capacitor operations

[zaidi289]

can any one explain me what will happen when i used capacitor for improving my lagging power factor during running process of induction motors capacitor operates but when machines shut down capacitor should aslo be shuted but what will happen if all capacitor will remain on even the plant is in running or shuted i know it will be capacitve load but does it effect my utility bills?

 

[davidbeach]

It depends. Whether or not it affects you utility bill depends on the utility tariff. Sometimes/some places it will, others it won't.

 

[zaidi289]

for load of 800 kw 600 kvar but what will happen if 1100 kvar will operate

 

[jraef]

What you are talking about is called "Bulk Correction" because you are correcting the total kVAR load with one system. The danger is that if the inductive loads go away, and you do not remove some of the PFC capacitance, then you over correct, which can raise your system voltage, cause resonance with other equipment that has capacitors, and in some cases, as davidbeach alluded to, end up costing you more in tariffs. But as he said, that last issue is totally dependent upon your local utility. Some change just as much penalty for over correcting as for under correcting.

A much better approach is what is called "At Load" PFC correction, where you mount the caps close to the motor so that they only come on when the motor is on. You must be careful with that on motors controlled by soft starters, they need separate capacitor control contactors, but that's not too difficult.

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[MikiBg]

Capacitor banks with load disconnected on inductive impedance (transformer+cable) can cause negative voltage drop, i.e. voltage raise that jraef mentioned. Even if it is within tolerance, that means shorter lifetime for insulation, and possible changes in motor current.
Another benefit of "At load" correction is the fact that reactive current won't flow upstream the motor cubicle, while with "Bulk correction" it will flow through all cables downstream of PCC (where bulk capacitor is connected), hence causing greater voltage drops and losses, as well as reduction in capacities. Why there is a bulk correction at all? Because it is usually cheaper than "at load" for same amount of total kvar installed.
Another important aspect is fixed or automatic correction choice. For variable loads there is a greater chance to over compensate with fixed banks connected.

 

[zaidi289]

another thing is that i have read in a book that "capacitor and inductor does not consumed real power means kilowatt actually all loads fall in these two extremes" so whats the reality of that.is it true and if it is true than how much load fall in between the capacitor and inductor.