What is Reactive power?
The reactive power moves between the power source and the load connected to the circuit. The reactive power can not be doing any useful work in the power system. so this reactive power is called useless power. The unit of reactive power is volt amper reactive(VAR).
Active power is useful to power which is supplied to run an induction motor, heat water, illuminate the electric bulbs. The reactive power provides very important functions for regulating the supply voltage.
The reactive power moves between the power source and the load connected to the circuit. The reactive power can not be doing any useful work in the power system. so this reactive power is called useless power. The unit of reactive power is volt amper reactive(VAR).
Reactive power is the imaginary power or apparent power that does not do any useful work but moves back and forth in transmission lines. Reactive power is produced from inductive and capacitive load and it is a byproduct of AC systems. Reactive power exists only when there is some phase displacement between the voltage and current in the power system. Reactive power is a product of voltage, current and sine of the angle between the voltage and current.
Synchronous generators are generating or absorb reactive power depending upon the DC excitation to its fields windings. When the field winding of the synchronous machine is overexcited it generates the reactive power. When the field winding of the synchronous machine is under excited its absorbs the reactive power.
The capacitive compensator generates the reactive power and the inductive compensator absorbs the reactive power. so Inductive and capacitive devices are used in series and shunt form to regulate the system voltage and stability.
Series capacitors are mostly used in transmission lines for generating reactive power went needed. Shunt capacitors are used in loaded substations to generate reactive power and keep the system voltage within the limits. Shunt reactors are used to absorb the reactive power and keep the system voltage down.
What is the use of reactive power?
The importance of reactive power is given below
1. Voltage control
2. To satisfy the reactive power demand
3. To reduce electric blackout
4. To produce magnetic flux
1. Voltage control:
The electrical equipment is designed to operate satisfactorily within the limits of rated voltage ±6 per cent at the consumer terminals. The increase and decrease in rated voltage take place due to the increase and decrease in the load in the power system.
If the load in the power system increase, then the voltage drop in the system increase which decreases the voltage at consumer terminals. Due to decrease in voltage which it affects the performance of equipment connected at consumer end like lamps, induction motors etc.
This increase and decrease in rated voltage at the consumer terminal are maintained by providing voltage control equipment at suitable places in the power system. The voltage profile of the power system is maintained by generating or absorbing the reactive power. The power system voltage can be increased by increasing the supply of reactive power and power system voltage decreased by decreasing the supply of the reactive power.
If the transmission lines are highly loaded where reactive power is more than that of supply, more currents are drawn from the generating source and receiving end voltage is decreased from the rated voltage. This reduced voltage cause tripping of generating stations, major equipment failure and overheating of induction motors.
In this condition, relays operate the reactive power equipment which increases the reactive power to bring the voltage at the rated voltage at the consumer terminal. The equipment used for reactive power generation is series reactors and series capacitors.
If the transmission line is lightly loaded where reactive power is less the that of supply, due to this receiving end voltage increases to a greater value. An increase in receiving end voltage cause the low power factor, insulation damage to the machine and generation station tripping. Under this condition, additional reactive power is compensated by reactive power compensation equipment such as shunt capacitors and shunt reactors.
2. To satisfy the reactive power demand:
The transformer and HVDC converter requires reactive power for their proper functioning. When the loads in the line increase, the voltage drop will take place.
Due to this voltage drop, more current will be drawn from the supply source to maintain the power supply which causes the line to consume more reactive power and again voltage drop takes place.
This voltage drop causes voltage collapse which is causes tripping of generators, unstable systems and tripping of other equipment connected with these lines.
The reactive power sources like series capacitors are connected to the load where reactive power is required. The voltage collapse problem is overcome by connecting a reactive power source at load ends.
3. To reduce electric blackout:
If the load in the transmission line increases, equipment connected to this line draws more current from the line so voltage drop increases and also reactive power demand also increases.
The transmission line also draws more current from the source and again voltage drop. Due to this voltage drop, system voltage becomes too low which is causes to trip the generating stations and equipment. Finally, the blackout of power takes place.
This power blackout is reduced by connecting reactive power sources at the load end where reactive power requirement is more.
4. To produce magnetic flux:
The inductive loads like transformers, induction motors and induction heating equipment require reactive power for producing the magnetic flux. every machine consumes reactive power for producing and maintaining the magnetic flux.
Due to this power factors reduces. For increases, the power factor capacitors are connected with this equipment.
Comparison table between Active power and Reactive power
| Active Power | Reactive Power |
|---|---|
| Active power is useful power that is consumed by the load. | The reactive power is useless power that is not consumed by the load. |
| Active power is a product of voltage, current and cosine of the angle between the voltage and current. | Reactive power is a product of voltage, current and sine of the angle between the voltage and current. |
| Active power is real power and it is measured in watts. | The reactive power is not real power and it is measured in volt-ampere reactive. |
| For representing the active power P letter is used. | For representing the reactive power Q letter is used. |
| The heat dissipates in the heater, the light that emits from lamps all are due to the active power. | The reactive power only determines the power factor of the power system. |
| The wattmeter is used to measure active power. | The VAR meter is used to measure the reactive power. |
| The active power can flow from source to load. | The reactive power can flow from the source to the load and back load to the source. |
| Active power is called real power, watt full power or true power. | The reactive power is called imaginary power, watt less power or complex power. |
| Active power can flow in one direction at a time. | The reactive power can flow in both directions in a time. |
| Active power can work both AC and DC circuits. | The reactive power can work only in DC circuits. |
| Active power converts electrical power into optical, thermal and mechanical. | The reactive power does not convert energy. it just produces magnetic flux in the induction machines. |
| In the power system, active power contributes to the current component which is the phase with the supply voltage. | In the power system, reactive power contributes to the current component which is out of phase with the supply voltage. |
| Active power exists in the DC circuits. | The reactive power can not exist in the DC circuits. |
| In a purely capacitive circuit, active power is zero. | In pure capacitive circuit reactive power is leading vars. |
| Active power in a purely inductive circuit is zero. | In the purely inductive circuit, reactive power is lagging vars. |
| Active power exists in purely resistive circuits. | Reactive power does not exist in purely resistive circuits. |
Difference between the active and reactive power:
1. Active power is useful power that is consumed by the load. The reactive power is useless power that is not consumed by the load.
2. Active power is a product of voltage, current and cosine of the angle between the voltage and current. Reactive power is a product of voltage, current and sine of the angle between the voltage and current.
3. Active power is real power and it is measured in watts. The reactive power is not real power and it is measured in volt-ampere reactive.
4. For representing the active power P letter is used. For representing the reactive power Q letter is used.
5. The heat dissipates in the heater, the light that emits from lamps all are due to the active power. The reactive power only determines the power factor of the power system.
6. The wattmeter is used to measure active power. The VAR meter is used to measure the reactive power.
7. The active power can flow from source to load. The reactive power can flow from the source to the load and back load to the source.
8. A active power is called real power, watt full power or true power. The reactive power is called imaginary power, watt less power or complex power.
9. Active power can flow in one direction at a time. The reactive power can flow in both directions in a time.
10. Active power can work both AC and DC circuits. The reactive power can work only in DC circuits.
11. Active power converts electrical power into optical, thermal and mechanical. The reactive power does not convert energy. it just produces magnetic flux in the induction machines.
12. In the power system, active power contributes to the current component which is the phase with the supply voltage. In the power system, reactive power contributes to the current component which is out of phase with the supply voltage.
13. Active power exists in the DC circuits. The reactive power can not exist in the DC circuits.
14. In a purely capacitive circuit active power is zero. In pure capacitive circuit reactive power is leading vars.
15. Active power in a purely inductive circuit is zero. In the purely inductive circuit, reactive power is lagging vars.
16. Active power exists in purely resistive circuits. Reactive power does not exist in purely resistive circuits.