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

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 AS/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.</p

A Classification of Single-Phase Transformerless Inverter Topologies for Photovoltaic Applications

© 2018 IEEE. In Photovoltaic (PV) applications, a transformer is often used to provide galvanic isolation and voltage ratio transformations. However, a transformer based inverter is bulky and has high conduction losses, therefore lead to a reduction in the inverter efficiency. To overcome this issue, the transformerless inverter topologies are addressed widely, but the main challenge of a transformerless inverter is common mode issue. Numerous topological modifications with their control and modulation techniques makes them difficult to follow, generalize and highlight the advantages and disadvantages. To address the issue, this paper gives an overview on transformerless inverter and classify them into subsection to discuss the merit and demerit of some of the major topologies. Five subsections based on common mode behavior, voltage clamping and decoupling techniques have been demonstrated (i.e., common ground, mid-point clamping, AC-decoupling, DC-decoupling and AC+DC decoupling). To verify the finding and for general consensus, major transformerless topologies are simulated using PLECS. A general summary is presented at the end to stimulate readers to acknowledge the problems and identify solutions

Transformerless Microinverter with Low Leakage Current Circulation and Low Input Capacitance Requirement for PV Applications

The inevitable depletion of limited fossil fuels combined with their harmful footprint on the environment led to a global pursuit for alternative energy sources that are clean and inexhaustible. Renewable energies such as wind, biomass and solar are the best alternative energy candidates, with the latter being more suitable for GCC countries. Besides, the energy generated from photovoltaic (PV) modules is one of the elegant examples of harnessing solar energy, as it is clean, pollutant-free and modular. Furthermore, recent advances in PV technology, especially grid-connected PV systems revealed the preeminence of using multiple small inverters called (Microinverters) over using the conventional single inverter configuration. Specifically, the break-even cost point can be reached faster and the system modularity increases with microinverters usage. Nonetheless, due to microinverter’s small ratings designers prefer transformerless designs because transformer removal achieves higher efficiency and power density. However, the transformer removal results in loss of galvanic isolation that leads to dangerous leakage current circulation that affects system safety. Another issue with microinverters is that since they are installed outside their bulky DC-Link electrolytic capacitor lifetime deteriorates the system reliability because electrolytic capacitor failure rate increases as temperature increases. Moreover, the DC-Link capacitor is used to decouple the 2nd order power harmonic ripples that appear in single-phase systems. Thus, the objective of this thesis is to design an efficient transformerless microinverter that has low leakage current circulation and low input capacitance requirement with a minimum number of active switches. In other words, the objective is to increase the safety and the reliability of the system while maintaining the high efficiency. Eventually, the configuration selected is the transformerless differential buck microinverter with LCL filter and it is modeled with passive resonance damping and active resonance damping control

Novel High Efficiency H-Bridge Transformerless Inverter for Grid-Connected Single-Phase Photovoltaic Systems

© 2018 IEEE. This paper proposes a new H-bridge type transformerless inverter for grid-connected photovoltaic (PV) application. The proposed H-bridge zero voltage switch controlled rectifier (HB-ZVSCR) inverter uses additional switches and diodes at the AC side with voltage clamping feature to the DC midpoint. Main characteristics of the proposed inverter are the high conversion efficiency and low leakage current, which make it a suitable candidate for grid-connected PV applications. The analysis and operating principles of the proposed inverter are discussed in details. This theoretical findings has been simulated using PLECS software to verify the common mode voltage (CMV) and leakage current behaviors and the results are compared with similar existing midpoint voltage clamping inverter topologies (i.e. HB-ZVR and HB-ZVR-D). Furthermore, power loss and efficiency of the proposed inverter have been evaluated and compared with existing topologies

Design and Evaluation of High Efficiency Power Converters Using Wide-Bandgap Devices for PV Systems

The shortage of fossil resources and the need for power generation options that produce little or no environmental pollution drives and motivates the research on renewable energy resources. Power electronics play an important role in maximizing the utilization of energy generation from renewable energy resources. One major renewable energy source is photovoltaics (PV), which comprises half of all recently installed renewable power generation in the world. For a grid-connected system, two power stages are needed to utilize the power generated from the PV source. In the first stage, a DCDC converter is used to extract the maximum power from the PV panel and to boost the low output voltage generated to satisfy the inverter side requirements. In the second stage, a DC-AC inverter is used to convert and deliver power loads for grid-tied applications. In general, PV panels have low efficiency so high-performance power converters are required to ensure highly efficient PV systems. The development of wide-bandgap (WBG) power switching devices, especially in the range of 650 V and 1200 V blocking class voltage, opens up the possibility of achieving a reliable and highly efficient grid-tied PV system. This work will study the benefits of utilizing WBG semiconductor switching devices in low power residential scale PV systems in terms of efficiency, power density, and thermal analysis. The first part of this dissertation will examine the design of a high gain DC-DC converter. Also, a performance comparison will be conducted between the SiC and Si MOSFET switching devices at 650 V blocking voltage regarding switching waveform behavior, switching and conduction losses, and high switching frequency operation. A major challenge in designing a transformerless inverter is the circulating of common mode leakage current in the absence of galvanic isolation. The value of the leakage current must be less than 300mA, per the DIN VDE 0126-1-1 standard. The second part of this work investigates a proposed high-efficiency transformerless inverter with low leakage current. Subsequently, the benefits of using SiC MOSFET are evaluated and compared to Si IGBT at 1200 V blocking voltage in terms of efficiency improvement, filter size reduction, and increasing power rating. Moreover, a comprehensive thermal model design is presented using COMSOL software to compare the heat sink requirements of both of the selected switching devices, SiC MOSFET and Si IGBT. The benchmarking of switching devices shows that SiC MOSFET has superior switching and conduction characteristics that lead to small power losses. Also, increasing switching frequency has a small effect on switching losses with SiC MOSFET due to its excellent switching characteristics. Therefore, system performance is found to be enhanced with SiC MOSFET compared to that of Si MOSFET and Si IGBET under wide output loads and switching frequency situations. Due to the high penetration of PV inverters, it is necessary to provide advanced functions, such as reactive power generation to enable connectivity to the utility grid. Therefore, this research proposes a modified modulation method to support the generation of reactive power. Additionally, a modified topology is proposed to eliminate leakage current

Analysis, Design, and Control of a Single-Phase Single-Stage Grid-Connected Transformerless Solar Inverter

As energy utilization is increasing with the rise in the world’s power demand, the traditional energy sources are depleting at a high pace. It has led to attention drawn towards inexhaustible energy resources. There is a huge augmentation in the power generation from renewable energy sources (RES) like wind, solar, hydropower, biomass, etc. to reduce the stress on conventional energy sources like fossil fuels, oil, gas, etc. There has been a steep increase in interest for wind and solar energy systems. PV energy has been growing swiftly in the past two decades which made it most demanded power generation system based on RES. This worldwide requirement for solar energy has led to an immense amount of innovation and development in the Photovoltaic (PV) market. The Conventional grid-connected PV inverter was either with DC/DC converter or without DC/DC converter. These inverters were isolated using a transformer either on the grid (AC) side as a low-frequency transformer or as a high-frequency transformer on the DC side. Elimination of the transformer leads to a galvanic connection between the grid and PV module. This gives rise to the flow of leakage current which is disastrous for the system when it exceeds a specific value. Thus, minimization of this leakage current after the removal of the transformer has been an interesting topic explored by many researchers. Many topologies have been proposed targeting reduction in this leakage current either by 1.) Directly connecting the PV negative with neutral of utility grid or 2.) Disconnecting the PV panel side from AC side. This generally involved addition of more switches or diodes or supplementary branches to disconnect during the freewheeling period. Generally, the above-mentioned ways lead to a reduction in efficiency due to increased losses or complex circuitry. The motivation of this thesis is to design a transformerless inverter for single-phase PV grid-tied system with a smaller number of devices and still has minimum ground current. It discusses the prevailing inverter topologies in detail and then explains the modes of operation of the proposed inverter. A simple control strategy has been derived and passive elements of the inverter are designed. The simulation results presented have validated the theoretical claims. The experimental results which are similar to simulation results are evidence that the proposed topology is suitable for PV grid-tied systems. Also, the dynamic modeling of the inverter has been done to derive the plant transfer function. Then, the Proportional Resonant (PR) controller has been designed to ensure the flow of sinusoidal current into the grid with zero steady-state error and constant sinusoidal grid voltage irrespective of load change. The simulation and experimental results achieved high performance which makes this topology successful and promising for grid-tied PV systems