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Over-correction of power factor


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Hello wangHL


Welcome to the forum.


It really depends on the conditions.

If you over correct using static correction (capacitors applied directly to the motor circuit and controleed by the motor starter) you are going to have major problems. The reactance of the capacitors and the reactance of the motor combine to create aresonant circuit. If you under correct, the resonant frequency is higher than line frequency. If you over correct, the frequency is less than line frequency.

If you switch OFF the motor (i.e. open the contactor) and the capacitors remain connected to the motor terminals, the motor will generate voltage as it slows down. The frequency is dependent on the motor speed. It wil pass through resonance and creat very high voltages. These high voltages will cause damage to the motor, switchgear and capacitors. Additionally, if the insulation breaks down with the high voltages, there can be severe torque transients generated also which can damage the equipment.


If you over correct with bulk correction, there is generally no problem provided that the supply is well loaded. The only negative is that the current is minimum at a correction of 1. As you increase the correction into a leading power factor, the current begins to rise again and you are not getting any benefit.


If the supply is lightly loaded and you over correct, you can get supply resonances which can generate high voltages and cause damage to equipment.


Power factor correction is only really beneficial under high load conditions, so it can be eliminated under light load conditions.


Best regards,

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  • 4 weeks later...


we use fixed (detuned) shunt capacitors with reactor (approx 4400Kvar) for PF correction in 25kv railway traction system.

as there are wide variations of load in traction application , i want to know consequences of over correction in pf or leading pf for some duration when system is not much loaded.


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Hello sumit


With induction motors, the magnetising current is essentially constant and independant of load. For this reason, it is usually OK to use a fixed capacitor for a motor. (static correction)


In your installation, I suspect that you are using some form of variable speed control and there will be rectifiers rather than induction motors connected to the supply. If this i9s the case, then there will be a poor power factor due to harmonics rather than inductive currents.


Power factor correction capacitors will improve displacement power factor only and will not improve distortion power factor.


Excessive capacitance across a supply can cause supply resonance issues when the load is light. Resonance will cause voltage surges triggered by transients.

If the load on the supply transformers is light, there will be low dampening of any supply resonance (power factor capacitors with supply inductance)


The problem with over correction of the supply is very different from over correction with static correction.

A poor leading power factor will draw the same current as lagging power factor so there is no advantage in over correcting.


Best regards,

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