During the operation of 380 Volt power networks, violations are possible that significantly affect the quality of the electricity supplied to the consumer. One of such deviations is phase imbalance, which manifests itself in their uneven distribution over the loads connected to the line. The result of this effect is a significant reduction in the power of the equipment included in the industrial three-phase network (transformers or motors, in particular). At home, it is fraught with damage to household appliances connected to one of the phases of the power grid of a country house. This is due to the fact that the voltage in it becomes greatly underestimated, or vice versa - exceeds the permissible norm. A number of organizational and technical measures have been developed to prevent the negative consequences of different voltages at 380 Volt phases.
Allowable misalignment rates
To limit the permissible voltage deviations due to phase imbalance, standards have been developed that regulate their values for industrial power networks. If these standards are exceeded, there is a real danger of failure of the power equipment connected to this line. Their exact values are given in the relevant GOSTs and other documents that determine the procedure for the operation of electrical equipment (in the PUE, for example).
In accordance with the standards, fixed ratios are established between the ratings of voltages and currents in the least and most loaded sections of the lines. For power distribution boards, it should not exceed 30%, and for inputs to private houses (ASU) - 15%. According to the current GOSTs, the permissible phase imbalance for individual lines with reverse currents cannot be more than 2 percent, and for the neutral conductor of the transformer - 4 percent.
Causes of phase imbalance in a three-phase network
There are several known reasons for the appearance of phase imbalance in three-phase networks, the main ones of which are considered to be:
- Uneven distribution of the operating power according to the loads connected to each of the phase lines.
- "Zero break", most often manifested in the burning off of the neutral.
- Other malfunctions in the station equipment or in local consumers connected to it.
In the first case, the power consumed by the linear load sharply increases (or decreases), which leads to a corresponding change in the current flowing in this branch.
In the absence of phase imbalance, current components of equal magnitude flow along each of the lines connected according to the "star" scheme. Their resultant in neutral due to vector addition of three separate components should theoretically be equal to zero. With an increase in consumption along one of the lines, the current components through it increase, as a result of which the neutral wire does not fulfill its function and violates the uniformity of the distribution of phase potentials.
In the event of a neutral break (zero burnout), a skew arises due to the fact that the neutral wire function is automatically transferred to one of the phase conductors; in this case, the voltage on all others is shifted upward. Disturbances in the operation of station equipment also lead to uneven distribution along the phase lines, but already on the side of the transformer "star", and not on the object connected to it (a country house, in particular).
Violation of symmetry in high-voltage networks
In high voltage networks, the appearance of undesirable asymmetry is associated with the presence of powerful single-phase loads or three-phase consumers with unequal phase distribution. Sources of distortion in industrial networks of 0.38-10 kV are various types of electric melting furnaces (ore thermal, induction and similar heating installations). The list of equipment that creates asymmetry should include inverter welding machines, characterized by high consumption currents and capable of disrupting the uniformity of load distribution.
Traction substations of railway transport are powerful sources of dangerous asymmetry, since modern electric locomotives are single-phase consumers of electrical energy. Their power reaches several hundred kilowatts, which only increases the likelihood of disturbances in the distribution of loads.
You can verify their presence with the help of special current clamps, through which it is possible to check circuits that operate with overload. If current values are found in one of the phases, which significantly exceed the permissible values, we can safely speak of the presence of a dangerous skew.
Negative effect of voltage and current imbalance
It is necessary to respond promptly to the appearance of phase asymmetry for the following reasons:
- In this case, there is a real threat of damage to devices connected to this network or a deterioration in their performance.
- This leads to disruptions in the operation of power sources (substation transformers, in particular).
- Another consequence of the abnormal phase distribution is a decrease in the service life of the plant equipment.
For the average consumer, the consequences of asymmetry translate into increased costs for electricity, repair of household appliances, and the possibility of injury. If the skew in the line is caused by the destruction of the neutral wire, the conditions for protection against electric shock deteriorate markedly. The bus of the grounding device (ZU), mounted on the transformer substation, is cut off; in the absence of a local loop, the user remains absolutely defenseless.
With the appearance of asymmetry in industrial networks, electricity consumption also increases, and the line equipment included in them experiences severe overloads. At distribution substations, oil consumption in transformers increases sharply, and control and distribution equipment can fail. All these threats ultimately lead to additional material costs associated with the need to repair or replace burnt-out equipment.
To prevent such situations, you will need to think in advance about effective measures to help prevent them. If it is not possible to avoid phase imbalance, you will have to use all possible ways to eliminate it.
Protection methods
To ensure the symmetrical operation of power networks and the normalization of the voltage value in each of the single-phase lines, special correcting devices are used. This function is most often performed by classic voltage stabilizers. However, these devices are not able to completely eliminate the unbalance in the supply circuits, since their purpose is to stabilize only one phase. For this reason, it is not possible to protect the entire three-phase network with such devices, as well as to eliminate the consequences of the skew.
Situations are not excluded when the stabilizers themselves become the cause of uneven distribution of electricity in phases.
To protect three-phase circuits from phase asymmetry, the following organizational and technical methods are used:
- high-quality study of the power supply project, taking into account the unevenness of the loads;
- the use of special devices with which it is possible to automatically align them (the so-called balancing transformers);
- correction of the current energy consumption schemes (if mistakes were made earlier).
Significant assistance in protection against asymmetry is provided by special blocking equipment (phase and voltage monitoring relays, for example), which disconnects the line when violations are detected.
Only timely measures taken will eliminate the phase imbalance in the network and eliminate the negative consequences of this phenomenon: protect equipment and household appliances from breakdowns.