Alstom MiCOM P846, Technical Manual

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Operation P846/EN OP/D
MiCOM P846
(OP) 5-15
OP
t asymmetry of the CVT measurements and to output an
exceeded. The asymmetry is detected with the following The DLO provides a mean to detec
alarm when a settable threshold is
index:
Asymmetry index:
Vn
Vp Vn
+
based on the positive and negative sequence voltages.
This function is activated in the 'CT AND VT RATIOS' column of the settings:
CVT Threshold: 0% to 100%
As an example, the followin se voltages
CVT Enable: Enabled or Disabled
g set of pha
Va
= 0.9 p.u.
∠
0
0
Vb
= 1.0 p.u.
∠
1200
Vc
= 1.0 p.u.
∠
2400
gives a stability index of
0.1/ 3
2.9 / 3 0.1/ 3
+
= 3.3%
Pole dead logic
Pole dead logic is used by the relay to
2.5
determine when the circuit breaker poles are open
(“pole dead”). A pole dead condition is determined by either monitoring the status of the
ng" logic. If a "CB
Open" signal is given the relay will automatically initiate a pole dead condition regardless of
o that a pole dead indication is still
circuit breaker auxiliary contacts (52a or 52b) or by measuring the line currents and voltages.
The status of the circuit breaker is provided by the "CB State Monitori
the current and voltage measurement. This is necessary s
given even when an upstream breaker is opened. In the event that the VT fails, a signal is
taken from the VTS logic (DDB 772 – Slow Block) to block the pole dead indications that
would be generated by the under voltage and undercurrent thresholds. The pole dead logic
diagram is shown in Figure 11.