Transformers - Their Losses & Reduction

 by

Milind Chittawar & G. Hari Kumar

Transformers are used in industries to reduce the voltage level as per the plant's requirement. As the life of the transformer is very high, care should be taken about the transformer designed losses before purchasing and installing at the plant premises.

 The operation of a transformer is based on 'Faraday's Law of Induction'. By the right choice of power level and location it is possible to establish an optimal use of transformers. In contrast to the ideal transformer, real transformer has losses. These are open circuit or iron losses and short circuit or copper losses. 

The open circuit losses are independent of the power, which is consumed by the transformer. These losses can be reduced by better design & improved material of construction i.e. raising the cross section of limb or using novel ferromagnetic material. The short circuit losses increase with the square of power. They can only be lowered by increasing the cross section of the conductors of the windings (i.e. by reducing the resistance).

 Normally open circuit losses are of the order of 0.2 to 0.5 % of the nominal power, whereas the short-circuit losses are 0.7 to 2.1% of the load on the transformer. In practice, transformers does not always operate at full load, it is some times more important to decrease the open circuit losses than short circuit losses.

 With the help of transformer load audit, it is possible to provide better quality of power to different load centers in the plant at high overall efficiency. Transformers should always be loaded optimally to get better efficiency. Maximum efficiency in Transformer occurs at a load, when the iron losses are equal to copper losses. Therefore proper load management of the transformers will lead to substantial savings in electricity bills.

 The % Loading, % Iron losses, % Copper Losses and All day Efficiency of a transformer can be calculated as follows:

 % Loading = Primary side Voltage (V) x Primary side Current / KVA of Transformer

 % Iron Loss = % Loading x Designed Iron loss in KW

 % Copper Loss = (1/% Loading) x Designed Copper loss in KW

 All Day Efficiency  =  [1 - (% Iron Loss + % Copper Loss) / ((PF x 100) + % Iron Loss + % Cu loss)] x 100

Where PF = Power Factor

 The following aspects can be helpful for energy savings in Transformers:

 It is recommended to quantify iron losses and Copper losses at various loading (say 20%, 40%, 60%, 80% & 100%) in terms of KWhr/day & Rs. per day. The transformer test reports can be referred for this purpose.

Primary side of the transformers can be switched off on holidays or periods of no load to save iron losses. Separate lighting transformer helps to switch off main transformer.

 Transformers should be always loaded to optimum level (load at which its efficiency is maximum). In case of multiple transformer and low loads feasibility to switch all the loads on some/a transformer for lowest total losses should be worked out (see case studies).

Transformers should not be oversized.

 While iron losses of a given transformer cannot be reduced, the load losses in transformers and cable losses can be reduced with demand control and with improvement of load power factor.

Amorphous core distribution transformers are more energy efficient than transformers made with silicon iron cores. However amorphous core type transformer are available for lower ratings only

By improving power factor of the transformer, the reactive power requirement of the plant can be reduced and hence the energy bills.

 Case studies on Transformer Load Management

 Case Study 1:

 The following is the case study where optimal load management of two transformers has lead to considerable savings: 

 As Load on Transformer 2 is lowest and the value is 24.75 %, by Transferring the total load on Transformer 2 to Transformer 1, 7884 Units can be saved per annum.

 Case Study 2 :

 A plant contains single transformer of 1000 KVA and is loaded with a 1000 KW load. The operating parameters of this are:

 % Loading = 90 %

Total Losses of Transformer = 13.6 KW

All day Efficiency = 98.6 %

By installing another 1000 KVA transformer and sharing the load equally on two transformers, the operating parameters are as follows:

 % Loading  = 50 % on each transformer

Total Losses for two transformers = 9.2 KW(4.6 KW + 4.6 KW)

All day Efficiency = 99.0 %

Savings in Losses = 13.6 - 9.2 = 4.4 KW.

 As the load is remains constant, the annual savings are 38544 KWh.

 Case Study 3:

 A steel plant contains three transformers of 1600 KVA each and are loaded with 43%, 43 % and 18% loading.  It is recommended to shift the total 18 % load on one transformer to second transformer. The losses before load management were:

 No Load Loss = 2 x 4.5 KW = 9 KW

Full Load Loss = 2 x 18 = 36 KW

Total Energy Losses = 88469 Kwhr/Yr 

Losses after load management are:

 No Load Loss = 4.5 KW

Full Load Loss =18KW

Total Energy Losses = 55907 Kwhr/Yr

 Savings in Energy losses = 32600 KWhr/Yr with out any investment.

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