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In this paper, a new modulation strategy based on the carrier-based switching strategy for the non-square direct matrix converters (MC) is proposed to control the active power of distributed generation (DG) units. In this strategy, the active power of DGs is controlled by the central input current control of the non-square direct MC independent from the voltage and frequency. Conventionally, each DG has a converter, and for supplying a load with N number of DGs, N number of converters are needed and each converter has its own modulation switching and control strategy to control the power output of each DG. Needless to say, in a microgrid with N number of DGs, the control strategy of each converter has more complex structure than that of a microgrid with one converter, and surely the former strategy entails more volume and price. Using the proposed converter in this paper, it is possible to supply a load with N number of DGs through one converter. Also, the power outputs of all DGs are controlled by a central control strategy. The proposed central control strategy is described and simulated for a typical microgrid. Experimental and simulation results validate the effectiveness of the proposed converter and the proposed strategy. The results demonstrate the applicability and efficiency of the system and verify the theoretical analysis. [ABSTRACT FROM AUTHOR]

The energy transition is aimed to take advantage of the operational flexibility of hydropower to extend the integration of intermittent renewable sources. Consequently, the hydrogenerators will have to operate in regimes far away from their designed best-point operation. In order to accurately assess the impact, this paper presents a useful approach to determine the overall operating efficiency of synchronous generators under intermittent operation. An accumulated average efficiency (AAE) model is proposed and compared against an alternative approach; the weighted average efficiency (WAE) model. It is found that the WAE approach produces unrealistic low efficiencies when the generator operates in synchronous condenser mode (SCM) for long periods. In general, the AAE supersedes the WAE for all the different load distributions that were investigated. This was further illustrated by a worked example and by constructing more complex load distributions. A load distribution dominated by SCM yields a difference as high as $33.18 \,\%$ , while an even distribution deviates $1.43 \,\%$ in their respective efficiencies. Finally, a yearly on-site measurement of our studied $103 \,\mathrm{MVA}$ generator’s concentrated load distribution revealed a discrepancy of $0.67 \,\%$ , which is a significant deviation considering what the operating regime would mean in terms of economic implications. [ABSTRACT FROM AUTHOR]

This paper presents a nonlinear dynamic simulation model of an ultracapacitor (UC) bank and the associated control system. The control system at hand consists of two levels: the lower level controls the inverter of the UC bank, while the upper control level is responsible for providing charging/discharging active power set points to be followed by the lower control level. This paper focuses on the development of the upper control level for frequency control. Specifically, two simulation case studies are developed so as to assess the performance of the proposed control framework. In the first case study the upper control level is developed using a classical Proportional-Integral-Derivative (PID) controller. In the second case study the upper control level is devised using a Model Predictive Control (MPC) algorithm based on internal linear prediction model of a nonlinear UC bank. In both cases, a nonlinear UC bank simulation model is used. The simulation case studies are modelled and tested in Matlab/Simulink. The response of the MPC-controlled UC bank is compared to the 3 existing PID-control algorithms for frequency control. The simulation results show that the MPC algorithm outperforms the conventional PID controllers. [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]

This paper presents an approach for systematically modeling a class of microgrid (MG) systems. The derived model accommodates grid-connected and islanded operation of the MG simultaneously, and allows modeling of converter-based as well as directly interfaced resources. Full modeling of virtual synchronous machines and actual synchronous generators is addressed. The originally nonlinear model is then converted to a linear model whose eigenvalues determine local stability of the MG. The model is used to analyze impacts of cross-coupling droop terms on the MG stability. The conclusions of this analysis are as follows. Addition of cross-coupling droop terms reduces the losses, compromises the stability in the grid-connected mode, and improves the stability in standalone mode. Therefore, for an MG that is intended to continuously operate at grid-connected and standalone mode, only a modest amount of cross-coupling terms is recommended to increase the efficiency without compromising the grid-connected stability. Simulation and experimental results are presented to verify the derivations. [ABSTRACT FROM AUTHOR]

Power sharing amongst sources can be improved in an islanded microgrid by utilizing the information of the network’s impedance. In this paper, an impedance based model reference adaptive system (Z-MRAS) scheme is proposed to estimate the impedance angle for converter based distributed generators (DGs) in the islanded mode. The focus is to enhance the proportional apparent power sharing between the DGs operated using generalized droop control (GDC). The performance of GDC depends on the effective tie-line impedance between the DGs and the proposed MRAS scheme provides the feedback of the estimated impedance angle. The accuracy of the impedance angle estimation is established for various tie-line $X/R$ ratios, and this results in enhanced power sharing performance for balanced, harmonic, unbalanced, dynamic and constant power load cases as well as parallel line and underground cable cases. The proposed method is validated through off-line simulations in MATLAB and controller hardware in loop (CHIL) using real time simulator (Opal-RT) including cases of meshed networks and DG plug and play for a modified CIGRE network. [ABSTRACT FROM AUTHOR]

The conventional model predictive power control suffers from heavy computational burden and large power ripple. To address the problem, this paper proposes a simplified three-vector-based model predictive direct power control, which can greatly optimize the control set. Firstly, a switching table is designed to exclude the redundant virtual voltage vectors (VVs). Only half of the candidate VVs are retained. Then, the effect of each VV on power is analyzed in detail. The deviation of reactive power is used as the evidence to further simplify the control set. In this way, the prediction behavior is reduced from 12 to 2, which effectively reduces the computational burden. In addition, the concept of power deadbeat is employed to determine the duty cycles of the two virtual VVs and one zero VV. Thus, the power ripples and current harmonics can be considerably suppressed. With this proposed method, the calculation complexity is reduced, and the steady-state performance is improved. The simulation and experimental results are given to verify the proposed method. [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]

This paper introduces a method which can accurately transfer active and reactive power between sets of three-phase windings of a multiphase induction machine. The machine has to have a multiple of three phases (3, 6, 9, etc.), with each three phases representing one set. The paper demonstrates first that the sets do not share the power in the same way as they share a torque. Therefore, the paper proposes a method that is capable of achieving accurate power sharing. This is ensured by considering the whole power that is transferred through the air-gap to/from a set, instead of considering only a power that is transferred between that set and a rotor shaft. A complete control algorithm of the proposed method is given. Experiments and simulations verify theoretical considerations and the proposed control. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed frequency control method for an off-grid inverter-based networked microgrid system (NMS) with loads, renewable generators and energy storage devices (ESDs). The frequency regulation problem is formulated as a model predictive control (MPC) problem that aims to maintain the supply-demand power balance within each microgrid and scheduled net power interchange among connected microgrids at minimal regulation costs. The charging/discharging efficiencies as well as energy capacities of ESDs, and constraints on bus voltages are also considered. The proposed MPC problem is solved by a distributed projection-based algorithm via information sharing between adjacent buses. The effectiveness of the proposed method is validated by simulations implemented in Matlab/Simulink. [ABSTRACT FROM AUTHOR]

The paper presents an approach for online centralized control in active distribution networks. It combines a proportional integral (PI) control unit with a corrective control unit (CCU), based on the principle of Model Predictive Control (MPC). The proposed controller is designed to accommodate the increasing penetration of distributed generation in active distribution networks. It helps in continuously satisfying the reactive power requirements of the transmission system operators (TSOs), while maintaining an acceptable voltage profile in the active distribution network, and simultaneously minimizing the total active power losses. The controller also ensures compliance to operation requirements of distribution network operators (DNOs). By replacing the full load flow (LF) calculation with sensitivities, derived from a linearized model of the network, the controller can work in real-time applications. Moreover, the computational burden of the proposed controller is reduced since the CCU is activated only when a voltage violation or considerable change of operation condition occurs. The performance of the proposed controller is demonstrated on a 11-kV test network with 75 buses and 22 distributed generators. [ABSTRACT FROM AUTHOR]

The paper presents three algorithms to synthesize a dynamic controller for a doubly-fed induction generator (DFIG). In each case, the pole placement approach is used to compute the parameters of a feedback controller that regulates the active and reactive powers produced using the rotor voltages. Due to the special structure of the DFIG's model, the analysis and design of the feedback control loop are simplified by transforming its representation into an equivalent system with complex coefficients and half the number of states. This approach yields analytic solutions of the problem with remarkable simplicity. The complex framework facilitates the direct placement of the DFIG system poles in the left half-plane, which ensures stability and performance of the closed-loop system. Additionally, this framework can be used to evaluate the robustness properties of the closed-loop systems. The ability of the synthesized controllers to provide a desirable dynamic and steady-state response is investigated through experiments on a laboratory testbed. [ABSTRACT FROM AUTHOR]

High penetrations of renewable energy sources (RES) in distribution grids lead to new challenges in voltage regulation. These challenges are not just limited to the steady-state voltage rise, but they are extended to rapid voltage changes due to wind speed variations and moving clouds, casting shadows on photovoltaic panels. According to EN50160 in low-voltage (LV) grids, the steady-state voltage should not exceed 1.1 pu (static characteristic), and rapid voltage changes should be kept less than 0.05 pu (dynamic characteristic). These two characteristics may limit the maximum amount of RES that can be installed in LV grids, called, respectively, the static hosting capacity (SHC) and dynamic hosting capacity (DHC). Although existing research just evaluated SHC in distribution grids, high-penetrated RES grids can be faced with such large voltage changes, which cause a smaller DHC than the SHC. This paper studies both SHC and DHC in distribution grids and proposes an adaptive controller for static synchronous compensators to regulate the steady-state and dynamic voltage while avoiding the unnecessary increase in the reactive power. The simulation results in some German distribution grids show considerable effects of the proposed adaptive controller on improving both SHC and DHC. [ABSTRACT FROM AUTHOR]

Due to the increasing penetration of distributed renewable energy generation systems, their participation in power system restoration is gaining a growing interest. For the black start of a system without synchronous generation, Wind Energy Conversion Systems (WECSs) must be able to control voltage and frequency. However, WECSs have been traditionally controlled as current sources. This paper proposes a decentralized control system for the black start of WECSs operating in parallel. In comparison with known methods for operation of distributed energy resources without synchronous generation, this paper specifically addresses the requirements of WECSs. Total wind power is regulated to match the demand during the black start process. This power is shared between WECSs according to the wind power available in each of wind turbine using a droop control characteristic between the voltage and active power. Using the so-called reactive power synchronization method, the WECSs can operate in synchronism even if wind power is significantly different in each wind turbine. Simulation results for a case study where two WECSs are connected in parallel are presented to validate the proposal. [ABSTRACT FROM AUTHOR]

This paper proposes a novel dual input nine-level inverter (DINLI) topology which requires only seven controllable devices. The dual inputs are un-identical DC voltage sources in the ratio 1:3. A simple control scheme is derived by appropriately modifying the phase disposition pulse width modulation technique to generate high-quality 9-level output from the DINLI topology. At any given instant, a maximum of four switches conduct out of which only a maximum of two switches operate at high frequency, bringing down the switching and conduction losses significantly and improving the overall system efficiency. The bidirectional DINLI (BDINLI) topology with reactive power capability can be realized by replacing the unidirectional devices with the controlled switches (i.e., total nine switches). In addition, the proposed DINLI topology does not require extra voltage balancing circuits which further improves the efficiency. The DINLI operation is verified through MATLAB simulations for grid-connected applications and its extension to an n-level, 3-phase application is also investigated. Experiments performed on a 1KVA laboratory prototype to validate the theoretical claims. [ABSTRACT FROM AUTHOR]

In this paper, design and optimization of a ladder-type single-sided linear induction motor (Ladder SLIM) for machine tool applications is investigated. High-speed linear induction machines suffer from the end-effect phenomenon, which can reduce the thrust and result in declined output characteristics. Although it is common to consider this phenomenon in high-speed applications, it is essential to take it into account in the design and analysis of low-speed low-air-gap linear machines. In addition, Ladder SLIMs have significant flux density ripples, and using skewed bars for secondary of the machine is a common solution for it. Therefore, providing required equations, an algorithm for designing a Ladder SLIM considering the end-effect phenomenon based on Duncan model is proposed, and effect of several design parameters on the output characteristics are analyzed. In order to find the optimum design parameters, a multiobjective optimization problem considering efficiency, power factor, and braking force due to the end effect is defined. This problem is optimized with two algorithms, interior point and genetic algorithms, and results are compared. Finally, the validity of the obtained solution is verified using a 2-D and 3-D time-stepping finite-element method. [ABSTRACT FROM AUTHOR]

In this paper, the control of a grid-connected wind energy conversion system (WECS), based on an induction machine fed by a matrix converter (MC), is presented. The MC is controlled using a space vector modulation algorithm with zero displacement factor at the input. Stability issues related to the operation of the WECS for the whole power and speed range are fully analyzed in this paper, using a simplified small-signal model. The implementation of a model reference adaptive system (MRAS) observer, for sensorless control of the proposed WECS, is also presented in this paper. The MRAS observer is implemented using the output voltage demand, avoiding the use of voltage transducers to measure the machine voltage. From the speed estimated by the MRAS observer, the electrical torque of the induction generator is regulated in order to drive the WECS to the operating point where the aerodynamic efficiency is maximized. Experimental results obtained with a 2.5-kW prototype are presented and fully discussed in this paper. [ABSTRACT FROM PUBLISHER]

This paper proposes a design procedure in addition to some optimal design guidelines for line start permanent magnet (LSPM) motors, independent from their ratings. Major transient and steady-state characteristics of LSPM motors, including magnet braking torque, synchronizing torque, efficiency, and power factor, are analyzed with respect to the main motor design parameters, i.e., motor back electromotive force, reactances, and saliency ratio. The contradicting behaviors of the motor characteristics with respect to the motor parameters are fully investigated. Design guidelines are developed to wisely compromise the performance characteristics by selecting optimal parameters. A design procedure considering the guidelines is proposed. By following the procedure and guidelines, an LSPM motor is designed and optimal values of the motor parameters are determined. Accordingly, a prototype LSPM motor is built and tested. The finite element method (FEM)-based simulation results in close agreement with the experimental results confirm superior steady-state and transient performance characteristics of the motor, thus validating the developed guidelines and the proposed design procedure. [ABSTRACT FROM PUBLISHER]

This paper mainly proposes a capacity configuration strategy for a hybrid wind farm with fixed-speed induction generator (FSIG)-based and permanent magnet synchronous generator (PMSG)-based wind farms during grid faults. In this paper, FSIG-based wind farm realizes low voltage ride through (LVRT) with the associated control of nearby PMSG systems as StatComs to provide the required reactive power. By establishing the mathematical model of hybrid wind farm containing network parameters, the operation characteristics of the FSIG-based wind farm during grid faults are analyzed in detail. Based on the characteristics and the critical clearing time of FSIG-based wind farm, a novel capacity configuration method for the hybrid wind farm is proposed to calculate the required reactive power of FSIG-based wind farm for a LVRT operation. Based on the capacity configuration result, the minimum installed capacity of PMSG-based wind farm could be determined. Taking into account the impact of reactive power compensation capacity and grid transmission line parameters, the coordinated LVRT capability of the hybrid wind farm is then analyzed. Finally, simulation and experimental results demonstrate the effectiveness of the proposed capacity configuration method and the coordinated operation performance of the hybrid wind farm respectively. [ABSTRACT FROM PUBLISHER]

This paper analyzes the influence of the doubly fed induction generator (DFIG) based wind farm (WF) participating in the frequency response (FR) on the oscillation modes of the power system and proposes a hybrid modulated active damping scheme to mitigate the potential power oscillations. The FR control would excite additional lightly damped electro-mechanical oscillation mode and deteriorate the damping for the primary interarea oscillation mode inside the power system. Moreover, the internal damping effect on the turbine torsional oscillation would be weakened resulting shaft damage risk if FR is required by the WF. The frequencies of those modes are usually close to each other. By integrated hybrid multiple active damping loops into the active and reactive power controller of the WF respectively, the damping control on all the oscillation modes can be improved at the same time. The analysis and effectiveness of the proposed scheme in this paper are verified by the time-domain simulations. [ABSTRACT FROM PUBLISHER]

This paper presents a planning model that utilizes mobile energy storage systems (MESSs) for increasing the connectivity of renewable energy sources (RESs) and fast charging stations (FCSs) in distribution systems (DSs). The proposed planning model aims at enabling high penetration levels of green technologies while minimizing the total DS cost that includes investment, operating, and emission costs. The proposed model determines the optimal MESS sizes and transportation schedules as well as the optimal sizes and locations of wind-based distributed generators (DGs), photovoltaic (PV) DGs, and FCSs. The model takes into account techno-economic and environmental factors in addition to the power variations of RESs, FCSs, and load demands. The proposed planning model is applied on two benchmark test systems (i.e., 33-bus and 69-bus DSs). To evaluate the efficacy of the proposed model, the results obtained from the model are compared to those obtained from a traditional planning technique. The comparison of the results demonstrates that the proposed model with MESSs successfully achieves a significant cost saving and increases the penetration levels of RESs and FCSs. [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]

Hybrid cascaded-parallel microgrid is becoming a new emerging structure to integrate multiple low-voltage power sources. This paper presents a unified distributed control strategy to implement power sharing control in hybrid cascaded-parallel microgrid under both resistive-inductive (RL) and resistive-capacitive (RC) load, where a sign function is introduced to automatically match load characteristic. Active power and reactive power regulators without frequency drop are developed, and low bandwidth communication network is employed to support power management and improve system redundancy. Furthermore, small signal model of hybrid cascaded-parallel microgrid with RL load and RC load is established. Also, small signal stability and dynamic performance of the proposed distributed control strategy is investigated. Simulation results show that the unified distributed control strategy is able to implement desirable power sharing under different load types with superior control performance. Also, the proposed control strategy is able to improve system redundancy and support plug-and-play operation of microgrid. [ABSTRACT FROM AUTHOR]

The capacity to provide ancillary services has become more and more important for generators in the distribution grid. New grid codes have been implemented which regulate inverter behavior regarding different aspects of system safety and stability. This mainly includes the frequency-dependent power reduction or the allocation of reactive power. These standards affect mains monitoring because the power control directly changes penetration after the generator has islanded. In this paper, equations for active and reactive power balance are set up and developed further. Non-Detection Zones for different control methods are shown. These are then described and interpreted in detail to show how different effects that result from grid codes interfere with one another. Frequency-dependent power reduction was found to change the size and shape of the Non-Detection Zones. Reactive power control results in a shift of the Non-Detection Zones. In fact, even the initial operating point before loss of mains has an impact on inverter behavior after islanding. Islanding operating points are sometimes undefined which leads to oscillation. An interpretation of the results and advice for future revisions of grid codes conclude the paper. [ABSTRACT FROM PUBLISHER]

Generic electrical simulation models of wind power generation have been developed as standards, such as the IEC 61400-27-1, to be used by wind industry, system operators, and academia for power system stability studies. In this paper, the IEC type 3 wind turbine model with wind turbine level voltage controller and with wind power plant level power factor controller is validated based on field measurements from a 52-MW wind power plant. In addition to the validation of the IEC type 3 wind turbine and wind power plant controller models, a comparison of the validation approaches, which are the full grid and play-back simulation, is provided together with a survey of the existing validation studies and recommendations for future modeling and validation tasks. The implemented IEC models are tuned to match the measurements accurately and the validated values for the control parameters of the reference wind power plant model are given. [ABSTRACT FROM PUBLISHER]

This paper presents a new single-source three-phase seven-level inverter with an inherent three-time voltage boosting capability for medium-voltage applications. The proposed topology is comprised of series connection of two switched-capacitor (SC) networks with an added half-bridge module per phase. Each of such integrated SC networks requires a single capacitor associated with three active power switches. The three-time voltage boosting feature of the proposed topology can reduce the dc-link voltage requirements by 50% in comparison to the traditional neutral point clamped (NPC), flying capacitors, active NPC (ANPC), hybrid clamped ANPC, and cascaded H-bridge topologies, and 75% to advanced ANPC topologies. It can also reduce the number of required switches and capacitors as well as their voltage stresses compared to those state-of-the-art topologies reported in the literature. By integrating “n” number of added SC networks in the proposed seven-level basic topology, the number of output voltage levels can be extended to 2n+3 per phase. A finite control set model predictive control algorithm is also derived to control the converter in both the active and reactive power exchanges modes without distorting the generated grid current quality. The capacitor voltage balancing is inherent in the proposed topology, and thus, there is no need for any additional voltage balancing circuit, which further reduces the control complexity. The performance of the proposed topology and its control algorithm are validated by several measurement results. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of a self-excited reluctance generating system by using active and reactive power balances. The FEA simulation results are employed to account for both saturation and cross magnetizing effect of $d$ - and $q$ -axis inductances. The procedure to determine the operating point with this method is proposed, and the influence of generator parameters, including stator resistance and inductances, are also investigated. Experiments are carried out at no-load and resistive load conditions separately, verifying the predicted values. In addition, the active and reactive power balance curves are also used to determine whether the generating system works at certain conditions. If there are no crossing points between the two curves, the generator is not able to generate. At last, it is shown that the capacitors and load resistors can be selected based on the power and reactive balances, achieving both high output power and high power factor. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed control strategy for coordination of distributed energy resources (DERs) in low-voltage resistive microgrids. The proposed framework consists of two level structures; primary and secondary control. Unlike the existing distributed control solutions, the proposed method is based upon the practical assumption of network impedance being resistive. The primary control level consists of a V-I droop mechanism, where GPS timing is used to synchronize the control agents. A consensus-based distributed secondary control method is introduced to improve the voltage regulation and load sharing accuracy of the V-I droop method. In the proposed approach, the d-axis component of the voltage is altered so as to regulate the average microgrid voltage to the rated value while guarantying proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper current and, accordingly reactive power sharing. The proposed control methodology accounts for the distribution line impedances. It features a plug-and-play environment; prior system knowledge is not required, and an arbitrary DER can enter the microgrid without any need for additional synchronization. An ac microgrid is prototyped to experimentally demonstrate the efficacy of the proposed method. [ABSTRACT FROM AUTHOR]

In AC microgrids, voltage source inverters (VSIs) have become widely employed to interface DC energy sources to the AC network. Apart from power generation, VSI-based energy sources can also perform ancillary services such as demand side management using conservation voltage reduction (CVR). In conventional distribution systems, CVR involves lowering the voltage levels of the system within permissible limits to reduce the amount of power consumed by certain loads. However, in autonomous microgrids, the application of CVR for demand side management has not been fully explored yet. Thus, in this paper, a CVR strategy for a droop-controlled VSI-based distributed generation (DG) operating in an autonomous microgrid is proposed. This strategy employs the deviation in DG output voltage magnitude and frequency, caused by an increase in power demand during peak and overloading conditions, to determine the amount of voltage compensation required to reduce the microgrid bus voltages. In addition, a controller to coordinate between the proposed CVR strategy and load shedding is also developed to operate the DGs during overloading conditions. The proposed strategy is verified with a microgrid test system, and its performance is validated on MATLAB/Simulink software environment. [ABSTRACT FROM AUTHOR]

This paper proposes a novel distributed control architecture for output power regulation of doubly fed induction generator (DFIG) based wind turbines (WTs) with on-site battery energy storage systems (BESSs). The proposed distributed control architecture receives information from adjacent WTs+BESSs to control the DFIG's grid side converter (GSC) and the storage unit for output active/reactive power regulation and energy management purposes. The proposed method guarantees the equal active power sharing and energy management of WTs+BESSs in various wind speed conditions. A distributed controller is also designed to respond to the reactive power demand and share the reactive load demand between the GSC and BESS controllers based on their capacities. Furthermore, a controller is designed to account for the communication delays caused by the communication systems based on the IEEE standards. The proposed method is tested on the modified IEEE 14-bus power system with 10 WTs+BESSs. [ABSTRACT FROM AUTHOR]

The increasing penetration of photovoltaic (PV) systems in distribution networks often causes overvoltage problems. One solution to address this issue is the provision of reactive power (RP) by the PV converters. This can cause increased power losses on the PV converters leading to additional operational costs. However, the manufacturers of commercially available PV converters provide data regarding the converter losses only under unity power factor (PF). The data are also limited regarding the technical details of the PV converters. This paper presents a methodology to estimate analytically the power losses in two-stage PV converters under an RP provision based on the efficiency curves at PF = 1 for different PV voltages and the limited information given in the PV converter datasheet. The losses are separately estimated for the dc–dc converter and the dc–ac inverter, because the losses on the former are not affected by the RP, while the losses on the latter are. The method is validated with field measurements of PV converter losses under RP provision and with detailed simulations. [ABSTRACT FROM AUTHOR]

Traditional droop control strategy has lack of the decoupling ability, so the efficiency and stability of the microgrid based on the traditional droop control strategy are easily affected by the uncontrollable coupling of the distributed generation (DG) units. In this paper, an improved droop control strategy incorporating the coupling compensation and virtual impedance is proposed to improve the efficiency and stability of the microgrid. Its coupling compensation comprises the angular frequency deviation compensation and voltage deviation compensation, which decreases the influence of the uncontrollable coupling. The virtual impedance can change the characteristics of the line impedance of the DG units and reduce their coupling. Therefore, the combination of the coupling compensation and virtual impedance can greatly reduce the power circulating-current of the microgrid and improve its dynamic performance. Then, an improved particle swarm optimization algorithm incorporating the multi-swarm and multi-velocity is taken to optimize the parameters of the coupling compensation with high accuracy and high reliability. Simulation and experimental results validate the high performance of the improved droop control strategy. [ABSTRACT FROM AUTHOR]

This paper studies the significant impact of distributed-energy-resource (DER) inverters (which are connected to the same node) on anti-islanding protection of DER-installations. As it concerns the harmonic injection-based schemes, the induced harmonic voltage component that is provoked by each inverter is compromised, as synchronous injection (injection at the same time intervals) among the inverters cannot be reassured. Therefore, harmonic injection-based anti-islanding schemes will fail if the harmonic voltage component falls below a certain level. In this framework, a novel metric (i.e., the upgrade factor, UF) is introduced, indicating the supplementary power (of DERs) that can be installed in the same point of common coupling without compromising the effectiveness of the harmonic injection scheme. As this metric enables the evaluation of harmonic injection schemes prior to the future integration of DERs, it could be a substantial addendum into the relevant standards that evaluates the anti-islanding features of grid-tied DER-inverters. Finally, this paper proposes a technique (implemented in the software level), which maximizes UF and reassures the effectiveness of the harmonic injection schemes under the power variations of installed inverters. Extensive simulations and experimental tests have been conducted that verify the effectiveness of the proposed strategy under various penetration-levels and islanding-network conditions. [ABSTRACT FROM AUTHOR]

In this paper, the interactions of the torsional dynamics of the wind turbines and power modulation for oscillations in DFIG-based wind generators are investigated. One major concern of active power modulation is its interaction with wind turbine’s torsional dynamics. A study case with wind turbine torsional dynamics modeled is derived from our earlier work. Both active power modulation and reactive power modulation controls are developed in this paper. The paper successfully demonstrates the presence of interaction between active power modulation and torsional dynamics and the absence of such interaction in the case of reactive power modulation. Linear system analysis and root loci approach are employed to demonstrate such interaction. Simulation results in MATLAB/Simulink further verifies the analysis. The major contribution of this paper is: 1) the demonstration of the potential disadvantage of wind generation's active power modulation; and 2) the identification of reactive power modulation in wind generation as an alternative for interarea oscillation damping. [ABSTRACT FROM AUTHOR]

Conventional loss of excitation (LOE) detection methods for synchronous generators may misoperate during power swings. In this paper, a novel method based on measured slip frequency at the terminal of synchronous generators is proposed to differentiate power swing conditions from LOE conditions. To enhance performance of the proposed method for noisy measured signals, a modified Prony method based on a combination of generalized total least-squares method and conventional Prony method is implemented to estimate slip frequency and an energy sorting technique is applied to distinguish the dominant frequency components of measured signals. Performance of the proposed method is evaluated under various operating conditions of a test system that is simulated in a co-simulation platform by linking CAPE and PSS/E software. Case study results illustrate that the proposed method can accurately differentiate power swing conditions from LOE conditions. [ABSTRACT FROM AUTHOR]

This paper presents a novel decentralized control for offshore wind farms connected to the onshore grid through a high-voltage dc link by means of a diode rectifier. The proposed control system is implemented in each wind turbine generator system (WTGS). The capacitor placed at the filter of the wind turbine front-end converter is used for the proposed control implementation. Frequency control is achieved by aligning the capacitor voltage vector along a reference axis rotating at the reference frequency. Then, a frequency-reactive power droop control allows the synchronization of all the WTGSs. On the other hand, this droop strategy also leads to total reactive power sharing among WTGSs without relying on communications. An additional secondary frequency control is also implemented to compensate the frequency deviation caused by the droop control. The proposed control system has been validated by simulation and results demonstrate the appropriate performance even during start-up and faults. [ABSTRACT FROM AUTHOR]

In this paper, the power-angle synchronization control of a grid-tied bidirectional dc–ac converter is investigated for low-voltage grids. The power flow equations for the low-voltage grid are analyzed and compensators are designed to ensure the decoupled control of active and reactive power. The proposed control system operates without the need for a phase-locked loop during balanced and unbalanced grid conditions. It is shown that during balanced conditions the compensators ensure stable operation controlling the desired power flow to and from the grid. It is also demonstrated that the compensators are immune to grid fluctuations and to a large extent can cater grid unbalanced conditions. Experimental results verify the performance of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper presents a need-based distributed coordination strategy (NDCS) for multiple electric vehicle (EV) storages in an islanded commercial hybrid alternating-current (AC)/direct-current (DC) microgrid. The control capacity of the interlinking converter is enhanced by incorporating combined power-droop and voltage-droop strategies to leverage the coupling of AC and DC voltages. Therefore, the AC bus voltage can be regulated simultaneously by regulating only the DC bus voltage without affecting the power-sharing capabilities of the converter. The NDCS is proposed to coordinate the EV storages to regulate the DC bus voltage. The main objective of the NDCS is to decide whether the coordination of the available EV storages is to be done in a decentralised or a distributed manner. The mathematical model and the algorithm to deploy NDCS are developed to realise its application to a real system. The effectiveness of the control system is verified in a commercial hybrid AC/DC microgrid comprising one photovoltaic unit and four EV storages directly connected to the DC bus via DC/DC converters and four distributed-generation (DG) units connected to the AC bus using the conventional droop-control scheme. The performance of both controllers is tested under variable irradiation, commercial loading, and various fault conditions. The results of the case studies demonstrate the efficacy of the overall system in terms of robustness for a variable generation-demand scenario, time delay, EV plug-and-play, and fault conditions. [ABSTRACT FROM AUTHOR]

This paper proposes a simplified and improved model predictive current control (MPCC) scheme for grid-connected bidirectional voltage source converter (GC-BVSC) when the unbalanced grid voltages occur. In conditions of unbalanced grid voltage, the current harmonic components of the GC-BVSC increase significantly, and twice grid-frequency ripple exist in both active power and reactive power, which influence the output power quality. To reduce harmonic currents and power fluctuations, an improved MPCC (IMPCC) method is proposed, and a two-step predictive model of GC-BVSC is put forward. The current compensation values are expressed by grid voltages and their quadrature signals that lag 90 electrical degrees in the αβ stationary coordinates system. The cost function reducing both the power ripple and current distortion is designed. Compared to conventional methods, phase-locked loop, pulse width modulation, and complex positive and negative sequence extraction of grid voltage are not required. In addition, IMPCC presents a better performance by reducing both grid-connected current harmonics and power ripple of GC-BVSC with flexible bidirectional power regulation capability. Simulation and experimental results on power electronics-professional (PE-PRO) platform verify the effectiveness of the proposed scheme under both single-phase and three-phase unbalanced grid voltages. [ABSTRACT FROM AUTHOR]

Conventionally, the converter of switched reluctance motor (SRM) is fed by the uncontrollable diode bridge rectifier (DBR), which leads to a low grid-side power factor (PF) and high current total harmonic distortion (THD). In this paper, an alternative solution for the grid-connected high-speed SRM drive system with improved PF is proposed. In the proposed drive system, the three-level active front end (AFE) is connected in cascade with the midpoint converter for SRM operation. A centralized strategy, which controls the AFE and SRM together, is proposed to govern the motor speed and grid-side PF by regulating the real power and reactive power of the system, respectively. Specifically, the real power, reactive power, and the voltage balancing of split capacitors are controlled by the model predictive directed power control (MP-DPC) algorithm, which significantly reduces the control complexity and guarantees the fast dynamic response. Consequently, satisfying speed regulation, high PF, low current THD, and bi-directional power-transfer capability are achieved. An idea-proofed testbench is constructed in laboratory, and the applicability of the proposed drive system is verified by a series of experimental results. [ABSTRACT FROM AUTHOR]

Droop control is the most common approach for controlling inverter-based micro-grids. The active power droop gain has always been considered as the main parameter for identifying the micro-grid stability margin. Increasing this margin improves the transient performance and provides robustness to the micro-grid for a wide range of operations. Previous work on droop control focused on the active power droop gain, which is required for accurate power sharing as well as for micro-grid stability assessment. This paper utilizes small-signal stability analysis to analyze the impact of the reactive power droop gain on micro-grid stability, which is ignored in previous work. Consequently, a micro-grid domain of stability chart is proposed and defined in the mpmax-nq plane, which represents the zone within which the micro-grid will maintain stable operation. The proposed domain of stability chart is utilized to assess and compare the impact of the conventional and proportional derivative (PD) reactive power droop controller on the micro-grid stability margin. The results show that there exists a reactive power droop gain at which the stability margin is minimum. Furthermore, it has been shown, through the domain of stability chart, that the PD reactive power droop controller is capable and sufficient to significantly increase the micro-grid stability margin while maintaining equal load sharing. Further, the domain of stability chart can serve as a useful tool for defining the micro-grid droop gain operational boundaries and for assessing and comparing inverter-based micro-grid control schemes. [ABSTRACT FROM AUTHOR]

A new transformer-less three-level grid-tied inverter is proposed in this article. By employing the switched-capacitor (SC) modules, a new topology is derived. The proposed topology has some features which are high efficiency, the capability of voltage boosting without utilization of any additional boost stage, and suppressing the leakage current by using common grounded strategy. In this article, a peak current control is employed to generate the gate pulses of power semiconductor switches and adjustment of both active and reactive powers. By doing so, the grid injected current is controlled and adjusted to have a proper quality. The operation modes, design consideration, and a comparison with some recently proposed grid-connected topologies are done through this paper. Finally, the performance and feasibility of the proposed inverter are confirmed with a 500 W experimental prototype. [ABSTRACT FROM AUTHOR]

This paper is the continuation of Part I in which an online reference limitation strategy for shunt-connected converters is presented. This limitation strategy is successfully validated in this Part II of the paper by means of experimental results obtained in a laboratory setup. Two different experiment sets are carried out in order to check the two application scenarios described in Part I. On the one hand, the limitation strategy is implemented in the STATCOM application reducing the reactive power reference. On the other hand, the grid balancer functionality is implemented together with the new limitation strategy reducing the current references. In this last scenario, two different experiments are shown: 1) a switching-on of the converter; and 2) a voltage variation at the point of common coupling. In both functionalities, the corresponding reference limitations are activated due to overcurrent and overvoltage cases and the limitation due to the dc bus voltage oscillations is validated in the grid balancer functionality. [ABSTRACT FROM AUTHOR]

This paper presents a new control strategy using model-based predictive current control (MB-PCC) for a doubly fed induction machine (DFIM) driven by an indirect matrix converter (IMC). This strategy proposes the control of rotor currents, whose references are calculated from active and reactive stator power set points and the dynamic model of the DFIM. The control strategy works well in the four P-Q operating regions of the DFIM. The grid synchronization process is carried out by setting the P-Q power set points to zero. The results include the DFIM synchronization procedure as well as the active and reactive power control at variable shaft speed to validate the feasibility of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a single-phase grid-connected wound rotor machine that has the single-phase stator and three-phase rotor windings. The machine has no auxiliary winding or capacitor unlike conventional single-phase machines. Nevertheless, it can operate in all four quadrants of the torque–speed plane with a three-phase inverter. For adjustable speed drives, an isolated three-phase inverter is applied to the rotor windings while the stator winding is directly connected to a single-phase source. The grid filter and rectifier of the conventional system are eliminated and the rotor-side slip rings can be also removed by the inverter integration. So the overall structure of the proposed drive system is simple and cost effective. In this paper, the proposed machine is modeled into a modified d–q model considering the absence of q-axis stator coil. Its characteristics are analyzed and vector control methods of the grid power factor, dc-link voltage, and speed are proposed. For more efficient control, the optimal rotor current set is calculated from the minimum copper loss condition. The system has wider operating areas than other single-phase drive systems. The feasibility of the proposed system is verified by experiments. [ABSTRACT FROM AUTHOR]

Rotor winding interturn short-circuit (RWISC) occurring in turbine generators has been plaguing power producers with respect to production safety and economy. RWISC is characterized by impaired fundamental component of the exciting magnetomotive force and resultantly reduced no-load electromotive force E0. In this paper, an inverse reasoning method is proposed to analyze RWISC. First, it is assumed that the generator's rotor winding is running in good condition and the generator's electromagnetic power (virtual power, VP) is inversely calculated from the obtained operational data on generators. Second, the difference between the virtual power and the actual electromagnetic power is calculated and then used to determine whether an RWISC occurs on the turbine generator. Three real cases of severe rotor winding shorts have demonstrated that this method proposed in this paper not only enables to diagnose RWISC, but also allows determining the RWISC level and its trending accurately. The method is appropriate for RWISC diagnosis, whether offline or online, and can be applied to make the generator sets in the power plant run safer, more economical, and more stable. [ABSTRACT FROM AUTHOR]

This paper presents a robust nonlinear distributed controller design for islanded operation of microgrids in order to maintain active and reactive power balance. In this paper, microgrids are considered as inverter-dominated networks integrated with renewable energy sources (RESs) and battery energy storage systems (BESSs), where solar photovoltaic generators act as RESs and plug-in hybrid electric vehicles as BESSs to supply power into the grid. The proposed controller is designed by using partial feedback linearization and the robustness of this control scheme is ensured by considering structured uncertainties within the RESs and BESSs. An approach for modeling the uncertainties through the satisfaction of matching conditions is also provided in this paper. The proposed distributed control scheme requires information from local and neighboring generators to communicate with each other and the communication among RESs, BESSs, and control centers is developed by using the concept of the graph theory. Finally, the performance of the proposed robust controller is demonstrated on a test microgrid and simulation results indicate the superiority of the proposed scheme under different operating conditions as compared to a linear-quadratic-regulator-based controller. [ABSTRACT FROM AUTHOR]

In this paper, a virtual mutual inductance approach is used to analyze the performance of transverse flux machines (TFMs). The virtual mutual inductance between the stator windings and the magnets’ equivalent currents is obtained by integrating the flux produced by the stator windings over the surface of the magnets. Key design parameters such as back electromotive force, torque, phase inductance, and power factor are readily calculated using the proposed methodology. This method is also used to optimize the geometry of a particular machine that provides an insight into the relationship between power factor and torque. Furthermore, the insights gained suggest a design approach that takes into account the power factor of TFMs, which may help unlock their potential through a tradeoff between torque density and power factor. The results obtained using the analytical model are verified using three-dimensional finite element analysis and experimental data. [ABSTRACT FROM AUTHOR]

This paper first derives a usable formula for frequency shift islanding detection methods on the basis of the parallel R, L, C load and the conclusions in current literature: the angle by which the total output current of the distributed resources (DR) units leads the point of common coupling (PCC) voltage must be conducted to have the same shifting direction as the load admittance angle during the variation of the frequency. According to the formula and multi-DR operation, the scenarios in which the classic frequency shift methods are applied to energy storage converters are analyzed. The results indicate that the setting of the angle by which the energy storage converter current leads the PCC voltage may need to be modified when running state changes. It results in the problems that the classic methods are not applicable for non-unity power factor (non-UPF) control and have to distinguish between generation mode and consumption mode for UPF control. On account of the problems, a coping strategy, i.e., an improved method, is proposed. The analyses indicate that the improved method is applicable in every state. The last simulations and experiments confirm the preceding conclusions. [ABSTRACT FROM AUTHOR]

In this paper, a combined reactive power modulation and grid current distortion improvement approach is proposed for an H6 transformer-less full-bridge single-phase photovoltaic (PV) grid-connected inverter. H6 transformer-less inverters with traditional modulation and control strategies may not satisfy the requirement of reactive power compensation or may result in more severe zero-crossing current distortion. Therefore, contrary to the traditional modulation, a novel reactive power injection space vector pulse width modulation (SVPWM) is proposed, which consists of two operation stages—inverter modulation and reactive power modulation. The implementation of SVPWM for reactive power modulation using a digital signal processor is also investigated. Furthermore, to suppress the current zero-crossing distortion in the reactive power injection mode, a global sliding mode function based on the proportion-integration-resonance current controller is designed, and the control law of the global sliding mode is derived. Using the segment modulation and grid current distortion improvement approach, the function of reactive current injection is implemented in commercial PV inverters, and the total harmonic distortion of the grid current is decreased significantly by more than 5% in the low-power segment, under the operating conditions of a lagging or leading power factor of 0.95. The effectiveness and feasibility of the proposed approach are verified through simulation and experiment using a 5-kVA prototype. [ABSTRACT FROM AUTHOR]