Surge arrester between step up transformer and circuit breaker

[Grinc]

Hallo everybody. First of all I would like to apologize for my English because it is not my native language. My question is about surge arresters and their location. I have read in a book that surge arrester is used between step-up transformer and circuit breaker to protect transformer against switching overvoltages. In my case all lines are underground and generator installed capacity is about 0,4 MW. I attach the drawing in appendix if my explanation isn’t clear. I would like to understand in what cases this surge arrester is needed between step-up transformer (in my case 0,4kV/10kV) and circuit breaker?

andris

 

[rterickson]

The arresters on the two outgoing circuits protect against lightning overvoltages out on the transmission system.

But looking inward, if there is any chance of overvoltage in your switchyard (i.e. lightning strikes to the bus, or switching surges), then you want arresters as close to the terminals of the GSU as possible (as shown in your drawing).

 

[Eleceng01]

I can think of several generating plants that I have worked at that have this configuration (when shown as a one-line).

Grinc, I seem to remember from a class that switching introduces high-frequency transients on the system (on a per phase basis the circuit is a RLC circuit) so it is best to have the surge arrestors between the switches and the transformer to protect it.

 

[Skogsgurra]

Tip for westom: Have a look at

http://en.wikipedia.org/wiki/One-line_diagram

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[PHovnanian]

First of all, I would like to apologize for my English because it IS my native language.

I agree with the OP's book and rterickson that the surge arrester at the main transformer is to protect it from switching surges. If all lines are underground (or enclosed), including the 10kV bus, then protection against lighting strikes to the bus are not a big issue. Some notation as to the generator/transformer connection (Wye, Delta, etc) and grounding schemes might give a bit more insight as to the susceptibility of your particular installation to switching overvoltages. Combinations of transformers and underground systems can also be susceptible to ferroresonance and the associated overvoltages.

 

[westom]

A bus does not connect to one side of the transformer - as the diagram shows. A bus connects to at least two transformer leads. Why is the diagram confusing? Which bus wire between generator and transformer also connects to which grounds? That would not be important for discussing a grid distribution. But that is essential for the OP's question.

Switching transients are not necessarily normal mode currents as some are assuming. Just another reason why each ground must be defined.

 

[stevenal]

Arresters are used to protect insulation from damage from overvoltage. The circled arrester is located where it will most effectively protect the transformer high voltage winding insulation. The transformer insulation is particularly expensive to fix.

Tip for westom: See Skogsgurra's post above.

 

[odlanor]

1-Lightning arrester never protect equipment against switching overvoltage.

2- Lightning shielding substation by shield wire or masts can reliably intercept the large strokes,
and is used to protect buswork from lightning flashover.

3-Lightning arrester protect equipments against surge voltage traveling waves that arrive at substation originated from strokes to strike
overhead transmission lines. Because of this , they prefer the name SURGE arrester.

4- Shielding may allow some smaller strokes to strike the buswork and equipment. Even though these strokes may not cause flashover,
they may damage internal insulation systems of transformers, etc., unless they have proper surge arresters mounted at their terminals.

5- If all lines are underground (or enclosed), including the 10kV bus, then the installation of lightning arrester is not justified.

6-If auxiliary transformer circuit (0,4kV)is connect to an overhead line , you should install surge arrester at generator side.

 

[magoo2]

odlanor,

1-Lightning arrester never protect equipment against switching overvoltage.

I don't believe this is true. Why would you put arresters on switched capacitor banks?

If you're quibbling on words, lightning arresters and surge arresters are the same animal! They were originally called lightning arresters in North America and were later called surge arresters. They are designed to handle more than lightning.

 

[prc]

Grinc-Surge Arresters may be required some time for cable connected transformers also-depending on cable length etc.Please see the two application guides for Surge arresters IEC 6099-5 and IEEE C62.22-1997 where it is dealt with.Surge arresters do protect equipments against switching surges.That is why catalogues give switching surge spark over voltage for arresters.Of course it will not protect equipment against power frequency overvoltages.

 

[ScottyUK]

magoo / odlanor,

Is there any confusion between a ZnO surge arrestor and an air-gapped arcing horn arrangement? I disagree with some of Odlanor's comments if they are applied to a ZnO type arrestor, but certainly some apply to an arcing horn.


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If we learn from our mistakes I'm getting a great education!

 

[Grinc]

Thank you all for your replies. There are many useful things to think about. From my point of view all switching equipments have their own withstand value against overvoltages hence surge arresters in underground cable systems are not required.
Prc - i search these application guides for Surge arresters IEC 6099-5 and IEEE C62.22-1997 in net but whithout any resaults for free reading. Perhaps you can give me any link ?

 

[stevenal]

"surge arresters in underground cable systems are not required"

While the cable insulation is not as valuable as the transformer insulation, it can still be very difficult to repair. Unlike oil, solid dielectric cable is not self healing. It also degrades with time, so that the cable will be more susceptible to damage from switching surges as it ages. These surges can reflect off terminations to double in magnitude. Arresters are commonly applied just as your one-line shows.

IEC and IEEE standards are not free.

 

[slavag]

What stevenal saied

 

[westom]

Grinc - stated previously were requirements such as longitudinal verse transverse currents that must be considered. As some noted, switching spikes and lightning spikes are similar - require same protection. And again, unless you define where both leads of a protector are connected, then nobody can say what it does and does not protect from.

MOV or spark gap perform the same function; but differently.

Another noted that transformer insulation requires protection. Yes. But from which type of transient? He does not say. Therefore does not define where both protector leads must be connected. Again, even interconnected grounds are unique / different. You do not just ground a protector. Connect it to the appropriate ground.

C62.xx defines waveforms for testing - would provide little useful information. None of these standards are free.

Your diagram is only describes a grid distribution. Is missing too much information to define how individual components are connected. Your question is about where both leads of a protector connect. That answer is not possible with a bus diagram devoid of important facts.

 

[odlanor]

Everyone,
Surge arrester is dimensioned in studies of insulation coordination to protect equipment from surge voltage traveling waves that arrive at substation originated from strokes to strike overhead transmission lines.

Imagine a waveshape like BIL(Basic Lightning Impulse) arriving at terminal of transformer(without arrester). It will be duplicated and reflected to the system. As consequence transformer , with same insulation as BIL, will have his insulation breakdown. In the sequence will happen some kind of short circuit.

If you install surge arrester at transformer terminal , they will divide the incident wave: one wave with the surge arrester discharge voltage size goes to transformer. Other one with same size but with inverted polarity is reflected to system reducing incident wave.

We define the basic characteristics of an OZn-surge arrester: maximum continuous operating voltage MCOV, duty-cycle voltage rating, discharge current and energy capability (kJ/kV).
What about switching overvoltages?:
The switching operations concerns in EHV systems :
Line energization, line reclosing, load rejection and transformer switching at no-load.

If you have one of this problem, you should calculate the energy capability of surge arrester for discharge the energy contained in this switching surge.
We are not worry to protect the equipment;-We are worry to protect the surge arrester!
Of course , if your surge arrester absorve switching energy it will offer some protection to the equipment. But the affirmative the surge arrester protect equipment from switching overvoltages can not be compared to surge arrester protect equipment from lightning discharge!

Magoo2,
I do not remember about switching capacitor banks. I can not answer this question.
Yes ! lightning arresters and surge arresters are the same animal! They were originally called lightning arresters in North America and were later called surge arresters. They are designed to handle surge voltage travelling waves NOT lightning.

Prc,
They are talking about residual travelling waves originated from an overhead transmission line finshed by a cable connected transformer. Surge arrester are installed at both junction of cable with overhead transmission line and transformer.
Surge arrester is dimensioned based on lightning discharge but must have switching sparkover capacity to support switching overvoltage of the system.

ScottyUK
At 138kV system and bellow it is common to install arcing horn at terminal of transformer instead of surge arrester. I prefer this solution.

stevenal
“surge arresters in underground cable systems are not required"
I am not talking about this.
If all lines are underground (or enclosed), including the 10kV bus, then the installation of lightning arrester is not justified. Like ScottyUK suggests, an arcing horn is sufficient!

 

[stevenal]

"If all lines are underground (or enclosed), including the 10kV bus, then the installation of lightning arrester is not justified."

I disagree for the reasons already stated. Perhaps the confusion lies in your use of the word "lightning arrester." "Surge arrester" is more accurate, since surges can come from other sources. We use arresters on UG feeds where we would not use them on equivalent OH. This is because of the higher investment and repair costs associated with UG.

Like ScottyUK suggests, an arcing horn is sufficient!"

I don't see where ScottyUK suggested that.

 

[odlanor]

stevenal,

An 15kV OZn - surge protection costs less then U$800,00. This is much less than a study in EMTP about that problem.
If we are talking in higher investment we should never
worry with this discussion.
In this case I would installed surge arrester without any study.

ronaldo

 

[Grinc]

To Stevenal:
I agree with you that old underground cables have bad insulation and swithcing surges can disrupte the insulation or speed up break-down of the insulation . So I understand that surge arrester is used to protect old cable insulation from swithcing surges. Am I right? Perhaps you have any formula to estimate the value of swithcing surge?