(Very) short time withstand voltages 3

[Power0020]

I am aware of the 1 min. short time withstand power frequency voltage for insulators, however, I wonder if there is a formula for times less than 1 minute e.g.1 sec or 0.5 sec AC withstand voltage?.

 

[RRaghunath]

No. We have 1 min power frequency withstand voltage and impulse withstand voltage. Impulse withstand is for a voltage pulse with specified rise and fall times.
Power frequency over voltage withstand whether it is for a minute or lower, should not cross the specified value.
So, I suppose even if it is for a second or half-a-second, the max power frequency voltage shall not exceed the specified voltage magnitude for 1-min withstand.

 

[prc]

Yes.There is formula for converting time versus peak of voltage for different insulation systems of products. eg; For oil filled transformers if one minute value is 1.0, for 30 minutes it is 0.85, one hour 0.82, and for 1.2/50 microsecond 2.65 etc.

 

[Power0020]

@prc, can you please point out a reference for the formula?

 

[prc]

There are some IEE papers on the topic. I shall search.

 

[prc]

1) Electra No.179 August 1998,Pages 39-49 " Temporary overvoltage withstand characteristics of FHV Equipment" Paper by CIGRE WG 33.10

2)CIGRE 1990-Papers 33-210 "Temporary over voltages :Causes, Effects and Evaluation" & 33-209 "Temporary over voltages and their influence upon the insulation level of the equipment"

3) TOV for Power Transformers as per (1) -

10 minutes- 1.13 pu
1Minute - 1.23 pu
0.1Minute - 1.42 pu
0.01 Minute -1.75pu

 

[cuky2000]

Some researchers published data for the relationship for more than 1 minute as can see in the enclosed link. . However, I do concur with RRaghunath that for insulators there is no known formula for less than 1 minute as prescribed by the standard for testing insulators at low frequency.

There is no known accepted procedure to certify the withstand rating of any insulator for less than 1 min at the power frequency values. Even if there is a possible correlation between the time-voltage applied, there is a long tradition (more than 100 years) using this norm for testing insulators at 1 min duration. There is not much motivation for research to find a reduced time test alternative.

_{NOTE: It should be noted that although the standard recommends for insulator 1-minute low-frequency test (wet & dry) for many components and apparatus, the objectives are quite different. For instance, a flashover for insulator in transmission line and substation are considered self-restored from an insulation coordination perspective, this is not the case for transformers.}