Usually, the power factor of power system lies between 0.8 and 0.9 lagging. However, sometimes its value falls below 0.8. In such cases it is desirable to take special steps to improve the power factor for economical reasons. In this article, we will discuss about power factor improvement equipments. This can be achieved by employing the following equipment :
- Static Capacitors
- Synchronous Condensers
- Phase Advancers
We know that static capacitor takes current which leads the applied voltage by nearly 900. Therefore, if capacitors is connected across an inductive load resultant quadrature component of current of the whole combination will be the difference of leading component of capacitor current and lagging component of load current. For 3-phase induction motor, the capacitors can be connected in delta or in star. However, practically capacitors are only connected in delta because in that case the size of each capacitor will be one third to that of the capacitors when they are connected in star. Static capacitors are invariably employed for power factor improvement in factories.Advantages
- They have small losses, hence no energy consumption.
- They require almost no maintenance.
- They can work under ordinary atmospheric conditions.
- They can easily installed as they are light and require no foundation.
- They have smaller life.
- They are damaged quickly when the voltage exceeds the rated value.
- The damaged capacitor can not be repaired.
A synchronous motor takes a leading current when over-excited and, therefore, behaves like a capacitor. An over-excited synchronous motor when run without any mechanical load and soley for the purpose of power factor correction is called synchronous condenser.
To improve the power factor an over-excited synchronous motor is connected in parallel with an industrial load operating at lagging power factor. The leading wattless component of current taken by synchronous motor partly or completely neutralizes the lagging wattless component of the original load current and thus overall power factor is improved.These method is used to improve the power factor of the load above 500kVA. Below this load static capacitor are more economical for the improvement of the power factor.
Synchronous condensers are used for the power factor improvement of transmission lines. Therefore, sysnchronous condensers are installed at the receiving sub-stations.
- By varying the field excitation, the magnitude of current drawn by the motor can be changed by any amount. This result, power factor can be controlled easily.
- The motor windings have high thermal stability to short circuit current.
- There are considerable power losses in the motor.
- The maintenance cost is high.
- Below 500kVA, it becomes costly.
- It produces noise.
Phase advancers are used to improve the power factor of induction motors. The low power factor of an induction motor is due to the fact that motor draws exciting current that lags behind the supply voltage by 900. If the exciting ampere turns can be provided from some other a.c. source, then the stator winding will be relieved of exciting current and power factor of the motor can be improved.
- The exciting ampere turns are supplied at the slip frequency, therefore, lagging kVAR drawn by the motor are considerably reduced.
- Phase advancer can be conveniently used where the use of synchronous motors is inappropriate.
- Phase advancers are not economical for motors below 200 H.P.