Alstom MiCOM P546 - 1.39 Special weak infeed logic for stub end transformer terminals

Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546 (OP) 5-101
OP
When enabled, the following signals are set by the df/dt logic according to the status of the
monitored function.
df/dt> Inhibit (DDB 592) - Inhibit all 4 stages when high
df/dt>1 Tmr. Block (DDB 593) - Block timer on 1st stage when high
df/dt>2 Tmr. Block (DDB 594) - Block timer on 2nd stage when high
df/dt>3 Tmr. Block (DDB 595) - Block timer on 3rd stage when high
df/dt>4 Tmr. Block (DDB 596) - Block timer on 4th stage when high
df/dt>1 Start (DDB 597) - 1st stage started when high
df/dt>2 Start (DDB 598) - 2nd stage started when high
df/dt>3 Start (DDB 599) - 3rd stage started when high
df/dt>4 Start (DDB 600) - 4th stage started when high
df/dt>1 Trip (DDB 601) - 1st stage tripped when high
df/dt>2 Trip (DDB 602) - 2nd stage tripped when high
df/dt>3 Trip (DDB 603) - 3rd stage tripped when high
df/dt>4 Trip (DDB 604) - 4th stage tripped when high
All the above signals are available as DDB signals for mapping in Programmable Scheme
Logic (PSL).
1.39 Special weak infeed logic for stub end transformer terminals
The t
rue weak infeed condition is when no current based protection element is sensitive
enough to operate. This is the case when zero or minimal generation is connected at that
terminal, and the prospective level of fault current flowing through the CT is insufficient for
any forward/reverse protection operation. In such cases, the fault will be cleared using either
POR or Blocking schemes and enabling WI Echo + Trip.
However, there could be a specific configuration as presented in Figure 72 that may not be
detecte
d by relay as a weak infeed condition, even if there is no generation at that end (left
side - relay R2).
Figure 72 Weak infeed configuration on stub-fed radial circuit (parallel line is out
of service)
The reason is a star earthed transformer which, in case of phase to ground and double
phase to ground faults, imposes a very low zero sequence impedance and almost infinite
positive and negative sequence impedance, i.e. behaving as a source of zero sequence
current only. In such a case, the zero sequence current Io will dominate over I
1
and I
2
at the
weak end, where all three phase currents will approximately equal Io (all in phase and equal
in magnitude). This is true for F1 earth faults at R2, and for F2 earth faults at R1 and R2.
The phase currents will be sufficient to pickup current level detectors in the MiCOM P54x,
and a true weak infeed condition will not be seen as such by the relay.
G
Zs
R1R2
F1
F2