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Electrical question No.31 - Power factor correction
Is there any limitation for using of capacitor in electrical systems as power factor corrector?
Author : ALI - From: IRAN
 
#1
Tue, December 2, 2008 - 20:22
The electricity supply utilities generally specify a power factor of 0.8 to 0.9.
Compensation beyond cos ϕ= 1 (over-compensation Qc > Q1) must be avoided as this gives rise to capacitive reactive power which stresses the conductors in the same way as inductive reactive power, and in addition, unwelcome voltage increases can occur.

According to IEEE Std 1036-1992, the maximum bank size is influenced by the following factors:

a) Change in system voltage upon capacitor bank switching.
b) Switchgear continuous current limitations.

When a capacitor bank is energized or de-energized, the fundamental system voltage increases or decreases, respectively. In order to have a minimal effect upon customer loads, this voltage change is often limited to a value in the range of 2% to 3%. This voltage change (dV) can be estimated by the following formula:

dv=Mvar/MVAsc .%100

where
Mvar is the Mvar size of the capacitor bank
MVASC is the available three-phase short-circuit MVA at the capacitor bank location

The continuous-current rating of switchgear used for capacitor bank switching may be a factor in choosing the capacitor bank size. The rating is usually determined by multiplying the nominal capacitor current by 1.25 for ungrounded operation and by 1.35 for grounded-wye banks.

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In motor compensation, the motor and capacitor are connected in parallel. They are switched on and off by the same switching device and are supervised by the same protective system. No discharging device is needed. The capacitor discharges through the motor windings. The switchgear must be selected according to the capacitor making current, and the electrical connections according to the compensated full-load current of the motor. The capacitor should be located in the immediate vicinity of the motor. To avoid over-compensation at part-load and self-excitation of the motor as it runs down after disconnection, compensation should amount to only 90 % of the open-circuit reactive power. This will give cos ϕ ≈ 0.9 at full load, and roughly 0.95 to 0.98 at no load.

Also direct connection of a capacitor to a transformer, together with which it is switched on and off, is possible and permissible on both the HV and LV sides.
If the capacitor is fitted on the low-voltage side of the transformer, in the case of networks having a high harmonics content, it is necessary to check whether a voltage resonance at a harmonic present in the network (usually the 5th and 7th harmonic) can occur between the capacitance of the capacitor and the leakage inductance of the transformer. The maximum capacitor rating can be defined approximately as



where SrT is the transformer rated power in kVA, and Qc the capacitor rating in kvar, and ukr the rated impedance voltage (in per cent) of the transformer and the feeding network, and v is the number of the highest critical harmonic.
 
Author : Hamid - From: IRAN
 
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