Operation Principle; Fundamental Principles Of The Restricted Earth-Fault Protection - ABB REG650 Technical Manual

Section 6
Differential protection
6.2.7
6.2.7.1
112

Operation principle

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
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
CTs are shown as connected together in order to measure the residual 3I
the sake of simplicity.
These three
zero-sequence
currents are not measured
L1
Power
L2
system
L3
Ifault
Return path via
power system
External
fault
region
External
fault
region
IEC09000107-3 V1 EN
Figure 44: Zero sequence currents at an external earth fault
rd
harmonic currents, are fully suppressed. Then the residual
zone of protection
I
zs1
I
zs1
I
zs1
Uzs
I + I +I = 3I
L1 L2 L3 0
3I = 3Izs1 I
0
(Summation in the IED)
Return path through
transformer
block operate
I
3I
N
0
block
ROA
Internal
block
region
1MRK 502 048-UEN A
44 and figure 45, where the three line
o
3Izs1
Current in the neutral
(measured as I
as a directional reference
-
= 3Izs1
N
because it has the same
direction for both internal
and external faults.
Zero-sequence differential
current for external fault
Idiff = abs(3I
Idiff = 3Izs1 - 3Izs1 = 0
fault
ROA = Relay Operate Angle
IEC09000107-3-en.vsd
Technical manual
current, for
L1
L2
L3
) serves
N
+ I )
0 N