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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]

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]

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 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]

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]

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]

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]

Since intermittent nature of distributed generation and uncertainty in load behavior, the existing secondary control are not sufficient to deal with robustness and fast response. In this paper, attacked from a cooperative based control perspective inspired to the tracking consistency paradigm, a robust nonlinear distributed secondary voltage controller is proposed to achieve fast compensation for the islanded microgrid. Employing sparse communication, the proposed control strategy can achieve a compromise between the conflicting goals of voltage regulation and reactive power sharing. As the distributed architecture only requires the information of its own and its neighbor, the use of centralized controllers is avoided and the flexibility and scalability of system can be enhanced. Finally, the stability analysis is provided and the simulation results are presented to verify control performance under rapid load changes, robustness under communication failure, and flexibility during plug-and-play operation. [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]

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]

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 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]

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]

In this paper, a three-phase series hybrid active filter (SEHAF) interconnected with photovoltaic (PV) system and dc–dc boost converter is proposed to minimize sag, swell, and harmonics caused due to nonlinear power electronic loads. The SEHAF consists of a voltage source inverter (VSI) with a capacitor connected across it to provide consistency in managing and compensating the reactive power. This minimizes the sag, swell, and harmonics present in the source and load voltages. With the integration of PV, the voltage across the dc-link capacitor of VSI is controlled effectively, which helps in better compensation. Reference current generation is done using the proposed robust extended complex Kalman filter (RECKF) technique. The performance of the PV-integrated-HAF is analyzed using a synchronous reference frame with proportional-integral (PI) as well as fuzzy logic controller (FLC) and is compared with the proposed RECKF technique. The PV-integrated hybrid power system is developed using MATLAB/SIMULINK. Further, real-time digital simulation using OPAL-RT OP5142 is also carried out to support the simulation results. It is observed that the proposed control scheme provides better harmonic compensation compared to conventional PI and FLC. [ABSTRACT FROM AUTHOR]

Direct power control (DPC) has gained much attention for medium-voltage applications due to its fast power control capability. However, a conventional static switching table based DPC can lead to unsatisfactory response when the converter is connected to an electric grid because it is commonly designed assuming ideal and constant voltages of the grid and the dc link. Therefore, the actual grid voltage and the actual converter dc-link voltage should be considered while designing the switching table. This paper proposes a novel dynamic switching table for the DPC, considering actual grid and dc-link voltages variation. The proposed dynamic switching table incorporates a new definition of power influence crossover angles of each converter space vector. The proposed scheme dynamically adapts the switching table by feeding forward the actual grid and dc-link voltages, and hence the converter's power is precisely controlled. The proposed technique is verified for the three-level neutral point clamped converters connected to a standard IEEE 9-bus test system. Moreover, the real-time simulations are performed in a real-time digital simulator, OPAL-RT. The obtained results verified the effectiveness of the proposed DPC strategy under different grid fault scenarios. [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 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]

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 focuses the analysis around a grid-connected converter that operates under unbalanced voltage and current conditions. Two different scenarios have been selected to validate the proposed algorithms. In the first scenario, the converter operates as a reactive power compensator, exchanging reactive power with the grid, and minimizing the dc bus voltage oscillations. The second scenario is more challenging, since it operates as a load balancer. In this application, the unbalanced current consumption of the load is compensated by the converter, exchanging positive and negative sequence currents with the grid. Another objective in this second scenario is to compensate the reactive power of the unbalanced load. It is important to highlight that in both scenarios, any maximum limit of the converter can be exceeded due to the specific unbalanced voltage and current conditions. Three variables are considered critical for the converter: 1) output ac current limit; 2) output ac voltage limit; and 3) dc bus voltage oscillation limit. Thus, this paper proposes a control method, which limits online the reactive power reference in the first scenario and the exchanged current references in the second scenario in order not to exceed any of the mentioned critical variables. The corresponding experimental results are shown in Part II of this paper so as to validate the limitation algorithm obtained by means of the mathematical analysis carried out in Part I. [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]

Decentralized methods for computing optimal real and reactive power setpoints for residential photovoltaic (PV) inverters are developed in this paper. It is known that conventional PV inverter controllers, which are designed to extract maximum power at unity power factor, cannot address secondary performance objectives such as voltage regulation and network loss minimization. Optimal power flow techniques can be utilized to select which inverters will provide ancillary services and to compute their optimal real and reactive power setpoints according to well-defined performance criteria and economic objectives. Leveraging advances in sparsity-promoting regularization techniques and semidefinite relaxation, this paper shows how such problems can be solved with reduced computational burden and optimality guarantees. To enable large-scale implementation, a novel algorithmic framework is introduced—based on the so-called alternating direction method of multipliers—by which optimal power flow-type problems in this setting can be systematically decomposed into subproblems that can be solved in a decentralized fashion by the utility and customer-owned PV systems with limited exchanges of information. Since the computational burden is shared among multiple devices and the requirement of all-to-all communication can be circumvented, the proposed optimization approach scales favorably to large distribution networks. [ABSTRACT FROM AUTHOR]

Doubly fed induction generators are suitable for systems having limited speed range as the overall control can be carried out by fractionally rated converters. However, brushes and slip-rings used in these generators reduce system reliability and demand greater maintenance. Dual stator winding induction generator (DSWIG), being brushless, removes this limitation. Two distributed windings are embedded in the stator and the rotor is squirrel-cage. One of the windings is interfaced to an uncontrolled rectifier and the other to a fractionally rated PWM converter. Uncontrolled rectifier degrades the power quality within the generation system. At the same time, reactive power demand in induction generators increases with loading. This paper deals with design and control of a standalone dc system based on DSWIG, where a combination of passive tuned filter and series capacitor is utilized to address the voltage regulation and power quality issue. Simulation results using MATLAB/Simulink and experimental results (obtained from a laboratory prototype) have been presented, compared, and discussed to demonstrate the effectiveness of the proposed alternative. [ABSTRACT FROM AUTHOR]

This paper presents a voltage and frequency control (VFC) and an average-value model (AVM) of a line-commutated converter for a rectifier station in an offshore wind farm (OWF) connected by a high-voltage direct current link. A capacitor bank is placed at the AC terminals of the rectifier station to perform VFC within the OWF. The proposed model uses the active and reactive power generated by the OWF as inputs, while the state variables are the voltage magnitude and phase angle at the capacitor bank bus. The proposed VFC is based on the orientation of the voltage vector at the capacitor bank bus toward a synchronous reference axis. It is then demonstrated that frequency control is achieved by regulating the reactive power balance at the capacitor bank bus, while voltage control is carried out by regulating the active power balance. Moreover, it is demonstrated that in a diode rectifier, although voltage cannot be controlled as in a thyristor rectifier, it is bounded within acceptable limits. In addition, small-signal study is performed to facilitate controller design and system stability analysis. VFC and the accuracy of the proposed AVM are validated by simulation, using both the proposed AVM and a detailed switching model. [ABSTRACT FROM AUTHOR]

This paper proposes one control paradigm that can operate in both grid-connected and islanded modes, hence does not need any sort of an islanding detection method. The proposed method automatically and seamlessly rides through a fault on the grid side, and controls the microgrid's voltage and frequency during islanded operation. During an islanded operation, it utilizes the combination of distributed generation-energy storage similar to the prime mover of a synchronous generator to control the frequency. A comprehensive active and reactive power control is proposed that minimizes the usage of a local fossil-fuelled auxiliary generator. The method is based on expanding the so-called non-detection zone to all operational scenarios including islanded mode, hence having small “undetectable” voltage and frequency deviation. As soon as the grid is reconnected, the distributed generator is automatically and seamlessly synchronized with the grid. This is achieved through keeping phase-locked loop as a part of the operation in islanded mode without altering its phase angle. The proposed method is validated using PSCAD/ EMTDC simulation. [ABSTRACT FROM PUBLISHER]

This paper proposes a hybrid pulse width modulation (PWM) based field weakening control strategy for a hybrid-inverter-driven open-winding (OW) permanent magnet synchronous motor (PMSM) system. The hybrid-inverter consists of two voltage source inverters, one is connected to a dc voltage source and named as main inverter (MI), and the other is connected to a floating capacitor and called compensation inverter (CI). A novel hybrid PWM technique is proposed for the hybrid-inverter, in which a double-vector-based PWM is applied to the MI to reduce the switching frequency, and the conventional space vector PWM is applied to the CI to compensate the reactive power. Furthermore, a field weakening (FW) control strategy based on the hybrid PWM technique is proposed to better utilize the dc-link voltage and extend the operation range of the OW-PMSM system. Simulation and experimental studies are carried out to verify the high efficiency and wide speed range of the proposed strategy. [ABSTRACT FROM PUBLISHER]

This paper proposes a combined wind farm controller based on Model Predictive Control (MPC). Compared with the conventional decoupled active and reactive power controls, the proposed control scheme considers the significant impact of active power on voltage variations due to the low $X/R$ ratio of wind farm collector systems. The voltage control is improved. Besides, by coordination of active and reactive powers, the Var capacity is optimized to prevent potential failures due to Var shortage, especially when the wind farm operates close to its full load. An analytical method is used to calculate the sensitivity coefficients to improve the computation efficiency and overcome the convergence problem. Two control modes are designed for both normal and emergency conditions. A wind farm with 20 wind turbines was used to verify the proposed combined control scheme. [ABSTRACT FROM PUBLISHER]

As penetration of distributed generation increases, the electrical distribution networks may encounter several challenges mainly related to voltage control. The situation may deteriorate in case of a weak grid connected to an intermittent source such as photovoltaic (PV) generation. Fast varying solar irradiance can cause unacceptable voltage variations that may not be easily compensated by slow-responding utility equipment. The PV inverter can be used to control the grid voltage by injecting/absorbing reactive power. A small-signal model is derived in order to study the stability of a PV inverter exchanging reactive power with the grid. This paper also proposes a method that utilizes the available capacity of the PV inverter to support the grid voltage without violating the rating of the inverter and the maximum voltage that the inverter's switching device can withstand. The method proposed in this paper is validated using PSCAD/EMTDC simulations. [ABSTRACT FROM PUBLISHER]

This paper proposes a model predictive direct power control (MPDPC) approach with reduced switching frequency for grid-connected voltage-sourced converters (VSC). This approach is proposed for both a real/reactive power controller (with a fixed input voltage) and a power port controller (with a variable input voltage). It uses two different cost functions: one for the steady state and one for transients. Utilizing these two cost functions leads to having a low switching frequency in the steady state and a fast response during transients. The proposed MPDPC also provides overcurrent protection of the VSC. In applying the proposed MPDPC to a power port, drawbacks of conventional voltage controllers are addressed by employing power estimation and set point modulation in the controller. Simulation and experimental results validate the proposed MPDPC and the voltage controller. [ABSTRACT FROM PUBLISHER]

In this paper, an enhanced hierarchical control structure with multiple current loop damping schemes for voltage unbalance and harmonics compensation (UHC) in ac islanded microgrid is proposed to address unequal power sharing problems. The distributed generation (DG) is properly controlled to autonomously compensate voltage unbalance and harmonics while sharing the compensation effort for the real power, reactive power, and unbalance and harmonic powers. The proposed control system of the microgrid mainly consists of the positive sequence real and reactive power droop controllers, voltage and current controllers, the selective virtual impedance loop, the unbalance and harmonics compensators, the secondary control for voltage amplitude and frequency restoration, and the auxiliary control to achieve a high-voltage quality at the point of common coupling. By using the proposed unbalance and harmonics compensation, the auxiliary control, and the virtual positive/negative-sequence impedance loops at fundamental frequency, and the virtual variable harmonic impedance loop at harmonic frequencies, an accurate power sharing is achieved. Moreover, the low bandwidth communication (LBC) technique is adopted to send the compensation command of the secondary control and auxiliary control from the microgrid control center to the local controllers of DG unit. Finally, the hardware-in-the-loop results using dSPACE 1006 platform are presented to demonstrate the effectiveness of the proposed approach. [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]

Conventional active islanding detection methods (IDMs) are designed for the inverter-based distributed generation systems (DGSs) with current control interface. Such strategies can hardly be extended to the DGS with power control interface because the power control loop can affect the IDMs by enlarging the nondetection zones (NDZs). This paper presents an active IDM based on negative-sequence power injections for the DGS with power control interface. Combining with the IDM, the power control of the DGS is achieved with two control loops. One is the positive-sequence power loop that satisfies the conventional power control requirements. The other is the negative-sequence power/current variation loop for the islanding detection. The positive and negative sequences are separated by a simple strategy based on an all digital phase-locked loop. Due to the differences between the grid impedance and the local load impedance, the percentage of the voltage imbalance (VI) at the point of common coupling is utilized to indicate the islanding operation. For the grid-connected DGS, the VI is dominated by the grid voltage, which is a constant. If the DGS is disconnected from the grid, the VI is determined by the injected negative-sequence power/current. The NDZs of the presented scheme with different system configurations, such as grid impedances, load quality factors, etc., are also analyzed in this paper. By injecting the negative-sequence power/current periodically, the NDZs resulting from the imbalance of the grid voltages or the local loads are further mitigated. For multi-DGSs, the IDM is still effective if combined with the conventional under/overfrequency-protection strategy. The simulation and experimental results verify the effectiveness of the IDM. [ABSTRACT FROM AUTHOR]

This paper studies microgrids where loads are supplied by parallel-connected inverters controlled by decentralized active power/voltage frequency and reactive power-/voltage-magnitude droop control laws. A paralleled ac system, such as a multiinverter microgrid, is susceptible to circulating currents due to differences in voltage magnitude, frequency, phase angle, or dc offset. Circulating currents due to differences in voltage magnitude and dc offset have been known issues reported in literature. However, an in-depth analysis of the problem is required to ascertain the deviation of the system-operating condition from the desired condition. This paper provides a mathematical model that predicts the effect of voltage-magnitude offsets on reactive power sharing between inverters. Simulation and experimental results verify the accuracy of the analytical results obtained from the mathematical model. We examine the effect of dc-circulating currents and propose a simple capacitor emulation control law implemented in software to eliminate dc-circulating currents. This solution is a possible alternative for hardware implementation to eliminate dc-circulating currents. The effectiveness of the capacitor emulation control law has been verified through experimental results. [ABSTRACT FROM PUBLISHER]

This paper explores and compares the performance of alternative voltage control strategies applied to doubly fed induction generator (DFIG). Different combinations of reactive power control of rotor- and grid-side converters are investigated for voltage-control purposes. Simulations are performed using detailed models built in DIgSILENT PowerFactory in order to illustrate the influence of controllers on transient stability and steady-state operation of the DFIG-based wind plant. This paper also proposes appropriate control strategies for different sets of network operating conditions and topologies. Operational limits, such as current margins and pulse-width modulation limits, are also taken into account. [ABSTRACT FROM AUTHOR]

The voltage source converters (VSC) are the key power interfaces between the individual ac grids and dc grids in the hybrid power system. When the ac grid is unbalanced, the grid currents are distorted and the harmonic is induced. Meanwhile, the oscillations in the active and reactive power are also increased, which may damage these interfaces. In order to ensure the safety of the converters and the grid facility, a flexible control strategy is proposed for the ac/dc hybrid grid in this paper. At first, the current harmonic for the VSC is theoretically analyzed under unbalanced conditions. Furthermore, the methods to separate the positive- and negative-sequence components of the ac grid voltages are summed up and analyzed. Based on the theoretical analysis, different protected control methods are compared. A new control method, which can flexibly control oscillations of the active and reactive power by introducing a parameter k, has been proposed. The estimation of maximum current in each phase is discussed; meanwhile, the relationship between the voltage ripple on dc link and the parameter k is given. Finally, the validity and advantages of the proposed method are verified by the simulating and experimental results. [ABSTRACT FROM PUBLISHER]

The fully rated converter of type 4 wind turbines is capable of providing dynamic voltage control during grid faults by injecting controlled reactive currents. This paper describes three different dynamic voltage control options during unbalanced grid faults: 1) the positive sequence voltage control with only a positive sequence reactive current injection and suppression of the negative sequence current; 2) the positive sequence voltage control with limitation of the positive sequence reactive current injection and suppression of the negative sequence current; and 3) the positive and negative sequence voltage control with both a positive and a negative sequence reactive current injection. These different control options are compared in simulations of a wind power plant connected to a meshed power system, including synchronous generators. It is shown that both the positive sequence voltage control with limitation and the positive and negative sequence voltage control can overcome the voltage rise and voltage distortion that can occur with pure positive sequence voltage control without limitation. Both of these options have a distinct fault response, where the positive and negative sequence voltage control results in a fault response that resembles the fault response of a synchronous generator with higher fault current contributions in the faulted phases. [ABSTRACT FROM AUTHOR]

The paper presents a collaborate control for the rotor-side converter (RSC) and grid-side converter (GSC) of a doubly fed induction generator (DFIG) generation system during network unbalance. In this study, the RSC is controlled to reduce the torque ripples, and three selectable control targets for the GSC, i.e., balanced total currents, and constant total active or reactive power into the grid from the overall system are identified to reduce the impacts of the negative-sequence voltage on the DFIG system performance. A reduced-order generalized integrator is employed, which is implemented in the positive synchronous reference frame. Based on the math model, the impacts of the limited dc voltage, as well as the rejection capability on negative-sequence voltage of the proposed control strategy, have been investigated. Finally, the simulation and experimental results are provided to demonstrate the effectiveness of the proposed collaborative control strategy. [ABSTRACT FROM AUTHOR]

In multibus islanded microgrids, the power quality requirements for different areas and buses can be different. This paper proposes a hierarchical control to realize optimal unbalance compensation for satisfying the power quality requirements in different areas. Primary and secondary controllers are applied to realize unbalance compensation for critical bus, and at the same time, to make distributed generators (DGs) equally share the compensation efforts. Tertiary control, which inherently is an optimization method, is implemented to adjust the compensating effort of each DG considering the voltage unbalance limits in local buses and DG terminals. This method realizes multipower-quality-level control in a multibus islanded system by optimally utilizing DGs as distributed compensators and saves the investment for additional compensation equipment. Hardware-in-the-loop results demonstrate the effectiveness of the method. [ABSTRACT FROM AUTHOR]

Cascaded brushless doubly fed induction machines have attracted much attention because of the elimination of slip rings and brushes. However, only limited works have been reported on the control strategies due to the complex machine modeling. Further developments of these kinds of machines are still under investigation, especially for wind energy applications. This paper proposes a new control strategy for cascaded brushless doubly fed induction generators (CBDFIGs) based on direct power control. The effects of voltage vectors on the output active and reactive powers are first investigated. A vector selection strategy is then proposed to achieve flexible power regulation. In addition, the behaviors of the CBDFIG under unbalanced grid voltage conditions are studied and a power compensation scheme is developed to improve the power quality. The effectiveness of the proposed control strategy is validated experimentally on a laboratory prototype. [ABSTRACT FROM PUBLISHER]

This paper presents the performance of a self-excited induction generator (SEIG) with a voltage and frequency controller (VFC) in a standalone microhydro power generating system. The VFC consists of a distribution static compensator (DSTATCOM) and a direct torque controlled variable frequency induction motor drive (VFIMD) operating as an electronic load controller (ELC). The DSTATCOM is an insulated-gate bipolar transistor-based three-leg voltage source inverter with a self-sustaining dc bus. The main objective of DSTATCOM is to regulate the system voltage through reactive power compensation. In addition to voltage regulation, the DSTATCOM compensates the harmonics generated by nonlinear loads as well as ELC. The ELC controls the system frequency through the active power balance. The VFIMD of the ELC drives a pump and it consumes the power in excess of consumer load power to maintain constant power generation at the SEIG terminals, which in turn regulates the system frequency. A prototype-proposed VFC is developed and tested with a variety of loads. Experimental results demonstrate that the proposed VFC can effectively control the system voltage and frequency. [ABSTRACT FROM PUBLISHER]

Paralleled operation of voltage-source inverters (VSIs) is currently achieved by using voltage/frequency droop control techniques which requires the knowledge of the system parameters. Otherwise, centralized control techniques with robust communication among VSIs controllers are also used. This paper presents a new control strategy which allows the load sharing between the power sources of an ac microgrid without centralized controller or any communication among the VSIs; only local measurements of voltage and output current are used. The dispatchable sources (e.g., fuel cells) of the microgrid are operated using voltage control with a direct droop scheme, and the nondispatchables or intermittent ones (e.g., wind turbine generators) are operated using power control with a complementary inverse droop scheme (D-Droop + I-Droop). The number of operating sources can be changed online without any modification needed on the VSI controllers. The proposed VSI controllers are based on the variable frequency adaptive linear neuron with frequency-locked loop for the VSIs system synchronization, voltage/power and signal estimation. Experimental results using field-programmable gate array devices for the implementation of each VSI control in the microgrid test bench demonstrated the validity of the proposition. [ABSTRACT FROM PUBLISHER]

In the single-phase photovoltaic (PV) grid-connected distributed generation (DG) system located at the end of the feeder, it is necessary to provide quickly active and reactive power to the local loads. In this paper, an improved reactive current detection and power control method are proposed to realize active power control and reactive power compensation. To detect quickly the loads reactive current, a fast reactive current detection method using the derivative and ip\!\!-\!\!iq algorithm is presented, and it can overcome the long delay in a conventional phase-shift method. In the inner current loop, the quasi-proportional-resonant (QPR) control method with grid voltage feed-forward is presented to control accurately the grid current with zero steady error, and to reduce the current distortion due to the frequency offset and distortion of the grid voltage. The inner loop stability is analyzed, and the proper parameters are selected. In the outer loop, proportional–integrator (PI) controller is applied to stabilize the dc-link voltage, and power feed-forward is introduced to speed up system response. Simulation and experimental results verified the validity of the proposed control method. [ABSTRACT FROM PUBLISHER]

This paper investigates the feasibility of the application of a hybrid HVDC transmission system for the grid integration of offshore wind farms. The proposed hybrid HVDC consists of a pulse width modulated current source converter (PWM-CSC) and a line-commutated converter (LCC). The PWM-CSC is connected to the offshore wind farm and the LCC connects the onshore grid. The hybrid topology takes advantages from self-commutated converters as well as LCCs. On the one hand, LCC-based HVdc is the most mature technology with the lowest power losses and lowest cost. On the other hand, PWM-CSC has the same features that a voltage source converter for offshore applications, i.e., the ability to operate without an external commutation voltage, reactive power control capability, and a relative small footprint. Moreover, both the PWM-CSC and the LCC are current source converters and hence the coupling can be effortlessly done. The control design for the entire system is presented and verified using numerical simulations. Simulations are performed using PSCAD/EMTDC under different conditions including changes in the wind speed and ac and dc faults. [ABSTRACT FROM AUTHOR]

This paper presented an expandable grid-connected power quality compensator with distribution generation interface for three-phase three-wire system. The proposed system not only can control the active power flow, but also mitigate load unbalance, harmonics, and manage reactive power. The power quality compensator can simultaneously perform the two functions of power control station and active power filter or either one. To make the system more flexible, the grid current, instead of output current of the converter, is shaped to be a sinusoidal current in phase with the grid voltage. The reference currents are generated by multiplying the voltage loop controller output by the corresponding grid voltage waveforms. Consequently, harmonic analysis or dq transformation, as required in the conventional design, is not necessary. In this manner, controlling the dc voltage level is equivalent to performing an active power balance for the whole system. As a result, no additional hardware and interface are required. Experimental results verified the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper deals with the compensation of magnetic saturation in line frequency transformer coupled, grid-connected power converters. The developed strategy originates from the choice of controlling the actual current injected in the grid (output of the line frequency transformer) instead of the input current of the transformer, as usually proposed. As a matter of fact, better power quality cannot neglect the direct control of the injected grid current. An effective solution providing zero steady-state error at grid frequency and superior disturbance rejection involves the use of a d– q current controller synchronous with the grid voltage. For this reason, the proposed saturation compensation strategy has been implemented inside the d –q reference frame. The downside is that closing the current feedback at the transformer output renders impossible to control the dc current at transformer input, leading to core saturation. A core saturation compensation strategy, based only on the monitoring of the output current distortion around voltage zero crossings, is here presented. Moreover, since the proposed solution does not rely on dc measures, offset problems are avoided. Experimental results confirm the effectiveness of the proposed control architecture and of the strategy adopted to avoid transformer core saturation. [ABSTRACT FROM PUBLISHER]

Regarding doubly fed induction generator (DFIG) operation, unbalanced and harmonically distorted grid voltage conditions have been treated as two separate control problems. This paper reports a solution for the rotor- and grid-side power converters, which allows one to keep the DFIG successfully in operation under both grid voltage conditions. The proposed solution is based on sliding-mode control (SMC). The rotor-side converter is commanded so that the electromagnetic torque and the stator reactive power remain free of fluctuations that arise during grid voltage disturbances. Meanwhile, the grid-side converter ensures both constant DC-link voltage and steady active power output from the overall system. The developed algorithms turn out being robust against parameter variations and of fast dynamic response. In addition, none of the converters need either voltage or current positive and negative sequences extraction. The simulation results presented demonstrate the appropriateness of SMC to face such disturbed scenarios. Finally, the stability proof of both converters’ control algorithms is provided in the appendices. [ABSTRACT FROM PUBLISHER]

This paper proposes an improved direct power control (DPC) strategy of grid-connected wind-turbine-driven doubly fed induction generators (DFIGs) when the grid voltage is unbalanced. The DPC scheme is based on the sliding mode control (SMC) approach, which directly regulates the instantaneous active and reactive powers in the stator stationary reference frame without the requirement of either synchronous coordinate transformation or phase angle tracking of grid voltage. The behavior of DFIGs by the conventional SMC–DPC, which takes no negative-sequence voltage into consideration, is analyzed under unbalanced grid voltage conditions. A novel power compensation method is proposed for the SMC-based DPC during network unbalance to achieve three selective control targets, i.e., obtaining sinusoidal and symmetrical stator current, removing stator interchanging reactive power ripples and canceling stator output active power oscillations, respectively. The active and reactive power compensation components are calculated via a simple method and the proposed three control targets can be achieved, respectively, without the need of extracting negative-sequence stator current components. Experimental results on a 2 kW DFIG prototype are presented to verify the correctness and validity of the proposed control strategy and power compensation method. [ABSTRACT FROM PUBLISHER]

This paper addresses a detailed design of a wind power plant and turbine slope voltage control in the presence of communication delays for a wide short-circuit ratio range operation. The implemented voltage control scheme is based upon the secondary voltage control concept, which offers fast response to grid disturbances, despite the communication delays, i.e., this concept is based on a primary voltage control, located in the wind turbine, which follows an external voltage reference sent by a central controller, called secondary voltage control, which is controlling the voltage at the point of connection with the grid. The performance has been tested using PSCAD/EMTDC program. The plant layout used in the simulations is based on an installed wind power plant, composed of 23 doubly fed generator wind turbines. The resulting performance is evaluated using a compilation of grid code voltage control requirements. The results show that fast response to grid disturbances can be achieved using the secondary voltage control scheme, and the fulfillment of the design requirements can be extended for a wide range of short-circuit ratios. [ABSTRACT FROM AUTHOR]