power transformer sizing 1

[144x]

Hi,
my question is what happens if a power transformer is loaded most of the time close to half its full capacity and only for a very short period close to its full capacity. this happens in a design where a solar farm is going to be connected to grid (230 kV). should any special design be considered for transformer in these cases?
thanks.

 

[waross]

A little less terminal voltage at full load. Check the regulation specs for the transformer.
Greater voltage drop on feeders under heavier loading.
Is there any related equipment that is very voltage sensitive?

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

 

[144x]

Thanks waross, let's assume there is no voltage drop as it is injecting power to the grid.

 

[prc]

Nothing special in design. But if you want to save money in investment, you can specify the short time load as an over load on transformer. A similar application is railway trackside transformers. These are generally specified for 50 % overloading for 15 minutes and 100 % overloading for 5 minutes.

 

[144x]

thanks prc, is "IEEE C57.159-2016 – IEEE Guide on Transformers for Application in Distributed Photovoltaic (DPV) Power Generation Systems" is applicable for such large transformers also? I do not have the standard though.

 

[waross]

let's assume there is no voltage drop as it is injecting power to the grid.

Dangerous assumption.
Voltage mismatch may cause reactive current to flow.

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

 

[Mbrooke]

This is a really good question. If this was a pole or padmount unit I would undersize it since thermal inertia in the oil would keep the unit cool during overloads- and enough time to cool off during low/normal load levels. However, I have not heard of it being done on power units outside of LTE and STE contingency action ratings.


However I would imagine you could do the same as with a pole top unit. Physics would say it is (should be) possible.

 

[prc]

144x, The IEE standard, you mentioned, covers transformers that are connected to solar inverters. Since you mentioned 230 Kv transformers, I thought of collector transformers feeding to grid with primary connected to collecting line from solar transformers. Over loading can be done on all transformers, provided you specify requirements clearly during procuring stage.

 

[144x]

Thank you prc, if there is any limitation or requirement to be considered on the design of inverter transformers arising from irradiation variable or cyclic heating and cooling shouldn't these be mirrored or considered on the collector transformers as well ?

 

[bacon4life]

Are you asking about the 230 kV transformer or the medium voltage transformers? Is your load profile just the typical solar insolation curve? Or would the short term loading at 100% only be due to the failure of redundant components?

Early wind farms using standard transformers experienced higher than expected failure rates due to cyclic loading. I haven't seen the new IEEE guide, but I would expect your transformer specification to include provisions to deal with thermal expansion/contraction cycles. Thermal cycling can impact the winding/paper system from thermal stresses as well as increasing the amount of air moving through the oil preservation system. From an maintenance point of view, be sure to account for increased nitrogen/desiccant usage.