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Power factor

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Let's consider that primary voltage U1 is not distorted, since capacity of a supply network is much more than the converter one.

 

 

Figure 3.3

Then power factor we shall determine as

where P1=3U1PhI1(1)cosφ(1) - the active input power transmitted to the converter by supply-circuit;

S1=3U1I1 - a full input power transmitted to the converter by supply-circuit;

I1(1) – rms value of first harmonic of a primary current of the transformer;

φ(1) - a shift angle of primary current concerning a mains voltage.

Reactive input power by the converter is

Full power transmitted by a network is

where Т - distortion power.

Distortion factor is

,

Therefore always

.

φ(1) increases with a hence Q1 increases too and P1 decreases. This is a reason of variation a mains voltage. As a primary current curve is distorted the highest harmonics of primary current create a voltage drop on reactances of supply system and main voltage may be distorted. Distortion power T is not only useless but it creates additional load for power system lines and consumer’s equipment.

 

For dependent inverters a is greater than p/2 and Р1 is less than 0 but for accounts Р1>0 and are considered.

Always there is χ < 1.

ν increases with a number of phases of the rectifier, with a load, with γ and with ха.

Some methods of increase χ:

- The artificial commutation of thyristors is applied to increase cosφ(1). It is commutation of thyristors before natural commutation by way inclusion in a switching contour of additional sources of EMF.

- Inclusion of additional thyristors with the purpose of a current cutoff flowing when voltage across the load is negative.

- Asymmetrical controlling;

- For improvement ν use also equivalent multiphase circuits.

 


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