Space Vector Sequences Research Articles

Optimal Sector-Based Sequential Model Predictive Control for Current Source Rectifier

Conventional model predictive control (MPC) methods for current source rectifier (CSR) use either a PI controller or weight factor-based methods, which require time-consuming tuning methods to develop an efficient controller. Moreover, the limitations on the bandwidth requirements of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter cause resonances and grid current distortions if a variable switching frequency-based predictive control is used. So, a new fixed switching frequency-based MPC algorithm has been developed to control the load and source current simultaneously, thereby facilitating the weight factor less operation and applying a predefined space vector sequence. Unlike the conventional single cost function methods applied to CSR, the proposed method uses separate cost function for each control variable. Simultaneously, to provide fixed frequency operation, duty ratio-based modulation of grid current control has been applied. A virtual impedance-based active damping has been embedded into the supply current control to suppress the oscillations caused by the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter and grid current harmonics. Simulation and experimental results have been supported to validate the improvements in terms of supply current total harmonic distortion (THD), steady state, and transient switching performance.

Three‐vector model predictive power control for three‐level T‐type rectifier based on dead‐beat control

This paper presents a three-vector model predictive power control with neutral-point voltage balance (TVMPPC-NPVB) and a dead-beat control for three-phase three-level T-type rectifier. Three vectors are adopted to reduce the current ripple, balance the neutral-point voltage and achieve unity power factor on the AC side simultaneously, while the conventional model predictive power control suffers from large current ripple, difficulty in designing the weighting factor and computational burden. In the proposed strategy, two cost functions are derived to select the optimal space vector sequence and reduce the computational burden. The action time of each vector is introduced in detail. The traditional PWM rectifier utilizes PI controller to regulate the output voltage. But PI controller tends to have a long transition time and big variation on output voltage when the load changes suddenly. The proposed dead-beat control is capable of solving these problems caused by conventional PI controller. Both simulation and experimental results are shown to confirm the effectiveness of the proposed method.

Optimal Space Vector Sequence Investigation Based on Natural Sampling SVM for Medium-Voltage Current-Source Converter

The device switching frequency of current-source converters (CSCs) in high-power medium-voltage (MV) drive applications is normally around 500 Hz. Conventional space vector modulation (SVM) features fast dynamic response but its output contains low-order harmonics with high magnitudes. Recently, a natural-sampling-based SVM (NS-SVM) with superior low-order harmonics performance has been proposed for MV CSCs. This study further investigates the low-order harmonics performance of different space vector sequences for MV CSCs based on NS-SVM. Dwell times equations based on NS-SVM for each space vector sequence with minimized switching frequency are analyzed and derived; comparisons of different vector sequences in terms of low-order harmonics and load/grid-side total demand distortion performance are thoroughly conducted. Among all the investigated sequences, sequence 2 (SQ2) is the best one; its linear two-step dwell times equations are provided for online implementation. Simulations and experiments are provided to verify its performance.

Continuous Space Vector Modulation for Symmetrical Six-Phase Drives

The two common three-leg, two-level voltage source inverters of a six-phase drive are able to produce 64 different switching states, representable as voltage space vectors in two orthogonal subspaces. The discrete output voltage of the inverter legs is usually controlled by means of a space vector modulation (SVM) technique, the aim of which is to produce the same volt-seconds as the continuous reference voltage vectors in both subspaces within a certain time frame (switching cycle). Using a simplified phase model of the machine consisting in each subspace of a voltage source and a leakage inductance, the phase currents are found to exhibit unwanted distortion harmonics. A proper sequence of voltage vectors has to be applied in order to reduce these harmonics. By allowing each inverter leg to change its output twice during the switching cycle (continuous modulation), 518400 different space vector sequences can be generated. This paper demonstrates that, among these sequences, only 11 need to be considered to modulate the reference vectors. Further simulation shows that, for a certain pair of reference vectors and leakage inductance ratio, only one of them reduces the distortion to a minimum. Due to the high computational effort, however, this optimization problem is not solvable online. To overcome this drawback, four different modulation strategies, the standard three-phase SVM, the fragmented sector mapping, the optimal interleaved and the approximated interleaved SVM, are derived from and compared with an offline optimization. Each strategy is evaluated regarding the resulting current distortion and the influence of the reference vectors as well as the inductance ratio. The harmonic current of these online modulation strategies is calculated in simulation and validated by experiments. Applying the proposed modulation techniques leads to a great reduction in the machine losses caused by switching.

Capacitor voltage balancing of a three‐phase neutral‐point clamped bi‐directional rectifier using optimised switching sequences

In this study, a modulation strategy is presented using minimum number of switching transitions of optimised switching sequences for balancing of DC-bus capacitor voltages of a three-phase neutral-point clamped bi-directional converter. This new approach is derived from the fact that in front end AC–DC converters, the location of supply voltage vector and reference current vector (magnitude and lag angle) vary depending upon the amount of reactive power compensation and the load current to be supplied by converter. Accordingly, the effect of small and medium vectors (using space vector pulse-width modulation) for same switching state varies in each sector and its regions resulting in DC-bus capacitor voltage unbalancing. This observation along with the detailed analysis of current pattern for both the capacitors within each sector and its regions and its effect on neutral-point voltage deviation forms the basis of proposed approach of maintaining optimised space vector sequences and changing the number of sampling periods within each sector and its regions, thus resulting in DC-bus capacitor voltage balancing. As a result, modified reference current vector spends different amount of time in alternate sectors (moves faster and slower in alternate sectors) but total time for one complete cycle is same as actual current vector. The converter exhibits excellent performance in terms of other critical parameters like unity input power factor, low input current THD, minimum switching losses, reduced ripple factor of the regulated DC output voltage and particularly, the neutral-point voltage deviation under dynamic load conditions both for rectification and inversion modes of operation.

FPGA Implementation of SVPWM Technique for Seven-Phase VSI

This paper presents the performance investigation of FPGA implementation of SVPWM technique for seven-phase voltage source inverter (VSI). Seven-phase voltage source inverters are dominantly used to supply the seven-phase drives which are used for high power applications. In the SVPWM technique a large space vectors are detailed. A voltage source inverter with the proposed SVPWM is simulated in Matlab/Simulink. The performance of the 7-phase VSI is analyzed in terms of fundamental voltage and THD. In the proposed work, SVPWM algorithm is described in high-speed integrated circuit hardware description language coding and implemented in an XILINX Spartan 3A FPGA processor. The selection of zero and active space vector sequence are discussed and the possibilities of realization on FPGA are analyzed.

Space Vector Sequence Investigation and Synchronization Methods for Active Front-End Rectifiers in High-Power Current-Source Drives

Space vector pulsewidth modulation (PWM) schemes for the active front end of a high-power drive normally produce low-order and suborder harmonics due to the low switching frequency and the drifting of synchronization between the PWM waveform and the rectifier input frequency. To provide a synchronized PWM and achieve the best harmonic performance, different space vector sequences suitable for a current-source converter are investigated in this paper. Details on how to achieve the waveform symmetries with a minimum switching frequency for each sequence are discussed. A thorough comparison of the harmonic performance of different space vector sequences is carried out. An optimum space vector modulation method by switching between two best sequences is proposed to achieve the best line-current total harmonic distortion with reduced switching losses. In addition, two synchronization methods, namely a PWM frame regulation method and a direct digital phase-locked loop synchronization method, are proposed. Both methods are equally effective in providing tight synchronization of the PWM waveform with the rectifier input frequency. The work has been verified in simulation and experiment.

Flux modulation for multilevel inverters

This paper presents a new flux modulation approach for the closed-loop control of the output voltage of multilevel inverters, using hysteresis with a rectangular bounding box in the synchronous d-q reference frame and a simple set of switching rules. The modulator is constrained to select only from the three active space vectors nearest to the target reference phasor, and consequently achieves a significantly improved harmonic performance compared to other hysteresis modulators. Furthermore, by shifting the reference origin on the vector diagram, the proposed modulator can achieve the same optimal space vector sequence and placement, and therefore the same harmonic performance, as the well-known centered space-vector modulation. Without major hardware modifications, the proposed flux modulation scheme can also be readily modified to achieve reduced-common-mode voltage switching, while retaining the same optimal vector sequence and placement. Also, since the modulator is a closed-loop system, it exhibits excellent robustness to parameter variations and other system disturbances. The performance and practicability of the new modulator have been verified both in simulation and experimentally.