In this paper, the switching cell concept is extended to isolated ac-ac converters, and a highly reliable double step-down ac-ac converter is proposed with high-frequency transformer (HFT) isolation. ...By using the switching cell structure and coupled inductors, the proposed converter has no commutation problem as it is immune from both short-circuit and open-circuit problems, even when all the switches are turned on or turned off, simultaneously. Therefore, it does not require pulse width modulation dead times along with bulky and lossy RC snubbers or voltage-sensing circuitry to implement soft-commutation strategies, resulting in high reliability and high quality output waveforms. The HFT in the proposed converter provides electrical isolation and safety which is required in applications such as dynamic voltage restorer (DVR), solid-state transformer (SST), etc., without the need for external bulky line frequency transformer. Moreover, all the passive components experience twice the switching frequency; therefore, their size can be reduced. The proposed converter is very suitable for application as DVR, to compensate both voltage sags and swells, owing to its ability to provide both inverting and noninverting outputs. A detailed theoretical analysis and operation of the proposed ac-ac converter are provided, and its applications as DVR and SST are also discussed. Experimental results with scaled-down prototype are also provided to verify its performance.
This paper presents a new power converter topology to suppress the torque ripple due to the phase current commutation of a brushless dc motor (BLDCM) drive system. A combination of a three-level ...diode clamped multilevel inverter (3-level DCMLI), a modified single-ended primary-inductor converter (SEPIC), and a dc-bus voltage selector circuit is employed in the proposed torque ripple suppression circuit. For efficient suppression of torque pulsation, the dc-bus voltage selector circuit is used to apply the regulated dc-bus voltage from the modified SEPIC during the commutation interval. In order to further mitigate the torque ripple pulsation, the 3-level DCMLI is used in the proposed circuit. Finally, simulation and experimental results show that the proposed topology is an attractive option to reduce the commutation torque ripple significantly at low- and high-speed applications.
The line commutated converter (LCC)-based high voltage direct current (HVDC) transmission system is an effective scheme for long-distance transmission of wind power. Commutation failure is a common ...failure of LCC-HVDC transmission system. During the commutation failure, the overvoltage of the sending alternating current (ac) grid will occur to disconnect the wind farm from the utility grid and limit the transmission capacity of the system. It is necessary to suppress the overvoltage of the sending ac grid under commutation failure to increase the transmission capacity of the LCC-HVDC system. In this article, a mathematical model of the transient overvoltage peak value of the sending ac grid under commutation failure is established. The mathematical model can quantitatively analyze the influence of the reactive power of the wind farm and the transmission capacity of the LCC-HVDC system on the overvoltage peak value of the sending ac grid. Based on the mathematical model, a power coordinated control strategy for wind farms is proposed to improve the reactive power support capability under grid overvoltage. The proposed control strategy can effectively suppress the overvoltage of the sending ac grid and increase the transmission capacity of the LCC-HVDC system. The accuracy of the mathematical model is verified by simulation, and the superiority of the power coordinated control strategy is verified by the control hardware-in-the-loop (CHIL) experiment.
In order to improve the speed precision and stabilization of the gimbal servo system of double gimbal magnetically suspended control moment gyro, a comprehensive analysis of the reason of ...electromagnetic torque ripples of brushless direct current motor with nonideal back electromotive force (EMF) drives in the conduction and commutation regions is presented. A novel automatic control method of torque is proposed. With this method, the current control rule is designed, and the duty cycle of pulse width modulation (PWM) is regulated in real time by measuring the wave function of back EMF. In order to eliminate the diode freewheeling of the inactive phase, the PWM_ON_PWM scheme is used. Simulation and experiment results are given to show that, compared with the conventional current control method, the proposed method can reduce the torque ripple effectively and improve the speed precision and stabilization as well.
Abstract
The failure of commutation will cause the abrupt change of the operating parameters of the DC system. In this paper, the mechanism and physical process of commutation failure are analyzed. ...In view of the changing characteristics of inverter side valve current and DC current in the case of commutation failure, two kinds of S transform entropy are introduced into the detection of commutation failure of DC system in combination with S transform entropy’s excellent signal change feature recognition ability. In addition, the detection performance of the two s-transform entropies is fully considered in the case of no commutation failure, critical commutation failure and commutation failure. The simulation results based on PSCAD/EMTDC preliminarily verify the validity and accuracy of the two kinds of S transform entropy in dc system fault detection, which provides a basis for the further application of S transform entropy in dc system fault detection field.
This paper presents a novel topology for the generation of adjustable frequency and magnitude pulsewidth-modulated (PWM) three-phase ac from a balanced three-phase ac source with a high-frequency ac ...link. The proposed single-stage power electronic transformer (PET) with bidirectional power flow capability may find application in compact isolated PWM ac drives. This topology along with the proposed control has the following advantages: 1) input power factor correction; 2) common-mode voltage suppression at the load end; 3) high-quality output voltage waveform (comparable with conventional space vector PWM); and 4) minimization of output voltage loss, common-mode voltage switching, and distortion of the load current waveform due to leakage inductance commutation. A source-based commutation of currents associated with energy in leakage inductance (termed as leakage energy) has been proposed. This results in soft-switching of the output-side converter and recovery of the leakage energy. The entire topology along with the proposed control scheme has been analyzed. The simulation and experimental results verify the analysis and advantages of the proposed PET.
Sparking is a common phenomenon in brushed DC motors. Early detection of sparks can prevent their aggravation and promptly adopt appropriate maintenance actions. In this work, we propose the ...application of a multifractal analysis to armature currents and stray flux signals to detect the presence of sparks in the commutation system of DC motors. Two methodologies are proposed in the paper, the first one using the spectral kurtosis of armature currents and the second one using the flux signal envelope. We apply this method to signals captured both under starting and at steady state to compare their suitability and analyze their feasibility to be used as the basis of the sparking detection method. Additionally, we also propose a quantitative indicator based on the variance of the complex cepstrum to determine the severity of the failure based on the cepstral analysis of both considered signals. The results demonstrate the potential of the approach for sparking detection and sparking level assessment in DC motors and its suitability for potential future incorporation into autonomous systems.
The commutation of 8.5, 2.5 kA thyristors is experimentally investigated in a wide range of operation conditions. The dependence of the commutation turn-off time <inline-formula> <tex-math ...notation="LaTeX">{t} _{q} </tex-math></inline-formula> on the commutation di/dt, reverse voltage during commutation, forward blocking dv/dt, and temperature is presented in the range of 20-<inline-formula> <tex-math notation="LaTeX">800 ~\mu \text{s} </tex-math></inline-formula>. The <inline-formula> <tex-math notation="LaTeX">{t} _{q} </tex-math></inline-formula> of phase control thyristors (PCT) is carried out for three application relevant combinations of the ON-state voltage <inline-formula> <tex-math notation="LaTeX">{V}_{T} </tex-math></inline-formula> and recovery time <inline-formula> <tex-math notation="LaTeX">{Q} _{\text {rr}} </tex-math></inline-formula> and compared with that of the bidirectional PCT (BiPCT). Growth coefficients of <inline-formula> <tex-math notation="LaTeX">{t} _{q} </tex-math></inline-formula> are provided for practical magnitudes of di/dt, dv/dt, and temperature to be used in the design of high-power converters.
A seven-level hybrid-clamped (7L-HC) converter is a promising option for medium-voltage drives due to its low number of devices and the ability to well balance all the capacitors. However, like other ...hybrid-clamped multilevel converters, the freewheeling paths during deadtime can produce unexpected output voltage levels. The unexpected output voltage can cross a few consecutive output levels, introducing a large dv / dt to the system. To address this problem, in this article, we conduct a detailed study of the mechanism that produces such undesirable voltages and propose a commutation scheme to coordinate the turn- on / off sequence of switches in the 7L-HC converters. The proposed method is based on an easy-to-implement bitwise operation of switching vectors, where the number of switching actions is not increased. The proposed method is validated by both simulation and experimental results.
In order to improve the performance of surface permanent magnet synchronous motor drives, a novel sensorless drive scheme is proposed in this paper, which is based on the virtual third-harmonic back ...EMF incorporating a fractional-N phase-locked loop (FN-PLL) and a synchronous frequency-extract filter (SFF). A simple low-pass filter based circuit is used to obtain the commutation points through the virtual third-harmonic back EMF. After that, an FN-PLL is applied to obtain continuous rotor position angles. On this basis, the SFF-based compensation is proposed to compensate the detecting error adaptively. Meanwhile, the field vector control loop for surface PM motor is proposed, which is based on the proposed rotor position detection link and compensation link. An experimental high-speed magnetically suspended blower platform is built. The effectiveness and the feasibility of the proposed method are validated with experimental results.
