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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]

Partial shading condition drastically reduces the maximum power output of photovoltaic array. Partial shading occurs due to several factors, such as flying birds, trees, and passing clouds. Many ways can be mitigated partial shading problems in photovoltaic (PV) array. One among the way is reconfiguration techniques, namely reconfigure the location of PV modules in PV array based on irradiance levels in order to distribute shading effects and increasing maximum power. This paper proposed an optimal SuDoKu reconfiguration pattern for $9\times 9$ total-cross-tied (TCT) PV array to improve maximum power under partial shading conditions. In this approach, the physical location of PV modules in TCT array are rearranged based on optimal SuDoKu style without altering the electrical connections, so that the shading effects can distribute over the array. Further, the performance of proposed pattern investigated with existing SuDoKu pattern under different shading conditions by comparing the global maximum power point, mismatch losses, fill-factor, and efficiency using MATLAB-Simulink. Based on the results of this paper, it concluded that the proposed optimal SuDoKu reconfiguration arrangement is reducing the wiring arrangement and increasing the shading distribution over the array as compared to SuDoKu arrangement. [ABSTRACT FROM AUTHOR]

This paper presents a high-gain modular input-parallel output-series dc-dc converter for photovoltaic integration with medium-voltage dc grids. A new full-bridge zero-current-switching (FB-ZCS) converter proposed in this paper is used as a fundamental building block. Existing FB-ZCS converters utilize transformer leakage inductance and a shunt-connected resonant capacitor for achieving smooth resonant current commutation and ZCS. The shunt-connected capacitor requires dedicated charging interval in every switching cycle and stores full-load-rated resonant energy at all load/input conditions, resulting in significant duty-cycle loss. Moreover, it suffers from energy leakage through transformer winding and requires overrating. All these factors adversely impact the operation range of the converter, which is important for achieving a wide maximum power point tracking range. Novel improvements in the proposed converter include minimization of duty-cycle loss and resonant capacitor leakage and the realization of the current-adaptive resonant energy storage mechanism, without using additional switches or auxiliary circuits. Operation of the proposed converter is presented and validated with simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a novel maximum output power point tracking (MOPPT) strategy for a three-phase dual-active bridge (DAB3) converter to integrate a large-scale photovoltaic (PV) energy generation into medium-voltage dc grids. The DAB3 converter is employed as a single-stage solution to step up dc voltage levels and perform the maximum power point tracking of the PV farm at the same time. In order to achieve a high weighted average efficiency over a wide operation range, design considerations of the stray inductance and transformer's turns ratio are presented to shape the soft-switching region of the DAB3 converter in order to match the area of the maximum power points for the PV panels in the given environmental conditions. To further maximize the energy yield of the entire PV power generation system, a novel MOPPT method is proposed to track the maximum output power point of the DAB3 converter. The MOPPT is implemented by a cascaded closed-loop voltage control with the PV current feedforward compensation to improve the dynamic performance. The effectiveness of the proposed control strategy is validated by both simulations and the experimental verification of a down-scaled laboratory prototype. [ABSTRACT FROM AUTHOR]

In this paper, an improved floating interleaved Boost converter is proposed for fuel cell applications. In order to achieve zero-ripple input current and keeping others merits such as high voltage gain and low voltage stress of the conventional floating interleaved Boost converter (FIBC) unchanged, a pair of input current ripple cancellation networks composed of a small inductor and a small capacitor are added into the conventional FIBC. Then, the operating principle and the steady-state and dynamic performances of the proposed converter are performed and compared with the conventional FIBC. Finally, a 240 W experimental prototype has been created for further evaluating the expected performances of the proposed converter. The simulation and experimental results demonstrate that the input current ripple of the proposed converter can be totally removed in all power range without significantly increasing the current stress and power loss. [ABSTRACT FROM AUTHOR]

This paper deals with the design of a converter-machine system considering a set of operating points (speed, torque) composing a routine cycle. From the 1-D analytical modelling of the Permanent Magnet Synchronous Machine (PMSM) and the power electronic converter, a design methodology minimizing the power losses-to-volume ratio is established where the flux weakening control structure is considered within the system sizing. Based on a deterministic approach developed for PMSM sizing only, we propose a methodology to include the converter losses in the machine design process. Hence, we demonstrate how the computing time of calculation can be significantly reduced limiting the calculation of the flux weakening mode to a few critical power points. This method is applied to a liquid piston mechanism as part of a Compressed Air Energy Storage (CAES) system. We then validate the converter-machine design method from a 2-D analysis applied to the most significant points. Finally, the PMSM design from both methodologies with and without considering the converter losses are compared. Results highlight how the converter affects the machine sizing and the distribution of power losses to reduce the total losses of the system. [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 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]

More than 170 countries have already established renewable energy targets to meet their national increasing energy demand and also to keep their environment sustainable. Due to a number of features, the use of the multi-megawatt solar photovoltaic (PV) power plants is becoming the preferred choice for escalating and updating the power systems all over the world. Moreover, the solar PV power plant is also the first choice for meeting rapidly growing demands; as it can be installed relatively quickly, say in 6–12 months, compared to that of the fossil-fuel-based plants that may require more than 4–5 years. The traditional low-voltage (288–690 V) converter-based system requires a step-up transformer and a line filter to interconnect a solar PV power plant with medium-voltage grids. Recently, the use of medium-voltage converters without a step-up transformer and a line filter has become more attractive for direct medium-voltage grid integration of solar PV power plants. This paper aims to review the necessity and the technical challenges in developing medium-voltage power electronic converters, including the converter circuit topologies and control techniques used in the development of medium-voltage converters to interconnect solar PV power plants to medium-voltage grids directly. In this paper, a comprehensive review of the current research activities and the possible future directions of research to develop medium-voltage converter technologies to provide for a cost-effective grid integration of solar PV power plants are presented. [ABSTRACT FROM AUTHOR]

A high step-up DC–DC converter with active switched LC-network is proposed in this paper, which can be used in the photovoltaic system as the front-end stage to generate the required DC bus voltage. Based on the transformerless DC–DC converter with an active switched inductor (ASL) network, this proposed converter introduces an active switched capacitor (ASC) network, where only one capacitor and one diode are added, while the voltage gain is efficiently increased by a compound of ASL and ASC networks. The structure of the proposed converter is simple and voltage stresses on the additional diode and capacitor are low. In addition, unlike other voltage-boosting structures that utilized switched capacitor, the proposed converter avoids the extreme instantaneous currents of capacitor. The detailed analysis of the proposed converter and performance comparison with recently published ASL network-based converters are presented. Finally, a 100 W, 20/200 V prototype circuit is operated to verify its performance. [ABSTRACT FROM AUTHOR]

An enhanced DC–DC converter is proposed in this paper, based on the combination of the Cuk and SEPIC converters, which is well-suited for solar photovoltaic (PV) applications. The converter uses only one switch (which is ground-referenced, so simple gate drive circuitry may be used), yet provides dual outputs in the form of a bipolar DC bus. The bipolar output from the DC–DC converter is able to send power to the grid via any inverter with a unipolar or bipolar DC input, and leakage currents can be eliminated if the latter type is used without using lossy DC capacitors in the load current loop. The proposed converter uses integrated magnetics cores to couple the input and output inductors, which significantly reduces the input current ripple and hence greatly improves the power extracted from the solar PV system. The design methodology along with simulation, experimental waveforms, and efficiency measurements of a 4-kW DC–DC converter are presented to prove the concept of the proposed converter. Furthermore, a 1-kW inverter is also developed to demonstrate the converter's grid-connection potential. [ABSTRACT FROM AUTHOR]

This paper presents system architecture and control scheme of a photovoltaic (PV) string inverter allowing seamless battery integration with the dc-series integration method. The architecture uses the partial-power processing universal dc–dc optimizer to have flexible power control by regulating the T-node compensation current. The universal optimizer is configured with a dual active bridge dc–dc converter allowing bidirectional power flow and galvanic isolation. Three operation modes are proposed depending on the input sources connected. Based on the operation mode, the universal optimizer and a dc–ac inverter switch their control reference to deliver the maximum PV power and perform battery current control. The steady-state and transient operation of the closed-loop control scheme have been demonstrated through controller hardware-in-the-loop tests to verify feasibility and effectiveness of the PV-battery series inverter architecture. [ABSTRACT FROM AUTHOR]

In this paper, some issues related to the impedance-based stability criterion, such as RHP poles in the minor-loop gain, how to specify source and load impedances, and node-dependent stability margins are discussed and the vector fitting method is used to improve the method. This results in an eigenvalue analysis of the system instead of using the Nyquist diagram, and leads to obtain more information about the system. Furthermore, a participation factor analysis can be used to identify the problematic subsystem. The proposed method is examined in the assessment of a stable and an unstable system, and simulation results are used for verification. [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]

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]

Multiphase (more than three-phase electric power) electric drive system is the focus of a significant research in the last decade. Multiphase power transmission system is also investigated in the literature because multiphase transformers are needed at the input of rectifiers. In the multiphase power transmission and multiphase rectifier systems, the number of phases investigated is a multiple of three. However, the variable speed multiphase drive system considered in the literature are mostly of five, seven, nine, eleven, twelve, and fifteen phases. Such multiphase drive systems are invariably supplied from power electronic converters. In contrast, this paper proposes technique to obtain seven-phase output from three-phase supply system using special and novel transformer connections. Thus, with the proposed technique, a pure seven-phase sine-wave voltage/current is obtained, which can be used for motor testing purposes. In addition, a seven-phase power transmission and rectifier system may benefit from the proposed connection scheme. Complete design and testing of the proposed solution is presented. Analytical analysis, simulation, and experimental verifications are presented in the paper. [ABSTRACT FROM AUTHOR]

A load-commutated inverter (LCI) fed synchronous motor (SM) is operated as an adjustable speed drive (ASD) in high-power applications. These drives are known as commutatorless motor (CLM) drives and posses many promising features like high efficiency, economic operation, and flexibility of control in high-power ratings. The CLM drives are used in compressors, blowers, fans, pumps, and mill drives for a range of industries as mining, water treatment plants, chemical, paper, textile, cement, rolling mills, and petrochemical plants. However, the power quality (PQ) problems at ac mains have been the concerns in these drives as the LCI has front-end thyristor converter injecting harmonics in the supply. This paper investigates various topologies for the mitigation of PQ problems in LCI-fed SM drives using multipulse ac-dc converters. A set of hybrid topologies is proposed, which use a combination of a passive filter with a multipulse converter to feed CLM. A basis for selection of a suitable ac-dc converter is presented for PQ improvement at the input mains of the LCI-fed SM drives. [ABSTRACT FROM AUTHOR]

This paper presents a direct frequency control for offshore wind farms (OWFs) connected to the diode-rectifier-based (DR-based) high voltage direct current (HVDC) link. The frequency control is guaranteed through reactive power balance at the DR station without a capacitor bank placed at the DR station; while AC-voltage magnitude is clamped by the DR, provided that the HVDC link voltage is imposed by the onshore inverter. The control system is implemented by a voltage source converter (VSC) connected to the DR station. The VSC is also used to compensate harmonic currents, leading to passive AC-filters being also removed from the DR station. The proposed control system provides comparative advantages in terms of DR station volume and OWF installation cost with other centralized controls where besides a VSC, a high-voltage large capacitor bank is needed to be placed at the DR station. A new average-value model (AVM) of the DR system is derived to test the proposed frequency control, while the stability of the system is evaluated through a small-signal analysis. The proposed control system performance and the accuracy of the proposed AVM are verified through a detailed switching model of the system using operation of a cascaded H-bridge VSC. [ABSTRACT FROM AUTHOR]

A new class of electronically commutated brushless motor, namely, the flux-switching (FS) motor is gradually emerging in power tools and household appliances. This motor offers advantages of both high-power density and relatively high efficiency. There are a number of papers introducing this new class of motor. However, almost all of these papers focused on the analysis, and not the design, of the FS motor. Thus, this paper presents the design and analysis of the FS motor. Design equations are analytically derived for initial calculations of the main dimensions, number of turns, and inductances of the FS motor. Furthermore, a comprehensive time-stepping finite element (TSFE)-behavioral model is developed and utilized for detailed analysis and design refinements of a prototype 8/4-pole FS motor. Finally, test results of the prototype motor are provided to verify the design equations and efficacy of the design methodology. [ABSTRACT FROM AUTHOR]

This paper presents a decentralized current control approach for a nine-phase wind turbine generator. This type of generator has three different three-phase stators sharing the same machine yoke and connected to the grid by means of three different voltage source back-to-back power converters. Due to the machine configuration, magnetic couplings are present between the three stators, complicating the design and implementation of the machine current controllers. Rather than a centralized control approach, this paper proposes a methodology to design a decentralized machine control to regulate the active and reactive power flowing through each stator independently. A complete dynamic analysis is performed in order to design the controller to reduce the coupling effects within the machine, while ensuring a proper dynamic performance. The control strategy is validated through simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a photovoltaic (PV) inverter architecture composed of stackable dc to three-phase ac converter blocks. Several such blocks, each containing a converter power stage and controls, are connected in series on their ac sides to obtain transformerless medium-voltage ac interfaces for PV power plants. The series-connected structure is made possible by a quadruple active bridge dc–dc converter that provides isolation between the PV input and each of the three ac-side phases within each block. Furthermore, since incoming PV power is transferred as constant balanced three-phase ac power, instantaneous input–output power balance bypasses the need for bulk energy storage. To streamline implementation and maximize system scalability and resilience, decentralized block-level controllers accomplish dc-link voltage regulation, maximum power point tracking, and ac-side power sharing without centralized means. The proposed architecture is validated by simulations of a PV string to medium-voltage ac system consisting of six blocks and on a proof-of-concept hardware prototype that consists of three cascaded converter blocks. [ABSTRACT FROM AUTHOR]

A dual-input high step-up DC/DC converter with zero voltage turn-off (ZVT) auxiliary circuit is presented in this paper. The cost of whole photovoltaic (PV) power could be decreased significantly by the proposed converter because it allows two PV panels to be connected at the same time. ZVT auxiliary circuit provides soft switching in a wide range of load variations with six passive components. Turn-off switching losses could be decreased and the efficiency of the whole converter could be improved. In , the operation principle and performance characteristics of the proposed converter have been analyzed and an 800 W experimental prototype has been built to validate the analytical results in. In , a dual-input maximum power point tracking (DMPPT) control algorithm has been designed for the proposed converter. Experimental results show that two PV panels can achieve the maximum power output state separately by the proposed DMPPT control algorithm. [ABSTRACT FROM AUTHOR]

The non-inverting buck-boost converter has emerged as one of the most promising topologies for distributed maximum power point tracking applications. Pressure to reduce cost and volume of photovoltaic module integrated converters often compels bootstrap capacitor type gate drivers. The bootstrap technique is a simple and economical solution to create a floating power supply for high-side gate drivers. However, it comes with a major challenge, as the low-side switch must be periodically turned on, to recharge the high-side bootstrap capacitor. In the buck and the boost mode, one of the switching legs of the converter must be held with the high-side switch on, which eventually discharges the high-side capacitor. When the non-inverting buck-boost input voltage is regulated by means of a single voltage loop, recharging the bootstrap capacitor causes an undesired ripple in the PV module voltage, degrading the regulation quality. In order to mitigate this problem, this paper proposes a cascaded control technique, entailing regulation of the PV module voltage and inductor current. The cascaded controller dramatically improves the rejection of the disturbance induced by the periodic recharge of the bootstrap capacitor, thanks to the fast action of the current loop, regulating the average value of the inductor current. Converter transfer functions and controller design are derived and validated. Experimental results clearly show that the cascaded control scheme outperforms the traditional voltage control scheme, resulting in a better regulation of the PV module voltage. This main advantage comes at the cost of an additional measurement of the average inductor current. [ABSTRACT FROM AUTHOR]

This paper presents a novel converter topology for six-phase switched reluctance motor (SRM) drives, which reduces the number of switches and diodes by half, compared with the conventional asymmetric half-bridge converter, but needs no additional energy storage component. A dynamic model of a six-phase SRM is developed in the MATLAB/SIMULINK environment and conventional current chopping and angle position control techniques are applied to the proposed converter, demonstrating successful operation across the full speed range with modified conventional control techniques, lower converter losses, and higher system efficiency compared with the asymmetric half-bridge converter. Experimental tests comparing two versions of the proposed converter with an asymmetric half-bridge are described and verify the predictions of the simulations. [ABSTRACT FROM AUTHOR]

Resonant switched-capacitor converters (SCCs) are being increasingly used in energy conversion systems due to their low cost, low weight, and high efficiency. The zero-current switching (ZCS) feature of an SCC, which can be achieved at a switching frequency below the resonant frequency, not only results in soft switching and, hence, low switching losses, but also reduces the electromagnetic interference caused by the converter. This paper proposes a cascaded step-down SCC (CSD-SCC) with an arbitrary number of converter stages and an exponential output-to-input voltage ratio. This SCC reduces the number of required converter stages for a low voltage conversion ratio and has a low ripple in the output voltage. A mathematical study is proposed to analyze the operating modes of this generalized CSD-SCC under ZCS operation. This study also enables the calculation of the output voltage, the output voltage ripple, and the efficiency for different switching frequencies and loads. The proposed analysis is validated via several simulation and experimental case studies, including a photovoltaic energy conversion system. The experimental CSD-SCC has two converter stages with an efficiency higher than  $\text{95}\%$. [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 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]

Wind turbine power converter system (WTPCS) is a crucial device in a wind energy conversion system. This paper presents the new failure rate models and a reliability evaluation technique for the (WTPCS) considering effects of wind speeds, which can be named by the multistate probability analysis method. The case studies on the WTPCS of a 2 MW wind turbine with a permanent magnet synchronous generator are conducted using wind speed data at Lauwersoog and Valkenburg wind sites in Holland. The results indicate that reliability of the WTPCS is affected significantly by variations of wind speed and the rated wind speed of wind turbine. The effectiveness of the proposed method is demonstrated by examples. [ABSTRACT FROM AUTHOR]

Open-end winding three-phase variable speed drives with dual-inverter supply have been extensively investigated for various applications, including series hybrid powertrains and propulsion motors. The topology is simple to realize while offering a higher number of switching states without the need for capacitor voltage balancing algorithms, when compared to the standard multilevel converters. This paper extends the open-end winding concept to a five-phase drive. A relatively simple space-vector modulation (SVM) algorithm, based on the already well-understood five-phase two-level drive SVM method, is developed. The proposed modulation technique has the advantage of being straightforward to implement and, like its two-level counterpart, is able to generate output voltages with minimum low-order harmonic content. The method generates up to 17-level output phase voltage and, therefore, offers superior harmonic performance when compared to the two-level five-phase modulation. The developed scheme is verified via detailed simulations and experiments, using a five-phase induction machine operating under open-loop V/f control. [ABSTRACT FROM AUTHOR]

This paper presents a robust natural-frame-based interfacing scheme for grid-connected distributed generation inverters. The control scheme consists of a dead-beat line-voltage sensorless natural-frame current controller, adaptive neural network (NN)-based disturbance estimator, and robust sensorless synchronization loop. The estimated uncertainty dynamics provide the necessary energy shaping in the inverter control voltage to attenuate grid-voltage disturbances and other voltage disturbances caused by interfacing parameter variation. In addition, the predictive nature of the estimator has the necessary phase advance to compensate for system delays. The self-learning feature of the NN adaptation algorithm allows feasible and easy adaptation design at different grid disturbances and operating conditions. The fact that converter synchronization is based on the fundamental grid-voltage facilitates the use of the estimated uncertainty to extract the position of the fundamental grid-voltage vector without using voltage sensors. Theoretical analysis and comparative evaluation results are presented to demonstrate the effectiveness of the proposed control scheme. [ABSTRACT FROM PUBLISHER]

In the field of wind energy, electrical generator solutions have converged on a small number of technologies for specific technical and economic reasons. This paper investigates whether a similar rationale exists within the field of oscillating water column, wave energy converters. The suitability or otherwise of the various generator options in the offshore marine environment is examined in detail. Each generator configuration is then modeled and the annual energy output for a typical wave climate assessed with consideration being given to the impact of speed control on the energy output. Finally, some conclusions are drawn regarding the relative suitability of the different generator options. [ABSTRACT FROM AUTHOR]

This paper introduces a new solution for doubly fed induction generators to stay connected to the grid during voltage sags. The main idea is to increase the stator voltage to a level that creates the required flux to keep the rotor side converter current below its transient rating. To accomplish this goal, a series compensator is added to inject voltage in series to the stator side line. The series converter monitors the grid voltage and provides compensation accordingly to accomplish this aim. Since the turbine and converter stay connected, the synchronization of operation remains established during and after the fault and normal operation can be resumed immediately after the fault is cleared. To keep the current at its minimum, a control strategy has been developed to keep the injected voltage and line voltage in phase during and after the fault. [ABSTRACT FROM PUBLISHER]

With the increasing penetration of wind turbines (WTs) grid utilities require extended reactive power supply capability not only during voltage dips but also in steady-state operation. WTs with doubly fed induction generators (DFIG) are able to control active and reactive power independently. The reactive power capability is subject to several limitations resulting from the voltage, current, and speed, which change with the operating point. This paper discusses the steady-state reactive power loading capability of DFIG-based WTs by taking into account the most important physical phenomena restricting the reactive power supply of DFIG-based WT systems. The active–reactive power diagram is systematically derived by considering the typical power–speed relationship and converter loading limits. The authors discuss also some special operating modes limiting the reactive power capability together with aspects of modeling and control that give rise to these limitations. [ABSTRACT FROM PUBLISHER]

Design and development process of solar cells can be greatly enhanced by using accurate models that can predict their behavior accurately. Recently, there has been a surge in research efforts in multijunction (MJ) solar cells to improve the conversion efficiency. Modeling of MJ solar cells poses greater challenges because their characteristics depend on the complex photovoltaic phenomena and properties of the materials used. Currently, several commercial complex device modeling software packages, e.g., ATLAS, are available. But these software packages have limitations in predicting the behavior of MJ solar cells because of several assumptions made on the physical properties and complex interactions. Artificial neural networks have the ability to effectively model any nonlinear system with complex mapping between its input and output spaces. In this paper, we proposed a novel Chebyshev neural network (ChNN) to model a dual-junction (DJ) GaInP/GaAs solar cell. Using the ChNN, we have modeled the tunnel junction characteristics and developed models to predict the external quantum efficiency, and I–V characteristics both at one sun and at dark levels. We have shown that the ChNN-based models perform better than the commercial software, ATLAS, in predicting the DJ solar cell characteristics. [ABSTRACT FROM PUBLISHER]

This paper develops a coordination control strategy for wind farms with line commutated converter-based HVdc delivery for participating in inertial response and primary frequency control. The coordination philosophy is to feedback the grid frequency and its derivative and adjust the delivery power of the HVdc link according to the feedback signals. The feedback loop employing the derivative of the grid frequency aims to improve the inertial response while the feedback loop employing the grid frequency deviation introduces a droop at the rectifier control loop. When the grid frequency is too high or too low, active power flow through the HVdc link will be ramped down or up. In turn, the wind generation will increase or decrease the blade angles to reduce or increase the captured wind power through pitch control. A case study demonstrates the effectiveness of the inertial enhancement and frequency droop in HVdc control. Simulation results in transient security assessment tool are given. [ABSTRACT FROM AUTHOR]

This paper presents an innovative control law for distributed dc generation supplied by a fuel cell (FC) (main source) and supercapacitor (auxiliary source). This kind of system is a multiconverter structure and exhibits nonlinear behavior. The operation of a multiconverter structure can lead to interactions between the controls of the converters if they are designed separately. Typically, interactions between converters are studied using impedance criteria to investigate the stability of cascaded systems. In this paper, a nonlinear control algorithm based on the flatness properties of the system is proposed. Flatness provides a convenient framework for meeting a number of performance specifications for the hybrid power source. Using the flatness property, we propose simple solutions to hybrid energy management and stabilization problems. The design controller parameters are autonomous of the operating point; moreover, interactions between converters are taken into account by the controllers, and high dynamics in disturbance rejection is achieved. To validate the proposed method, a hardware system is realized with analog circuits, and digital estimation is accomplished with a dSPACE controller. Experimental results with small-scale devices (a polymer electrolyte membrane FC of 1200 W, 46 A and a supercapacitor module of 100 F, 500 A, and 32 V) in a laboratory corroborate the excellent control scheme during a motor-drive cycle. [ABSTRACT FROM AUTHOR]

Doubly fed induction generator (DFIG) technology is the dominant technology in the growing global market for wind power generation, due to the combination of variable-speed operation and a cost-effective partially rated power converter. However, the DFIG is sensitive to dips in supply voltage and without specific protection to “ride-through” grid faults, a DFIG risks damage to its power converter due to overcurrent and/or overvoltage. Conventional converter protection via a sustained period of rotor-crowbar closed circuit leads to poor power output and sustained suppression of the stator voltages. A new minimum-threshold rotor-crowbar method is presented in this paper, improving fault response by reducing crowbar application periods to 11–16 ms, successfully diverting transient overcurrents, and restoring good power control within 45 ms of both fault initiation and clearance, thus enabling the DFIG to meet grid-code fault-ride-through requirements. The new method is experimentally verified and evaluated using a 7.5-kW test facility. [ABSTRACT FROM PUBLISHER]

This paper investigates multiple single-phase converters for multiple single-phase motor drives from a dc-link voltage. The dc-link voltage is obtained from primary energy sources such as photovoltaic, battery, utility grid, or by a combination of these. This paper points out the best configuration for different scenarios. The proposed systems allow functions such as soft start, independent adjustment of motor speeds, or group of motors operating at different rated frequencies. Furthermore, a control technique has been proposed to maximize the utilization of the dc-link voltage and minimize shared leg currents. Several relevant characteristics of the converters are addressed, such as voltage capability, shared-leg currents, and power ratings. Experimental and simulated results are presented. [ABSTRACT FROM AUTHOR]

This paper presents a modulation and controller design method for paralleled Z-source inverter systems applicable for alternative energy sources like solar cells, fuel cells, or variable-speed wind turbines with front-end diode rectifiers. A modulation scheme is designed based on simple shoot-through principle with interleaved carriers to give enhanced ripple reduction in the system. Subsequently, a control method is proposed to equalize the amount of power injected by the inverters in the grid-connected mode and also to provide reliable supply to sensitive loads on- site in the islanding mode. The modulation and controlling methods are proposed to have modular independence so that redundancy, maintainability, and improved reliability of supply can be achieved. The performance of the proposed paralleled Z-source inverter configuration is validated with simulations carried out using Matlab/Simulink/Powersim. Moreover, a prototype is built in the laboratory to obtain the experimental verifications. [ABSTRACT FROM AUTHOR]

This paper presents a new sensorless vector control system for high performance induction motor drives fed by a ma- trix converter with nonlinearity compensation and disturbance observer. The nonlinear voltage distortion that is caused by commutation delay and on-state voltage drop in switching device is corrected by a new matrix converter modeling. The lumped disturbances such as parameter variation and load disturbance of the system are estimated by the radial basis function network (RBFN). An adaptive observer is also employed to bring better responses at the low speed operation. Experimental results are shown to illustrate the performance of the proposed system. [ABSTRACT FROM AUTHOR]

This paper presents a novel dual-mode time-sharing cascaded sinusoidal inverter. It is composed of a time-sharing buck-type current-fed converter and time-sharing sinusoidal pulse-modulated full-bridge inverter. Compared with the conventional time-sharing cascaded sinusoidal inverter, it has higher power density and better control character. [ABSTRACT FROM AUTHOR]

Due to the high integration of renewable energy, the power systems may become more susceptible to unbalanced grid voltages, grid frequency disturbances, harmonic resonances and others. Therefore, several works have pointed out that the ancillary services provided by the energy storage systems can minimize these issues. The combination of energy storage and static synchronous compensators (ES-STATCOM) has been proposed to reduce the cost. Modular multilevel converters (MMC) are featured as a good solution for ES-STATCOM realization, due to their high efficiency, modularity and inherent fault-tolerant structure. However, the study of the integration of the MMC based ES-STATCOM in a complex power system composed of power electronic converters, transmission system, renewable plants, transformers, etc, leads to a high computational burden. Therefore, this work presents a detailed design and implementation of three efficient simulation models of the MMC ES-STATCOM. The performance of these models compared during a grid frequency support case study by a 100 MVA/33 kV ES-STATCOM. In addition, the performance of these models is evaluated during a battery charging process and a grid symmetrical fault. The models are compared through the following aspects: processing time, normalized integral of absolute error, state-of-charge estimation and active and reactive power dynamic response. [ABSTRACT FROM AUTHOR]

Direct methanol fuel cells (DMFC) have been widely researched for applications in portable electronics due to their use of liquid fuel for easy storage and transportation compared to gaseous hydrogen. However, DMFC’s performance is strongly affected by methanol crossover that significantly degrades the energy efficiency at low output power, and is characterized by an increasing efficiency at increasing output power. The maximum efficiency point (MEP) is inherently difficult to track due to the commonly unknown methanol crossover rate, but since it is experimentally found to be located very close to the maximum power point (MPP), it is suggested that the MPP can be practically viewed as the quasi-MEP and an alternative tracking approach based on the MPP is proposed for the purpose of MEP tracking. In this paper, a fuel-cell-oriented MPP tracking algorithm based on resistance matching is developed, implemented, and tested in the context of a DMFC/supercapacitor hybrid energy system. To account for the generally slow fuel cell dynamics, the DMFC is constantly tracked at the MPP, while any surplus or deficit power is absorbed or delivered by the supercapacitor bank. The detailed formulation of the algorithm and the power flow design and realization are also discussed. [ABSTRACT FROM AUTHOR]

An ac/dc/ac power converter is an important device used to extract power from variable speed permanent magnet wind generators and feed it into the grid. This paper describes how these converters incorporate maximum power point tracking based on its power feed to the grid at different wind speeds. Using the permanent magnet generator voltage, grid current, and grid voltage samples, the proposed system achieves an enhanced dynamic behavior. This feature effectively prevents the grid from “boost” charging the dc side of the H-bridge inverter at the start of operation. Since small wind turbines normally do not have expensive pitch control mechanisms, a thyristor-based “dump-load circuit” is employed to protect the turbine from high wind speed operation when disconnected from the grid. The thyristor controller also protects the inverter from high dc voltage input from the wind generator at high wind speed. Preliminary results are included using a laboratory 2-kW prototype converter. [ABSTRACT FROM PUBLISHER]

Superconducting magnetic energy storage (SMES) systems are getting increasing interest in applications of power flow stabilization and control in the transmission network level. This trend is mainly supported by the rising integration of large-scale renewable energy power plants into the high-power utility system and by major features of SMES units. In a SMES system, the power conditioning system (PCS) is the crucial component for controlling the power exchange between the superconducting coil and the ac system. The dynamics of the PCS directly influences the validity of the SMES in the dynamic control of the power system. This paper describes a novel PCS scheme of SMES to simultaneously perform both active and reactive power flow controls. Moreover, a detailed model of the SMES unit is derived and a three-level control scheme is designed, comprising a full decoupled current control strategy in the d–q reference frame with a novel controller to prevent PCS dc bus capacitors’ voltage drift/imbalance. The dynamic performances of the proposed systems are fully validated by computer simulation. [ABSTRACT FROM AUTHOR]

This paper details the transient operation of a wind energy conversion system (WECS) used simultaneously as an active filter and power generator. This study is intended to address the system response to two types of transient phenomena: voltage dips (fast transients) and wind speed variations (slow transients). The system response to voltage dips is governed by the electrical system dynamics and control method and results in the evaluation of the WECS low-voltage ride through capability. The study of the system response to wind speed variations requires a complete mechanical model to be included. Simulation results are presented for a typical WECS, and a discussion is carried out dealing with the generalization of the present work to other configurations. [ABSTRACT FROM AUTHOR]

This paper proposes a new converter protection method, primarily based on a series dynamic resistor (SDR) that avoids the doubly-fed induction generator (DFIG) control being disabled by crowbar protection during fault conditions. A combined converter protection scheme based on the proposed SDR and conventional crowbar is analyzed and discussed. The main protection advantages are due to the series topology when compared with crowbar and dc-chopper protection. Various fault overcurrent conditions (both symmetrical and asymmetrical) are analyzed and used to design the protection in detail, including the switching strategy and coordination with crowbar, and resistance value calculations. PSCAD/EMTDC simulation results show that the proposed method is advantageous for fault overcurrent protection, especially for asymmetrical faults, in which the traditional crowbar protection may malfunction. [ABSTRACT FROM AUTHOR]

This paper aims to calculate the optimal operation of offshore wind farms (OWF) working with wind turbines based on doubly fed induction generator (DFIG) technologies, and with a high-voltage dc transmission connection. The objective of the optimization problem is to maximize the active power output of the arrangement, while considering certain factors, i.e., the restrictions imposed by the available wind power, the capability curve of the DFIG, the dc-link model, and the operative conditions. The accomplishment of this aim implies setting, adjusting, and operating the system under study in order to produce a reliable and cost-efficient electric energy supply. A realistic simulation test case is performed to evaluate the proposed method, and the optimal operation analysis takes into account different wind speeds and high-voltage dc-link lengths. The results show the effectiveness of the proposed method, demonstrating the advantages of using the reactive control performed by the DFIG to manage the operational requirements of the dc link. [ABSTRACT FROM AUTHOR]

Vector control of the doubly fed induction generators (DFIGs) depends on accurate information from a stator voltage phase lock loop (PLL) and a rotor position encoder for the reference frame transformations required by theory. The effect of erroneous information due to noise picked up by commonly used hardware PLLs and incremental position encoders was analyzed in this paper. The analysis shows that erroneous information can give rise to destructively large currents due to: 1) the DFIG system wrongfully perceiving very large P and Q references; and 2) system instability. It is shown that instability does not arise from inside the inner current feedback loop, but from the outer feedback loop that regulates the complex power. Simulation and experimental test results validate the analysis. The experimental results are taken from a 1.5-MW DFIG intended for a wind turbine. A simple scheme to protect against the erroneous information has been tested successfully in a brief trial period. [ABSTRACT FROM AUTHOR]

This paper examines the possibility of providing reactive power support to the grid from wind farms (WFs) as a part of the ancillary service provisions. Detailed analysis of the WF capability curve is carried out considering maximum hourly variation of wind power from the forecasted value. Different cost components are identified, and subsequently, a generalized reactive power cost model is developed for wind turbine generators that can help the independent system operator (ISO) in managing the system and the grid efficiently. Apart from the fixed cost and the cost of loss components, a new method is proposed to calculate the opportunity cost component for a WF considering hourly wind variations. The Cigré 32-bus test system is used to demonstrate a case study showing the implementation of the developed model in short-term system operations. A finding is that higher wind speed prediction errors (a site with high degree of wind fluctuations) may lead to increased payments to the WFs for this service, mainly due to the increased lost opportunity cost (LOC) component. In a demonstrated case, it is found that 2340 $/h is paid to the WF as the LOC payment only, when the wind prediction error is 0.5 per unit (p.u.), whereas 54 $/h is the expected total payment to the WF when the prediction error is 0.2 p.u. for its reactive power service. [ABSTRACT FROM AUTHOR]