We have been using GE's frame 9FA gas turbines in number of projects. This is a Typical GCB scheme that i am referring to. We are using three winding VT's for the measurement as well as protection of the generators. But in all the projects we are taking primary winding of the VT as ungrounded star, one secondary winding as grounded star and the other tertiary winding of the VT as delta connected. The delta connection is as per GE's recommendation. Kindly clarify why the delta winding is required?

 

Gitesh... is the tertiary set, the one you called delta-connected) open-corner connected?

Regards, Phil Corso

 

Dear phil,

The tertiary winding is not open corner connected. It is delta connected. I got the info that it is being used for balancing. But how?

 

Gitesh... strange application! A tertiary-winding is generally used in a 3-phase transformer to overcome thru-fault and zero-sequence problems.

With corner-closed, as you observed, then how is the polarizing-voltage source derived for directional ground/earth-fault relays?

Are you sure the "VTs" are conventional voltage transformers having a primary-winding with three-secondary-windings, and not CVTs or CCVTs which are Capacitive-Voltage-Transformers?

Regards, Phil Corso

 

Dear phil,

The VT's are not CVT's . These are three winding VT's. The primary winding of the VT is being used for both metering as well as protection. Accuracy class of the star winding is 3P/0.5.
Also, we do not have any directional earth fault protection(67N). I simply do no understand that why the GE would suggest to keep an extra delta winding in VT? Also, this winding is not being used anywhere in the protection/metering function.

 

Gitesh... can you provide detail regarding the primary kV, secondary V, and capacity in VA?

Phil

 

Dear phil,

Primary kv=16.5

secondary1 V =110/sqrt(3)
Secondary2 V=110

Secondary1 VA=50
Secondary2 VA=50

 

Gitesh… I contacted a couple of CT manufacturers (GE included) but was unable to get a response.

I believe the supplier established three secondary windings as a standard. Thus, the delta-winding open-corner configuration is available for those situations requiring a polarizing source of voltage. If not used, as in your case, the corner is closed!

My concern is that if there is no earthed/grounded static-shield between the primary and secondary windings then the unused winding is exposed to static-coupling which may result a charge on the winding's terminals.

Regards, Phil Corso

 

Dear phil,

Thanks for the efforts that you have made to get the answer. But the sec2 delta configuration is not VT manufacturer's standard. It was possibly due to GE's remarks that we have kept one delta winding. But the purpose is not known.

And also, it is not open delta, it is delta. The delta winding is only feeding the voltage supervision relay. It is not being used for any other purpose.

Regards Gitesh

 

Gitesh... FYI, because of the confusion that often arises, open-delta, to me, means the use of just 2 transformers connect in a "vee!"

Phil

 

Dear phil,

But I still have not got answer to my question....plz reply

 

Gitesh... my answers were based on your statement, "... this winding is not being used anywhere... " But, now you reveal it is used to feed a voltage supervision relay!

Why not ask the person(s) who provided you with the info about it being used for balancing?

Regards, Phil

 

hey,
u should know this dear friend

In single phase transformer there are mainly two windings, but sometime a 3rd winding known as tertiary winding is also used. Normally tertiary winding is connected in Delta connection.
Importance of using tertiary winding in a transformer are given below:

1) To supply small additional load at a different voltage.

2) To supply phase compensating devices such as capacitor for power factor improvement which can work at different voltages.

3) In Delta connected tertiary winding reduces Zero sequence impedance and allow adequate current for operation of protective device in order to limit voltage imbalance which may be produced when in balance in load.

4) It indicate voltage in high voltage transformers.

 

Ranjan... I believe you missed the point of Gitesh's original question.

He(?) asked if the delta-connected tertiary-winding provided a "balancing" function since it was not used to provide any protection/metering function.

Subsequently, he noted it was connected to a voltage supervision relay!

Regards, Phil Corso

 

Hi Gitesh.

to answer your question, the open delta is connected to trap the harmonics from the supply and make sure the quality of power supply is meeting the requirements.

regards
Farza.