Improved power sharing with a back-to-back converter and state-feedback control in a utility-connected microgrid

The primary function of microgrid (MG) is to share the power accurately among different distributed generation (DG) units according to the load demands. The droop control methods are normally used for appropriate power sharing. However, these control techniques suffer badly due to the feeder impedances and uncontrollable power coupling issues. Many techniques have been proposed to share power accurately, most of them rely on communication link. In this work a localized/communication free signal generating mechanism is proposed. The mechanism injects the transient reactive power in frequency droop characteristics by observing the changes in microgrid. This injection disturbs the actual active power sharing which is helpful for the computation of reactive power sharing errors. These errors are minimized by utilizing the proportional integral (PI) controller. The PI controller modifies the y-intercept and the slop of Q-E (Reactive Power –Voltage) droop characteristic for the correction of reactive power sharing error. Moreover, the parameters of droop and PI controller of inner control loop are optimized by Particle Swarm Optimization (PSO) to obtain high precision and reliability. The proposed technique has the following salient features:1. Accurately shares the reactive power without the information of feeder line impedances2. This accurate reactive power sharing scheme has been started locally without the need of communication signal from the central controller. Thus, there is no delay in monitoring the notable changes in microgrid caused by DGs. 3. The careful selection of PI & droop controller parameters through PSO makes the system more stable as compared to the conventional methods in terms of transients and steady state responses.

This paper describes a control method for distributed generation (DG) units to implement active power sharing and frequency recovery simultaneously in an islanded microgrid. Conventional active power–frequency ( P – f ) droop control is used for the DG controller, and the frequency deviation is recovered by the DG itself via self-frequency recovery control, without requiring secondary frequency control. Because the electrical distance (impedance) from each DG unit to a point where the load demand changes differs among DG units, the instantaneous frequency deviations may differ between DG units. These differences are fed into the integrators of the self-frequency recovery control and may result in errors in active power sharing. To solve this problem and share active power more accurately, a compensation control method is developed for active power sharing, which considers the droop coefficients of each of the DG units. Simulation results show that the proposed control method is effective.

To manage the microgrid effectively, it is important to get the power information among multiple distributed generation (DG) units. However, when the droop control method is adopted, as the corresponding DG units cannot be simply modeled as VF or PQ buses, the conventional power flow algorithm may become inapplicable. To solve this issue, a time domain iteration (TDI) based power flow algorithm is hereby proposed. Firstly, a microgrid model is prepared, containing a network model and several DG unit models. Then, the proposed TDI is executed for power flow calculation, which mimics the real-time operation of microgrids. In each iteration, the DG unit models input voltages and currents to the network model; then the network model changes its state accordingly and feeds related parameters back to those DG unit models. As the DG unit models simulate the behavior of actual DG units, the proposed algorithm is not limited to the droop control governed microgrids. Moreover, as the TDI has definite physical meaning, convergence problems can be avoided. Finally, the validity of the proposed power flow algorithm is verified through the Matlab simulation results from an 8-bus microgrid system.

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Introduction. Constant changes in electrical system loads lead to increased power losses and voltage drops, requiring effective strategies to improve grid performance amid changing power demands. Problem. Many studies assume constant loads when determining optimal locations for distributed generation (DG) units, when in reality, loads change throughout the day. These changes affect network performance and require efficient solutions that adapt to changes in loads demand to maintain system efficiency and stability. Goal. This research aims to optimize the locations and sizes of DG units to reduce power losses and optimize voltage profile, taking into account changes in loads hourly over a 24-hour period. Methodology. The study analyzes 24 hourly scenarios using 2 optimization techniques: the conventional particle swarm optimization (PSO) algorithm and the hybrid-dynamic PSO algorithm. A multi-objective function is adopted to reduce power losses and improve voltage profile at the same time. Results. The modified IEEE 33 bus system was used to verify the effectiveness of the proposed method. The hybrid-dynamic PSO algorithm has shown superior performance in reducing active and reactive losses compared to the traditional algorithm. It also contributed to a significant improvement in the voltage profile, demonstrating its high efficiency in dealing with changes in loads demand during time. Scientific novelty of this work lies in the integration of hourly load changes into the process of allocating DG units and using a hybrid-dynamic PSO algorithm that combines the benefits of PSO traditional and adaptation mechanisms, leading to realistic and more efficient improvement. Practical value. This methodology enhances the performance of the smart grid by reducing power losses and voltage deviation under daily load, ultimately reducing operational costs and improving grid reliability. References 28, tables 4, figures 10.

Control technique for enhancing the stable operation of distributed generation units within a microgrid

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes. Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method. Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.

Rejectable deep differential dynamic programming for real-time integrated generation dispatch and control of micro-grids

Advancements in power conversion efficiency and the growing prevalence of DC loads worldwide have underscored the importance of DC microgrids in modern energy systems. Addressing the challenges of power-sharing and voltage stability in these DC microgrids has been a prominent research focus. Sliding mode control (SMC) has demonstrated remarkable performance in various power electronic converter applications. This paper proposes the integration of universal droop control (UDC) with SMC to facilitate distributed energy resource interfacing and power-sharing control in DC microgrids. Compared to traditional Proportional-Integral (PI) control, the proposed control approach exhibits superior dynamic response characteristics. The UDC is strategically incorporated prior to the SMC and establishes limits on voltage variation and maximum power drawn from the DC–DC converters within the microgrid. A dynamic model of the DC–DC converter is developed as the initial stage, focusing on voltage regulation at the DC link through nonlinear control laws tailored for Distributed Generation (DG)-based converters. The UDC ensures voltage stability in the DC microgrid by imposing predetermined power constraints on the DGs. Comparative evaluations, involving different load scenarios, have been conducted to assess the performance of the proposed UDC-based SMC control in comparison to the PI control-based system. The results demonstrate the superior efficiency of the UDC-based SMC control in handling dynamic load changes. Furthermore, a practical test of the proposed controller has been conducted using a hardware prototype of a DC microgrid.

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.

With high penetration of power electronics-based distributed generation (DG) units, the decentralized control strategy based on droop control with virtual impedance has been widely studied in recent years. But the conventional virtual impedance cannot properly deal with the massively increasing of nonlinear loads. Therefore, an improved virtual impedance method based on second-order general-integrator (SOGI) is presented. Besides, a further value correction of reactive power in droop control is also proposed in this paper. The proposed approaches have been applied and verified in a parallel system formed by two DG units connected with nonlinear loads. The form of SOGI and derivative formula of corresponding virtual impedance in the system has been analyzed in detail. The obtained experimental results show the proposed approaches can achieve less output voltage and current distortion of DG units with nonlinear loads, and also realize good performance of power sharing.

Distributed generation units with small energy sources, such as fuel cells, micro-turbines, and photovoltaic devices, can be connected to utility grid as alternative energy sources besides providing power to their local loads. The distributed generation units are interfaced with utility grid using three phase inverters. With inverter control, both active and reactive power pumped into the utility grid from the distributed generation units can be controlled. Reactive power flow control allows the distributed generation units to be used as static VAr compensation units besides energy sources. This work presents a distributed generation unit control technique which provides robust voltage regulation with harmonic elimination under island running mode and decoupled active and reactive power flow control under grid-connected mode. The control technique, which combines discrete-time sliding mode current control, robust servomechanism voltage control, and integral power control, enables seamless switching between island mode and grid-connected mode and guarantees sinusoidal line current waveform such nonlinear local load. The P Q coupling issue is addressed and the stability of the power control loop is proved using Lyapunov direct method.

Grid-connected inverters based on droop control have been widely used in distributed generation (DG) systems, as they can ensure smooth transition between grid-connected mode and islanded mode. However, the traditional active power-frequency (P-ω) and reactive power-amplitude (Q-E) droop control method is vulnerable to the grid voltage distortion, because of the following two reasons. Firstly, the inverter under droop control behaves like a controlled voltage source connected in parallel with utility grid, and the grid current is not directly controlled. Secondly, the line impedance interlinking the inverter and the grid is usually very small, so a slight distortion in grid voltage can lead to serious distortion in grid current. To suppress grid current distortion, this paper proposes to employ harmonic virtual impedance (HVI), which is effective only at harmonic frequency, to increase the equivalent impedance between inverter and grid. Compared with the existing control methods, the proposed HVI control scheme does not need to detect grid voltage and it is easy to be implemented. Comprehensive simulation and experimental results are provided to validate the effectiveness of the proposed method.

In this paper a transition control strategy for parallel voltage source inverters (VSIs) operating in a microgrid system is presented. In the proposed control method, VSIs regulate the power flow in standalone (SA) and grid connected (GC) modes through a hybrid control architecture without needing of an external communication link between them. The hybrid controller uses the current control in GC mode and in droop based voltage controller in SA and it switches autonomously and smoothly between them depending on the mode of operation. During the transition periods between the two modes of operation, the current control and droop control participate with different in formulating the inverter output voltage. To ensure high power quality during transition, one of the VSIs is designated as a dispatch unit to facilitate interconnection with utility grid and achieve proper waveform between the modes of operations. The dispatch unit is controlled always using droop control and the parameters of which are adjusted based on the current mode of operation. The proposed control technique provides proper performance, stabile operation, and fast dynamic response in the microgrid system during the transition mode of operation. The performance of the proposed control technique has been verified with simulation and experimental studies.

This paper presents a new voltage control strategy for an electronically-interfaced distribution generation (DG) unit that utilizes a voltage-sourced converter (VSC) as the interface medium. The control strategy is based on the concept of voltage-controlled VSC (VC-VSC) rather than the conventional current-controlled VSC (CC-VSC). The proposed VC-VSC 1. enables operation of a DG unit in both grid-connected and islanded (autonomous) modes, 2. provides current-limit capability for the VSC during faults, 3. inherently provides an islanding detection method without non-detection zone, 4. provides smooth transition capability between grid-connected and autonomous modes, and 5. can accommodate ride-through capability requirements under a grid-connected mode. This paper also investigates performance of the proposed VC-VSC strategy based on an eigenanalysis in MATLAB, and time-domain simulations in the PSCAD/EMTDC environment.