1, increase the power loss of the line
In the three-phase four-wire power supply network, when the current passes through the line conductor, there will be an electrical energy loss due to the impedance, and the loss is proportional to the square of the passing current. When the low-voltage power grid is supplied with three-phase four-wire system, the unbalanced three-phase load is inevitable due to the existence of single-phase load. When the three-phase load is unbalanced, the neutral line has current passing through it. In this way, not only the phase line has a loss, but also the neutral line has a loss, thereby increasing the loss of the grid line.
2, increase the power loss of the distribution transformer
Distribution transformer is the main power supply equipment of low voltage network. When it runs under the unbalanced condition of three-phase load, it will cause the increase of distribution loss. Because the power loss of the configuration changes with the unbalance of the load.
3, the transformer output is reduced
The structure of the winding is designed according to the load balance operation condition, the performance of the winding is basically the same, and the rated capacity of each phase is equal. The maximum allowable output of the transformer is limited by the rated capacity of each phase. If the transformer is operating under the condition of three-phase load imbalance, the light load phase has surplus capacity, so that the output of the transformer is reduced. The degree of output reduction is related to the unbalance of the three-phase load. The larger the three-phase load unbalance is, the more the output of the transformer is reduced. For this reason, the transformer is operated when the three-phase load is unbalanced, and its output capacity cannot reach the rated value, its standby capacity is correspondingly reduced, and the overload capacity is also reduced. If the transformer is operated under overload conditions, it is easy to cause the transformer heating, and even cause the transformer burning loss in serious cases.
4, the distribution of zero sequence current
The zero sequence current will be generated when the three-phase load is unbalanced, and the current will change with the degree of the three-phase load unbalance. The greater the degree of unbalance, the greater the zero sequence current. If there is zero sequence current in the running transformer, zero sequence flux will be generated in its core. (There is no zero sequence current on the high voltage side) This forces the zero sequence flux to pass through the tank wall and the steel component only as a channel, and the magnetic permeability of the steel component is low, that is, when the zero sequence current passes through the steel component, hysteresis and eddy current loss will be generated, thus causing the local temperature of the steel component to increase and heat up. The aging of the modified winding insulation is accelerated due to overheating, resulting in reduced equipment life. At the same time, the existence of zero sequence current will also increase the loss of the configuration.
5, affect the safe operation of electrical equipment
The configuration is designed according to the three-phase load balancing operation condition, and the resistance, leakage reactance and excitation impedance of each phase winding are basically the same. When the transformer operates in three-phase load balancing, its three-phase current is basically equal, and the voltage drop of each phase inside the transformer is basically the same, then the three-phase voltage output of the 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 transformer operates when the three-phase load is unbalanced, the three-phase output current is not the same, and the neutral line will have current through. Therefore, the neutral line generates impedance voltage drop, which leads to neutral drift, resulting in changes in the voltage of each phase. The voltage of the first phase with a heavy load decreases, while the voltage of the first phase with a light load increases. Power supply under the condition of voltage imbalance, that is, it is easy to cause the user electrical equipment of the high voltage one-phase connection band to burn out, and the user electrical equipment of the low voltage one-phase connection band may not be used. Therefore, when the three-phase load is unbalanced, the safe operation of the electrical equipment will be seriously jeopardized.
6, the motor efficiency is reduced
The output voltage will be unbalanced in three phases when the load is unbalanced in three phases. Since the unbalanced voltage has three voltage components: positive sequence, negative sequence and zero sequence, when the 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 plays a braking role. However, because 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 negative sequence magnetic field, the output power of the motor will be reduced, and the efficiency of the motor will be reduced. 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 for the motor to operate under the condition of three-phase voltage imbalance.