The rise in renewable energy has increased the use of DC/AC converters, which transform the direct current to alternating current. These devices, generally called inverters, are mainly used as an interface between clean energy and the grid. It is estimated that 21% of the global electricity generation capacity from renewable sources is supplied by photovoltaic systems. In these systems, a transformer to ensure grid isolation is used. Nevertheless, the transformer makes the system expensive, heavy, bulky and reduces its efficiency. Therefore, transformerless schemes are used to eliminate the mentioned disadvantages. One of the main drawbacks of transformerless topologies is the presence of a leakage current between the physical earth of the grid and the parasitic capacitances of the photovoltaic module terminals. The leakage current depends on the value of the parasitic capacitances of the panel and the common-mode voltage. At the same time, the common-mode voltage depends on the modulation strategy used. Therefore, by the manipulation of the modulation technique, is accomplished a decrease in the leakage current. However, the connection standards for photovoltaic inverters establish a maximum total harmonic distortion of 5%. In this paper an analysis of the common-mode voltage and its influence on the value of the leakage current is described. The main topologies and strategies used to reduce the leakage current in transformerless schemes are summarized, highlighting advantages and disadvantages and establishing points of comparison with similar topologies. A comparative table with the most important aspects of each converter is shown based on number of components, modes of operation, type of modulation strategy used, and the leakage current value obtained. It is important to mention that analyzed topologies present a variation of the leakage current between 0 to 180 mA. Finally, the trends, problems, and researches on transformerless grid-connected PV systems are discussed.

Leakage Current Reduction in Single-Phase Grid-Connected Inverters—A Review

Department of Electrical Engineering, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Department of Electrical Engineering, Qatar University, Doha, Qatar Power Electronics and Renewable Energy Research Laboratory, Department of Electrical Engineering, University of Malaya, Kuala Lumpur, Malaysia School of Software and Electrical Engineering, Swinburne University of Technology, Melbourne, Victoria, Australia

https://onlinelibrary.wiley.com/doi/pdf/10.1002/2050-7038.12730

Recent trends and review on switched‐capacitor‐based single‐stage boost multilevel inverter

This article proposes two generalized multilevel inverter configurations that reduce the number of switching devices, isolated DC sources, and total standing voltage on power switches, making them suitable for renewable energy sources. The main topology is a multilevel inverter that handles two isolated DC sources with ten power switches to create 25 voltage levels. Based on the main proposed topology, two generalized multilevel inverters are introduced to provide flexibility in the design and to minimize the number of elements. The optimal topologies for both extensive multilevel inverters are derived from different design objectives such as minimizing the number of elements (gate drivers, DC sources), achieving a large number of levels, and minimizing the total standing voltage. The main advantages of the proposed topologies are a reduced number of elements compared to those required by other existing multilevel inverter topologies. The power loss analysis and standalone PV application of the proposed topologies are discussed. Experimental results are presented for the proposed topology to demonstrate its correct operation.

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Reduced Switch Multilevel Inverter Topologies for Renewable Energy Sources

The power converters are taking responsibility to produce high-quality output voltage and current waveforms from the various PV sources. Recently, PV applications focus on high-frequency ac power distribution (HF-AC-PD). In particular, dc/ac switched capacitor multilevel inverters (SCMLIs) significantly reduce the power conversion stages and components, which result in high conversion efficiency. This article proposes a new SCMLI topology with a reduced number of switches and a self-balanced floating capacitor (FC) voltage. The proposed topology is configured as a single dc source and three dc sources. The proposed topology generates thirteen level output voltage waveforms, and the output voltage ( $V_{o}$ ) is two times higher than the input voltage ( $V_{in}$ ). The discharging time is less than the recently suggested topologies. Furthermore, the voltage stress on switches is reduced to 50% concerning the load voltage. Thus, a remarkable reduction in the bulky size of the FC is achieved. The simulation results are discussed for different fundamental frequencies of 360 Hz, 400 Hz, and 1 kHz and considering various loads. The proposed converter prototype is designed in the laboratory for 2 kW, and the results are presented. The results show a good agreement in terms of total harmonic distortions, power loss, and efficiency.

Switched Capacitor-Based 13L Inverter Topology for High-Frequency AC Power Distribution System

Switched-capacitor-based multilevel inverters have been gaining increasing attention in recent years in view of their voltage-boosting capability and capacitor self-balancing properties. However, the existing topologies, which comprise frontend switched-capacitor cells and a backend H-bridge, result in longer discharging duration in some of their switched-capacitors. In addition, the adoption of backend H-bridge introduces common-mode voltage and necessitates multiple dc sources for voltage level extension. To resolve these problems, this article proposes a novel topology comprises a T-type inverter and n cascaded switched-capacitor cells. It requires only a single dc source and it manages to mitigate the common-mode voltage by connecting the neutral of ac output to the mid-point of dc link. All switched-capacitors in the proposed topology are ensured to be charged for at least half of the fundamental period to alleviate the capacitor voltage ripples and current spikes. The operation of the proposed topology is analyzed and experimental results are presented for validation.

Switched-Capacitor-based Modular T-type Inverter (SC-MTI)

In this paper, a multisource switched capacitor based boost multilevel inverter is proposed. The proposed topology features less number of components, high gain, self-charging capability of capacitors, reduced size of filter components. The modes of operation of the proposed converter and design guidelines of converter components are discussed. A comprehensive comparative study of the proposed converter topology with the topologies in the literature is given. Analytical expression for capacitor energy utilization (CEU) factor for the proposed topology is derived and the feasibility of the proposed converter with respect to CEU for the designed power rating is verified. The analytical performance of the proposed converter topology is verified through simulation studies. The common mode voltage and leakage current in the proposed topology are analyzed. Experimental prototype of the proposed converter topology with two sources and two capacitors for power rating of 500 W is developed and tested with resistive, inductive, and nonlinear loads. The experimental and simulated results are in close agreement with the analytical steady-state performance of the proposed converter.

Multisource Switched Capacitor Based Boost Multilevel Inverter for Photovoltaic-Based Systems

In this paper, a deadbeat control is proposed with reduced steady-state error in grid connected inverter system. The proposed method takes into consideration the losses occurring in the inverter during each switching cycle. Initially, a method to track the reference current is proposed and the same is extended for direct power control. The proposed method does not require phase-locked-loop, transformation into rotating reference frame and output voltage sensor thereby improving the transient response. The proposed method is verified for a single-phase grid connected switched capacitor based multilevel inverter. The simulation results of proposed current control are presented for step change in current reference. The proposed deadbeat direct power control is verified for PV connected system with varying irradiance.

An Improved Deadbeat Direct Power Control for Grid Connected Inverter System

In this paper, a loss modulated direct power control method is proposed for a single-phase grid connected photovoltaic system. The proposed method employs the concept of adding the modulated signal corresponding to converter losses with the modulated signal corresponding to power transfer with ideal inverter. The proposed method is explained with the model of grid connected voltage source inverter with and without converter losses. The implementation of the control method does not require phase-locked-loop resulting in improved efficiency and response time. Further, an algorithm is proposed to generate the reference power required to implement the direct power control with maximum power point tracking for multisource photovoltaic system. The proposed control method and algorithm is implemented with switched capacitor based boost multilevel inverter. The simulation results under varying irradiance and reactive power are presented. The results show the good tracking performance of maximum power point along with real and reactive powers.

Loss Modulated Direct Power Control for Single-Phase Grid Connected Multisource Fed Photovoltaic System

In this paper, the design criterion considered for switched capacitor based boost multilevel inverters (SCBMLI) is presented. Variants of SCBMLI topology is proposed in literature for several applications. In this paper, evaluation of the SCBMLI variant and PWM technique selected for an application is done by defining a parameter called capacitor energy utilization factor. This factor is the ratio of charging and discharging energies calculated analytically from the given parameters of the topology and the components employed. Generalized analytical model for the above factor is derived. The proposed model for a variant of SCBMLI topology is verified in simulations. Basic considerations for designing and selecting a SCBMLI topology and semiconductor devices are presented. Limitations of the design methodologies available in the literature are explained. The effect of capacitor charging interval and non-idealities of the components ( $R_{ds, on}$ and $R_{ESR}$ ) on the converter behavior is explained. The limitations of the SCBMLI topologies and their design criterion are discussed through analytical and simulation results.

Switched Capacitor Based Boost Multilevel Inverters: Design Criterion and Limitations

In this paper, a shifting carrier based maximum power point tracking (MPPT) algorithm for multisource converters is proposed. This MPPT algorithm is explained for both low frequency (constant carrier) and high frequency (triangular) carrier signals. The proposed algorithm is applied for a single stage multisource switched capacitor based multilevel inverter for grid connected and stand-alone photovoltaic applications. This converter features high gain, less component count, self capacitor charge balancing, better dispersed value of sources and increased source utilization. The importance of selecting an appropriate pulse width modulation technique for switched capacitor based inverters in order to maintain the capacitor charging time is explained. The proposed technique is able to track the maximum power under varying irradiance levels. The experimental results of the proposed algorithm is presented. The MPPT technique is analyzed and verified in simulation for stand-alone and grid connected applications using MATLAB/Simulink platform.

Satish Ramaiah

Papers

Multisource Switched Capacitor Based Boost Multilevel Inverter for Photovoltaic-Based Systems

An Improved Deadbeat Direct Power Control for Grid Connected Inverter System

Loss Modulated Direct Power Control for Single-Phase Grid Connected Multisource Fed Photovoltaic System

Switched Capacitor Based Boost Multilevel Inverters: Design Criterion and Limitations

Maximum Power Point Tracking for Photovoltaic-Fed Multi-Source Switched Capacitor Based Multilevel Inverter