Rated voltage of lightning Arresters 1

[prc]

When LAs are used on the delta tertiary of transformers 110% rated voltage arresters are used, being ungrounded system.So in a 33 kV tertiary, 45 kV LAs are used to get 36 kV continuous power frequency voltage withstand performance. If a delta/star transformer is fed from a star/star (132/33 kV) feeding transformer (with secondary star neutral solidly grounded or low impedance earthed through an NGR) what should be the rating of 33 kV LAs to be used with delta/star transformer? In that case, are phase to phase arresters necessary in addition to phase to ground LAs?

 

[stevenal]

See IEEE C62.22.
Lightning stresses your insulation to ground, so I'm not sure what benefit line to line arresters would provide.

 

[prc]

Please see clause 5.2.3.5 of above standard,C62.22-2009 Application Guide for Metal Oxide arresters. This gives reason and details of phase to phase arrester application for delta connected terminals. The reason why phase to phase arresters are required for unearthed system is when a surge impinge on one phase, the other phase gets induced surge of opposite polarity and then the phase to phase voltage may exceed the BIL of the winding.

 

[stevenal]

Agreed. The system you described above is not unearthed.

 

[magoo2]

Normally, the system configuration feeding the transformer determines what arrester rating is needed and not the transformer connection. Since the 33 kV is a grounded wye connection, you shouldn't have to use a phase to phase rating for the arrester. You can use a lower rated one.

If you have distributed generation on the star side of the 33 kV delta wye transformer, then you have to consider the possibility of backfeed from an ungrounded source and will need a phase to phase rated arrester.

 

[prc]

Thank you magoo2. That was my understanding also. I wanted a reassurance. So only phase to ground arresters are required with line to ground voltage as max operating voltage. Phase to phase arresters can be skipped.
When power flow is from unearthed Y to delta, why you consider higher voltage rating for arresters? Still the 33 kV side neutral remains earthed.

 

[magoo2]

prc
For the backfeed case, once the feeder circuit breaker operates, assuming a single-line-to-ground fault, one corner of the delta winding will be grounded and the other two phases will be at phase-to-phase potential.

 

[cuky2000]

The surge arrester will be connected phase to ground but rated phase-to-phase.

In the event of a ground fault, the unfaulted phase voltage will be raised up to the phase-to-phase voltage as shown in the Coefficient of Grounding (COG =1.73). See the info below for detail info in this matter

 

[stevenal]

I assume you are not allowing the DG to island after the feeder trips and removes the neutral ground reference. Transfer tripping or other methods of tripping the DG would be employed. The corner ground condition exists until the DG clears. Look at the temporary overvoltage (TOV) curve of your arrester to determine if it can withstand the overvoltage for this period. You may or may not need to increase the duty cycle rating to get the proper TOV. I agree the arresters should be connected line to ground.
This really has nothing to do with whether or not the load prior to the fault is a backfeed condition. Forward feed would be the same.