Accurate Current Sharing in a DC Microgrid Using Modified Droop Control Algorithm

TL;DR

The paper addresses inaccurate current sharing in DC microgrids with parallel DC/DC converters caused by unequal cable resistances. It introduces an adaptive, online modified droop control that updates the droop gains using local voltage and current measurements and a low-bandwidth communication of currents, guided by the condition $|I_j - K_j I_{ave}| < \gamma$. The approach is validated through MATLAB/Simulink simulations and experimental tests on a two-converter bench, showing accurate sharing across varying loads and cable resistances, with robust performance when resistances change. The method offers a simple, scalable solution that improves sharing accuracy and efficiency in DC microgrids, with potential applicability to systems featuring heterogeneous converter ratings and limited communication capabilities.

Abstract

Due to the increasing popularity of DC loads and the potential for higher efficiency, DC microgrids are gaining significant attention. DC microgrids utilize multiple parallel converters to deliver sufficient power to the load. However, a key challenge arises when connecting these converters to a common DC bus: maintaining voltage regulation and accurate current sharing. Unequal cable resistances can cause uneven power sharing and lead to power losses. Conventional droop control methods, which employ a virtual resistor to address this issue, have limitations in achieving good performance across the entire converter operating range. This paper proposes a modified droop control algorithm to address this issue. This method modifies the virtual resistor in a way that ensures power sharing aligns with each converter-rated capacity. The algorithm is simple to implement and uses local measurements to update the droop gain. This paper presents simulation studies and experimental tests to analyze the performance of the proposed method, considering scenarios with equal and unequal converter ratings. The results successfully validate the accuracy and effectiveness of this innovative approach.

Figures (10)

• Figure 1: A simple equivalent circuit for parallel DC/DCs in a DC microgrid.
• Figure 2: General control block diagram of parallel converters.
• Figure 3: V-I characteristic of the converters with and without using droop control.
• Figure 4: The proposed algorithm for power sharing.
• Figure 5: Case study I: voltages and currents of the converters before and after activation of droop control with $R_{L1}=1~\Omega$ and $R_{L1}=2~\Omega$: (a) converter current, (b) terminal voltages, (c) bus voltage.