Judgment Methods and Countermeasures for Three-phase Unbalance (3)
(3) What are the hazards of three-phase imbalance
1. The power loss of the increasing line
In the three-phase four-wire power supply network, when the current passes through the line wires, there will be power loss due to the existence of impedance, and the loss is proportional to the square of the passing current. When the low-voltage power grid is powered by a three-phase four-wire system, due to the existence of a single-phase load, it is inevitable that the three-phase load will be unbalanced. When the three-phase load is running unbalanced, the neutral line has current flowing through it. In this way, not only the phase line is lost, but also the neutral line is lost, thereby increasing the loss of the grid line.
2. Increase the power loss of distribution transformers
The distribution transformer is the main power supply equipment of the low-voltage power grid. When it operates under the unbalanced three-phase load, it will increase the loss of the distribution transformer. Because the power loss of the distribution transformer varies with the unbalance of the load.
3. Distribution transformer processing reduction
When the distribution transformer is designed, its winding structure is designed according to the load balance operation condition, the 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 is operated under the unbalanced three-phase load condition, the light-loaded one phase has a surplus capacity, which reduces the output of the distribution transformer. The degree of output reduction is related to the unbalance of the three-phase load. The larger the three-phase load unbalances are, the more the distribution transformer output is reduced. For this reason, when the distribution transformer runs when the three-phase load is unbalanced, its output capacity cannot reach the rated value, its spare capacity is also reduced accordingly, and the overload capacity is also reduced. If the distribution transformer operates under overload conditions, it will easily cause the distribution transformer to heat up, and even cause the distribution transformer to burn out in severe cases.
4. Distribution transformer produces zero-sequence current
When the distribution transformer operates under the unbalanced three-phase load, a zero-sequence current will be generated. This current will change with the unbalanced degree of the three-phase load. The greater the unbalanced degree, the greater the zero-sequence current. If there is a zero-sequence current in the distribution transformer in operation, zero-sequence magnetic flux will be generated in its iron core. There is no zero-sequence current on the high-voltage side, which forces the zero-sequence magnetic flux to pass only through the tank wall and steel components. However, the magnetic permeability of steel components is low. When the zero-sequence current passes through the steel components, hysteresis and eddy current losses will be generated, so that the local temperature of the distribution-transformed steel components will increase and generate heat. The winding insulation of the distribution transformer is rapidly aged due to overheating, resulting in a reduction in the life of the equipment. At the same time, the existence of a zero-sequence current will also increase the loss of the distribution transformer.
5. Affect the safe operation of electrical equipment
The distribution transformer is designed according to the three-phase load balance operating conditions, and the resistance, leakage reactance and excitation impedance of each phase winding is basically the same. When the distribution transformer operates in a three-phase load balance, its three-phase currents are basically equal, and the voltage drop of each phase inside the distribution transformer is 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 is not equal, and the three-phase voltage drop inside the distribution transformer is not equal, which will inevitably lead to the three-phase unbalance of the output voltage of the distribution transformer.
At the same time, the distribution transformer operates when the three-phase load is unbalanced, the three-phase output current is different, and the neutral line will have current passing through. As a result, the neutral line produces an impedance voltage drop, which causes the neutral point to drift and causes the voltage of each phase to change. The voltage of one phase with heavy load decreases, while the voltage of one phase with light load increases. Supplying power under the condition of voltage imbalance, that is, it is easy to cause the user's electrical equipment with a high-voltage one-phase connection to burn out, while the user's electrical equipment with a low-voltage one-phase connection may be unusable. Therefore, when the three-phase load is unbalanced, the safe operation of the electrical equipment will be seriously endangered.
6. Reduced motor efficiency
The distribution transformer operates under the unbalanced three-phase load condition, which will cause the three-phase unbalance of the output voltage. 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 braking effect. But 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. However, due to the braking effect of the negative sequence magnetic field, the output power of the motor will be reduced, thereby reducing the efficiency of the motor. At the same time, the temperature rise, and reactive power loss of the motor will also increase with the unbalance of the three-phase voltage. Therefore, it is very uneconomical and unsafe to operate the motor under the condition of a three-phase voltage imbalance.
