Power Factor Improvement Equipment for a Plant

The typical range of the power factor of the whole load on a large generating station is in the region of 0.8 to 0.9. However, sometimes it is lower and in such circumstances it is generally desirable to take special steps to improve the power factor. This can be achieved by using the following equipment:

·     Static capacitors

·     Synchronous condenser

·     Phase advancers

Static Capacitors

The power factor can be improved by connecting capacitors in parallel with the equipment operating at lagging power factor. The capacitor (generally termed to as static capacitor) draws a leading current and partly or completely neutralises the lagging reactive component of the load current. This consequently raises the power factor of the load. For three-phase loads, the capacitors can be connected in delta or star as illustrated in the figure below:

Static capacitors are often used for power factor improvement in factories.

Advantages of using Static Capacitors for Power Factor Improvement

·     They require little maintenance as there are no rotating parts.

·     They can be easily installed as they are light and require no foundation.

·     They have low losses.

·     They can work under ordinary atmospheric conditions.

Disadvantages of Static Capacitors

·     They are easily damaged if the voltage exceeds the rated value.

·     They have short service life ranging from 8 to 10 years.

·     Once the capacitors are damaged, their repair is uneconomical.

Synchronous Condenser

A synchronous motor takes a leading current when over-excited and therefore, behaves like a capacitor. An over-excited synchronous motor running on no load is known as synchronous condenser. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralises the lagging reactive component of the load. Hence the power factor is improved. This is illustrated in the figure below:

The 3Φ load takes current IL at low lagging power factor cos ΦL. The synchronous condenser takes a current Im which leads the voltage by an angle Φm. The resultant current I is the phasor sum of Im and IL and lags behind the voltage by an angle Φ. From the diagram above, it is clear that Φ is less than ΦL so that cos Φ is greater than cos ΦL. Therefore, the power factor is increased from cos ΦL to cos Φ. Note the reactive power taken by a synchronous motor depends upon the dc field excitation and the mechanical load delivered by the motor. The maximum leading power is taken by a synchronous motor with maximum excitation and no load. Synchronous condensers are typically employed at major bulk supply substations for power factor improvement.

Advantages of Using Synchronous Condenser Technique

·     The varying field excitation, the magnitude of current drawn by the motor can be changed by any amount. This helps in achieving step-less control of power factor, which is in contrast to static capacitors, where the power factor improvement, can only be done in steps by switching the capacitors in various groupings.

·     The motor windings have high thermal stability to short circuit currents.

Disadvantages

·     The maintenance cost is high.

·     There are considerable losses in the motor.

·     It produces noise.

·     As synchronous motor has no self-starting torque, therefore, auxiliary equipment has to be provided for this purpose.

·     With the exception of sizes above 500 kVA, the cost is greater than that of static capacitors of the same rating.

Also Read: Types of Busbar Arrangements in Grid Stations and Substations

Phase Advancers

The phase advancers are used to improve the power factor of induction motors. The low power factor in an induction motor is a result of the fact that its stator winding draws exciting current which lags behind the supply voltage by 90°. If the exciting ampere turns can be provided from some other ac source, then the stator winding will be relieved of exciting current and the power factor of the motor can be improved. This is achieved by the phase advancer which is simply an ac exciter. The phase advancer is mounted on the same shaft as the main motor and is connected in the rotor circuit of the motor. It provides exciting amperes turns to the rotor circuit at slip frequency. By providing more ampere turns than needed, the induction motor can be made to operate on leading power factor like an over-excited synchronous motor.

Advantages of Phase Advancers

·     Since the exciting ampere turns are supplied at slip frequency, hence, lagging kVAR drawn by the motor are considerably reduced.

·     Phase advancers can be conveniently used where the use of synchronous motors is not allowed.

Disadvantage of Phase Advancers

·     They are not economical for motors below 200 H.P.

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