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
