Power transformer derating 1

[AntonioPerez]

According to IEEE standard and others a derating factor must be applied to consider the effect of worse ambient and site conditions (altitude, ambient temperature) over an oil immersed power transfomer. As I understand, to assure the transformer won't suffer any damage, the transformer protections shall be set for the incomming current to the transformer is limited to avoid a higher power than the derrated power. So the transformer won't never be able to supply the nominal power in case there is a derating factor.

I would appreciate your answers. Thanks in advance.

 

[edison123]

I would appreciate your answers.

Ok, what are your questions?

Muthu

http://www.edison.co.in

 

[AntonioPerez]

I would like to know if your understanding about this issue is the same as mine. I'm calculating the current for the incomming cable to the transformer and I need to know if I have to consider the nominal power or the derated power.

Thanks.

 

[cuky2000]

Transformer and ancillary equipment such as cooling systems, surge arresters, and bushings are subjected to be derated/uprated by the ambient temperature and altitude. The transformer output capacity will be impacted by both ambient temperature and altitude. The external dielectric strength (BIL & BSL) and associated external clearances are subjected to be impacted by the altitude due to the insulation reduction of the air at a lower density above the sea level.

The effectiveness of the ambient air to remove/carry away the heat from a transformer is reduced at higher altitudes due to air`s lower density at those elevations. Both ANSI & IEC provide a curve or an equation to determine the altitude derating factor. As a rule of thumb, the transformer capacity is reduced by 0.3% for every 100 meters are above 1000 meters.

The following curve is an interpretation of the transformer power rating capacity derating/uprating factor depending upon the 24 Hr. average ambient temperature. It should be noticed that for short term rating there is directive in the standard for safely overload the transformer above the nominal nameplate rating of the unit.

 

[jghrist]

Typically, overcurrent protection of power transformers is not used for tripping on overloads. Overload protection is achieved by monitoring loads. Transformers can withstand short time overloads without loss of life because of the long thermal time constant. You can use overcurrent to alarm for overloads, but tripping for overloads would normally be manual or based on temperature. Monitoring loads will allow manual reduction of loads.

Overcurrent relays or fuses are normally used to keep time of operation for fault currents below the transformer damage time-current curve.

 

[waross]

I suggest using the normal power rating for the cable ampacity.
Many of the factors influencing transformer derating may also apply to the cable ampacities. In both cases the ability to reject heat is a major factor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

Derating can also go the other way in cold temperature climates. A Generator Step Up Transformer connected to a gas turbine driven generator might operate 10% or more above its nominal rating in cold weather. The cooler, denser air also lets the turbine produce more MW. If that happens, the cables better be designed to carry the additional load. If they are underground cables the cooler air will not have much of an impact on cable rating.

 

[cuky2000]

Hi rcwilson,

This is a good point. I concur with you that for small generator units with underground feeders, the temperature of the earth, a few feets below-grade, remain relatively stable in all seasons.

For a large generator, could be a significant economy sizing the GSU, generator circuit breaker and even the Isophase bus using this approach. A little more challenger will be required to apply this concept to a steam turbine generator units.

For a hydropower transformer with a water heat exchanger, the ambient temperature may not be as critical as a transformer with an air-cooled radiator because of the relatively constant water temperature.

Should be noted that altitude does not impact the dielectric rating of the transformer winding since this is not exposed to the air

 

[prc]

IEE Standard on Transformers are based on a maximum ambient air temperature of 40C and a 24 hour average ambient temperature of 30C. A maximum oil temp rise & maximum average winding temperature rise of 65C and a max winding hot spot temperature rise of 80C are specified for these conditions. In case your ambient conditions are within above (40&30C ) no derating is required. It is assumed that any excess ageing occurring ( with 30C as reference) at 40C is compensated when unit is working at 20C or lower temperatures. But if at site, any of these (30& 40C) are exceeded, then oil and winding rises are to be reduced to the same extent ie rating has to reduced or cooler size to be increased.

Transformers are normally designed for operation up to an altitude of 1000 metres. In case you are shifting the unit to a higher altitude, check with manufacturer. MVA rating has to de de-rated and bushings are also to be changed. This is because the external BIL get reduced at higher altitudes.