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Auto change over scheme

Auto change over scheme

Auto change over scheme

(OP)
Dears
I faced a typical problem i want to mention to you & seek any guidance or logic.

We had done changeover in two incomer & one buscoupler configuration to work on one incomer in 13.8 KV system, thus one incomer & buscoupler were closed.AFter doing the job it was observed that incomer was not closing & Synchrocheck failure appeard in Siemens relay (7SJ....)

The 13.8 Kv voltage was matching, but at 34.5 KV level the voltage was 34.1 & 34.4, after adjusting the voltage it became 34.18 & 34.3. Trying at that time the breaker was closed.Why did this thing happened when the 13.8 KV side voltage were matching.

thanks in advacne  

RE: Auto change over scheme

The phase angles of the incomers were not matched?

You need to provide more information. How does the 34.5kV relate to the 13.8kV? Shall we assume the relay is on the 13.8kV system?
 

Alan
----
"It's always fun to do the impossible." - Walt Disney

RE: Auto change over scheme

Possibly voltage regulation in the loaded transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Auto change over scheme

It's not that unusual for there to be a phase difference between two sources nominally 'in phase' and with good voltage matching. Local generation nearby can cause this effect if there is sufficient impedance between the local bar and the rest of the network. Heavy loads on a line with significant reactive impedance would give a similar effect relative to an unloaded line.

If you have an accurate voltmeter and access to the secondaries of a three-phase VT group on each bar then you should be able to construct a vector diagram showing the magnitude and relative phase displacement of the two supplies. With just two single phase measurements to the check sync relay you could still have a reasonable go at estimating the phase displacement. Compare that with the setting on the 25 relay. Post your findings.
 
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Auto change over scheme

(OP)
Dear Alehman,
The phase angle must have been checked during commissioning,The incomers were working normally & also the change over was done previously also.The chaneg over was done at 13.8 KV switchgear.There are step down transformers from 34.5 Kv to 13.8 Kv.Also other set of Transformers  is from 34.5Kv to 4.16 Kv.The realy is on 13.8 KV switcgear.

RE: Auto change over scheme

zaza,
Thanks for the additional information. Perhaps during commissioning the phase angles matched within the relay settings but as load was added, the system impedances resulted in mismatch. See Scotty's comments above.

Alan
----
"It's always fun to do the impossible." - Walt Disney

RE: Auto change over scheme

Zaza.
Im with Scotty and Alan.
Please attach setting of 25 relay. What are
Umin/Umax, dU ,dPfi and dFr settings?
According to your information your setting is very sensetive, you have open "window" of settings,
about dU=0.05Unom and dPfi about 10-15deg.
Best Regards and Good Luck.
Slava

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