A Distributed Control Scheme for Cyber-Physical DC Microgrid Systems

Droop control is the basic control method for load current sharing in dc microgrid applications. The conventional dc droop control method is realized by linearly reducing the dc output voltage as the output current increases. This method has two limitations. First, with the consideration of line resistance in a droop-controlled dc microgrid, since the output voltage of each converter cannot be exactly the same, the output current sharing accuracy is degraded. Second, the dc-bus voltage deviation increases with the load due to the droop action. In this paper, in order to improve the performance of the dc microgrid operation, a low-bandwidth communication (LBC)-based improved droop control method is proposed. In contrast with the conventional approach, the control system does not require a centralized secondary controller. Instead, it uses local controllers and the LBC network to exchange information between converter units. The droop controller is employed to achieve independent operation, and the average voltage and current controllers are used in each converter to simultaneously enhance the current sharing accuracy and restore the dc bus voltage. All of the controllers are realized locally, and the LBC system is only used for changing the values of the dc voltage and current. Hence, a decentralized control scheme is accomplished. The simulation test based on MATLAB/Simulink and the experimental validation based on a 2 × 2.2 kW prototype were implemented to demonstrate the proposed approach.

The paradigm shift in electrical power grids and the increased interest towards decentralisation has opened a new window in the design, control and theoretical analysis of small scale power systems, i.e. microgrids, which aim at the integration and utilisation of renewable energy sources, energy storage systems and responsive loads at a local scale. Given their DC nature, DC microgrids have attracted significant interest as they provide a natural interface to the main grid by avoiding additional conversion steps. The aim of this thesis is to design and analyse novel hierarchical control schemes, both at the primary and secondary control level, that guarantee tight voltage regulation, accurate power sharing, current or voltage limitation, and present a straightforward approach towards deriving stability conditions for DC microgrids that incorporate nonlinear loads. As the microgrid configuration is paramount in the theoretical analysis, a rigorous method of computing the admittance matrix is developed that facilitates the stability analysis of DC microgrid systems supplying a constant impedance (Z), constant current (I) or constant power (P) load. This method is particularly useful as it permits the factorisation of the admittance matrix, while separating the singular matrices. In this way, the closed-loop asymptotic stability proof can be more easily approached by isolating the singularities and then, employing straightforward linear algebra tools, such as quadratic eigenvalue problem (QEP) theory, to derive the stability conditions. As stated in the technical specifications of every source, a crucial issue is represented by the ability to protect itself and its interface device (power converter) during faults, transients and unrealistic power demands. That is why current-limiting control is often combined with the primary control, i.e. often droop control, in DC microgrids to ensure the desired unit protection. In this thesis, novel current-limiting droop controllers areformulated for DC microgrids consisting initially of multiple unidirectional DC/DC boost converters and, later on, bidirectional DC/DC boost and three-phase AC/DC converters, that integrate different distributed generation units with the local grid. In contrast to the traditional approaches that use small-signal modelling, here, the accurate nonlinear models of the converter units are taken into account to prove the boundedness and the current-limiting property, by employing Lyapunov methods and the ultimate boundedness theory. Exponential stability at the desired equilibrium point is mathematically guaranteed, and further analysed from a graphical perspective, providing insights of the load’s effect onto the system performance and stability. To address the shortcomings that conventional droop control approaches introduce, i.e. inaccurate power sharing and significant load voltage drop, a DC microgrid architecture that takes into account the power converter dynamics of distributed generation units is deployed under decentralised primary and distributed secondary control scheme, in a hierarchical control framework. At the primary control layer, a novel current-limiting droop control scheme is implemented in a decentralised manner, whereas at the secondary control layer a fully distributed controller that performs a voltage restoration and improves the power sharing is deployed using a nearest-neighbour coupling communication network. By investigating for the first time both the dynamics of the converters with the nonlinear load and the two-layer control, singular perturbation theory is applied to analytically guarantee the stability of the entire DC microgrid. Finally, apart from the desired overcurrent protection, since DC capacitors are customary used at the output of each converter unit to stabilise the output voltage, they also introduce a maximum voltage limit. Hence, as the need for protection against overvoltages has emerged, droop controllers with inherent overvoltage protection are also proposed for parallel and meshed configuration networks. The upper limit of the voltage of each source is rigorously proven using ultimate boundedness theory. Asymptotic stability to the desired equilibrium for the closed-loop system is analytically guaranteed, and detailed conditions are derived to guide the control design. In order to validate the effectiveness of the different control methodologies developed in this thesis, both simulation and experimental testing are performed for each one of the methods, and are also compared with the conventional approaches to highlight their superiority.

The DC microgrid is connected to the AC utility by parallel bidirectional power converters (BPCs) to import/export large power, whose control directly affects the performance of the grid-connected DC microgrid. Much work has focused on the hierarchical control of the DC, AC, and hybrid microgrids, but little has considered the hierarchical control of multiple parallel BPCs that directly connect the DC microgrid to the AC utility. In this paper, we propose a hierarchical control for parallel BPCs of a grid-connected DC microgrid. To suppress the potential zero-sequence circulating current in the AC side among the parallel BPCs and realize feedback linearization of the voltage control, a d-q-0 control scheme instead of a conventional d-q control scheme is proposed in the inner current loop, and the square of the DC voltage is adopted in the inner voltage loop. DC side droop control is applied to realize DC current sharing among multiple BPCs at the primary control level, and this induces DC bus voltage deviation. The quantified relationship between the current sharing error and DC voltage deviation is derived, indicating that there is a trade-off between the DC voltage deviation and current sharing error. To eliminate the current sharing error and DC voltage deviation simultaneously, slope-adjusting and voltage-shifting approaches are adopted at the secondary control level. The proposed tertiary control realizes precise active and reactive power exchange through parallel BPCs for economical operation. The proposed hierarchical control is applied for parallel BPCs of a grid-connected DC microgrid and can operate coordinately with the control for controllable/uncontrollable distributional generation. The effectiveness of the proposed control method is verified by corresponding simulation tests based on Matlab/Simulink, and the performance of the hierarchical control is evaluated for practical applications.

An adaptive droop control with constant voltage regulation is proposed for the power and voltage management of a DC microgrid (DCMG) with multiple power sources, such as a utility grid (UG), a distributed generator (DG), an energy storage system (ESS), and an electric vehicle (EV). In the proposed scheme, the droop characteristics for the UG, ESS, and EV are adaptively changed according to electricity price conditions and state-of-charge (SOC) levels in order to optimize the DCMG operation flexibility as well as electricity cost. The proposed control method not only ensures the power-sharing of DCMG reliably without the use of a communication link, but also regulates the DC bus voltage stably at the nominal value. To achieve this, the proposed scheme consists of primary control and secondary control. The primary control is used to achieve power-sharing in the decentralized DCMG, while the secondary control is used to overcome the disadvantage of conventional droop control, i.e., to remove DC bus voltage deviations. Decentralized power management is also presented to enhance the DCMG system reliability in the presence of uncertainties such as DG generation power, the ESS and EV SOC levels, the grid and EV availabilities, the load demand, and electricity price conditions. The effectiveness of the proposed scheme is demonstrated in a comprehensive simulation and experiment under various conditions. The test results clearly confirm the control flexibility and overall performance of the proposed control scheme for decentralized DCMG system.

Stability analysis of islanded DC microgrid for the proposed distributed control strategy with constant power loads

In a DC microgrid, DC-DC converters are widely used as linking interfaces between DC power sources (or energy storage units) and the DC bus. Typically, linear controllers are widely adopted for these source DC-DC converters to control the power generation of the sources. However, the system's stability can hardly be guaranteed when constant power loads (CPLs), whose input impedances are negative, are connected to the microgrid due to unexpected interactions between sources and CPLs. In addition, as the variation of DC bus voltage is limited to a specific range by electrical standards, which indicates constrained states of the DC-DC converters, conventional nonlinear controllers would cause deteriorated power quality in static and dynamic processes. This paper proposes a novel strict state-constrained sliding mode control strategy for the source DC-DC converters to address the stability and power quality issues of the DC microgrid. The design methodology, stability analysis, and implementation of the proposed control strategy are discussed in detail. A 3-kW buck/boost converter, which is widely used as the source DC-DC converter in a DC microgrid, is designed and established in the lab. The simulation results verify the effectiveness of the proposed control strategy.

Constant power loads (CPLs) introduce negative impedance in direct current microgrids (DCMGs), which is a major challenge. This negative impedance can significantly reduce the overall damping of the system, making it less stable and harder to control. To address this issue, output virtual resistance (VR) shaping is commonly employed to enhance system damping and improve power‐sharing amongst distributed generators (DGs). The technique proposed in this work involves an adaptive variation of the DG virtual output resistance () linearly with the output current. This shows improved power sharing between sources. The work compares the small signal stability criteria and the minor loop gain methods for linear, non‐linear, and inverse droop controllers to determine the controller parameters with constant power loads. The control scheme is extensively tested through simulations for four different droop control schemes. The work also validates the DCMG performance when the DERs work with different droop controllers (heterogenous of controllers) to assess constant power load penetration, performance in meshed configurations, and DG plug‐and‐play operations. Additionally, improved power sharing performance was validated through a controller hardware in the loop (CHIL) based implementation.

DC Microgrids (MGs) are increasingly popular in recent years to support distributed generation based power systems. This paper focuses on the improvement of the load sharing control strategy and presents a decentralized control scheme, namely improved digital average current sharing (IDACS). Compared to the conventional hierarchical droop control scheme, the IDACS shows the structural difference and demonstrates the advantages of high modularity, automatic load sharing, low current deviation, and robust voltage regulation. In the proposed IDACS scheme, each primary converter includes both current controller and voltage controller to compensate voltage uniformity, which is caused by droop control scheme. Based on the foundation of digital average current sharing (DACS), a comprehensive approach is developed to design controllers for DC-DC converters by using anti-windup strategies. The improved performance is validated by simulation and demonstrates low voltage regulation and fast recovery time.

Grid connected DC Microgrid (DCMG) with wind turbine and solar PV distributed generations (DG), battery storage system and constant power loads (CPL) which are integrated via power electronic converter is presented in this paper. In DCMG interfaced with multiple parallel converters the stability of dc bus can be deteriorated by DG output power intermittency and microsources converter output impedance and load converter input impedance mismatches. Thus to solve these problems, this paper presents an autonomous control using decentralized control scheme based on dc bus voltage as means of communication is proposed in this paper. Droop control strategy for power sharing among the different sources and to stabilize bus voltage is employed. The stability analysis of the system is performed using Lyapunov stability criteria to determine the qualitative behavior of the system based on the simplified equivalent source transmission line. The control method verified via MATLAB/SIMULILK simulation.

In the DC microgrid system, the bidirectional DC/DC converter is one of the most important components; thus, research on its control strategy has attracted widespread attention. Firstly, the single bidirectional DC/DC converter based on a sliding mode (variable structure) control (SMC) strategy exhibits an inherent contradiction between the reaching time and the chattering phenomenon. In order to address this problem, an SMC strategy based on the improved exponential reaching law was designed. This control strategy modifies the constant-speed reaching term and introduces the system state variable to indicate the chattering level, which not only improves the dynamic performance of the bidirectional DC/DC converter but also suppresses the chattering problem. Secondly, the bidirectional DC/DC converter group based on the traditional droop control strategy exhibits an inherent contradiction between load power allocation and bus voltage stabilization. In order to address this problem, an improved droop control strategy that takes the line impedance characteristics into account is proposed. This control strategy modifies the traditional droop control strategy by introducing virtual resistance and uses DC bus voltage information to replace the line impedance value. This ensures the accuracy of power allocation and stability of the DC bus voltage simultaneously. Finally, the stability of each designed strategy is verified individually. The combination of the two control strategies is applied to a group of bidirectional DC/DC converters group to conduct a semi-physical simulation experiment, and the results verify that the proposed control strategies are effective and feasible.

DC microgrids are well known as a proper solution to link different DC sources, such as photovoltaic panels and wind turbines, directly to DC loads. Along with their advantages, they suffer from an imbalance state of charge (SOC) in their energy storage units (ESUs), improper current‐sharing between ESUs, and DC bus voltage deviation. This study proposes a novel control strategy for DC microgrids, which not only balances ESUs SOC and shares current between ESUs proportional to their capacity but also, reduces DC bus voltage deviation significantly. In this way, the same droop curve is defined. Then, three virtual resistances are considered. The first one is set based on the lines resistance and ESUs capacity to share current among the ESUs properly. As the lines resistance changes affect the proportional current‐sharing, the second virtual resistance is intended to relieve this effect. The third one is suggested to balance the ESUs SOC. The small‐signal stability analysis is performed, which demonstrates that the proposed method has stability in all operating conditions. Simulation results in MATLAB/Simulink confirm that with the aid of the proposed control method, not only proper current‐sharing and ESUs SOC balancing are obtained, but also DC bus voltage deviation is effectively diminished.

The integration of information and communication technologies (ICTs) has provided modern smart grids, such as direct current (DC) microgrids (MGs), with various advantages, e.g., the possibility of large-scale integration of renewable energy sources and complex control schemes. This extensive deployment of ICTs, however, makes DC MGs prone to a variety of cyber threats, e.g., false data injection (FDI) attacks. On this basis, this paper develops online detection and mitigation schemes to counter FDI attacks against DC MGs. Initially, in the offline training of the detection scheme, the density-based spatial clustering of applications with noise (DBSCAN) method is used for extracting prescriptive features of DC MG measurements and clustering them in healthy and under-attack operations under several uncertainties in DC MG operations. These features are used to train an adaptive network-based fuzzy inference system (ANFIS) architecture for the online identification of FDI attacks. Afterward, the trained ANFIS is used to detect FDI attacks and obtain the attack vectors benefiting from the real-time measurements estimated by an optimal state observer. Finally, an adaptive model predictive controller (AMPC) is developed to update the power-sharing commands—and mitigate the FDI attack—using the ANFIS’s obtained attack vectors. The effectiveness of the proposed framework is evaluated for several realistic FDI attack scenarios and different uncertainties. To show the scalability of the proposed detection and mitigation methods, a DC MG with eight converters is also studied in the presence of FDI attacks and uncertainties using real-time simulations.

In this paper, a novel approach for robust and autonomous dc bus voltage control in a dc microgrid has been proposed. A nonlinear disturbance observer (NDO) only with local information is designed firstly to track power disturbances caused by load changing or the volatility of the renewable sources in the dc microgrid. With the observer, voltage droop method has been adopted in achieving power sharing and autonomous operation of a dc microgrid. Furthermore, an improved current feedforward is developed to suppress wide fluctuations of the dc bus voltage during transients. In addition, a notch filter has been used to effectively eliminate the second harmonic ripple components in the inductor currents of the droop control units to improve the power quality. The effectiveness of the proposed control scheme is verified on a laboratory prototype including two bidirectional dc-dc converters supplied by two programmable dc power supply systems, a dc load with a resistor connected to the dc bus via a buck type dc-dc converter and a three-phase unbalanced ac load.

A novel droop controller for DC microgrid systems, consisting of multiple paralleled sources feeding a constant power load (CPL), is proposed to achieve the desired voltage regulation and accurate load power distribution, while ensuring an overvoltage protection for each source. CPLs are well-known to exhibit negative impedance characteristic due to their nonlinear behaviour, which may cause instability to a DC microgrid if the necessary impedance inequality criteria are not satisfied. In this paper, a new droop control scheme is proposed to limit the voltage of each source below a desired bound, achieve tight voltage regulation and power sharing, and guarantee closed-loop system stability with the existence of a CPL. The upper limit of the voltage of each source is rigorously proven using ultimate boundedness theory, while after a suitable manipulation of the admittance matrix of the microgrid, analytic conditions of stability are obtained to guide the control parameters design. To validate the theoretical design and analysis, a detailed simulation is performed of a DC microgrid equipped with the proposed controller.

Due to the uneven distribution of virtual resistance and line impedance in the DC micro-grid, the traditional droop control has the contradiction between current sharing and voltage regulation. In this paper, the droop control based on the discrete consensus algorithm is adopted. Each power generation unit adopts adjacent communication, and the reference value of the DC bus voltage is corrected according to the mean value after the iterative of the system voltage, thereby compensating for the voltage drop caused by the droop control. The photovoltaic (PV) cell adopts the maximum power tracking (MPPT) control. The battery adopts the constant voltage control with current threshold, which can keep the DC bus voltage stable, and the outlet current is prevented from being too large to shorten the service life of the battery. Stable operation of the DC micro-grid is realized. Finally, the PV/battery DC micro-grid model is built in Simulink. When the environmental conditions and load change, the micro-grid can ensure the DC bus voltage constant and the active power balance, which verifies the effectiveness of the proposed control strategy.