In electrical design, reactive power compensation cabinets are centrally installed on the low-voltage side of the power distribution room. The corresponding compensation installation capacity is marked on the drawings. However, due to the differences in the rated voltage of the capacitors, the access voltage at the compensation cabinet, and the reactance of the series reactors, the actual output capacity of the capacitor bank is not the installed capacity.

Example1.

Design Selection: The capacitor has a rated voltage of 480V and a rated capacity of 30kVar. When connected to a 400V voltage level system, the output capacity is: Qc = 30 × (400²/480²) = 21kVar

Example2.

Design Selection: The capacitor has a rated voltage of 480V and a rated capacity of 60kVar. After connecting the capacitor branch in series with a 7% reactance, it is connected to a 400V voltage level system. The compensation output capacity of the capacitor + reactance is: Qc + l = 60 × (400²/480²) / (1 - 7%) = 45kVar

Example3.

Design Selection: The reactive power compensation cabinet is designed with 8 branches. Each branch's capacitor has a rated voltage of 480V and a rated capacity of 50kVar. Each branch has a 7% reactor connected in series to suppress the 5th harmonic. Connected to a 400V voltage level system, what is the output capacity of the reactive power compensation cabinet?

Answer: The total installed capacity of the capacitors alone is:

Qc1 = 50kVar × 8 = 400kVar

The total output capacity of the reactive power compensation cabinet is:

Qc2 = 400kVar × (400² ÷ 480²) ÷ (1 - 7%) = 299kVar

Example4.

Design Selection: The reactive power compensation cabinet is designed with 8 branches. Each branch's capacitor has a rated voltage of 480V and a rated capacity of 50kVar. Each branch has a 14% reactor connected in series to suppress the third harmonic. Connected to a 400V voltage level system, what is the output capacity of the reactive power compensation cabinet?

Answer: The total installed capacity of the capacitors alone is:

Qc1 = 50kVar × 8 = 400kVar

The total output capacity of the reactive power compensation cabinet is:

Qc2 = 400kVar × (400² ÷ 480²) ÷ (1 - 14%) = 323 kVar

In summary, the reactive power capacity required by the power distribution system is a fluctuating value. In practical electrical design, it is recommended to calculate the reactive power compensation capacity based on a power factor of 0.95 during load calculations. This should be used to design and select the reactive power compensation cabinet to compensate for reactive power losses caused by the rated voltage of capacitors, the connection voltage at the compensation cabinet, and the reactance rate of series reactors. If there are stringent requirements for compensation capacity and harmonic control, it is recommended to select a reasonable reactive power compensation installation capacity after detailed calculations.