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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]

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]

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 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 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]

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]

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]

A control paradigm is proposed in this paper for decentralized power balance in hybrid AC/DC Microgrids (MGs). In this technique, the AC and DC sub-grids can transact energy from or towards the other side based on their loading conditions. Hybrid MGs use an interlink converter (ILC) as a back-to-back bi-directional converter at the Point of Common Coupling (PCC) between the sub-grids. The main duty of the ILC unit is to maintain the loading balance between the sub-grids. For this purpose, a superimposed AC voltage on the network's nominal DC voltage is utilized in the DC sub-grid. Also, the AC sub-grid is equipped with the conventional f / P droop control. The frequency of the superimposed AC voltage in the DC sub-grid and the AC network frequency in the AC sub-grid are used for estimating the loading percentage of each side. As a result, the ILC unit acts as a load for one side and concurrently as a source for the other side. The ILC controller adjusts the participation of this unit in a fully decentralized manner. Simulation studies via PLECS test the efficacy of the suggested control system. • Decentralized control strategy for Hybrid AC/DC Microgrids. • Simultaneous accurate load sharing and voltage regulation. • Energy management and balance id Islanded AC/DC Microgrids. • Plug-and-Play (PnP) feature for Distributed Generation (DG) units in DC Microgrids. [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]

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]

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]

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]

• Studying the ageing effects of AC preheating methods on LiFePO 4 (LFP) cells. • The ageing impact of shallow charge-discharge cycles, microcycles, on LFP cells. • High-frequency microcycling has a minor degradation impact on LFP cells. • Low-frequency microcycling causes accelerated degradation. • Voltage polarization induced by AC is the key indicator of any likely ageing effect. Superimposed Alternating Current (AC) imposed by electric machines and power electronics components in renewable energy systems and electric vehicles (EVs) exposes Li-ion batteries (LIBs) to high-amplitude AC harmonics (i.e., ripple currents). These AC perturbations make LIBs experience shallow charge-discharge cycles or microcycles, resulting in significantly increased battery charge throughput. Recently high amplitude AC-only waveforms have also attracted significant attention as they can be used to warm up/preheat EV batteries in low-temperature environments. To understand what happens when the amplitude of AC-bearing duty cycles is pushed to higher levels, beyond the recommended constant current value set by the manufacturer, a set of fresh LiFePO 4 (LFP) cells were subjected to high-amplitude AC-only profiles at low and high frequencies for extended periods. Subsequently, LFP cells were cycled on dynamic current profiles (superimposed AC profiles) for ∼ 200 days, representing real situations encountered by LIBs in renewable energy and electromobility systems. Long-term aging results revealed that as long as the AC current induces a voltage oscillation that remains below the calculated threshold polarization overpotential, the cycle-life of LFP cells is not significantly affected for AC duties as high as the recommended constant current values. Furthermore, for high-frequency AC, a significant amount of microcycling is effectively filtered out by the battery's double-layer capacitance. However, for low-frequency superimposed AC, for which most of the AC current flows through the charge-transfer resistance, higher charge throughput induced by microcycles causes accelerated degradation. [ABSTRACT FROM AUTHOR]

• Studying the effects of AC perturbation on degradation mechanisms of lithium-ion batteries. • High-frequency AC has negligible ageing effects; slightly improved cell lifetime. • Low-frequency AC of sufficient amplitude accelerates degradation rate. • Voltage polarization induced by AC is the key indicator of any likely ageing effect. With the rapidly growing markets for electric vehicles and renewable energy systems, the complex duty cycles imposed by electric machines and power electronics components are now a common feature of battery service. As a result, lithium-ion, increasingly the battery of choice, must cope with superimposed alternating current (AC) across a broad range of frequencies. To advance understanding of how AC may influence the ageing of Li-ion batteries (LIBs), this work uses electrochemical impedance spectroscopy to investigate the interaction of AC with key aspects of LIB operation (charge-transfer, properties of the solid electrolyte interphase, and lithium-ion diffusion), along with analysis of capacity performance to quantify the main modes of degradation. For a set of fresh LiFePO 4 cells, subjected to nearly 200 days of service, comparison of DC profile with coupled DC + AC duty reveals that high-frequency superimposed AC does not contribute to ageing, with some evidence that it can actually enhance the battery life. In contrast, low-frequency superimposed AC accelerates both loss of lithium inventory and loss of active material (as the most pertinent degradation modes of LIBs) and consequently induces greater capacity loss. This important correlation between AC frequency and extent of degradation was confirmed in AC-only studies, which showed that the voltage polarization induced by AC perturbation is the key indicator of any likely effect. These findings can serve as a guide for deciding whether a given combination of frequencies and amplitudes is likely to have an adverse impact. [ABSTRACT FROM AUTHOR]