Surge Capacitor on Unit Transformer 1

[Umtully1]

Hi All,

So I'm working on a generating station upgrade project and the utility has asked for surge capacitors on the low voltage side of the unit transformers. I've never seen this in my 4 shorts years of engineering, but neither has my mentor.

Could anyone shed any light on why they're asking for capacitors at the transformer? Note, that these are in addition to the surge capacitors found at the generator terminals.

Cheers,
Tully

 

[mgtrp]

The theory is that in the event of a lightning strike or similar with a fast rise time, the transformer will act more like a capacitor instead of a transformer, and voltage will be transferred from the HV to the LV via capacitive coupling between the windings potentially causing damage. The addition of a capacitor on the LV of the transformer is meant to create a capacitive voltage divider to limit the amount of overvoltage.

 

[magoo2]

A surge capacitor tends to slope the waveshape of the incoming surge. They are generally applied with dry type insulation like motors and generators, but I've never heard them applied for a unit transformer which is generally an old filled transformer. Dry type insulation is vulnerable to surges with high rates of rise because the surge doesn't distribute uniformly across the windings. Oiled filled transformers are much more tolerant of surges with high rates of rise compared to dry types.

If you can find it, there's an old reference by Bewley which demonstrates the effects of surge capacitors. I can look up the title if you are interested.

 

[mgtrp]

See section 18.5.2 of this book for a better explanation of the transfer of voltage through a transformer. The surge capacitance added to the LV side of the transformer increases the Cs term in equation 8.7a, which makes Vtc smaller.

This practice of adding a transformer seems uncommon to me - magoo2 has described the normal situations where surge capacitors are applied - and it's not something that I've ever worried about requesting. If you're particularly worried about it, I have buried away at the office somewhere a calculation done by someone showing their assumptions in calculating the transferred voltage, which could give you some guidance in performing a calculation for your own circumstances.

 

[prc]

Separate surge arrester or capacitor is not required for LV side of GSU because 1) the surge impedance of generator will dampen the transferred surge 2) The LA and capacitor provided for generator will help in reducing transferred surge.

But separate arrester and capacitor is required for transformer LV when a generator circuit breaker is used, as then during the breaker open condition, the transferred surge to LV may exceed the BIL of the transformer.Usually GCB manufacturer provide this in the breaker itself for the protection of transforner.

 

[Pouria1978]

I'm agree with prc. surge capacitors are used to reduce the rate of rise of the Generator Circuit Breaker recovery voltage. generally the TRV duty of GCBs are different from typical circuit breakers as stated in IEEE C37.013 standard. surge capacitors are supplied by the GCB manufacturers and you can request them for supplying.

 

[odlanor]

I do not agree with prc:

1-surge arrester is required for generator to reduce transferred surge originated from the system impinged by lightning strokes,
and capacitor to dampen the transferred surge. Last one is not necessary if you have stepup transformer connected NEAR to generator.

2-The LA and capacitor provided for generator will help in reducing and damping transferred surge.

3-Only LA is required for transformer LV when a generator circuit breaker is used, as then during the breaker open condition, the transferred surge to LV may exceed the BIL of the transformer.

4-Others case I can not justify now.

 

[prc]

odlanor, I dont think you are disageeing with me. The LA and capacitor are provided to protect generator winding insulation from transferred surges from HV.This will automatically protect transformer LV terminals also. LA will cut off voltage and capacitor will reduce the transferred voltage and also dampen the wave.

 

[odlanor]

prc,
Your statement suggests that we need capacitor to dampen surge at LV transformer.
you DO NOT Need it! This refers to my item 4.

 

[Umtully1]

Ok, let me see if I understand the reasoning:
The client would ask for a surge capacitor on the LV side of the GSU transformer, because there is the possibility, when the generator CB is open, that a lightning strike (or similar) surge event could cause the voltage surge to be transferred from the HV to LV windings of the transformer exceeding the BIL rating?

And since we have a generator CB I think this make sense. And this surge capacitor can be installed with the gen. CB?

Thanks for the help!
Tully

 

[odlanor]

Umtully1

Have you seen any transmission transformer with capacitor in secondary because of BIL?

 

[Umtully1]

@Odlanor
Not that I'm aware of, but then I wasn't specifically looking for one. Surge arrestors are common on almost all outdoor equipment though.

 

[prc]

Let me clarify:

1) Today the leading manufacturers of GCB are ABB and Alsthom.Both of them provide a capacitor( about 130 nF) on both sides of the breaker.In addition Alstom provide LA on both sides and ABB on transformer side only with generator side as option.The capacitors seem required from Breaker operation angle.In addition this gives protection to the generator windings from transferred surges from HV

2) In cases where GCB is not used, with the transformer LV directly connected to generator, LA and capacitors are provided for generator protection. Some times when generator installtion is not ready, project authorities go for back charging to see that the transformer is healthy ie energising from HV with LV open.For that operation, transformer manufacturer always insists
for the generator protection capacitor and LA in circuit.There were many failures during back charging of GT due to transferred switching surges causing flash overs on LV side

3) Theoretically it is proved and personally I have seen at test bed- capacitors do reduce the transferred surge to LV and also dampen the surges.Please see Annexure B to IEC 60076-3 Power Transformers. Qoute-"A reduction of the inductively transferred overvoltage component can be effected either by resistive damping through a surge diverter or by modification of the oscillation with capacitiveloading. If capacitors are used, the capacitance value has usually to be of the order of tenths ofmicrofarads. (They will therefore automatically eliminate the capacitively transferred
component as long as the circuit inductance is low.)" Page 101 of standard ed2.0

4) Why it is not used in transmission trnsformers? The transferred surges exceed the BIL when the voltage ratio is high( case with GT and tertiaries in large auto-transformers) In Europe (at least in Germany) it used to be a practice to provide surge absorbers(capacitors) for the protection of tertiary winding of EHV auto-transformers. In india also we used to provide, but later dropped by increasing the tertiary BIL by one higher step.

 

[odlanor]

prc,
I'm sorry, but I think that closing this thread requires a clarification of the leading manufacturers of GCB : ABB and Alsthom.

 

[davidbeach]

leading manufacturers of GCB are ABB and Alsthom.

leading manufacturers of GCB : ABB and Alsthom.

I'm sorry, I don't see what the clarification might be.

 

[odlanor]

See the ABB manufacturer's conclusion in section 7 page 8/9 of paper attached

 

[odlanor]

[highlight #FCE94F]Protection Against Transient Overvoltages Transferred Through the Step-up Transformer[/highlight]
Transient overvoltages hitting the high-voltage terminals of a step-up transformer are transferred both capacitively and inductively to the low-voltage side; the initial portion of voltage surges is thereby transferred via capacitive coupling. It is obvious that capacitively transferred overvoltages may become a problem at high voltage ratios of the step-up transformer. When the generator circuit-breaker is open the relatively large phase-to earth capacitance of the generator stator winding is disconnected from the system. The highest transferred surges will therefore appear under these circumstances. To mitigate this effect the connection of surge capacitors with a capacitance of at least 100 nF to the low-voltage side of the step-up transformer is deemed to be mandatory. [highlight #FCE94F]Modern SF6 generator circuit-breakers are generally equipped with capacitors to limit the rate-of-rise of the transient recovery voltage. These capacitors at the same time function as surge capacitors.
Since surge arresters are low cost elements they are normally included in the overvoltage protection scheme without any further investigations.[/highlight]It is however of utmost importance that both surge capacitors and surge arresters are connected to the generator busbar between the low-voltage terminals of the step-up transformer and the generator circuit-breaker (or rather the associated disconnector) so that they remain effective also when the circuit-breaker is open .
CIGRÉ 2004 Overvoltage ABB-Protection in Power Stations with Generator Circuit-Breakers

 

[prc]

odlanor,you had mentioned quote "I do not agree with prc" Now I see you are quoting fromm ABB paper some what same as I contributed.So before concluding this thread I will be happy to know whether you are still disagreeing with me on the view expressed by me.

You also demanded a clarification on my statement "leading manufacturers of GCB are ABB and Alsthom" Iam confused as David felt. You want clarification- On what basis I have arrived at it or what relevance these names has on the subject under discussion or something else.If you clarify your query,I shall try my best to answer.No offence meant please.

 

[odlanor]

prc
My viewpoint is:
1-surge arrester and capacitor is required for generator connected direct to the system;
2- surge arrester is required for generator connected to the system by stepup transformer;
3- When generator circuit breaker is used, surge arrester is required for generator, and capacitor for circuit breaker(TRV rate of rise)

2)In practice, these guidelines are not defined this way because:
The real origin need to adopt surge arrester AND / OR capacitor to protect surges traveling wave voltage resulting from lightning in generator AND / OR transformer AND / OR generator breaker is completely masked by the fact that
1-cost medium voltage these devices are insignificant compared with the power equipment to be protected.
2-The calculations for sizing them are difficult; it's easier to transfer responsibility to the manufacturers of the equipment or adopt a standard, which results in an installation generic rather than specific to the project;

Prc and David
about manufacturers, sorry if gave rise to some improper interpretation;
my english is very poor, I merely copy your text and complete my point of view, without regard to the comsequences.