3.3.2 Power factor

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

i₁

i₁₍₁₎

I_d

2π

π

U₁

γ/2

γ

α

Figure 3.3

Then power factor we shall determine as

where P₁=3U_1PhI₁₍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  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  is greater than /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 х_а .

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.