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Hybrid energy storage system (HESS) consisting of battery and supercapacitor (SC) is an effective approach to alleviate voltage stability problems brought by the fluctuation of renewable resources in a dc microgrid. This paper proposes a novel distributed control scheme for multiple HESSs based on a leaderless consensus protocol to realize the power splitting between batteries and SCs and thus regulate the dc bus voltage. Without assigning leaders among the batteries and SCs, each SC or battery has equal priority in participating in voltage or state of charge (SOC) regulation using only neighboring information exchange instead of global communication. This distributed approach makes the system robust against any physical failure or communication fault only if the remaining communication network is connected. Consensus for the state variable associated with the battery SOC, the SC terminal voltage, and the dc average bus voltage are all achieved in this scheme, and thus the voltage quality and the energy storage lifetime can be improved. In addition, the small-signal stability of a four-HESS system is investigated, and the impact of the control parameters on the system is analyzed. Simulations and experimental studies are conducted to demonstrate the effectiveness of the proposed control scheme in an islanded dc microgrid. [ABSTRACT FROM AUTHOR]

Grid connected voltage source converters are inevitable means now for expanding and supporting power systems. Nevertheless, their harmonic injection and high frequency switching may cause degradation of power quality and detraction of the equipment reliability. Therefore, the converters injected harmonics must observe the corresponding standards, while lower switching frequencies and rapid transient responses are desirable. In this paper, a new combined control method with a flexible switching table is presented for the converters to enhance the power quality and transient response. The method employs a virtual flux in connection with a combined control for the first time. In comparison with the conventional virtual flux direct power control, the proposed method provides better power quality, reduced THD and faster transient response, in addition to less grid parameters dependency. Compared to the conventional combined control method, it determines more accurately the converter voltage vectors, while avoiding grid voltage measurement and cumbersome controller tuning. [ABSTRACT FROM AUTHOR]

This paper introduces a new search-based method for coordinated control of voltage (Volt) and reactive power (Var) devices for conservation of voltage reduction to achieve an effective demand and power loss reduction while keeping voltage profile within limits on a distribution system. The method searches for the best control settings for the Volt–Var devices as the operating conditions change for real-time operation. The main advantage of the proposed method is that it provides global optimal solution. When the number of control devices increases on an active distribution system with distributed generation, the search space will increase combinatorially. The method has a computationally efficient search scheme to reduce the search space systematically. The computation effort is further reduced by adopting the decoupled Volt–Var approach. To demonstrate the computational efficiency of the proposed method, test results are compared with a conventional Volt–Var scheme. The control update considered is 15-min interval. The simulations are done on modified IEEE 13 node unbalanced distribution circuit by using MATLAB™ and a power flow analysis program (OpenDSS). [ABSTRACT FROM AUTHOR]

A phase-shift pulse-frequency modulation (PS-PFM) for a full-bridge LCC resonant converter is proposed in this paper to reduce both switching frequency variation range and reactive power of a traditional PFM LCC resonant converter. Like the PFM LCC resonant converter, a PS-PFM LCC resonant converter has a wider zero voltage switching operation range than a phase-shift modulated LCC resonant converter. By integrating a voltage-controlled carrier generator, the proposed PS-PFM modulator can adjust phase-shift angle and switching frequency simultaneously, resulting in one more degree of control freedom to optimize the steady-state performance of an LCC resonant converter. A detailed analysis of the proposed PS-PFM LCC resonant converter is performed and a step-by-step parameter design procedure is given. A 500-W/48-V prototype with 100–200 V input voltage is built to verify the theoretical analysis. The maximum efficiency of the prototype reaches 95.3%. [ABSTRACT FROM AUTHOR]

This paper introduces a data-driven method to compute voltage magnitude, and power loss sensitivity coefficients in unbalanced low voltage network with high photovoltaic (PV) distributed generation penetration. Implementation of the proposed method is based on the Least-Squares estimator and does not require knowledge of network model parameters, but only requires measurement of voltage magnitude, active, and reactive power. To overcome the data collinearity problem, the sensitivity coefficients are estimated using ridge regression. To track changes in the network operating conditions, the coefficients are updated using the Recursive Least-Squares and to avoid instability in the estimation process during low excitation periods, the method uses the concept of directional forgetting in which old data are discarded only when new information are available. Effectiveness of the proposed method is demonstrated by a case study where the impact of active and reactive power injection is estimated. Besides that, it is also shown how sensitivity coefficients can be useful to establish how each photovoltaic inverter can contribute to set a cooperative control, by achieving losses minimization and ensuring voltage level within limits set by the standards. [ABSTRACT FROM AUTHOR]

This paper employs a single-phase-shift modulated dual-active-bridge converter (DABC), and establishes a small-signal model, which involves its voltage transformation ratio. Then, through a voltage transformation ratio feedback loop, which is feedforward compensated by the port voltages and currents, closed-loop control of the transformation ratio of the DABC is realized. The proposed controller can make the DABC operate in constant transformation ratio mode, and act as an ideal dc transformer in an active dc distribution network. Based on the bidirectional voltage support capability of the proposed dc transformer, the different buses of an active dc distribution network can mutually provide voltage support for each other. Besides, the proposed dc transformer can realize the normalized decentralized power coordination between the energy storage systems at different buses of the active dc distribution network. The feasibility of the proposed dc transformer is verified by several hardware-in-loop tests. [ABSTRACT FROM AUTHOR]

Voltage violation is an important factor that restricts grid-connected photovoltaic system. The utilization of PV plant's reactive power capability is an effective measure to mitigate voltage violation. This paper proposes two-stage voltage control strategy to enhance the voltage support capability of photovoltaic grid-connected system. In first stage, adaptive gains are set according to the maximum reactive capacity of each PV plant in order to distribute reactive power among PV plants more reasonably, avoiding certain PV plants operating in the limit state for a long time. In second stage, when the reactive power capacity is insufficient, the priority of active power curtailment for each PV plant is calculated in real time. Active power curtailment control will be performed in PV plants with high priority to released more reactive power capacity for voltage support. The simulation results carried out by PSCAD/EMTDC software demonstrated that this strategy can effectively solve the voltage violation problem. Meanwhile, the proposed strategy not only can exploit reactive capacity of each PV plant as much as possible to avoid certain PV plants operating in the limit state for a long time, but also can reduce the amount of active power curtailment when reactive power adequacy is insufficient. [ABSTRACT FROM AUTHOR]

This paper describes a new steady-state model to represent the synchronous machines, generators or motors, in unbalanced power flow studies. Also some power and voltage controls, which can be applied in the machines, are modeled. The groundings of the synchronous machines can be represented. Most power flow studies represents the synchronous machines in a very simplified way, such as conventional fixed power injections in a bus (PQ bus) or as a voltage controlled bus (PV bus). This representation can present good results for power flow studies in balanced electrical systems, but in unbalanced electrical systems can lead to considerable errors due to particularities of the electromechanical behavior of the machine. In the proposed model, it is possible to represent the internal losses of the machines, an internal voltage source in phase-domain, defining the internal voltage module and a fixed angular difference between phases. The controls developed allow to specify the three-phase active and reactive power demanded or consumed by the machine and to control the voltage in a bus in different ways, dependent on how the control systems are in practice. The model was applied to the IEEE13, IEEE34, IEEE123, and IEEE8500 test feeders, and proved to be robust and more accurate than conventional models in situations of imbalance. [ABSTRACT FROM AUTHOR]

This paper addresses the optimal allocation of the electric spring (ES) smart load device in radial distribution systems for voltage regulation. An ES can transform a connected noncritical load into a smart load providing voltage support and voltage suppression functions. A risk-limiting ES planning method is proposed to obtain the optimal ES configuration (number, locations, capacities, and types) to mitigate the voltage violations caused by uncertainties in renewable distributed generation. The forecasting uncertainties are placed into multi-state discrete levels to reduce scenario numbers. The voltage regulation capability of different types of ES are quantified and compared to determine the optimal allocation model. The optimal ES installation plan is identified as per the minimal total cost and lowest voltage violation risk. The proposed optimal allocation method is validated on a modified IEEE 15-bus distribution network. [ABSTRACT FROM AUTHOR]

In this paper, we propose a distributed voltage control in power distribution networks through reactive power compensation. The proposed control can operate in a distributed fashion where each bus makes its decision based on local voltage measurements and communication with neighboring buses, always satisfy the reactive power capacity constraint, drive the voltage magnitude into an acceptable range, and minimize an operational cost. We also perform various numerical case studies to demonstrate the effectiveness and robustness of the controller using the nonlinear power flow model. [ABSTRACT FROM AUTHOR]

Precise power sharing considered is necessary for the effective operation of an Autonomous microgrid with droop controller especially when the total loads change periodically. In this paper, reactive power sharing control strategy that employs central controller is proposed to enhance the accuracy of fundamental reactive power sharing in an islanded microgrid. Microgrid central controller is used as external loop requiring communications to facilitate the tuning of the output voltage of the inverter to achieve equal reactive power sharing dependent on reactive power load to control when the mismatch in voltage drops through the feeders. Even if central controller is disrupted the control strategy will still operate with conventional droop control method. additionally, based on the proposed strategy the reactive power sharing accuracy is immune to the time delay in the central controller. The developed of the proposed strategy are validated using simulation with detailed switching models in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Replacing the phase shifting transformers of interphase power controller with a reduced rating dual unified power flow controller (UPFC) results in a unified interphase power controller (UIPC) with potential for enhanced performance. However, in most cases, the UPFC is controlled as a static phase shifter (SPS), producing marginal improvements. A control approach that allows the minimization of the power ratings of the series voltage source converters (VSCs) of the UIPC was recently proposed. For that, one has to define four control parameters, as opposed to only two in the SPS-based UIPC. This can be done off-line, by means of an optimization procedure that splits the desired transmission line current among the capacitive and inductive branches of the UIPC. In this paper, a control scheme is presented for the implementation of the novel technique. The current sharing factors obtained from the optimization are stored in a lookup table. The gating signals for the VSCs are generated using Park's transformation so as to synthesize the optimal UIPC inductive and capacitive branch currents with proportional resonant controllers and sinusoidal pulse width modulation. The transmission line angle, as well as the magnitude and phase of the desired transmission line current with respect to the voltage at the receiving end voltage, is assumed to be provided by a transmission system operator. Experimental results that include steady state and transient conditions are provided to prove its feasibility. [ABSTRACT FROM AUTHOR]

The term Virtual Synchronous Machine (VSM) commonly refers to the emulation of synchronous machines operation by using power electronics converters. However, a wider application of this control technique is still pending on further refinement of its design aspects and digital implementation. In this paper, a systematic approach to design the VSM parameters is introduced. First, a simplified third-order small-signal model of a VSM based on the conventional model of synchronous generators is developed. Unlike the existing works published in literature, the proposed model takes into account the effects of inductive and resistive weak grids. The model is then used to design the controller parameters by appropriately placing the poles of the closed-loop system. VSM robustness is studied for weak grids dominated by either inductance or resistance, and the aspects of digital control implementation are addressed. All the control system features and improvements were tested on a 15 kW prototype of a battery-supported VSM connected to a weak grid. [ABSTRACT FROM AUTHOR]

Microgrid (MG) concept is considered as the best solution for future power systems, which are expected to receive a considerable amount of power through renewable energy resources and distributed generation units. Droop control systems are widely adopted in conventional power systems and recently in MGs for power sharing among generation units. However, droop control causes frequency fluctuations, which leads to poor power quality. This paper deals with frequency fluctuation and stability concerns of  f-P droop control loop in MGs. Inspired from conventional synchronous generators, virtual damping is proposed to diminish frequency fluctuation in MGs. Then, it is demonstrated that the conventional frequency restoration method inserts an offset to the phase angle, which is in conflict with accurate power sharing. To this end, a novel control method, based on phase angle feedback, is proposed for frequency restoration in conjunction with a novel method for adaptively tuning the feedback gains to preserve precise active power sharing. Nonlinear stability analysis is conducted by drawing the phase variations of the nonlinear second-order equation of the δ-P droop loop and it is proved that the proposed method improves the stability margin of  f-P control loop. Simulation results demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

A resilient power system is capable of providing fast and efficient recovery from outages caused by natural or man-made attacks. After a major blackout, transmission, and distribution system operators (TSOs and DSOs) coordinate to restore the system in a timely and reliable manner. However, these two systems are usually operated separately without considering capabilities and limitations of each individual network, or their mutual impacts. This could prolong the restoration process and lead to unpractical outcomes. Therefore, a distributed strategy for integrated transmission and distribution systems restoration (ITDSR) has been developed in this paper. Based on the alternating direction method of multipliers (ADMM), the distributed algorithm is able to provide the coordinated restoration for both TSOs and DSOs by sharing limited information of boundary buses in an iterative procedure. The ADMM-ITDSR model includes convex ac power flow model and three-phase unbalanced branch flow model. The ITDSR strategy coordinates the operation of distributed energy resources in terms of serving high priority loads across different networks. The effectiveness and advantage of developed models and algorithms are validated and demonstrated through testing of the integrated IEEE test cases, such as 14-bus transmission with 13-node distribution systems, and 39-bus transmission with 123-node distribution systems. [ABSTRACT FROM AUTHOR]

A growing number of inverter interfaced rooftop photovoltaic and battery systems are changing suburban distribution systems. The potential of reactive power support from this equipment is attracting attention as a way of addressing the systems’ voltage magnitude problems. However, if the voltage is controlled by a conventional droop-based strategy, common radial configuration at suburban feeders can cause an uneven allocation of the reactive power compensation among the inverters. A droop control strategy is developed in this paper using two parameters of the steady-state Thévenin equivalent model of the system seen by each inverter. The Thévenin based droop control permits even allocation of the reactive power compensation. A sufficiently invisible low-level probing is utilized by each inverter to robustly identify Thévenin reactance and Thévenin source in steady state. The inverter's output power is adjusted by the two identified parameters. Demand changes are addressed. Interference by the neighbor inverters’ simultaneous probing and compensation is also considered. Simulations results on the IEEE 33-bus and a large-scale system show the efficacy of the proposed strategy. [ABSTRACT FROM AUTHOR]

Line-commutated-converter-based HVdc (LCC-HVdc) is extensively used in modern power systems. Most of the LCC-HVdcs are implemented with the control strategy termed marginal current control method (MCCM), while their operation is sensitive to disturbance from the ac part of the system. This paper originally reveals the mechanism of two types of oscillations induced by the MCCM after ac disturbances, i.e., the current boundary induced oscillation and the current error induced oscillation. A coordinated marginal current control method (CMCCM) is proposed, aiming at suppressing the aforementioned oscillations and enhancing HVdc recovery performance. Compared to the MCCM that responds only to dc voltage, the CMCCM additionally considers ac voltage, ac current, and power factor of the converter. AC grid's parameters are also taken into account. With a more considerate design, the empirical current boundary determined by the MCCM is replaced with an analytical one to coordinately control dc current. Therefore, better performance of HVdc is achieved. Simulations in RTDS reveal that the proposed CMCCM can suppress the oscillations, speed up HVdc recovery process, and moderate transient over voltage of HVdc. [ABSTRACT FROM AUTHOR]

Voltage control plays an important role in the operation of electricity distribution networks, especially with high penetration of distributed energy resources. These resources introduce significant and fast varying uncertainties. In this paper, we focus on reactive power compensation to control voltage in the presence of uncertainties. We adopt a chance constraint approach that accounts for arbitrary correlations between renewable resources at each of the buses. We show how the problem can be solved efficiently using historical samples analogously to the stochastic quasi-gradient methods. We also show that this optimization problem is convex for a wide variety of probabilistic distributions. Compared to conventional per-bus chance constraints, our formulation is more robust to uncertainty and more computationally tractable. We illustrate the results using standard IEEE distribution test feeders. [ABSTRACT FROM AUTHOR]

This paper investigates the need of coordinated operation of conservation voltage reduction (CVR) in the presence of electric vehicle (EV) penetration in the active distribution network. In order to analyze the impact of both the technologies (CVR and EV), a time horizon-based model predictive Volt/VAR optimization (VVO) methodology has been introduced in smart grid framework. The proposed VVO methodology operates in centralized as well as local controls under different time scale of operation, including cloud transient effects on solar photovoltaic (PV) power output. Moreover, the control algorithms also consider the uncertainties in load demand and PV power generation. The VVO methodology has been validated with and without presence of EV loads in the distribution network. The VVO includes the impact of different EV charging loads having the ability of participation in reactive power support at selected charging points. This is also referred to as vehicle-to-grid operation in terms of reactive power dispatch only. Besides, the voltage and VAR regulation through smart inverters of PVs and EV charging station has been fruitfully utilized in global as well as local domain. A real-time Volt/VAR droop based controller has been introduced to control the smart inverters reactive power dispatch. To validate the developed methodology, a real-time cosimulation framework, using real-time digital simulator and Python interface, has been built. The proposed model predictive VVO algorithm has been tested and validated on a modified IEEE 34 bus test system. The simulated results reveal that significant CVR energy savings and losses reduction has been achieved without violating the system constraints. The voltage control algorithm works well in both slow and fast time scales. [ABSTRACT FROM AUTHOR]

Grid-connected voltage source converter is used to control the flow of active power from renewable energy sources with several functionalities, such as harmonic mitigation, reactive power compensation, and power factor improvement. The reference current calculation and its accuracy decide the performance of the control algorithms during harmonic disturbances. Fundamental signal extraction techniques are utilized to overcome the disturbances present in the system voltage and current. This paper presents a novel orthogonal current decomposition control for load current decomposition into active and reactive components. These current components are obtained through character of orthogonal vector in the inner product space. The block diagram of the proposed technique is implemented with the help of adder, multiplier, and windowed integrator. Phase voltage and load current for each phase are used as inputs to this functional diagram for getting desired fundamental active current signal. Subsequently, adjustable dc-link voltage control is also presented to diminish the switching losses in the converter and to avoid the false tripping of the grid-connected system during susceptible grid conditions. The proposed control technique is simulated in MATLAB/Simulink and verified through developed hardware prototype using ARM Cortex M4-based microcontroller (STM32F407VGT6) using Waijung environment. [ABSTRACT FROM AUTHOR]

This paper proposes an extended Kalman filter (EKF) based control strategy for fault ride-through operation in two-stage grid-connected photovoltaic (GPV) system. Unlike the conventional controllers for ride-through operation, the proposed strategy does not compromise with power quality improvement features in the system while enabling ride-through operation. The controller accounts for nonlinear loads in the system, grid harmonic-currents elimination, and grid-currents balancing even during the harmonic/distorted grid voltages. The IEEE Standard-1547.4 compels the distributed resource to ride-through during voltage disturbances caused by faults. For the ride-through operation, a limit is imposed on PV active power injection to prevent inverter overcurrents and dc-link energy aggregation, which reduces the lifetime of dc-link capacitor. The reactive power is fed to the grid, as per the depth in voltage-sag. The derated PV array power is supplied in cases where the inverter cannot handle the utmost PV-power. The power quality improvement is ensured using EKF state estimator, which precisely estimates the fundamental load currents. In distribution network with modern nonlinear loads, especially at far radial ends, the grid voltages are prone to huge diversions and the proposed controller provides a possible solution to maintain active/reactive power support and maintain power quality in the network. The effectiveness of the strategy is demonstrated through simulations and experiments. Under all disturbances, the harmonic content in grid currents is observed within limits, in accordance with the IEEE Standard-519. [ABSTRACT FROM AUTHOR]

This paper presents the design and experimental validation of disturbance observer (DO)-based control for grid-tied photovoltaic (PV) inverters fed by a dc–dc boost converter considering unbalanced grid voltages. The control scheme uses multiples controllers that are developed based on employing DO technique along with a feedback-linearizing (FBL) control. The main controller is designed to regulate the active and reactive powers injected into the grid via controlling the grid-tied inverter. Under unbalanced grid voltages, the active and reactive power is composed of a dc component as well as an ac component that oscillates at twice the grid frequency. Therefore, the unknown perturbation, representing the mismatch between the nominal and actual system, can be treated as a sum of a constant offset and a sinusoidal disturbance. To ensure accurate control of the active/reactive power, a DO is designed to estimate the unknown perturbation, which is then canceled by a FBL control. By considering the control input limitation in the observer synthesis, an antiwindup compensator arises naturally in the control law, which attenuates the effect of the windup phenomenon during control saturation. Moreover, a simplified representation of the DO is proposed to facilitate real-time implementation. The proposed approach for the power control can be used in both balanced and unbalanced systems since it can reject constant and sinusoidal disturbances. Experimental tests were performed to investigate the ability of the proposed control scheme to achieve good transient and steady-state performances. [ABSTRACT FROM AUTHOR]

Distributed power generators in islanded microgrid usually adopt droop control strategy or virtual synchronous generator (VSG) control strategy to simulate Q–U droop characteristic of synchronous generators for voltage support ability enhancement and reliability improvement. However, because the distributed power generators’ capacities and locations are random, they cannot share reactive load proportional to their rated capacity, resulting in overload protection, and even system instability. In order to improve reactive power sharing precision, voltage control precision, and system stability simultaneously, an improved reactive power sharing algorithm combining virtual impedance and virtual capacitor is proposed in this paper. First, the three existing open-loop reactive power sharing strategies are summarized and the effect of their parameters on reactive power sharing error, steady-state voltage deviation, and system stability are analyzed. Afterwards, based on the analysis results, a reasonable configuration of virtual impedance and virtual capacitor values are put forward to realize the control targets of reducing steady-state reactive power sharing error, improving voltage control precision and system stability meanwhile. Finally, simulation and experimental platforms are built to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper presents a new five-level T-type nested neutral point clamped (T-NNPC) converter. The proposed five-level T-NNPC topology is very attractive for medium-voltage applications as it can work in a wide range of voltages without the need of devices in series, and fewer components compared to the other five-level topologies. A model predictive control (MPC) strategy is also developed for the proposed converter to control the output currents at different output frequencies, and control flying capacitor voltages. A discrete-time model of the converter is developed and the control objectives that are the output currents and flying capacitor voltages are defined regarding switching states. A cost function is defined to minimize the deviation of the predicted values of the control objectives from their desired values. During each sampling time, the best switching state is selected and applied to the converter. The performance of the proposed five-level T-NNPC and the developed MPC strategy is studied in the MATLAB/Simulink environment, and the feasibility of the converter is evaluated experimentally. [ABSTRACT FROM AUTHOR]

In this paper, we design a voltage-modulated direct power control (VM-DPC) for a three-phase voltage source inverter (VSI) connected to a weak grid, where the phase-locked loop (PLL) system may make the system unstable if the conventional vector current control (VCC) method is applied. Compared with the conventional VCC method, the main advantage of the proposed VM-DPC method is that the PLL system is eliminated. Moreover, in order to inject the rated real power to the weak grid, the VSI system should generate some certain amount of reactive power as well. An eigenvalues-based analysis shows that the system with the proposed method tracks its desired dynamics in the certain operating range. Both simulation and experimental results match the theoretical expectations closely. [ABSTRACT FROM AUTHOR]

It is well-known that in ac-grid-connected or motor-driving converters, current behavior directly sets the exchange of power between the two. Moreover, in corresponding current plants, most of the disturbance (formed by grid voltage or back EMF and the dc-link voltage) may be measured/estimated and appropriately canceled, reducing the current regulation problem to tracking challenge only. The latter is still of extreme importance since the amount of active/reactive power exchanged is proportional to current magnitude and phase. Recently, a method for deriving proportional-resonant controller structure and coefficients according to desired tracking behavior of ac signal amplitude, applied to typical power converter current loop, was proposed based on the fact that if ac signal envelope is perceived as dc signal, its transient behavior may be easily shaped utilizing well-known approaches employed in dc systems loop shaping while keeping zero phase tracking error at all time. This paper extends the proposed methodology by introducing in-depth analysis of the resulting loop gain shaping, applying practical issues, such as actuator delay, damping, and finite word length to the system, yielding a closed set of hands-on design guidelines. Experimental results are also given, providing a two-fold value: successfully validating the presented material and demonstrating that some of the previously reported outcomes (based on simulations) cannot be achieved in practice due to the above mentioned real world constraints. [ABSTRACT FROM AUTHOR]

This paper proposes a novel secondary control strategy for the power-electronic-based ac microgrid. This approach restores the voltage and frequency deviations by utilizing only local variables with very high bandwidth. This is realized with a finite control set model predictive control technique that is adopted in the inner level of the primary control of voltage source converters. In the outer level of the primary control, droop control and virtual impedance loops are exploited to adjust the power sharing among different distributed generation (DGs). As inner control level operates with a very high bandwidth, need for filtering of the calculated active and reactive powers in the outer level of the primary control is insignificant. Therefore, the secondary control can be operated with a far superior bandwidth compared to the case when the conventional cascaded linear control is used. Merits of the proposed approach are investigated analytically with the help of the describing function methodology that allows the quasi-linear approximation of the inner control level. Finally, simulation and experimental results are presented. [ABSTRACT FROM AUTHOR]

In single-phase AC charging system for electric vehicles (EVs), the single-phase pulse width modulation (PWM) rectifier with deadbeat predictive current control (DPCC) is characterized by fast current dynamic response and simple algorithm. However, in steady state, the performance of the DPCC is affected by the circuit parameters mismatch and control delay, which will result in the AC side power-factor degradation and the input current distortion. Moreover, the performance of dynamic response to load mutation can be improved, as the traditional DPCC adopted PI controllers in the voltage loop. In this paper, an improved deadbeat control method is proposed. In the current loop, the AC side voltage compensation parameter is proposed to improve the power factor and reduce the total harmonic distortion (THD). The voltage loop uses an active disturbance rejection controller (ADRC), which improves the dynamic performance of the system. The feasibility and effectiveness of the proposed deadbeat control method is verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

The power system planning and protection studies are becoming more challenging due to the rapid increase in penetration levels of converter-interfaced renewables. Type-IV wind turbine generators (WTGs) and photovoltaic panels are interfaced to the grid through a full-scale converter, and their short-circuit current contributions are mainly designated by the converter control and associated current limits. This paper proposes a new phasor domain modeling approach for the wind parks (WPs) with Type-IV WTGs, using the concept of control-based equivalent circuits. The proposed model precisely represents the detailed electromagnetic transient (EMT)-type model in steady state, and is able to account for the fault-ride-through function of the WTG control as well as its specific decoupled sequence control scheme in addition to the traditional coupled control scheme. Although the collector grid and WTGs inside the WP are represented with their aggregated models, the overall reactive power control structure of the WP is preserved by taking the central WP controller into account. The accuracy of the proposed model is validated through detailed EMT simulations. [ABSTRACT FROM AUTHOR]

Unity grid power factor is essential for the indirect matrix converter (IMC) applications due to the grid-friendly requirement. The existing closed-loop grid power factor control methods usually consist of complex structures, resulting in high requirements both in hardware and software aspects. In this paper, a low-cost phase-angle compensation method is proposed to reduce the costs. Based on the modeling for the IMC, the influences from the system parameters on the grid power factor angle are analyzed. Then, a virtual adaptive resistance model is denoted and calculated with the least mean square algorithm to generate a virtual voltage signal, which is consequently utilized to generate the phase compensation angle by a newly designed proportional-integral closed loop to regulate the grid power factor angle. Compared to the typical existing method, the proposed low-cost phase-angle compensation method requires only one grid current measurement in hardware and reduces 18.9% computational burden in software, while the advantages being adaptive to the system parameters and suitable for a wide load-range are reserved. Simulation and experimental research for the proposed method is carried out to validate its steady-state and dynamic-state compensation effectiveness under various load conditions. [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]

This paper proposes a distributed control scheme to compensate for droop-induced frequency deviations in autonomous microgrids. In this scheme, no extra direct frequency control and proportional-integral compensation are employed to remove the frequency deviations; that is, the deviations are compensated instantaneously. To reduce the communication burden, the scheme is then equipped with a need-based (event-triggered) data exchange strategy. An event-triggering mechanism is introduced, which highly reduces the amount of communications in both transient and steady-state stages and ensures that the intervals between consecutive communication instants are positive (i.e., the system is Zeno-free). Stability and equilibrium analyses of the resultant system considering the whole system dynamics are provided, as well. Effectiveness of the proposed controller for different cases is verified by simulating a microgrid in MATLAB/SimPowerSystems software environment. [ABSTRACT FROM AUTHOR]

This paper presents the resolution of the optimal reactive power dispatch (ORPD) problem and the control of voltages in an electrical energy system by using a hybrid algorithm based on the particle swarmoptimization (PSO) method and interior point method (IPM). The IPM is based on the logarithmic barrier (LB-IPM) technique while respecting the non-linear equality and inequality constraints. The particle swarmoptimization-logarithmic barrier-interior point method (PSO-LB-IPM) is used to adjust the control variables, namely the reactive powers, the generator voltages and the load controllers of the transformers, in order to ensure convergence towards a better solution with the probability of reaching the global optimum. The proposed method was first tested and validated on a two-variable mathematical function using MATLAB as a calculation and execution tool, and then it is applied to the ORPD problem to minimize the total active losses in an electrical energy network. To validate the method a testwas carried out on the IEEE electrical energy network of 57 buses. [ABSTRACT FROM AUTHOR]

Reactive power injection in distributed generation inverters is an useful ancillary service for grid supporting purposes. For grid-feeding converters, the slope control method is the most common voltage regulation strategy used in local (communication-less) applications. Despite its simplicity, this method offers limited dynamic properties in scenarios with changing operation conditions. In this sense, this paper presents an adaptive slope voltage control that provides an improved transient performance against operating variations. To derive the control configuration, a control-oriented mathematical model is developed. The accuracy of the modeling and the performance of the proposed control are validated by selected experimental results. [ABSTRACT FROM AUTHOR]

Direct power control (DPC) has been proposed as an effective control method for pulsewidth modulated (PWM) converters. The recent development focuses on the integration of the model predictive (MP) control and DPC to obtain better steady-state performance. In most MP-DPCs, duty cycle optimization (DCO) is a common technique to achieve the power error minimization. However, the periodic occurrence of a duty cycle larger than one (D > 1) deteriorates the performance of the PWM converter by inducing current spikes and power surges. To address this problem, this paper conducts an in-depth examination on the “D > 1” case common in MP-DPC using DCO. It is found the “D > 1” case is caused by the wrong control action of the active power in the starting period of each sector. Furthermore, a new table containing an additional pre-calculated interval in every sector is developed to eradicate the “D > 1” case and its associated power quality issues. Both simulation and experimental results are provided to demonstrate the effectiveness of the new table. [ABSTRACT FROM AUTHOR]

The intermittent behavior of wind power generation results in a fast variation of both frequency and voltage magnitudes of isolated microgrids with high wind power penetration. In this paper, a hybrid energy microgrid with wind energy conversion system (WECS) and diesel synchronous generators are analyzed during wind gust conditions. Superconducting magnetic energy storage (SMES) is connected at the same node of the WECS to mitigate the intermittent wind power generation. A developed fuzzy logic controller has been applied to achieve active power sharing. The damping of voltage fluctuation depends on SMES reactive power, which is controlled by a voltage source converter. The studied microgrid comprises 33 nodes, two diesel SGs, and two WECSs of squirrel cage induction generator type at 30% penetration level. The results demonstrate the effectiveness of the proposed control strategy to mitigate the frequency fluctuations during the wind speed gusts by smoothing power output from WECSs/SMES system. Moreover, voltage control is achieved by injecting reactive power from the SMES system. All simulations are performed by MATLAB/Simulink package. [ABSTRACT FROM AUTHOR]

For the purpose of controlling the power system and in particular predicting voltage collapse, the use of stability indices has become a mandatory operation. Nevertheless, it would be essential to consider reliable ones when managing the power system. Otherwise, considerable damage can occur leading to blackout phenomenon. In the light of this paper, an improved reactive line index will be designed to identify sensitive areas in transmission networks. Based on Pi-model of the line, the proposed index takes into account the shunt admittance of the lines as well as the angle gap between sending and receiving buses. The principal aim of this index is the online control of the reactive power flow and voltage stability prediction. Within this framework, the novel index is compared to those found in the literature, and has proven to be more appropriate than simplified indices and to be more sensitive than other index that considered the same model of line. To indicate this effectiveness, two power systems have been deployed, namely IEEE 6 bus and new England IEEE 39 bus test system. All simulations were performed using PSAT software. [ABSTRACT FROM AUTHOR]

This paper presents the voltage‐controlled oscillator (VCO) less phase‐locked loop (PLL)‐based control of voltage source converter. It is used in the distributed power generation system for improved power quality. This control extracts the fundamental component from the sensed load current using three phases to alpha–beta transformation without using VCO, unlike synchronous reference frame–PLL, where fundamental component extraction needs two transformations with the use of VCO. Owing to the absence of VCO in the proposed method, the computational burden of trigonometric operations is very less that results in a faster dynamic response under variable conditions. This characteristic makes it suitable for the distributed power generation system, where fast changes occur frequently. In this study, wind‐based distributed generation system has been taken for study, which comprises of self‐excited induction generator feeding non‐linear load with varying electrical conditions. Using the proposed control, the reactive power compensation, harmonics reduction and load balancing are carried out in the system. In addition to it, neutral current compensation is also accomplished with the help of star–delta transformer connected at the load end. The system performances are investigated under steady state as well as in transient state and results are found satisfactory. [ABSTRACT FROM AUTHOR]

The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency area and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultraweak grid from the point of view of system stability. Finally, experiment results validate the stability analysis. [ABSTRACT FROM AUTHOR]

Inductive power transfer (IPT) techniques are becoming popular in battery charging applications due to some unique advantages compared to the conventional plug-in systems. A high-performance IPT charger should provide the battery with an efficient charging profile consisting of constant charging current and constant charging voltage. However, with a wide load range, it is hard to realize the initial load-independent constant current (CC) and the subsequent load-independent constant voltage (CV) using a single IPT converter while maintaining nearly unity power factor and soft switching of power switches simultaneously. This paper systematically analyzed the characteristics of an LCC–LCC compensated IPT converter and proposed a design method to realize the required load-independent CC and CV outputs at two zero-phase angle frequencies. The design also combats the constraints of an IPT transformer and input voltage, thus facilitating the use of a simple duty cycle control operating at two fixed frequencies for both CC and CV operations. The design criteria, control logic, and sensitivities of compensation parameters to the input impedance and load-independent output are discussed. Finally, an IPT battery charger prototype with 1 A charging current and 24 V battery voltage is built to verify the analysis. [ABSTRACT FROM AUTHOR]

The VIENNA rectifier is increasingly being considered as the active front-end converter in many applications that do not require regeneration or controllable input reactive power. In such applications, this rectifier is normally operated at unity source displacement factor, which derates its nominal voltampere (VA) rating. Moreover, in the “source voltage oriented” reference frame that is normally used for control, this rectifier has complex operating limits on the rectifier terminal voltage (imposed by the operating conditions), which needs to be computed online to avoid the line currents from going through zero-crossing distortions. Also, the source voltage has to be either measured or estimated, which further increases the cost and/or computational burden. This paper proposes a control method that eliminates the requirement of the input voltage sensors (or estimators) and makes the limits on the rectifier terminal voltage far simpler to compute. The control method is implemented in the input line current oriented reference frame, considering unity displacement factor at the rectifier input terminals, which minimizes the VA rating of the rectifier (for a given rated load power). The proposed sensorless current-oriented control approach is compared with the conventional voltage-oriented control strategy through experimental results. [ABSTRACT FROM AUTHOR]

[Display omitted] • A continuous-time sub-hourly central optimization method is proposed. • A continuous-time hierarchically-coordinated inverter model is proposed. • Bernstein polynomials are extended to the modeling of any time interval. • Properties of Bernstein polynomials are proposed for solution space transformation. The photovoltaic (PV) integration brings both fast photovoltaic generation (PVG) variations and a large number of PV inverters to the distribution networks (DNs). The management of the PVG variation and the PV inverter is an urgent problem. The discrete-time model can't describe PVG variations during the schedule interval and the regulation potential of the PV inverter is limited by a fixed dispatching value. This paper proposes a continuous-time voltage control method based on hierarchical coordination for high PV-penetrated DNs. In the central schedule, based on the Bernstein polynomial method, continuous-time models of PVGs are established and reference values of local inverter control curves are dispatched as continuous-time functions in a sub-hourly timescale. In the local control, reactive power and active power curtailment of PV inverters are set as the reference values, i.e., the corresponding values of the continuous-time functions, every 30 s to optimize the central dispatching objective under the voltage constrains, and deviate from the reference values according to both the local control curve and voltage measurement only when the real-time bus voltage violation occurs. In addition, the cooperation between inverter and mechanical equipment in the continuous-time framework is also considered, and the Bernstein polynomial method is developed to the continuous-time modeling of voltage control in DNs. The simulation shows that the proposed method has lower maximum voltage magnitude and lower energy loss compared with discrete-time central schedule method. [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]

This paper proposes a novel open-winding brushless doubly-fed reluctance generator (OW-BDFRG) with dual two-level converters in order to reduce the converter rating and switching frequency for large-scale wind turbine applications. The new converter topology is equivalent to a three-level converter directly connected to the control winding of typical BDFRG. The OW-BDFRG system with this topology structure requires lower converter rating and switching frequency, and has a more flexible control mode, better operation performance, and fault redundancy capability. For the OW-BDFRG, this paper also proposes a new control scheme combining direct power control (DPC) with sliding mode variable structure (SMVS) control to implement the power tracking. The voltage-vector switching table of DPC is redesigned according to the error signals of active and reactive powers of the power winding, as well as the sector location of control winding flux. The active and reactive powers of the OW-BDFRG can be directly decoupled and independently controlled by properly selecting the switching voltage vectors. The novelty of this paper lies in an OW-BDFRG topology driven by dual two-level converters to improve the system characteristics, and the use of SMVS control to improve the DPC accuracy and robustness to parameter variations. Finally, the effectiveness of the proposed system is verified through simulation and experimental studies. [ABSTRACT FROM AUTHOR]

In order to provide a constant current to the Light-emitting diodes (LEDs) and to minimize the size and cost of the driver circuit in ac/dc offline LED drivers, electrolytic capacitors are conventionally used due to their high energy density and low cost. However, electrolytic capacitors are sensitive to operating temperature and have much shorter lifetime than the LED semiconductor devices. This paper proposes an improved control scheme that is capable to control the output LED current and regulate the dc-link capacitor's energy simultaneously for a family of single-stage soft-switched high power factor LED driver topologies. Each of these topologies consists of an integrated power factor correction (PFC) circuit. Due to the proposed controller's capability to control the LED current directly via variable frequency control and the dc-link capacitor's voltage via duty ratio control, the required energy storage capacitance can be significantly reduced, thereby allowing small size film capacitor to be used as the energy storage capacitor. Simulation results are provided on two different LED driver topologies, one with an integrated boost PFC stage and the other one with an integrated buck–boost PFC stage, to verify the performance of the proposed control scheme. Experimental results are also been provided on a 12-W prototype to highlight the merits of this paper. [ABSTRACT FROM PUBLISHER]

Droop control strategy is widely applied in ac microgrid to realize the seamless mode transition and achieve average power sharing among different inverters. However, there is a lack of analysis about the operation performance of droop control strategy in grid-connected mode. In this paper, grid fluctuation and control accuracy of reactive power are illustrated to be the main influence factors for power flow control according to the small-signal model. The feedforward of grid frequency and voltage magnitude is proposed to mitigate the impacts of grid fluctuation on power flow control. Since the feedforward items are achieved through the synchronous reference frame phase-locked loop with the input of filter capacitor voltage, extra sensors for grid voltage are avoided. Moreover, voltage magnitude control is introduced to enhance the control accuracy of reactive power flow. In addition, modified droop control loops are also proposed to improve the system stability. With the implementation of the proposed control scheme, improved power flow control and enhanced system stability are achieved with the feature of seamless mode transition maintained. Finally, the major contribution of this paper is verified by a laboratory prototype. [ABSTRACT FROM PUBLISHER]

This paper extends the voltage-doubler concept to the single-phase SEPIC rectifier in discontinuous conduction mode and, as a consequence, novel rectifiers are proposed. A comparison with the classic SEPIC rectifier shows that the proposed converters can either provide reduced voltage stress on the semiconductors for the same output voltage level or supply double the gain of the output voltage with the same voltage stress. The proposed voltage-doubler SEPIC rectifiers provide a high power factor with no current control and they can be applied in order to improve the static gain of the structure, which can make the SEPIC rectifier suitable for applications that require a high voltage level. In this paper, the generic structure based on a three-state switch, four different implementations of the proposed concept (including bridgeless structures), steady-state analysis, a dynamic model, system control, and experimental results are presented. The proposed rectifiers were verified by experimental results obtained with a prototype built with the following specifications: 1000 W output power, 220 V input voltage, 400 V output voltage, and 50 kHz switching frequency. Peak efficiency of 95.84%, THD of 2%, and power factor of 0.9997 were obtained and, most importantly, double the gain of the output voltage was verified. [ABSTRACT FROM AUTHOR]

In wind energy conversion systems, the fundamental frequency step-up transformer acts as a key interface between the wind turbine and the grid. Recently, there have been efforts to replace this transformer by an advanced power-electronics-based solid-state transformer (SST). This paper proposes a configuration that combines the doubly fed induction generator-based wind turbine and SST operation. The main objective of the proposed configuration is to interface the turbine with the grid while providing enhanced operation and performance. In this paper, SST controls the active power to/from the rotor side converter, thus, eliminating the grid side converter. The proposed system meets the recent grid code requirements of wind turbine operation under fault conditions. Additionally, it has the ability to supply reactive power to the grid when the wind generation is not up to its rated value. A detailed simulation study is conducted to validate the performance of the proposed configuration. [ABSTRACT FROM AUTHOR]

Due to the increasing penetration level of microgrids (MGs), it becomes a critical issue for MGs to help sustaining power system stability. Therefore, ancillary services, such as the low-voltage ride-through (LVRT) capability should be incorporated in MGs in order to guarantee stable operation of the utility grid during grid faults. In this paper, a LVRT control strategy based on positive/negative sequence droop control is proposed for grid-interactive MGs to ride-through voltage sags with not only inductive/resistive, but also complex line impedance. By using the proposed control strategy, MGs can support the grid voltage, make profits, and also ride-through the voltage dip during the whole fault period. A two-layer hierarchical control strategy is proposed in this paper. The primary controller consists of voltage and current inner loops, a conventional droop control and a virtual impedance loop, while the secondary controller is based on a positive/negative sequence droop scheme which is able to coordinate the power injection during voltage sags. Experimental results are obtained to verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

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