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

Study on Parallel Operation for Distributed Generation Inverter based on Droop control Kim Hyun-jun Department of Electrical Engineering Graduate School, Myongji University

Study on Parallel Operation for Distributed Generation Inverter based on Droop control Kim Hyun-jun Department of Electrical Engineering Graduate School, Myongji University Directed by Professor Han Byung-moon

 When the droop control is conducted through the inverter for distributed generation, active/reactive power coupling occurs because of the resistive component of the output impedance. In case of long distance distribution line, the voltage drop caused by the line impedance between the PCC and output voltage changes initially designed property curves of the Q-V droop. So, the control performance of Q-V droop is deteriorated and it causes an error of reactive power control and distribution. Therefore, a new droop control, which is to improve the power coupling and the accuracy of reactive power, is proposed in this paper. Its effectiveness is proved by the performance evaluation that relates to unbalanced power compensation, which should be prepared by the inverter for distributed power, random switch and system reconnection that are required when the drive mode is converted. In general, the droop control is used for the high voltage system, so that resistive component of line impedance is ignored because of significant inductive However, the line impedance has more significance in resistive component than inductive component in the low power distribution system, so that the power coupling occurs. To solve this problem, many of the improved droop control method have been proposed in various ways. Most typical method is the virtual impedance type. The virtual impedance type readjusts the output impedance to have inductivity, even though there is a significant resistive component caused by the output impedance of the inverter and low voltage line impedance. Therefore, it is effective to remove the power coupling that is generated by the resistive component. However, increased virtual impedance can also increase reactive power control or distribution error. In addition, though the virtual impedance type prevents power coupling, the reactive power control or distribution error caused by the line impedance can not be compensated. Therefore, there should be an additional control method to improve the reactive method and distribution accuracy influenced by the line impedance. There are two types of general methods; One is Q-VPCC droop control type, which directly measure the PCC voltage using the communication and conduct droop control. The other is indirect Q-V voltage droop control type that estimates the drop control of line impedance and compensated it. In the Q-VPCC droop control type, each inverter is controlled by identical PCC voltage, so that it is not influenced by the line impedance and enables accurate reactive power control and distribution. However, additional voltage sensor to measure PCC voltage should be installed, and inverters for distributed power are located far away from the PCC, thereby requiring communication line to deliver the information of PCC voltage. Therefore, indirect voltage droop control method has been proposed to make up for the defect of direct voltage type. The approximated value of the voltage drop component caused by the line impedance is calculated by active/reactive power, so that the PCC voltage can be estimated and it enables accurate reactive power control and distribution. However, most indirect voltage types use approximated value of voltage drop caused by active/reactive power, so that it is only effective for small line impedance. Since the large value of line impedance can lead to significant error of approximated voltage drop value, reactive power control and distributed error cannot be completely improved. It also requires the information of the line impedance. Therefore, a new droop control, which is to solve defects of the power coupling, reactive power control, and distribution is proposed in this paper. The proposed method conducts the droop control using the dq transformation. Since the droop control is conducted on the dq coordinate, the Q-V droop voltage control method becomes similar to the DC droop control and the power coupling is removed. Therefore, output impedance component in the normal condition, since the DC voltage value is controlled. In other words, the power decoupling is enabled without adding virtual impedance to the droop controller. In addition, errors of reactive power control and distribution that occurs because of the error of voltage drop in the line impedance can be compensated by calculating accurate voltage drop value on the dq coordinate. Using the proposed droop control method, unbalanced load compensation, random switch performance, and system reconnection issues were handled that were also fundamental performance index for the inverter of distributed power. Finally, the PSCAD/EMTDC simulation and two of 5 kw prototype inverters for distributed power were produced and utilized in the experiment of theoretical verification.

 Keyword Droop Control, Seamless Transfer, Inverter-based distributed generator(DG), Reconnection, Energy Storage System, Reactive power sharing compensation Unbalanced Voltage compensation

Seminário Mecanismos de Qualidade no Setor Fotovoltaico 10/08/2018 - Tarde

Seminário Mecanismos de Qualidade no Setor Fotovoltaico 10/08/2018 - Manhã

Transmissão de energia elétrica em corrente contínua - Prof. Dr. Kleber -Departamento de engenharia elétrica da UNIVERSIDADE FEDERAL DE CEARA-BRASIL