P.F Measurement

How P.F is measured?

Mathematically, the power factor can be defined as

Power factor = KW / (V x I x 1000), where KW = Power, V = voltage, C = current

or

Power factor = cosφ, where φ is the angle between Current and Voltage

So, there are two ways, the power factor can be measured:

(1) Measure the KW, voltage (V) and current (I). Then calculate the power factor from the formula

(2) Measure the angle difference (φ) between the current and voltage. Then calculate power factor from the formula

The power factor measuring device also follows the same calculation. For more detail see the 'Measuring power factor' section from the wikipedia http://en.wikipedia.org/wiki/Power_factor

Power factor is a term related to the load. If you measure the power factor at the incoming of the 'LV panels in large industrial units', you measure the average power factor of the loads connected to the panel.

I am really confused about the logic of applying capacitor stages?

Capacitor is used for improving the power factor but the required KVAR value of the capacitor depends on the power factor and amount of loads. Since the load of a system fluctuates, if you use capacitor of fixed KVAR, it may happen that the KVAR is not enough at the peak load and larger than the required at off-peak load. So, the capacitor banks are generally divided in stages in the PFI and the number of stages is automatically connected or disconnected based on the power factor value. If power factor is less than the preset value, additional stage is connected or if it is more than the set value, the connected stage is automatically disconnected. The ultimate goal is to keep the power factor within a predefined range.

Why in Large industrial units (in LV) P.F is normally kept up to 0.95

For large industries, the utility company generally charge a penalty if the power factor is kept below a certain value which is generally in the range of 0.95. So, if the industries maintain the power factor equal or above this value (0.95), they can avoid the penalty due to low power factor.

See these previous threads:

http://cr4.globalspec.com/thread/54893

http://cr4.globalspec.com/thread/54230

http://cr4.globalspec.com/thread/56760

- MS