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This paper proposes a voltage compensator for converters in multi-converter DC microgrids (MG), which enhances the DC-MG behavior under fault conditions. Among the four existing categories in the classification of the faults, this paper focuses on those faults that affect voltage generation of converter-based distributed generation units (DGUs). Such faults conceptually occur from the terminal voltage reference to actual voltage generated by DGUs. To compensate the adverse effects of the faults, an adaptive scheme is proposed, which considers the most general case of the faults by using the time-varying multiplicative and additive fault models. The proposed scheme benefits from an integrated structure, which does not require separate fault detection, isolation, and identification blocks. As an advantage of the proposed approach, it is designed independent of the primary and secondary controllers of DGUs, and hence its performance is not affected by using different primary and secondary controllers. The effectiveness of the proposed scheme is verified under various fault conditions and uncertainty in the system parameters. Using the proposed scheme, the results show that the performance of system under the fault condition is similar to that of the normal operation. • An adaptive voltage control compensator scheme is proposed for DC microgrid. • The considered faults affect the voltage generation of DGUs. • The faults are modeled by time-varying multiplicative and additive components. • The proposed scheme does not need any separate fault detection and isolation modules. • To validate the conducted analysis, various case studies are provided. [ABSTRACT FROM AUTHOR]

• An accurate admittance model for the grid-connected converter with droop control is proposed. • The frequency coupling phenomenon is induced by the asymmetry of the droop controller for the grid-connected converter. • The outer control loop will impact the frequency-coupled phenomenon, but has slight effects on positive and negative sequence admittance of VSCs. • The proportional gain of the current control loop impacts obviously the frequency coupling phenomenon. The droop control scheme is widely applied for the paralleled operation of the grid-connected converter. Yet, the frequency-domain admittance model for the grid-connected converter with droop control is still inaccurate due to the complex coupling relation of the droop control loop, the voltage control loop, and the current control loop. To address this problem, the coupling relationship of these control loops is analyzed in this paper, and then an accurate admittance model for the grid-connected converter with droop control is proposed based on the complex vector modeling method in both dq -frame and αβ -frame. The influence of different control parameters on the admittance model is analyzed further. The proposed model accurately describes the admittance characteristics of the grid-connected converter with droop control, and reveals the frequency coupling phenomenon caused by the droop control loop. The frequency response measured by frequency scan and simulation results verify the effectiveness of the established model. Finally, the impedance interactive stability analysis results show that the grid-connected converter with droop control is prone to instability in stiff grid conditions. © 2017 Elsevier Inc. All rights reserved. [ABSTRACT FROM AUTHOR]

For islanded microgrids with inverter-based distributed generators (DGs), this article focuses on the fast finite-time secondary voltage and frequency control problem considering system uncertainties. The distributed controller is designed based on the command filtered backstepping procedure, and a disturbance observer is constructed to estimate the lumped system uncertainties (including parameter perturbations, unmeasurable variables, etc.). In addition, the singularity issue derived from differentiating the virtual control law is avoided by using a smooth switch function. The proposed controller guarantees that the tracking errors of voltage and frequency converge to a small region around the origin in a finite time, while achieving desired active power sharing ratio. Case studies are conducted on a microgrid system to demonstrate the effectiveness of the proposed strategy in the presence of load changes and parameter perturbations. • This paper aims at achieving fast finite-time secondary control of islanded microgrids. • The proposed method is free of chattering and singularity issues. • The disturbance observer is constructed to enhance the robustness of voltage control. • Case studies are conducted to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

• Propose a SMPC based voltage regulation scheme to coordinate DG, OLTC, SVR and CBs considering uncertainty of DG outputs. • Use the time-correlated scenario generation method in the proposed SMPC framework. • Consider the single-phase regulation effect of OLTC, DGs and CBs. With the increasing penetration of renewable energy resources, the uncertainty of renewable energy resources output introduces challenges to voltage control in distribution networks (DNs). To solve these problems, this paper proposes a double-time-scale voltage control scheme for unbalanced DNs based on stochastic model predictive control (SMPC) to coordinate multiple voltage control devices. In fast-time-scale control, the active and reactive power outputs of distributed generators (DGs) are optimized to minimize voltage deviation, active power curtailment and reactive power fluctuation. In the slow-time-scale control, optimal tap changes of on-load tap changers (OLTC), step voltage regulators (SVR), and switched capacitor banks (CBs) are calculated to reduce voltage deviation and tap operations. Additionally, in the fast-time-scale control, the SMPC-based scheme adopts a scenario generation approach to represent the uncertainty of DG output. Considering the temporal correlation of the DG output, different prediction scenarios and scenario probabilities are obtained by the empirical cumulative distribution function fitted by the historical measurement data. In this paper, the effectiveness of the proposed double-time-scale voltage control scheme is verified by the unbalanced IEEE123 node system. The case study demonstrates that the proposed voltage scheme can effectively keep the three-phase voltages in unbalanced DN within the secure operation range and achieve better voltage profiles. [ABSTRACT FROM AUTHOR]

[Display omitted] • A physics-informed fast voltage control method of distribution network is proposed. • It develops a novel deep reinforcement learning algorithm with graph knowledge. • The proposed algorithm uses multiple agents with continuous action space. With the recent rapid uptake of photovoltaic (PV) resources, the overvoltage condition during low load periods or intermittent renewable generation is one of the most critical challenges for electricity distribution networks. Traditional model-based local or centralized control methods that rely on proper system parameters are difficult to mitigate rapid changes in power systems. Utilizing model-free/data-driven multiagent deep reinforcement learning (MADRL) has been recognized as an effective solution to active voltage control. However, existing MADRL-based control approaches trained solely on data are agnostic to the underlying real-world physics principles. Therefore, this paper aims to incorporate physical knowledge of regional distribution networks into MADRL's decision-making. The main contributions of this paper are summarized as follows. First, a novel physics-informed MADRL -based distributed voltage control method is proposed, which is still under a centralized training and distributed execution framework and only requires local measurements. Second, graph neural networks are employed to help MADRL agents learn graph knowledge (node features and topological information). Further, transformer is introduced to extract discriminative representations and ensure agents' cooperative control. Third, we adopt a physics-guided architecture of neural networks in the actor network to stabilize the training process and improve sample efficiency. Finally, simulation results based upon modified IEEE 33-bus and 141-bus networks validate the proposed method's effectiveness, robustness and computation efficiency. [ABSTRACT FROM AUTHOR]

• The proposed voltage control method provides optimal voltage profiles in various LVDC and MVDC electric railway systems by minimizing inter-area voltage drops. • The proposed voltage control method maximizes the release of the line capacity and minimizes the power losses by minimization of inter-area current. • The proposed voltage control method is a decentralized and distributed voltage control method that employs the DC voltage of DC electric railway networks rather than telecommunication systems. • The proposed voltage control method benefits from simplicity, cheapness, high reliability, expandability and modularity. • Any changes in the power, number and location of trains, traction substations, distributed generators, ESS's and the length of electric lines in LVDC and MVDC electric railway systems cannot perturb the performance of the proposed voltage control method. The medium voltage DC (MVDC) electric railway system is one of the most interesting options for future high-speed rail (HSR) lines. The dc voltage control is one of the most critical challenges of MVDC electric railway systems due to various reasons such as simultaneous, continuous and considerable changes in the power and location of trains, regenerative brake energy of trains, long power lines, the application of ESS's and RES's. In this paper, a voltage control, i.e., the optimal-voltage voltage-power (OVVP) control method, is presented for MVDC electric railway systems. OVVP control method is a practical, reliable, automatic, and modular distributed voltage control method. The proposed voltage control method satisfies the nearest voltage to the nominal value in each node and the smoothest voltage profile in different MVDC electric railway systems. Although the analyses carried out in this paper analytically prove the significant performance of the presented voltage control method, different challenging scenarios are considered to verify OVVP control method more. In addition, the results of the proposed voltage control method are successfully validated by GAMS software. [ABSTRACT FROM AUTHOR]

• The core of the novelty of NV-MAIS approach relies on the fact that we are using shunt reactors for both voltage regulation and frequency oscillation damping. To the best knowledge of the authors, there has not been reported the use of the shunt reactors in frequency oscillation damping and the NV-MAIS strategy is the first of a kind of such an approach with grid commissioning and installation in high voltage 735-kV transmission level. The Hydro-Québec transmission system is among the most extensive and complex power grid in North America due to its very long lines (over 1000 km) joining the generation facilities in the north of the Québec province to the main centers of consumption in the south. Since the 1990s, Hydro-Québec has adopted a defense plan, consisting of various types of protection solutions including different remedial action schemes (RAS) and automatic action strategies, to increase the stability and reliability of its transmission system against extreme contingencies. One of these protection strategies is the automatic switching of shunt reactors (MAIS) 1 MAIS is the acronym of the French name of the project as M anoeuvre A utomatique d' I nductance S hunt (MAIS). which has been in operation since 1997. The goal of the MAIS system is to keep the voltage profile of the whole 735-kV grid in an acceptable range to increase the transmission capacity. The MAIS system at each substation is entirely independent from neighbour substations and will close or trip shunt reactors just based on local measurements. As a periodical upgrade in all existing protection solutions in the Hydro-Québec power system, new functionalities have been added into the existing MAIS system to develop its new version to improve both the voltage profile and the frequency response. This paper presents this new version including the algorithm and the concept behind the new additional functionalities. The simulation results show the efficiency and performance of the proposed method. On that basis, a deployment on Hydro-Québec's grid to replace almost all the existing MAIS system with this new version will be realized by 2030. [ABSTRACT FROM AUTHOR]

This paper deals with a novel hybrid transformer configuration for MV/LV three-phase distribution systems. The configuration involves a standard distribution transformer with an open-end secondary winding, which is connected to the LV grid and a four-legged inverter with a floating DC bus. Unlike previous hybrid transformers, the proposed configuration eliminates the need for a bidirectional converter and auxiliary transformer windings as well. The fourth leg of the inverter is exploited to manage the neutral line current. The main advantage of the proposed configuration is its ability to provide phase voltage control at the distribution grid edge. This enables the compensation of various grid voltage anomalies such as sags, swells and some kinds of interruptions. The effectiveness of the proposed configuration is demonstrated through simulation and experimental results, highlighting its promising performance in real-world scenarios. [ABSTRACT FROM AUTHOR]

• An unprecedented very large-scale T&D dataset covering Texas is made publicly available. • The sheer size of this dataset is intended to provide a challenge for the scientific community. • This lays the foundation for demonstrating the coordination of T&D. • It can help foster the improvement of algorithms and tools for years to come. Distributed energy resources, such as rooftop solar, have rapidly expanded in recent years, given declining costs and the desire to reduce carbon emissions. With more energy resources located in the lower-voltage distribution system, it is increasingly helpful to utilize combined transmission and distribution (T&D) system models to analyze interactions between these normally-distinct subsystems. This paper proposes a methodology for creating very large-scale, highly detailed, combined T&D systems that are synthetic—that is, free from non-public data—yet still realistic. The methodology creates very large-scale combined T&D systems by merging the most up-to-date techniques for creating synthetic distribution feeder networks with the latest methods for building synthetic, meshed bulk-power transmission networks. This methodology is demonstrated on a T&D system geolocated in Texas, and benchmarked with co-simulation results. Validation demonstrates that the resulting syn -texas-TDgrid synthetic test system realistically represents characteristics found in actual networks, addressing the lack of available T&D test systems. With over 15,000 feeders and 46 million electrical nodes, this T&D dataset has applications for research in optimal power flow algorithms, voltage control, reconfiguration, and T&D coordination schemes under high adoption of distributed energy resources. [ABSTRACT FROM AUTHOR]

• Propose a new real-time distributed voltage control scheme for MV&LV DNs based on ATC method. • Coordinate multiple DGs at different voltage levels to guarantee secure operation of the entire DN. • Design a dynamic allocation approach to determine the weighting factors for the optimization problem of MV&LV DNs. This paper proposes a new real-time distributed voltage control (RTDVC) scheme for medium and low voltage (MV&LV) distribution networks (DNs) based on the analytical target cascading (ATC) method. In this scheme, photovoltaic (PV) units at the LV level, on-load tap changer (OLTC) and distributed generation (DG) clusters at the MV level are optimally coordinated to regulate the bus voltages in MV&LV DNs to be close to the nominal value while minimizing the number of operations of OLTC and smoothing the reactive power outputs of DGs. Based on the ATC method, the large-scale strongly coupled voltage control problem of MV&LV DNs is decomposed into several subproblems while guaranteeing the optimality of the primal voltage optimization problem. The decomposed subproblems are iteratively solved in parallel in the MV and LV controllers, which achieves optimal voltage control in a distributed manner and effectively reduces the computation burden. An MV&LV test system with two 20 kV feeders and 39 0.4 kV LV grids was simulated to demonstrate the control performance of the proposed distributed voltage control scheme. [ABSTRACT FROM AUTHOR]

Previous research indicates that post-fault grid-forming (GFM) converters only work in current limiting control (CLC) mode. However, converters integrating the current saturation algorithm (CSA) can operate in both constant voltage control (CVC) and CLC modes. This paper firstly derives the angle-switched model of the grid-connected system governed by CSA and virtual synchronous generator (VSG). Then, combining the fault and three types of post-fault trajectories, critical clearing angle is analytically formulated through the Segmental Equal Area Criterion (SEAC). Finally, a range of values for the saturated current angle is proposed to ensure that the system is stabilized in CVC mode. Notably, the optimal saturated current angle, which is the minimum value within the range, provides the maximum critical clearing angle. • An angle-switched model for a virtual synchronous generator equipped with the current limit is established. • Segmental equal area criterion is proposed and analytical formulations are derived for the critical clearing angle under three post-fault operational states. • Optimal saturated current angle control is proposed to enhance transient stability by configuring the saturation current angle. [ABSTRACT FROM AUTHOR]

• Microgrid is a small grid that uses renewable energy sources to generate power. • Performance of microgrid may deteriorate due to the uncertainty in load dynamics. • A controller is proposed to control the voltage, current and power of a microgrid. • The controller is designed for both single and three phase islanded microgrid. • Performance of the controller is measured and compared against various loads. Future grids will consist of many small and large microgrids that will require complex control of grid interaction as well as islanding operation. This paper addresses the control of islanded microgrids under dynamic system conditions. In this paper, a robust model reference modified adaptive proportional integral derivative (PID) controller is designed for voltage and current control augmentation of islanded microgrid against various load dynamics. The variation of load dynamics may cause the unsafe operation of the microgrid. The objective of the designed controller is to confirm the safe operation against voltage and current control under different load dynamics. The controller is designed based on selecting the desired reference model and controlling parameters. The effectiveness of the proposed controller is evaluated against the presence of load dynamics, harmonic sources, asynchronous machines, nonlinear and unknown loads. The results show that the proposed controller is capable of providing high tracking performance and safe operation for both the single and three phase islanded microgrid system. [ABSTRACT FROM AUTHOR]

Highlights • The applied control strategy does not depend on any tuning parameter. • The converter does not require any separate output filter. • The series transformer impedance is controlled indirectly by flux (voltage) injection to its secondary. • This system does not require any signal analysis tool for voltage disturbances extraction. • The "smart branch" can conduct the harmonics directionally. Abstract A novel "Smart Branch" is introduced in this paper which compensates for various power quality disturbances. This "Smart Branch" includes a series transformer which its impedance is controlled indirectly by voltage injection to its secondary. By using a finite control set-model predictive controller (FCS-MPC), the "Smart Branch" can be controlled adaptive, flexible and straightforward. The "Smart Branch" is installed at the PCC, where the load current and the PCC voltage can be measured locally without using a communication channel. The "Smart Branch" controller tracks pure sinusoidal waveforms as its references. Thus the "Smart Branch" hardly produces harmonics and therefore its output filter can be eliminated. The controller can be used for multi-objective optimization of power quality in power systems and especially in microgrids. A droop based method is developed for adjusting the objectives' weightings in the cost function of the MPC. In the last part of the paper, by using the extensive simulations and scenarios, the figure of merits of the "Smart Branch" is proved. [ABSTRACT FROM AUTHOR]

• Impacts of stator voltage, DC voltage and magnetic flux on transient stability are analyzed. • VSynC-DFIG model is simplified as a well-studied synchronous generator. • A new procedure to calculate critical clearing angle of VSynC-DFIG is proposed. Transient stability of virtual synchronous controlled (VSynC) equipment is usually analyzed without considering the dynamics of the AC voltage controller. However, neglecting the impact of stator voltage controller dynamics in doubly fed induction generator (DFIG) wind turbine can lead to inaccurate stability assessment results. In this paper, the quantitative analysis of the impact of non-forced magnetic flux and DC voltage fluctuations on transient stability is conducted. Based on these analyses, a simplified DFIG model is proposed, which is approximately equivalent to a synchronous generator with excitation regulator. Then, a discretized step-by-step calculation procedure is introduced for the proposed model. Moreover, the influence of controller parameters on transient stability is analyzed based on the calculation results. Finally, simulations and experimental validations are carried out to demonstrate the effectiveness of the proposed model. [ABSTRACT FROM AUTHOR]

• Voltage control plays a crucial role in the restructuring of modern power systems especially with high share of renewables and controlled resources. • An optimal control is proposed based on a proper linear quadratic integral control. • Voltage time response of aperiodic type without oscillations is assured. • Feasibility and robustness analyses on actual regional networks and test systems. The network voltage control together with the regulation of reactive power, has always been recognized as vital feature of power systems. It is aimed at containing the bus voltage within acceptable limits for guaranteeing system security, first and then assessing power quality. The voltage control is a key field in which several Transmission System Operators are currently investing resources to be well prepared in managing power systems with a high share of renewable energy sources, overcoming the drawbacks of the existing architectures. As is well known the voltage control is realized through a hierarchical structure, which is based on classical primary control, secondary control, and tertiary optimization. The secondary control is concerned with the regional level, implying proper coordination at both the geographical and temporal level. Due to its central role in power system operation, the secondary voltage control cannot be excluded from the effects of the dramatic changes that power systems are experiencing worldwide, even though regulators work by implementing well consolidated methodologies and strategies which show excellent performances. The motivation for this study in fact relies on the observation of some oscillatory phenomena in some areas of the Italian power system characterized by a high number of regulating resources highly coupled each other on the basis of the grid topology: despite these oscillatory phenomena are not so frequent, with the increasing of regulating resources enslaved by secondary regulation (such as renewable-based power plants) could potentially be worsened in the near future. Thus, in this paper, an optimal control strategy is proposed by tailoring a proper linear quadratic integral control which allows guaranteeing the desired profile for pilot buses voltage, ensuring a time response of aperiodic type without any oscillations. The aim of the proposed approach is to preserve the strengths of the existing secondary voltage regulation while enhancing the overall stability so enabling the progressive large penetration of renewable energy sources-based power plants and minimize the occurrence of oscillatory phenomena especially in highly electrically coupled areas. The robustness of the proposed regulation is verified first with respect to the WSCC 9-bus test system and successively on an actual regional area of Italian transmission network. [ABSTRACT FROM AUTHOR]

• The aim of this paper was to improve the ride-through capability under an line-commutated converter (LCC) AC system fault in a three-terminal hybrid DC transmission system. The main conclusions drawn from this paper were as follows: • The full bridge submodule(FBSM) could output a negative level, so that the DC outlet voltage of the inverter station could be adjusted significantly without any additional requirements for the reactive power reserve of the AC system. • Current margin control was proposed to be adopted for the load center side of MMC 2 when a slight fault on the AC side of LCC to avoid the interruption of LCC transmission power. • The non-load center terminal MMC 1 adopts voltage margin control when a severe fault on the AC side voltage of LCC to avoid cutting off many important loads and a frequency collapse. This paper considers a three-terminal hybrid DC transmission system in which the rectifier station adopts line-commutated converter (LCC) while the inverter station adopts parallel two-terminal modular multilevel converter (MMC). The focus of this paper is on the improvement of ride-through capability of LCC AC system in the presence of a fault. A strategy for improving the fault ride-through capability based on local information is proposed. First, the mathematical models of different types of converters in a hybrid DC system are established, and the influence of AC faults at the sending end on the DC transmission system is discussed. Second, in the presence of a slight fault on the AC side of LCC, the current margin control is proposed for the load center side MMC 2 , where the MMC 2 maintains the DC port current as the minimum allowable value. Third, if a serious fault on the AC side of LCC causes a significant AC voltage drop, it is proposed that the non-load center terminal MMC 1 adopts voltage margin control. In addition, the MMC 1 takes over the DC system voltage control so as to maintain MMC 2 to absorb minimum power from the DC system, and prevent the load center connected by the MMC 2 from removing too many important loads or causing a frequency collapse. Last, a large number of simulation results verify the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

The design of Shipboard Power Systems (SPSs) needs careful consideration of a variety of system specifications, operating constraints and design requirements under different operating scenarios and mission profiles to maintain a stable, reliable and economically efficient operation. Especially, for the state-of-the-art DC-based SPSs, with the extensive use of power electronic apparatus and the wide adoption of intelligent ship modernization techniques, the complexity and heterogeneity of on-board electrical systems significantly increase. Therefore, the corresponding specifications, regulations, and requirements that are tailored specifically for DC-based SPSs need to be developed to address the requirements of electric ship design, simulation, control, optimization, management, and automation. Given the limited amount of existing published works in this field, this paper aims to bring together different aspects of model-based electric ship design. It provides a thorough survey of design specification and requirement criteria along with their mathematical formulations that are of different importance, physical natures and time scales to address the transient and steady-state studies for the latest Medium-Voltage DC (MVDC) SPSs. This paper can be considered an attempt to provide a comprehensive study for the SPS research community from academia, industry, and Navy to facilitate various aspects of design, development, and deployment of the MVDC SPS applications. [ABSTRACT FROM AUTHOR]

This paper analyzes the problem of selecting the optimal access point in power grid for the offshore wind farm (OWF), which is increasingly integrated into the onshore power system by use of voltage-source converter based high voltage direct current (VSC-HVDC) transmission. The development of the selecting method includes three steps. First, three major factors, which have significant impacts on deciding the optimal location of the onshore VSC, are introduced, i.e., voltage support from onshore VSCs, voltage fluctuation caused by the change of wind power and construction costs of the VSC-HVDC transmission system. Secondly, three evaluation indexes that are corresponding to the three factors are put forward. Thirdly, in the face of this multi-objective programming problem, a linear weighted model is proposed in this paper and the comprehensive weight method based on the theory of information entropy and the analytic hierarchy process (AHP) method is adopted to get the weight coefficients. The method is carried out in a practical power system in China. The results demonstrate that the proposed methodology is an effective way for selecting the optimal access point of the OWF. [ABSTRACT FROM AUTHOR]

Recently, hybrid Modular Multilevel Converter (MMC) composed of Half-Bridge Sub-Module (HBSM) and Full-Bridge Sub-Module (FBSM) is becoming a preferable topology in overhead line based HVDC transmission. Assuming that two switches are paralleled in each HBSM and FBSM to double the current ratings, this paper proposes an enhanced hybrid MMC topology based on the improved series–parallel HBSMs and two-port FBSMs. The two types of Sub-Module (SM) segments are obtained from direct mirror flipping of all the traditional SMs, but with more degrees of freedom for control. The proposed topology maintains the dc-fault ride through capability and can minimize the requirements of capacitor voltage sensors and control hardware. Then the nearest level capacitor voltage control strategy suitable for the new topology is proposed and implemented by controlling the specific switches to parallel adjacent capacitors for voltage equalization, and the spike currents during the parallel procedure can be suppressed by the distributed arm inductors, this is defined as capacitor voltage self-balancing in this paper. The effectiveness of this work has been validated in terms of steady and transient states on a two-terminal hybrid MMC based HVDC system. [ABSTRACT FROM AUTHOR]

This paper simulates an islanding network including wind turbine, battery energy storage systems (BESS), and load. The purpose is to control voltage and frequency of the load following wind speed variations by proper control of BESS. A decentralized control scheme including two control loops is designed on BESS. One control loop is implemented for voltage regulation and the other loop is designed for frequency control. Both loops are equipped with PI (Proportional–Integral) type controllers as internal controllers. Furthermore, both loops are equipped with supplementary stabilizers as external controllers. The internal controllers regulate frequency and voltage and the external stabilizers enhance stability. This paper optimally tunes all the parameters of internal controllers and external stabilizers at the same time. The problem for tuning a large number of the design variables is mathematically expressed as a mixed integer nonlinear optimization programming and solved by modified-adaptive PSO technique. The proposed methodology is simulated on a typical standalone network including wind turbine, BESS, and load. The accurate model of BESS and wind turbine is incorporated to cope with real conditions. Moreover, in order to demonstrate the real-world results, non-linear time domain simulations are carried out in MATLAB software. The results verify that the proposed control scheme can efficiently utilize BESS to control voltage, regulate frequency, and damp out oscillations under wind and load variations. [ABSTRACT FROM AUTHOR]

In this paper a new nonlinear regulation for synchronous generator excitation control is presented. This method comprises the nonlinear Interconnection and Damping Assignment (IDA) method together with the generator voltage feedback giving a nonlinear Automatic Voltage Regulator (AVR). First, the IDA excitation control is introduced in the third order salient pole synchronous generator mathematical model represented as a Port Controlled Hamiltonian (PCH) system. Thereafter, a generator voltage feedback signal is added. It is proven that adding the feedback to IDA control preserves a PCH structure of a system and the system is therefore passive. In the paper it is also proven that the gained system is stable and that the reference operating point corresponds to the minimal energy point of the system. Finally, the gained IDA AVR is tested with seventh order synchronous generator model and compared to classical generator excitation controllers. The simulations show the efficient tracking of generator voltage reference value, as well as the maintenance of stability in case of grid-side short-circuit occurrence. The main advantages of the proposed controller are efficiently generator voltage tracking and attenuation of electromechanical oscillations in case of a large disturbance, such as grid-side short-circuit. [ABSTRACT FROM AUTHOR]

This paper proposes a novel approach to TSO–DSO coordination in day-ahead operation planning for the procurement of ancillary services (AS) such as congestion management and voltage control. The approach overcomes two major issues hindering practical applicability of existing TSO–DSO coordination mechanisms, namely the computational complexity due to the large number of times the TSO and DSO problems are resolved as well as the inability to map analytically the cost and amount of flexibility at DSO level into TSO problem. To this end, a two-fold novelty of our approach consists of the decomposition of the overall AS problem into two sequential stages, which seek the procurement of: (i) "active power related" AS (i.e., for congestion management) and then (ii) the "reactive power related" AS (i.e., for voltage support), respectively as well as a fast approximation of the cost of aggregated active and reactive power flexibilities of active distribution systems (ADSs). Unlike existing mechanisms, the proposed approach considers real-world challenging aspects such as the N-1 security at TSO level and operation uncertainties at both TSO and DSO levels. Accordingly, the approach relies on tailored versions of stochastic multi-period AC security-constrained optimal power flow (S-MP-SCOPF) at TSO level and stochastic multi-period AC optimal power flow (S-MP-OPF) at DSO level, which are formulated as non-linear programming (NLP) problems. The importance and performance of the proposed approach are illustrated on a power system connecting a 60-bus transmission system (under 33 N-1 contingencies) with five 34-bus ADSs. • This paper proposes a two-stage approach to TSO–DSO coordination in day-ahead operation. • The approach procures optimally ancillary services for managing congestion and voltages. • A new mapping cost vs amount of aggregated flexibility in distribution grid is proposed. • The paper considers real-world challenges such as operation uncertainties and the N-1 security. • The approach relies on tailored versions of stochastic multi-period AC optimal power flow. [ABSTRACT FROM AUTHOR]

This paper focuses on the voltage control of grid-forming Voltage Source Converters (VSCs) connected to AC networks. The work aims to identify the best pairing between voltage control structures and usual ac filter topologies. The two filter topologies considered are the inductive (L) and the inductive–capacitive (LC) filter. The control structures studied are the Cascaded Control (CC, cascaded voltage and current loop) and the direct Voltage Control (VC, without current loop). To select the best control-filter pair, the following methodology is suggested. First, for each pair, a voltage control parameter sweeping methodology is developed to find suitable designs considering the following limits: minimum bandwidth, maximum converter voltage, point of connection voltage deviation and maximum current. Second, this action is repeated for different filter parameters and network Short-Circuit Ratios (SCRs). Finally, based of the obtained results, the most relevant control-filter pairs can be selected, prioritizing them according to the fulfilment of the defined limits. To conclude, time-domain simulations are carried out to validate pairing selection. • The paring of voltage control and AC filter for grid-forming converters is presented. • The tuning method optimizes control against disturbances within converter limits. • The analysis considers several SCR, filter parameters and converter limits. [ABSTRACT FROM AUTHOR]

• A frequency regulation method for a two-stage PV system by controlling DC voltage is proposed, which is coordinated with the enhanced virtual inertia control (VIC) of the DC capacitor. • An active DC voltage recovery control is designed at the inverter to restore the frequency support ability after primary frequency modulation. • The proposed method can fully use the available power to improve the frequency nadir, rate of change of frequency, and voltage recovery ability. The current frequency regulation methods for a photovoltaic (PV) system cannot balance frequency support and primary control performances. This paper proposes a frequency regulation method for a two-stage PV system by controlling DC voltage, which is coordinated with the enhanced virtual inertia control (VIC) of the DC capacitor. The power reserve control of the PV arrays is improved considering the characteristics of the VIC, thereby strengthening the frequency regulation ability of the PV system. An active DC voltage recovery control is designed at the inverter to restore the frequency support ability after primary frequency control. With MATLAB/Simulink software, the performance of the proposed method in dealing with different load sizes, load change trends, and DC voltage recovery coefficient is verified by comparing it with other traditional methods in different scenarios. The proposed method can fully use the available power to improve the frequency nadir, rate of change of frequency, and voltage recovery ability. [ABSTRACT FROM AUTHOR]

• New soft-switching Dual-input DC-DC boost converter. • Checking the voltage gain and getting an acceptable result. • Converter load voltage control. • Experimental verification is provided. This paper proposes a new non-isolated dual-input zero-voltage switching (ZVS) dc-dc boost converter with medium voltage gain. Two input ports of the converter are current-fed and suitable for a unidirectional dc power source, and a chargeable dc energy storage. The proposed converter basic circuit composed of three cascaded power switches, which at their connection points join to the converter three inductors and a switched capacitor voltage multiplier (VM) cell is accompanied. Using this VM cell the converter voltage gain achieves two times of that in the conventional boost converter and also half voltage stress across the power switches. Through the converter two input inductors powers from a unidirectional source (like PV) and a chargeable battery source can be accepted. Taking a triangular resonant current at the switching frequency, the converter third inductor aids to reach ZVS for the power switches. The converter power switches turn-on with ZVS and the power diodes stop conducting with ZCS in all operation conditions. As a result, low switching power losses, reduced reverse recovery issue, and high efficiency are accessible especially for higher input and output powers. The converter power switches are switched in PWM manner with two individual duty ratios, allowing to regulate the output voltage and control the extracted power from the input unidirectional port. The proposed circuit has been analyzed in detail to find its different operating modes, determine its capacitor currents, estimate its power losses components and design its control system. Behavior of the proposed system is also examined by a low-power laboratory prototype tested under four different practical operation conditions. The proposed converter benefits from simple and compact structure, current-fed input ports, low number of power switches and magnetic elements, low voltage stress across the power switches, medium voltage gain and high efficiency. [ABSTRACT FROM AUTHOR]

The usage of distributed generation sources and the segmentation of power systems into microgrids brings several advantageous features, such as energy independence, lower transmission losses, and improved energy management flexibility. Still, the expected steady abolition of traditional centralized networks and their centralized control schemes introduces several operational difficulties, that can compromise (micro)grid's stability and reliability. Consequently, several approaches to how microgrids should be operated have been proposed, albeit with several salient deficiencies. Precise power exchange, control of frequency and voltage excursions, applicability on different grid topologies, and the possibility to integrate grid codes, which are very important control aspects, are not jointly enabled in a general case. This paper proposes a novel approach, based on a specific goal function and corresponding decentralized control that addresses the mentioned problems and guarantees the microgrid's stability by constraining the frequency and node voltage deviations, simultaneously supporting the desired active power exchange between prosumers. Unlike droop-based and droop-derived control schemes, the proposed approach aims to inherently preserve the frequency and voltage levels within the limits defined by the grid operator or future microgrid codes, thus preventing the generation unit's unwanted disconnection during transients and disturbances. In addition, the control algorithm is independent of network topology, can be applied in both islanded and non-islanded microgrids, and secures system scalability. To confirm the effectiveness of the proposed control strategy, a number of simulation results are presented and thoroughly discussed. • A novel seamlessly scalable self-stabilizing microgrid control framework is proposed. • Guaranteed upper bounds on deviations of node frequencies and voltages are provided. • Precise active power exchange between nodes is enabled. • Priorities of the proposed scheme can be adjusted according to user preferences. • Comprehensive tests in relevant operational scenarios are performed. [ABSTRACT FROM AUTHOR]

• Transient response improvement of grid-connected converter is discussed. • Universal utility range (frequencies and magnitudes) are considered. • *Corresponding plug-in disturbance observer design is proposed. • Converter dynamics is enhanced without existing controller redesign. It is well-known that dynamics of single-phase grid-connected power converters employing PI or type-II DC link voltage controllers is limited by the maximum tolerable value of utility-side current total harmonic distortion (THD). Moreover, universal DC link voltage controller must be capable of supporting both 50 Hz and 60 Hz utilities and universal range of magnitudes (85V rms –265V rms). As a result, DC link crossover frequency value (and hence corresponding settling time) may vary significantly in case grid voltage magnitude variations are not compensated in the controller. In order to cope with the above-mentioned deficiencies, the paper suggests a design methodology for a novel dual-frequency-tuned utility magnitude insensitive plug-in disturbance observer (DOB), supporting universal utility frequencies and magnitudes so that the transient response of a grid-connected converter is improved without altering the existing PI or type-II DC link voltage controller coefficients. These characteristics are attained by robust modification of DC link voltage loop plant to possess notch-like properties around two frequencies of interest and treating the grid voltage magnitude value as non-measurable uncertainty translated into input disturbance and subsequently cancelled. Analytical design guidelines for obtaining PI + DOB controller coefficients are provided for desired values of utility-side current THD and DC link voltage loop phase margin (PM). Simulations and experimental results are provided to validate the feasibility of proposed design methodology. [ABSTRACT FROM AUTHOR]

• A novel improved negative feedback PLL (NFPLL)-modified synchronous reference frame (MSRF)-based control circuit is proposed for shunt active power filter (SAPF). • The performance of the proposed NFPLL-MSRF scheme under different source and load conditions has been deeply analyzed through a series of simulation-based experiments. • A detailed performance comparison of the proposed NFPLL-MSRF scheme with conventional approaches is presented. • The performance of the proposed control technique is validated using OPAL-RT 4510 real-time digital simulator platform. The use of nonlinear devices introduces current and voltage harmonics into power distribution networks. The presence of harmonics, an undesirable power quality issue, causes transformer overheating, malfunctioning of sensitive loads, power losses, and significantly reduced distribution system efficiency. Therefore, harmonics mitigation in power systems has become absolutely essential and is widely addressed, utilizing shunt active power filters (SAPFs) around the globe. The performance of SAPF in generating adequate compensating current depends on the working of three control loops used for the purpose of reference current generation, dc-link voltage control, and switching pulse generation. The existing approaches for reference current generation suffer from inherent complexity and the inability to extract positive sequence signals required for obtaining synchronizing angle during source disturbance conditions. Therefore, this paper proposes a novel negative feedback phase-locked loop-based modified synchronous reference frame technique to overcome the drawback of the existing approaches. The effectiveness of the proposed approach is investigated under ideal and non-ideal grid voltage conditions with different load perturbations. The proposed control strategy is first tested using a number of simulation-based experiments and, thereafter, validated using OPAL-RT 4510 real-time digital simulator. Compared to other existing approaches, the proposed technique has offered improved results in terms of reduced total harmonics distortion. [ABSTRACT FROM AUTHOR]

• A comprehensive dynamic model that takes into account all state variables that can affect VSM stability, is presented. This model results in a more effective and stable global performance of the proposed VSM through the nonlinear relations between the state variables. • A new global stable VSM is proposed using the dynamic state variable model and Lyapunov method, which can provide a superior frequency and voltage magnitude control for a WG-connected microgrid by exerting the globally stability evaluation to the initial VSM. • A performance index known as the AFRE is introduced to demonstrate how the proposed VSM outperforms the initial model of VSM. • A two-stage process is taken into account for adjusting the LCs. In the first stage, the variation of LCs of angular frequency and d-component current in the frame of two- and three-dimensional curves is used to analyse the LCs of control functions. The second stage establishes an evaluation based on a Hyperbola curve to determine the impact of the Lyapunov coefficient on the reduction of the VSM current errors. Virtual synchronous machines (VSMs) have been widely investigated for improving the inertia of power systems and providing reliable frequency control against rapid penetration of renewable energy sources. But the drawback of these studies is the lack of global stability evaluation of VSMs in various operating conditions. In this paper, a new global stable VSM is proposed using the Lyapunov method to attain a stable frequency and voltage magnitude for a weak grid-connected microgrid. The proposed Lyapunov-based VSM uses an initial model of VSM, that incorporates small-signal linearization and the swing equation of synchronous generators. Using a comprehensive dynamic model with state variables including the VSM currents, angular frequency and the VSM input voltage, the Lyapunov method is then applied to the initial VSM in order to attain the proposed VSM with the desired global stability. To further illustrate the proposed VSM's advantages over the initial VSM, an index known as the angular frequency regulation efficiency is presented. The Lyapunov coefficients are also adjusted using a two-stage process. The first adjustment process is based on the proposed VSM control function errors. In this regard, the control function Lyapunov coefficients are analyzed relying on the Lyapunov coefficient variation trend of angular frequency and d-component current in the frame of two- and three-dimensional curves. Moreover, a Hyperbola curve–based assessment is established for determining the Lyapunov coefficient effects on the decrement of the VSM current errors. MATLAB/SIMULINK environment is utilized to validate the effective and superior performances of the proposed VSM acquired from the global stability feature. [ABSTRACT FROM AUTHOR]

In this paper, an approach for placement of voltage regulators (VRs) in traditional distribution systems by considering a local controller model is presented. The main aims of this paper are controlling the voltage level in its permitted range and decreasing the costs imposed to the distribution system companies, such as costs that stem from power losses, VRs’ investment and maintenance. Genetic algorithm (GA) has been used as a tool to determine the number, location and rated power of VRs. Since in traditional distribution systems, tap position determination of VRs is achieved by local controllers, local controller model is established to determine tap operations. A 70-bus distribution system is considered to prove the value of the presented approach. Effectiveness of the proposed approach and ineffectiveness and infeasibility of conventional approaches are presented in numerical studies. The presented approach allowed to eliminate voltage violation in all load conditions and a reduction of power losses of about 6% for the maximum load level. [ABSTRACT FROM AUTHOR]

This paper proposes a method to compensate the network voltage unbalance using the doubly fed induction generator (DFIG) wind turbine connected to a dynamic voltage restorer (DVR). In the configuration employed, a DVR based on a voltage-source converter is connected in series between the generator and the grid. The proposed control uses symmetrical components to calculate the negative sequence current to be injected in the network to compensate the voltage unbalance at the Point of Common Coupling (PCC), and the voltage to be applied at the stator, to avoid torque oscillations. The contribution of this paper consists on proposing a coordinated strategy for compensating the voltage unbalance at PCC and avoiding torque oscillations in the machine. Due to the coordinated use of all resources available for compensation, the proposed method increases the operational range of the existing methods. Basically, the strategy of the proposed method consists on using the remain capacity of the grid side converter of the DFIG for injecting compensating current and, when it reaches its limit, the rotor side converter is controlled to inject compensating current through the machine stator to the grid. The torque oscillations due to the unbalanced operation of the machine is avoided by applying a series voltage by the DVR. Simulation results have validated the proposed method, which maximizes the compensation at the PCC voltage unbalance and guarantees the machine operation with low oscillation in the electromagnetic torque. [ABSTRACT FROM AUTHOR]

This paper proposes a robust control method for power sharing among dispatchable electronically-coupled distributed energy resources (DERs) in an autonomous microgrid. The DER units operate in parallel to meet the total aggregated load demand. The aggregated load is assumed to be uncertain, unbalanced and/or nonlinear, thus it is considered as a source of unmodeled dynamics. The main objective of this paper is to design an H ∞ robust controller to enable tracking reference commands of the voltages at the output of the DERs in the presence of unmodeled dynamics. In addition, the H ∞ control design problem is combined with other control methods, such as constrained optimal control and regional pole placement, to achieve an optimal control objective, and to improve transient response. The optimal controller is achieved by minimizing an upper bound on H 2 performance. The regional pole placement is achieved by defining a linear matrix inequalities (LMI) region. This multiobjective H 2 / H ∞ problem, combined with regional pole placement are expressed as a set of LMIs, and solved by using standard convex optimization algorithms. To show the effectiveness of the proposed method in maintaining the desired voltage at the DER outputs, in presence of unbalanced and nonlinear loads, simulation results are provided for a single DER unit, and the performance of the proposed control strategy is compared with three different controller strategies. Power sharing among multi DER units in autonomous networks is then demonstrated through digital time-domain simulation studies using MATLAB/SimPowerSystems Toolbox. [ABSTRACT FROM AUTHOR]

Voltage amplitude is one of the most important parameters of electrical energy, particularly from the viewpoint of sensitive loads connected to the grid. The dynamic state in a power grid (voltage sags/swells/interrupts) originating from dynamic loads, uncontrolled power flow, switching effects, adverse weather condition, etc., might cause defects in sensitive loads and financial losses. The annual damages in Europe due to voltage perturbations is estimated at 10 billion €. This paper presents a three-phase voltage conditioner and power flow controller without DC energy storage unit for the control of phase and voltage amplitude. In comparison to conventional solutions, such as DVR or UPFC, the power electronic AC/AC converter is based on a bipolar matrix-reactance chopper. The main aim of this paper is to analyse the basic properties of the presented solution, especially from the point of view of the ability to compensate voltage sag/swell and interruption and to control phase shift in bidirectional power flow control. The studies have been carried out on a three phase system with passive and active load (two-source system). On the basis of the authors’ research, it can be stated that the described solution gives the possibility to compensate deep voltage sag (50% of nominal voltage), overvoltages (up to 150% of nominal voltage) and single phase interruption. Simultaneously the solution allows independent control of the phase angle of output voltage in the range ± π/3. The paper presents an operational description, theoretical analysis, simulation and the experimental test results of a 1 kVA laboratory model using a conventional PWM control strategy. [ABSTRACT FROM AUTHOR]

This paper presents an optimal coordinated voltage control scheme for preserving long-term voltage stability of power systems. Linear quadratic integral (LQI) controller is employed to construct this scheme. Also, this paper considers the detailed dynamic model of doubly-fed induction generator (DFIG) wind turbine, synchronous generators, over excitation limiter (OEL) and under-load tap changer (ULTC) system, which are important elements influencing voltage stability of power systems. The proposed approach at each time instance involves following two major steps: First, the power system nonlinear equations are linearized and optimal controllers are obtained by LQI technique. Then, in the second step the system dynamic behavior is investigated via time-domain simulations by applying the attained optimal control signals at the first step. The impact of the proposed coordinated voltage control scheme is evaluated by time domain simulations on a well-known test system, under variable wind speed and fault conditions. [ABSTRACT FROM AUTHOR]

• A systematic planning method to allocate dynamic VAR sources is proposed. • The concept of voltage control area is proposed to partition a bulk power system. • A zone-based voltage supporting index is defined in the site selection process. • A zone-based distributed optimization framework is put forward. In this paper, a practical planning method to allocate dynamic VAR sources (DVSs) using the concept of voltage control areas (VCAs) is proposed to enhance short-term voltage stability of power systems. Firstly, based on the defined transient distance index, a spectral clustering method is employed to partition a system into VCAs. Then, candidate DVS sites in each zone are determined by the proposed voltage supporting index. In each VCA, the capacity of candidate sites is preliminarily estimated by use of the dichotomy method to decide the most effective control bus and the initial capacity guess, based on which further system-wide optimization is implemented using particle swarm optimization (PSO) to obtain the final allocation scheme. Case study on the New England 39-bus and 68-bus test systems along with the comparisons with the existing methods confirm the effectiveness and advantages of the proposed methodology. [ABSTRACT FROM AUTHOR]

• Sensorless control method to establish a DC-link voltage. • Removal of the outer control loop to reduce the inverter control complexity. • Modelling and control of the grid-connected inverter. • Implementation of incremental conductance based MPPT algorithm to search an operating point for the boost converter. In this paper, a novel control approach for a grid-connected photovoltaic system (GCPVS) has been proposed. This novel approach introduces a sensorless DC-link voltage control for two-stage three-phase GCPVS. The two-stage system with the proposed control approach includes an intermediate (DC-DC boost) converter and pulse width modulation (PWM) strategy based voltage source inverter (VSI). In the first-stage control, the intermediate converter is incorporated with the maximum power point tracking (MPPT) technique. The incremental conductance MPPT technique is implemented to enhance the voltage level of the PV array under variable irradiance condition. Further, the second-stage control of VSI deals with DC-link voltage regulation (outer-loop) and current control (inner-loop) matching with the conventional scheme. Distinctively, the proposed scheme avoids the outer-loop and controls the inner-loop. Hence, the reduction of DC-link high voltage sensor minimizes the cost as well as the size of the system. However, the DC-link voltage will remain maintained through the power balancing. Hence, the removal of the outer-loop controller enhances the system stability and the dynamic response of the system under variable irradiance conditions. The proposed GCPVS is simulated in MATLAB/Simulink using sim power tools to verify the system performance and the dynamic behaviour at different irradiance levels of the system. [ABSTRACT FROM AUTHOR]

• System uncertainty is mitigated when promoting the renewable energy consumption. • Various active management elements are integrated in the optimal power flow (OPF). • A more precise and efficient SOCP model for the OPF is presented. • Decentralized P / Q (V) control strategies are proposed for real-time regulation. This paper presents a multi-stage management scheme for renewable-rich medium voltage distribution network (MVDN) to promote the renewable energy consumption and mitigate the system uncertainty. In the first stage, a day-ahead (DA) dispatch model is proposed to promote the renewable distributed generation (RDG) consumption and decrease the power loss. In the second stage, a model predictive control (MPC) based rolling optimization model for intra-day (ID) operation is proposed to minimize the mismatch of the interactive power at the upstream Grid Supply Point (GSP) between the DA schedule and ID operation. Multiple active management elements such as network reconfiguration and soft open point are integrated and a novel second order cone programming (SOCP) model for the centralized optimal power flow in the DA and ID stages is presented. In the third stage, a decentralized P / Q (V) control strategy of RDG inverter for real-time (RT) voltage regulation is proposed to ensure system safety and mitigate the fast voltage fluctuation. The effectiveness of proposed management scheme is demonstrated in a test system integrating a standard network and real-world data of load and RDG output profiles. [ABSTRACT FROM AUTHOR]

• A distributed voltage control scheme is proposed to optimally regulate voltages in distribution networks. • Distributed information synchronization is realized based on consensus protocols. • Distributed model predictive control is used to coordinate regulation of DGs and OLTC. • Two control modes are designed for both normal and dynamic conditions. This paper proposes a distributed voltage control (DVC) scheme for smart distribution networks with high penetration of inverter-based distributed generators (DGs), aiming to optimally coordinate DG units and on load tap changer (OLTC) transformer to regulate the voltages within the feasible range. The proposed scheme consists of two important parts: (1) distributed information synchronization (DIS) framework and (2) distributed model predictive control (DMPC)-based voltage control scheme. The DIS framework is established based on the consensus protocols to synchronize the specific information about the critical bus voltages and potential OLTC actions. The DMPC-based voltage control scheme is presented, in which each DG unit only exchanges information with its immediate neighbors and solves the local optimal control problem. Two control modes are designed to better deal with different operating conditions. In the normal mode, only the reactive power outputs of DG units are optimized to mitigate the voltage deviations. In the corrective mode, both of the active and reactive power outputs of DG units are optimally controlled to correct the severe voltage deviations. To mitigate the mutual interaction between the DGs and OLTC, the potential actions of OLTC are predicted and considered in the optimization problem of each units. The control performance of the proposed scheme was demonstrated using a real medium-voltage (MV) distribution network with two feeders under both normal and large-disturbance conditions. [ABSTRACT FROM AUTHOR]

• A two-layer control strategy for the operation of LV grids is proposed. • A method for voltage regulation and voltage unbalance mitigation is developed. • A method to implement DSO policies for active power curtailment is developed. • An OLTC control is proposed to increase the overall green energy production. • Impact of several control schemes on the maximum DRES penetration is evaluated. In this paper, a coordinated control strategy is proposed to effectively address the voltage regulation issue in active unbalanced low-voltage (LV) distribution networks. The proposed control scheme is based on a two-layer centralized approach, consisting of the local and the overlaying control layers. The former is embedded at the hardware level of the distributed renewable energy sources (DRESs). Therefore, it is continuously active, aiming to prevent network overvoltages and to mitigate voltage unbalances based explicitly on local measurements. The overlaying control strategy is implemented at a central controller, at the distribution system operator (DSO) level, acting supplementary to the local control scheme to increase the total injected green energy and to implement DSO policies for the reallocation of the active power curtailment. The validity of the proposed two-layer control strategy is demonstrated on a real Slovenian LV network by means of time-series simulations. Additionally, the impact of the proposed control on the maximum DRESs penetration level is thoroughly evaluated, while its performance is compared against other conventional control strategies, revealing its superiority in terms of voltage regulation, voltage unbalance mitigation and increased green energy production. [ABSTRACT FROM AUTHOR]

Highlights • Proposing a stabilization technique for matrix converters. • Proposing a combination of the damping resistor and the input digital filter. • Providing the stability analysis based on the small signal model of the converter. • Presenting a bidirectional power flow control between the main grid and a microgrid. • Definition of the matrix converter stability regions based on the system parameters. Abstract In this paper, the importance of the matrix converter stabilization in bidirectional power flow control is investigated. A stabilization technique for matrix converters based on a combination of the damping resistor and the input digital filter is proposed. The suggested technique is applied to a direct matrix converter employed as an interface between two power sources as the main grid and a microgrid to realize a stable four-quadrant power flow control process. The control method is based on the voltage oriented control strategy using anti-windup proportional-integral controllers. The stability analysis using the small-signal model of the converter is presented and simulated numerically. The proposed stabilization and bidirectional power flow control methods are validated by experimental tests on a prototype. [ABSTRACT FROM AUTHOR]

This paper presents the design of local PI regulators to control voltage and active power of the Distributed Energy Resources (DERs) in grid-connected Active Distribution Networks (ADNs). The design is developed based on the proposed Multiple-Input Multiple-Output (MIMO) model of ADN. Firstly, the Relative Gain Array (RGA) matrix is used to address the problem of input–output pairing and to show that the voltage control loops are coupled whereas the active power control loops are decoupled. Subsequently the method of the Effective Open-Loop Transfer Function (EOTF) is used to decompose the multi-loop voltage control system into a set of equivalent Single-Input Single-Output (SISO) loops. Then, each PI regulator is independently designed to fulfill some control objectives dictated by the designer. The proposed design procedure is simple and straightforward. The fixed-parameters PI regulators facilitate the integration of DERs into traditional distribution networks which are usually operated without or with limited communication infrastructure. The simulation results presented for various scenarios of two different ADN confirm that performance objectives and regulation at the desired set-points are achieved in a robust sense. • Voltage and active power PI control of DERs. • Square MIMO model of the ADN oriented to the design. • Analysis of the internal and external interactions among control loops. • Independent design based on the concept of Effective Transfer Function. • Robust stability and robust performance in presence of parameters uncertainty and disturbances. [ABSTRACT FROM AUTHOR]

• Developing impedance models of IBR and SM considering detailed control for stability analysis of severe grid conditions. • Adverse control interaction of SM/SC and GFL IBR may occur by high IBR penetration during system-split situations. • The low-frequency non-passivity of GFL IBR and SM/SC can adversely participate their control interaction. • Oscillation risk in SM and GFL IBR system can be mitigated by retuning their control, but may induce other stability issue. • Through changing part of the IBRs from GFL to GFM, both grid-support services and oscillatory stability can be ensured. Motivated by the potential transient oscillation risks in inverter based resource (IBR) dominated power systems, the paper is to identify the unwanted control interactions between traditional synchronous machines (SMs) and IBRs during system-split situations. Modal impedance analysis method is applied to reveal oscillation risk and the underlying mechanism, while electromagnetic transient (EMT) simulation analysis is used to observe reliable oscillation phenomenon. Impedance models of IBRs considering both grid-following (GFL) and grid-forming (GFM) control schemes and SMs equipped with automatic voltage regulator (AVR) and power system stabilizer (PSS) are developed for grid conditions with large frequency and/or voltage deviations, and validated by EMT simulations. Potential adverse control interactions within an exemplary test system are analyzed taking into account the impacts of: 1) renewable penetration level; 2) converter control scheme (GFL or GFM); 3) converter control tuning; 4) SM control setting; 5) grid-support capabilities of converter-interfaced loads. It has been observed that oscillation instability at around several Hz can occur by high penetration of GFL controlled IBRs after system-splits, which can be mitigated by redesigning the AVR & PSS control, or retuning the power/voltage control loops and phase-locked loop (PLL) of GFL converter. However, such measures might inevitably induce other stability issues, e.g. synchronization instability. From the oscillation analysis, it has also been revealed, through replacing part of the GFL converters with GFM converter(s), the oscillation mode at several Hz can disappear, owning to the varied impedance including better damping of GFM than GFL for the frequencies between 1 and 10 Hz. [ABSTRACT FROM AUTHOR]

It is crucial for hybrid DC microgrids (MGs) to employ power management strategies that take into account the dynamic characteristics of all energy sources in the system. This paper proposes a novel complex droop control (CDC) for voltage control units in hybrid DC MGs, which enables flexible power management by providing the capability to adjust steady-state and dynamic power sharing in addition to damping provision by leveraging a multifunctional droop function. To accurately tune the splitting frequencies that determine the performance of dynamic power sharing, a novel design procedure based on a reduced-order DC MG model is proposed. To gain insight into the dominant oscillatory modes and the impact of the proposed control on the stability of the hybrid DC MG, a system-level small-signal analysis is also conducted. Offline and real-time simulations of the hybrid DC MG subjected to pulsed-power load variations reveal that the CDC can effectively share dynamic power, while improving voltage regulation and damping. • A droop control method for dynamic power sharing between heterogeneous sources of DC microgrids. • Reduced-order modeling of hybrid DC microgrids to design droop gains. • Small-signal stability analysis provides information on dominant oscillatory modes. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed finite-time event-triggered secondary control for islanded DC microgrids (MGs). The proposed controller can provide accurate current sharing and critical bus voltage regulation in finite time without the need for periodic communications. The finite-time feature improves the convergence speed and transient response of the secondary control while the event-triggered behavior reduces the burden on the communication network. The proposed secondary control scheme is validated in MATLAB/Simulink and HIL testbed utilizing OPAL-RT and Raspberry Pis. • The finite-time event-triggered secondary controller for a DC MG is proposed. • The critical bus voltage restoration and accurate current sharing are achieved. • MATLAB/Simulink and HIL simulation testbeds validate the proposed controller. • An improved transient response with reduction on communication burden is presented. [ABSTRACT FROM AUTHOR]

• An efficient coordination method of DOCRs and DisRs is developed considering five critical fault locations. • A new objective function for DisRs and DOCRs coordination problem is proposed. • A new time-voltage-current characteristic of DCORs is proposed in the optimization problem with two new decision variables. • The miscoordinations of DOCRs and DisRs are totally eliminated compared with the published works. The power transmission systems are mainly protected by distance relays (DisRs) as a primary protection and directional overcurrent relays (DOCRs) as a backup protection. These two types of relays have a different operation principle that, sometimes, leads to miscoordination problems. In this paper, a new approach for DOCRs and DisRs coordination considering the DOCRs time-voltage-current characteristic (TVC-C) is proposed to fully eliminate the miscoordination problems found in the already published works. The coordination problem is formulated as a mixed-integer nonlinear programming (MINLP) considering a new proposed objective function. Furthermore, two new decision variables related to TVC-Cs are proposed namely: the type and the voltage control variable. An enhanced version of the hybrid GSA-SQP method is proposed to solve the developed optimization problem. In order to check the applicability of the proposed relays coordination method on small and large and more complex systems, the IEEE 9-bus and IEEE 39-bus are used for this task. The obtained results are compared with the published works which show the superiority of the proposed method in terms of satisfaction of all coordination constraints. [ABSTRACT FROM AUTHOR]

• Propose a distributed voltage control (DVC) scheme for unbalanced distribution networks with DGs. • Use preventive and corrective control modes to coordinate maximum energy usage of DGs and voltage regulation. • Design a novel mode switching method to eliminate severe voltages fluctuations caused by frequent mode switching. • Realize voltage information synchronization based on consensus through information exchange between adjacent controllers. This paper proposes a distributed voltage control scheme for unbalanced distribution networks (DNs) with single-/three-phase distributed generations (S/T-DGs) based on distributed model predictive control (DMPC). The active and reactive power outputs of each phase of DG units are optimally coordinated to regulate three-phase voltages of all buses to be close to the nominal value and mitigate voltage fluctuations. According to the operation conditions of three-phase DNs, two control modes are designed: preventive and corrective modes. In the preventive mode, the measured voltages of each phase are within the predefined limits, only the reactive power outputs of DG units are optimally adjusted to minimize the voltage deviations of each phase of all critical buses from the nominal value and mitigate the reactive power variations; while in the corrective mode, both the active and reactive power outputs of DG units are optimized to minimize the voltage deviations and the curtailed active power of DGs. The relationship between voltage variation of each phase and power injection is determined using an efficient analytical sensitivity calculation method. The DG controller only exchanges information with adjacent controllers and solves the local DMPC problem. A modified Finnish distribution network with 6 SDGs and 2 TDGs was used to verify the control performance of the proposed distributed voltage control scheme. © 2017 Elsevier Inc. All rights reserved. [ABSTRACT FROM AUTHOR]

Multi-Input Converters (MIC) are an advancing technology suitable for sustainable energies, due to its ability to incorporate several Renewable Energy Sources (RES). Various structures and topologies of DC–DC converters are developed recently to ensure the highest performances of hybrid RES. The MIC are a typical coupling system which has several operating modes. Thus, their management system and controller design become complicated. This paper reports on the design, modelling, and control of a non-isolated double-input boost converter based on fuzzy supervision strategy. In fact, this work describes the design of a multi-input multi-output (MIMO) supervisor with power sharing abilities. The supervisor decides the percentage of the power for each input source. The studied converter is proficient for the integration of power sources individually or simultaneously. First, the state space model of the dual input single output converter based on the average model technique is presented. Second, an intelligent fuzzy management system is proposed. Meanwhile, the power control is based on sliding mode control (SMC). The stability of the system with the proposed SMC is proved via Lyapunov theory. Finally, a comparative study is conducted betweendifferent controllers in the supervised and non supervised modes, theses controllers (PI and SMC) have been used to control the voltage and the current loops. The validity of the suggested supervisor and its performances is tested experimentally on a laboratory prototype using dSPACE 1104 real time digital board. Experimental results demonstrate the robustness and the flexibility of the Fuzzy supervisor based on SMC (FSMC) compared to one which based on PI control(FPI) under different operating conditions. • Design a global smart fuzzy supervisor controller using Lyapunov approach and associated with a sliding mode control to ensure the power balance in the system and output voltage stabilization. • A comparative study was carried out between the conventional PI controller and the sliding mode control to control the voltage and current loops. • The fuzzy management and supervision system have been validated both by simulation and experimentation. • This approach gives satisfactory results. The supervision and control scheme, performs very well despite the imposed dynamic conditions where different disturbances such as input voltage variations, load variations and reference voltage variations occur during the experiment. [ABSTRACT FROM AUTHOR]

This paper proposes the application of fictitious reference iterative tuning (FRIT) method to optimize the gains of dc voltage controller of grid connected photo-voltaic (PV) system. It may be difficult to achieve good control of dc voltage using conventional PI controller having only one-degree-of-freedom (1-DOF) due to the trade-off between overshoot (in step response) and disturbance response. In this paper, the optimal control of dc voltage is proposed with improved disturbance response by implementing 2-DOF PI controller structure. FRIT method has been programmed in MATLAB based upon the particle swarm optimization (PSO) algorithm. The fundamental idea related to FRIT method is the extraction of input and output data, reference model setting and range of controller gains. The performance of dc voltage control for the optimized 2-DOF PI controller is also compared with the fuzzy logic controller (FLC) response. [ABSTRACT FROM AUTHOR]

This paper considers a three-phase, low voltage community network with both grid-connected and autonomous modes of operation, which is composed of a group of residential houses and some single-phase, converter-interfaced distributed energy resources (DER) with equal and arbitrarily scattering of the DERs among the phases. The vigorous operation of such a microgrid system is examined with the proposed management techniques. In such a network, it is highly probable for one phase to have a high generation capacity while the other phases experience a higher demand; a technically challenging problem for a network operating in autonomous mode. In this paper, it is proposed for the single-phase DERs of such a system to operate under a droop-based voltage control technique while an appropriate technique is proposed to facilitate the transmission of the excess power from one phase to other phases. The proposals are validated by extensive digital simulations in PSCAD/EMTDC for several scenarios to demonstrate the feasibility of operating such a system and the efficacy of the proposed techniques. [ABSTRACT FROM AUTHOR]

In this paper, STATCOM performance for voltage–reactive power control is investigated by comparing different tuning methods, used to evaluate gain parameters of STATCOM controller in presence of high probabilistic uncertainty in input wind power and reactive power load demand. To control voltage transient response in least time, reactive power demand is managed by STATCOM. The conventional methods for tuning gain parameters of STATCOM controller do not satisfactorily operate in case of random disturbances and therefore, advanced controllers such as Genetic Algorithm (GA), Artificial Neural Network (ANN) and Adaptive Neuro Fuzzy Inference System (ANFIS) are required. The main contribution of the paper is: (i) Investigation of STATCOM performance in presence of high probabilistic uncertainty with step changes in input wind power and reactive power load demand, (ii) system studies during dynamic conditions with composite load model in lieu of static load model in the system, (iii) comparison of voltage control and STATCOM reactive power using various tuning methods. Results comparison through all tuning methods show that advanced tuning methods are able to preserve optimal performances over wide range of disturbances using Integral of Square of Errors (ISE) criterion. [ABSTRACT FROM AUTHOR]