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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.

A novel distributed secondary layer control strategy based on average consensus and fractional‐order proportional‐integral (FOPI) local controllers is proposed for the regulation of the bus voltages and energy level balancing of the energy storage systems (ESSs) in DC microgrids. The distributed consensus protocol works based on an undirected sparse communication network. Fractional‐order local controllers increase the degree of freedom in the tuning of closed‐loop controllers, which is required for DC microgrids with high order dynamics. Therefore, here, FOPI local controllers are proposed for enhanced energy balancing of ESSs and improved regulation of the bus voltages across the microgrid. The proposed control strategy operates in both islanded and grid‐connected modes of a DC microgrid. In both modes, the average voltage of the microgrid converges to the microgrid desired reference voltage. The charging/discharging of ESSs is controlled independent of the microgrid operating mode to maintain a balanced energy level. The performance of the proposed distributed control strategy is validated in a 38‐ V DC microgrid case study, simulated by Simulink real‐time desktop, consisting of 10 buses and a photovoltaic renewable energy source.

The DC microgrid can provides an effective solution for integration of various loads, distributed generations(DGs),and distributed storages. With the expansion of DC microgrid capacity, coordinated control between multiple distributed generations has become a research hotspot. The conventional droop control has low accuracy of load current sharing due to the inconsistent parameters in the DC microgrid. To solve this problem, based on the fuzzy algorithm, this paper designs a distributed hierarchical control structure of DC microgrid, and sets corresponding control strategies at each layer to achieve multi-source dynamic coordination. Firstly, the influence of line resistance on load current sharing is analyzed in detail. Then, the fuzzy controller is designed to dynamically change the virtual resistance. Finally, the secondary compensation control is used to eliminate bus voltage deviation. The simulation model is built by Matlab/Simulink and the simulation results verify the effectiveness of the designed control strategy.

This paper presents primary and secondary control of islanded microgrid. In conventional primary droop control, there is a problem of poor voltage regulation of microgrid. The drawback of conventional primary droop control is improved by voltage shifting based primary control. Voltage shifting based primary control is not completely satisfied by the voltage regulation of microgrid. Also, there is current sharing problem in both types of primary control. So, in secondary control, both current sharing and voltage regulation are achieved by slope adjusting and voltage shifting simultaneously. In this secondary control method, average value of current, voltage, and droop resistance of two neighboring converters is calculated then controlled by additional layer of distributed secondary control over both primary controls. By adjusting droop coefficient of different converters making same value of output impedance of converter, current sharing and voltage regulation are achieved. Also, in transient state, current sharing is achieved by using this secondary control technique. The performance of proposed microgrid is verified by MATLAB simulation.

In order to achieve the flexible and efficient utilization of distributed energy resources, microgrids (MGs) can enhance the self-healing capability of distribution systems. Conventional primary droop control in microgrids exhibits deviations in voltage and frequency and lacks research on voltage–frequency control during network reconfiguration. Therefore, this paper investigates the control strategy of secondary control for voltage and frequency during the process of reconstructing distribution networks to operate in the form of microgrids after faults. Firstly, the mathematical model of three-phase voltage-source inverters with droop control is analyzed. Then, inspired by the generator’s secondary frequency control strategy, an improved secondary control method based on droop control is proposed, and the effectiveness of the improved droop control strategy is verified. Finally, simulation examples of distribution networks with various dynamic boundary topology changes are designed to simulate a relatively stable control result, validating the proposed strategy.

This paper proposes a novel State-of-Charge (SoC)- based power allocation strategy for photovoltaic (PV)-based DC microgrids, leveraging an adaptive consensus algorithm to ensure SoC balance across Battery Energy Storage System (BESS) nodes. The primary control scheme employs inner-loop current droop control to mitigate disturbances arising from PV operation. The SoC-based power allocation strategy serves as a secondary control mechanism, designed to maintain a uniform SoC level across all BESS nodes via the adaptive consensus algorithm. This algorithm is compatible with a range of secondary control types, thereby mitigating battery degradation. Specifically, the stability and compatibility of the SoC-based power allocation strategy are rigorously proven by constructing a Lyapunov function. The proposed control strategy is simulated in MATLAB/SIMULINK and in an OPAL-RT-based real-time simulation environment to demonstrate its effectiveness, in which the proposed adaptive consensus algorithm can accommodate two different secondary control strategies.

This paper discusses about a cooperative secondary control strategy for accurate load sharing in DC microgrids. Since accuracy in DC voltage regulation and power sharing can't be achieved simultaneously in a non-negligible impedance network, an adaptive droop has been proposed to address proportionate power sharing of distributed DC sources via a secondary controller. A fixed droop along with a secondary controller generally introduces a loading mismatch between converters while accomplishing voltage regulation at load bus. Hence, an adaptive droop has been proposed to improve power sharing along with a tertiary update used for fine-tuning. Further, a voltage observer has been used to achieve global average voltage regulation at all source buses. The performance of the proposed control strategy is validated in MATLAB/SIMULINK environment for various communication as well as converter adversities.

In this study, to enhance the system reliability under false data injection (FDI) attacks and DC-link voltage (DCLV) sensor failures, a hybrid control strategy for a DC microgrid (DCMG) based on the Lagrange extrapolation and voltage observer is proposed. Under normal conditions without FDI attacks or DCLV sensor failures, the DCMG system works in a distributed control scheme. To enhance the reliability of the system under the DCLV sensor failure or FDI attack, the DCMG system utilizes a hybrid control strategy that combines distributed control with decentralized control. The hybrid control strategy is achieved by the proposed detection algorithms for FDI attacks and DCLV sensor failures. The detection of FDI attacks is accomplished by comparing the predicted secondary controller output based on the Lagrange extrapolation with the actual one. When a power agent detects an FDI attack, its control mode is switched to decentralized control by using the proposed hybrid control strategy. The DCLV sensor failure detection algorithm to enhance system reliability against DCLV sensor failures is achieved by comparing the estimated DCLV with the measured one from the voltage observer. Upon detecting a DCLV sensor failure, the operation of the power agent is switched to the current control mode to sustain the system operation even under DCLV sensor failures. The proposed detection algorithms are simple, effective, and precise, operating without mutual interference that deteriorates the detection accuracy. Simulation and experiments are carried out under various uncertain test conditions to validate the reliability and effectiveness of the proposed control strategy.

Lyapunov-based distributed secondary frequency and voltage control for distributed energy resources in islanded microgrids with expected dynamic performance improvement

As a consequence of the increasing demand for electricity and environmental issues, the generation of electrical energy from renewable energy sources has improved in recent times. The renewable energy sources are connected with power grids all around the world. The increasing part of distributed energy resources in the current power system, has formed new chances and tests. Microgrid is the primary stage of future smart grid. This paper generally investigates the switching structures of microgrid reliant upon orthodox power system droop control. Microgrid droop switch schemes are deliberated in specifics for improving the understanding in microgrid control. This paper reviews droop control strategy of DC microgrid. As DC microgrid goes on developing, the control strategies of DC microgrid draw more and more attention. An ample survey of variety of issues associated with droop control strategies of DC microgrid is presented in this paper.

Stable current sharing and voltage balancing in DC microgrids: A consensus-based secondary control layer

As the conventional AC power grid is evolving with the integration of renewable energy resources (PV and wind) and power electronic converters, we discover more technical and economic values for DC microgrids (MGs), particularly with electric vehicles (EVs) and battery energy storage systems (BESSs) changing the load landscape. This paper presents a double-layer centralized hierarchical control framework to achieve the DC bus voltage regulation and power coordination of a PV-BESSs based DC MG, which operates in fully islanding condition. The proposed control strategy consists of two levels: 1) The primary control can achieve the DC bus voltage regulation by modifying the commonly-used inner and outer PI control loops; 2) The secondary control considers the characteristics of different types of BESSs. The latter is also responsible for load sharing by generating power/current reference signals among different DGs and DC converters. The advantage of the proposed control strategy is to eliminate the droop control as well as the conventional secondary control, which is designed to restore the voltage deviation caused by droop equations in a three-level hierarchical control system. The simulation results show that the proposed control strategy is effective to stabilize the DC bus voltage and it can share the dynamic power variation within DC MG.

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.

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.

The hierarchical control strategy enables seamless integration of power electronics-based distributed generators (DGs) into microgrids (MGs). This work presents a distributed coordinated secondary voltage and frequency control strategy of islanded MGs, utilizing distributed model predictive control (DMPC) approach. The Proposed approach is entirely distributed in nature, with plug and play features and requires minimal communication network with information exchange between immediate neighbors. This technique facilitates efficient information exchange and obviates the need for centralized control. Droop control is the primary controller for all the DGs, which leads to the deviations in DG voltage and operating frequency from standard nominal values. Secondary control enables the restoration of DG voltage and frequency to their nominal values. Input–output feedback linearization (IOFL) is used to transform nonlinear dynamics of inverter-interfaced DGs into first-order linear dynamics between input and output. Compared to some published works, the single-integrator dynamics eliminates the need to compute the derivative of the output voltage. Furthermore, DMPC has been used for all DG units as secondary controller, to achieve voltage and frequency regulation. This approach takes advantage of distributed architecture with optimal control actions and enhances reliability and flexibility of MG operations. Moreover, the proposed approach ensures that voltage and frequency of overall system are restored to its reference value while maintaining accuracy in real power sharing. Effectiveness of proposed control scheme is validated with simulations conducted on islanded MG test system using MATLAB/Simulink toolbox.