VTdrive Reactive Power Compensation
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VTdrive Reactive Power Compensation - 1

Reactive Power can best be described as the quantity of “unused” power that is developed by reactive components, such as inductors or capacitors in an AC circuit or system. In a DC circuit, the product of “volts x amps” gives the power consumed in watts by the circuit. However, while this formula is also true for purely resistive AC circuits, the situation is slightly more complex in an AC circuits containing reactive components as this volt-amp product can change with frequency. In an AC circuit, the product of voltage and current is expressed as volt-amperes (VA) or kilo volt-amperes (kVA) and is known as Apparent power, symbol S. In a non-inductive purely resistive circuit such as heaters, irons, kettles and filament bulbs, etc. their reactance is practically zero, and the impedance of the circuit is composed almost entirely of just resistance. For an AC resistive circuit, the current and voltage are in-phase and the power at any instant can be found by multiplying the voltage by the current at that instant, and because of this “in-phase” relationship, the rms values can be used to find the equivalent DC power or heating effect. However, if the circuit contains reactive components, the voltage and current waveforms will be “out-of-phase” by some amount determined by the circuits phase angle. If the phase angle between the voltage and the current is at its maximum of 90o, the volt-amp product will have equal positive and negative values. In other words, the reactive circuit returns as much power to the supply as it consumes resulting in the average power consumed by the circuit being zero, as the same amount of energy keeps flowing alternately from source to the load and back from load to source. Since we have a voltage and a current but no power dissipated, the expression of P = IV (rms) is no longer valid and it therefore follows that the volt-amp product in an AC circuit does not necessarily give the power consumed. Then in order to determine the “real power”, also called Active power, symbol P consumed by an AC circuit, we need to account for not only the volt-amp product but also the phase angle difference between the voltage and the current waveforms given by the equation:VI.cosΦ. Then we can write the relationship between the apparent power and active or real power as:

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VTdrive Reactive Power Compensation - 2

Note that power factor (PF) is defined as the ratio between the active power in watts and the apparent power in volt-amperes and indicates how effectively electrical power is being used. In a non-inductive resistive AC circuit, the active power will be equal to the apparent power as the fraction of P/S becomes equal to one or unity. A circuits power factor can be expressed either as a decimal value or as a percentage. But as well as the active and apparent powers in AC circuits, there is also another power component that is present whenever there is a phase angle. This component is called...

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VTdrive Reactive Power Compensation - 3

consume reactive power which takes up space on the transmission lines since larger conductors and transformers are required to handle the larger currents which you need to pay for. Reactive Power Analogy with Beer In many ways, reactive power can be thought of like the foam head on a pint or glass of beer. You pay the barman for a full glass of beer but only drink the actual liquid beer which is always less than a full glass. This is because the head (or froth) of the beer takes up additional wasted space in the glass leaving less room for the real beer that you consume, and the same idea...

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VTdrive Reactive Power Compensation - 4

Industrial customers, on the other hand, which use 3-phase supplies have widely different power factors, and for this reason, the electrical utility may have to take the power factors of these industrial customers into account paying a penalty if their power factor drops below a prescribed value because it costs the utility companies more to supply industrial customers since larger conductors, larger transformers, larger switchgear, etc, is required to handle the larger currents. Generally, for a load with a power factor of less than 0.95 more reactive power is required. For a load with a...

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