from the electrical department...
PT secondary voltage electrical power system
 Posted by mahesh on 25 June, 2012 - 6:18 am
in one of the hydro power project, i am having 3.3 KV generator which feeds to 33KV grid from 3.3/33KV trafo. i had a problem in PT secondary voltages which is used for synchronization. when we charge 3.3 KV bus from generator side at that time the PT secondary voltage is 110V between phases and 63 between phase and neutral. when we back charge the 3.3 KV bus from grid for synchronization purpose at that time same PT shows 110V between phases and 93V between phase and neutral. please resolve the quire why this is going to happen?

Thanks

 Posted by Phil Corso on 25 June, 2012 - 9:10 am
Mahesh... how many PTs are used on each bus? Two or three?

Regards, Phil Corso

 Posted by Eric Delgado on 25 June, 2012 - 5:30 pm
We need more information about the connection for the PTs, primary and secondary, they are connected to ground or earth, are you using two or three PTs?

 Posted by Phil Corso on 1 July, 2012 - 2:50 pm
Mahesh... usually only two PT-derived voltages are compared by the Synchronizer unit: one on the Gen-side of the Gen's CB; the other on the Bus-side of the CB.

Or, is the Bus-side voltage derived from a 3rd PT when back-feeding from the grid?

Phil

 Posted by pragatri on 27 July, 2012 - 10:20 am
This is some strange problem. From what I understood from information given by you, it is clear that when fed from generator the P.T. readings are normal. It means P.T. ratio & connections are completely alright.

Before synchronizing, however when fed from 33 KV side (which I believe must be star winding) the same P.T. shows abnormal phase voltage. It is however not clear, whether all 3 P.T.s are showing same voltage. Normally for synchronizing one P.T. signal is taken. So I assume your observation is only for single P.T. If 3.3KV side is delta then phase voltage & line voltages should be equal & this problem should not occur.

However, if you have star star transformer, then phase voltages will not be equal to line voltages. If it is so then if 33KV side star winding has some neutral shifting problem ( may be due to inter turn shorting & neutral is not solidly grounded) then phase voltages induced in 3.3 KV star side will be unequal but line voltages will be equal.

It will help you if you measure all 3 phases PT readings at 3.3KV & 33KV side.

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