figure 73: Ref Protection For Solidly Earthed System - GE P642 Technical Manual

P64x
2.1

RESISTANCE-EARTHED STAR WINDINGS

Most distribution systems use resistance-earthed systems to limit the fault current. Consider the diagram below,
which depicts an earth fault on the star winding of a resistance-earthed Dyn transformer (Dyn = Delta-Star with
star-point neutral connection).
Source
V00669

Figure 72: REF Protection for resistance-earthed systems

The value of fault current (I
The value of earthing resistance (which makes the fault path impedance negligible)
●
● The fault point voltage (which is governed by the fault location).
Because the fault current (I
fault.
A restricted earth fault element is connected to measure I
protection. The overall differential protection is less sensitive, since it only measures the HV current I
I is limited by the number of faulty secondary turns in relation to the HV turns.
S
2.2

SOLIDLY-EARTHED STAR WINDINGS

Most transmission systems use solidly-earthed systems. Consider the diagram below, which depicts an earth fault
on the star winding of a solidly-earthed Dyn transformer.
Source
V00670

Figure 73: REF Protection for solidly earthed system

In this case, the fault current I
The leakage reactance of the winding
●
● The impedance in the fault path
The fault point voltage (which is governed by the fault location)
●
In this case, the value of fault current (I
A restricted earth fault element is connected to measure I
protection.
P64x-TM-EN-1.3
87
I
F
I
S
I
F
) depends on two factors:
F
) is governed by the resistance, its value is directly proportional to the location of the
F
87
I
F
I
S
I
F
is dependent on:
F
) varies with the fault location in a complex manner.
F
Chapter 8 - Restricted Earth Fault Protection
1.0
Current p.u.
(x full load)
Pickup
0.2
64
Winding not protected
Fault position from neutral
directly. This provides very sensitive earth fault
F
10
8
Current p.u.
6
(x full load)
4
2
64
Fault position from neutral
directly. This provides very sensitive earth fault
F
I
F
I
S
20% 100%
(Impedance earthing)
S
I
F
I
S
20% 40% 60% 80% 100%
(Solid earthing)
. The value of
169