Induction motor accounts for more than half of the industrial load and are responsible for the poor power factor. The power factor of an induction motor varies with the load on the motor. Lightly loaded motor has a very poor power factor. Poor power factor results in reduced capacity of transformers, cables and other equipments and are also responsible for higher power losses because of increased current flow. They also cause additional voltage drop.
Motivation for power factor improvement:
Utilities usually encourage the consumers to improve the power factor of their installation by incorporating power factor based penalty/rebate in the power tariff structure.
Figure shows the Electricity Bill of a Hotel for the month of Dec 2016. The premises, which is fed from 11 kV, is having an average power factor of 0.63 (as can be seen in the picture) for which the consumer has to pay a penalty of 28,759 INR ( a hefty amount indeed, for sheer casual approach)
Power factor may be billed as one, or a combination, of the following:
1. A penalty for power factor below a set limit. The penalty goes on increasing as the power factor goes down,
2. A rebate for keeping the power factor above a given value,
3. A charge on monthly Reactive power (kVAr), and
4. A flat charge for the maximum demand recorded during the billing period in terms of apparent power (kVA).
Power factor improvement using Static Capacitors:
Static capacitors are among one of the commonly used solutions to improve the power factor. They may be provided on each individual induction motor or on the main bus bar depending upon the consideration for cost, effectiveness, ease of operation etc.
Automatic Power Factor Controllers (APFC) are employed on the main bus bar and helps to improve the overall power factor of the establishment. Capacitor banks on networks with harmonics may produce resonance. So special care should be taken while installing capacitors on such circuits. The capacitance bank of the APFC is divided into steps because the load may vary during the day. If the capacitor bank rating is 200 kVAr and 8 steps are available in the controller, then the step size of the APFC is 25 kVAr. A simple APFC consists of similar step size.
Key elements of a small APFC:
Small APFCs are usually wall mounted. The key elements of any small APFC are:
1. Power Factor Controller,
2. Capacitor bank,
3. Auxiliary transformer,
5. Thermal cut off switch,
6. Fans for cooling,
7. Bus bars, and
8. Circuit Breaker (optional).
The Power Factor Controller provided inside the APFC automatically maintains the power factor (p.f) of the premises at a set value. It senses the power factor based on the input current and voltage. The controller has both auto and manual mode for switching the capacitor steps. In the auto mode, the controller automatically switches the steps to maintain the set power factor. In the manual mode, the operator is able to switch the steps. On failure of supply, all the capacitors are switched out. On restoration of power supply, the capacitors are switched on according to the load requirements. The controller has the following features:
1. The pre-set value for automatic p.f correction is adjustable between 0.9 lag to 0.95 lead.
2. Facility to manually set the power factor between 0.8 lag to 0.9 lead,
3. Adjustable step switching timing,
4. Indicators for capacitor steps connected, system p.f, set p.f, and other parameters.
The controller automatically cycles the capacitors on a first on/first off basis so as to ensure equal usage of each capacitor. The step switching time is provided to ensure a capacitance step/unit is discharged before its switching to reduce the inrush current.
APFC does not operate when the thermal cut off switch is ON. This switch is a temperature sensitive switch which shuts off the power supply to the APFC when the temperature inside the enclosure exceeds the pre-set temperature limit. In this case only the fans provided for cooling operates. As soon as the temperature becomes normal, the APFC unit again starts functioning.
To disconnect the APFC from the network, circuit breakers are preferred over fuses. A current transformer (CT) of proper specification has to be used so that it monitors both the load current and the APFC current. The CTs usually used have a secondary current rating of 1 or 5 A and burden of 15 VA. The rating for primary ratio is so selected that the CT can handle the total line current i.e. load and of APFC current along with the transients during starting. When connecting the CTs, the secondary of each CT should be short circuited otherwise the CT may be damaged.
APFC units are built for indoor operations and hence should be well protected from dust, moisture and temperature. The APFC unit should be kept in a well ventilated space.
Some of the checks while Installing an APFC:
As an electrical engineer engaged in installation of electrical equipments and substations, one has to make sure that the transportation of APFC was safe. This can be done by checking the packing of the unit. After removal of packing visually inspect the exterior and interior of the APFC. Any loss or damage during the transportation should be immediately reported to the manufacturer or their representative. The APFC unit comes with the instruction manual and wiring diagram which must be referred while installing the unit.
Careful handling is also necessary during the installation process also. The equipment has usually two nameplates, one outside and the second one inside the enclosure. These nameplates carry the necessary information like nominal voltage, frequency, APFC capacity etc which are very useful while installing.
All Automatic Power Factor Controllers (APFCs) are full factory tested. Separate earth connection should be provided to each APFC. All cables, wires and switchgear should be rated 1.5 times higher than the nominal current rating of the APFC and should be well coordinated with current rating of the back-up fuses.
Working with an APFC:
While working with electrical equipment, one has to be careful. The case is the same for APFC. Do not attempt to work on a live APFC. After switching off the power supply to the APFC, always wait for 5 minutes before working on the unit (by the time the power capacitor gets discharged). Always verify that the capacitors are discharged. AC capacitors are charged at a higher voltage than the nominal network voltage. Always wear protective hand gloves and eye-protection while working on electrical installations. To ensure safe access, the power supply of each individual APFC must be isolated before opening. Check for residual voltages, because of capacitor, before working inside the APFC.