In trasformerless grid-connected photovoltaic (PV) systems, common-mode voltage (CMV) fluctuations cause leakage current flow through the stray capacitance of the PV panels. Shoot-through states in a quasi-Z-source inverter (q-ZSI) increase the amplitude of high-order harmonics of CMV. In this paper, by using the modulation technique based on odd pulse width modulation and minor change in the Z network of the three-phase q-ZSI, the leakage current is blocked. No extra semiconductor element is added. By the proposed technique, CMV is kept nearly constant during switching cycles. The experimental results for CMV analysis in a $\text{1}\;{\text{kW}}$ prototype are presented to verify the theoretical analysis.

Three-Phase Quasi-Z-Source Inverter With Constant Common-Mode Voltage for Photovoltaic Application

This paper describes the design and implementation of the first medium-voltage impedance measurement unit (IMU) capable of characterizing in situ source and load impedances of dc and ac networks (4160 V ac, 6000 V dc, 300 A, 2.2 MVA) in the frequency range of 0.1 Hz–1 kHz. The IMU comprises three power electronics building blocks (PEBBs), each built using 10-kV SiC MOSFET H-bridges. The modularity of the PEBBs allows for both series and shunt perturbation injection modes to be realized, as both injection modes are needed to accurately predict the stability of the electrical system. The effectiveness of the proposed impedance identification approach is experimentally verified on medium voltage power grid.

Medium-Voltage Impedance Measurement Unit for Assessing the System Stability of Electric Ships

Training the weighting factors of model predictive control in multiobjective problems is a time consuming and sophisticated process. In this article, conventional model predictive control (CMPC) has been developed as supervised learning model predictive control (SLMPC) to cancel common-mode voltage (CMV) in a three-phase neutral-point-clamped (NPC) inverter, while other control objectives are desirably tracked. SLMPC is accurately and quickly trained through the artificial bee colony (ABC) algorithm to optimize the controller weighting factors. Using the optimized weighting factors, transient response is minimized and CMV is surpassed. After training the weighting factors, SLMPC containing the optimized waiting factors is applied to the three-phase NPC inverter without considering the ABC algorithm in the control loop. By applying the optimized weighting factors to the cost function, SLMPC has been evaluated under several experimental and simulation tests to show that desired control objectives, particularly CMV suppression, have been attained. The proposed training process can be generalized and used for MPC cost functions with more control objectives to obtain the best possible performance.

Supervised Learning Model Predictive Control Trained by ABC Algorithm for Common-Mode Voltage Suppression in NPC Inverter

The advancement in modulation strategies brings emergent solutions to suppress the leakage current in three-phase two-level transformerless grid-connected voltage source inverters (VSIs). However, most of the techniques are analyzed based on reduced common-mode voltage pulsewidth modulation (PWM) in motor drivers but fail to satisfy the specific requirement of grid-connected applications, such as wider power factor operation and lesser current distortion. To overcome such substantial drawbacks, this paper proposes a novel scalar PWM method for transformerless grid-connected VSIs. First, presented through a generalized scalar approach with unified zero-sequence voltage generation, the method is simple to implement and favored in practice. Besides, systematic and comprehensive mathematical analysis is illustrated to exhibit the validity of implemented method in reducing leakage current and shows its advantages over whole power factor range in terms of dc-link current harmonic, output current quality, and switching losses. Finally, simulations and experimental results are carried out to verify the effectiveness and superiority of the proposed method.

A Novel Scalar PWM Method to Reduce Leakage Current in Three-Phase Two-Level Transformerless Grid-Connected VSIs

This article proposes a common-mode voltage (CMV) attenuation method for the three-phase current source inverter (CSI)-fed permanent-magnet synchronous machine drive. The key of this method is to introduce an auxiliary circuit branch connected in parallel with the dc link. Based on the dedicated switching strategy, the zero-voltage-switching conditions are provided for the main switches in the CSI. In particular, the zero current vector is newly constructed without utilizing shoot-through operation in the CSI. Thus, the CMV in the traditional CSI drives is attenuated significantly. Compared with the existing research on CMV suppression, the proposed method will not compromise the modulation index range or increase output current harmonics. Meanwhile, the overshoot voltage can be clamped by the auxiliary power circuit in the dc link under open-circuit fault in the power switches. The system configuration, operating principle, analysis of operation modes, as well as the control scheme are described in detail. Both simulation and experimental results are presented to verify the performance of the proposed method.

A Soft-Switching Current-Source-Inverter-Fed Motor Drive With Reduced Common-Mode Voltage

With wide application of inverters in modern industry, common-mode voltage (CMV) problems invoked severe negative effects. Hardware and software solutions have been proposed to reduce the CMV. Compared with hardware CMV mitigation solutions, software strategies based on pulse-width modulation (PWM) modifications have aroused widespread attention for their cost-effective and control flexible advantages. Up to date, various reduced CMV PWM (RCMV-PWM) strategies have been reported, such as active zero state PWM, remote state PWM, near state PWM, phase-shifted carrier PWM, carrier peak position modulation and so on. For convenience of understanding and utilising the existed RCMV-PWM strategies, this study conducts a review on this topic. According to the principle of CMV reduction algorithms, the study divides the reported RCMV-PWM strategies into three different categories. It also presents comparisons of CMV reduction techniques and output performances (like CMV value, output ripple and linearity range) between different strategies. Meanwhile, CMV suppression principles, problems in implementation and improvements of each technique are described in detail. Moreover, the inherent characteristics and development tendencies of the RCMV-PWM techniques are discussed so as to offer reference for further research.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-pel.2015.1019

Review on pulse-width modulation strategies for common-mode voltage reduction in three-phase voltage-source inverters

In a variable speed drive (VSD) system, high-frequency switching of power devices will cause large common-mode (CM) voltage during operation. While the CM voltage acts on the parasitic capacitance of the system, it will generate serious CM current on the motor side. It is necessary to model the VSD system so as to analyze and forecast the CM current theoretically. However, an essential condition of modeling CM current is to extract the CM impedance on the motor side of the system. In this paper, a novel extraction method for the CM impedance of an inverter-fed induction motor is proposed. First of all, time-domain waveforms of the CM voltage and current on the motor side of the VSD system are measured, and then mathematical calculations are performed to obtain the spectrum characteristics of the CM impedance on the motor side of the system. Compared to the traditional extraction methods of the impedance parameter by an impedance measurement or numerical calculation, the method proposed in this paper does not require laborious modeling of the motor or cable, and the extraction process is much simpler and easier. Moreover, in traditional methods, the motor impedance parameters are usually obtained in a nonworking state, while the proposed extraction method can be conducted in working state. Thus, the CM impedance extracted by the proposed method will be more accurate. Through comparing measurement and the simulation results of the CM current on motor side, the proposed CM impedance extraction method for an inverter-fed induction motor is validated.

Extraction of Common-Mode Impedance of an Inverter-Fed Induction Motor

In this paper, a comprehensive method is proposed to derive the boost DC–DC converter from a given gain formula. The given gain formula is obtained by analyzing, generalizing, and summarizing previous boost structures in the literature. The analysis is based on the volt-second balance theory of inductors. Thus, the gain formula is only related to two elements, the connection between components and the duty cycle of the switches. Therefore, the topology corresponding to the gain can be derived and synthesized according to the connection of the inductors and the capacitor components during the commutation of the switch to meet the demand for different boost converters in different applications. Meanwhile, all the three-order gain formulas generated by this method are analyzed and three of them are selected for topology simplification and focus analysis, and the correctness of the selected topologies is verified by the simulation results and experimental results.

/pdf/a-topology-generation-and-synthesis-method-for-boost-lzbrfsy3.pdf

Huan Zhao

Papers

Review on pulse-width modulation strategies for common-mode voltage reduction in three-phase voltage-source inverters

Extraction of Common-Mode Impedance of an Inverter-Fed Induction Motor

A Topology Generation and Synthesis Method for Boost Converters Based on Inductive Volt-Second Balance Theory