Abstract
For the purpose of accurate power sharing and the state of charge (SOC) balancing of each energy storage unit (ESU), a novel secondary control scheme which regulates the virtual impedance is proposed herein. However, there is a coupling relationship between the power sharing and voltage restoration process. In this study, the secondary control scheme could eliminate that effect so that the virtual impedance was only influenced by the system distribution and no longer affected by system initialization and noise. The proposed secondary strategy contains four controllers, including the current sharing controller, the SOC balancing controller, the virtual impedance correction controller, and the local reference voltage controller. Through the proposed scheme, the ESU with a higher SOC will give more power, whereas the lower one will give less power. The steady-state analysis for the uniqueness of the droop coefficient is given. Besides, the small-signal analysis of the ESU was thereby performed to ensure stability. Finally, the effectiveness of the proposed strategy was verified through MATLAB/Simulink.
Highlights
The microgrid (MG) is the development trend of the modern electric power system, since the renewable energy power generation is one of the available means to solve the global energy crisis and decrease environmental pollution [1,2,3]
The deviation of state of charge (SOC) between neighborhood node i and j is generated by the PI controller to produce a correction term for modifying the droop coefficient, which can be written as δdsoc = G pis
To realize the power sharing and SOC balancing, the main contribution of the proposed control strategy is to achieve the stability of virtual impedance, so that the virtual impedance is only influenced by the system distribution and no longer affected by system initialization and noise
Summary
The microgrid (MG) is the development trend of the modern electric power system, since the renewable energy power generation is one of the available means to solve the global energy crisis and decrease environmental pollution [1,2,3]. Given the difference between the line impedances, there are still some limitations in using droop control to equalize the current; i.e., droop control can influence the output voltage drop, which may cause damage to the power quality [18,19] To solve this problem, a secondary control method should be applied to realize power sharing between each ESU and to restore the average reference voltage [20]. There are four designed controllers in secondary control level, the aim of which is to balance SOC, achieve accurate power sharing proportionally, restore the virtual impedance, and maintain the local reference voltage of each energy storage unit.
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