Power Transformer BIL Rating 2

[PQ]

Hi, we have purchase a 2500kVA 13.8kV/440V Power Transformer connected delta-wye. The Primary High Voltage BIL rating is only 95kV... The Utility where we will connect the later is requiring a 110kV BIL at 13.8kV. Could someone explain to me the risk of installing a Transformer @ 95kV BIL to there system? The utility will allow us to connect to their system but we will shoulder any risk that whould happen to the system. Can someone explain to me this risk

 

[jbartos]

Suggestion: Reference (or similar one):
Donald G. Fink, H. Wayne Beaty “Standard Handbook for Electrical Engineers,” 13th Edition, McGraw-Hill, Inc., 1993,
page 10-38 par. 101 "Basic-Insulation Level (BIL) Reduction. The need to demonstrate absence of significant partial discharges in operation is increased for higher circuit voltages where improved surge-arrester characteristics have encouraged a continuing trend toward BIL reduction. Because of progressively decreasing margins between the conventional induced test voltage and operating voltage, new standards for transformers rated 115kV and above (please, notice that your transformer is 13.8kV only) require a 1-h induced voltage test with continuous monitoring of the partial-discharge levels to demonstrate the soundness of the insulation. During this test all parts of the insulation system must be overstressed to a degree corresponding 150% of maximum system voltage at the high-voltage terminals (see ANSI/IEEE C57.12.00-1987)
This means that better arresters allow lower BIL. However, the transformer partial-discharge should be more carefully tested for the transformer with lower BIL.
The risk is that the lower transformer BIL will more likely experience insulation decay than the higher BIL transformer. However, the modern arresters should enable the lower BIL transformer to be protected and be approximately equivalent to the higher BIL transformer.
Your case will require a focus on the better transformer arresters.
Visit

http://www.hubbellpowersystems.com/powertest/tips_news/pdfs_best/EU1202-H.pdf

http://www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-3006/c-4.pdf

http://grouper.ieee.org/groups/tran.../Dielectric Tests Rev4 with Max Sys Volts.ppt

(indicates 95kV as a preferred BIL)
etc. for more info

 

[RajT]

The BIL on the Tx you have purchased is in line with standard BIL rating specified for that voltage (ie 13.8kV)as per IEC (and perhaps ANSI as well).
You should stay calm and ask formally the utility to explain their technical objection to the BIL of 95kV.
If they say that the region has a higher levels of lightning strikes then you could arrange a surge arrester to be fitted.
If on the other hand they try to fob you off then you could decide not to take any further action on adding a surge arrester.
Note that a BIL of 95 kV is a respected value specified on say 90-95% transf sold world wide as most Tx manuacturers will confirm.

 

[busbar]

It is not clear in the original posting whether the transformer has liquid or dry dielectric. In the first case, IEEE Std C57.12.00-2000 §5.10 Insulation levels seems by no means trivial in ratings and commensurate test levels for liquid-insulated transformers.

[IEEE C57.12.01 is the corresponding standard for dry units.]

Aside — In any case, careful selection of surge arresters and their installation is in order—particularly with respect to lead routing.

 

[gwgroup]

Standard IEC 60076-1.
The rated withstand voltages of the winding are:

A rated short-duration power-frecuency withstand voltage for the line terminals according to Table III. based in current practice in the USA: 34 KV

A rated lihtning impulse withstand voltage for the line terminals according to Table III. based in current practice in the USA: BIL 95 KV for Distribution transformers and 110 kV for Other transformers.

Your power transformer is OK.

 

[alehman]

95kV is standard, with 110 being optionally available on some models. I suspect the utility's engineer is ignorant of standards. I suppose it's possible they've designed their system insulation for 110kV throughout, but it seems unlikely. I would argue the point. As busbar said, lighting protection should be coordinated with BIL rating, but it is important to have regardless.

One possible point of confusion - the standard "chopped wave" test voltage for 95kV BIL is 110kV.

http://geindustrial.com/products/brochures/sst_brochure.pdf

 

[rconnett]

A general observation:

Pwer transformers normally use 110 kV BIL, distribution transformers are associated with 95 kV BIL. The 95 kV BIL is very common, especially on underground systems which are not exposed to lighting.

If you have particular concerns, I recommend performing (or having someone perform) an insulation coordination study.

I have successfully applied both voltages.

 

[jghrist]

The only danger to the utility system if you use a 95 kV BIL transformer, and all of their equipment is 110 kV BIL, is that there may be a slight reduction of system reliability. This reduction would come about because part of their system (your transformer) would have a lower BIL and be more susceptible to a lightning flashover. If you have surge arresters on your transformer, the reduction in system reliability would be negligible.

If you are in North America, I doubt seriously if all of the utility's equipment is 110 kV BIL. Normally 110 kV BIL is only used for substation equipment on 13.8 kV systems.

You will have a higher probability of lightning failure on your transformer than if you had a 110 kV BIL transformer, even with arresters, because the protective margin will be smaller with 95 kV BIL. However, an adequate protective margin can be achieved with either BIL.

If anything, your transformer having a lower BIL would reduce the probability that the utility would have a lightning flashover of their equipment because you would be the weak point for a nearby lightning stroke.

 

[GerH]

Had similar issues connecting MV ( 11kV) generators directly to grid.

I understand there are two aspects:
Firstly a utility designs on the basis of a uniform capability to withstand transient over-voltages without breaking down. This is particularly important for switching surges as they can be expected with high frequency.
If your transformer has a lower breakdown level than the specified value then other customers on the utility will have impaired service. Fitting a surge arrester does not help this situation, in fact it can make the situation worse for the utility. Don't expect your local utility engineer to change the rule for you.

Second issue is that your equipment may be damaged by events on the grid. You can protect your transformer using suitable surge arresters. As arresters are quite inexpensive it should always pay to take this precaution except to bear in mind that most surge arresters fail unsafe ( i.e. a faulty arrester can be open circuit and not evident). In making this evaluation you could look at the type of utility your are connected - if it is entirely cabled underground from the higher level step-down transformer ( e.g. in urban area) then the risk/magnitude of transients is low. Worst case would be direct connection from overhead lines without shielding conductors in an area with high frequency of lightning.

If it is an oil filled transformer it has better ability to withstand transients and a degree of self-healing. Check with the manufacturer and he may be able to advise if what if any margin exists in the design i.e. you may already have higher BIL than what is stated on the rating plate.

 

[jghrist]

GerH,

How would surge arresters make the situation worse for the utility? A properly operating surge arrester will shunt the surge to ground and prevent power-follow current that might cause a trip.

 

[stevenal]

Sometimes a higher BIL is specified to gain creepage. Salt and polution are reasons to increase the creepage.

 

[GerH]

jghrist,

What the utility engineers said was that surge arresters transient recovery voltage can be low enough to pass some voltage reduction on to the local grid and some types of surge arresters can fail to short. I do know of some cases where surge arresters blew out which would indicate to me that a sustained breakdown fault is possible.

 

[jbartos]

Suggestion: There may also be an economical reason for 11OkV BILL transformer. If this transformer lasts longer (has a higher life expectancy), and if it is more expensive, then a break even analysis is necessary to perform. The 110kV BIL transformer may turn out to be the better deal for the Utility.

 

[xabproject]

Dear PQ,
Is the 110kV BIL requirement for the HV Bushing or the Transformer HV Winding? The reason I ask you this question is that in some cases I have seen the HV Bushing BIL higher than the Transformer HV Winding BIL - for example HV Bushing BIL of 1050kV, transformer HV Winding BIL of 950kV.
best regards,

 

[jghrist]

GerH,
I do not understand what is meant by the surge arresters transient recovery voltage. If the arrester does not recover it's full impedance after a surge, it will fail.

If you mean by "fail to short" that they do not shunt the surge current, then the situation would be the same as if there were no arrester, not worse.

Yes, sometimes arresters fail, but in areas of high lightning activity, studies have shown that reliability will be higher with more arresters on the system.

 

[PQ]

Guys, thanks for the information. However what type of arrester should we use a distribution type or a station type? what is the difference between the two? We have experirnce a distribution type arrester which blows up (the line and ground wire connection where detached on the arrester terminal) during heavy rains.. Is it normal for a 27kV arrester connected to 34.5kV system to function this way? is this a normal operation that during a flashover it detaches the wire from its terminal....

 

[jghrist]

Station type arresters will handle a higher surge current than a distribution type. Protective margins (governed by equivalent front-of-wave crest voltage and discharge voltage) will be better with station type. You can get similar protective margins to a station type for less money by using riser-pole arresters, but the surge current capability will not be as high.

If the surge current is too high for the arrester, it will fail. Lighting is a probabilistic phenomenon; there is no upper limit to surge current, just a lower probability for higher currents. Distribution arresters have ground lead isolators that disconnect the ground lead in case of internal failure. This allows the circuit to be reenergized because the arresters fail shorted otherwise.

For a power transformer, I'd get station type arresters and have them mounted on the tranformer. The lead length (both phase and ground) is very important in determining the effective protective margin.

 

[busbar]

An appropriate reference may be IEEE Std C62.22-1997 …Application of Metal-Oxide Surge Arresters for Alternating-Current Systems