Distributed Secondary Controller to Minimize Circulating Current Flowing Among Sources in DC Microgrid

Abstract

In the islanded mode of operation of a DC microgrid, the main objective is to achieve proportional sharing of load power among sources and to maintain the source voltage within the specified limit. The droop control technique is widely used to achieve these objectives. Nevertheless, the performance of the traditional droop control technique becomes poor in the case of DC Microgrid including sources having unequal values of nominal voltages. The difference in nominal voltages arises due to errors in the measurement of the voltage feedback signal. The unequal values of nominal voltages lead to errors in the proportional sharing of the load current among the source converters. The mismatch in nominal voltages leads to a flow of circulating current among sources. To minimize this circulating current flowing among the sources, various distributed secondary controllers are suggested in the literature. However, these controllers require prior information of circulating current flowing among sources or accurate measurement of line parameters. Further, the applicability of these controllers is valid for the radial configuration of the DC microgrid. To resolve these issues, a distributed secondary controller is proposed in this paper which shifts the droop characteristics of the source parallel to the voltage axis. The proposed controller does not require prior information of circulating current and no estimation of line parameters. The proposed controller requires the evaluation of proportional current to minimize the circulating current. The proportional value of current for a given source is calculated using the formation of power ratings of sources and initial values of their droop gains. The proposed controller ensures the accurate sharing of the load current among source converters in case of a microgrid having sources of unequal capacity or communication link failure. Further, the voltage regulation is maintained within the defined limit. A small signal model is derived to show the effect of the variation of parameters of the proposed controller on the stability of the DC microgrid. The efficacy of the proposed controller is validated with the help of results capture using the Controller Hardware-in-Loop (CHIL) approach which includes a Real-Time Digital Simulator (RTDS) and Digital Signal Processor (DSP).

Keywords

• DC microgrid
• secondary controller
• load sharing
• stability analysis
• circulating current