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Reactive Power Basics - KBR

Reactive power is also known as magnetizing power. It oscillates between the consumer and the energy provider at twice the network frequency, and accordingly loads cables, fuses and transformers.

In practical operation, reactive current compensation in commercial and industrial power networks is an issue that often raises many questions. For technicians, the term compensation describes the interaction between different parameters which - in the best case scenario - cancel each other out. The objective of this is to reverse the negative effect of an interfering physical parameter with a second parameter. In our case, we want to compensate inductive with capacitive reactive power.
Electrical energy generated by power stations or through regenerative methods is transformed into largely usable energy, such as light, heat or kinetic energy, depending on the consumer. Some consumers require inductive reactive power from the energy supply network to create a magnetic field. Typical inductive consumers are motors and transformers. The active power resulting from the product of voltage and current is billed by the energy provider as consumed energy in kWh. Things are different with reactive power. It changes between provider and consumer and is not "consumed" in the literal sense.

Why does the energy provider bill the reactive energy?

The degree of load created by network transformers, transmission lines and power plants is expressed as apparent power
(S). It is calculated from the active power (P) and reactive power (Q).

S = √ P2 + Q2

As can be seen from the formula, the transmission equipment of the network operator is additionally loaded by the reactive power. To keep the current-related losses to a minimum and to guarantee economic energy transport, network operators stipulate a minimum power factor cosφ. This describes the ratio of active to apparent power.

cosφ = P over S

Energy meters for commercial and industrial use not only measure the active energy but also the reactive energy, which is billed in accordance with the electricity supply agreement. For most energy supply networks, a cosφ of 0.9 is specified. Here, 50% of the consumed active energy obtained from the power supply network may be taken as reactive energy free of charge in the billing period.

Other reasons for reactive current compensation:

Thus, the main objective of compensation is to reduce the reactive current costs billed by the energy provider to "zero". Another reason for reactive power compensation is to reduce the current load. Let's take a closer look at the formula for active power:

P = U x I x cosφ x √3

If we apply it to the current, this results in the following formula:

I = P over U x cosφ x √3

The current thus depends on the power factor cosφ. Let's calculate the current reduction using an example:

An additional consumer with a power consumption of 35 A is to be connected to a sub-distribution unit with 250 A at an outgoing line. The following values were measured:

U = 400 V
I = 238 A
cosφ = 0.72
P = U x I x cosφ x √3 = 400 V x 238 A x 0,72 x √3 = 118.700 W

If you increase the power factor to cosφ 0.97 by compensation, the current is reduced from 238 A to:


Improving network quality

Reactive power compensation is also used for improving the network quality. In modern industrial installations, consumers with power electronics (e.g. frequency converters) are used for energy efficiency measures. The input current of these "linear consumers" is no longer sinusoidal. As a result, network feedback is created as harmonic voltage. This can cause malfunctions in the consumers connected to the same network. By using a compensation system as an absorption circuit, the harmonic voltage level can be reduced, rectifying the disturbance in the consumers. The principle of an absorption circuit system corresponds to that of a detuned reactive power compensation system with the resonance frequency close to the interfering harmonic frequency.
Another possible application is renewable energy generators, such as solar and wind power plants. According to applicable laws, these energy generation plants feeding energy into the public grid with an output of more than 100 kW have to contribute to keeping the voltage constant. If the network voltage drops, the voltage can be increased by switching on capacitors. A distinction is made between medium-voltage and low-voltage systems. In low-voltage systems, a Q / P characteristic curve has to be compensated, in medium-voltage systems, a Q / U characteristic curve.

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