In general, the term Power Factor represents the ratio of power actually used to the power actually supplied and varies with the losses encountered in a particular system.

What are the causes of a low power factor?

All inductive circuits within a distribution system require current for the purpose of the excitation of magnetic fields. This applies to:

• Induction motors
• Transformers
• Induction furnaces
• Welding plant
• Induction regulators
• Fluorescent lighting
• High Bay Discharge lighting
• Solenoids
• Electric Clocks

All require excitation currents to establish the magnetic field necessary for the function of each item of plant. Magnetic fields are a fact of electrical life and must be lived with.

Disadvantages

The following are some of the disadvantages that occur because of the presence of inductive circuits. First of all, the calculation for energy required at the load is:

Watts (W) = Volts (V) x Amps (A) x Power Factor

Initially, as the power factor falls below unity the current in the system increases and in so doing causes the system
voltage to decrease with the following effects:

1. Lower voltage on lighting will result in reduced lumen output.
2. Induction motors will run at reduced speeds (increased slip) which will necessitate increased currents to meet the required loads.
3. Because of the increased currents the I2R power loss increases in cables and windings leading to overheating and consequent reduction in equipment life.
4. Capacities of contacts, switches, circuit breakers and fuses may be exceeded with reduction in working life.
5. Efficiency as a whole suffers because more of the input is absorbed in meeting losses.

Advantages of Improved Power Factor

1. Ensures that the rated voltage is applied to motors, lamps etc. to obtain optimum performance.
2. Decreased losses in circuits and cables.
3. Decreased losses in distribution transformers.
4. Ensure maximum power output of transformers is utilised and not used in making-up losses.
5. Enables existing transformers to carry additional load without overloading or the necessity of capital cost of new transformers.
6. To obtain the financial benefits which will result from lower maximum demand charges.

Simplified Graphical Presentation

Power factor is defined as the Cosine of the angle between W and VA shown above i.e. Cos (W/VA). Thus using trigonometry, for any two given figures, the rest can be worked out. So, if we know the consumption (kWh) and the reactive power (kVArh) we can calculate the Power Factor.

How to Improve Power Factor

How do we approach the problem of improving the power factor? The usual method of making a system capacitive is achieved by introducing static capacitors which consist of chlorinated diphenyl impregnated paper dielectric elements in a sealed case, into the circuits either in the load-source (i.e. in a Sub-Station) or adjacent to the inductive plant.

Due to the fact that these electrostatic capacitors take a leading current they can be used to compensate for the lagging currents of the inductive circuits. When connected in circuit the capacitors act as a reservoir for energy which can be interchanged between the dielectric field of the capacitor and the magnetising needs of the inductive plant.

Other methods of power factor correction include synchronous motors and synchronous condensers. Synchronous motors are excited by direct current and do not therefore impose a lagging current for magnetising purposes on the system. These machines are intended primarily for situations where constant speeds are necessary over a wide range of loads, but in addition are able to operate at power factors between unity and 0.8 leading. This feature enables the system generally to benefit and an improved power factor results. However, unless the speed control properties are essential, the high cost of these machines would, for power factor correction purposes only, be quite uneconomic. Synchronous condensers are used purely for situations where larger amounts of corrective kVAr are required and carry no mechanical load. These are not usually considered for normal industrial purposes.

Example:

If the consumption was 100,000 kWh and reactive consumption 65,000 kVArh, then, using the methodology earlier, the power factor will be 0.838. If we need to get this above 0.95 to prevent penalty charges, capacitance should be installed. This can be calculated by the installer.

Control of Capacitors

Control of static capacitor banks is carried out by means of contactor equipment which in turn is controlled by a sensing relay. Basically a single phase current and voltage supply is applied to the relay such that at unity power factor the current and voltage vectors are displaced by 90°.

Changes in this angular displacement either lagging or leading are sensed by the relay which then switches the contractors which connect the reactive kVAr required for correction of the power factor, in or out of the system. Control relays can be single or multi-stage depending on the extent of the capacitor equipment in use.

In a smaller installation the capacitor bank would probably be located adjacent to the incoming supply. In progressively larger installations the capacitors would be positioned at different load centres i.e. adjacent to distribution boards. In practice however, it is more economical to group capacitor banks together, using multi-stage control, possibly in a sub-station (which also reduces the possibility of interference to relays).

Individual correction should then be limited for example to motors of 37 kW and above. In this case control relays would not be required as capacitors would be switched in and out with the operation of the motor. Individual correction can also be an advantage with welding plant.

Level of Correction

Having established that a need for power factor correction exists, what level of correction should be applied?

This depends to a large extent on the geographical location of the plant in question. Whilst improvements in plant efficiency are desirable it is the savings that accrue from lower tariff/contract charges that invariably dictate whether or not an installation shall be carried out. The power factor below which penalty charges are applied varies according to the grid charging zone and is between 0.85 and 0.98.

In general the considered optimum lies between 0.95 and 0.98 lagging. At this level both factors of tariff and efficiency are covered.

Maintenance

With regard to maintenance, as there are no moving parts to a capacitor, very little can go wrong. Capacitors can be checked by measuring the current in each phase, which should be approximately the same as shown on the rating plate. The only other maintenance consists of cleaning capacitor bushings and checking that connections are tight.

Attention should be given to switchgear and contactor contacts approximately every three months to ensure that there is no pitting or burning. Circuit breakers, where fitted, should be operated at least once each week to keep contact surfaces clean.

Under all normal circumstances the reactive kVAr control relays require no maintenance, but if any doubt exists get the manufacturer to check.

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