Bidirectional DC/DC and SOC Drooping Control for DC Microgrid Application

Abstract

In DC microgrids, distributed energy storage plays a key role in stabilizing the DC bus voltage. The bidirectional DC/DC converter in the distributed energy storage system should be designed according to the voltage level and electromagnetic isolation requirements, and multiple energy storage units should be coordinated for load current distribution according to the state of charge (SOC). This paper proposes a SOC power index droop control strategy by communication lines to coordinate the fast and high-precision distribution of load current among multiple energy storage units, and the SOC between energy storage units quickly converges to a consistent state. Considering that communication lines are susceptible to interference, this paper further proposes an improved SOC power index droop control to overcome the effects of communication line failures. Considering the high cost of the energy storage unit, it should be connected to the DC microgrid in layers to achieve a reasonable allocation of resources in practical applications. In order to provide high-quality power to a large power grid, the quantification standards of the DC bus fluctuation range and the working range of each converter are further discussed to maximize the stability of the DC bus voltage and grid-connected power fluctuation.Keywords: bidirectional DC/DC converter; DC microgrid; distributed energy storage; SOC (state of charge) droop control IntroductionDistributed energy storage is the key issue to solve the issue of grid-connected renewable energy generation. For example, it can improve the ability of the grid to accept wind and photovoltaic (PV) power [1-3]. A typical DC microgrid structure is mainly composed of a distributed generation unit, an energy storage unit, a load cell, and a grid-connected converter [4][5][6], as shown in Figure 1. DC microgrid research focuses on stabilizing the DC bus voltage to ensure the power balance of the system. To stabilize bus voltage fluctuations and solve energy supply volatility issues, adding energy storage devices can improve the device's voltage sag and the inrush issues caused by load-switching, changes in natural conditions, and instantaneous faults in DC microgrid systems; this improves the reliability and scheduling flexibility of the distributed generation grid connection. Using low bandwidth communication control reduces the long-distance stability and DC microgrid distribution of the system due to network delay packet loss and other issues. To avoid the risk caused by long-distance communication control, each unit is divided into three control layers according to the normalized voltage of DC bus and coordinated control of various units [7]. Compared with the two-level converter, the three-level converter has only half of the switches to change state peer cycle, and the voltage stress on the switch is only half of the bus voltage [8,9].

DC Microgrid Planning, Operation, and Control: A Comprehensive Review

“…It is indentified that several research gaps could be filled with cutting edge technologies and control for further improvement of voltage regulation and power sharing. Although different droop techniques, such as robust droop [150], [151], universal droop [152], distributed droop [153], [154], adaptive droop [155], optimal droop [156], [157] , are currently dominant in DC microgrids, further investigations may improve the performance of such controllers.…”

A Review on State-of-the-Art Power Converters: Bidirectional, Resonant, Multilevel Converters and Their Derivatives

“…These converters are normally used as a linkage between the energy storage components and the grid utilities or DC bus [132]; they can also be used in EVs, microgrids, and vehicle-to-grid (V2G). BDCs are normally connected with energy storage components such as supercapacitors or batteries, with a wide range of voltage variation during operation.…”

A Novel State-of-Charge-Based Droop Control for Battery Energy Storage Systems to Support Coordinated Operation of DC Microgrids

“…Zeng et al [19] also proposed to balance the SoCs by comparing the SoCs of BESSs with the average SoC, while three different parameters are used to achieve desirable balancing speed and for accurate regulation [19]. Zhang et al [22] also used the average SoC information to ensure that the SoC equilibrium is reached quickly [22]. In [19,21,22], the communication between BESSs is required to calculate the average SoC.…”

“…Zhang et al [22] also used the average SoC information to ensure that the SoC equilibrium is reached quickly [22]. In [19,21,22], the communication between BESSs is required to calculate the average SoC.…”