Reactive power is measured in VAR, where V stands for volts, A for amperes, and R denotes reactive. In power systems, particularly in transmission networks, reactive power is typically expressed in megavolt-amperes reactive (MVAR), where 1 MVAR equals one million VAR.
What is reactive power?
The reactive power is not a useful power. This is actually the power which oscillates between the source and the reactive elements, like inductors or capacitors, connected in the power system. Unlike active power, which is represented with watts or megawatts, reactive power does not do any real work. Instead, it gives support to the voltage level and enables the power to flow from one point to another point.
When there are a number of inductive loads connected in the system, those inductive loads consume reactive power, which has to be supplied from the generating station. As we know, when a current passes through an inductor, the current lags behind the applied voltage across the inductor. In a power system, the same thing happens. If the system voltage is represented by ( V ) and the current flowing through the system is represented with capital ( I ), then the active power will be \( VIcos\theta \), where, \( \theta \) is the angle between voltage ( V ) and current ( I ). This means \(\theta \) is the angle by which the current in the system lags behind the system voltage, and \( cos \theta \) is the power factor.
If the inductive load of the system becomes large enough, the current will lag more behind the system voltage, means, angle \( \theta \) is increased. That means, power factor, \(\cos \theta \) is decreased. This condition is called a poor power factored condition.
When the network is fully resistive, with no inductive or capacitive load connected to the system, or the equipment or network connected to the system does not inherit any capacitive or inductive components, the power will be pure active power. However, when an inductive load is connected in the network, or the network itself has some inductive characteristics, the current lags behind the system voltage.
How do we express reactive power?
The active power, as we told, is measured as the product of system voltage, current, and the power factor, which means \[ P = VIcos\theta \]
The reactive power is measured as the product of system voltage, current and \( \sin\theta \). The total power, which is called apparent power, has to be supplied from the generation system. The apparent power is measured in volt-ampere or mega volt ampere (MVA).
Devices like transformers, induction motors, and transmission lines, which have their own inductive effects, and equipment that has magnetic coils, impose inductive loads on the system. Because of this, the current lags behind the voltage and consumes reactive power.
Effects of Reactive Power in Power Network?
Now we come to the effects of reactive power. Reactive power directly influences the voltage level.
Poor Voltage Regulation : – If you increase the inductive load, the power factor will decrease. So, to transmit a specific quantity of power from one point to another in the power system, we need more current, because power is nothing but the product of voltage, current, and the power factor. \[ P = VIcos\theta \] The expression of voltage drop in a transmission system is given by \[ \Delta V = I(X\sin\theta + R\cos\theta ) \] That will also increase if current ( I ) is increased. So, voltage regulation of the system becomes poor.
Increased Power Loss : – Excessive reactive power reduces power factor thereby increases the current as already told. Increased current means increased \(I^2R \) loss. So, a low power factor means more ohmic power (\(I^2R \)) loss as heat, or this is simply the additional power loss.
Reduced Transmission Capacity : – Transmission system capacity is also affected by reactive power. If the load increases the reactive power of the system, the current required to transmit a fixed amount of power from one point to another also increases. Increased current requires conductors with a higher cross-sectional area. In other words, if the conductors size is not adjusted, the current-carrying capacity of the system will be derated.
Power Triangle
Now, we come to the most basic thing, that is, the power triangle. The power triangle is a right-angle triangle whose base is the active power, perpendicular is the reactive power, and hypotenuse is the apparent power. That means, apparent power the sum of active power and reactive power.
