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Grid-forming inverters (GFMIs) are recognized as a prominent driver toward achieving renewable energy-rich power grids. Unlike grid-following inverters (GFLIs), which are controlled as current sources, GFMIs are controlled as voltage sources. In GFMIs, dynamic control of the magnitude ($V_{\mathrm{c}}$) and angle ($\theta$) of the point of common coupling (PCC) voltage is used to achieve active ($P$) and reactive ($Q$) power transfer across a line. However, independent control of $P$ and $Q$ via $V_{\mathrm{c}}$ and $\theta$ becomes challenging due to the coupling between $P$ and $Q$ loops. The coupling becomes severe as the resistance-to-reactance ratio of the grid impedance and the power angle between the GFMI and the grid voltages are increased. This article proposes a novel multivariable controller to completely decouple $P$ and $Q$ loops in GFMIs. The proposed multivariable controller could be designed based on the prevalent control structures for GFMIs such as droop controller, swing equation-based virtual synchronous generator (VSG) controller, zero steady-state error reactive power controller, and fixed steady-state error reactive power controller. The additional cross-channel decoupling controllers in the proposed multivariable controller provide superior decoupling action over the existing decoupling methods, such as the virtual inductor-based method. An $\mathcal {H}_\infty$ -based method is adopted to tune the proposed multivariable controller parameters, where the straightforward formulation of the desired closed-loop dynamics based on the open-loop system is clearly shown. The decoupling performance of the controller is experimentally validated extensively. The experimental results show that the proposed controller results in superior performance over the prevalent decoupling methods, such as the virtual-inductor decoupling method. [ABSTRACT FROM AUTHOR]

In this paper, a droop-based sliding mode controller (DBSMC) is designed for power sharing in an isolated DC microgrid (DC MG), with improved nonlinear droop model, in the presence of bounded structured uncertainties and external disturbances. The proposed DBSMC strategy does not require any communication link. The design process of the proposed controller is such that the trade-off between the two factors proportional power sharing and precise voltage regulation reaches the minimum possible. The creative design of the controller greatly reduced the control action stress, in such a way that, the occurrence of various uncertainties in the nonlinear droop model is compensated by applying small intelligent changes in the control parameters, using a fuzzy approximator. To evaluate the efficiency of the designed DBSMC controller, simulations are performed on an isolated DC MG with DC-DC buck configuration in MATLAB software. Finally, the real-time digital simulation results utilizing real-time digital power system simulator (RTDS) corroborate the performance of the proposed DBSMC controller. The simulation results obtained from MATLAB and the RTDS verify the robust, accurate and fast function of the proposed droop-based controller in power sharing of the isolated DC MG in the presence of variable operating conditions, parametric uncertainties and external disturbances. [ABSTRACT FROM AUTHOR]

This paper proposes a feedback linearization controller for a grid-following inverter (GFLI) that uses a conventional Phase-Locked Loop (PLL) as the synchronization unit. The proposed controller enhances the GFLI synchronization by expanding the PLL domain of attraction to the whole plane that is limited to a small region around the equilibrium point in a conventional PLL, provided that the grid impedance and voltage are known. Linearizing the overall closed-loop system, the proposed controller provides linear system attributes for the PLL, leading to an infinite domain of attraction and only one equilibrium point. Additionally, a state-feedback controller is integrated within the feedback linearization controller so that it enables the system to have an adjustable dynamic response. Finally, it is shown that the system is robust against parametric uncertainties. The performance of the proposed control design is validated in Matlab/Simulink and experiment. It is verified that the proposed controller expands the domain of attraction and enhances the system dynamic response. [ABSTRACT FROM AUTHOR]

Weak grid scenarios and low-inertia systems are emerging issues in power systems, which could lead to voltage and frequency instabilities. Synchronous Condensers (SynCons) have recently drawn renewed attention as a promising solution to provide system strength and inertia support. Even though the exciter control of SynCons is a well-established technology, further developments are required to guarantee the stability of post-fault operations, in particular, in weak grids. This paper proposes a data-driven approach for designing higher-order optimized exciter controllers to meet this requirement. A pseudo-random binary sequence (PRBS)-based system identification method is used to obtain frequency response data of the power system, from the exciter point of view, which is then fed into the proposed optimal control design procedure. The proposed exciter controller is tested for voltage ride-through and fault scenarios in a single machine infinite bus (SMIB) case and the IEEE 39-bus test system to assess its performance compared to the conventional AC1A exciter controller. The results obtained through simulation tests carried out using the PSCAD/EMTDC software verify that the proposed exciter controller guarantees the post-fault stability in both strong and weak grids. [ABSTRACT FROM AUTHOR]

This paper proposes a comprehensive method to estimate non-synchronous inertia in an AC microgrid system by analyzing measured frequency transients. Non-synchronous inertia includes inertial support from various inverter-interfaced renewable energy sources and energy storage devices in the form of virtual or emulated inertia. In the proposed method, a multi-step approach is used to identify a matrix of parameters that estimates the overall non-synchronous inertial response. Firstly, motivations are drawn from the fundamental system requirements of adequacy in RoCoF and maximum deviation in frequency, for any credible contingency. A systematic classification of various contributors to the non-synchronous inertial response is also carried out. System-identification studies are then performed to identify various component-models that contribute to the non-synchronous inertia. The component models include a faster RoCoF-based response model, a relatively slower frequency deviation based model, and in some cases, a slow, integral-of-frequency based model. Lastly, the concept of releasable kinetic energy and non-linear estimation is used to identify the capability of the virtual inertia sources and improve the accuracy of the overall estimate. The developed methodology is tested in a microgrid network hosting a mix of renewable energy generations and battery energy storage systems. The estimate of non-synchronous inertia will help system operators to accurately estimate the maximum frequency deviation and the time to reach the maximum deviation for any credible contingency. The estimate can also help in deciding the type of virtual inertia that will best maintain frequency stability in the microgrid system. [ABSTRACT FROM AUTHOR]

Islanded microgrids require power reserve with fast primary-frequency response to operate stably. Primary-frequency response from generators or batteries incurs high costs. Alternatively, primary-frequency responsive loads reduce costs by allowing generators and batteries to operate more flexibly. In industrial microgrids, which comprise a significant portion of the microgrid market, electrical motors are a significant portion of the total load. However, existing demand response mechanisms are not suitable for motor loads in microgrids. In this article, a novel grid-supporting motor drive controller is proposed. This controller can be retrofitted to motor drive loads to provide fast primary-frequency response. A reduced-order analytical model of the motor drive load is derived, which can be directly integrated into system-wide frequency stability studies. Additionally, the primary-frequency response time of the motor drive is assessed, analytically and through EMT simulation, and is less than $\text{100 ms}$ , which is comparable to large battery energy storage systems. Finally, the behaviour of a microgrid system that includes two large motor drive loads is assessed during various large disturbances through EMT simulation. [ABSTRACT FROM AUTHOR]

Islanded microgrids have low real and reactive power generation capacity and low inertia. This makes them susceptible to large frequency and voltage deviations, which deteriorate power quality and can cause frequency or voltage collapse. Grid-supporting battery energy storage systems are a possible solution as they are able to respond quickly to changes of their real and reactive power set-points. In this paper, a data-driven grid-supporting control system for battery energy storage systems, which requires no changes to the inverters inner real and reactive power control loops compared with a conventional grid-supporting inverter, is proposed. Tuning the data-driven controller does not require a dynamic model of the microgrid. Instead, the frequency response of the microgrid is identified and used directly to optimally tune the controller for H∞ performance and robustness criteria. The performance of the data-driven controller is verified through real-time software-in-the-loop electromagnetic-transient simulation, where it is compared with an inverse-droop controller and is shown to significantly reduce voltage and frequency deviations. [ABSTRACT FROM AUTHOR]

In this paper, a stable fuzzy controller in the form of linear matrix inequalities (LMIs) is designed for an electric vehicle (EV). The proposed controller evaluates the stabilization of the EV speed with robust disturbance rejection approach throughout both transient and steady states and in the presence of external disturbances and parametric uncertainties. In order to achieve an efficient control, EV's battery voltage, as control input, and EV speed, as system output, are constrained. The proposed control strategy is based on Takagi–Sugeno (T-S) fuzzy model and the parallel distributed compensation (PDC) fuzzy controller. Proposed controller can be practically implemented due to low computational load of control input that is owing to the offline process of obtaining the feedback gains, very small amplitude of control input and low power consumption. To evaluate the performance of the designed controller, simulations in five steps are conducted on an EV equipped with a brushed direct current (BDC) motor as a case study in MATLAB simulation environment. Ultimately, the real-time digital simulation results using real-time digital power system simulator (RTDS) confirm the efficiency of the proposed input-output constrained robust disturbance rejection stable fuzzy controller (IOCRDRSFC) in stabilizing the EV speed. The simulation results obtained from MATLAB and the RTDS confirm the energy-efficient and robust performance of the proposed controller in quick stabilization of the EV speed in the presence of all structured and unstructured uncertainties. [ABSTRACT FROM AUTHOR]

This paper presents an improved method to estimate the available inertia in an islanded AC microgrid. Inertia estimation is carried out based on measured frequency response for any arbitrary disturbance that occurs in the system. Modifications are made to the conventional swing equation-based curve-fitting method to obtain an accurate estimate for a system with high penetration of renewable generations. A polynomial curve fit over the total power generation is introduced to estimate the size of the disturbance accurately. Additionally, a variable order polynomial fit is carried out over the measured frequency, which not only improves the estimate of inertia but also helps to refrain the influence of network topology and size/location of the disturbance. The test microgrid system considered is a modified Standard IEEE distribution network, which consists of radial feeders and distributed generations. Firstly, the proposed method is tested on a system with only synchronous generations to assess the accuracy of the estimate. This is followed by the integration of Type 3 and Type 4 wind turbines, and a PV array within the microgrid system. Virtual inertia control is then implemented in the wind turbines to obtain inertial support. Estimation study of the microgrid system with virtual inertia is then carried out. The developed estimation method can accurately estimate the inertia provided by the synchronous sources within the generation mix. Finally, from all the results and observations, the inertia estimation process in a microgrid system is segregated into synchronous and nonsynchronous inertia estimation. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) have established their position as the choice of next-generation high voltage converters. These converters, however, require a bulk number of sensors in each phase for sub-modules (SMs) voltage balancing. A limited number of studies are found in the literature where the number of sensors is reduced by incorporating observers to estimate the SM voltages. Nonetheless, most of these methods require accurate measurements of the arm voltage. Thus, the dependence on sensors is not completely eliminated. In this paper, a Kalman-filter-based estimation of SM voltages is presented, which does not require any voltage sensor to measure the arm voltage. Therefore, this method considerably simplifies the implementation of MMCs. Extensive simulations and experimental test cases are conducted to validate the performance of the proposed SM voltage estimation method. The performance assessments verify the accuracy and robustness of the proposed method for various test scenarios performed over a wide range of sampling frequencies. [ABSTRACT FROM AUTHOR]

This paper proposes a feedback/feedforward control strategy to attenuate the dc-link voltage variations in a reduced-capacitor battery energy storage system. It also identifies the impact of dc-link voltage variations on a battery energy storage system's response dynamics and power capabilities. It shows that addition of a feedforward controller to the existing feedback controller improves the dc-link voltage regulation and consequently the response dynamics of the reduced-capacitor battery energy storage system. It formulates a power setpoint change as a disturbance to the battery energy storage control system. The battery energy storage system is analyzed using an averaged-value modeling technique. Small-signal analysis is then used to tune feedforward controller parameters. The performance of the proposed controller is evaluated via an electromagnetic transient software (PSCAD/EMTDC), and the findings are experimentally validated using a laboratory-sized test bench. [ABSTRACT FROM AUTHOR]

A Z-source series circuit breaker topology, which allows bidirectional power flow and has the ability to autonomously disconnect dc faults, is introduced. This topology allows current flow in the forward and reverse directions through the use of a diode bridge. The diode bridge allows response to faults on either the source or load side with only a single controlled switch. No additional passive components are required when compared with the unidirectional series Z-source circuit breaker topology. Analysis is performed to find the fault conditions that cause the bidirectional circuit breaker to trip when operating in a single-load power system. Then, using the simulation platform MATLAB/Simulink, operation of the bidirectional circuit breaker is evaluated for both source- and load-side faults. To experimentally validate the findings, an experimental prototype has been implemented. This prototype is used to confirm the circuit breaker's function in both the forward and reverse directions in addition to its response to source-side faults. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has attracted significant interest for medium-/high-power energy conversion applications due to its modularity, scalability, and excellent harmonic performance. One of the technical challenges associated with the operation of the MMC is the circulation of double-frequency harmonic currents within its phase legs. This paper proposes a circulating current control strategy in a double-frequency rotating reference frame, which, contrary to the existing solutions that are based on approximate/inaccurate models, relies on an experimentally identified nonparametric model of circulating currents to determine the coefficients of the controller. Minimizing the squared second norm of the error between the open-loop transfer function of the system and a desired one, the coefficients of the controller are determined. To guarantee the stability of the closed-loop system, the minimization problem is subjected to a few constraints. The validity and effectiveness of the proposed control strategy is confirmed, and its dynamic performance is compared with that of an existing solution by experimental results. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is a potential candidate for medium/high-power applications, specifically for high-voltage direct current transmission systems. One of the main challenges in the control of an MMC is to eliminate/minimize the circulating currents while the capacitor voltages are maintained balanced. This paper proposes a control strategy for the MMC using finite control set model predictive control (FCS-MPC). A bilinear mathematical model of the MMC is derived and discretized to predict the states of the MMC one step ahead. Within each switching cycle, the best switching state of the MMC is selected based on evaluation and minimization of a defined cost function. The defined cost function is aimed at the elimination of the MMC circulating currents, regulating the arm voltages, and controlling the ac-side currents. To reduce the calculation burden of the MPC, the submodule (SM) capacitor voltage balancing controller based on the conventional sorting method is combined with the proposed FCS-MPC strategy. The proposed FCS-MPC strategy determines the number of inserted/bypassed SMs within each arm of the MMC while the sorting algorithm is used to keep the SM capacitor voltages balanced. Using this strategy, only the summation of SM capacitor voltages of each arm is required for control purposes, which simplifies the communication among the SMs and the central controller. This paper also introduces a modified switching strategy, which not only reduces the calculation burden of the FCS-MPC strategy even more, but also simplifies the SM capacitor voltage balancing algorithm. In addition, this strategy reduces the SM switching frequency and power losses by avoiding the unnecessary switching transitions. The performance of the proposed strategies for a 20-level MMC is evaluated based on the time-domain simulation studies. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted. [ABSTRACT FROM AUTHOR]

This paper proposes a vector control strategy for LCL-filter-based grid-connected voltage-source converters (VSCs). The proposed control strategy is inherently capable of attenuating the resonance phenomenon of such systems. This is an advantage over the existing methods, which require additional damping techniques. Moreover, the proposed vector control strategy is able to fully decouple the direct (d) and quadrature (q) components of the current in a rotating reference frame. The design procedure comprises a constrained optimization-based loop shaping. It utilizes the multi-input multi-output (MIMO) nonparametric model of the system along with a high-order linearly parameterized MIMO controller to form an open-loop transfer function matrix. Minimizing the second norm of the error between the open-loop transfer function matrix and a desired one, the coefficients of the controller are optimally determined. Conducting several reference tracking scenarios, the performance of the proposed vector controller is evaluated both by means of time-domain simulation studies in MATLAB/Simulink and experimental results. [ABSTRACT FROM AUTHOR]

Cross regulation is the main technical challenge of a single-inductor multiple-output (SIMO) dc-dc converter. This paper proposes a multivariable digital controller to suppress the cross regulation of a single-inductor dual-output (SIDO) buck converter in continuous conduction mode (CCM) operation. The controller design methodology originates from the open-loop shaping of the multi-input multi-output (MIMO) systems. The control design procedure includes: 1) determination of a family of nonparametric models of the SIDO converter at operating points of interest, 2) determination of the class of the controller, and 3) system open-loop shaping by the convex minimization of the summation of the square second norm of the errors between the system open-loop transfer function matrices and a desired open-loop transfer function matrix. The proposed controller minimizes the coupling between the outputs of the SIDO converter and provides satisfactory dynamic performance in CCM operation. This paper describes the theoretical aspects involved in the design procedure of the controller and evaluates the performance of the controller based on simulation studies and experiments. [ABSTRACT FROM AUTHOR]

This paper proposes a multivariable digital control design methodology for the voltage regulation of an islanded single distributed generation (DG) unit microgrid and its dedicated load. The controller design methodology is based on a family of spectral Multi-Input Multi-Output (MIMO) models of the microgrid system and performs open-loop shaping and system decoupling simultaneously by a convex optimization approach. The control design procedure includes: (i) the determination of a family of nonparametric models of the system at various operating points, (ii) the determination of the class of the controller, and (iii) system open-loop shaping by convex minimization of the summation of the square second norm of the errors between the system open-loop transfer functions and a desired open-loop transfer function. Based on the proposed design methodology, two dq-augmented voltage controllers are proposed to regulate the load voltages of a single-DG-unit microgrid. The proposed controllers guarantee the robust stability and satisfactory dynamic response of the system in spite of load parametric uncertainties and also the presence of nonlinear load. This paper describes the theoretical aspects involved in the design procedure of the controllers and evaluates the performance of the controllers based on simulation studies and experiments. [ABSTRACT FROM PUBLISHER]

This paper presents a vector control strategy for regulating the current of grid-tied voltage source converters (VSCs) in a rotating reference frame. The proposed approach is based on shaping the open-loop and closed-loop transfer matrices of the system. Solving a constrained convex optimization problem, the shaping is achieved, which guarantees the stability of the closed-loop system. The designed controller results in the desired dynamic performance and decouples the direct and quadrature (dq) current axes. The structure of the proposed controller is similar to that of its predecessors and consists of four proportional–integral controllers. The performance of the method is evaluated based on simulation and experimental results. It is confirmed that its dynamic performance is better than that of the previously proposed approaches, and it results in the decoupled current axes. [ABSTRACT FROM PUBLISHER]

Low system voltage in traction networks, mainly caused by active power absorption of locomotives, adversely affects the performance of the locomotives and also the power transmission capability of the catenary line. This paper introduces a voltage support scheme to compensate for the adverse effects of low system voltage. The proposed method is based on the injection of reactive power through the current-controlled line-side converter of locomotives. Comparing the catenary voltage with its reference value, the error is fed to a high-order controller. The controller generates the quadrature (q)-axis reference value of a current control strategy, which is responsible for the reactive power injection. To design the high-order controller, adopting the nonparametric models of the system at various locations, an optimization-based loop-shaping approach is used. The loop shaping guarantees the stability and the acceptable performance of the closed-loop system for various locomotive positions in the network. The performance of the proposed control strategy is evaluated based on simulation results in MATLAB/PLECS environment. Moreover, implementing a scaled-down laboratory setup, the performance of the proposed scheme is experimentally evaluated. [ABSTRACT FROM AUTHOR]

This paper proposes a voltage support scheme for traction networks, which compensates for the voltage drop along the catenary line to which locomotives are connected. The proposed method is based on the injection of capacitive reactive power through the current controlled active line-side converter of locomotives. Comparing the catenary voltage with its reference value, the error is fed to a gain-scheduled PI-controller. The controller generates the q-axis reference value of the converter current, which is responsible for reactive power injection. The gain scheduling is carried out through identifying the parameters of the catenary line. The catenary parameters identification is performed by harmonic current injection and analyzing its impact on the catenary voltage. The performance of the proposed control strategy is evaluated in MATLAB/PLECS environment for a traction network consisting of one locomotive and two substations and moreover, for a two-locomotive three-substation traction network. [ABSTRACT FROM PUBLISHER]

This paper presents a linear direct–quadrature current control strategy for voltage source converters (VSCs) in a rotating reference frame (RRF). The described method is based on multivariable-proportional–integral (PI) regulators and provides fast dynamics and a zero steady-state error. Contrary to the well-known conventional PI-based control strategies in RRFs, the presented method provides practically decoupled axes with a superior disturbance rejection capability. Moreover, its implementation is relatively simple and does not impose excessive structural complexity compared to its conventional PI-based competitors. The method is applicable to both single- and three-phase systems and also to anisotropic three-phase systems, e.g., synchronous motors with different direct and quadrature impedances driven by VSCs. Implementing a three-phase test system, the performance of the presented method is experimentally evaluated. [ABSTRACT FROM AUTHOR]

This paper analytically determines the nondetection zone (NDZ) of an active islanding detection method, and proposes a solution to obviate the NDZ. The method actively injects a negative-sequence current through the interface voltage-sourced converter (VSC) of a distributed generation (DG) unit, as a disturbance signal for islanding detection. The estimated magnitude of the corresponding negative-sequence voltage at the PCC is used as the islanding detection signal. In this paper, based on a laboratory test system, the performance of the islanding detection method under UL1741 anti-islanding test conditions is evaluated. Then, determining the NDZ of the method and proposing the countermeasure, the existence of the NDZ and the performance of the modified method to eliminate the NDZ is verified based on simulation results in PSCAD/EMTDC software environment and experimental tests. [ABSTRACT FROM AUTHOR]

This paper presents an alternative way for the current regulation of single-phase voltage-source dc–ac converters in direct–quadrature (dq) synchronous reference frames. In a dq reference frame, ac (time varying) quantities appear as dc (time invariant) ones, allowing the controller to be designed the same as dc–dc converters, presenting infinite control gain at the steady-state operating point to achieve zero steady-state error. The common approach is to create a set of imaginary quantities orthogonal to those of the real single-phase system so as to obtain dc quantities by means of a stationary-frame to rotating-frame transformation. The orthogonal imaginary quantities in common approaches are obtained by phase shifting the real components by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In the proposed approach of this paper, the orthogonal quantities are generated by an imaginary system called fictive axis, which runs concurrently with the real one. The proposed approach, which is referred to as fictive-axis emulation, effectively improves the poor dynamics of the conventional approaches while not adding excessive complexity to the controller structure. [ABSTRACT FROM PUBLISHER]