Analysis and Control Strategy for Three-Phase Power Balancing Using Electric Springs

Kwan-Tat Mok, Siu-Shing Ho, Siew-Chong Tan, Shu-Yuen Ron Hui
The University of Hong Kong

The power flow imbalance among different power phases is a general phenomenon in three-phase AC power utility grids. For example, the single-phase loads fed by individual power phases in residential building can cause imbalanced power consumption. The existence of power imbalance can cause excessive negative-sequence and zero-sequence currents circulating in the power system that can affect the system reliability and power quality. This research work presents a general analysis and a control strategy that enables electric springs (ES) to mitigate the negative–sequence and zero–sequence currents in unbalanced three-phase power systems. The analysis indicates that under certain load conditions, power balance can be restored without the need for active power from the ES. Outside such conditions, the theory can pinpoint the precise operating point at which power balance can be achieved with the minimum active power from the ES. Thus, the optimal use of energy usage and battery size for providing/storing active power can be realized. The outcomes of this research work are included in the following.

1. This work provides an in-depth analysis and a control strategy for electric springs (ES) to mitigate the negative–sequence and the zero–sequence currents in three-phase unbalanced power systems.
2. The proposed control strategy provides a method to allow precise calculation of the required ES voltage on each power phase such that (i) the ES can restore the power balance and (ii) the active power delivered from the battery storage can be minimized.
3. The theoretical analysis has been verified under a 3-kW laboratory-scale three-phase-power experiment setup.