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The overmodulation scheme plays an essential role in the dynamic performance of the current regulation under voltage limitation. This article analyses how the nonlinearity of the voltage hexagon affects dynamic overmodulation. Most dynamic overmodulation schemes had considered the voltage hexagon as the limit of the output voltage to maximize the voltage utilization. However, these schemes do not show uniform dynamic performance according to the rotor position on the voltage hexagon. To overcome this limitation, this article proposes an overmodulation scheme that is robust against the rotor position variation. The computer simulation and experimental results verify that the proposed method has the consistent performance regardless of the rotor position with less d-axis current overshoot keeping the fast dynamics. [ABSTRACT FROM AUTHOR]

When not all the converters are enabled in a parallel ac/dc power conversion system, some current flows through the disabled converters. This current not only distorts the current waveform of the ac side but also causes extra loss and unintended power flow from the ac to the dc side. In this article, the reason for this current flow is identified, and the total current according to the number of paralleled converters and its operating conditions are analyzed. A new pulsewidth modulation (PWM) method is proposed to minimize the current. And, the zero-sequence voltage and the consequent current flowing through the disabled converters under the proposed PWM method are compared to those under the conventional PWM methods. As a result, it was identified that discontinuous space vector PWM (DPWM) is not a proper PWM method because it causes a large current at some operating conditions. According to both simulation and experimental results, the magnitude of the current flows through disabled converters under the proposed PWM was about 40% less than that under sine PWM and DPWM, and it was about 10% less than that under continuous space vector PWM. [ABSTRACT FROM AUTHOR]

For stable and efficient drives of interior permanent-magnet synchronous machines (IPMSMs), stator flux should be observed across a wide speed range. However, distortions such as a dc offset and low-order harmonics are contained in the estimated flux due to various reasons. In this paper, a distortion-minimizing flux observer is proposed to suppress the effects of dc offset and harmonics by implementing frequency-adaptive observers. The proposed frequency-adaptive flux observer is designed to extract a fundamental flux while keeping a simple structure even if the rotating speed of the IPMSM varies. The fundamental flux could be estimated by the proposed current- and voltage-model based flux observer, which is implemented with the frequency-adaptive observer and harmonic extractors. Moreover, parameter sensitivity and model transition are discussed to improve the performance of the proposed observers. The validity and effectiveness of the proposed methods are verified with simulation and experimental results. [ABSTRACT FROM AUTHOR]

In this paper, a current controller for a parallel operation of inverters is described. When inverters operate in parallel, it is a well-known fact that the inverter current is composed of the average current (AVC) and the zero-sequence circulating current (henceforth ZSCC), which inevitably flows between inverters under the parallel operation. Considering this, previous studies concentrated on reducing ZSCC. However, in this paper, it is found that the inverter current has another component, a differential current, in addition to the two components of AVC and ZSCC. Based on these three decomposed current components, three equivalent circuits are derived from n-paralleled inverters. To minimize the differential current and improve the regulation performance of the total output phase current, a decomposed current control scheme is proposed from equivalent circuit models. The proposed algorithm is applied to a system consisting of three paralleled two-level inverters with small shared inductance whose dc links are connected in common. Additionally, synchronous pulsewidth modulation is employed as the PWM method for each inverter. The proposed current controller is compared with two conventional current controllers, one where each inverter controls its own currents and the other where only AVC is controlled. Through experiments, it is shown that these conventional controllers have limitations and the proposed current controller overcomes these limitations. The effectiveness and feasibility of the proposed control scheme are verified through experimental results. Especially, both theoretical analysis and experiment results verify that the proposed method is robust with regard to parameter errors. [ABSTRACT FROM AUTHOR]

In signal injection sensorless drives, the injection voltage cannot be reduced under a certain value. The lower limit of the injection voltage is mainly enforced by the inverter nonlinearity that causes distortion of the injection voltage and degradation of the position estimation performance. After analyzing the inverter nonlinearity during the voltage injection, it is revealed that parasitic capacitances of insulated-gate bipolar transistors have positive effects on inverter output linearity and sensorless control. Based on this analysis, this paper proposes a method to reduce the injection voltage by connecting additional capacitors to the output terminals of the inverter. Simulation and experimental results are provided to verify the effectiveness of the proposed idea. In the experiments, using the capacitor-integrated inverter, it is shown that the injection voltage can be reduced by more than half without degrading the position estimation performance. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is a promising candidate for voltage-sourced-converter-based high-voltage direct current (VSC-HVDC) transmission. The dc short-circuit fault management is a crucial issue especially for overhead line transmission, where the nonpermanent dc short-circuit faults occasionally occur. In this paper, a comprehensive dc short-circuit fault ride through (FRT) strategy is proposed for a hybrid MMC which combines half-bridge submodules and full-bridge submodules. By the proposed method, the hybrid MMC-based VSC-HVDC system is able to ride through a pole-to-pole short-circuit fault supporting the ac grid as a static synchronous compensator. Moreover, the transmission system can still transmit one-half of the rated active power in case of a pole-to-ground short-circuit fault without increasing power semiconductor stresses or requiring higher insulation levels of the electric equipment. The proposed method ensures successful regulation of arm currents and submodule capacitor voltages during the FRT process. Validity of the proposed strategy is verified by both computer simulation and experiment results. [ABSTRACT FROM PUBLISHER]

It is important to improve the overall efficiency of a photovoltaic (PV) inverter when it is connected to the grid. Fundamentally, the conversion efficiency from dc to ac power of an inverter is important. However, in the presence of partial shading, maximum power point tracking (MPPT) on PV modules is more important than the conversion efficiency. In this paper, a new control method for a three-level inverter is proposed. With the proposed method, each dc-link voltage of the three-level inverter can be asymmetrically regulated. When PV modules are split into two and each split module is connected to the respective dc-link capacitors of the inverter, the asymmetric control can be helpful because separate MPPTs are possible. The effectiveness of the proposed method was examined through experiments with a T-type three-level inverter, where each dc-link capacitor was supplied by a PV simulator emulating two separate PV modules under different shading conditions. [ABSTRACT FROM AUTHOR]

This article discusses the feasibility of signal-injec- tion sensorless control (SISC) in low saliency surface-mounted permanent-magnet synchronous motors (SMPMSMs). At first, dynamic inductances of a target SMPMSM according to rotor position and dq-axes currents are measured from an experiment. The current response induced by the injected voltage is theoretically calculated based on the inductances. The induced current signal in every operable point on dq current plane is evaluated with a newly defined index variable. An optimal current trajectory for the SISC is proposed after the evaluation. By adopting optimal trajectory control instead of maximum torque per ampere control, the performance of the SISC in the SMPMSM can be conspicuously improved at the cost of slightly increased electrical loss. The proposed method is verified with experimental results. [ABSTRACT FROM AUTHOR]

The nonlinearity of an inverter could be regarded as the output distortions of the inverter. In this paper, voltages in a trapezoidal form were utilized to compensate for the nonlinearity of an inverter. Undesirable harmonic distortions in the overall system were mitigated through the modulating shape of the trapezoidal voltage. For trapezoidal modulation, a dedicated modulator was designed to facilitate adaptive compensation for a wide operating range. The compensation effects from the proposed method were examined in a practical system, in which a permanent magnet synchronous motor (PMSM) was driven without any position sensor. Additionally, it was shown that the proposed method could enhance the sensorless control of the PMSM by reducing the output distortions of the inverter. [ABSTRACT FROM PUBLISHER]

Proposes a space-vector pulse width modulation strategy that can reduce the number of common mode voltage pulses in a three phase boost rectifier/inverter system using a synchronized switching sequence. Problems caused by common mode voltages created by high-speed pulse width modulation; Kinds of switching alignment; Test results of peak motor leakage current at various motor speeds.

This article proposes an online maximum torque-per-Ampere (MTPA) control of an interior permanent-magnet synchronous motor (IPMSM) based on the square-wave voltage signal injection. Because the proposed method does not rely on a position sensor, it is applicable to sensorless IPMSMs. The proposed algorithm consists of a dynamic inductance estimator, frequency-adaptive flux observer, and an MTPA tracking controller. The proposed inductance estimation utilizes the pulsating square-wave voltage injection, which can maximize the injection frequency, and the dynamic performance can be preserved during the signal injection. Through the online flux and inductance estimation, the magnetic saturation of the IPMSM and the temperature variation can be fully considered in the MTPA control. The conventional rotor position estimator is replaced with the MTPA tracking controller, which does not rely on offline commissioning parameters. The proposed algorithm can operate under both the torque control and speed control mode, and accurate torque control on MTPA curve can be achieved. The simulation and experimental results demonstrate that the proposed method accurately tracks the MTPA point up to 200% of the rated torque in the mid- and high-speed region. [ABSTRACT FROM AUTHOR]

Presents information on a study which focused on a stator transient inductance estimation scheme used to guarantee the performance of high-speed operation of an induction machine. Voltage and current constraints; Machine inductance detuned effect in flux-weakening region; Simulation results; Conclusions.

Presents a direct torque controller of a synchronous reluctance motor (SynRM) based on the stator current vector. What is torque control; Methodology used to develop the torque controller; Validation of the performance of the controller.

Presents a study on a current controller with both the satisfactory steady-state characteristics and fast transient response, using d-q axis crosscoupling. Comparison between the proposed current controller and the synchronous frame proportional-integral (PI) controller; Effectiveness of the proposed current controller; Analysis of conventional d-q synchronous regulator.

Discusses the drive technique without rotational position sensor for high-performance permanent magnet synchronous machine (PMSM) drives. Detailed information on PMSM drives; Balanced phases PMSM is assumed to have; Description of the PMSM.

Proposes a current controller that guarantees the fastest transient response. Optimal control voltage; Current regulation in the three-phase pulse width modulation (PWM) converter; Conventional synchronous PI regulator; Performance of the dc link voltage control.

This article proposes a precise identification scheme of an interior permanent-magnet synchronous motor (IPMSM) flux linkage through experimental tests considering motor nonlinearities, such as the magnetic saturation and spatial harmonics. In the proposed scheme, the flux linkage is identified based on the voltage equation of IPMSM. The fundamental and harmonic electromotive force in the rotor reference frame is obtained in real time by applying a discrete Fourier transform with the phase compensation of the time delay due to digital control and pulsewidth modulation. The effect of the inverter nonlinearity is compensated using an experimentally obtained lookup table. Based on the identified flux-linkage map, a flux-based IPMSM model that accurately simulates motor behavior can be constructed. The effectiveness of the proposed flux-linkage identification scheme is verified through the comparison of simulation and experimental results. [ABSTRACT FROM AUTHOR]

This article analyzes the instantaneous flux trajectory of a permanent-magnet synchronous machine (PMSM) in the six-step operation. Not only the flux dynamics but also the instantaneous voltage vector are included in the analysis. Based on the analysis, a time-optimal voltage vector transition scheme is proposed for the six-step operation of a PMSM. In the proposed scheme, a few steps are added in the transition of the voltage vectors to bring additional degrees of freedom, and they can enhance the dynamic performance conspicuously under the given current constraint. Simulation and experimental results are provided to verify the proposed scheme. For the stepwise torque command, the proposed scheme saves the settling time more than 50% with 60% reduction of the peak current compared to the conventional transition method. [ABSTRACT FROM AUTHOR]

In this article, a method to estimate the temperature of an NdFeB magnet in an interior permanent magnet synchronous motor (IPMSM) has been developed in medium and high speed operating conditions. Fundamental reactive energy is newly brought out and directly used in the developed method to estimate the magnet temperature accurately, which results in higher sensitivity with respect to the magnet temperature variation compared to the conventional methods. It is because the variation of the reactive energy caused by the magnet temperature includes variations of inductances in addition to that of the magnet flux linkage. In the proposed method, a stator resistance error effect caused by a stator temperature is inherently removed without any additional temperature sensor on stator winding. Moreover, the proposed method is robust to an inverter nonlinearity effect and ac resistance effect. As a result, the proposed method can estimate the magnet temperature accurately in real time, even in load variation, regardless of the stator temperature variation. The magnet temperature estimation error has been less than 3.7 °C in the online estimation for 10 000 s under wide speed and torque variations. [ABSTRACT FROM AUTHOR]

This article proposes a compensation method for position estimation error under high-frequency signal-injection sensorless control (SISC), especially for heavily saturated interior permanent-magnet synchronous motors (IPMSMs). In the proposed method, the current reference is tilted so that the controller sees more desirable side of the inductance of IPMSM under control. With the proposed method, the position error can be fully compensated and the torque under SISC is limited not by the inductance characteristics of motor but current limit of the inverter and motor itself. Simulations and experiments are performed to verify the proposed method. [ABSTRACT FROM AUTHOR]

A hybrid modular multilevel converter (MMC) consists of both half-bridge submodule (HBSM) and full-bridge submodule (FBSM) in each arm. It has several advantages over other MMC topologies, such as low system loss, reduced cost of the system, but still maintains fault ride-through capability against a solid dc-side fault. However, the hybrid MMC may have a difficulty of submodule (SM) voltage balancing under variation of its dc-bus voltage. Since the voltage unbalance of SMs results in SM capacitor overvoltage and low reliability of the system, the voltage of SMs, which can be also represented as energy of SMs, should be balanced at all time. In this paper, to extend the narrow adjustable range of dc-bus voltage due to the voltage/energy unbalance, new feedback and feedforward energy balancing controllers with revised sorting algorithm are proposed. The feedback control ensures the zero steady-state error, while the feedforward control provides fast dynamic response in the energy balancing control. By applying the proposed control scheme, the hybrid MMC can keep the voltage balance between HBSMs and FBSMs in wider range of dc-bus voltage. The full-scale computer simulation results and down-scale experimental results demonstrate the validity of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper describes a dc-link voltage control method of a single-phase back-to-back cascaded H-bridge inverter (SBCI) for a medium-voltage motor drive system. The main advantage of the SBCI topology over the conventional regenerative cascaded H-bridge topology with a three-phase active front-end (AFE) is a simple system structure, which is composed of an input transformer, a power cell, a current sensor, etc. However, the challenging points of the SBCI are larger voltage ripple in the dc-link capacitor and imbalance of dc-link voltages of each phase. The asymmetric dc-link voltage of each power cells could cause unstable operation such as over-modulation due to the lack of the dc-link voltage of a particular phase and result in over-voltage or under-voltage faults. In this paper, the control strategy of the dc-link voltage for the SBCI that uses the negative-sequence voltage of the converter is described. The proposed control method is verified with a computer simulation whose target is a 6.6-kV–1.25-MW medium-voltage drive system. Also, through the experimental setup with the prototype SBCI whose power rating is 16.2 kVA, the dc-link voltage of each AFE has been controlled within a 0.5% error of its reference value at the full load. [ABSTRACT FROM AUTHOR]

In this paper, a resonant damping circuit to suppress the switching ringing of full SiC mosfet is proposed. It reveals that the ringing phenomenon caused by parasitic impedances of switching circuit can be damped out using air core PCB transformer which has a properly designed secondary side circuit. The design method for PCB transformer and the secondary circuit are developed considering the physical dimension applied to the PCB transformer inserted between full SiC mosfet module and snubber capacitor. Experimental results using 1200 V–180 A full SiC mosfet module validate the design method and the performance of the proposed air core PCB circuit. Thanks to the damping circuit, the resonant component due to the switching ringing has been reduced to a half compared to that of system without damping circuit. [ABSTRACT FROM AUTHOR]

In this paper, a comprehensive control scheme for a delta-connected cascaded H-bridge (CHB) converter-based static synchronous compensator (STATCOM) is presented, especially focusing on improving dynamic performance by novel feedforward control method. The method can conspicuously improve the dynamics of zero-sequence circulating current regulation of delta-connected CHB STATCOM especially under grid fault condition as well as load unbalance without excessive dc cell capacitor voltage fluctuation. The validity of the proposed method is demonstrated by mathematical proof. Finally, the full-scaled simulation results and the down-scaled experimental results verify that stable operation is guaranteed for both emulated grid and load unbalance conditions. [ABSTRACT FROM AUTHOR]

In the voltage-source-converter-based dc system, the short-circuit current increases rapidly due to neighborhood dc-link capacitance. The protection system using a high-speed fuse has been applied in the industry to protect before the current freewheeling that ruins a semiconductor device. For proper protection system design, the capacitor-discharge I2t must be analyzed and set higher than the melting I2t of the fuse by securing sufficient capacitance. However, due to the implicit characteristics of the I2t function, it is difficult to calculate the required capacitance directly considering unknown fault resistance. In this article, the normalized I2t map of capacitor-discharge is presented as a calculation reference. After calculating the base values, the normalized I2t value can be calculated easily from the map without complicated formula. Since it is expressed as a function of the damping factor (half of the normalized resistance) and normalized capacitor's voltage, it helps I2t design with the range of fault resistance and voltage drop simultaneously. The proposed normalization and its result were verified through the experimental tests under various fault resistance. [ABSTRACT FROM AUTHOR]

In a maritime dc power system, the rectified dc-bus voltage of a permanent magnet synchronous generator is generally regulated by the pulsewidth modulation (PWM) operation of an active ac/dc converter. This article proposes a dc bus voltage regulation strategy exploiting the six-step operation. Under the six-step operation, the switching loss of the converter can be significantly reduced, compared to the PWM operation. Moreover, due to the higher modulation index and the less flux-weakening current of the six-step operation, the conduction loss can also be reduced. By employing diode rectification mode at light load, the conduction loss induced by the non-negligible ripple current of the six-step operation could be minimized. This article implements a controller for the proposed dc bus voltage regulation strategy and verifies its validity with simulation and experimental setup. According to the simulation and the experimental test, the converter efficiency at the rated load is enhanced by 0.6% and 0.4%, and the converter loss is reduced by up to 70% and 19%, respectively. [ABSTRACT FROM AUTHOR]

A current source inverter (CSI) requires a capacitor filter for the commutation of switching device as well as for attenuating switching harmonics. Hence, the CSI-fed ac machine has a second-order system in the continuous time domain. This paper presents a design methodology for the closed-loop current controller of the CSI-fed ac machine drive system. A multiloop current controller design using a pole/zero cancellation method is employed with a transfer function matrix. To decouple the cross-coupling terms which cause mutual interferences between the d- and q-axes in the synchronous reference frame, two types of controller are proposed and implemented using different decoupling method. Additionally, active damping methods are incorporated to enhance the stability of the system. A stability analysis in discrete-time domain is investigated to verify the feasibility of the proposed closed-loop current controller. To evaluate the effectiveness of the proposed current controller, computer simulations and experimental tests were performed and the results are discussed. [ABSTRACT FROM AUTHOR]

A matrix converter interfaces with three-phase ac voltage source, which creates three different virtual dc-link voltages. In this paper, a new pulsewidth modulation (PWM) method for the matrix converter is proposed to generally use these three virtual dc-link voltages. By using the two higher line-to-line voltages as the virtual dc-link voltages, the proposed method can create the identical switching sequences to those created by the conventional space vector PWM (SVPWM) method. Moreover, the proposed method is able to select the appropriate virtual dc-link voltage, according to the PWM purpose and/or load condition. In this paper, the virtual dc-link voltages adjacent to the output voltage reference are introduced to reduce output-current switching ripples. The feasibility of the proposed PWM method was verified by a computer simulation and experimental results. The output voltage and current waveforms produced by using the proposed method can be equal or better than those created using the conventional SVPWM method. Moreover, a new switching sequence that reduces output-current switching ripple can be easily adopted within the proposed PWM method. The harmonic characteristics of output current created using the proposed method are markedly improved over those using the conventional PWM method. [ABSTRACT FROM PUBLISHER]

Presents information on an overmodualation technique for interior permanent magnet synchronous machine (IPMSM), with focus on the concept of torque control. Implementation of the technique to a inverter system; Characterization of the operation of the electrical drive system with a current controller; Difference between the machine terminal voltage and the machine-back electromotive force (EMF); Effects of current regulation performance on torque production.

In this paper, a novel filter structure to suppress circulating currents is proposed based on the sequence of pulsewidth modulation voltage harmonics for high-power interleaved motor-drive systems. The sideband harmonics can be divided into positive-, negative-, and zero-sequence components. Through 120° interleaving among three paralleled inverters, a majority of positive- and negative-sequence harmonics are phase shifted. By these phase shifts, the differential-mode circulating currents between the paralleled ac–dc converters can be suppressed with the proposed filters, which have an identical structure to the three-phase common-mode chokes. Such a structure highly benefits the design and manufacturability for high-power applications where a choice of magnetic-core shapes is limited. Compared to coupled inductors (CIs) with cyclic-cascade or monolithic configurations, the number of magnetic cores can be reduced by one-third. Peak flux-linkages of the proposed filters and the conventional CIs were compared to estimate the size reduction in case of flux-limited designs. Considering the whole modulation index range, which may be required in the motor-drive systems, the maximum flux-linkage can be reduced by 50%. Prototype filters are built and showed a 33% reduction in weight and size compared to the conventional CIs. The validity of the proposed filter is verified through the simulation and a small-scale experiment. [ABSTRACT FROM AUTHOR]

Cascaded H-bridge (CHB) inverter is the most widely used topology for a medium-voltage drive system due to the high degree of modularity, easier implementation of medium output voltage, and the ability to continuous operation under the cell fault condition. Because each power cell of CHB should have isolated dc source, multiwinding input transformer and three-phase active front end (AFE) are generally used for regenerative applications. The whole system can be simplified by replacing the three-phase AFE with single-phase AFE. However, if the control strategy of normal operation is adopted under the cell fault condition, input power imbalance among three phases inevitably occurs. In that situation, not only faulty cells, but also some unscathed cells should be excluded with giving up the maximum capability of the system, not to deteriorate grid current. This paper proposes a control scheme against the cell fault condition of the single-phase AFE CHB. By applying the proposed control scheme to the system, dc-link voltage of each cell and grid current are well regulated without imbalance even under the cell fault conditions. Finally, it can minimize the number of undamaged cells which should be turned off and maximize the capability of the system under the cell fault condition. Simulation and experimental results are provided to verify the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

This letter presents a modular multilevel converter (MMC) for bipolar medium-voltage direct-current (MVDC)) distribution systems. By employing a zigzag transformer as a grid interface transformer with a dedicated operation method, the operation of two dc poles of an MMC can be fully decoupled. Consequently, the MMC can provide full bipolar-operation capability. No additional component is required, and no penalty of increased power rating is imposed on the grid interface transformer. The scheme based on the proposed topology is competitive and cost-effective for emerging MVDC distribution systems. Feasibility of the proposed scheme is verified by both simulations and down-scale experiments. [ABSTRACT FROM AUTHOR]

This paper addresses the inverter nonlinearity of a three-level T-type inverter. The main causes of the nonlinearity are pulse shaping, pulse skipping due to narrow pulses for the dead time, and voltage drop of the switching devices. Although the same causes have existed in two-level inverters, the pulse skipping phenomenon and results are quite different in the case of the three-level T-type inverter. The effects of pulse shaping and skipping have been investigated and discussed. In addition, the voltage drop of the switching devices has been considered in conjunction with the conduction path of the T-type inverter. Compensation methods based on the pulse width modulation (PWM) of T-type inverter to alleviate the inverter nonlinearity have been proposed from research results. The proposed methods could be easily implemented by adding appropriate offset voltages to the voltage references of the inverter in PWM. Furthermore, a neutral voltage balancing method of a three-level T-type inverter has been proposed in conjunction with the proposed PWM methods using offset voltage. Through extensive experimental tests, the fifth- and seventh-harmonic components of the current are conspicuously reduced along with the even harmonics. The neutral voltage of the inverter can be balanced effectively with the proposed offset voltage adjustments as well. [ABSTRACT FROM AUTHOR]

This paper introduces a new multilevel converter topology for a hybrid HVDC system comprising line-commutated converter (LCC) and voltage source converter (VSC). Among the existing modular multilevel converter (MMC) topologies for the hybrid HVDC, a mixed MMC structure with half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs) has characteristics of reduced system cost, low operation loss, but still keeping capability to cope with dc short-circuit fault. However, it is very difficult for the conventional hybrid MMC structure to balance the submodule capacitor voltages under dc-bus voltage sliding since each MMC arm is a mixture of HBSMs and FBSMs. To solve the defect of the conventional hybrid MMC structure, an asymmetric mixed MMC, in which one arm is made of series-connected HBSMs and other arm is made of FBSMs, is devised. The proposed asymmetric MMC can regulate the dc-bus voltage freely without uncontrollable submodule capacitor voltages. The problems of the conventional MMC structure and the validity of asymmetric MMC are verified by both computer simulation and experimental results. [ABSTRACT FROM AUTHOR]