1,Real power(P) measured in watts
2.Apparent power(S) measures in volt amperes
3.Reactive power Q measured in reactive volt
Definition of power factor :
In electrical engineering, the power factor of an AC electrical power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit.It refers to the fraction of total power (apparent power) which is utilized to do the useful work called active power.
- Low power factor results when KW is small in relation to KVA.
- The inductive loads causes large KVAR in the systems which includesTransformer , Induction motor, Induction generator (wind mill generators), High intensity discharge lightening. These inductive loads consume major portions of the power consumed in the industries.
- Reactive power (KVAR) required by the inductive load increases the amount of the apparent power (KVA) in the distribution system. This increase in the reactive and the apparent power results in the large angle and thus the cosine (or power factor) increase.
- Real power is given by P = VIcosφ. The electrical current is inversely proportional to cosφ for transferring a given amount of power at a certain voltage. Hence higher the pf lower will be the current flowing. A small current flow requires a less cross-sectional area of conductors, and thus it saves conductors and money.
- From the above relation, we see having a poor power factor increases the current flowing in a conductor, and thus copper loss increases. A large voltage drop occurs in the alternator, electrical transformer, and transmission, and distribution lines – which gives very poor voltage regulation.
- The KVA rating of machines is also reduced by having a higher power factor, as per the formula:
Hence, the size and cost of the machine are also reduced.
This is why the electrical power factor should be maintained close to unity – it is significantly cheaper.
Disadvantages of low power factor
Let us consider, load as P is supplied at terminal voltage V and at power factor cosΦ by a 3-phase balanced system then load current is given by
Where P is the real power (watt) From the above expression for a given load, it is clear that if the power factor is low, the load current will be higher. The larger the load current due to low power factor results in the following effects
1) Effect on transmission lines: For the fixed active power to be transmitted over the line, the lower the power factor, the higher will be the load current to be carried by the line. Since the maximum permissible current density of the line conductor is fixed, the cross –sectional area of the conductor is to be increased in order to carry larger current. This results in an increased volume of the conductor material which in turn increases the capital cost of transmission lines.
2) Further, Increases in the current causes increase in the line losses with a reduction in the efficiency of the line. Also the line voltage regulation is poor.
3) Effect on transformer: A reduction in the current increase in the line losses with a reduction in the efficiency of the line.
4) Voltage regulation becomes poor at low power factor. Current at low lagging power factor causes a greater voltage drop in alternators, transformers, and transmission lines causing to have low power supply at the receiving end. To keep the receiving end voltage within permissible limits, extra equipment (i.e., voltage regulators) is required that increases the overall cost of the system.
5)Effect on switchgear and bus bar: The lower the power factor at which a given power is to be supplied, the larger is the cross –sectional area of the bus bar and the larger is the contact surface of the switchgear
6) Effect on generators: With a lower power factor, the KW capacity of agenerator is reduced. The power supplied by the exciter is increased. The generator copper losses are increased, which results in low efficiency of the generator.
7) Effect on prime movers: When the power factor is increased, the alternator develops more reactive KVA i.e. the reactive power generated is more. This requires a certain amount of power to be supplied by the prime mover. So, a part of prime mover capacity is idle and it represents a dead investment. The efficiency of the prime mover is reduced.
8) Effect on existing power system: For the same active power, the operation of an existing power system at a lower power factor necessitates the overloading of the equipment during full load.
Describe the range of power factor and meaning of lagging and leading power factor.
The power factor is defined as the ratio of the real power absorbed by the load to the apparent power
In case of perfectly sinusoidal waveform P,Q and S can be expressed as the vectors that form a vector triangle such that
If is the phase angle between the current and the voltage then the power factor is equal to the cosine of the angle
- Since the units are consistent, the power factor is by definition a dimensionless number between -1 to 1.
- When the power factor is 0, the energy flow is entirely reactive and the stored energy in the load returns to the source in each cycle.
- When the power factor is 1 all the energy supplied by the source is consumed by the load.
- power factors are usually stated as lagging or leading to show the sign of
- Capacitive loads are leading and the inductive loads are lagging.
1) Single phase capacitor start and capacitor run motor can be used as electric drives for better power factor
2)Three phase induction motor and transformer can be loaded to its higher load condition so that power factor at higher load is more
3) If load is shared by three phase induction motor and three phase synchronous motor then synchronous motor can be run in over excitation mode by increasing its excitation so that it runs with leading power factor and induction motor will run at lagging power factor, then overall power factor of the system improves
Q. Poor power factor reduces the handling capacity of the plant. Justify your answer.
Ans:
- Poor power factor reduces the handling capacity of all the elements of the system
- For low value of power factor (lagging) increases the KVAR i.e. reactive component i.e. reactive component of the system and hence full power is not utilized and hence power handling capacity reduces.
- All the above drawbacks of lower power factor suggest that P.F. must be improved at least up to a value of 0.8, 0.85.
- For the industrial consumers, the power supplying company insist on P.F. improvement. The power tariffs are revised to impose penalties if the P.F. is poor, lesser than 0.8.
- They are advised to install Power factor improvement devices.
