Improved P-f/Q-V and P-V/Q-f Droop Controllers for Parallel Distributed Generation Inverters in AC Microgrid Chethan Raj Da,⁎, D.N. Gaonkara, Josep M. Guerrero-aDepartment of Electrical and Electronics Engineering, NITK, Surathkal, Mangalore, India bDepartment of Energy Technology, Aalborg University, Aalborg

Improved P-f/Q-V and P-V/Q-f Droop Controllers for Parallel Distributed Generation Inverters in AC Microgrid Chethan Raj Da,⁎, D.N. Gaonkara, Josep M. Guerrerob

 aDepartment of Electrical and Electronics Engineering, NITK, Surathkal, Mangalore, India bDepartment of Energy Technology, Aalborg University, Aalborg, Denmark joz@et.aau.dk

Abstract-Distributed generation inverters are generally operated in parallel with P-f/Q-V and PV/ Q-f droop control strategies. Due to mismatched resistive and inductive line impedance, power sharing and output voltage of the parallel DG inverters deviate from the reference value. This leads to instability in the microgrid system. Adding virtual resistors and virtual inductors in the control loop of droop controllers improve the power sharing and stability of operation. But, this leads to voltage drop. Therefore, an improved P-f/Q-V and P-V/Q-f droop control is proposed. Simulation results demonstrate that the proposed control and the selection of parameters enhance the output voltage of inverters.
Keywords-Distributed generation inverters, droop control, microgrid, output impedance, virtual resistors, virtual inductors.

 1. Introduction Distributed generation (DG) systems use renewable energy resources such as wind, solar, tidal energy, and some non-renewable energy sources such as fuel cells, gas turbines, microturbines, and generators [1]. As compared to traditional power systems, DG systems are decentralized and highly flexible [2]. Hence, accounts for reduced transmission cost and improved stability and reliability of power systems [2]. The distributed power supply in DG systems is not controllable. When directly connected, causes negative impact on the power grid [3]. To avoid this adverse effect on the power grid, United States Electrical Reliability Technology Solutions Consortium has studied the role of distributed power in low-voltage power grids and proposed the concept microgrid [4],[5].

Microgrid can be categorized as AC, DC and AC-DC microgrids [5], [6],[7]. In AC microgrids the parallel operation of DG inverters can be divided into wired and wireless parallel control. Wired parallel control include circular chain control (3C) [8], centralized control [9], and master slave control [10], among others. Wired parallel control strategy uses interconnected signal lines for communication between the DG inverters. However, too many communication signal lines leads to a complex structure of the microgrid and inhibits expansion. In order to solve the signal line problem of wired interconnection control, a wireless parallel control strategy 2 based on active and reactive power droop control is proposed [11], [12]. Wireless control includes droop control [10], [12],[33] reverse droop control [14], hierarchical droop control [12], improved droop control [15], [16], [17] and virtual power droop control [18], among others.

Droop control in a microgrid has a broad application prospects, as it does not require physical communication links and easy to achieve plug and play operation [11]. The traditional P-f/Q-V droop control and P-V/Q-f droop control as the research background, the main research is summarized on the following aspects: Droop decoupling control strategy [12], droop coefficient self-tuning optimization algorithm [19], virtual impedance control [20], [21]. In an inductive line environment, droop control can achieve better results. But, mostly for microgrid voltage level of 10 kV the line impedance is resistive, thus affects the droop control performance. The use of traditional droop control method makes it difficult to achieve precise power sharing and circulation suppression [22]. A variety of improved droop control methods are proposed. In [23], [24] an improved droop control is proposed by designing control parameters, so that the inverter output impedance is always inductive. However, this method has a limited range of effective output impedance adjustment. In [25], by adding differential links in the traditional droop control equation, the power sharing of the parallel DG inverter is quickly stabilized. But this leads to harmonic amplification and output voltage distortion. Virtual impedance method [20], [21], [26] is adopted for parallel DG inverters to improve power sharing under different line conditions. However, virtual impedance does not completely eliminate the influence of line impedance and increases the voltage drop. In an actual microgrid system, differences in parameters and line impedance, makes active and reactive powers not completely decoupled, thus affecting the accuracy of the droop controllers.
In view of the aforesaid problem, by amplitude frequency characteristics analysis, different control parameters effects on the output impedance of DG inverters and appropriate control parameters are selected. In order to solve the parameters differences and uneven distribution of power between the parallel DG inverters in a microgrid, virtual resistors and inductors are added into the control loop of the droop controllers. The introduction of virtual resistors and inductors cause DG inverter output voltage to drop. In order to reinstate the effect, an improved P-f/Q-V and P-V/Q-f droop with secondary control is proposed. The paper is organized as follows. In Section 2, power flow characteristics of droop control is presented for DG inverters. In Section 3, dual loop control parameters are altered using virtual resistors and inductors for improving power sharing between DG inverters and also secondary control is proposed to improve the voltage deviations. In Section 4, simulation results are presented. Finally, the concluding remarks are deliberated in Section 5.

LINK
https://www.researchgate.net/publication/325231811_Improved_P-fQ-V_and_P-VQ-f_Droop_Controllers_for_Parallel_Distributed_Generation_Inverters_in_AC_Microgrid