Causes of asymmetric modes in electrical networks

A symmetrical three-phase voltage system is characterized by voltages identical in magnitude and phase in all three phases. In asymmetric modes, the voltages in the different phases are not equal.

Asymmetric modes in electrical networks arise due to the following reasons:

1) uneven loads in different phases,

2) incomplete operation of lines or other elements in the network,

3) different line parameters in different phases.

Most often, voltage imbalance occurs due to the inequality of the phase loads. Since the main cause of the voltage imbalance is the phase difference (unbalanced load), this phenomenon is most characteristic of low-voltage electrical networks of 0.4 kV.

In urban and rural networks of 0.4 kV, voltage asymmetry is mainly caused by the connection of single-phase lighting and low-power household electrical consumers. The number of such single-phase power consumers is large and they must be evenly distributed over phases to reduce unbalance.

In high-voltage networks, asymmetry is caused, as a rule, by the presence of powerful single-phase electrical receivers, and in some cases three-phase electrical receivers with uneven phase consumption. The latter include arc furnaces for steel production. The main sources of asymmetry in industrial networks 0.38-10 kV are single-phase thermal installations, ore thermal furnaces, induction melting furnaces, resistance furnaces and various heating installations. In addition, asymmetric electric receivers are welding machines of different power. Traction substations of electrified AC railway transport are a powerful source of asymmetry, since electric locomotives are single-phase electrical receivers. The power of individual single-phase electric receivers currently reaches several megawatts.

There are two types of asymmetry: systematic and probabilistic or random. Systematic asymmetry is caused by non-uniform constant overloading of one of the phases, probabilistic asymmetry corresponds to non-constant loads in which different phases are overloaded at different times depending on random factors (periodic asymmetry).

Incomplete operation of network elements is caused by a short-term disconnection of one or two phases during a short circuit or a longer disconnection during staged repairs. A single line may be equipped with phasing control devices which disconnect the faulted phase of the line in cases where the automatic reclosing operation fails due to a sustained short circuit.

The majority of stable short circuits are single-phase.In this case, the interruption of the damaged phase leads to the preservation of the other two phases of the line in operation.

In a network with an earthed neutral power supply on a line with an incomplete phase can be acceptable and allows you to abandon the construction of a second circuit on the line. Half-phase modes can also occur with transformers turned off.

In some cases, for a group composed of single-phase transformers, in the event of an emergency shutdown of one phase, it may be acceptable to supply two phases. In this case, the installation of a spare phase is not required, especially if there are two groups of single-phase at the substation transformers.

The inequality of the parameters of the phase lines occurs, for example, in the absence of transposition along the lines or its extended cycles. Transpose supports are unreliable and a source of crashes. Reducing the number of transposition supports along the line reduces its damage and increases reliability. In this case, the alignment of the linear phase parameters deteriorates, for which transposition is usually applied.

Effect of voltage and current imbalance

The appearance of voltages and currents of the reverse and zero sequence U2, U0, I2, I0 leads to additional power and energy losses, as well as voltage losses in the network, which worsens the modes and technical and economic indicators of its operation. The currents of the reverse and zero sequences I2, I0 increase the losses in the longitudinal branches of the network, and the voltages and currents of the same sequences - in the transverse branches.

The superposition of U2 and U0 leads to different additional voltage deviations in different phases. As a result, voltages may be out of range.The superposition of I2 and I0 leads to an increase in the total currents in individual phases of the network elements. At the same time, their heating conditions deteriorate and productivity decreases.

The imbalance negatively affects the operational and technical-economic characteristics of rotating electrical machines. The positive sequence current in the stator creates magnetic fieldrotation with synchronous frequency in the direction of rotation of the rotor. Negative sequence currents in the stator create a magnetic field that rotates relative to the rotor at double synchronous frequency in the opposite direction of rotation. Due to these two-frequency currents, a braking electromagnetic torque and additional heating, mainly of the rotor, occur in the electric machine, which leads to a reduction in the life of the insulation.

In asynchronous motors, additional losses occur in the stator. In some cases, in the design, it is necessary to increase the rated power of the electric motors, if no special measures are taken to balance the voltage.

In synchronous machines, in addition to additional losses and heating of the stator and rotor, dangerous vibrations can begin. Due to unbalance, the service life of transformer insulation is shortened, synchronous motors and capacitor banks reduce reactive power generation.

The voltage imbalance in the supply circuit of the lighting load leads to the fact that the luminous flux of the lamps of one phase (phases) decreases, and that of the other phase increases, and the life of the lamps decreases. Unbalance affects single-phase and two-phase electrical receivers as a voltage deviation.

Common damages caused by asymmetry in industrial networks include the cost of additional power losses, increase in renovation deductions from capital costs, technological damage, damage caused by a decrease in the luminous flux of lamps installed on phases with reduced voltage, and a reduction of the life of lamps installed on phases with increased voltage, failure due to a decrease in reactive power generated by capacitor banks and synchronous motors.

The voltage imbalance is characterized by the negative sequence coefficient of the voltages and the zero ratio of the voltages, whose normal and maximum permissible values ​​are 2 and 4%.

Balancing network voltages comes down to negative sequence current and voltage compensation.

With a stable load curve, a reduction of the system voltage imbalance in the network can be achieved by equalizing the phase loads by switching part of the loads from an overloaded phase to an unloaded one.

Rational redistribution of loads does not always allow reducing the voltage unbalance coefficient to an acceptable value (for example, when part of powerful single-phase electric receivers do not work according to technology all the time, as well as during preventive and major repairs). In these cases, it is necessary to use special balloons.

A large number of balun circuits are known, some of them are controlled depending on the nature of the load curve.

To balance single-phase loads, a circuit consisting of inductance and capacitance… The load and the capacitance connected in parallel with it are connected to the line voltage. The other two line voltages include an inductance and another capacitance.

For balancing two- and three-phase unbalanced loads, a circuit of unequal capacitance of capacitor banks connected in a delta is used. Sometimes baluns are used with special transformers and autotransformers.

Since the baluns contain capacitor banks, it is advisable to use circuits where the mode is both balanced and Q is generated to compensate for it. Devices for simultaneous mode balancing and Q compensation are under development.

The reduction of unbalance in four-wire city networks of 0.38 kV can be carried out by reducing the zero-sequence current I0 and reducing the zero-sequence resistance Z0 in the network elements.

The reduction of the zero-sequence current I0 is mainly achieved by redistribution of the loads. Load equalization is achieved by using networks in which all or part of the transformers operate in parallel on the low voltage side. A reduction of the zero-sequence resistance Z0 can be easily realized for 0.38 kV overhead lines, which are usually built in areas with low load density. The possibility of reducing Z0 for cable lines, i.e. increasing the cross-section of the neutral conductor, must be specifically justified with appropriate technical and economic calculations.

The connection scheme of the windings of the distribution transformer has a significant influence on the voltage imbalance in the network.6-10 / 0.4 kV.Most distribution transformers installed in networks are star star with zero (Y / Yo). Such distribution transformers are cheaper, but have a high zero-sequence resistance Z0.

To reduce the voltage imbalance caused by the distribution transformers, it is recommended to use star-delta with zero (D / Yo) or star-zigzag (Y / Z) connection schemes. The most favorable for reducing the asymmetry is the use of the U / Z scheme. Distribution transformers with this connection are more expensive and very labor-intensive to manufacture. Therefore, they must be used with a large asymmetry due to the asymmetry of the loads and the zero-sequence resistance Z0 of the lines.