Permissable loss of load for contingency

[Mbrooke]

What is the the typical allowable loss of load (in MW) for a bus fault or breaker failure? Any established standards be it NERC or IEC?

 

[bacon4life]

NERC TPL-001-4 allows any amount of radial service to be interrupted if it is necessary to clear the fault. The voltage level and circuit configuration determine whether any amount of non-consequential load can also be tripped.

 

[Mbrooke]

Thank you, I am digesting it now. By non consequential they mean system stability, ie any fault can not cause system oscillations or voltage collapse outside of the faulted circuits cleared?

 

[bacon4life]

Non-consequential load is slightly different. Non-consequential load is the load shed that was not a direct result of clearing the fault. Most typically, it would be load that is planned to be tripped by a Remedial Action Scheme in order to avoid instability or overloads.

 

[Mbrooke]

So it is permissible to have a bus fault clear say 60MVA of radial load and then shed another 60MVA of looped load to prevent depressed voltages or mitigate thermal overloads? The rule for me is that transformers, lines, cables, reactors and the like must result in no load lost under N-1. In North America my understanding is that a 30 minute time frame then exists to prepare the system for another contingency (N-1-1).


This I understand well, but because operating experience across the globe has shown bus faults to be some of the lowest occurring contingencies- and breaker failure rare- the idea is that these could be classified as an extreme contingency rather than a normal contingency.

 

[bacon4life]

For P2.2 or 2.3 contingencies, shedding another 60 MVA of looped load would be acceptable for HV but not EHV lines. For a bus tie breaker fault, shedding load for all voltages is allowed.

In the past, some operators assumed they could overload a facility by any amount as long as operator action could fix it within 30 minutes. The 30 minute timeframe has drastically changed with the recent versions of NERC standards. For all "credible" contingencies, the system must be operated such that all post-contingency flows will stay under the applicable equipment ratings. Many utilities have begun using 15-30 minute emergency ratings to allow higher short term loading while operators take actions. Possible cascading is screened for by assuming equipment loaded beyond the emergency rating will trip out.


If a condition occurs outside of a "credible" contingency, TOP-007-0 R2 applies:

R2. Following a Contingency or other event that results in an IROL violation, the Transmission
Operator shall return its transmission system to within IROL as soon as possible, but not
longer than 30 minutes.

 

[Mbrooke]

So 345kv overloads can not be mitigated by shedding 138kv loads for a 138kv bus fault?

 

[bacon4life]

I assume the voltage level where the fault occurs is the determining factor.

 

[Mbrooke]

What do they mean by "interruption of firm transmission service" and "none consequential load loss"?