Reactive and Under Voltage Relation?

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Reactive and Under Voltage

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Dear Friend,

You draw the VECTOR DIAGRAM, OR REFER the VECTOR DIAGRAM. It is simple to understand.

DHAYANANDHAN.S

In practical power systems, a busbar can be represented by an fixed voltage source of zero impedance in series with an inductor and a resistor, which is the simple equivalent circuit of a transformer. Rough typical values for small distribution transformers 9a few 100s of kVA) would be inductor voltage drop = 0.04, resistor voltage drop = 0.01 at rated current with a busbar voltage of 1.0 (all these values are times rated busbar voltage). The regulation curve is drawn below..

The curve is drawn for full rated current.

Consider full rated current with 0.8 power factor lagging, the regulation is about 3.0 % (it would be about -1.5%, voltage increase, with 0.8 leading power factor). In both cases, the active current is 0.8 x rated, while the reactive current is 0.6 x rated.

For the lagging case....

Take away the active current, it leaves 0.6 x rated current at 0 p.f. lagging - the regulation will be 4% x 0.6 = 2.4%, since regulation is proportional to current. That is only 3% to 2.4% change not very much, 0.6%.

Now try taking away the lagging current, this leaves 0.8 x rated at unity power factor. The regulation will be, from graph, 1% x 0.8 = 0.8% - a 2.2% change from 3%.

So taking away 80% rated active current has affected the voltage by about 1/4 of the amount you get by taking away 60% rated current reactive 90 degree lag.

You can do similar workings for leading power factor.

The conclusion is that changing reactive current affects busbar voltage much more than the same change in active current. Consequently, changes of active current do affect busbar voltage but they are of much less concern than reactive current changes.

Note that if the power factor stayed at unity, the voltage regulation would be very good, <1%, even though the inductive reactance is 4%. As transformers get bigger, reactance % and regulation get bigger, to about 12% or more, but the resistive % gets smaller - the predominant effect of reactive current over active current upon regulation increases.

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