Distributed Control to Ensure Proportional Load Sharing and Improve Voltage Regulation in Low-Voltage DC Microgrids

Abstract

DC microgrids are gaining popularity due to high efficiency, high reliability and easy interconnection of renewable sources as compared to ac system. Control objectives of dc microgrid are: (i) ensure equal load sharing (in per unit) among sources and (ii) maintain low voltage regulation of the system. Conventional droop controllers are not effective in achieving both the aforementioned objectives simultaneously. Reasons for this are identified to be the error in nominal voltages and load distribution. Though centralized controller achieves these objectives, it requires high speed communication and offers less reliability due to single point of failure. To address these limitations, this paper proposes a new decentralized controller for dc microgrid. Key advantages are high reliability, low voltage regulation and equal load sharing, utilizing low bandwidth communication. To evaluate the dynamic performance, mathematical model of the scheme is derived. Stability of the system is evaluated by eigenvalue analysis. The effectiveness of the scheme is verified through detailed simulation study. To confirm the viability of the scheme, experimental studies are carried out on a laboratory prototype developed for this purpose. Controller Area Network (CAN) protocol is utilized to achieve communication between the sources.

On Secondary Control Approaches for Voltage Regulation in DC Microgrids

“…Droop schemes have been employed to control the load sharing among dispatchable energy units in dc MGs [10], [21]- [23]. Droop controlled converters in dc MGs can be modeled as an ideal voltage source in series with a droop resistance [22], [23].…”

“…In short feeders the resistance of lines and voltage drop on the lines are small, hence, the differences in voltage levels will be small. Here the following approaches, which employ different voltage regulation schemes, have been considered:  Approach I: regulating the average voltage of generator busses [6], [10], [11], V1, V4 in The effects of the different control approaches on voltage regulation are explained in the following.…”

“…In centralized schemes, the voltage of localized loads connected to a common bus or the voltage at the coupling point into the utility grid should be regulated [9], [10]. On the other hand, in decentralized methods, the average voltage of generator busses (busses with voltage source converter), is controlled [6], [7], [10]- [12], [16] In the centralized secondary approach, it is considered the loads are localized at a common buss and the secondary controller regulates the voltage of the common bus at a reference value. A central control unit measures the voltage of common buss and send the set point voltage to the voltage controlled converters [8]- [10].…”

“…On the other hand, in decentralized methods, the average voltage of generator busses (busses with voltage source converter), is controlled [6], [7], [10]- [12], [16] In the centralized secondary approach, it is considered the loads are localized at a common buss and the secondary controller regulates the voltage of the common bus at a reference value. A central control unit measures the voltage of common buss and send the set point voltage to the voltage controlled converters [8]- [10]. Since the reliability and resiliency of the system in the presence of a central controller is questionable due to the communicating among converters and the central control unit, some distributed methods are presented [8], [11].…”

From DC nano- and microgrids towards the universal DC distribution system - a plea to think further into the future

“…As the bulk grid is becoming more of an "internet of energy", there should be a way of communication between different terminals in order to enable control strategies. Generally, control strategies can be categorized into centralized, decentralized, distributed and hybrid [12]. Communication is used in centralized and distributed control, whereas no communication is generally used in decentralized and hybrid control.…”