In this blog, we are going to discuss Power Factor, True Power, Reactive Power, Apparent Power, the role of power factor. Since the power factor plays a vital role in power transmission. Distribution systems are typically made up of a combination of various resistive, inductive, and capacitive loads.
Resistive loads include devices such as heating elements and incandescent lighting. In a purely resistive circuit, current and voltage rise and fall at the same time. They are said to be “in phase.
Power drawn by a resistive circuit is converted into useful work. This is known as the true power in a resistive circuit. True power is measured in watts (W), kilowatts (kW), or megawatts (MW). In a DC circuit or in a purely resistive AC circuit, true power can easily be determined by measuring voltage and current. True power in a resistive circuit is equal to system voltage (V) times current (I).
for example, an incandescent light (resistive load) is connected to 120 VAC. The current meter shows the light is drawing 0.833 amps. In this circuit, 100 watts of work is done (120 VAC x 0.833 amps).
Inductive loads include motors, transformers, and solenoids. In a purely inductive circuit, current lags behind voltage by 90°. Current and voltage are said to be “out of phase.” Inductive circuits, however, have some amount of resistance. Depending on the amount of resistance and inductance, AC current will lag somewhere between a purely resistive circuit (0°) and a purely inductive circuit (90°).
Capacitive loads include power factor correction capacitors and filtering capacitors. In a purely capacitive circuit, current leads voltage by 90°. Capacitive circuits, however, have some amount of resistance. Depending on the amount of resistance and capacitance, AC current will lead voltage somewhere between a purely resistive circuit (0°) and a purely capacitive circuit (90°).
Circuits with inductive or capacitive components are said to be reactive. Most distribution systems have various resistive and reactive circuits. The amount of resistance and reactance varies, depending on the connected loads.
Just as resistance is opposition to current flow in a resistive circuit, reactance is opposition to current flow in a reactive circuit. It should be noted, however, that where frequency has no effect on resistance, it does effect reactance. An increase in applied frequency will cause a corresponding increase in inductive reactance and a decrease in capacitive reactance.
Energy in a reactive circuit does not produce work. This energy is used to charge a capacitor or produce a magnetic field around the coil of an inductor. Current in an AC circuit rises to peak values (positive and negative) and diminishes to zero many times a second. During the time current is rising to a peak value, energy is stored in an inductor in the form of a magnetic field or as an electrical charge in the plates of a capacitor. This energy is returned to the system when the magnetic field collapses or when the capacitor is discharged.
Energy in Reactive Circuits
Power in an AC circuit is made up of three parts; true power, reactive power, and apparent power. We have already discussed true power.
Reactive power is measured in volt-amps reactive (VAR). Reactive power represents the energy alternately stored and returned to the system by capacitors and/or inductors. Although reactive power does not produce useful work, it still needs to be generated and distributed to provide sufficient true power to enable electrical processes to run.
Not all power in an AC circuit is reactive. We know that reactive power does not produce work; however, when a motor rotates work is produced. Inductive loads, such as motors, have some amount of resistance.
Apparent power represents a load which includes reactive power (inductance) and true power (resistance). Apparent power is the vector sum of true power, which represents a purely resistive load, and reactive power, which represents a purely reactive load.
A vector diagram can be used to show this relationship. The unit of measurement for apparent power in volt-amps (VA). Larger values can be stated in kilovolt-amps (kVA) or megavolt amps (MVA).
Power factor (PF) is the ratio of true power (PT) to apparent power (PA) or a measurement of how much power is consumed and how much power is returned to the source. Power factor is equal to the cosine of the angle theta in the above diagram.
In purely resistive circuits, apparent power and true power are equal. All the power supplied to a circuit is consumed or dissipated in heat. The angle of theta is 0° and the power factor is equal to 1. This is also referred to as unity power factor.
In purely reactive circuits, apparent power and reactive power are equal. All power supplied to a circuit is returned to the system. The angle theta is 90° and the power factor is 0. In reality, all AC circuits contain some amount of resistance and reactance. In a circuit where reactive power and true power are equal, for example, the angle of theta is 45° and the power factor is 0.70.
Power Factor Problems
It can be seen that an increase in reactive power causes a corresponding decrease in power factor. This means the power distribution system is operating less efficiently because not all current is performing work. For example, a 50 kW load with a power factor of 1 (reactive power = 0) could be supplied by a transformer rated for 50 kVA. However, if the power factor is 0.7 (70%) the transformer must also supply additional power for the reactive load. In this example, a larger transformer capable of supplying 71.43 kVA (50 ÷ 70%) would be required. In addition, the size of the conductors would have to be increased, adding significant equipment cost.
Leading and Lagging Power Factor
Since current leads voltage in a capacitive circuit, power factor is considered leading if there is more capacitive reactance than inductive reactance. Power factor is considered lagging if there is more inductive reactance than capacitive reactance since current lags voltage in an inductive circuit. Power factor is unity when there is no reactive power or when inductive reactance and capacitive reactance are equal, effectively canceling each other.
It is usually more economical to correct poor power factor than to pay large utility bills. In most industrial applications motors account for approximately 60% or more of electric power consumption, resulting in a lagging power factor (more inductive than capacitive). Power factor correction capacitors can be added to improve the power factor.