Power Factor Correction Capacitors in Power Factor Correction

Power factor is the key factor to evaluate how effectively electric power in a power system is being used. If the power factor is high (unity), then the effectiveness of usage of electric power in a power system is more. As the power factor reduces, the less effectively electric power is being used in power system. There are different reasons due to which the power factor is being reduced, to improve power factor there are different power factor correction methods. Efficient and typically used power factor correction method utilizes capacitors for power factor improvement. But, primarily we must know what power factor is.

What is Power Factor?

Power factor can be defined as the ratio of active power to the apparent power. Generally, it is called as the cosine of angle between the voltage and current. Why do we use cosine of angle between voltage and current? This is because; consider power triangle derived from the phasor diagram of voltage/current.

Current Vector Diagram

The angle between load current and supply voltage is same as that between the active power and apparent power in the power triangle. Power factor is a dimensionless number and its value is always in closed interval of -1 to 1.

Power Vector Diagram

So, if the cosine of the angle between voltage and current is one, then power factor becomes unity. Unity power factor represents that electric power is being used most effectively. But, in practical power factor is always less than unity due to inductive or capacitive loads it is always lagging or leading. For pure resistive loads only unity power factor exists which is practically impossible. As the power factor decreases, the effectiveness of electric power being used reduces. So, in order to improve factor (to make power factor improve towards unity), there are different power factor correction techniques.

Power Factor Correction

Reasons for low power factor are: inductive loads such as transformers, induction generators, high intensity discharge lights, and induction motors; inductive loads that are acting as source of reactive power.

Reasons for power factor correction are: to reduce utility fees by reducing peak active power demand, to eliminate power factor penalty, to increase system capacity by reducing system losses, to increase voltage level in electrical system.

Power factor correction can be defined as the method of improving power factor value to make it reach unity or nearby unity value, such that the angle between voltage and current reduces. If the voltage and current are in phase, then there will be no power loss, entire power can be used to give supply to loads. High power factor reduces the transmission losses and improves voltage regulation at the load end. There are different power factor improvement methods such as capacitor power factor correction, synchronous power factor correction, filter power factor correction, and active boost power factor correction. Before discussing about power factor correction method in detail, we must know how to calculate power factor and how to derive power factor formula.

Power Factor Formula

Power factor is also called as displacement power factor (DPF) or fundamental power factor (FPF). From power triangle we have active power P, reactive power Q, and apparent power S as shown in the figure.

Power Triangle

Active power is called as useful power or true power or real power used for supplying loads and is derived as follows.

Active Power Formula

The apparent power is given as

Apparent Power Formula

Where, S represents apparent power which is defined as the magnitude of the oscillating component of instantaneous power measured in units of VA or KVA.

Q represents reactive which is proportional to the energy stored in the power system and measured in units of VAR or KVAR.

Power factor gives the relation between active power and apparent power (volt-amperes).

Displacement Power Factor Formula

Power Factor Improvement Using Capacitors

Power factor correction capacitors are used for improving the power factor in the power system applications where inductive machines or electric motors are used. Due to inductive nature of these motors, power factor reduces. Hence, by using a capacitor which is having leading power factor property improves the power factor or diminishes the effect of inductive motors on power factor.

These capacitors provide capacitive reactance for improving power factor. Power factor improvement capacitors can be used only for power factor improvement of linear loads and are unable to provide for non-linear loads. As the load changes the power factor also changes in a power systems due to inductive loads which causes inductive reactance that decreases the power factor. This inductive reactance cancelled by the capacitive reactance provided by the capacitor banks. Thus, by the cancellation of inductive reactance of inductive loads and capacitive reactance of capacitive banks improves power factor. While designing care must be taken to avoid instability due to cancelling out of inductive reactance and capacitive reactance.

Motor Current composed of Load Current and Magnetizing Current

Consider loads like motors whose current is composed of both load current and magnetizing current. Magnetic current is used for establishing magnetic field for spinning the motor. This magnetizing current is constant, doesn’t vary with load, it lags with voltage sine wave by 90 degrees.

To stop delivering magnetizing current component along the cable that is used to feed motor.

Consider magnetizing current that is equal in magnitude and apart from each other at 180 degrees compared to the magnitude of current drawn by power factor correction capacitor.

Magnetizing Current and Capacitor Current

If we compare these to system voltage, then the voltage leads the magnetizing current by 90 degrees and voltage lags the power factor correction current by 90 degrees. If magnetic is building up, then these power factor correction capacitors supply the demanding current by discharging.

Thus, the motor and power factor correction capacitor bank provides the current (magnetizing current) associated to the magnetic field. Hence, cable is intended only for delivering the required load current. Therefore, the power factor can be improved using the power factor correction capacitors with the inductive loads.

ABB Power Factor Correction Capacitors

ABB power factor correction capacitors are classified into two types based on the type of correction. They are: Fixed power factor correction capacitor system and automatic power factor correction system.

Fixed Power Factor Correction Capacitor

Single Step Fixed Capacitor Correction System

A few capacitors having identical ratings or different ratings are connected to form a capacitor bank that is inserted in parallel on the network of the system. It is also called as single step capacitor bank scheme as the capacitor bank is energized by a contactor in single step that supplies all capacitors. In this fixed correction system, inrush current peak is equal to 30 times of the capacitor bank nominal current.

Automatic Power Factor Correction Capacitor

Multi Step Automatic Capacitor Correction System

Several capacitor banks having identical ratings or different ratings are energized separately in multiple steps based on the power factor value that is to be corrected. An electronic device is used to detect power of the steps to be energized for activating the relevant contactors. In automatic correction system, inrush current peak depends on the power of the steps that are already on duty. Here, inrush current peak is almost 100 times of the step’s nominal current that is to be energized.

Power factor can be defined as ratio of active power to the apparent power. Power factor can be improved using different methods. Here, in this article power factor correction using power factor correction capacitors is discussed in brief along with ABB power factor correction capacitors. Are you aware any other frequently used power factor correction method? If yes, then share your technical knowledge by posting your comments in the comment section below.