×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Surge arrester on neutral terminal of transformer
2

Surge arrester on neutral terminal of transformer

Surge arrester on neutral terminal of transformer

(OP)
Hello guys,

In a project we are participating, there is a transformer wye-wye grounded on both sides, the HV side is grounded through a surge arrester, while the secondary (LV) side is through a resistor.

My question is:
Has anybody dealt with something similar (H0 terminal earthed through a surge arrester)?

I will thank if somebody had some clues or references to understand the application and some possible effects on the protections scheme.

Regards,

RE: Surge arrester on neutral terminal of transformer

Never heard of this but would be glad to get some explanation as well ...

RE: Surge arrester on neutral terminal of transformer

2
... the HV side is grounded through a surge arrester...

your HV is ungrounded!

When a  travelling wave, originated from lightning discharge,  is impinged in the HV terminal transformer,  surge arresters will protect each winding.With operation of surge arrester refrated wave equal discharge voltage surge arrester goes to the neutral.
If neutral is solidly grounded, refrated wave vanish in earth.
If neutral is ungrounded refrated wave will be refleted to phase winding with double value. Surge arrester is necessary at neutral to avoid this.

   

RE: Surge arrester on neutral terminal of transformer

I agree.  High side neutral is protected by a surge arrester not grounded by one.

RE: Surge arrester on neutral terminal of transformer

Yup, above two are absolutely correct.

On first glance, one may wonder why neutral bushing needs any surge protection at all since it is so deep inside the transformer and surges tend to diminish as they travel through the winding.  But it is also a point where travelling waves may do some very weird things. I do recall seeing a severely damaged neutral bushing on a wye winding once which was attributed to surge and I wondered how it could happen.  It was subject of long discussion on alt.engineering.electrical in the 1990's. The details have faded from my memory.

=====================================
(2B)+(2B)'  ?

RE: Surge arrester on neutral terminal of transformer

I found the thread I was thinking of:

http://groups.google.com/group/misc.industry.utilities.electric/browse_thread/thread/f87359c8c04c7b7c/457c25f3dc8a0456?lnk=gst&q=electricpete+x0+bushing#457c25f3dc8a0456

The problem is described in the 2nd post at the top.

I should have mentioned the winding of interest (low voltage in our case) was ungrounded wye... and got severe damage to the X0 bushing.  Suggests a good reason to include arestor on neutral bushing of ungrounded wye.   

=====================================
(2B)+(2B)'  ?

RE: Surge arrester on neutral terminal of transformer

I seem to recall seeing this configuration before - the Middle East I think, maybe Kuwait(?). If I recall correctly the grid there is geographically small and has a heck of a lot of generation on it. The system fault level with all the dYN* GSU transformers is excessive, so on some generating units the neutral is switched out but a surge arrestor is across the neutral switch.

Of course, this could be somewhere else entirely!
  

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

RE: Surge arrester on neutral terminal of transformer

ScottyUK,

You're right, I can confirm this being used in Eastern Europe as well, to cater for high fault level, usually for GSU you'll have a fast switch to change between neutral grounded through sure arrester (a resistor) and bolted.

For 132/33 or 132/11 kV Yod-11 distribution transformers, it's the same, if you have 3 transformers on the same 132 kV busbar, you'll have one bolted and the other two grounded through a surge arrester (resistor) because of the fault levels, except there's no fast switch required. Not all transformers are OK to be grounded through a surge arrester, depends on level of insulation of the neutral.  

May you grow up to be righteous, may you grow up to be true...

RE: Surge arrester on neutral terminal of transformer

I guess 345kV-Y winding transformer of Korean System are connected to ground by surge arrester and disconnect switch in parallel. That means, transformer can operate ungrounded or solidly grounded.

 

RE: Surge arrester on neutral terminal of transformer

I doubt any utility is running 345 kV level transformers with neutral ungrounded, as a system operator you shall switch any transformer out asap, if neutral link is no longer grounded.
Neutral grounded through surge arrestor, doesn't mean it's ungrounded.

May you grow up to be righteous, may you grow up to be true...

RE: Surge arrester on neutral terminal of transformer

There are no universal utility operating practices. And "grounded" through an arrestor is ungrounded.  

RE: Surge arrester on neutral terminal of transformer

@davidbeach

Agreed, is likely to find power system nonsense management nowadays

Have you ever seen any surge arrester in service, without a ground terminal?
 
 

May you grow up to be righteous, may you grow up to be true...

RE: Surge arrester on neutral terminal of transformer

Interesting.lume,can you give the ratings(KVA and KV) of the windings and the rating of the neutral arrester employed?How you size the arrester for such an application.

Recently I came across a case from East Europe where for single phase  420 kV GSU they are adopting 245kV voltage class for the star neutral. I could not understand the resaon then.But in India in 420 and 765 Kv systems, GSU neutral class is 38KV AC ( Test voltage).If fault current is the issue, it would have been a problem here also, being a much bigger system.Waht can be the issue?

Last pete, how you store all these old knowledge nuggets?Question from a computer illiterate.

RE: Surge arrester on neutral terminal of transformer

I have no idea what you might mean by "nonsense management".  Different utilities have different practices.  Different parts of the world have different practices.  There are IEC practices that seem completely alien to me as I am sure that there are IEEE/ANSI practices that seem weird to the IEC world.  None of it is necessarily wrong or nonsense.

Just because the arrestor has a ground terminal doesn't mean that it makes for a grounded transformer.  At least not an effectively grounded transformer.  If each phase to neutral/ground voltage is less than the turn on voltage of the arrestor it is essentially open, no ground reference at all.  If I would to connect one that way, I'd use an arrestor rated for phase-phase voltage anyway so that ground faults wouldn't result in conduction until such time as the fault was cleared.  That would be a good way to burn out arrestors.

RE: Surge arrester on neutral terminal of transformer

m3ntosan,
I did not find Korean paper about my affirmation.
Attached is a 380kV transformer T5 & T6 ungrounded in the British system.
I guess those transformers are used in the system with multiple grounded with high level of ground fault current. If you ungrounded transformer, you reduce short circuit to ground.

prc,
A Y winding solidly grounded of  a transformer shall have insulation gradually reduced from phase terminal up to neutral terminal.
A Y winding ungrounded of  a transformer shall have insulation full from phase terminal up to neutral terminal.


 

RE: Surge arrester on neutral terminal of transformer

odlanor,

That diagram is definitely not based on the British grid - our voltage levels are 132kV, 275kV, 400kV, and our system transformers all have a solidly earthed neutral at 132kV and above.
 
  

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

RE: Surge arrester on neutral terminal of transformer

ScottyUK,
it was extract from Network Protection & Automation Guide page chap2-4-15, new version of english eletric protection book.  

RE: Surge arrester on neutral terminal of transformer

(OP)
Hello,

I thank all of your oppinions and very usefull points.

This is a transformers for an industrial customer (it will be only load, no generation inside!).
It is a 115/13.8 kV rated power of 25 MVA, according to the information is YNyn0 and it is a "policiy" of the customer to install the surge arrester to "earth" the H0 terminal.
Up to now, I have no information available about the surge arrester.

As you mentioned, I also think the surge arrester is installed on the H0 terminal only to protect it from some voltage transients that may ocurr on the transmission network.
The result is a transformer protected against these transients and neutral ungrounded, because the surge arrester does not allow an effective connection to earth, isn't it.

Best Regards

 

RE: Surge arrester on neutral terminal of transformer

@odlanor,

In the British system is not permitted to operate with transformer neutral ungrounded, at least at the transmission level, needs to be switched out asap, even if you lose demand...

@davidbeach

Please refer to EPRI Copper book and EPRI fault level management, grounded through a surge arrester is grounded, low or high resistance grounded depending on surge arrester characteristics.
 

May you grow up to be righteous, may you grow up to be true...

RE: Surge arrester on neutral terminal of transformer

@lume7006,

the ones I've oprerated were similar sizes and voltage levels, the surge arresters were rated for 72 kV, for Y windings at 110 kV. It's the same path to earth, by operating an isolator and/or earth switch you're getting the surge arrester in/out the circuit.

May you grow up to be righteous, may you grow up to be true...

RE: Surge arrester on neutral terminal of transformer

Quote:

Please refer to EPRI Copper book and EPRI fault level management, grounded through a surge arrester is grounded,low or high resistance grounded depending on surge arrester characteristics.  
I don't think many would agree with that. Surge arrester is basically an open circuit until a certain overvoltage appears accross it.  Are you talking about the EPRI copper book... which chapter?
 

=====================================
(2B)+(2B)'  ?

RE: Surge arrester on neutral terminal of transformer

Hi, its standard practic in lot of systems.
Reason, reduce a level of ground fault.
Scotty is right, in smaller system with lot of generation for example

RE: Surge arrester on neutral terminal of transformer

I agree with electricpete, when a neutral bushing has a surge arrester it is ungrounded and protected by the surge arrestor. Conduction is only possible with excessive overvoltages to minimise insulation damage.

RE: Surge arrester on neutral terminal of transformer

Lume7006(1)
...because the surge arrester does not allow an effective connection to earth at 60Hz frequency.
Example
If neutral  is solidly grounded: BIL reduced 450kV at phase terminal winding  and graduated  up to 60kV at the neutral.
Maximum overvoltage  between good phase during  a  ground fault= 0.80 *112 =89.6 kV
Ohio Brass OZn surge arrester selected for phase terminals:
Minimum continuous overvoltage = 88kV
Duty cycle =  108kV
Discharge voltage for 10kA = 244kV

If neutral transformer is ungrounded:  BIL shall be 550kV
Maximum overvoltage  between good phase during  a  ground fault= 1.05 *112 =117.6 kV
Ohio Brass OZn surge arrester selected for phase and neutral terminals:
Minimum continuous overvoltage = 115kV
Duty cycle =  144kV
Discharge voltage for 10kA = 332kV

lume7006(2)
1-    If phase terminals surge arrester were 144kV then refrated wave of 332kV will be refleted to phase winding with double value(664kV) and if 72kV surge arrester fails, 60kV insulation of winding will break !
2-    Insurance company of transformer will not pay for this accident because transformer was not sized in accord to standards!

m3ntosan
My example refers to 380kV of NPAG book and 345kV Korean system.
There are some utilities in Brazil(few) that only requires the generating producer using transformer with isolated neutral to connect to system for avoiding increase the fault-to-ground 138kV system.

RE: Surge arrester on neutral terminal of transformer

Grounding transformer neutral via surge arrester create a hybrid connection. Under normal operating conditions grounded or ungrounded the transformer performance are very similar. However; during asymmetric switching or ground fault scenarios, the system very fast could be connected to ground (earth) reducing the overvoltage in the unfaulted phase(s).
 
Wye-wye transformer connection in HV systems has limited applications. Grounding Y-Y transformers via surge arrester is not a common practice in the ANSI marketplace. The risk exposure vs. benefits is difficult to justify for high reliable installation.
 

RE: Surge arrester on neutral terminal of transformer

lume7006(2)
ERRATA
1-    If phase terminals surge arrester were 144kV then refrated wave of 332kV will be refleted to phase winding with double value(664*60/450 kV) and if 72kV surge arrester fails, 60kV insulation of winding will break !

cuky2000
if you sized Grounding transformer neutral for
X0/X1 < 3 - system effectively grounded
3 < X0/X1 < 10 - system middle grounded (1-phase earth fault current less then 3 phase fault)
 X0/X1 > 10  - system ungrounded.
 

RE: Surge arrester on neutral terminal of transformer

Hi edlanor,

X0/X1= infinite (no path for the zero sequence) Under normal operating conditions - ungrounded
X0/X1< 3 during phase to ground fault if the arrester operate.
 

RE: Surge arrester on neutral terminal of transformer

cuky,
What would be the advantage of this hybrid connection vs solid grounding of H0?  How would you size the arrester?  It would have to have a low breakdown voltage and be able to withstand ground fault current.
 

RE: Surge arrester on neutral terminal of transformer

Hi jghrist,

More than overcurrent, the concern is mainly overvoltage issues during ground fault, switching operation or transient conditions.

Theoretically, the advantage of a hybrid grounding system is to stabilize the system voltage in the event of transient conditions and allows specify Y-Y transformer with lower than full BIL to reduce initial costs.

There is not large world wide experience grounding transformers via surge arresters and there is no standard or guidelines to address the surge arrester sizing for this application. One sizing method is modeling the system using a transient program such as EMTP but, this also costly and requires challengers such as determining accurate parameters such as saturation curves, capacitance, etc for the transformer model.

As I mentioned earlier, the benefit in term of performance and cost reduction may be difficult to justify the risk of grounding the transformer via surge arrester.

 

RE: Surge arrester on neutral terminal of transformer

cuky2000   
X0/X1 > 10 meand high impedance grounded with BIL and surge arrester equivalent to ungrounded.

gentlemen,
 All HV or EHV system are solidly grounded.

If you have only HV/EHV source stepup transformer solidly grounded with all stepdown transformer of substation system ungrounded you have either a system solidly grounded.

BUT with less shortcircuit to ground then

If you have all HV/EHV transformer solidly grounded.



 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources