Hello All, I am looking for more i

[Rocketir]

Hello All,
I am looking for more information regarding the characteristics of and protection of autobanks. I have a 21 kV 4 Wire feeder feeding an autobank as well as other connected load. The autobank is a 21/12 auto Y connected with H0/X0 grounded and closed tertiary. The 12 kV side is also 4 wire.
I had a situation where a single phase of the 21 kV line on the source side of the autobank opened with no sustained fault. A single disconnect blew open. The circuit breaker at the distribution substation opened and reclosed and held, the circuit was abnormally lightly loaded. We received no out of power calls after that. The next day, in talking with the operations guys they told me that during the patrol the autobank was found to be hot to the touch. I checked a SCADA controlled switch that is directly on the source side of the auto bank and it showed high ground amps. We suspected that one cable feeding the autobank was open or that the autobank was failing. At this point we tried to isolate the autobank. When we opened the switch to the autobank we started receiving out of power calls on the 21 kv circuit. Somehow the autobank was energizing the open phase on the 21 kv. How is this possible? No one was out of power until we isolated the autobank. We are thinking that the open phase was back feed through the tertiary and supported the 21 kV load because of the light loading.

A little diagram

OCB ------- Blown Disconnect -----------additional 21 kV load----eol
|
SCADA Switch
|
AutoBank
|
line recloser
|
12 kV load
Any thoughts? I am trying to analyze what happened. I am currently looking at symmetric components to try to figure this one out.


Jasjit Khangura P.E.
Electric Distribution Engineer

 

[busbar]

Looks like the delta tertiary turned the bank into a nice zero-sequence source, effectively supporting the floating primary phase.

{excerpt, IEEE C62.92.4-1991}
If the high-voltage wye-delta neutral of the transformer bank is connected to the primary circuit neutral, the transformer bank may experience the following:
1) Circulating current in the delta in an attempt to balance any unbalanced phase-to-neutral loads connected to the primary line
2) Acting as a grounding bank and supplying fault current to any fault on the circuit to which it is connected…

 

[jghrist]

Look at winding connection diagrams. This is a clearer way than sequence diagrams to see how the delta winding supports the voltage of the open wye winding (at least for me).

 

[jbartos]

Suggestion: Visit

http://wwwrsphysse.anu.edu.au/~bdb112/engn3225/Chapter_2_Transformers_revB.pdf

for: the tertiary winding purpose. It is essentially for the 3rd harmonic trap. It normally will not affect the zero sequence currents.

http://www.mathworks.com/access/hel...rsys/threephasetransformerthreewindings.shtml

etc. for more info
The systematic testing/troubleshooting will be needed. A computer simulation of that power distribution section would also help.