Series reactor vs Phase Angle Regulator

[Mbrooke]

We have a situation where two 132 kv lines link between two 400kv-132kv substations feed a dozen 132-33/11kv substations. Under certain contingencies such as the loss of 400kv autotransformers or the loss of 400kv lines linking these two 400-132kv substations, the remaining 132kv lines begin to thermally overload. Generation dispatch change does not resolve the excessive load flow. Currently the problem is mitigated by a protection relay package which trips the 132kv buss tie breakers at the remote 132-33kv substations turning two circuits into 4 radial transmission lines.

The concern is that should a fault occur on any one of these transmission lines during radial operation, up to 85MW of load will be lost and 175MW for the loss of both lines in the same right-of-way. Under normal conditions this loss of load is mitigated via operating all 132kv line protection and buss tie breakers in these 132-33/11kv substations normally closed. During a fault POTT and zone selective deferential relaying only trip the breakers immediately in between the faulted line resulting in no loss of transmission supply to power distribution transformers. Distribution feeders are only exposed to a voltage sag of 6 to 20 cycles during normal clearing time. Running these lines normally closed is very advantageous from a service continuity and reliability goals stand point.


The two options being considered are the insertion of a series reactor in each 132kv line bay or the insertion of a phase angle regulator (PAR) on each line.

The only other option is a theoretical relaying system which closes the opened 132kv breaker during a fault and then opens only the breakers between the fault segment like seen during normally closed operation however I have never heard or even seen of such. (Though I do ask if it exists).

 

[davidbeach]

Why not a RAS (Remedial Action Scheme) to address the overloading when it happens, not before, and then moves the open point if/as necessary post fault?

 

[Mbrooke]

How might RAS work in this case to reduce or change load flow over these 2 lines?

 

[davidbeach]

Do you have a one-line? Even one where you've named the stations A, B, C, etc. and the breakers are A1, A2, etc. Would make it easier to discuss how the RAS approach might help.

 

[Mbrooke]

I will sketch one up right now.

 

[bacon4life]

Series reactors are certainly used to address this type of situation. I am under the impression that phase angle regulator are very complex transformers more suited to redirecting normal flow rather than just being used to address a very specific contingency. How large are the overloads? Can all of the loads still be served with adequate voltage with a reactor large enough to mitigate the overloads?

A RAS that reconfigures 12 132 kV substations and 3 400 kV circuits sounds quite complicated vs reactors. Additionally, reactors would allow continuity of service whereas the RAS would involve a momentary interruption. Also, if you are in North America the NERC standards require a fair amount of compliance effort for RAS installations.

 

[Mbrooke]

But what does RAS specifically do in this case? It sounds like an alternative to reactors and PARs, but to be honest I am not well versed in it.

 

[davidbeach]

Essentially a RAS can do anything that can be done. Most commonly they will be used to open and close breakers; generation ramping is another common function. Think of the RAS as the automation of what a really sharp dispatcher would do instantaneously in response to a certain set of system conditions. A RAS that addresses a stability problem may need to operate extremely rapidly, sub-cycle decisions, while a RAS that addresses a thermal problem has more time to respond.

 

[bacon4life]

Sounds like you have two 400 kV substations A & I, with 132 kV stations B through H in between them. A RAS could:
1. Open the line between D & E if the flow on either A-B or H-I exceed the thermal rating.
2. If a fault occurs on any section, move the normal open to restore loads. For example if a fault occurred between C&D, the breakers at C&D would still open by their regular protection, then the RAS would close the breakers between D&E, reenergizing the loads at D.
This sound pretty close to what utilities are experimenting with on the distribution system for automatic restoration.

 

[Mbrooke]

I drew up a sketch of the segment in question. S1 through S7 are the substations in between "A" and "B". "o" next to disconnects and breakers means "normally open", "O" is the breaker at S3 substation which is closed unless current goes above the lines thermal limits.