When the load is switched on, the phase voltages at the output of the transformer will decrease to varying degrees due to the different loads, taking into account the voltage drop in the transformer windings (vectors OA´, OV´, OC´ - phase voltages; vectors А´В´ , В´С´, С´А´ - linear stresses). As can be seen from the figure, not only the phase but also the line voltages change.

2. The voltage drops in the transformer windings are insignificant, but the neutral conductor is "damaged"

The interaction of phase and linear voltages can be represented in the form of an equilateral triangle (figure below) with vertices "A", "B", "C" and center at point "0". The vectors AB, BC and CA (located on the side of the triangle) are linear voltages (380V).

The vectors (solid lines) drawn from the center of the triangle to its vertices - 0A, 0B and 0C - are phase voltages. Under symmetrical loading, they are equal to each other 0A = 0B = 0C and are offset from each other by an angle of 120 °. In this case there is no phase voltage imbalance.

One of the causes of phase imbalance is "bad zero", when the resistance of copper between the zero point of the transformer and the zero point of the load is unacceptably high or, even worse, when there is a "break" of the neutral conductor. In such cases, due to the fact that many users, including single-phase, are connected to the network, at any random point in time it can be expected that the loads in different phases will differ from each other. Moreover, even if the single-phase loads are the same in size, it is not possible to switch them on or off under load synchronously. The difference in phase loads in size and nature creates conditions for imbalance of phase voltages.

Graphically it will look like this (dotted lines in the figure above): point 0 in the center of the triangle, from which the vectors of ideal phase voltages of 220V (0A, 0B and 0C) originate, is shifted relative to the center of the triangle to point O´. The phase voltage vectors themselves are shifted at any angle to each other. The voltage of each of the phases varies from a value of 220 V, for example, to 190V, 240V and 230V, respectively.

In this case, the phase imbalance (phase voltages), as a rule, is characterized by the invariance or uniformity of the linear voltages of the source and a significant difference in the magnitude of the phase voltages. That is, the triangle formed by the linear voltage vectors remains equivalent, which means that the value of the three linear voltages corresponds to 380V, and there may be slight tolerances.

3. The voltage drops in the transformer windings are significant and the neutral conductor is "damaged".

This is the worst situation, leading to a significant phase imbalance. Two factors act simultaneously: uneven phase load and high resistance (interruption) of the neutral conductor. Phase voltage imbalances have a major impact on equipment performance. The main part of the three-phase consumers (consumers powered by linear voltage) are electric motors. The control and monitoring system for starting such three-phase consumers, as a rule, is connected to the phase voltage. In case of phase imbalances, the control system for starting the electric motor, which controls the duration and the fact of starting, is unstable, ie. spontaneously issues start or stop commands. The range of variation of the phase voltage is strictly regulated by the operating documentation (as a rule, no deviation of more than ± 7.5 ÷ 10% of the nominal) is allowed. If the misalignment has exceeded the permissible limit, then the release control system fails. When the phase voltage level is restored, the next start occurs, and so on.

It is known that the starting mode of an induction motor is characterized by short-term operation of the stator windings in short-circuit mode. Frequent restarts will lead to significant overheating of the insulation and will significantly increase the power consumption of the network. The possible negative consequences of this mode of operation are either failure to start or damage to the equipment due to burnout of the motor windings.

For single-phase users, low voltage is the cause of low light in lighting fixtures, prolonged starting of motor devices, computer malfunctions and more. High voltage causes damage to electrical receivers due to deterioration of insulation, disconnection of protective devices, blown fuses.

According to the information provided by the measurement results, a small voltage asymmetry (eg up to 2%) of the induction motor terminals leads to a significant increase in power losses (up to 33% in the stator and 12% in the rotor), which in turn causes additional heating of the windings and reduction of the service life of their insulation (by 10.8%), and with imbalances of 5%, the total losses increase by 1.5 times and the corresponding increase in current consumption. Additional losses due to voltage asymmetry have nothing to do with motor load.

The long-term allowable power for motors up to 7 kW with a voltage imbalance of 5% is reduced compared to the nominal by 10 - 15%, and with an imbalance of 10% - by 25 - 45%. Another negative effect of voltage imbalance is the appearance of additional vibrations, as a result of which the service life of individual parts of the motor, including its windings, is reduced. In symmetrical mode, the main cause of vibration is the imbalance of the rotating parts, misalignment of the shafts. At unbalanced voltages additional vibrations occur, which are commensurate with or greater than the vibrations in symmetrical mode, and the total vibration may exceed the allowable level. Calculations show that in some cases the allowable voltage imbalance is limited not by the heating conditions but by the mechanical overload conditions during the vibrations of the motor housing.

If the phase imbalance cannot be eliminated by even distribution of the load on the phases and at the same time its presence leads to disruption of technological processes, its elimination is usually ensured by including a powerful three-phase voltage stabilizer (actually three independent single-phase stabilizers) or installation of a special three-phase balancing transformer.

However, the inclusion of phase stabilizers does not really solve the problem, as they themselves provoke the asymmetry of the three-phase system. In addition to their main disadvantage, three-phase voltage regulators consume a significant amount of electricity and require significant maintenance costs, as they have low reliability - both electromechanical and electronic voltage regulators have rapid wear and frequently damaged parts, which makes such a decision to reduce phase imbalance expensive.

Therefore, a balancing transformer is usually preferred, as it performs the following functions:
- elimination of phase voltage imbalance,
- evenly distributes the loads in the phases;
- provides set value of phase voltages;

However, the inclusion of a balancing transformer leads to a number of additional problems:
- due to the fact that the balancing transformer is switched on in case of interruption of the mains wires, in case of its damage the power supply of the whole enterprise is disrupted;
- the balancing transformer, according to its principle, must operate in no-load mode, which leads to the fact that it becomes a powerful source of reactive power of inductive nature in the network of the enterprise, which must be further compensated.
- the balancing transformer has a significant weight and dimensions, for example a balancing transformer 160 kVA has a weight of 250 kg, the dimensions are 710x610x640 mm.