ABB REG650 ANSI, Technical Manual

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6.2.7 Operation principle
6.2.7.1 Fundamental principles of the restricted earth-fault protection
Restricted earth-fault protection, low impedance function (REFPDIF) detects earth
faults on earthed power transformer windings, most often an earthed star winding.
REFPDIF is a winding protection of the differential type. Since REFPDIF is based on
the zero sequence current, which theoretically only exists in case of an earth fault,
REFPDIF can be made very sensitive regardless of normal load currents. It is the
fastest protection a power transformer winding can have. Remember that the high
sensitivity and the high speed tend to make such a protection unstable. Special
measures must be taken to make it insensitive to conditions for which it should not
operate, for example, heavy through faults of phase-to-phase type or heavy external
earth faults.
REFPDIF is of the low impedance type. All three-phase currents, and the neutral point
current, must be fed separately to REFPDIF. The fundamental frequency components
of all currents are extracted from all input currents, while other eventual zero sequence
components, such as the 3
rd
harmonic currents, are fully suppressed. Then the residual
current phasor is calculated from the three line current phasors. This zero sequence
current phasor is then added to the neutral current vectorially, in order to obtain
differential current.
The following facts may be observed from figure 44 and figure 45 , where the three line
CTs are shown as connected together in order to measure the residual 3I
o
current, for
the sake of simplicity.
Return path via
power system
L2
L3
L1
L2
L3
I
N
=
-
3Izs1
Uzs
Current in the neutral
(measured as I
N
) serves
as a directional reference
because it has the same
direction for both internal
and external faults.
zone of protection
I
zs1
I
zs1
I
zs1
Ifault
ROA
block
I
N
External
fault
region
L1
Zero-sequence differential
current for external fault
Idiff = abs(3I
0
+ I
N
)
Idiff = 3Izs1 - 3Izs1 = 0
3I
0
block
block
operate
Return path through
transformer
I
L1
+ I
L2
+I
L3
= 3I
0
3I
0
= 3Izs1
(Summation in the IED)
Power
system
External
fault
region
Internal
fault
region
ROA = Relay Operate Angle
3Izs1
IEC09000107-3-en.vsd
These three
zero-sequence
currents are not measured
IEC09000107-3 V1 EN
Figure 44: Zero sequence currents at an external earth fault
Section 6
1MRK 502 048-UEN A
Differential protection
112
Technical manual