Sizing grounding transformer connected for delta side of Generator transformer when Generator circuit breaker is open


Thread Starter

Seema Saran

Pl. help me in sizing the grounding transformer for delta side of Gen. Trf. (525/23kV, 720MVA, YNd1). We have a Generator circuit breaker connected between Gen. Trf. and Generator. There is also a tap-off to Unit Transformer connected between Gen. Ckt. Breaker and Gen. Trf. Generator is 776 MVA, 23kV and grounded through an NGT with resistor. When Generator Circuit breaker is open (Off, the delta side of Gen. Trf. is ungrounded. Our Client wants us to provide a grounding transformer with isolator which will open when Gen. Ckt. Breaker is closed. It will be required only during back-charging from the grid when GCB is open. How to size the grounding transformer, isolator and what protection is to be provided for this grounding transformer.
Mr Phil,

I have re-checked the vector group and transformer voltage. They are correct. Pl. tell me what should be the correct parameters according to you. Are you anticipating any problem?

Regards, Seema
Seema... disregard my earlier question. I misunderstood '525' value. To be clear about your requirement:

1) You want a dis-connectable grounding-source on the 23kV system remaining between the GSU's delta-winding and the CB when the CB is open?

2) Furthermore, please confirm that Unit-Xfmr 23kV-winding is connected between the generator's terminals and its CB?

3) Is the 23kV-side of the system mainly comprised of isolated-phase bus-duct?

4) To start do you know the total capacitance-to-ground of the 23kV system including the GSU's delta-winding, the 23kV bus-duct, if any, 23kV cable, if any, and other acessories!

5) Where are the GSU's protective and metering CTs located on the 23kV-side?

6) Ditto for generator's protective and metering CTs and PTs on the line-side of the CB!

Regards, Phil Corso

PS: a simplified Single-Line-Diagram, even if hand-drawn, would be better!
Mr. Phil,

My pointwise reply to your queries are as follows:

1. Yes

2. Isolated phase busduct Tap-off to Unit Aux. Transformer 23kV side is provided between Gen. Trf. and Gen. CB.

3. Yes, 23kV side is Isolated phase bus-duct only.

4. Capacitance/Phase of Generator Transformer 23kV winding (in μF)= 0.00628
Capacitance/Phase of Unit Auxiliary Transformer HV winding, (in μF) = 0.000982
Capacitance/phase (in μF) for Main Run IPBD, = 0.005931
Capacitance/phase (in μF) for UAT Tap-Off Run IPBD = 0.000258

5. GSUT CTs & PTs are located in GSUT marshaling kiosk.

6. For generator, PTs are located in GCB cubicle and CTs are located in Generator.

I do not know how to upload any drawing in this forum. Pl. help.

<b>moderator's note: you cannot upload drawings to this forum. You need to find an upload site and give people the url for uploads.</b>
Mr Phil,

The following are the NGT parameters:
NGT Parameters:

Voltage Rating (in volts) 23000 : 240
Rating (cont.) of NGT in KVA at 40 deg. C ambient temp.: 75
Duty Cycle (in minute): 10
Selected Short time rating of NGT (in KVA) :200
Sec.Resistor Parametes
Ohmic Rating (in ohms) at 40 deg.C ambient temperautre : 0.20
Current Rating (in Amps) : 800

Regards, Seema

Dear Seema,

please note that Grounding Transformer is not required when GCB is open. As when GCB is opened Generator is getting isolated, but GT is feeding the Unit Loads (while back charging) through Unit Auxiliary Transformer (UAT). The UAT LV side is being non-effectively grounded through resistor.
Dear Gitesh,

Generally, we do not provide a separate grounding transformer for delta side (LV side) of Gen. Trf. and UAT (HV side). Only measurement of ground fault is done through open delta PT scheme and respective breakers are opened in the event of fault.

Here we are talking about delta side of GT and delta side of UAT. Both will be left ungrounded if GCB is opened and NGT gets disconencted.

In this project, client specifically wants that a separate grounding transformer to be supplied.
Dear Seema,

You can propose to provide Pre-Synchronising Earth Fault Protection (59NGT) to know if there is any earth fault while GCB is open. the 59NGT shall get its supply from the open delta VT. the open delta VT shall be connected after GCB towards GT side.
Dear Gitesh,

Already this scheme has been proposed, but client wants a permanent grounding arrangement with grounding transformer and resistor for delta side of GT and UAT when GCB is open.
Dear seema

I think u don't need any grounding transformer. Because when CB is open, at that time the power is feed from GT to UAT (i mean back charging) and the UAT Low voltage side may be grounded (see the vector group of UAT).

I am sure about this bcoz we have the same design in our power plant.
Dear Seema

I think the calculations for the above problem is completed, can you send the SLD & final calculations for my reference at shaileshchvk at gmail dot com.

I have asked my senior about the same, he says we have to calculate fault current distribution in delta-star transformer, using the derived current value on can calculate weather the differential relays provided will work properly or not (I mean stability of relay to be validated)

if the relay operation is not in stable region then we need to install a grounding transformer.

but plz do send the final calculations which you have done for the present case.

I just found this thread an I have a similar case.

A HRG generator connected to the delta side of a step-up transformer in 11 kV. At 11 kV I have 3 distribution transformers (11 kV to 0,42 kV). All the connections with cables (no busducts).

The concern is what happens when the generator circuit breaker is opened, then the 11 kV level becomes ungrounded.

If I want a ground for that situation a NGT YNd11 (open delta with resistor in series) is proposed.

Could anyone provide references about how to size that NGT or if the NGT could be avoided?

Abes... the procedure was given to Seema, but I am not at liberty to disclose the outcome related to his specific situation.

Regarding your situation, can you furnish the MVA/kVA capacity of the 11kV generator, each 11kV motor (if any), and each 11kV transformer. Also, the size/length of 11kV cable, if substantially greater than say, 95mmq/200m 200m.

Actually a hand-drawn sketch or SLD is preferable!

Reards, Phil Corso (cepsicon[at]AOL[dot]com)