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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 maximum power point should be continuously tracked in a photovoltaic (PV) inverter. However, the maximum power point could be found at very low dc-link voltages around sunrise or sunset. Then, the overmodulation (OVM) of the inverter has to be considered to keep the power generation even at low dc-link voltages. In this paper, a series of implementation schemes is proposed to keep the current regulation, even if the inverter is under the OVM. These schemes can be helpful to increase the operating time of the PV inverter, which correlates with the daily amount of power generation. After being examined in a small-scale prototype inverter, the proposed method has been tested with a 250-kW PV inverter installed at a proving ground. In a sunset, the operation time of the inverter has been extended by about 12 min, and the generated energy has been increased by 234 Wh, when compared to the normal operation without the proposed methods. [ABSTRACT FROM AUTHOR]

This paper conducts modeling and controller design for a SEries-connected Output Universal Link (SEOUL) converter. The SEOUL converter was proposed to interface multiple distributed power sources to the grid with a reduced amount of reactive components. It employs a high-frequency (HF) transformer instead of a bulky line-frequency transformer like solid-state transformers (SSTs). In addition, it does not use a large dc-link capacitor at the input side and reduces the size of the input filter. This is inherently possible due to its original operating characteristic. The input phase current is not actively controlled but synthesized indirectly by HF power transfer between input and output sides of the SEOUL converter. Therefore, it is important to properly control the HF power transfer part, on which this paper focuses. An analytic model is developed, and a control strategy is proposed based on the model with a decoupling control method. The developed model and the proposed control method were verified by experiments. [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]

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]

This paper presents a novel flux-weakening strategy for an induction machine driven by an electrolytic-capacitor-less inverter. In the electrolytic-capacitor-less inverter, the dc-link voltage is fluctuating at six times the frequency of the input three-phase source due to its small dc-link capacitance. Hence, the decoupling of the fluctuation and maximum utilization of the dc-link voltage is a major issue in the electrolytic-capacitor-less inverter. In this paper, the cost function is set to increase the voltage utilization of the inverter for the flux-weakening operation of an induction machine. With the proposed flux-weakening strategy, the operating speed of the induction machine is extended above the base speed without any stability problems. The experimental results show the effectiveness of the proposed strategy. [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]

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, the theoretical analysis of the position estimation error in position-sensorless operation using pulsating square wave signal injection is presented. The purpose of this paper is to analyze the phenomenon that the position estimation performance varies depending on the injection frequency. Mathematical derivation supported by simulation and experimental results shows that, in no-load condition, the voltage distortion induces position estimation error with 3rd-order harmonics if the frequency of the injected square wave is the same to the switching frequency. While, if the frequency is a half of the switching frequency or heavier load condition in either injection frequency, the effect of nonideal characteristics of inverter disappears and there is no 3rd harmonics in the position estimation error. Therefore, only the effect of cross-coupling inductance is reflected in the estimation error. As the result, the position estimation error appears in the form of 6th-order harmonics due to the spatial harmonics of inductances, and the error is relatively small compared with the switching frequency injection case. The theoretical analysis was verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

In this paper, it is described how the circulating current flows between inverters that have isolated dc link such as cascaded H-bridge (CHB) topology when they are operating in parallel. In this case, zero sequence circulating current, which normally flows through shared dc link, cannot flow between inverters in parallel. However, circulating current is provoked by asynchronous switching instant of devices, difference of dc-link voltages of parallel inverters, and unbalanced impedance. The circulating current should be suppressed by a sharing inductor that is inserted between inverters. Generally, all of the inverters should synthesize the same output voltage reference for load current control to minimize the size of this sharing reactor. However, this conventional method cannot guarantee diminishing circulating current in the transient state and even in the steady state. In this paper, in order to reduce sharing reactance, after deriving circulating current model from general case of n-parallel operation, circulating current control method is devised based on the model. This proposed algorithm is applied to active front end five-level-CHB inverter system for medium voltage drive. Simulation and experimental results are provided to verify the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

In the square-wave voltage signal injection sensorless drive of the interior permanent-magnet synchronous machine, the injection voltage can be distorted by the inverter nonlinearity effects especially when the injection voltage is low. If that happens, a high-frequency (HF) current signal which contains the rotor position information could be also distorted, which directly leads to an error in the position estimation. This paper analyzes the effects of the inverter nonlinearity on injection voltage, on induced current ripple, and on the position estimation performance in sequence and proposes a voltage injection method to minimize the impact of the inverter nonlinearity by the regulation of the HF current ripple. By simulations and experiments, the performance of the proposed method has been verified. The experimental results show 19.7% reduction of position estimation error under 15% of rated voltage signal injection without increasing the current ripple at a certain operating condition. Moreover, the performance of position estimation is kept uniform regardless of the loading condition. [ABSTRACT FROM AUTHOR]

This paper proposes a new sensorless method for a synchronous machine with an inverter integrated rotor (SMIIR). The SMIIR is a newly developed machine based on the wound rotor synchronous machine (WRSM) but has no brushes and slip-rings. The conventional high-frequency signal injection sensorless methods are based on a physical magnetic saliency. Therefore, the conventional methods cannot be applied to the SMIIR since it has no adequate physical magnetic saliency. The proposed sensorless method suggests a virtual resistance saliency created by the rotor-side inverter, of which the saliency ratio and axes can be arbitrarily set. Based on this virtual saliency, the rotor position information can be included in the relationship between the high-frequency stator voltage and stator current independent of the machine parameters or operating conditions. Because the SMIIR inherently injects a high-frequency voltage for the power transfer, there is no need to inject additional voltage for the proposed high-frequency sensorless control. The initial $d$-axis detection algorithm using the rotor-side inverter without magnetic saturation is also proposed. The feasibility of the proposed sensorless method was verified by the experimental results of the prototype SMIIR. [ABSTRACT FROM AUTHOR]

Six-step operation has many advantages in permanent-magnet synchronous machine (PMSM) drives such as maximum power utilization and widened flux-weakening region. However, due to the maximum utilization of inverter output, saturation of current regulator makes it difficult to maintain instantaneous current control capability. Accordingly, in most of conventional research studies, the six-step operation has been implemented by voltage angle control without current regulation, whose dynamic performance is quite unsatisfactory. This paper proposes a control scheme for the six-step operation of PMSM with enhanced dynamic performance of current control. By collaborative operation of dynamic overmodulation, flux weakening, and a technique for enhanced dynamic performance, the six-step operation is realized without losing instantaneous current control capability. Simulations and experiments are carried out to verify the effectiveness of the proposed control scheme. The experimental results show 17% extension of the constant torque region and 27% enhancement of torque capability at three times of the base speed. [ABSTRACT FROM AUTHOR]

If a small dc-link capacitor is used in the dc link fed by a single-phase ac source, then the dc-link voltage severely fluctuates at twice of the source frequency. To handle this fluctuation, the concept of “average voltage constraint” is proposed in this study. On the basis of this concept, a flux-weakening scheme, generating the d-axis current reference (ids^{r}^{\ast}) for the interior permanent-magnet synchronous machine, is devised. The q-axis current reference (iqs^{r}^{\ast}) is modified for the unity power factor operation in the viewpoint of an ac source without an additional sensor. The proposed scheme has been applied to the inverter-driven 1-kW compressor of an air conditioner. From the experimental results, it has been verified that the compressor operates well at the required operating condition, regardless of the severe fluctuation of dc-link voltage, because of the reduced dc-link capacitance. The frequency spectrum of the ac source current reveals that the harmonics of the source current meet the regulation of IEC 61000-3-2 Class A and that the overall power factor is above 96% without any additional circuit, such as an input filter and a power factor correction circuit. [ABSTRACT FROM PUBLISHER]

This paper presents a nine-phase permanent-magnet synchronous motor (PMSM) drive system based on multiple three-phase voltage source inverters. The nine-phase PMSM was developed as a traction motor for an ultrahigh-speed elevator. The mathematical model of the motor was simplified through symmetry of the system. Using the simplified model, the drive system can be controlled by the well-known d-q control theory. The feasibility and validity of the drive system were experimentally demonstrated at the world's tallest elevator test tower. Moreover, an additional experiment was performed to ensure the fault-tolerance capability of the system. [ABSTRACT FROM PUBLISHER]

This paper proposes an isolated universal-link ac–ac power converter suitable for grid connection. The proposed power converter can have multiple ports to connect various loads or electric energy sources, and the configuration can be arranged according to the situation. The proposed power converter does not need to employ the large interface inductor at input side and the huge electrolytic capacitor at dc link. Moreover, it utilizes high-frequency transformers for the galvanic isolation instead of bulky line-frequency transformers. These characteristics of the proposed power converter result in the reduction of the system volume and weight remarkably. The proposed power converter is the modular structure, and an H-bridge works as a basic module of the converter. By stacking the modules, the power converter can be adapted to the high-voltage grid and the various types of loads and/or sources. This paper addresses the structure of the proposed power converter and the fundamental principle of power flow. The operation of the proposed converter is verified by both computer simulations and experimental results with a laboratory-level prototype system. [ABSTRACT FROM AUTHOR]

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]

In this paper, the characteristics of ripple current of a multimegawatt drive system fed by a pulse-width modulation (PWM) inverter are investigated. Based on the investigation, a novel PWM scheme is devised to reduce the switching loss of the inverter under an identical amount of torque ripple of a motor. The proposed PWM is composed of two parts. The first is minimum torque ripple PWM (MTRPWM), which is optimized in terms of the minimum torque ripple in high modulation index (MI). The second determines the minimum switching frequency while holding the average torque ripple less than the allowable value under various operating conditions. It is termed reduced switching frequency PWM (RSFPWM). Consequently, by simply changing the PWM method from conventional fixed switching frequency continuous PWM to the proposed PWM, MTRPWM and RSFPWM, simulation results show that the switching loss of the inverter driving a 3.5 MW induction motor is reduced to 51.6% while keeping the torque ripple in the allowable boundary, where MI is 1.12. This approach is applied to small-scale experimental sets consisting of an interior permanent magnet synchronous machine (IPMSM) motor-generator set, and an induction machine MG set in an effort to verify the validity and feasibility of the proposed method. Based on these experimental results, the cost indexes of the IPMSM and the induction machine, as defined in this paper, decrease by up to 66% and 77%, respectively, when the proposed method is applied when MI is 1.12 compared with the outcome with the conventional PWM. [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]

In this paper, a synchronous motor with an inverter integrated rotor (SMIIR) is introduced, and its control scheme is proposed. This presents a new breed of synchronous motor in that it integrates an inverter inside the rotor. The basic operational principle and control strategies are discussed. Given these new strategies, the SMIIR can be considered as a synchronous motor which changes its field according to the harmonic voltage of the stator inverter. The feasibility of the proposed structure of the drive system and its operation principle are evaluated through experimental results. [ABSTRACT FROM AUTHOR]