Development of an alternative droop strategy for controlling parallel converters in standalone DC microgrid

Shebani, Muamer M. (2021) Development of an alternative droop strategy for controlling parallel converters in standalone DC microgrid. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Most of parallel-connected DC-DC converters schemes are based on a high-bandwidth communication network to achieve minimum circulating current, proper load current sharing, and acceptable voltage regulation. However, in DC microgrids, the use of communication network can be costly and unsuitable considering the data reliability and cost investment because the load and renewable energy sources are connected to the point of common coupling. Therefore, the droop control as a decentralized method has gained more attention. However, the challenge for the conventional droop method is to overcome the issue of circulating current, poor load current sharing, and the drop in DC grid voltage due to the droop action. This thesis develops and tests an approach for minimizing the circulating current, as well as improving the voltage regulation and the load current sharing for the droop method. The developed approach is based on the concept of synchronized switching, which is implemented using an alternative droop strategy for controlling different sizes of parallel-connected DC-DC boost converters. In this thesis, synchronous switching, based on an optimized controller, is presented to eliminate the initiation of circulating current and minimize the ripple in the output current for parallel-connected boost converters. Furthermore, a modified droop method, including the cable resistance, is introduced. The modified droop method uses the measurements of the voltage and current at the point of common coupling to estimate the voltage set point for each converter locally. The communication network is eliminated by utilizing the modified droop method because, in the proposed method, there is no current and voltage measurement data transmitted from one converter to the other converter. Additional loop control is also applied for equal current sharing between parallel converters to overcome the issue of mismatch in parameters of the parallel converters. The additional loop control is added to improve the load current sharing in the modified droop control. The modified droop control method with additional loop control is verified using MATLAB/SIMULINK and validated with experimental results. However, the droop action of the modified droop and different cable resistances degrades the voltage regulation and load current sharing. Therefore, an improved droop method, which utilizes the virtual droop gain and voltage droop control gain, is proposed to overcome the problem of load current sharing and voltage regulation. The virtual droop gain compensates the differences in the cable resistances, and the voltage droop control gain regulates the voltage at the point of common coupling. This maintains the common DC bus at its rated value. The effectiveness of the improved droop method is demonstrated by MATLAB/Simulink and Laboratory prototype results. Finally, the proposed method is utilized in a standalone DC microgrid. An example of a DC microgrid of a residential building powered by a PV solar system illustrates the feasibility and the effectiveness of the proposed methods.

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