Technical Articles
With increased emphasis on power quality and significant incentives / penalties associated with power factor improvement, most customers are trying to operate their plant electrical system at near unity power factor.
In order to achieve near unity power factor, all sources of reactive power need to be identified and fully compensated. One such element that consumes reactive power is the transformer. Reactive power is consumed by transformers through the no load magnetizing current and through the leakage reactance. This article is aimed at helping customers size capacitor banks for transformer compensation.
The power factor on the HT side (source side) of a transformer depends upon the following:
The load side power factor is compensated by employing APFC panels and/or by providing individual compensation to connected loads. Transformer compensation on the other hand needs a different approach.
The equivalent circuit of a transformer is as shown below:
Where: VP is the source voltage, VS is the load voltage, IO is the no load current, IW is the no load watt oss current, IM is the magnetizing current, RNL is the no load resistance, Lmag is the magnetizing inductance, Lleakage is the leakage inductance, RT is the winding resistance.
As can be seen from the equivalent circuit, the inductive elements, namely Lmag and Lleakage contribute to the VAR consumption of the transformer
The magnetizing kVAR consumption is a function of the rated voltage and the magnetizing current of the transformer. The no load current of a transformer varies between 0.5 % and 2.5% of the full load current depending on the design of the transformer and the operating flux level. The magnetizing current is around 80% of the no load current and thus varies between 0.4% and 2%. It is safe to assume a value of 1-1.2% for distribution transformers.
Thus the kVAR required to compensate for the magnetizing current of the transformer is around 1- 1.2% of the transformer kVA rating.
The kVAR requirement due to leakage reactance is a function of the square of the current and the leakage reactance. At full load the voltage drop across the leakage reactance is equal to the impedance voltage (%Z impedance). The reactive kVAR consumption is equal to the product of impedance voltage and load current.
Where IL is the load current and XLeakage is the leakage reactance; QX is the kVAr
Typically, for a 3 phase transformer,
Where V2 is the secondary voltage.
% Loading is assumed to be 50% to 75%. Thus for % Z=5%, QX works out to 5%* (75%)2 i.e. 3%
Thus the kVAr requirement to compensate for the leakage reactance of the transformer is around 3% of the kVA rating of the transformer.
The total kVAr required to compensate for the reactive power consumed by the transformer is around 4% to 4.25% of the kVA rating of the transformer.
Sourav Dasmodak,
Product Management & Marketing (Powergear - ACB)Product Owner of Air Circuit Breaker (ACB) of Lauritz Knudsen for Domestic & International Market. I can talk to you about Electrical Products' Sales, Business Development, Market Expansion, Cracking Critical Strategic Account, handling Key Account & of course how to develop & motivate Channels along with the organizational growth. Having near about one and a half decade of experience across the country with major electrical manufacturers (Top 4).
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