Introduction to Electromagnetic Compatibility

In photovoltaic (PV) applications, a transformer is often used to provide galvanic isolation and voltage ratio transformations between input and output. However, these conventional iron- and copper-based transformers increase the weight/size and cost of the inverter while reducing the efficiency and power density. It is therefore desirable to avoid using transformers in the inverter. However, additional care must be taken to avoid safety hazards such as ground fault currents and leakage currents, e.g., via the parasitic capacitor between the PV panel and ground. Consequently, the grid connected transformerless PV inverters must comply with strict safety standards such as IEEE 1547.1, VDE0126-1-1, EN 50106, IEC61727, and $\text{A}S/N$ ZS 5033. Various transformerless inverters have been proposed recently to eliminate the leakage current using different techniques such as decoupling the dc from the ac side and/or clamping the common mode (CM) voltage (CMV) during the freewheeling period, or using common ground configurations. The permutations and combinations of various decoupling techniques with integrated voltage buck–boost for maximum power point tracking (MPPT) allow numerous new topologies and configurations which are often confusing and difficult to follow when seeking to select the right topology. Therefore, to present a clear picture on the development of transformerless inverters for the next-generation grid-connected PV systems, this paper aims to comprehensively review and classify various transformerless inverters with detailed analytical comparisons. To reinforce the findings and comparisons as well as to give more insight on the CM characteristics and leakage current, computer simulations of major transformerless inverter topologies have been performed in PLECS software. Moreover, the cost and size are analyzed properly and summarized in a table. Finally, efficiency and thermal analysis are provided with a general summary as well as a technology roadmap.

Transformerless Inverter Topologies for Single-Phase Photovoltaic Systems: A Comparative Review

This paper presents a pulse width modulation (PWM) technique for the minimization of the leakage current in the grid-connected/stand-alone transformerless photovoltaic (PV)-cascaded multilevel inverter (CMLI). The proposed PWM technique is integrated with the MPPT algorithm and is applied to the five-level CMLI. Furthermore, using the proposed PWM technique the high-frequency voltage transitions in the terminal and common mode voltages are minimized. Thus, the proposed PWM technique minimizes the leakage current of the PV array and electromagnetic interference filter requirement in the system without addition of any extra switches. Furthermore, this paper also presents the analysis for the terminal voltage across the PV array and the common mode voltage of the inverter based on the switching function. Using the given analysis, the effect of the PWM technique can be analyzed, as it directly links the switching function with the common mode voltage and leakage current. Also, the proposed PWM technique requires reduced number of carrier waves compared to the conventional sinusoidal pulse width modulation technique for the given CMLI. Complete details of the working principle and analysis with the support of simulation and experimental results of the proposed PWM technique are presented in this paper.

Analysis of the Modulation Strategy for the Minimization of the Leakage Current in the PV Grid-Connected Cascaded Multilevel Inverter

Spread-spectrum (SS)-based pulsewidth modulation (PWM) techniques play an important role when tackling the electromagnetic interference (EMI) problem in the switch-mode power supply (SMPS) domain. They achieve the best EMI noise reduction and emerge as a promising and very effective solution to comply with electromagnetic standards. The current work provides an insight into the SS techniques: its history, its types, and mainly its ability to mitigate the EMI problem in the SMPS domain. Moreover, it gives a short review of the different SS-based PWM techniques going from the most basic and simple, i.e., the periodic modulation, to the most developed and complex, i.e., the random and chaotic modulations. Besides, a short review of the different implementation methods was addressed and they were categorized into analog and digital implementation techniques where its pros and cons were provided. Finally, a new fast digital implementation method based on a dsPIC33F microcontroller is presented. Its effectiveness was proved and its advantages over the reviewed implementation techniques were recognized. It reached a 20 dBuV EMI level reduction when the digital chaotic and periodic profiles were embedded into a boost converter prototype.

A Review of Spread-Spectrum-Based PWM Techniques—A Novel Fast Digital Implementation

Transformerless grid-connected inverters (TLI) feature high efficiency, low cost, low volume, and weight due to using neither line-frequency transformers nor high-frequency transformers Therefore, TLIs have been extensively investigated in the academic community and popularly installed in distributed photovoltaic grid-connected systems during the past decade This article analyzes and summarizes the state of the art of TLI techniques, three rules of maintaining constant common-mode voltage (CMV) of TLIs at switching frequency have been concluded from a generic CMV analysis model at the beginning Second, suppression methods of leakage current (LC) and dc current injection (DCCI), so-called two key challenges for first-generation TLIs, have been classified and discussed in detail, respectively Finally, future trends and some challenges of TLIs based on wide bandgap devices named second-generation TLIs have been presented in this article

Overview of Transformerless Photovoltaic Grid-Connected Inverters

Transformerless photovoltaic (PV) grid-connected inverters have become more and more popular in distributed PV power generation systems due to the advantages on high efficiency, low cost, and small size. However, common-mode (CM) current in the transformerless PV inverters can result in serious electromagnetic interference and insecurity, which will further reduce the reliability of the PV inverter systems. In this paper, an improved H5 topology, namely H5-D topology, is proposed, in which a clamping diode is added on the basis of H5 topology to eliminate the CM voltage fluctuation in H5 topology. Further, the PSIM simulation results of the H5-D topology and H5 topology are given and compared, especially, the performance of H5-D topology for CM current suppression is presented and analyzed concretely. Finally, the experimental prototypes of the H5-D topology and H5 topology are built and tested, and the experimental results validate the advantages of the H5-D topology. The proposed H5-D topology provides a new practical topology for distributed PV grid-connected power generation systems.

An Improved H5 Topology With Low Common-Mode Current for Transformerless PV Grid-Connected Inverter

Passive electromagnetic interference filters (PEFs) are the most common way to solve electromagnetic interference (EMI) problems in power converters. However, PEFs bring additional volume, weight, and cost for power converters, especially common-mode (CM) inductors in PEFs, and it is a tricky issue for the high-power-density converters that must meet the electromagnetic compatibility specification. In order to design compact PEFs for power converters with pulsewidth modulation (PWM), a volume reduction method of CM inductors with chaotic PWM (CPWM) is proposed in this article. First, the mechanism that CPWM reduces the CM inductance by increasing the corner frequency is analyzed. Second, the relationship between the reduction of EMI spectrum magnitude by CPWM and the decrease of the CM inductance is quantitatively calculated. Third, utilization rate of the magnetic core η is defined to reasonably compare the size of the different inductors under traditional PWM and CPWM. Finally, the proposed design method of CM EMI filters is applied into a dc–dc converter and a dc–ac inverter, respectively, to verify its effectiveness and feasibility. In a dc–dc converter with the switching frequency 100 kHz, 275 W, the volume of the CM inductor and the volume of CM EMI filters are reduced by 63.5% and 48.3%, respectively, by using the proposed compact EMI filter.

A Compact EMI Filter Design by Reducing the Common-Mode Inductance With Chaotic PWM Technique

Chaotic sinusoidal pulse width modulation (SPWM) can be used to suppress the electromagnetic interference in power converters. In this paper, a mathematical method for estimating power losses in insulated-gate bipolar transistors and freewheeling diodes occurring under chaotic frequency modulation SPWM (CFM-SPWM) switching is proposed. This method is based on the features of the chaotic mapping and uses semiconductor datasheet information to calculate the power losses in a discrete way. Analytical expressions and approximations for estimating the losses for the cases of SPWM and CFM-SPWM switching, respectively, are presented. This paper includes a quantitative comparison of the losses under the two previously mentioned cases. It is confirmed that CFM-SPWM does not result in higher power losses when it greatly improves electromagnetic compatibility of power converters, and hence, its adoption will not require any modified converter thermal design. Experimental results taken from a single-phase ac–dc converter are presented to prove the validity of the proposed method and associated results.

On Thermal Impact of Chaotic Frequency Modulation SPWM Techniques

The modular multilevel converter (MMC) is one of the most critical implementation forms of high voltage direct current transmission. However, the electromagnetic interference (EMI) problems in MMC are unavoidable and complex due to the nonlinear switching behavior of a considerable number of submodules. In this article, the state of the art of EMI research in MMC is reviewed. The characteristics of MMC EMI sources are studied by analyzing the topology and modulation method of MMC and the dynamic turn-on and turn-off processes of the power electronic devices. The standard evaluation and the modeling methods of MMC EMI emission are reviewed by analyzing the EMI propagation paths. In addition, the EMI reduction methods of MMC at sources and along the propagation paths are summarized. Finally, the MMC EMI research is concluded and the future prospects are presented.

A Review of EMI Research in Modular Multilevel Converter for HVDC Applications

This paper proposes a common-mode electromagnetic interference (CM-EMI) spectrum calculation method based on double Fourier series for photovoltaic (PV) inverters with chaotic sinusoidal pulsewidth modulation (CSPWM). In this paper, the CM-EMI of a single-phase PV inverter is first analyzed, and the equivalent circuit of the CM-EMI in the single-phase PV inverter is established, then the CSPWM is employed to suppress the CM-EMI in PV inverters. Moreover, the proposed EMI spectrum calculation method based on double Fourier series is applied in evaluating the CM-EMI spectrum of the single-phase PV inverter. Finally, the simulation and experimental results validate the EMI suppression effectiveness of the CSPWM and the correctness of the proposed EMI spectrum calculation method. Therefore, the proposed EMI spectrum calculation method provides a theoretical support for CSPWM to reduce EMI.

Zhichang Yang

Papers

An Improved H5 Topology With Low Common-Mode Current for Transformerless PV Grid-Connected Inverter

A Compact EMI Filter Design by Reducing the Common-Mode Inductance With Chaotic PWM Technique

On Thermal Impact of Chaotic Frequency Modulation SPWM Techniques

A Review of EMI Research in Modular Multilevel Converter for HVDC Applications

Common-Mode Electromagnetic Interference Calculation Method for a PV Inverter With Chaotic SPWM