At present, in the medium and low voltage distribution network system of the State Grid, the three-phase load system changes randomly. These loads will cause three-phase imbalance in the distribution system. The three-phase load imbalance will lead to the imbalance of three-phase voltage and current in the power supply system, cause negative sequence voltage and negative sequence current of the power grid, affect the power supply quality, increase line loss and reduce power supply reliability.

        Influence of three-phase unbalanced system:

      The three-phase load imbalance will increase the transformer and line loss. Through theoretical calculation, it can be proved that the transformer loss in the case of imbalance is 3 times that in the case of balance, and the line loss is 6 times that in the case of balance.

        When the three-phase load is unbalanced, the heavy load phase current is too large, which is easy to cause overheating of winding and transformer oil. Lead to accelerated winding insulation aging and oil quality deterioration, quickly reduce the service life of the transformer, and even burn the winding.

        When the three-phase load is unbalanced, the heavy load phase current is too large, which may cause the temperature of the phase conductor, switch and other components to rise sharply, resulting in burning and burnout. And because the cross section of neutral wire is generally smaller than that of phase wire, considering the existence of zero sequence harmonic current in power grid, the probability of neutral wire burnout is higher.

        The unbalanced operation of three-phase load will cause excessive zero sequence current of transformer. The hysteresis and eddy current loss caused in the oil tank wall or other metal components of transformer will heat these components, resulting in abnormal temperature rise of local metal parts of transformer, which will lead to transformer operation accident in serious cases.

        The three-phase load imbalance of low-voltage power grid will cause excessive corresponding phase current at the high-voltage side, resulting in tripping and power failure of high-voltage line and large-area power failure. At the same time, frequent tripping of switchgear in substation will reduce the service life.

        When the three-phase load is unbalanced, the output current of each phase is not equal. In order to avoid single-phase overcurrent, the transformer must be used with capacity derating. At the same time, due to the unequal three-phase voltage drop inside the distribution transformer, it will inevitably lead to the three-phase imbalance of the output voltage of the distribution transformer. The voltage of one phase with negative load decreases, while the voltage of one phase with light load increases, which is easy to burn out the user's electrical equipment of one phase with high voltage, while the user's electrical equipment of one phase with low voltage may not be used.

        A large number of harmonic loads generated in the operation of a large number of frequency conversion equipment and rectifier equipment not only seriously lead to the imbalance of power grid, but also seriously affect the normal operation of other power grid equipment, and even lead to the damage of power grid power supply and electrical equipment.