Resistance-Earthed Star Windings; Solidly-Earthed Star Windings; Figure 88: Ref Protection For Resistance-Earthed Systems - GE MiCOM P40 Agile Technical Manual

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sequence current to flow through the neutral line, resulting in uneven currents in the phases, which could cause
the protection to maloperate. By measuring this zero sequence current and placing it in parallel with the other
three, the currents are balanced, resulting in stable operation. Now only a fault inside the star winding can create
an imbalance sufficient to cause a trip.
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
I
S
V00669

Figure 88: 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.
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87
I
F
I
F
64
) depends on two factors:
F
) is governed by the resistance, its value is directly proportional to the location of the
F
Chapter 7 - Restricted Earth Fault Protection
1.0
Current p.u.
(x full load)
Pickup
0.2
Winding not protected
Fault position from neutral
directly. This provides very sensitive earth fault
F
I
F
I
S
20%
100%
(Impedance earthing)
. The value of
S
165

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