Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 70

The full range of load regulation of a power transformer is the change in secondary voltage Vs, expressed in percentage of rated Vs for a specified power factor. This occurs when the rated MVA output at a specified power factor is reduced to zero, with Vp maintained constant. One can use these approximate formulae to calculate ΔV at any operating load and power factor:

(2.6)

For instance: Transformer: 30 MVA as a MVA_b = 1 pu, having Z = 9% (0.09 pu) impedance

on 30 MVA base, efficiency 99.5% at rated load.

 Total loss = (1 − 0.995) × 30 MVA = 150 kW. The total loss includes no load and load losses.

With this information, we can calculate transformer resistance R, followed by calculating the reactance X. After that, we calculate the voltage regulation.

Transformer resistance R is calculated from the load losses. Since we do not have that figure we will assume that the load loss is equal to 80% of total loss. The rest is the no load loss.

Assume operating load of 25 MVA (MVA load pu = 0.833 pu) at power factor pf = 0.85.

 R or Load loss in % = 0.8 × 100 = 0.4%. Z is given as 9%

 

 Pload = MVApu × cos ϕ = 0.833 × 0.85 = 0.71 pu

 Qload = MVApu × sin ϕ = 0.833 × 0.52 = 0.43 pu

 ΔVpu = (0.004 × 0.71 + 0.0899 × 0.43) = 0.0414 pu or 4.1 % . From Eq. (2.6).