ABB RELION REX640 Technical Manual page 883

1MRS759142 C
REX640
Technical Manual
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.
R1
Z<
S
CB1 line section 1 CB2
A
B
C
GUID-4ACB8FB3-0FB1-4351-986B-3565B265A689 V1 EN
Figure 473: 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 large 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 large 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 837, Table 838 and
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.
"No filter" option does not block the single-phase earth-faults,
therefore this option should be used with caution.
R2 R3
Z< Z<
M
CB3
Table 839 summarize the available phase preference logic
Section 4
Protection functions
R4
Z<
R
line section 2 CB4
877