HV Circuit Breaker Spec: Shunt Reactor Switching Application

[switchnlight]

We have been experiencing failures of a particular design of 72.5kV (Alstom FX10) Sf6 breakers installed on our 66kV system. Two units are used to switch 36MVar shunt reactors connected to the 66kV bus, two are on the transformers and the other five are on feeders. These breakers were commissioned in 1990/91, however failures started within two years of commissioning, the most recent one being in mid 2004 on one of the reactors. Other failures have been: 2 on feeders and 1 on the transformer. We are in the process of completely replacing these breakers and would appreciate help from the experts to answer the following questions:

1. Is it okay to use the same spec for a breaker to be used for shunt reactor, transformer and line switching applications?
2. How do you design an SF6 breaker in order to eliminate breaker reignition?
3. What are some of the major mechanisms by which shunt reactor breakers suffer damage and how do you mitigate against these?
4. Does the rated breaking current of a circuit breaker have any bearing on the application for which it is to be used?

I would greatly appreciate people shading light on these issues.

 

[rbulsara]

Not an expert on shunt reactors, but what is the failure mode?
What happens?

 

[switchnlight]

The mode of failure is one where the interupting chamber explodes. In two of the fauires two interrupters exploded and the debris affected the third column which was partialy damaged in the process. Now my concern was that perhaps these breaker may not have been properly dimensioned for this kind of application. Thats why am trying to seek guidance from the forum experts on any info that might help us come up with a ideal spec for the replacement units. The existing units are rated at 20kA symmetrical, with a making capacity of 50kA.

 

[rbulsara]

Not an expert in this kind of system, but I suspect perhaps the breakers are OK for what it says on its nameplate but may be being mis-applied. A sytem transient analysis may be in order (which I am sure is done). Shunt reactors on a 72 kV system is uncommon from what I gather, unless you have long capacitive (cable) lines. Has resonance been ruled out? Could be improperly sized reactors?

 

[shirui]

It seems to me that magnitude of the transient recovery voltage (TRV) and/or rate of rise of recovery voltage (RRRV) exceed the breakers's capability. My
1. Generally speaking it is ok but in your case, You should do simulation to check TRV and RRRV of these breakers. Does the breaker fail on the feeder when opening single phase to ground fault? Is there any fault when breakers on transformer and reactors fail?
2. As per IEEE std. C37.015 IEEE application guide for shunt reactor switching, the auxiliary circuit that can be applied to the circuit breaker to limit overvoltages inludes opening resistors, metal oxide surge arresters, and synhcronous opening control devices.
3. Is your shunt reactor directly grounded? Chopping overvoltage and reignition overvoltage may ocur during reactor switching.
4. It should be problems of breaker capability of TRV and RRRV. I believe the breaking capability for three phase fault should be checked during system design.