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

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

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

H-Bridge Zero-Voltage Switch Controlled Rectifier (HB-ZVSCR) Transformerless Mid-Point-Clamped Inverter for Photovoltaic Applications

A single-phase transformerless mid-point clamped H-bridge zero-voltage switch-controlled rectifier inverter topology is proposed in this paper for photovoltaic (PV) systems to address the issue of common mode (CM) voltage and leakage currents. Apart from the full H-bridge inverter, the proposed voltage clamping circuit consists of two switches and a full-bridge diode which clamps the AC terminal to the DC midpoint (consisting of two DC-link capacitors) during the freewheeling period. As a result, the common mode voltage is held constant which makes it suitable for the grid-connected PV system. The operating principle and CM effect of the proposed topology are analysed and compared with the conventional topologies. This is followed by the thermal analysis and loss calculation, which shows that the proposed circuit is more efficient over the conventional topologies. Validation is carried out using MATLAB-Simulink using the PLECS toolbox followed by a scale down prototype of 1.5 kW. It is shown that the proposed inverter has the 98±1% efficiency over a wide range of loads with a peak efficiency of 98.96%, and the total harmonic distortion of the output current relatively low (≤1.8 %). The leakage current (icm) is measured for different values of parasitic capacitance that reaches a maximum of 16.65 mA for 330 nF capacitor under consideration which is well below the limit set by different safety standard

Highly Efficient Gan-Based Single-Phase Transformer-Less Pv Grid-Tied Inverter

Growing energy demand and environmental concerns have led to an increased interest in renewable energy resources to provide a sustainable and low carbon emission energy supply. Among these renewable energy resources, photovoltaic (PV) systems have been the focus of many scientific researchers. The most vital component of a PV system that needs to be improved is the power converter. Grid-tied transformer-less inverters have gained a lot of interest in recent years because of their higher efficiency, reduced volume and lower cost compared to traditional line transformer inverters. This dissertation discusses single-phase transformer-less inverter challenges and provides solutions that could lead to a next generation, high performance, grid-connected, single-phase transformer-less inverter. A new topology with new current paths is proposed to increase efficiency and reduce the leakage current. A comparison study of the proposed topology and multiple transformer-less inverters is carried out in terms of leakage current, power losses and efficiency. This dissertation also investigates the impact of emerging Gallium Nitride (GaN)-based power devices on a single-phase transformer-less inverter in terms of efficiency, high switching frequency capability, volume and cooling efforts. GaN device structure, as well as static and dynamic characterization, are discussed. Furthermore, this dissertation studies GaN power devices - reverse conduction capability to provide the proposed inverter with reactive power control. Existing PWM techniques cannot provide a freewheeling path in the negative power region to generate reactive power in a single-phase transformer-less inverter. Thus, a new PWM technique is proposed to provide new modes of operation to achieve reactive power generation capability in the proposed inverter. Due to the increased penetration of PV systems into the grid and the updated grid codes concerning PV systems, next-generation PV systems will be required to have several features like high efficiency, high power quality, voltage regulation and fault ride through capability. This dissertation also explores these future requirements for PV system integration into the grid. To comply with the new grid codes and to enhance the PV inverter capability, a simple and flexible multifunctional control strategy is developed to provide PV inverters with advanced functions that will support the grid. The simulation results validate the theory that the proposed topology reduces the conduction losses of the system. The conduction losses, switching losses, and thermal analysis at different output powers and switching frequencies verify the benefits of replacing Silicon (Si) MOSFET with Gallium Nitride (GaN) HEMTs. Moreover, the use of GaN HEMTs provides superior performance at higher frequencies when compared to their Si counterparts. Consequently, the filter volume is reduced, heatsink requirements are also reduced, and the cost is lowered. Furthermore, the simulation results validate the improvement of the proposed high efficiency transformer-less inverter with the new pulse width modulation (PWM) techniques to generate reactive power. The results also prove the effectiveness of the multifunctional control strategy to provide maximum active power injection, ride through faults, and support the grid by providing reactive power during grid faults. The high efficiency PV inverter equipped with advanced functions is the key to providing a reliable and cost-effective future grid tied to a PV system that can improve power quality