Advantages of Surge Arrester on Sec.of T/F or Downstream Switchgear 2

[SilverArc]

Hi,

Could anybody advise me the advantages of having a surge arrester on secondary bushing of a transformer(44/13.8 KV) or the switchgear which is being fed from this.

I think, the reasons could be on lines of switching voltage or lightening protection.

I would appreciate a word.

Thanks

 

[alehman]

It depends. You normally want the surge arrestor closest to what you are trying to protect. If the load equipment is very critical and sensitive, get the surge suppressor close to the loads. If you see the transformer as most critical, put it there. You should also look at the relative sensitivities of the transformer, switchgear and loads to surges. Sometimes more than one is a better solution.

 

[dpc]

In most cases the low side arresters are not as critical as the high side, but they are cheap protection against all sorts of transients including switching surges. Also, if this is distribution substation transformer, lightning surges can come from either side. The transformer windings are the most vulnerable part of the system when it comes to surges, so high and low side arresters are usually a very good investment - as long as they are properly grounded.

 

[Bahram7]

I agree with dpc. Also High side lightning surges may be transfered to the low side by windings capacitive coupling. Utilities usually do not take a chance and put a low side arrester on power transformers.

Bahram7

http://www.Simtech-Intl.com

 

[SilverArc]

Thanks all for your input.

In my case, the HV side of transformer has already bushing arresters. This is an industrial installation.

The sec.(13.8 KV) is fed to a switchgear. Now, what I am trying to get to basically at a theoretical level is that having either a surge arrester at switchgear bus or sec. of transformer or at both locations, which would make sense.
How this is theoretically justfied.
I understand this in case of a lightening strike, the HV bushing arrester will divert it to ground but how about the switching phenomena. How switching can impact low voltage side.
If you are aware of some literature available that can explain a theoretical basis for interaction of a switching transient and surge arrester. Please advise.
Thanks

 

[dpc]

I would definitely put a surge arrester at the transformer, and probably another one at the switchgear.

Vacuum circuit breakers can create significant switching transients due their rapid arc quenching. So the transients can originate on the low voltage side as well as the high side.

Proximity is the number one factor when applying surge arresters.

Try this:

http://search.abb.com/library/ABBLi...LanguageCode=en&DocumentPartID=&Action=Launch

 

[SilverArc]

Thanks dpc.

I will go through this pdf document and probably will help me getting a better picture of the whole switching mechanism.

I have another question to add here. I got this info from utility for the primary of transformer.

Base MVA-100
69 KV

I(3Ph)-4000 A
I(1Ph)-3183 A

Pos. seq. Impedance- .091+J.0325

Zero Seq.Impedance- .07+J .25
I was wondering, how come zero sequence reactance so much higher than all other parameters?

Is there any explanation to it.

Thanks

 

[dpc]

Zero sequence impedance includes the ground return path, so it can be significantly higher. Any chance the transformer has a grounding reactor?

 

[SilverArc]

Thanks dpc.

It is a feed from utility, and I dont know, if their system is resistance grounded or not. But mostof the utility substations are solidly grounded and I dont know why utilities ground them in this fashion might be a trend thing I guess.

Do you think, is there any other reason for zero sequence impedance to be higher than positive sequence impedance ?

 

[bjenks]

From IEEE Red Book 141-1993 :Zero sequence= ZG0 + 3ZGR "Currents of zero sequence, being in phase, do not add up to zero at the end terminal as do both the positive and negative sequence currents. They add arithmetically and return to the source via an additional circuit conductor."

Do you think the zero sequence you got from the utility is of the primary of the transformer or could it be the Thevenin equivalent of the impendence seen by the primary of the tranformer? If the latter then that could explain it.

 

[SilverArc]

Thanks. I wonder, why these small things could be so complicated at times.

Utility normally provides:
3 Ph( SC)
1Ph (SC)
and
Z(+ve)where Z(-ve) will be same as z(+ve)
Z(zero Sequence)

This line this is coming to our installatin is certainly from a sec. of a 115 kv/69 KV transformer.
AS our input is 69 KV.

I guess that means this sec. of utility transformer is resistance or reactance grounded. This is the only way to justify.

Anyways,
How utilities decided that they will have transformers solidly grounded or resistance grounded ?

I understand this issue in industrial and commercial installations but how this works in case of
utilities ?
Thanks