Increasing Capacity of a Transformer

INCREASING TRANSFORMER CAPACITY

When electric power is supplied to a locality, city or an area, transformers that are capable of handling the required power demand ,are installed. These power transformers are rated in MVAs or KVAs .

The capacity for a given transformer, refers to how much electrical load can be put on this transformer. Say for a 230 / 12 volts transformer having 60 VA capacity ( Single phase) ,the maximum load can be 5 Amps on 12 V side. If the load is exceeded, the transformer will get overheated and may burn.

Based upon the load requirement, appropriate transformer size is chosen for supplying electricity to a region.

E.g.: A transformer rated 20 MVA can satisfy the electricity demand of an area with a load requirement of 20*0.8=16 MWatt. Where 0.8, is assumed to be the power factor. (Apparent power*power factor=Real power)

Similarly, if XYZ society has a power demand of 30 MWatt, then a power transformer of capacity 30/0.8= 37.5~40 MVA needs to be installed.

However, what if the load requirement of a particular region increases suddenly? This can happen during festivals like Deepawali. Then should we go for replacing the older transformer with another one, having a higher capacity? That would be odd ☹ …..It seems that power distribution companies have a way to solve this problem!

The temperature in power transformers is the most important limiting factor for their loading .If somehow we could limit the heating up of transformers, we can load the transformer more! 😊

Interesting ain’t it? But hold on, we cannot load transformer infinitely!!! There is a limitation. The winding hotspot temperature should not exceed a particular value(say 98 degree celcius).This hotspot temperature is specified in the datasheet of transformer.

Most power transformers are oil-immersed. Oil helps in cooling and also ,provides insulation medium between transformer winding and transformer tank. Transformer bodies also have a component called ‘Radiator’. These are projected from the main tank and increase the surface area for dissipating heat of the oil . Below is a picture of how radiator look like.

Here’s a nameplate of a Transformer manufactured by Schneider Electric:

We can see that the nameplate mentions two different types of Rated power:

For ONAN cooling type: 20 MVA

For ONAF cooling type: 25 MVA

So, the same transformer can be loaded more (uptil 25 MVA ) if we change its cooling method from ONAN type to ONAF type. 

Practically, there are 4 types of Transformer cooling methods:

ONAN: Oil Natural Air Natural (Causes least cooling)

By Air Natural method the generated heat in the transformer is cooled by the circulation of natural air. Natural cooling happens due to convection of oil fluid.

ONAF: Oil Natural Air Forced

In this method, the heat generated is cooled by the forced air circulation method. Fans and blowers near radiators help transformer to cool fast.

 OFAN: Oil forced Air Natural

In OFAN cooling system the oil is forced to circulate within the closed loop of transformer tank by means of oil pumps. Natural air cooling happens.

OFAF: Oil forced Air forced (causes best cooling)

The oil is forced to circulate within the closed loop of transformer tank by means of oil pumps. Also, fans are used for further cooling.

Now consider a transformer X. It can be operated at a maximum capacity of 16 MVA in ONAN, 20 MVA in ONAF and 25 MVA in OFAF cooling techniques. The maximum capacity at which it can run depends on how effectively we cool it!      

The same transformer can be used to supply even higher loads just by increasing the cooling.

Hence ,next time, we see the load shoot up, we  just switch on the fans near transformer radiator. This increases its capacity and now it could handle more power! 😊😊

BY SHIVANI KUMARI

www.linkedin.com/in/shivani-kumari-7b1990173