Improved Reactive Power Sharing for Parallel-operated Inverters in Islanded Microgrids

Abstract

The unequal impedances of the interconnecting cables between paralleled inverters in the island mode of microgrids cause inaccurate reactive power sharing when the traditional droop control is used. Many studies in the literature adopt low speed communications between the inverters and the central control unit to overcome this problem. However, the losses of this communication link can be very detrimental to the performance of the controller. This paper proposes an improved reactive power-sharing control method. It employs infrequent measurements of the voltage at the point of common coupling (PCC) to estimate the output impedance between the inverters and the PCC and then readjust the voltage droop controller gains accordingly. The controller then reverts to being a traditional droop controller using the newly calculated gains. This increases the immunity of the controller against any losses in the communication links between the central control unit and the inverters. The capability of the proposed control method has been demonstrated by simulation and experimental results using a laboratory scale microgrid.

A Communicationless PCC Voltage Compensation Using an Improved Droop Control Scheme in Islanding Microgrids

“…In this case, the measured feeder resistance and inductance for the controller calculation are 0.075Ω and 1.5mH, respectively. According to (6), the virtual impedance of DG unit 1 should be set to 0.125Ω and 1.5mH for achieving the proper power sharing.…”

“…However, this case is not always valid since the line impedance in low voltage microgrids is not highly inductive [14]- [16]. Therefore, the traditional droop control scheme faces the challenges of the coupling between the real and reactive power control and they cannot achieve effective power sharing [6]- [10], [14]- [17]. To solve this issue, a virtual impedance can be inserted into the control loop to regulate the equivalent impedance to be inductive or resistive [6]- [9], [14]- [17].…”

“…Therefore, the traditional droop control scheme faces the challenges of the coupling between the real and reactive power control and they cannot achieve effective power sharing [6]- [10], [14]- [17]. To solve this issue, a virtual impedance can be inserted into the control loop to regulate the equivalent impedance to be inductive or resistive [6]- [9], [14]- [17]. It has been demonstrated that the flexible operation of virtual impedance can effectively decouple the power control and improve the reactive power sharing accuracy [14], [15], [17].…”

Supervisory Control for Power Management of an Islanded AC Microgrid Using a Frequency Signalling-Based Fuzzy Logic Controller

“…Droop control [22], [23] is used in all the three DC/AC inverters as a frequency responsive technique. The details of power management based on droop control and bus frequency signalling techniques will be discussed in the next section.…”

“…It adds extra cost and complexity in controlling the overall power system. Furthermore, the communication affects the expandability of the power system [8]. The design of an effective coordination strategy becomes a challenging task if communication between different units is not used because there will be neither a central energy management system nor a direct interaction between the different units.…”