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Rotor position estimated with high-frequency (HF) voltage injection methods can be distorted by voltage errors due to inverter nonlinearities, motor resistance, and rotational voltage drops, etc. This paper proposes an improved HF square-wave voltage injection algorithm, which is robust to voltage errors without any compensations meanwhile has less fluctuation in the position estimation error. The average position estimation error is investigated based on the analysis of phase harmonic inductances, and deduced in the form of the phase shift of the second-order harmonic inductances to derive its relationship with the magnetic field distortion. Position estimation errors caused by higher order harmonic inductances and voltage harmonics generated by the SVPWM are also discussed. Both simulations and experiments are carried out based on a commercial PMSM to verify the superiority of the proposed method and the investigations in this paper. [ABSTRACT FROM AUTHOR]

Non-line-start permanent magnet synchronous machines (NLSPMSMs) are normally considered not able to operate stably under grid drive. However, this paper proposes an effective control strategy to achieve soft start and synchronous switching of NLSPMSMs from inverter to grid drives so that to extend the application field. The system electromechanical model during switching is deduced and analyzed numerically to verify the stability under grid drive. For the control strategy, model-based sliding-mode observer is adopted during soft start for low-cost position sensorless drive, and two digital phase lock loops are used to obtain the phases of grid and inverter voltages for phase tracking. Experiments are carried out on five NLSPMSMs with different rated powers and velocities, and test results show that all these prototypes can be safely switched to grid drive and robust to load variation. Tested waveforms are coincident with numerical analysis, and the system efficiencies of NLSPMSMs under inverter and grid drives are compared. [ABSTRACT FROM AUTHOR]

The papers in this special section are devoted to the topic of solutions for power quality management. Although the power quality problems, such as, reactive and harmonic currents, distortion and fluctuations in the supply voltage, are not new to power system and power electronics researchers, the ever changing landscapes in the power generation and distribution systems have given rise to additional power quality challenges. The growing interest in the smart grid concepts, vehicle to grid operation, inverter based large scale photovoltaic (PV) and wind integration, distributed generators (DGs) as ancillary service provider, multilevel inverters for high power applications and so on is making the power quality aspects more prevalent. [ABSTRACT FROM PUBLISHER]

Parallel operation is an effective way to improve the capacity of full power converter in permanent-magnet direct-drive wind power generation system. But it causes the zero-sequence circulating-current (ZSCC), which brings current discrepancy, current waveform distortion, power losses, and electromagnetic interference (EMI), etc. The paper proposes a new topology of full power converters which are composed of n full power converters. The n converters have the same structures and are in parallel with each other. Besides, the average models of the system are analyzed and the fundamental mechanism of ZSCC generation is given. Based on the above analysis, the control strategy of the system is designed. The simulation and experiment results of two parallel full power converters show the feasibility of the proposed theory and control scheme. [ABSTRACT FROM PUBLISHER]

Under weak grid condition, negative influence is brought in on system stability because the grid impedance and the grid-connected converter control system are coupled to each other. The key factor affecting the stability of the system is the perturbation of the voltage fluctuation at the point of common coupling (PCC) to the phase-locked loop (PLL) and the dc-link voltage loop. In this article, the impedance model of a three-phase grid-connected converter, including the current loop, PLL, and dc-link voltage loop in the dq frame, is adopted, because it includes coupling information of the converter and reflects the transferring relationship among grid-connected system variables. Based on the transferring relationship, this article analyzes the perturbation paths of PCC voltage and proposes the corresponding compensation methods, i.e., the PLL and dc-link voltage loop compensation terms, which are inserted into the q and d axes of the current loop controller, respectively. The impedance analysis results show that the proposed method can effectively improve the system stability. In addition, the experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

DC-link electrolytic capacitor critically affects the lifetime of the motor drive system. This paper proposes an inverter power control strategy based on dc-link voltage regulation for the electrolytic capacitor-less interior permanent magnet synchronous motor drive system. The dc-link electrolytic capacitor is replaced by a small film capacitor and the power factor correction circuit is also eliminated. Hence, the inverter power should be regulated effectively to achieve high input power factor and low grid current harmonics. The inverter power control loop based on proportional resonant (PR) controller is established to regulate the inverter power into sinusoidal wave. The PR controller can be designed to achieve a high gain at the desired frequency and the parameters are easy to design due to the simple structure. In addition, a power compensation method based on the dc-link voltage regulation is used to diminish the error of inverter power control, which does not depend on the precise calculation and is easy to realize. The effectiveness of the proposed method is demonstrated by the experimental results on an air conditioner. The input power factor can reach 0.992 and the harmonics of grid current are considerably lower than the requirement of EN61000-3-2 standard. [ABSTRACT FROM PUBLISHER]

The overmodulation strategy plays an important role in the field weakening control for high-speed permanent magnet synchronous motor (PMSM) drives. However, the voltage jumps between the output voltage vectors (OVVs) in high overmodulation regions result in the deterioration of both current performance and loading capability. In order to solve this problem, a novel overmodulation strategy based on the voltage vector space division is proposed. The overmodulation regions are divided by the amplitude and the phase of reference voltage vectors to guarantee the continuity of OVVs. When the overmodulation region changes, the voltage jump can be efficiently suppressed, which contributes to eliminating current ripples. In addition, the proposed parallel projection replaces the traditional center projection to increase the modulation index, which can also improve the fundamental voltage amplitude, and make a reduction of the field weakening current and its current overshoot. By improving the utilization of the dc bus voltage, the higher capability of the output torque can be achieved in the field weakening region. Finally, the effectiveness of the proposed strategy is verified on a high-speed surface-mounted PMSM platform by the experimental results. [ABSTRACT FROM AUTHOR]

High-frequency resonance (HFR) in the modular multilevel converter (MMC) has been reported in recent years frequently, yet, existing focuses on HFR are limited to the specific control strategy or grid condition, from which the conclusion may be not applicable if the control or grid changes. In this article, a unified MMC high-frequency impedance model (UHFIM) is proposed to fill this gap. By changing the gain of the model, UHFIM can reflect the high-frequency characteristics of the MMC under different control strategies in a unified form, which provides a theoretical basis for the general HFR suppression method. Analysis based on UHFIM reveals the worst cases of the HFR risk and the constraint of low-pass filter (LPF) order on the convergence of MMC high-frequency damping. In order to suppress HFR, this article proposes a joint design method for LPF and passive damper, which can completely remove the HFR risk from the entire high-frequency range without sacrificing too much dynamic performance of MMC, and it works no matter how the grid impedance varies. Besides, in the case that the damper cannot be installed, an alternative HFR suppression method based on delay time adjustment is also developed. Finally, the proposed unified model and the HFR suppression methods have been verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

Communication links, as a part of coordinated control in dc microgrids (MGs), play an essential role in achieving high operational performance. In this article, a bipolar topology based on series modular structure of dc MG is designed, and a complementary dual-window dc bus interacting (CDBI) method which combines droop control with antidroop control is proposed to improve the robustness of system and against communication failure. In the proposed CDBI method, distributed generations regard dc buses as common communication channels for information exchange, and the coordinated control within the dc MGs can be realized through predesigned sequences of voltage pulses without extra communication links. Finally, the proposed symmetrical bipolar structure and communication-free coordinated control scheme have been verified in various cases through rapid control prototype. The experimental results show that the bipolar-type dc MG controlled by the proposed CDBI method can supply high-quality power for a long time even in the case of communication failure. [ABSTRACT FROM AUTHOR]

The power density can be improved and the system cost can be saved in the reduced dc-link capacitance motor drive system. The line inductance can also be diminished to reduce the system volume further. In previous research works, the active damping control methods mainly concentrated on the drive system equipped with a larger line inductor. In this article, a dc-link voltage estimation feedback-based active damping method is proposed to suppress the grid current harmonics and improve the drive system stability with low line inductance. The dc-link voltage ripple caused by the LC resonance is analyzed to evaluate the direct dc-link voltage feedback-based active damping control method. To improve the damping performance, the resonant component of the dc-link voltage caused by the LC resonance could be obtained through the grid current to establish the feedback control loop. The sampling delay should be compensated to improve the control performance. The stability of the drive system is evaluated by the Nyquist plots of the impedance model, and the grid current is also analyzed, which could satisfy the harmonic requirements of IEC-61000-3-2. Experimental results are performed to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This article proposes a coordinate impedance compression strategy for high frequency inverter systems. Based on a two parallel inverter structure, the strategy adopts two methodologies: 1) a passive LC network to adjust the current and voltage seen by each inverter module, and 2) an active modulation of the inverter input voltage and output voltage phase to adjust the load current distribution. For the proposed strategy, the resistance and reactance can be compressed coordinately, which can help the inverter to operate in high efficiency load range. The Half-Bridge Class D inverter is taken as an example to be analyzed. A 10-MHz prototype is built and the experimental results verify the correctness and feasibility of the proposed strategy. [ABSTRACT FROM AUTHOR]

This letter proposes a switched capacitor (SC)-based continuously adjustable modular bidirectional dc–dc converter. The structure of the proposed converter consists of three types of modules, series–parallel SC converter module, regulating module, and resonant SC converters module. A basic configuration of three modules working in buck operating mode is analyzed in detail. The boost mode operation is symmetrical to the buck mode. The regulation module provides all regulation capability, and the other two modules operate at fixed duty cycle and provide the main conversion ratio. A prototype of the basic configuration was built to verify the theory. The peak efficiency of the prototype is 97.15%. [ABSTRACT FROM AUTHOR]

In meshed high-voltage direct current (HVdc) grid, the current flows through the lines cannot be controlled with sufficient freedom without additional power electronics-based devices, namely current flow controller (CFC). In this article, a novel hybrid modular interline CFC is proposed based on H-bridge submodules and thyristor valves. Due to its modularity, the proposed CFC is particularly suitable for applications requiring high voltage and large power capacity. The circuit structure, operation principle, and parameter design are presented. In addition, a control strategy is developed for the proposed CFC. Simulations are carried out using a four-terminal meshed HVdc grid to verify the effectiveness of the topology and its control strategy. A downscaled prototype is built, which further validates the proposed CFC. [ABSTRACT FROM AUTHOR]

The nonlinearity of the voltage source inverter (VSI) leads to the error between output voltage and reference voltage, and causes the phase current harmonics. It is necessary to compensate its influence in the fields such as parameter identification, sensorless control, and servo drives. In this article, an industrial solution for VSI nonlinear compensation method based on lookup table (LUT) is introduced. The characteristics of semiconductor devices, equivalent dead-time and influence of parasitic capacitance effects are calculated quantitatively though injecting increase and decrease current during self-commissioning process. The critical current value for parasitic capacitor and the variation of equivalent dead time are verified through different distorted voltage curves. Considering dc-link voltage variation is irresistible for industrial products, 3-D linear interpolation is adopted to compensate the variation of VSI nonlinearity under different voltage conditions. Proposed LUT method does not require VSI datasheet and complicated calculation formula during implementation. Experiments are carried out on a commercial permanent magnet synchronous motor drive system to verify the proposed LUT method. [ABSTRACT FROM AUTHOR]

The overmodulation-based nonlinear voltage extension could increase the maximum output torque of the induction motor (IM) in the field-weakening (FW) region. Since the voltage distortion increases with the deepening of overmodulation degree, the extreme nonlinear voltage extension (six-step voltage) is only worthwhile in maximum torque demand conditions. However, the conventional overmodulation-FW scheme always adopts the six-step voltage regardless of torque demand, leading to severe torque ripple. To solve this problem, the minimum-nonlinear-voltage (MNV) method is proposed to satisfy the torque demand with minimum overmodulation-voltage nonlinearity. The difference between the limiting and the feedback values of the torque current is used for suppressing the nonlinear voltage extension during a nonmaximum torque demand state. The quantitative relation between the torque ripple and torque demand of the MNV method is calculated and compared with that of the conventional six-step-FW method. Analysis shows that the MNV method can eliminate the torque ripples of low and medium torque demand conditions completely and partially, respectively. The effectiveness of the proposed method is verified experimentally on a 3.7 kW IM platform. [ABSTRACT FROM AUTHOR]

In this letter, a high-power step-up dc/dc converter dedicated to dc offshore wind farms is proposed, with the combination of half-bridge submodules, thyristors, diodes, and transformer. Compared with the other promising dc/dc topologies, the device numbers, costs, losses, and energy storage requirement are greatly reduced. The operation principle is introduced. The effectiveness and validity are confirmed by simulation and experimental results. [ABSTRACT FROM AUTHOR]

The overmodulation canimprove the utilization of dc-link voltage in the induction motor (IM) field-weakening (FW) control, thereby extending the maximum output torque. Nevertheless, the high modulation index (MI) will cause severe torque ripple. To figure out the relation between the torque ripple and MI, the amplitude of torque ripple with different MI is calculated quantitatively. Then, a torque quality evaluation function (TQEF) is designed to evaluate the torque quality in overmodulation-FW region by considering both fundamental torque extension and torque ripple suppression. On this basis, a modulation index optimization module is proposed for the maximum TQEF value realization. The analysis shows that the studied method could achieve the optimal trade-off between the fundamental torque extension and the torque ripple suppression. The TQEF value of the studied method is higher than that of the conventional six-step voltage method and the inscribed circle voltage method, meaning that the torque quality is optimized. Finally, the effectiveness of the studied method is verified experimentally on an advanced RISC machines (ARM)-based 3.7 kW IM platform. [ABSTRACT FROM AUTHOR]

Medium-voltage (MV) dc collection system is a promising solution for future integration of large-scale renewables at remote places. This requires a high-power dc–dc converter to step up the voltage and connect with the high voltage direct current transmission line. In this article, a novel modular high-power dc–dc converter is proposed, with the combination of half-bridge and full-bridge submodules (SMs), as well as thyristors and diodes. It alternatively configures the SMs into parallel connection with the MV side to effectively share high current stress while series connection with the high voltage side to share the high voltage stress. It presents attractive features such as high device utilization rate, low cost, high step-up ratio, high efficiency, and small footprint. The circuit structure, operation principle, and the control schemes are discussed. Simulation results demonstrate the effectiveness, and the experimental results from a scaled down prototype further prove the proposed concept. [ABSTRACT FROM AUTHOR]

In the transition process from the base-speed region to the field-weakening region of induction motor (IM), the voltage margin would suddenly disappear due to the limited inverter voltage. This insufficient voltage margin would deteriorate the current control of IM, causing severe voltage saturation and current ripple. To address this problem, a circular arc voltage trajectory method is proposed with the smooth transition-combined field-weakening controller. The voltage and current behavior are analyzed quantitatively during the transition process. The theoretical analysis shows that the studied circular arc trajectory can suppress the sudden decrease of the voltage increasing rate in the transition region, leading to the reduced voltage saturation and current ripple. The sufficient voltage margin and smooth current can be realized at the cost of < 10% voltage utilization ratio. The effectiveness of the studied method is verified by contrast experiments on an ARM-based 3.7-kW IM platform. [ABSTRACT FROM AUTHOR]

Bipolar LVdc distribution is a promising solution for better integration of renewables and enhancement of system efficiency and reliability, but it also faces the challenge of pole voltage imbalance. By simple modifications of the bidirectional CLLC dc/dc converter, this article presents a dc/dc converter with integrated pole voltage balance capability. The converter presents less number of semiconductors, smaller overall footprint, and lower power loss compared to the employment of additional pole voltage balancers. Operating principle of this dc/dc converter and its inherent pole voltage balancing capability are explained, while further control scheme is also developed to compensate the small pole voltage difference caused by the dead time effect. Simulation results with 150 kW/±375 V bipolar output demonstrate the effectiveness of the converter, and the experimental results from a 500 W/±50 V downscaled prototype further prove the proposed concept. [ABSTRACT FROM AUTHOR]

Simple but effective output voltage/current control schemes are essential for electric vehicle wireless charging systems (EVWCSs). This article proposes an EVWCS using interleaved boost converter (IBC) and flat solenoid coupler (FSC). The closed-loop control between the primary and secondary is realized by adjusting the duty cycle of the IBC. The currents and losses in the primary are reduced in the proposed system because the dc bus voltage in the primary is increased by using the IBC. The input current ripple is diminished owing to the interleaved structure. This article also proposes a parameter optimization method to obtain the optimal FSC. Without loss of generality, the total area of the transmitter (Tx) and receiver (Rx) is assumed to be constant, and the Tx is assumed to be larger than the Rx. A scale-down prototype whose output power is 500 W was built in the laboratory to verify the theoretical analysis. The output voltage of the prototype kept constant when the misalignment was 200 mm in the winding direction or 100 mm in the magnetic direction. A highest system efficiency, from dc input to dc output, of 90.1% was achieved when the power transfer distance was 170 mm. [ABSTRACT FROM AUTHOR]

The remarkable inductance nonlinearities of synchronous reluctance motors (SynRMs) could result in operating-point-dependent high-frequency (HF) current responses, and more HF losses with audible noises with conventional constant amplitude high-frequency signal injection (HFSI) methods applied. This article proposes an adaptive pseudorandom high-frequency square-wave voltage injection (AP-HFSVI) based sensorless control method for SynRM drives at low-speed operation. An adaptive HF current control strategy is applied using pseudorandom injection patterns to adapt to the nonlinear characteristic of SynRM inductances. The magnitude of the injected HF voltage signals can be adjusted adaptively with the load variation online via HF current adaptive control scheme, which processes and regulates the HF current components to be constant. Hence, the HF losses and audible noises can be effectively suppressed due to the decreasing magnitude of injected signals. The current power spectra density of the AP-HFSVI method is analyzed and compared with the conventional HFSI method. The effectiveness and superiority of the proposed sensorless control method are verified with experiments on a 3-kW SynRM drive platform. [ABSTRACT FROM AUTHOR]

In this article, an ac/dc converter based on the stacked boost circuit and dual-mode LLC circuit is proposed. In the power factor correction (PFC) part, two boost circuits are stacked to halve the input voltage and the inductor can be shared. The stacked boost circuit can achieve PFC and soft-switching characteristics with open-loop control with the complementary driving signal. In the dc/dc part, a dual-mode LLC circuit is adopted based on two stacked half-bridge structure. The proposed ac/dc converter can be utilized in wide input voltage variation conditions. A prototype is proposed in the laboratory and the experimental results verify the feasibility of the proposed converter. [ABSTRACT FROM AUTHOR]

DC microgrids (MGs) have gained significant attention for its remarkable advantages of integrating renewable energy sources and electronic loads. In order to improve the operational performance of the dc MG in both steady and transient states, a fully distributed fixed-time-based secondary controller is proposed to remove the dc voltage deviation and provide proportional current sharing simultaneously. The secondary control of each distributed generation (DG) consists of a fixed-time consensus-based current and voltage regulators that use relative information from the neighboring DGs through a sparse communication network. In contrast to the existing controllers, the proposed controller is distinguished with its accelerated convergence time, which does not rely on the initial values. Besides, the stability of the proposed method is verified through the Lyapunov function. A simulation model is built in PLECS to prove the effectiveness of the proposed control strategy. Some scenarios are introduced to validate the link-failure resiliency, plug-and-play capability, the scalability, and the feasibility under various time delays. Meanwhile, a dc MG experimental prototype is built to verify the controller performance. [ABSTRACT FROM AUTHOR]

This paper addresses a fully distributed hierarchical coordination controller, which minimizes the global total generation cost (TGC) for a cluster of DC microgrids (MGs) through handling the optimal power-sharing between MGs. It consists of local and global tertiary controllers, in which a two-layer communication network is modeled to share information with a reduced complexity as one or some distributed generators (DGs) of each MG are pinned from the local network to broadcast the information between MGs at the global network. Under this control structure, the global tertiary controllers, based on dynamic consensus technique, generate the global optimum incremental cost (IC) reference to attain the entire cluster's economic operation. Moreover, using pinning-based leader-follower consensus algorithm, the local tertiary controllers assign MG average voltage reference at which the ICs of all DGs are matched at the global reference to ensure optimal operation of the cluster with considering DGs generation boundaries. Finally, the nominal voltage set points of each DG can be determined by the secondary controller to guarantee the generation-demand power balance. The feasibility of the proposed controller is demonstrated by simulation and experiment under various test scenarios. [ABSTRACT FROM AUTHOR]

Three-phase input diode rectifier interior permanent magnet synchronous motor (IPMSM) drives with small-volume film capacitors have many merits, such as high reliability, high power density, and low cost. However, in regenerative braking mode, the dc link may suffer from overvoltage due to the application of slim film capacitors. In this paper, a novel antiovervoltage strategy is proposed by controlling the motor voltage using stator current vector orientation. The relationship among the braking torque, the dc-link capacitance, and the antiovervoltage ability is analyzed quantitatively. Based on the analysis of principle, a closed-loop antiovervoltage regulation scheme is presented, which is easy for applications. Additionally, this method has the merit of low motor parameter dependence. Meanwhile, the small signal model of the electrolytic capacitorless IPMSM drive is derived for the coefficient design of the voltage controller. The proposed algorithm is finally implemented on a 2.2-kW electrolytic capacitorless IPMSM drive platform to verify the effectiveness of the antiovervoltage control scheme. [ABSTRACT FROM AUTHOR]

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) connected offshore wind farms could suffer serious challenges: DC overvoltage during a fault and post-fault DC voltage restoration. In this paper, to mitigate DC overvoltage, a two-stage voltage drop control scheme is proposed for offshore MMC to realize a fast active power reduction, by coordinating with a voltage-dependent active current control of individual wind turbines. The voltage drop depths are designed by considering the offshore de-loading efficiency and the stability of WTs. To improve the post-fault DC voltage recovery performance, an adaptive voltage rise control scheme is proposed to coordinate the recovery of onshore and offshore active power. The recovery trajectory of DC voltage is formulated and further employed to design the onshore active power ramp rate for uninterrupted operation of WTs. Case studies have been conducted on two test systems to validate the proposed method. [ABSTRACT FROM AUTHOR]

Electrolytic capacitorless vector-controlled interior permanent-magnet synchronous motor (IPMSM) drives have many advantages, such as higher reliability, longer lifetime, and lower cost. However, the dc-link may suffer from the overvoltage during motor transient process due to the use of slim film capacitors in the diode rectifier front end. The conventional antiovervoltage method by controlling the quadrature axis current will lead to energy control error, which causes large dc-link voltage control error. In this paper, a novel antiovervoltage control scheme with coefficient autoregulation is proposed based on the concept of system loss. A dual antiovervoltage controller is designed to enhance the voltage control performance by controlling both the direct and quadrature axis currents to achieve more effective active braking. The coefficients of the dc-link voltage controller are autoregulated to maintain the enough stability margin during the regenerative braking. By using the proposed method, the dc-link overvoltage phenomenon can be avoided while guaranteeing the maximum copper loss during the braking process. The proposed algorithm is ultimately verified on an electrolytic capacitorless IPMSM drive. [ABSTRACT FROM AUTHOR]

This paper presents a six-step mode-combined field-weakening control scheme to improve the maximum output torque of induction motor (IM) in the voltage-extension region. Compared with the conventional inscribed circle and hexagon methods, the proposed six-step mode method can increase the utilization of dc-link voltage, leading to extended maximum output torque capability. However, the six-step mode operation will inevitably result in torque ripple and current harmonic. To figure out the quantitative relation between the torque extension and ripple, an “equivalent-circuit”-based method is proposed for theoretical torque analysis in six-step mode. To cut off the main current harmonic (sixth harmonic), a second-order generalized integrator based band-stop filter is designed for the feedback current in the current loop. The designed filter can achieve IM stable operation in a six-step mode without affecting the bandwidth of the current loop. The experimental results from a 3.7-kW IM setup verify the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Due to the requirements of multiple working conditions for induction motor in high-speed flux-weakening operation, conventional “maximum torque oriented” flux-weakening controller reveals its own limitations. This paper focuses on holistic torque performance optimization for multi-condition operation. Torque ripple and dynamic response under two kinds of specific working conditions (high-speed braking and load step change) are explicitly analyzed. The theoretically precise maximum torque point is derived to extract the possible output torque for deep flux-weakening operation. Further, an operating point selected flux-weakening controller (OPS-FC) is proposed. It retains the maximum output torque capability for step acceleration and full-load conditions, as well as holding other novel features for the changing working conditions. First, the selected operating point is automatically manipulated for torque ripple alleviation under non-maximum torque condition. Second, since the voltage reference in proposed OPS-FC is directly constructed, the system robustness is enhanced. Finally, speed range is extended to the deep flux-weakening operation, and the complex vector decoupling part of d-axis current regulator is first applied and shared as a flux-weakening path for closed-loop control. Hence, the complicated PI regulator tuning is averted. The proposed scheme is experimentally verified on a 3.7 kW commercial platform. [ABSTRACT FROM AUTHOR]

The instability issue is an important concern in the reduced dc-link capacitance motor drive system, which is caused by the interaction of the LC resonance and the negative impedance of the constant power load (CPL). Harmonics of the grid current also need to be concerned to meet the requirement of the application standards. In this paper, the impedance model of the drive system is established considering the dc-link voltage fluctuation, which is applied to evaluate the drive system stability. The grid current harmonics issue could also be analyzed by the grid input impedance model. A novel grid current feedback based stabilization control method is proposed to stabilize the drive system, which could formulate the Nyquist plot of the drive system. Besides, the grid current harmonics could also be suppressed effectively by adopting the proposed stabilization control method. Experimental results are performed to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Reduced dc-link capacitance interior permanent magnet synchronous motor (IPMSM) drives have many advantages, such as longer lifetime, reduced cost, and higher power density. However, due to the dc-link voltage fluctuation, the dc-link voltage utilization rate and the torque ripples are two major issues in flux-weakening operation region. It is important to consider these two issues comprehensively when a flux-weakening control method is adopted. In this paper, the IPMSM torque ripples caused by the dc-link voltage fluctuation and the overmodulation are analyzed in depth. In order to reduce the torque ripples and increase the utilization rate of the dc-link voltage, a novel adjustable maximum voltage based voltage closed-loop flux-weakening method is proposed. By switching between the minimum dc-link voltage control mode and the extended dc-link voltage control mode, the torque ripples can be reduced effectively and a nearly maximum utilization of the dc-link voltage can be realized. The proposed flux-weakening control method only depends on the dc-link voltage and the stator current vector reference. Experimental results on a 2.2-kW IPMSM drive platform are provided to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In distributed generation, grid-connected and islanded modes are essential, but usually they have different control cores to satisfy their respective control targets. Transferring between the cores may therefore lead to prominent disturbances, if the cores are complex and noticeably different in structures. To introduce seamlessness, some techniques have been recommended, but they usually lack unification of control blocks for the front dc–dc converter and rear dc–ac inverter. Moreover, when connected to the grid, some seamless techniques do not control the grid current directly. Hence, its dynamics may be slow, in addition to distortion caused by grid-voltage harmonics. When islanded, most techniques also use two control loops to regulate a drooped voltage across local loads. This double-loop structure works fine, but it has more state variables to change during a transfer of modes, which may not be as seamless. An alternative technique has thus been proposed here, whose purpose is to identify a simple unified control core, which will remain unchanged during the mode transfer. The transfer may then be viewed as a normal change of power reference, instead of an internal change of control structure. Hence, seamless operation may be ensured, as it is tested in simulations and experiments. These tests have additionally shown that the proposed scheme can regulate the grid current directly when connected to the grid, without compromising the droop-voltage regulation when islanded. [ABSTRACT FROM AUTHOR]

Inductive power transfer (IPT) systems designed with conventional methods offer optimum performance only at specific operational points. These methods are not applicable for dynamic IPT systems where both the coupling and load change throughout the operation. This study proposes a novel particle swarm optimization (PSO)-based parameter design method and aims to obtain a steady output voltage regardless of the coupling and load. The proposed method addresses the constraints of the conventional methods where the compensation capacitors/inductors are designed to resonate with certain other inductors/capacitors at the system's operational frequency. The S/CLC (primary series, secondary capacitor–inductor–capacitor) compensation topology is chosen in this research as it offers many appealing characteristics. The procedure to design an IPT system using the proposed method is presented. Two IPT systems, one designed with the proposed scheme and the other using conventional methods, are theoretically and experimentally compared in terms of fitness, voltage variation ripple (VVR), and power transfer efficiency (PTE). It is demonstrated that the fitness and the VVR of the conventional system are 1400.8% and 372.3% higher than those of the proposed system. [ABSTRACT FROM AUTHOR]

The information of the initial rotor position is essential for smooth startup and robust control of permanent magnet synchronous machines (PMSMs). RoTating Voltage Injection (RTVI) methods in the stationary reference frame have been commonly adopted to detect the initial rotor position at standstill without any position sensors. However, although the Pulsating sqUare-wave Voltage-Injection (PUVI) method performs better in estimation time and accuracy, it is rarely used because the estimation result may converge to the q-axis. In this paper, this fault convergence is avoided by modifying the initial states of the position observer, and the PUVI method can finally be used for the robust initial rotor position detection. Modified signal processing techniques are proposed for both RTVI and PUVI methods for better implementations in fixed-point processors and easier observer gain designs. Detailed comparisons between these two methods are provided. Furthermore, two position estimation observers, i.e., the proportional–integral observer and the extended state observer are compared, and their parameter tuning methods are studied as well. Both simulation and experimental results are provided for verifications. [ABSTRACT FROM AUTHOR]

Circulating current is an undesirable phenomenon in modular multilevel converter which should be attenuated to reduce current stress and power losses. This letter presents a comparative study of the active and passive circulating current suppression methods. Voltage penalty of the active circulating current control is derived, and design of the passive circulating current filter parameters is provided. Simulations and experiments are performed to validate the analysis. [ABSTRACT FROM PUBLISHER]

Traditional droop-controlled system has assumed that generators can always generate the powers demanded from them. This is true with conventional sources, where fuel supplies are usually planned in advance. For renewable sources, it may also be possible if energy storage is available. Energy storage, usually as batteries, may however be expensive, depending on its planned capacity. Renewable sources are therefore sometimes installed as nondispatchable sources without storage. This may not be viable for remote grids, where renewable sources may be the only or major type of sources. In those cases, traditional droop scheme may not work well when its demanded power cannot be met by some renewable sources due to intermittency. When that happens, the system may become unstable with some sources progressively brought out of generation. To avoid such occurrence, an enhanced dual droop scheme is proposed for general two-stage converters with front rectifiers or dc–dc converters for conditioning powers from renewable sources and rear inverters for channeling powers to remote grids. Unlike the traditional droop scheme, the proposed dual droop scheme uses both dc-link voltage and generated powers for determining the required control actions, which have subsequently been proven stable by small-signal analysis. Experimental results have also verified the effectiveness of the dual droop scheme. [ABSTRACT FROM AUTHOR]

This paper proposes an asymmetric mode control for a modular multilevel converter (MMC) operating at low-frequency and low-output-voltage conditions. Differently from conventional symmetric operations, affected by large capacitor voltage ripples caused by the large dc-link voltage, with the proposed asymmetric mode MMC operations, one arm does not produce output current and applies the major dc-link voltage, whereas the other arm sustains almost all output current with low dc-link voltage. Variations of the power level in arms remain bounded and the MMC does not need any other power exchange technique such as high-frequency or harmonic current injection in a low-frequency/low-voltage condition to suppress the capacitor voltage ripple. The asymmetric control realization, and arm energy regulation and its optimization are presented in detail. Experimental and simulation results show that the proposed method is suitable for motor drives operating under 20 Hz. [ABSTRACT FROM AUTHOR]

This paper addresses the zero-sequence circulating current control in the multiparalleled three-phase voltage-source inverters. The model of the zero-sequence circulating current in the N-paralleled (N\;\geq\;3) inverters is derived. It is shown that the circulating current is not only susceptible to the mismatches of circuit parameters, but it is also influenced by the interactions of circulating current controllers used by other paralleled inverters. To eliminate these adverse effects on the circulating current control loop, a coordinate control strategy for the N-paralleled inverter is proposed based on the zero-vector feedforward method with the space-vector pulse width modulation. Moreover, a virtual inverter method is introduced to facilitate the implementation of the proposed controller, which decouples the interactions of circulating current controllers in the paralleled inverters. Finally, experimental results obtained from three-paralleled, grid-connected inverters validate the effectiveness of theoretical analysis and proposed approach. An effective mitigation of the circulating current is demonstrated for the paralleled inverters under the unequal operating conditions. [ABSTRACT FROM PUBLISHER]

In high-speed aircraft, an altitude adjustment system is an important auxiliary part that needs rapid power support. In this paper, an ultrahigh-speed generator is described. However, the ultrahigh-speed generator's safe operation and smooth power supply to the load can be threatened by mismatch between the open-loop generator and fickle load conditions. In this study, a power converter with a paralleled energy-matching link was developed, which can broaden the safe operation zone and ensure the generator's safe operation as well as the load's smooth power supply under relatively light-load conditions. This power converter can also ensure the power supply voltage under relatively heavy-load conditions. The validity of the proposed power converter was verified by experiments. [ABSTRACT FROM AUTHOR]

Droop control has commonly been used with distributed generators for relating their terminal parameters with power generation. The generators have also been assumed to have enough capacities for supplying the required power. This is, however, not always true, especially with renewable sources with no or insufficient storage for cushioning climatic changes. In addition, most droop-controlled literatures have assumed a single dc–ac inverter with its input dc source fixed. Front-end dc–dc converter added to a two-stage photovoltaic (PV) system has, therefore, usually been ignored. To address these unresolved issues, an improved droop scheme for a two-stage PV system has been developed in this paper. The developed scheme uses the same control structure in both grid connected and islanded modes, which together with properly tuned synchronizers, allows mode transfer to be seamlessly triggered. Moreover, the proposed scheme adapts well with internal PV and external grid fluctuations, and is, hence, more precise with its tracking, as compared with the traditional droop scheme. Simulation and experimental results have verified these expectations, and, hence, the effectiveness of the proposed scheme. [ABSTRACT FROM PUBLISHER]

This paper proposes an improved distributed secondary control scheme for dc microgrids (MGs), aiming at overcoming the drawbacks of conventional droop control method. The proposed secondary control scheme can remove the dc voltage deviation and improve the current sharing accuracy by using voltage-shifting and slope-adjusting approaches simultaneously. Meanwhile, the average value of droop coefficients is calculated, and then it is controlled by an additional controller included in the distributed secondary control layer to ensure that each droop coefficient converges at a reasonable value. Hence, by adjusting the droop coefficient, each participating converter has equal output impedance, and the accurate proportional load current sharing can be achieved with different line resistances. Furthermore, the current sharing performance in steady and transient states can be enhanced by using the proposed method. The effectiveness of the proposed method is verified by detailed experimental tests based on a 3 × 1 kW prototype with three interface converters. [ABSTRACT FROM AUTHOR]

This paper presents a novel field-weakening strategy combined with a modified discrete speed-adaptive full-order observer to improve the performance of speed-sensorless induction machine drives in the field-weakening region. The robust field-weakening controller regulates the field current and the clamp of torque current according to the machine operating mode. Compared with the conventional inverse proportion to speed method and optimal flux level calculation method, the proposed controller shows the superiorities over maximum torque selection and parameter robustness. Moreover, a discrete speed-adaptive full-order observer is designed to estimate the rotor speed. However, when the machine operates at high speed, two poles of the conventional forward Euler approximation-based observer are outside the unit circle, which leads to system instability. To solve the conflict between stability, accuracy, and computational burden, a modified Euler approximation is used to discretize the observer. A comparative study of different approximations substantiates the presented approximation. The effectiveness of the proposed solution is demonstrated experimentally on an industrial induction machine drive. [ABSTRACT FROM AUTHOR]

The requirement of a large number of sizeable capacitors in traditional modular multilevel converter (MMC) submodules is seen as a major barrier hampering its widespread use in medium-voltage applications. To reduce the value of capacitance, this paper presents a modified MMC (M-MMC) topology, which offers inherent alleviated capacitor voltage fluctuations compared with traditional MMC. Its operating principle, modulation, and capacitor voltage balancing strategies are described in detail. Moreover, a novel capacitor voltage fluctuation suppression scheme is also proposed to further reduce the second-order capacitor voltage ripples. Finally, both a three-phase prototype of M-MMC and traditional MMC have been built in the laboratory. The effectiveness of the proposed M-MMC topology and control methods is experimentally verified by comparing with the traditional MMC. [ABSTRACT FROM PUBLISHER]