Current Limiting Reactors to reduce fault currents

[rockman7892]

I'm reviewing a short circuit study for the system on the attached one-line and find that the 69kV bus as well as several downstream 13.8kV Switchgear lineups are overdutied due to the fact that there is a significant amount of generator contribution on the 69kV bus in addition to the two utility lines. There are varying degrees of overduteid equipment ranging from 105% to 150% depending on location and existing gear ratings.

Worst case results were modeled with all generator units running and downstream switchgear with Tie Breakers normally closed which represents normal operating conditions.

I'm trying to determine what solutions exist outside of having to replace switchgear such as the insertion of CLR's or other current limiting devices. One thought is to put a CLR on the 69kV tie however this likely will only satisfy the 69kV gear (and not for case where all gens may be connected to single bus)but would still leave downstream switchgear overdutied due to NC tie breaker. For each switchgear lineup I'm assuming that a CLR would also need to be provided at each tie breaker along with the one in the 69kv tie.

What about putting CLR's on the 13.2kV side of the generator circuits? Would this likely cause voltage stability issues along with other dynamic stability issues?

Others have also suggested the use of a Triggered Current Limiter (TCL) make by G&W to act similar to a current limiting fuse to limit let through current. I've seen some application papers suggest using these in parallel with a CLR to achieve the benefits of current limiting as well as system continuity. Does anyone have any experience with these or suggest a location where these may provide the most benefit (generator circuits ? )

 

[davidbeach]

Put the CLRs in your model and see what happens. I’d try to keep them at the 69kV level and not tied to the generator terminals. CLiPs (or whatever they’re calling them today) are a mixed bag. If you have to replace them more than once or twice over the life of the installation you’ll probably wish you’d chosen a different approach. Don’t be in the vicinity when a CLiP does it’s thing.

 

[RRaghunath]

Did you work with nameplate %Z of transformers while estimating the short circuit currents or did you take the datasheet values?
Nameplate values are 'as tested' values during factory acceptance test and wwe don't need to apply negative tolerance on these values while estimating the short circuit currents.
If the SC currents are exceeding the busbar rated fault levels, I would think the practical/simple approach could be decoupling the two 69kV busbars by keeping the buscoupler open.
The 69kV busbars will remain coupled at 138kV busbars upstream through the 138/69kV transformers and this should not pose a problem I suppose.
If necessary, AVRs of all OLTCs can be set to work to minimise the circulating currents through parallel transformers (though I don't think this will be necessary).

 

[crshears]

My utility employs these for this purpose in various configurations.

At stations with two parallel supplies, I've seen it done two different ways.

One places a CLR in the secondary circuit of each transformer [meaning in series with the secondary breaker].

At other sites a CLR is placed in the tie bus such that any fault that occurs in the station is supplied by < 2 pu of normal fault current infeed capability, thus eliminating the overduty; special logic and control circuitry has been implemented to automatically close a CLR bypass switch whenever one of the transformer secondary breakers is opened. At these latter sites, back-to-back feeder tie switches can be closed without restriction, but when being opened there must be < 12 MVA difference between the loadings of the two supply transformers before the open operation is authorized. If the delta is > 12 MVA one transformer must be unloaded during the switch opening operation.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[rockman7892]

Thanks for the responses.

The most logical place to insert CLR at 69kV seems to be Bus Tie, or keeping the bus tie open. This would not take care of the issues at the downstream 13.8kV Switchgear since they are operated with tie breaker NC and would have contribution combining from both A and B 69kV buses. It seems that CLR's would also be required in the tie breakers at the 13.2kV switchgear to reduce fault levels as well. I'm wondering what the practicality of installing a CLR on a tie breaker/bus that is internal to the gear would look like?

A couple of additional questions:

1) Is there a particular approach to modeling or performing short circuit analysis with CLips or is it simply a matter of using let-through curves provided by manufacturer similar to a fuse? Do these have to be applied as part of a tested/listed combination like is required with a fuse in order to take advantages of reduced let-through for SC evaluation purposes?

2) As davidbeach stated putting the CLR's at terminals of the 13.2kV gen's is prob not a good idea from a stability standpoint but what about putting a CLip at these generator outputs at 13.2kV. These CLips only go up to 38kV so applying at 69kV is not an option. Any downside to applying these at gen output at 13.2kV?