Power-Capacity-Based Bus-Voltage Region Partition and Online Droop Coefficient Tuning for Real-Time Operation of DC Microgrids

Abstract

Multiple-voltage-region control, in which the busvoltage range is divided into several regions, is usually implemented for dc microgrid operation in distributed manner. Voltage/power droop relationships are imposed for active power sharing among slack terminals. Conventionally, threshold voltages for voltage region partition are determined with fixed percentage of variation around the nominal value, which may result in unevenness of droop coefficients in different regions. If system droop coefficient is too high, significant bus-voltage step change due to load variation will occur. On the other hand, significant power sharing error among slack terminals will be induced if the droop coefficient is too low. In this paper, a compromised solution with power-capacity-based bus-voltage region partition is proposed to equalize the droop coefficients in different regions. However, the droop coefficients are determined based on the rated power capacity of system units. Busvoltage discontinuity appears when the power capacity reduces in actual implementation. To eliminate the voltage discontinuity, online droop coefficient tuning according to the real-time power capacity is implemented. Algorithms for local power capacity estimation of solar photovoltaic (PV) and battery energy storage have been proposed. A lab-scale dc microgrid has been developed for verification of the proposed methods.Index Terms-DC microgrid, droop coefficient, online tuning, power capacity estimation, voltage region partition.

A Novel Autonomous Current-Sharing Control Strategy for Multiple Paralleled DC–DC Converters in Islanded DC Microgrid

“…Due to the diversity and decentralization of distributed energy, power converters are usually used to connect them in order to realize energy conversion and management. How to achieve parallel operation, accurate load current sharing between power converters, and maintain stability has become a research hotspot in microgrids [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. With the increasing demand of DC power at load side and the rapid development of distributed DC power sources such as photovoltaic cells, wind power, storage batteries, fuel cells, and supercapacitors, the research on DC microgrids has been gradually increasing in recent years [7][8][9].…”

“…To study the parallel operation problem of distributed generations is essentially to study the current sharing control of multiple paralleled DC-DC converters. Accurate current sharing between paralleled converters and no deviation regulation for bus voltage are two important control objectives in islanded DC microgrid [13][14][15][16][17][18][19][20][21][22][23]. At present, the current-sharing strategies that are most commonly used are master-slave control and peer-to-peer control.…”

“…This method does not need any communication, but its droop characteristics would cause serious voltage drop, and the uncertain line impedance would also affect the current sharing accuracy. Then, some optimization methods with variable droop coefficients were proposed [16][17][18]. According to the output current range, voltage deviation information, and the defined droop index of the converter, the optimal value of virtual impedance is calculated and then different droop coefficients are set to optimize the current sharing among the micro-sources.…”

Decentralised control method for DC microgrids with improved current sharing accuracy

“…The main objective of a coordinated control system is to prevent over stressing of any energy source and properly distribute the total power load to each DG in proportion to their rated powers [7,8]. To realise this, the so called droop control which employs own electrical parameters as global indicators is widely applied in MG systems due to its decentralised characteristic [9][10][11][12].…”

Real-Time Minimization Power Losses by Driven Primary Regulation in Islanded Microgrids

“…The continuous adjustment of droop regulation coefficients has been mentioned in some papers. In [18], an online droop coefficient adjustment is illustrated for a direct current (DC) microgrid. The paper shows the negative impacts of too high or too low droop coefficients on the bus-voltage and proposed an online tuning of droop coefficients to eliminate the bus-voltage discontinuity.…”