Transformer BIL vs Altitude

[Freshsurfer]

Hi, I need to calculate the BIL for a transformer that will work at 3000 meters over sea level.

According the tension class for this transformer the BIL at sea level is 250kV, how can I calculate the BIL at 3000 m?

 

[prc]

It will be reduced by 20 % ( Clause 16 of IEC60076-3 & IEEE C57.12.00)

 

[cuky2000]

Beware that only the external exposed bushing BIL will be derated by altitude. The winding BIL will not be impacted by the altitude.

 

[Freshsurfer]

In order to avoid the need to derate the BIL for the connection terminals of a transformer is possible to encapsulate its connections?

 

[PHovnanian]

"encapsulate its connections"

Yes. But then the BIL of the encapsulation would have to be considered.

 

[RRaghunath]

The requirement to derate is due to thinning of air at that altitude.
Hence, whichever insulation is exposed to air requires to be derated,
i.e. whether encapsulated or inside an air insulated cable box.

Rompicherla Raghunath

 

[Freshsurfer]

Thanks for your clarifications, there is another issue, is possible to attend a required BIL by installing a surge arrester?, we have a transformer with 60kV of BIL on a system where a 75kV of BIL is required?

 

[HamburgerHelper]

If it is exposed connections, I am guessing it is just the minimum spacing between phases or phases to something grounded?


Looking at a Siemens document, I am surprised that thermal derating is more as you go up in altitude.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[Freshsurfer]

Yes HamburgerHelper, the distances between the terminal connections were derated. Now I´m with another understanding about the impulse test to verify the BIL design value.

Why we perform the impulse test between each phase and ground and not between phases?

Since an impulse test is by example, a 60kV front wave applied between the each winding head and the ground for about 60us,

We cant state that on the case of a lighting discharge hits this transformer the place where an isolation would breaks will be inside the winding?

By the way based on this, we can state that on the external connections, the electrical distances between the transformer terminals are quite higher than those inside the winding's, so that, an applied voltage test on the transformer is going to be enough to test the equipment?

 

[prc]

Normally when transformers are designed for a higher altitude than 1000 metres, depending on altitude, a higher voltage class bushing will be provided, though transformer test voltage remains the same at factory. But if you are providing an oil filled cable box or plug-in type bushings, there is no need for extra margins for altitude.

Higher the altitude, air density comes down reducing the effectiveness of cooling by convection, main mode of transformer cooling. So to take care of the margins specified in standards, a lower temperature rise will be ensured at factory.

An impulse test will check both internal and external insulation of transformer for an incoming surge from the grid. Theoretically you can use a surge arrester of lower cut off value to allow you to use a transformer of lower BIL. To my knowledge utilities never go for that.

 

[Freshsurfer]

How can I select a right surge arrester to protect the transformer against a 75kV impulse on 4.16kV nominal voltage system?