Three-Phase Unbalanced Hazards and Treatment

Three-phase imbalance is an important indicator of power quality. Although there are many factors that affect the power system, normal imbalance is mostly due to asymmetry of three-phase components, line parameters or loads. Since the three-phase load factor is not certain, the three-phase voltage and current at the power supply point are easily unbalanced and cause line losses. Not only that, it will also have an adverse impact on the motor at the power supply point and endanger the normal operation of the motor. Therefore, if the three-phase imbalance exceeds the tolerance of the distribution network, the safe operation of the entire power system will be affected.

Causes of three-phase imbalance in distribution network

The first reason is the unreasonable distribution of three-phase loads. Many workers who install meters and connect electricity do not have professional knowledge and concepts about three-phase load balance. Therefore, they do not pay attention to controlling the three-phase load balance when connecting electricity. They just blindly and casually connect and install circuits. Table, which to a large extent causes the unbalance of the three-phase load. Secondly, most circuits in our country combine power and lighting, so when single-phase electrical equipment is used, the efficiency of electricity consumption will be reduced. This difference further aggravates the inconsistency of the three-phase load of the distribution transformer. equilibrium condition.

The second reason is the changing electrical load. Reasons for unstable electricity loads include frequent demolitions, meter relocations or increases in electricity users in Land II; and the instability of temporary and seasonal electricity consumption. Such uncertainty and non-concentration in total amount and time make the electricity load have to change with the actual situation.

The third reason is the weakening of monitoring of distribution transformer loads. In the management of distribution network, the management issues in three-phase load distribution are often ignored. In the detection of distribution network, the three-phase load of distribution transformer is not regularly detected and adjusted. In addition, there are many factors that cause three-phase unbalance, such as the influence of lines and the imbalance of three-phase load moments, etc.

The dangers of three-phase imbalance

1. Increase the power loss of the line

In a three-phase four-wire power supply network, when current passes through line conductors, electrical energy loss will inevitably occur due to the existence of impedance, and the loss is proportional to the square of the passing current. When the low-voltage power grid uses a three-phase four-wire system to supply power, it is inevitable that the three-phase load will be unbalanced due to the presence of single-phase loads. When the three-phase load is running unbalanced, current flows through the neutral line. In this way, not only the phase lines have losses, but also the neutral lines have losses, thereby increasing the losses of the power grid lines.

2. Increase the power loss of distribution transformers

Distribution transformers are the main power supply equipment of low-voltage power grids. When they operate under unbalanced three-phase load conditions, distribution transformer losses will increase. Because the power loss of the distribution transformer changes with the imbalance of the load.

3. The distribution transformer output is reduced

When designing the distribution transformer, its winding structure is designed according to load-balanced operating conditions, its winding performance is basically the same, and the rated capacity of each phase is equal. The maximum allowable output of the distribution transformer is limited by the rated capacity of each phase. If the distribution transformer operates under unbalanced three-phase load conditions, the light-loaded phase will have excess capacity, thereby reducing the output of the distribution transformer. The degree of output reduction is related to the unbalance of the three-phase load. The greater the unbalance of the three-phase load, the greater the reduction in distribution transformer output. For this reason, when the distribution transformer operates when the three-phase load is unbalanced, its output capacity will not reach the rated value, its reserve capacity will be reduced accordingly, and its overload capacity will also be reduced. If the distribution transformer is operated under overload conditions, it will easily cause the distribution transformer to heat up, and in severe cases, it may even cause the distribution transformer to burn out.

4. Distribution transformer generates zero sequence current

When the distribution transformer operates under unbalanced three-phase load conditions, a zero-sequence current will be generated, which will vary with the degree of unbalanced three-phase load. The greater the unbalance, the greater the zero-sequence current. If there is a zero-sequence current in a running distribution transformer, zero-sequence magnetic flux will be generated in its core. (There is no zero-sequence current on the high-voltage side) This forces the zero-sequence magnetic flux to pass through the tank wall and steel components only, and the steel components have low magnetic permeability. When the zero-sequence current passes through the steel components, hysteresis and eddy currents will be generated. Loss, thereby causing the local temperature of the steel components of the distribution transformer to increase and generate heat. The winding insulation of the distribution transformer ages faster due to overheating, resulting in reduced equipment life. At the same time, the existence of zero sequence current will also increase the loss of distribution transformers.

5. Affect the safe operation of electrical equipment

The distribution transformer is designed according to the three-phase load balanced operating condition, and the resistance, leakage reactance and excitation impedance of each phase winding are basically the same. When the distribution transformer operates when the three-phase load is balanced, its three-phase currents are basically equal, and the voltage drop of each phase inside the distribution transformer is also basically the same, so the three-phase voltage output by the distribution transformer is also balanced. If the distribution transformer operates when the three-phase load is unbalanced, the output current of each phase will not be equal, and the three-phase voltage drops inside the distribution transformer will not be equal, which will inevitably lead to the three-phase imbalance of the output voltage of the distribution transformer. At the same time, when the distribution transformer operates when the three-phase load is unbalanced, the three-phase output current is different, and current will flow through the neutral line. This causes an impedance voltage drop in the neutral line, causing the neutral point to drift and causing the voltage of each phase to change. The voltage of the heavily loaded phase decreases, while the voltage of the lightly loaded phase increases. If power is supplied under unbalanced voltage conditions, it is easy to cause the user's electrical equipment connected to the high-voltage phase to burn out, while the user's electrical equipment connected to the low-voltage phase may not be usable. Therefore, unbalanced operation of three-phase loads will seriously endanger the safe operation of electrical equipment.

6. Motor efficiency decreases

When the distribution transformer operates under unbalanced three-phase load conditions, the three-phase output voltage will be unbalanced. Since the unbalanced voltage has three voltage components: positive sequence, negative sequence, and zero sequence, when this unbalanced voltage is input to the motor, the rotating magnetic field generated by the negative sequence voltage is opposite to the rotating magnetic field generated by the positive sequence voltage, which acts as a brake. effect. However, since the positive sequence magnetic field is much stronger than the negative sequence magnetic field, the motor still rotates in the direction of the positive sequence magnetic field. Due to the braking effect of the negative sequence magnetic field, the output power of the motor will inevitably be reduced, resulting in a reduction in motor efficiency. At the same time, the temperature rise and reactive power loss of the motor will also increase with the imbalance of the three-phase voltage. Therefore, it is very uneconomical and unsafe for the motor to operate under unbalanced three-phase voltage conditions.

Technology to improve three-phase imbalance in distribution network

1. Pay attention to the reasonable distribution of three-phase loads

Regarding the distribution of three-phase loads, electric power workers should carefully collect and record relevant data in actual work to achieve a state where the electrical load can be predicted to a certain extent. Secondly, a better three-phase balanced distribution problem can be achieved by installing a balancing device. In some areas that use low-voltage three-phase four-wire systems, reactive power compensation devices to adjust unbalanced current can be added to solve the various consequences of unbalanced current phenomena that often occur in the power grid. Such a device can not only compensate for the reactive power of the system, but also adjust the effect of unbalanced active currents. In addition, according to the different load moments in the actual situation, appropriate adjustment of the wiring method also has a certain impact on the reasonable distribution of the three-phase load.

2. Methods to control unbalanced current in three-phase loads

According to the compensation principle of unbalanced current susceptance, at any certain moment, there are mainly three-phase unbalanced loads that are not grounded. Then each of their phase loads can be connected in parallel with the same resistor and capacitor. Therefore, under the guidance of the unbalanced current control susceptance compensation theory, the equivalence of different properties can be analyzed to determine the reactive power compensation amount between phases and relative to ground. When distribution transformers need to compensate for unbalanced currents, the following principles should be met. First, it should be noted that current management should have two contents, one is to compensate the power factor, and the other is to adjust the three-phase current imbalance. The two together determine the reactive power required for compensation. The second point is that during actual project construction, a fully capacitive management method should be adopted to distinguish it from inductance compensation to avoid serious over-compensation. The third point is that it is necessary to consider that the load will change with time. Based on this characteristic, the compensation amount should also be appropriately adjusted according to the change of the load. The fourth point is reflected in the limitation of the number of switching of device switches and compensation equipment. The optimization plan of the whole day must be strategically managed during the design. In short, when setting the proportional adjustment coefficient, it is necessary to consider both the power factor limiting conditions and the over-compensation limiting conditions.

3. Add detection and adjustment of three-phase load

Regular inspection of three-phase loads is also very necessary. After reasonable distribution and control of three-phase compliance, relevant departments should initiate testing work. The balance of electricity cannot be absolute, but can only be achieved by trying to achieve a relative balance. In actual testing work, each department should use the balance measurement indicators formulated by the state and relevant departments as a standard to conduct professional testing results. Record and analyze, and regularly detect the load current of each phase, so as to promptly detect some three-phase unbalanced conditions. When parts with potential safety hazards are discovered during the inspection process, adjustments and modifications must be made in a timely manner. We should also be more vigilant about areas where no problems were found during the inspection. After the test is completed, it is not only necessary to organize and analyze the data, but also provide timely feedback. The feedback here mainly refers to the adjustments that need to be made to the three phases inferred based on the test results, and the prediction of the possibility of using new technologies in the three phases. Through reasonable detection and in-depth analysis of the detection results, we can avoid the occurrence of imbalances to the greatest extent and reduce the occurrence of electrical accidents.

HZ-600H Transformer Winding Deformation Tester