Zig-Zag Transformer Neutral Current 1

[Engineer1916]

Hello Folks,
I have a generating station with three 2.5MW gas units. We have high resistance grounding on the gensets and they are connected to 3 wye-delta step up transformers. At high voltage delta side a neutral will be achieved using a zigzag grounding transformer. I am trying to figure out the size for that zig-zag and have following questions.
1. Available fault current changes with time because of subtransient, transient and synchronous impedance of generators. What fault current value should I use to size the zig-zag?
2. If you want to limit the available fault current through zig-zag's internal impedance, how do you calculate that? And how to report it to the manufacturer for biddig?

Thank you in advance for any help.

 

[xnuke]

1. Subtransient
2. See IEEE Std C62.92.4; there's a procedure for zig-zag grounding transformer calculations in it. Also, see IEEE Std 32 for the short-term rating multipliers used to reduce the VA rating required.

It sounds like these are unit-connected generators. If so, why are the GSU transformers wye-delta? They should be delta-wye (which gets you your HV neutral connection point). HRGs will do next to nothing to reduce ground fault currents if the GSU transformers are wye-delta with the wye solidly grounded. Are they ungrounded wyes? A sketch would be nice.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Engineer1916]

xnuke,
Thank you very much for your response.
1. Can you please provide a reference to how you come up with subtransient? I am proposing 10 sec rated zig-zags, Using subtransient current as "In" in this case will provide me with a transformer capable of carrying subtransient fault current for 10 sec. But we know that the zig-zag will not see the subtransient current for more than a couple of cycles. I would really appreciate it if you can point me towards a document.
2. I have looked at both these standards. IEEE std C62.92.4 has an example to calculate the resistance of zig-zag. Which is strange to me, why are we calculating resistance and not impedance? How to calculate the total impedance of zig-zag transformer to limit the fault current.

Yes these are unit-connected generators. The GSU transformers are wye-delta, with wye ungrounded. So the fault current is limited on the primary side of transformer by HRGs. I am not using delta-wye and not using HV neutral connection point because this will provide multiple return points for L-G fault on secondary side. So for easy L-G fault detection i am using wye-delta ungrounded.

 

[Parchie]

Engineer1916 (Electrical)

Neutral Grounding transformers are only required to carry the neutral fault current from inception and until your breakers isolate the generator from the bus. Please refer to Westinghouse T&D Reference Book for grounding transformer overload factors (depending on the different time ratings). For a grounding transformer rated for 10 seconds, the overload factor is 6.4 (divide your estimated NG transformer value by this).

 

[Engineer1916]

Parchie,
Thank you for the reference. I will check it out. I understand that Neutral Grounding transformer will carry the neutral current from inception until breakers trip. But my question is, what fault current should I use for sizing? As the fault current value is changing with time i-r subtransient, transient and synchronous. Also assymetrical fault current vs symmetrical fault current.

 

[Parchie]

@Engineer1916,
Sorry for the late reply. First, you have to estimate the zero sequence impedance of your generator. These are consisting of zero sequence capacitances to ground of all elements connected to your generator like:
[ol 1]
[li]Generator Winding ,[/li]
[li]Generator Connection/cable ,[/li]
[li]Generator Transformer ,[/li]
[li]Transformer cable ,[/li]
[li]Generator Surge Arrestor and[/li]
[li]other Capacitances![/li]
[/ol]Once you have these, compute the capacitive reactance and you use the rated line to neutral voltage of your generator to get your neutral current:
(V_LN X Field-forcing factor)/total capacitive reactance to ground = Io
Multiply by 3 to get 3Io = the neutral current on ground fault. Use the calculated 3Io to get your grounding transformer kVA rating = Vphase X 3Io X 1.732;
Apply the reduction factor to finally get the correct kVA rating of the neutral grounding transformer.

 

[prc]

1.The latest national standards for Grounding transformers are IEC 60076-6 and IEE C57.32-2015 (earlier No.32)
2.Query is about the neutral grounding transformer on HV side. Why generator capacitance calculation?
3.You have to decide the fault current to which you wish to reduce. Usually it is limited to 25 % of max 3L fault current.
4.IEEE 62.94.4-2014 Guide for the Application of grounding transformer -Part IV Grounding Transformer. Annexure A gives sample calculation.
5.Yes it is bit strange. You will find only R of the resistor contributes in the calculation of current and grounding transformer impedance has no effect.

 

[prc]

Sorry,I messed up IEEE number. It is 62.92.4 and not 62.94.4. Also please ignore item 2 &3 of my comments. I confused with neutral grounding reactor. Attached is a calculation for a similar case.

 

[Engineer1916]

prc,
Thank you very much for the calculations.

Parchie,
Why are we calculating the Io based on generator parameters? There is wye(ungrounded)-delta in between generator and the fault point. wouldnt the fault on high side be seen as phase fault by generator?