What is Power Factor?
Power factor(Cos ϕ) is ratio of active power(P = VI Cos ϕ) to apparent power(Q = VI). The active power is measured on volt-ampere and apparent power is measured in kilo volt-ampere. Power factor is the cosine of the angle between the applied voltage in volt and current in ampere flowing through a.c circuit.
Power factor improvement methods
The following three equipment are used for the improvement of power factor
1. Static capacitors
2. Synchronous condenser
3. Phase advancers
1. Static capacitors
The power factor can be improved by connecting the group of capacitors in parallel with the power system equipment which is running at a lagging power factor or low power factor. These capacitors are known as static capacitors.
The static capacitors draw the leading current from the source and cancel the lagging reactive component of the load current of the power system which is increased the power factor of the power system from lagging to leading.
In three-phase loads, the capacitors are connected in delta or star conditions for improvement of power factor. In big factories static capacitors are used for the improvement of power factors.
Static capacitors are a source of reactive power. The static capacitors are reduced the phase difference between the applied voltage and current flowing through the circuit.
Advantages of static capacitors:
1. The static capacitors have very low losses.
2. The static capacitors require very less maintenance because it does not have any rotating parts.
3. The static capacitors can easily be installed because static capacitors are light in weight and do not require any foundations.
4. The static capacitors are works in ordinary atmospheric conditions.
Disadvantages of Static capacitors:
1. The static capacitors are having a very short service life ranging from 8 to 10 years.
2. The static capacitors can easily be damaged if voltage increased from the rated value.
3. The repairing of static capacitors are not economical when its fails.
2. Synchronous Condenser
The synchronous condenser is nothing but a three-phase induction motor running at no load. When the synchronous motor's field winding is overexcited, it takes a leading current and the synchronous motor behave like a capacitor. An over-excited synchronous motor is known as a synchronous condenser.
The synchronous condenser is used in major substations which are supplying the major power. The synchronous condenser draws the lagging current from the supply system or supplies the reactive power to the supply system. For inductive load, Synchronous condensers are connected at the load side for improving the power factor.
The reactive power taken by the synchronous motor depends upon the field excitation and mechanical load delivered by the motor. When the load is zero and excitation is maximum, maximum leading power is taken by the synchronous motor.
Advantages of synchronous condenser:
1. In the synchronous condenser, by varying the field excitation of the field winding, the magnitude of reactive power drawn by the induction motor can be changed by any amount which helps in achieving step-less control( Power factor improvement in case of static capacitors can be achieved by step by step switching the capacitors in a grouping. But in the case of a synchronous condenser any amount of capacitive reactance can be provided by changing excitation of field winding) of power factor.
2. The synchronous windings have higher thermal stability for short circuit currents.
3. In case of synchronous condenser fault can be removed easily.
4. Synchronous condenser supply as well as absorbs the reactive power.
Disadvantages of Synchronous condenser:
1. In the synchronous condenser, there more losses take place in the motor.
2. In synchronous condenser maintenance cost is very high.
3. The synchronous condenser produces a very higher noise.
4. The synchronous condenser size above the 500kva, the cost is very larger as compared to the static capacitor of the same size.
5. The synchronous motor does not have any self-starting. so other equipments are required for starting the synchronous motor.
3. Phase advancers
Phase advancers are used for supplying the reactive power and improving the power factor of the three-phase or single-phase induction motor. The reason for the low power factor in induction motor is: stator winding of induction motor draws the excitation current which lags behind the supply voltage by 90 degrees.
If the excitation current of the stator winding is provided from other a.c sources then the stator windings are relived from the exciting current and the power factor of the motor is improved. This work can be done by the phase advancer which is one type of a.c exciter.
The phase advancer is placed on the same shaft of the main induction motor and the phase advancer is connected with the rotor circuit. The phase advancer provides the excitation amper turn to the rotor circuit of the induction motor at slip frequency.
By placing a more number of excitation ampere-turns than required to stator winding of induction motor, it is running at the leading power factor as the over-excited synchronous motor runs.
Advantages of Phase advancer:
1. In the induction motor, exciting ampere-turns are provided at slip frequency. so lagging Kvar drawn by the motor is reduced.
2. The phase advancer is used where the use of a synchronous motor is not admissible.
Disadvantages of phase advancer:
1. The phase advancer is not economical for the motor with less than 200 horsepower.
Advantages of power factor improvement:
1. Due to the improved power factor, the kilowatt capacity of prime movers are improved.
2. The improved power factor increased the KW capacity of transmission lines and transformers.
3. The improved power factor increased the efficiency of the power system.
4. Due to improved power factor, cost per unit reduced.
5. The improved power factor increased voltage regulation of the power system.
6. The improved power factor reduced the load current so power losses are reduced.