A Control Strategy for Cascaded H-Bridge Based STATCOM with Unequal Capacitive DC Links Based on Model Predictive Method

Static Synchronous Compensator (STATCOM) is a shunt grid connected converter used for reactive power/current compensation in electrical networks. Owing to its advantages, cascaded H-bridge inverter with unequal capacitive dc links is utilized in STATCOM application in this paper. In STATCOM application, the voltages of capacitors have to be controlled in addition to reactive current/power control. The studied inverter is composed of cascaded H-bridge cells with capacitive and unequal voltages which makes the regulation and balancing the voltages of capacitors, challenging. In this paper, a strategy based on model predictive control is proposed to meet the control objectives. First, the discrete-time model of the inverter is derived to predict the control variables and then a multi-objective cost function is developed. The input of the multi-objective optimization is the inverter switching states and the proposed control method controls the active and reactive power exchanged with the grid and the voltages of capacitors in each cell. Also, it mitigates the switching frequency in the high-voltage cell. The simulation results are provided on a 27-level asymmetric CHB based STATCOM.