Arrester selection for system with neutral grounding resistor 1

[yinger]

Normally our 13kV wye systems are solidly grounded. But a customer has asked for a 400 amp 10 second NGR. I think I need to change from a L-N rated arrester to L-L. Is there an IEEE paper that confirms this? Is it it the IEEE Green Book? What are the drawbacks to using the higher rated arrester?

 

[dpc]

Yes, I think you're correct. For any impedance grounded system, the arrester rating must be based on L-L voltage. I'm not sure of a standard on this, but this is the guidance given by ABB and other arrester manufacturers.

The main drawback is the reduced protection for the equipment. However, if you look at transformer BIL ratings, there is generally a lot of margin, even with L-L rated arresters until you get up in EHV range.

 

[cuky2000]

For a resistance grounded system the maximum voltage expected to be applied on the surge arrester will be between voltages levels LL and LN.

Under sizing surge arrester could provide excessive discharge and early failure in the system. On the other hand, over sizing arresters may not provide proper degree of protection.

One accurate methodology is to size the surge arrester using a transient network analysis or a transient program analysis such as EMTP. Since those tools are not easy an alternate method to size SA is to follow the guidelines of the IEEE Std. C62. or similar guidelines published by surge arrester manufacturers.

A simplified overview of this option require to multiply the max overvoltage by the coefficient of grounding (COG) at the transformer location determined based on the sequence impedance values using normalized curves published in this standard. Thermal capacity and protective margins of 15% or 20% should not be exceeded. See the enclose site for further references.

http://www.hubbellpowersystems.com/powertest/literature_library/pdfs4lib/OB/EU1045-HR3.pdf

SUGGESTIONS:
For small transformer particularly on MV application, is OK to follow the manufacturer recommendations using guidelines from the surge arrester manufacturer. (Check Table II page 12 on the enclose site).

For large units probably the project could afford an Insulation coordination study. In some cases this is a cost effective option with minimum risk is using units rated for less than full BIL/BSL backed up by a responsible insulation coordination study.

 

[busbar]

Resistor application is probably more of a industry-dependent variation. Where not common with a significant part of the system of overhead lines, shielded cables often dictate the resistor to limit ground-fault cable-shield damage.

Discussion at IEEE Std C62.22-1997 Application of Metal-Oxide Surge Arresters for Alternating-Current Systems, with supporting info at IEEE C62.92.4-1991 Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution

 

[mwilke]

I use a 12,470 volt (WYE) resistance grounded system. The Industrial Power Systems Handbook by Beemans states the lightning arrestors should be sized at 125% of the L-L voltage. This has worked well for over 25 years. Generally, if lower value arrestor have been used (in equipment furnished by others)the arrestors fail during ground fault tests or ground faults.

mwilke

 

[cuky2000]

Dear Mwilke,

Selecting surge arresters as 125% of the L-L voltage should be re-evaluated. I suggest some caution on the application of this recommendation from D. Beeman reference. This classical handbook was edited in 1955 were air gap and silicon carbide technology for arrester dominates the market. Today, most likely we have to specify metal-oxide arrester witch have different parameters.

In the event of ground fault for a wye resistance grounded transformer, the maximum voltage on the un-faulty phase should be expected in a rage coorespondent to an effective grounded and ungrounded systems in the following range:

80%. V_LL < Vmax < 100%V_LL

Under-sizing arrester may lead to premature failure. Over sizing arrester on the other hand, may not protect transformer. For proper arrester selection, consider apply the guideline of IEEE Std. C62.22.

 

[rowbotth]

The NGR is ALWAYS used to limit Ground Fault Current. It is always installed between the star point of the transformer (or generator, or source of power) and ground, or earth.

As it is installed at this location, the maximum voltage which can be impressed across it is the Phase to Ground voltage. So in a 4.16/2.4 kV system, you would use the 2.4 kV to calculate resistor size.

I'm not familiar with your 13kV system. So you get to calculate the actual Phase to Ground voltage yourself.

HR.

 

[vandax]

In our 13.8 kV distibution line system with 20 Ohm NGR, we kept to use the 15 kV voltage rating, and 12.75 Maximum Continous Over Voltage. The MCOV is determined by possibility of voltage increment in unfaulted phases when single phase to ground occurred.

This all is according to ANSi and manufacturer reccommendation. You may contact/download hubbell or Cooper for detail.

Regards

 

[Kiribanda]

Yinger,

From the data given it seems you are going to have a low resistance grounded system with an allowable let through current rating of 400 Amps. So any ground fault relay should trip within 10 sec.Therefore the arrester rating depends on the Earth Fault Factor(E.F.F.-as per IEC60071) of the new system. It can be easily calculated on the network and to be verified whether system satisfies (X0/X1)>0, (X0/X1<3) and (R0/R1)>0 for an effectively grounded low resistance (400 A )system.Then the arrester rating should be little higher than E.F.F.multiplied by Nominal Line to ground (Phase) voltage.The duty of the arrester should also be 10 sec only.
If we select a higher rating, then that voltage will never occur on the said system which means an unnecessary expense in my opinion.In other words for a High resistance grounded system where the E.F.F. is very high, will naturally have expensive high arrester ratings. :->

 

[jbartos]

Suggestion: Visit

http://www.eng-tips.com/viewthread.cfm?spid=242&newpid=242&sqid=39720

http://www.ipst.org/TechPapers/2001/IPST01Paper136.pdf

etc. for more info