load is unbalanced, is it better to use three-phase capacitors or to use single-phase capacitors?

When the load is unbalanced, is it better to use three-phase capacitors for simultaneous switching on/off or to use single-phase capacitors for separate phase switching? Under the condition of load balance, the performance of various forms of reactive power compensation devices is theoretically the same. However, when the load is unbalanced, the performance will be quite different. Now, we will make a theoretical comparison of several types of reactive power compensation devices.

If the reactive power compensation device that uses the simultaneous switching of three-phase capacitors is employed for compensation, since the inductance values of the three phases are different, the compensation can only be carried out based on the inductance value of the A phase. Otherwise, over-compensation will occur in the A phase. By comparing the data obtained from practice, it can be seen that after the three-phase capacitors are put into use, although the currents of all three phases have decreased, the imbalance degree among the three phases has actually intensified. Before the capacitors were installed, the current ratio of phase C to phase A was 2:1, but after the installation, the current ratio of phase C to phase A became 2.24:1. The zero-line current did not change before and after the installation of the capacitors, because the zero-line current is caused by the unbalanced current. Since the three-phase capacitors do not have a zero-line connection, the zero-line current cannot change. Therefore, the reactive power compensation device with simultaneous switching of three-phase capacitors is not suitable for application in systems with severely unbalanced three-phase currents.

If a reactive power compensation device that uses single-phase capacitor phase-by-phase switching for compensation is employed, based on the data comparison obtained from practical applications, it can be observed that after the corresponding single-phase capacitor is put into use, the currents of all three phases have decreased, while the imbalance degree among the three phases remains unchanged. Before the capacitor was installed, the current ratio between phase C and phase A was 2:1 compared to the capacitor. After the capacitor was installed, the current ratio between phase C and phase A remained 2:1. The neutral line current decreased after the capacitor was installed, because the unbalanced reactive current in the neutral line was eliminated, leaving only the unbalanced active current component. Thus, it can be concluded that in systems with severely unbalanced three-phase currents, the use of the reactive power compensation device with single-phase capacitor phase-by-phase switching is more effective than that of the device with simultaneous three-phase capacitor switching.