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For the three-phase parallel voltage source inverter systems with common dc link, several control methods were developed to suppress the common-mode circulating current. The common-mode ripple current is the main disturbance source of the common-mode circulating current control system that forms part of the parallel discontinuous pulsewidth modulation (PWM) inverter system. In this paper, a real-time analysis method for the common-mode ripple voltage of the three-phase discontinuous PWM inverter is proposed, whereby the amplitude of the common-mode ripple current can be estimated correctly, the hysteresis width of the circulating current controller can be adjusted to be as small as possible, and consequently, the rms circulating current is minimized as well. The simulation and experimental results are presented to confirm the theoretical analysis method of the common-mode ripple current and the performance of the proposed common-mode ripple current estimator. Index Terms--Common dc link, common-mode ripple current, parallel, pulsewidth modulation (PWM).

This paper reports a large-scale eddy-current analysis of conductive frame of large-capacity inverter. The eddy-current loss obtained by the electromagnetic field analysis is used as a heat source for the thermal analysis. Then, the suitable configuration of the frame for the suppression of temperature increase is investigated and designed. The hybrid finite-element and boundary-element (FE-BE) method is very suitable for the analysis of a complicated-shaped frame because it does not require mesh division for a free space and can easily treat eddy current. For the reduction of large computational costs, the fast multipole method (FMM) is introduced. Furthermore, in order to improve the convergence characteristic of iterative methods for system matrix, we develop the application of the IDR(s) method to the minor iterative preconditioning technique (MIP) and evaluate the performance. Finally, some numerical results that demonstrate the effectiveness of the developed method are presented. Index Terms--Coupled electromagnetic-thermal analysis, eddy-current, fast multipole method, hybrid finite-element and boundary-element method, IDR(s), large-scale analysis.

This paper presents a two-control loop design considering the nonlinear time-varying characteristics of a single-phase grid-connected photovoltaic (PV) full-bridge central inverter. The control scheme design is based on the energy-balance modeling of the PV system and enables the design of a voltage loop linear discrete controller ensuring the stability of the system for the whole range of PV array operating conditions. A set of experimental results carried out on a laboratory prototype is provided to validate the proposed approach. Index Terms--Photovoltaic power systems, power control, power electronics.

This paper examines the application of a diodeclamped multilevel inverter (DCMLI)-based distribution static compensator (DSTATCOM) connected to a three-phase, four-wire (3p4w) distribution system. The DCMLI has an inherent unbalancing problem among its dc capacitor voltages. Additionally, when the load contains a dc part, the neutral point of the compensator also becomes unbalanced. In this paper, the effects of different loading conditions on the dc capacitor voltages of the inverter are studied. Two new control circuits are proposed for equalizing the capacitor voltages. Different control techniques are presented for the equalizing circuits. Equations of the proposed equalizing controllers are developed corresponding to the control of the switching devices. Comparative studies of different control techniques and the performances of the two controllers are performed. Simulation studies are performed using PSCAD/EMTDC to validate the efficacy of the proposed equalizing controller circuits. Index Terms--Capacitor voltage equalization, dc-dc converter control, diode-clamped multilevel inverter (DCMLI), distribution static compensator (DSTATCOM), equalizing circuit.

Islanding detection and prevention is a mandatory requirement for grid-connected distributed-generation systems. In this paper, we propose an antiislanding technique based on the proportional power spectral density of the voltage period at the point of common coupling (PCC). The concept of proportional power spectral density (PPSD) is introduced as a normalized measure that can be used for islanding detection. We show that if the voltage period measured at the PCC is filtered and used to set the command period of the inverter, then an islanding condition results in a distinct PPSD. The technique is implemented and demonstrated using a 4-kW inverter and verified in simulation. Index Terms--Antiislanding, distributed generation (DG), grid-interconnection, islanding detection, power spectral density.

This paper presents a modified predictive current control strategy which allows one to have control over the spectrum of the load current. The proposed method uses a model of the system to predict the behavior of the current for each possible voltage vector generated by the inverter. For that purpose, at each sampling interval, signal predictions are evaluated using a cost function that quantifies the desired system behavior. The cost function used in this work evaluates the filtered error of the load currents. The inclusion of a filter for the load error allows one to manipulate current spectra. Thus, by designing this filter appropriately, the load spectrum can be shaped. The performance of the proposed control strategy is verified by simulation and experimental results. Index Terms--Current control, frequency control, frequency domain analysis, inverters, predictive control.

A novel dead-time elimination method is presented in this paper for voltage source inverters. This method is based on decomposing of a generic phase-leg into two basic switching cells, which are configured with a controllable switch in series with an uncontrollable diode. Therefore, dead-time is not needed. In comparison to using expensive current sensors, this method precisely determines the load current direction by detecting which anti-parallel diode conducts in a phase-leg. A low-cost diode-conduction detector is developed to measure the operating state of the anti-parallel diode. In comparison with complicated compensators, this method features simple logic and flexible implementation. This method significantly reduces the output distortion and regains the output RMS value. The principle of the proposed deadtime elimination method is described in detail. Simulation and experimental results are given to demonstrate the validity and features of this new method. Index Terms--Dead-time, gate drive, harmonic, phase-leg, pulsewidth modulation (PWM) control.

This paper focuses on the development of multilevel hysteresis current regulation strategies. Two such strategies have been discussed and some modifications in their control tasks have been proposed to achieve more reliable and improved performance. In general, the multiband concept has been used while making the proposals. The hysteresis band size considerations have also been presented by taking into account the desired and existing system conditions. The proposed modulation schemes have been applied to a five-level flying-capacitor inverter, whose operation under hysteresis current control mode is much less established. A new method of flying-capacitor voltage balancing is proposed which ensures balanced flying-capacitor voltages and, at the same time, maintains the desired current profile. It uses a time-based approach for controlling the capacitor voltages and achieves appreciable voltage spectrum under wide range of load power factor conditions. The performance of the proposed strategies is confirmed through both simulation and experimental investigations. Index Terms--Flying-capacitor multilevel inverters (FCMLI), flying-capacitor voltage, hysteresis, multilevel inverter, time-based control.

Commutation failure, which is a very frequent dynamic event in HVDC inverters, can deteriorate the availability of HVDC links and thus affect the performance of the power system. Most commutation failures are caused by voltage reduction due to ac system faults. In this paper, efforts are made to lower the effect of commutation failure on the power system, and the remedy is implemented in a fuzzy controller. In order to achieve control flexibility, the output of the fuzzy controller is made appropriate by setting a self-adjusting proportional factor. The validity and effectiveness of the fuzzy controller is verified by simulation which shows that the fuzzy controller can decrease commutation failure frequency induced by ac system faults. Index Terms--Commutation failure, fuzzy control, HVDC transmission, power system.

Islanding is one important concern for grid connected distributed resources due to personnel and equipment safety. Inverter-resident active islanding detection methods (IDMs) inject disturbances into the supply system and detect islanding based on the responses of locally measured parameters. Frequency drifting IDMs are very effective in systems with a single inverter but it is believed that their effectiveness might be reduced in multi-inverter systems due to mutual interferences. This paper discusses the effect of inverters with passive IDM on the islanding detection capabilities of inverters with active frequency drifting IDMs and also interferences between the latter. Non-detection zones (NDZs) of multi-inverter systems in a load parameter space are used as analytical tool. Simulation and experimental results are provided to validate the NDZs of the multi-inverter systems with active frequency drifting IDMs. Index Terms--Distributed generation (DG), inverters, islanding detection, non-detection zones, photovoltaic systems.

A survey of eight different advanced Regular-Sampled pulsewidth-modulation (PWM) strategies is presented and compared for three-phase inverters. The PWM strategies represent modifications of the basic Regular-Sampled technique and are designed to closely reproduce the 'exact' switching angles and performance of the well-known Harmonic Elimination, Harmonic Minimization, Space Vector Modulation, High-Frequency, and Hysteresis Band PWM strategies. Each strategy is compared, and the advantages and disadvantages of each are emphasized in terms of complexity and implementation. Experimental and computer-aided design simulation results are presented to show the differences between the various strategies and to confirm the validity and accuracy of each of the techniques. Index Terms--AC motor drives, electric machines, harmonic distortion, pulsewidth-modulation (PWM), regular sampling.

In inverter-fed motor coils, surge voltages with the rise time of several tens or hundreds of nano-second may cause partial discharge (PD) and degradation of electrical insulation performance of the inverter-fed motor coils. This paper discusses PD inception characteristics as well as PD propagation characteristics after PD inception for magnet wire of inverter-fed motor under surge voltage application. Experimental results firstly revealed that PD inception voltage (PDIV) decreased with the increase in the length of enamel-coated wire, which was evaluated in terms of the stressed wire contact length under surge voltage application, i.e. size effect. We proposed a regression line for the size effect on PDIV for the electrical insulation design of inverter-fed motor coils. Secondly, PD propagation characteristics were also investigated under the higher voltage application, and their mechanisms were discussed in terms of generation probability of initial electrons, space charge behavior in the wedge-shaped air gap, charging on the enamel surface and so on. Index Terms--Inverter-fed motor, inverter surge, partial discharge, inception voltage, initial electron, charge behavior.

The current source inverters may become direct competitors of the voltage source inverters thanks to the voltage control techniques. The paper proposes an improved voltage control technique for current source inverters, that chooses the current vectors relying on bang-bang controllers. This technique is compared to a well-established one using current space vector modulation and synchronous-frame proportional and integral controllers. The presented controller is explained and validated by means of off-line as well as real time simulations and experimental tests. The numerical results prove that the proposed control technique produces less distorted voltage waveforms in terms of total harmonic distortion and spectrum. According to the real-time simulations performed the proposed control technique is computationally less expensive than the benchmark. Finally, the experimental results obtained confirm the feasibility of the proposal. Index Terms--Current source inverter (CSI), power conversion, pulse width modulated power converters, voltage control.