Secondary Control Scheme for Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid

Abstract

The concept of microgrid hierarchical control is presented recently. In this paper, a hierarchical scheme is proposed which includes primary and secondary control levels. The primary level comprises distributed generators (DGs) local controllers. The local controllers mainly consist of power, voltage and current controllers, and virtual impedance control loop. The central secondary controller is designed to manage the compensation of voltage unbalance at the point of common coupling (PCC) in an islanded microgrid. Unbalance compensation is achieved by sending proper control signals to the DGs local controllers. The design procedure of the control system is discussed in detail and the simulation results are presented. The results show the effectiveness of the proposed control structure in compensating the voltage unbalance.

Methodology for the analysis of voltage unbalance in networks with single‐phase distributed generation

“…Based on these developed models, items that have different impedances for different phase sequences are identified and used to construct sequence networks 4 respectively. The unbalance of the distribution lines is modelled and addressed using positive-, negativeand zero-sequence impedances [17], i.e., Z +, Z -and Z 0 in Fig.…”

Tertiary control of voltage unbalance compensation for optimal power quality in islanded microgrids

“…Positive sequence active and reactive power (P + and Q + ) can be extracted by using low pass filters (LPF) [15], [16]. The calculated P + and Q + are then used by droop controller for P + /Q + sharing control, as defined in [15]:…”

“…The total compensation efforts are regulated by secondary control to ensure required power quality in CB which conventionally is equally distributed to each DG unit [15], while (14) is actually a simple but effective approach that maintains the total amount of compensation efforts and adjusts the compensation effort of respective DG. The optimization is aimed at optimal distribution of _ N S total V .…”

Tertiary control for optimal unbalance compensation in islanded microgrids

“…The output active and reactive power of the inverter is first calculated based on the instantaneous power theory [8]. Positive sequence active and reactive power (P + and Q + ) can be extracted by using low pass filters (LPF) [4]. The calculated P + and Q + are then used by droop controller for P + /Q + sharing control.…”

“…In addition, the compensation efforts can be shared among DGs so as to distribute the current of a single compensator under severe unbalance conditions. hierarchical control is proposed so as to compensate unbalance voltage in the point of common coupling (PCC) in an islanded system [4]. However, the power quality in DG terminals and other buses is sacrificed.…”