Automatic Power Factor Correction

Power factor correction

Power factor correction means approaching the power factor of an AC circuit using equipment that absorbs or supplies reactive energy to the circuit. Typically, power factor correction can be performed using the capacitor and synchronous motor in the circuit. Power factor correction will not change the actual amount of power, but will reduce the apparent power and total current absorbed by the load. Power factor correction

The phase shift between the voltage and current of the circuit is known as the power factor. It is represented by the cosine of the angle φ. The power factor represents the fraction of the total energy consumption to perform useful works and the remaining energy is stored as magnetic energy in the inductor and in the circuit capacitor. The power factor value is between -1 and +1.

The most economic value of the power factor is between 0.9 and 0.95. If the power correction factor value is less than 0.8 (approximately), then it absorbs more current from the load. The large current increases the losses and requires a large conductor, which increases the cost of the system. Loss can be reduced by correcting the system power factor.

Power factor correction methods

Power factor correction methods are mainly classified into two types, namely using the capacitor or the synchronous capacitor.

Power factor correction by means of the capacitor bank

In the three-phase system, the power factor improves when the star or delta capacitors are connected. The banks connected to thepower-factor-correction-images star and the triangle are shown in the following figure.

Power factor correction by synchronous capacitor

The power factor can also be corrected by installing the specially designed induction motor known as the synchronous capacitor. The synchronous capacitor operated without mechanical load and is connected in parallel with the load. Absorbs and generates reactive power (Var) by varying the excitation of the winding of the motor field.

The synchronous capacitor is used to improve the power factor in series. The output of the phase modifier can be varied without problems. The synchronous condenser has some disadvantages, as it is expensive and its installation, maintenance and operation are also not easy.

Power factor improvement

If the power factor is low or low, it must be improved or corrected. It can be improved by injecting a main current into the circuit to neutralize the effect of the delayed current. The power factor can be improved by using static capacitors or synchronous motors.

The main and delayed power factors are the two main terms associated with the power factor of the AC electrical system. The crucial difference between the main and the delayed power factor is that, in the case of the main power factor, the current carries the voltage. power factor can also be corrected by installing the specially designed induction motor known as the synchronous capacitor. The synchronous capacitor operated without mechanical load and is connected in parallel with the load. Absorbs and generates reactive power (Var) by varying the excitation of the winding of the motor field.

The power factor is a crucial property of AC electrical systems. It is nature without dimensions.

Used for single-phase and three-phase AC circuits. It is the ratio of actual or actual power to apparent power in AC systems.

In DC circuits, it is possible to evaluate the power of the circuit by finding the product of the voltmeter and the readings of the ammeter.

While in the case of the AC circuit, this multiplication of these two provides the apparent power but not the actual power. This is because in AC circuits, the total power supplied, i.e. the apparent power, is not used exclusively by the circuit.

And the power actually used by the circuit is known as actual power.

More simply, the power factor is the cosine of the phase difference between V and I.

The power factor of AC circuits with linear loads is between -1 and 1. It is generally believed that if a system has a power factor closer to 1, such systems are said to be stable.

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The most important requirement of the protection relay is reliability. They remain inactive for a long time before a fault occurs; but if an error occurs, the relays must respond instantly and correctly.

The relay must only function under the conditions for which the relays are put into service in the power supply system. There may be some typical conditions during the fault that some relays should not be operated or operated after a certain defined delay, so the protection relay must be able to select the appropriate condition for which it would be operated.


The relay equipment must be sensitive enough to function reliably when the fault level exceeds only the predefined limit.


The protection relays must operate at the required speed. Adequate coordination is required in the various protection relays of the power supply system so that a failure in one part of the system does not disturb another healthy part. The fault current can flow through a part of the sound part as they are electrically connected, but the relays associated with that sound part should not function faster than the relays in the defective part, otherwise an interruption without failure could occur. desired sound system. Again, if the relay associated with the faulty part does not work in a timely manner due to a defect in it or for other reasons, only the next relay associated with the healthy part of the system should be used to isolate the error. Therefore, it should not be too slow, which can cause damage to the equipment, nor should it be too fast, which can cause unwanted operation.

Important elements for the protection of the electrical system

Control substation

It mainly consists of a bulk oil circuit breaker, a minimum oil circuit breaker, an SF6 circuit breaker, an air jet circuit breaker and a vacuum circuit breaker, etc. Various control mechanisms are used in the switch, such as solenoid, spring, pneumatic, hydraulic, etc. The switch is the main part of the protection system in the power supply system and automatically isolates the defective part of the system by opening its contacts.

Protective equipment

It consists mainly of power system protection relays, such as current relays, voltage relays, impedance relays, power relays, frequency relays, etc., based on operating parameters, defined timed relays, reverse timed relays, steps etc. as differential relays, overflow relays, etc. During the fault, the protection relay issues a trip signal to the associated switch to open its contacts.

Station battery

All circuit breakers in the power supply system operate on direct current. Since DC power can be stored in the battery and if a situation occurs where there is a total power cut in the input, the circuit breakers can work to restore the situation using the battery power of the storage station. Therefore, the battery is another essential element of the power system. At some point it is known as the heart of the electrical substation. An electrical substation battery or simply a station battery containing multiple cells accumulates energy during the period of availability of the AC power supply and discharges when the relays operate, so that the switch in question intervenes at the time of the failure of the AC power input.

There is always the possibility that an electrical power system suffers from abnormal peaks. These abnormal overvoltages can be caused by various reasons, such as the sudden termination of a heavy load, lightning pulses, switching pulses, etc. These surges can damage the insulation of various equipment and insulators in the power system. Although, all overvoltage voltages are not strong enough to damage the insulation of the system, however these overvoltages should also be avoided to ensure proper operation of the power supply system.

All these types of destructive and non-destructive anomalous peaks are removed from the system by surge protection.


The surge voltages applied to the energy system are generally transient in nature. The transient voltage or voltage rise is defined as the sudden sizing of the voltage at a high peak in a very short duration.

Protection against electrical faults such as short circuits, line to ground faults and line to line faults. The MPCB can stop any electrical fault below its breaking capacity.

Motor overload protection, when a motor absorbs electrical current above the value of its plate for a long period of time. Overload protection is normally adjustable in MPCB.

Protection against phase imbalances and phase loss. Both conditions can seriously damage a three-phase motor, therefore the MPCB stops the motor in any case as soon as the error is detected.

Thermal delay to prevent the engine from starting immediately after an overload, giving the engine time to cool down. An overheated engine can be permanently damaged if it is restarted.

Switching the motor circuit: MPCBs are normally equipped with buttons or dials for this purpose.

Fault reporting – Most models of motor protection circuit breakers have an LED display that lights up whenever the MPC intervenes. This is a visual indication for nearby personnel that an error has occurred and that the electric motor should not be reconnected until the error is resolved.

Automatic reconnection: some MPCB models allow to insert a cooling time in case of overload, after which the motor will restart automatically.

Electric motors are expensive equipment, so the role of the motor protection switch is very important. If an engine is not adequately protected, it may be necessary to perform costly repairs or even completely replace the equipment. An electric motor properly protected with an MPCB will have a much longer service life.

Operating principle of the motor protection switch

The motor protection switch can be considered a sub-type of thermal magnetic circuit breaker, but with additional functions specially designed to protect electric motors. The basic operating principle is similar to all other circuit breakers.

Thermal protection is used to protect the electric motor from overloads. It is based on an expansion and contraction contact that disconnects the motor if excessive current is detected. It is very important to know that thermal protection has a delayed response, to allow high input currents when starting a motor. However, if the engine cannot be started for any reason, thermal protection will trip in response to the extended input current.

Magnetic protection is used in the event of a short circuit, line fault or other high current electrical fault. Unlike thermal protection, magnetic protection is instantaneous; to immediately disconnect dangerous fault currents.

The main difference between MPCB and other circuit breakers is that MPCB can provide protection against phase imbalance and phase loss. Three-phase circuit motors require three active conductors with balanced voltages to function effectively. An imbalance of more than 2% will be harmful to the life of the engine. If one of the phase voltages is suddenly lost, the effect is even more damaging because the motor will continue to run with only two phases. The motor protection switch is able to detect these conditions by measuring the differences between the phase voltages and immediately shuts down the motor when they occur. It is important to note that the imbalance of the phase current is normal in three-phase systems that provide separate single-phase loads, but it is unacceptable when the three-phase circuit supplies an electric motor.

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