The relationship between active, reactive and total power is determined by the phase angle between current and voltage in the network, more precisely by the value of the cosine of this angle - cosφ (power factor).

It follows from the power triangle that for a total power S, the greater the reactive power Q that passes through the alternator or transformer, the less the active power P that can reach the receiver.

Thus, at cosφ = 0.95 the reactive power is 33% of the consumed active power, at cosφ = 0.7 the value of the reactive power is practically equal to the value of the active power, and at cosφ = 0.5 it exceeds 1.7 times , significantly increasing the active losses in the network.

Thus, the reactive power does not allow full use of the total designed power of the alternating current sources to generate useful electricity. The same applies to electrical networks. The current that can be safely passed through an electrical network is determined mainly by the cross section of its conductors. Therefore, if part of the current passing through the network goes to generate reactive power, this automatically leads to a reduction in the active current that ensures the creation of active power that can be transmitted through this network.

The useless circulation of the reactive component of electrical energy between the AC source and the receiver, due to the presence of reactance in it, leads to the loss of a certain amount of energy that is lost in the conductors of the entire electrical circuit. The increased load of the network with reactive current also leads to a decrease in the mains voltage, while the sharp fluctuations in the reactive power - to fluctuations in the mains voltage. At the same time, with a decrease in cosφ, for the transformers the capacity of the active power decreases due to the increase of the reactive load, and for the consumers of electricity of the enterprise the productivity decreases and their functionality may even be impaired.

To compensate reactive power, special compensating devices are used, which are sources of reactive energy, mainly of capacitive nature. The principle of compensation is illustrated in fig. 2.

The reactive power, due to the inductive or capacitive nature of the load, is compensated in the immediate vicinity of the load Q (see Fig. 2), which excludes its negative impact on the power supply of the enterprise.

Synchronous compensators, electric motors and capacitor units are used to compensate reactive power in an enterprise with an inductive nature of the load of energy consumers.

According to the results of studies conducted in various sources, it was found that large capacity units and the worst technical and economic indicators in the range of small (up to 10 MVA) compensation capacities compared to capacitor units virtually exclude the use of synchronous compensators in grids of the larger number of enterprises and the use of synchronous motors is not cost-effective for low voltage networks (up to 1000 V), as well as for high voltage networks with energy consumption below 1500 kW.

Therefore, capacitor modules are currently widely used to compensate for reactive power in enterprises, with the advantages being:
- low specific losses of active power (own losses of modern low-voltage cosine capacitors do not exceed 0.5 W per 1000 VAR, while in synchronous compensators this value reaches 10% of the nominal power of the compensator, and in synchronous motors operating in excitation mode - up to 7%);
- easy installation and operation (no base required);
- relatively low capital investment;
- possibility to choose the necessary power for compensation;
- possibility for installation and connection at any point of the electricity transmission network;
- lack of rotating parts and as a result - noise during operation;
- low operating costs;

In this case, both simple banks of cosine capacitors and adjustable capacitor banks are used, in which by correcting the value of the power factor its correction is performed by connecting or disconnecting the required number of capacitor banks.
However, in existing devices when using capacitor units (including controlled) there are a number of problems that significantly reduce their performance and reliability:
- small number of switching steps and, as a consequence, low compensation accuracy;
- the possibility of "overcompensation" by reducing the reactive power in an inductive network (ie generating reactive power of a capacitive nature);
- in the adjustable capacitor units, the banks of the three-phase cosine capacitors are controlled by sensors installed in one of the phases, which significantly reduces the accuracy of compensation for the other phases;
- the possibility of damage to the cosine capacitors due to the presence of a high level of harmonics in the network with nonlinear load of consumers (pulse stabilizers and power converters, etc.), which appear harmonics with higher currents, comparable in size to the main harmonic ;
- the possibility of damage to the cosine capacitors due to the formation, together with the load inductance, of successive oscillator circuits close in resonant frequency to the frequency of one of the high harmonics. This leads to a significant increase in the current of the capacitors and significantly reduces their service life. The overvoltage resulting from the resonance of the capacitor unit elements and the load can damage their insulation.

In addition, the existing capacitor blocks practically do not solve the problem of reducing the phase imbalance. The harmonic suppression function, when capacitor modules are used, can only be provided with a separately purchased harmonic filter tuned for higher harmonics (usually 3rd, 5th, 7th, 11th, 13th).