Zhiyong Zeng

Modeling, Modulation, and Control of the Three-Phase Four-Switch PWM Rectifier Under Balanced Voltage

The modeling, modulation, and control of the three-phase four-switch (TPFS) PWM rectifier are investigated in this paper. Three space vector pulse width modulation methods using different equivalent zero vectors are developed, where sector identification and the trigonometric function are not required. Then, the high-frequency model for the current ripple analysis is proposed, and the effects of three SVM approaches on the ac current ripple are investigated. According to the analytical results, the method introducing the smallest current ripple is selected. With the optimized SVM approach, a control-oriented model, considering the capacitor voltage oscillation and deviation, is built in the dq synchronous frame to facilitate the controller design. Furthermore, a control strategy implementing the proportional controller is developed to eliminate the capacitor voltage deviation. Meanwhile, the dual-loop control of the TPFS is not affected by the proposed strategy as the capacitor voltage deviation is eliminated. Finally, a novel linear modulation index function is defined to reject the low-frequency harmonic current introduced by the overmodulation. Experimental results demonstrate that excellent current performance is achieved with comprehensive considerations of the modeling, modulation, and control strategy.

The Comprehensive Design and Optimization of the Post-Fault Grid-Connected Three-Phase PWM Rectifier

The comprehensive design and optimization, including the optimized modulation approach, detailed modeling, and reliable control algorithm, is presented in this paper to guarantee the stable operation of the postfault three-phase pulsewidth modulation (PWM) rectifier. The effects of the space vector modulation approaches on the capacitor current are investigated. Based on the analytical results, the method introducing the smaller capacitor current is selected as the optimized modulation approach, which is of paramount importance to avoid the permanent failure of the dc-link capacitors. Then, with the optimized modulation approach, a control-oriented model for the post-fault PWM rectifiers is derived based on the α - β stationary frame, where the capacitor voltage oscillation and deviation are taken into account. The proposed model-based controllers do not require the phase-locked loop (PLL), and the elimination of the PLL means that the post-fault rectifiers can never lose synchronization with the grid. Furthermore, an equation of the required dc voltage for linear modulation is proposed, and using the proposed equation, a correct dc voltage is selected to reject the overmodulation. The experimental results validate the effectiveness of the proposed comprehensive design and optimization. Finally, a conclusion can be drawn from the experimental results-that the dc-link capacitor current and the linear modulation should be taken into account when selecting the dc voltage in the post-fault operation.

Hybrid Space Vector Modulation Strategy for Torque Ripple Minimization in Three-Phase Four-Switch Inverter-Fed PMSM Drives

Three-phase four-switch (TPFS) inverters are generally applied as cost-reduction topologies for permanent-magnet synchronous motor (PMSM) drives because of their reduced number of switching devices. However, undesirable torque ripples are produced by the inverter-fed PMSMs due to the application of nonsinusoidal voltages. Because the torque ripples are strongly influenced by the employed pulse width modulation (PWM) strategy, two commonly used switching sequences in TPFS inverter-fed PMSM drives are fully investigated based on the root mean square value of the torque ripples, in which the effects of the different equivalent zero vectors on the torque ripples are presented. Then, a hybrid space vector modulation (SVM) strategy is proposed to minimize the torque ripples by alternatively using the two equivalent zero vector synthesis approaches during a fundamental period. The sector division of the proposed hybrid SVM strategy is determined by the location of the stator current vector, which is quite different from the methods used in other SVM methods. Then, a simplified sector identification method is proposed to reduce the computational burden. The experimental results demonstrate that the proposed hybrid PWM strategy can effectively reduce torque ripples in TPFS inverter-fed PMSM drives.

Performance Analysis of the Zero-Voltage Vector Distribution in Three-Phase Four-Switch Converter Using a Space Vector Approach

The ac current ripple, the common voltage (CMV), and the current stress on the dc-link capacitor are defined as the critical performance indicators of the three-phase four-switch (TPFS) converter, and the effect of the zero-voltage vector distribution on these crucial indicators is investigated comprehensively in the proposed paper. A unified space vector modulation (SVM) approach, including the special pulsewidth-modulated (PWM) signal logic, is developed to facilitate the performance analysis of the zero-voltage vector distribution. Then, a novel approach based on the nonorthogonal stationary frame is proposed to facilitate the sector identification. Furthermore, because of the asymmetry of the harmonics of the three-phase output voltages, an overall current performance indicator, the total three-phase ac current ripple, is defined to assess the overall performance of the three-phase ac current. In addition, the dc-link capacitor current and CMV per sampling period are investigated; different ineffective duty ratio allocations result in the varied CMVs and capacitor currents. Moreover, due to the complexity of the Fourier analysis method, the root-mean-square (rms) values of the crucial indicators are adopted to evaluate the relative merits of various zero-voltage vector distributions. The proposed rms value equations are straightforward for the evaluation of the effect of the zero-voltage vector distribution on these crucial indicators. Experimental results confirm the accuracy and usefulness of the proposed performance analysis.

Comprehensive Analysis and Reduction of Torque Ripples in Three-Phase Four-Switch Inverter-Fed PMSM Drives Using Space Vector Pulse-Width Modulation

As a result of their reduced number of switches, three-phase four-switch (TPFS) inverters are generally applied as cost-reduction topologies for permanent magnet synchronous motor (PMSM) drives. However, the torque ripples of PMSM severely deteriorate the performance and reliability of the entire system. Hence, comprehensive considerations for torque ripple reduction, including high- and low-frequency torque ripples, are elaborated considering TPFS inverter-fed PMSM drives. The second-order torque harmonics produced by dc-capacitor voltage fluctuations are first demonstrated, and a very simple compensation method is presented by introducing a novel nonorthogonal coordinate transformation. Then, to evaluate the effects on the high-frequency torque ripples of space vector modulation (SVM) schemes, three SVM schemes for TPFS inverter-fed PMSM drives are assessed based on the torque ripple root-mean-square value. Consequently, the preferred SVM scheme is obtained for high-frequency torque ripple minimization. Moreover, the linear modulation range of the TPFS inverter-fed PMSM drive is derived considering capacitor voltage fluctuations, therein avoiding the low-frequency torque ripples caused by overmodulation. Meanwhile, an adaptive capacitor voltage offset suppression method is proposed to fully exploit the dc-link voltage. The experimental results demonstrate the validation and effectiveness of the proposed analysis and methods for torque ripple reduction.

Space-Vector-Based Hybrid PWM Strategy for Reduced DC-Link Capacitor Current Stress in the Postfault Grid-Connected Three-Phase Rectifier

A hybrid pulse width modulation (PWM) strategy for the postfault grid-connected three-phase rectifier is proposed in this paper to achieve a reduced current stress on the dc-link capacitors. The capacitor current per sampling period is investigated, which consists of the effective duty ratio-based currents and the ineffective duty ratio-based currents. The currents introduced by the ineffective duty ratio are highly dependent on the allocation of the ineffective duty ratio. Therefore, this degree of freedom of the ineffective duty ratio allocation can be utilized to minimize the capacitor current. Then, the effect of the ineffective duty ratio allocation on the amplitudes of the capacitor current is analyzed. Based on this analysis, the optimized ineffective duty ratio allocations are proposed, where the ineffective duty ratios are flexibly allocated with respect to the vector angle of the ac current. Furthermore, a novel distribution of the vector plane is developed for the proposed hybrid space vector modulation (HBSVM) strategy. Moreover, the root-mean-square (RMS) values of the capacitor current are used to evaluate the performance of the proposed hybrid PWM strategy. The analytical results demonstrate that the proposed HBSVM strategy results in the reduced capacitor current RMS, which avoids a failure of the dc-link capacitors. Finally, the experimental results validate the effectiveness of the proposed HBSVM strategy.

Improved voltage vector control for a stand-alone DFIG system based on predictive current regulation algorithm

This study proposed an improved voltage vector control strategy based on predictive current control (PCC) for stand-alone doubly fed induction generator (DFIG) systems. The proposed control scheme implements close-loop control of both d-axis and q-axis stator voltage in a synchronously rotating reference frame, which regulates the magnitude and the phase of stator voltage precisely. Compared with previous voltage control schemes, this method provides fast and accurate tracking ability of stator voltage, resulting in excellent steady-state and dynamic performance of the system. Moreover, complex delay time compensation in PCC is no longer needed with the recommended strategy, intensely reducing the calculation burden. Simulation results for a 160kW stand-alone DFIG system are presented to validate the effectiveness of the proposed method.

Direct Stator Flux Vector Control Strategy for IPMSM using a Full-order State Observer

A direct stator flux vector control scheme in discrete-time domain is proposed in this paper for the interior permanent magnet synchronous motor (IPMSM) drive to remove the proportional-integral (PI) controller from the direct torque control (DTC) scheme applied to IPMSM and to obtain faster dynamic response and lower torque ripple output. The output of speed outer loop is used as the desired torque angle instead of the desired torque in the proposed scheme. The desired stator flux vector in dq coordinate is calculated with a given amplitude. The state-space equations in discrete-time for IPMSM are established, the actual stator flux vector is estimated in deadbeat manner by a full-order state observer, and then the closed-loop control is achieved by the pole placement. The stator flux error vector is utilized to calculate the reference stator voltage vector. Extracting the angle position and amplitude from the estimated stator flux vector and estimating the output torque are eliminated for the direct feedback control of the stator flux vector. The proposed scheme is comparatively investigated with a PI-SVM DTC scheme by experiment results. Experimental results show the feasibility and advantages of the proposed control scheme.

Small-signal discrete-time modeling of digitally controlled three-phase PWM boost rectifier under balanced voltage

Purpose The model for digitally controlled three-phase pulse width modulation (PWM) boost rectifiers is a sampled data model, which is different from the continuous time domain models presented in previous studies. The controller, which is tuned according to the model in continuous time domain and discretized by approximation methods, may exhibit some unpredictable performances and even result in unstable systems under some extreme situations. Consequently, a small-signal discrete-time model of digitally controlled three-phase PWM boost rectifier is required. The purpose of this paper is to provide a simple but accurate small-signal discrete-time model of digital controlled three-phase PWM boost rectifier, which explains the effect of the sampling period, modulator and time delays on system dynamic and improves the control performance. Design/methodology/approach Based on the Laplace domain analysis and the waveforms of up-down-count modulator, the small signal model of digital pulse width modulation (DPWM) in the Laplace domain is presented. With a combination of state-space average and a discrete-time modeling technique, a simplified large signal discrete time model is developed. With rotation transformation and feed-forward decoupling, the large-signal model is decoupled into a single input single output system with rotation transformation. Then, an integrated small signal model in the Laplace domain is constructed that included the time delay and modulation effect. Implementing the modified z-transform, a small-signal discrete-time model is derived from the integrated small signal model. Findings In a digital control system, besides the circuit parameters, the location of pole of open-loop transfer function is also related to system sampling time, affecting the system stability, and the time delay determines the location of the zero of open-loop transfer function, affecting the system dynamic. In addition to the circuit parameters discussed in previous literature, the right half plane (RHP) zero is also determined by the sampling period and the time delay. Furthermore, the corner frequency of the RHP zero is mainly determined by the sampling period. Originality/value The model developed in this paper, accounting for the effect of the sampling period, modulator and time delays on the system dynamic, give a sufficient insight into the behavior of the digitally controlled three-phase PWM rectifier. It can also explain the effect of sampling period and control delay time on system dynamic, accurately predict the system stability boundary and determine the oscillation frequency of the current loop in critical stable. The experimental results verify that the model is a simple and accurate control-oriented small-signal discrete-time model for the digitally controlled three-phase PWM boost rectifier.

An optimized algorithm for SVPWM based on three-phase stationary frame

The conventional space vector pulse width modulation (SVPWM), including coordinate transformation, sector identification, and active time calculations of voltage vectors, needs lots of complex irrational number operations such as trigonometric function calculations, which introduces challenges for real-time digital control chips. Considering the shortcomings of traditional algorithm, this paper presents an optimized algorithm based on three-phase stationary frame. In the proposed algorithm, the reference voltage vector is projected onto the three-phase stationary frame firstly, then a unified formula is obtained by utilizing the principle of symmetric, where only normal mathematical operations of modulation wave is required. With the unified formula, the duty ratios of three bridge-arms will be calculated directly. Comparing with conventional algorithm, the proposed algorithm is more suitable for digital system owing to the characteristic of easier programming, faster operating speed, and higher real-time ability. The simulation and experimental results show the validity and feasibility of the proposed algorithm.