Excessive Reactive Power- Electrical Engineering

1) overexcited synchronous generators 2) synchronous condensers 3) excessive capacitance 4) improperly set transformer taps

In discussions on lagging and leading power factor, no one could tell me exactly what happens when the power factor goes from lagging to leading. I believe usual practise is to correct to 95% rather than unity. Is there really a problem in going to a leading power factor?

G Scott

Yes.

Read post #3.

You will only see an advantage if you get the power factor as close to 100% as possible. If you have an inductive load you need to keep adding capacitors to balance the load. If you add too many capacitors the power factor will go past the 100% mark and start reducing again.

The diagram at the right indicates a voltage (shown in red), a leading current (shown in green) and a lagging current (shown in blue).

The power you get is

P = vi cos Ø

If you have an inductive load like the blue current you need to add capacitors to bring the current in phase with the voltage. If however you add too much capacitance the angle of the current will continue to advance until you end up with something like the green current.

As you can see once the current is in phase with the voltage and Ø becomes 0 cos Ø will be 1 and the power factor will be 100% .

Adding capacitors to correct an inductive load is an expensive exercise so there will be a point of maximum return, usually when the power factor is around 95%. Nearly all loads are inductive but if you have a capacitive load then the reverse applies and you will need to add some sort of inductive load to bring the phase of the current back in line with the voltage. To have a capacitive load is however rare and even so if you over correct you will end up going past the in phase angle and going backwards.

There are a number of other situations that may influence the power factor.

You state the power factor is poor when load flow is conducted.

If you mean when the machinery is "turned on" you should consider if you have a large number of lightly loaded AC machines. They will draw their excitation current but the load (torque) percentage will be low, hence a lagging power factor.

If you have variable speed drives the AC drives should have near unity power factor.

However, If they are converting the AC to DC to drive DC machines the power factor is loosely equal to the output voltage, ie: half volts, half speed, 0.5 power factor.

This is a typical situation with Paper machine drives. The machine is sometimes built for 5000 fpm and then run at 2500 fpm for a particular product. Under that condition the paper machine power factor is very close to 0.5.

Adding capacitors to the AC supply side for the power converters will probably void your warranty and destroy the SCR's. The capacitors make the system very "stiff" and will source a high dI/dt that may rupture the devices.

Correcting to 1.0 power factor usually is not cost effective. If you can get it to .95 the utilities usually do not penalize you.

Over compensating your system can actually cause its own set of problems if the leading power factor is by capacitors. The caps can source very high currents for short times and may start damaging contacts in starters etc. They can also create a tuned LC circuit and oscillate as transients are applied (from variable speed drives, dropping loads on and of the system etc.) which in turn may create transient overvoltage conditions. If you have automatic capacitor switching there can be some interesting transients generated as the caps come on and off line.

If you are serious about power factor correction you need to enlist the services of a specialist familiar with the techniques and pitfalls with them.

Most electrical machines produce power by using magnetic devices in their construction (motors, coils, chokes, inductors, etc). These all cause 'lagging' currents.

What this means, is that the maximum sinusoidal current occurs slightly later than the maximum sinusoidal voltage. The difference is the phase angle - Ø. The cosine of which is equal to the power factor. The biggest phase angle occurs when magnetic machines run on light load. Thus the power factor improves as machines are loaded up.

The power factor can be corrected (but is quite costly as has been explained by others - especially to make it exactly '1') but USERS do not bother because the 'reactive' part of the power is not normally paid for (it is not 'measured' by the meter). You only pay for V.I.cosØ.

Unfortunately the power station has to generate V.I - so you might have to pay a penalty for a permanently bad power factor if you are a large user.

The charge would be unusual because on a national scale there are sure to be users of synchronous motors (over-excited to produce a leading current) that average things out.

In Canada large users pay for KWH, a penalty for peek demand, and a penalty for power factor. They are all measured.

Small consumers as home owners and some light industry only have the KWH charge.

This is usually determined by your utility and the local price structure.