Alstom MiCOM P846, Technical Manual

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P846/EN OP/D Operation
(OP) 5-10
MiCOM P846
OP
Fortunately, even though this critical operating point is feasible, it is very sensitive for short
lines (30 km), the typical sensitivities being 45% per degree of angle shift and 25% Mvar per
unit of voltage ratio. The above considerations suggest therefore that even though the critical
operating point is feasible, it is difficult to achieve and even more difficult to sustain in
steady-state. Generally, the critical point will be matched transiently, and then the relatively
large sensitivity of the Power-angle and/or Power-voltage around this point will force the
power system to quickly exit the point. However, the greater the available accuracy in
measuring the current and power, the lower the transit time will be through this small singular
domain.
Transient behaviour:
As was mentioned earlier, there are a number of variables that are calculated during the
preprocessing stage and one of the most important ones at the core of the DLO is a
∆ -filter, which bears a physical meaning during a line disconnect process. It roughly
corresponds to the rate of change of the positive sequence active power over a suitable time
window. The behavior of DeltaP, where P is the active power, is illustrated in figure 8 for two
different situations:
•
The power transfer is ramping from a positive direction to a negative direction, thus
passing by zero in a manner that may mislead a crude open-line detector. At 4s, the
power ramp crosses zero and DeltaP reaches a maximum value 30ms later.
Interestingly, while the maximum value of DeltaP varies from 0.02 to 0.4 as the ramp
slope is increased from 0.5 p.u./min to 60 p.u./min, this maximum is reached nearly at
the same time, i.e. about two cycles after zero-crossing.
•
At various power transfer levels ranging from 1% to 100% of the SIL, the line is suddenly
opened. DeltaP then increases sharply from its zero-steady-state value to its maximum
value close to 1. Whatever the prior power transfer level is, the later valu
ched
about 2.5 cycles after the switching.
From th denly with a
breaker results in "large" maximum values of DeltaP, while power transfer reversal through
slow ramps results in comparatively low maximum values. The threshold to discriminate
between these two situations is about 0.5 and the time needed before an unambiguous
decision is about 2 cycles.
This principle is used for transmission lines in general. However, for the specific case of tie-
lines that are subject to fast power reductions due to breaker switching of major lines in their
neigh
ce
e is rea
ese empirical observations, it clearly appears that opening a line sud
borhood or fast ramping of dc/dc conversion equipment, the DLO provides a 'Tie-Line
factor' in its settings (refer to paragraph 1.2.1 of Settings section of this manual) to redu
the sensitivity of the
∆ -filters and to avoid spurious open line detection of such tie-lines.
P4061ENa
Figure 8: DeltaP filter response to power flow reversal (left) and DeltaP filter
response at breaker opening (right)