The phase preference logic influences only the selection of the phase-earth loops
and has no effect on the selection of the phase-to-phase loops. The influence is
seen in the RELEASE_PE monitored data. The logic does not affect the RELEASE_PP
monitored data or START_GFC/STARTS_GFC outputs.
All schemes, except "No filter", require a cross-country fault detection XC_FLT =
TRUE; otherwise all the phase-to-earth measuring elements are blocked resulting in
RELEASE_PE value "No fault". This guarantees that tripping at a single-phase earth
fault in the high-impedance earthed network is blocked.
If a cross-country fault is detected, XC_FLT = TRUE and only a single fault loop
can be identified as faulty by the faulted phase selection function, then that loop
is directly selected for measurement. In such a case, simultaneous tripping of both
faulted feeders during a cross-country fault is expected.
Example
Consider a cross-country fault where phases A and B become faulted
simultaneously. All relays, R1, R2, R3 and R4, are equipped with an "Acyc C-A-B" phase
preference scheme.
A
B
C
S R
Z<
Z<
Z<Z<
R1 R2 R3 R4
CB1 line section 1 CB2 CB3 line section 2 CB4
M
Figure 532: Cross-country fault (or double earth fault) in a high-impedance earthed
system
The expected operations of the relays are:
• R1: The cross-country fault cannot be detected as the residual current does not
flow through relay R1. The phase-to-phase loop AB is measured in the forward
direction but the estimated impedance represents an average impedance
between two fault locations and therefore is not seen in zone Z1.
• R2: The cross-country fault is detected as a high residual current flows through
relay R2. Phase A earth fault is seen in forward direction and phase B earth fault
in reverse direction. Based on "Acyc C-A-B" phase preference scheme, phase A is
selected for measurement. Relay R2 operates without delay and opens the CB2
circuit breaker.
• R3: The cross-country fault is detected when a high residual current flows
through relay R3. The phase A earth fault is seen in reverse direction and phase
B earth fault in forward direction. Based on the "Acyc C-A-B" phase preference
scheme, phase A is selected for measurement. As the fault direction does not
coincide with the zone direction, relay R3 does not operate.
• R4: Same as relay R1
As a result of the circuit breaker CB2 operation, the cross-country fault is
transformed into an ordinary single-phase earth fault.
Table 915
,
Table 916
and
Table 917
summarize the available phase preference logic
schemes and their operation in different fault scenarios. The logic is selected with
the
Ph Prf mode Hi Z GFC
setting. In case the logic is not required, then the "No
filter" option should be selected.
Protection functions
1MRS759142 F
958 REX640
Technical Manual
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