Due to various disadvantages of low power factor, the power supply companies insist on a power factor of 0.8 or above for industrial installations. To improve the power factor of an installation generally a capacitor bank is used. The power factor improvement using capacitor bank is very common in an industrial installation.
Most Economical Power Factor
The investment in power factor equipment is relatively high as the p.f. approaches to unity because capacitors of much larger rating are required for bringing about the same improvement in p.f. when it is near unity than when it is low.
In actual practice, it is quite economical to work with an average p.f. of 0.92 – 0.95, as the advantages accruing from higher p.f. are counterbalanced by the additional investment on capacitors.
Power Factor Improvement Using Capacitor Banks
The individual capacitor is known as a ‘unit’. The common ratings of capacitor units are 5, 10, 15 and 25 kVAR.
When a number of such ‘units’ are assembled in series, parallel, or series-parallel combination to achieve the desired current and voltage ratings are known as Capacitor Banks.
The three-phase capacitor banks can be connected in star grounded, star ungrounded and in delta arrangements.
The ungrounded star connection is generally used because protection is easier. In this method, the fault current in case of a fault in any unit in one phase is restricted by the capacitors in sound phases. So, we can use smaller fuses and less protection material. This facility is not available with star-grounded or delta connections.
Individual or Group Correction
Capacitors may be used for individual correction or group correction.
In individual correction method, p.f. of each appliance is corrected by a separate capacitor.
This method is suitable where the individual loads are heavy and scattered, or where there are a few large motors amongst a large number of small motors.
In a group correction method, the power factor of a number of machines is improved by a common capacitor bank. It is suitable in the following cases:
- Where load shifts extremely on different feeders.
- The voltage of individual motors may not be suitable for capacitors.
- Where a large number of small motors are installed.
Control of Capacitors
The common methods of control of capacitors are as under:
- By the motor control-gear in case of a capacitor connected directly across a motor.
- Manual control by switches or circuit-breakers.
- Automatic control.
Capacitors can be controlled by switches or circuit-breakers. Specially designed switches for control of capacitors should be used for this purpose only. The current rating of the switch must be at least twice the rated current of the capacitors.
In the case of manual control, it must always be ensured that the capacitor is switched on after the load and is switched off before the load, otherwise, due to over-compensation there will be a dangerous rise in voltage.
Automatic Control of Capacitors
In large installations, where the capacitors are connected in large groups, there is always a possibility of over-compensation on light loads.
The leading power factor creates same problems as that of lagging power factor, moreover, there is a possibility of a dangerous rise in voltage due to over-compensation.
Therefore, where the load does not follow any regular pattern, automatic control of capacitors becomes necessary. However, the simplest type of control should be adopted. The automatic control may be performed by:
- Voltage sensitive relay
- Current sensitive relay
- APFC relay
Where capacitors are used primarily for voltage improvement, voltage sensitive control should be used.
Current sensitive control is used where the voltage is regulated by some other method and p.f. is practically constant over wide range variations in load.
APFC relay (Automatic Power Factor Control Relay) control is used where p.f. varies considerably with variations in load.
Generally, APFC (Automatic Power Factor Control) panels are used in industries to control the power factor. In these panels, capacitors are connected and disconnected automatically with the help of APFC relay and capacitor duty contactors (CDC).
KVAR Calculation of Capacitor Bank to Improve Power Factor
kVAR rating of capacitor bank required for power factor improvement can be calculated by the method shown in the example below.
Example: An industrial consumer is operating 3-phase, 10 KW induction motor at a lagging p.f. of 0.8 and a source voltage of 400 Vrms. He wishes to raise the p.f. to 0.95 lagging.
Solution: Motor input P = 10 KW = 10000 W
Initial power factor (cos ø1) = 0.8 (lagging)
ø1 = cos-1 0.8 = 36.87o
Required power factor (cos ø2) = 0.95 (lagging)
ø2 = cos-1 95 = 18.195o
VAR rating of required capacitor will be
= P(tan ø1 – tan ø2)
= 10000(0.75 – 0.3287) = 4210.3 VAR = 4.21 kVAR
Shunt Capacitors Connected Directly to Motors
The capacitors may be connected directly across the terminals of a motor. In this case, the kVAR rating of the capacitors must not exceed the magnetizing kVAR of the motor, otherwise, a dangerously high voltage will be generated when the motor is coming to halt. Where a motor stops immediately after disconnection from supply, the kVAR rating of the capacitors may be exceeded.
Thanks for reading about power factor improvement using capacitor bank.