Hierarchical Control Scheme for Voltage Unbalance Compensation in Islanded Microgrids Mehdi Savaghebi 1 , Josep M. Guerrero 2,3 , Alireza Jalilian 1 , and Juan C. Vasquez 3 1 Center of Excellence for Power System Automation and Operation, Iran University of Science and Technology, Iran 2 Department of Automatic Control and Industrial Informatics, Technical University of Catalonia, Spain 3 Institute of Energy Technology, Aalborg University, Denmark savaghebi@iust.ac.ir , joz@et.aau.dk , jalilian@iust.ac.ir , juq@et.aau.dk Abstract— The concept of microgrid hierarchical control is presented, recently. In this paper, a hierarchical scheme which includes primary and secondary control levels is proposed for islanded microgrids. The primary control level consists of DG local controllers. Local controller of each DG comprises active and reactive power controllers, virtual impedance loop and voltage and current controllers. The secondary level is designed to compensate the voltage unbalance at the load bus (LB) of the islanded microgrid. Also, restoration of LB voltage amplitude and microgrid frequency to the rated values is considered in the secondary level. These functions are achieved by proper control of distributed generators (DGs) interface converters. The presented simulation results show the effectiveness of the proposed control structure in compensating the voltage unbalance and restoring the voltage amplitude and system frequency. Keywords-distributed generation; microgrid; hierarchical control; voltage unbalance compensation I. INTRODUCTION Voltage unbalance can result in adverse effects on equipment and power system. Under unbalanced conditions, the power system will incur more losses and be less stable. Also, voltage unbalance has some negative impacts on equipment such as induction motors, power electronic converters and adjustable speed drives (ASDs). Thus, the International Electrotechnical Commission (IEC) recommends the limit of 2% for voltage unbalance in electrical systems [1]. A major cause of voltage unbalance is the connection of unbalanced loads (mainly, single-phase loads connection between two phases or between one phase and the neutral). Compensation of voltage unbalance is usually done using series active power filter through injection of negative sequence voltage in series with the power distribution line [2]-[4]. However, there are a few works [5]-[8] based on using shunt active power filter for voltage unbalance compensation. In these works, voltage unbalance caused by unbalanced load is compensated through balancing the line currents. On the other hand, it is well-known that the Distributed Generators (DGs) often consist of a prime mover connected through an interface converter (e.g. an inverter in the case of dc-to-ac conversion) to the ac power distribution system. The distribution system may be the utility grid or the local grid formed by a cluster of DGs which is called microgrid. The main role of DG inverter is to adjust output voltage phase angle and amplitude in order to control the active and reactive power injection. In addition, compensation of power quality problems, such as voltage unbalance can be achieved through proper control strategies. In [9]-[12], some approaches are presented to use the DG for voltage unbalance compensation. A method for voltage unbalance compensation through injection of negative sequence current by the DG has been proposed in [9]. By applying this method, line currents become balanced in spite of the unbalanced loads presence. However, under severely unbalanced conditions, a large amount of the interface converter capacity is used for compensation and it may interfere with the active and reactive power supply by the DG. The approach presented in [10] is based on controlling the DG as a negative sequence conductance to compensate the voltage unbalance at the microgrid DGs output. In this approach which is implemented in the synchronous (dq) reference frame, compensation is done by generating a reference for negative sequence conductance based on the negative sequence reactive power. Then, this conductance is applied to produce the compensation reference current. In [11] some improvements are made to the approach of [10]. Also, it is noteworthy that the control system of [11] is completely designed in stationary (αβ) reference frame. A similar control structure is applied for a grid-connected DG [12], where a PI controller is used to follow the reference of voltage unbalance factor. The methods proposed in [10]-[12] are designed for compensation of voltage unbalance compensation at the DG terminal, while usually the power quality at the load bus (LB) is the main concern; since, sensitive loads maybe connected to LB. Thus, in this paper the concept of microgrid secondary control [13] is applied to compensate the voltage unbalance at LB in an islanded microgrid. Also, restoration of LB voltage amplitude and microgrid frequency to the rated values is considered in the control structure. Proportional-integral (PI) controllers are used to generate the reference control signals at the central secondary controller. These references are sent to DGs local controllers by low band-width communication (LBC).