Compensation of droop control in DC microgrid with multiple distributed generators

DC microgrid is a feasible and effective solution to integrate renewable energy resources, as well as to supply reliable electricity. The control objective of DC microgrids is to maintain the system's stable operation, low-voltage regulation, and proportional load sharing among the multiple distributed generators. Compared to the high-bandwidth communication-dependent master-slave control, droop control is an effective method to implement the control of DC microgrids without the requirement of communication. Droop control is an output impedance programming method, in which the output current decreases linearly with the decrease of output voltage. The load sharing is automatically achieved. However, in the real applications of lowvoltage DC microgrids, the nominal reference offsets and unequal cable resistances require trade-offs to be made between voltage regulation and load sharing. Thus some compensations need to be performed so as to solve this problem. This chapter discusses the methods to compensate the voltage error introduced by droop control as well as the unequal load sharing due to the transmission lines and the nominal voltage reference offsets. The compensation methods in the literature using low-bandwidth communication are reviewed and a unified compensation framework is proposed using the common current. In this scheme, the voltage deviation and the unequal load sharing are compensated separately. The common current is generated in each local controller by using the local module currents shared in a dedicated low-bandwidth communication line. The contents of this chapter are organized as follows: Section 10.1 overviews and compares the active current sharing control and the droop control in DC microgrids; Section 10.2 analyses the limits of the basic droop control under the condition of nominal voltage offsets and unequal connecting cable impedances; Section 10.3 reviews and classifies the different compensation methods from the literature; Section 10.4 analyses voltage and load sharing performance of the proposed method, and investigates the boundaries of the compensation parameters to maintain system stability. In Section 10.5, some simulations are conducted in the MATLAB/Simulink environment. In Section 10.6, experimental tests are performed on a laboratory-scale test bench to verify the previous proposed theoretical analysis.