INVESTIGATION ON GRID CONNECTED TRANSFORMERLESS PV INVERTER

Abstract

The crisis of conventional energy sources is increasing daily, a major challenge faced by our society. The main reason for today’s research focus in the direction of renewable energy sources, which guarantee to produce energy, without depleting the healthy environment. Solar energy is one of the trending renewable energy sources, which can produce energy without affecting the environment. Solar energy must be converted into electrical energy, which is utilized by households and industry. This led to the development of PV panel, which is the medium for energy conversion from solar to electrical (DC power). The DC power generated by the panel is processed through power electronic converters, which provide the electrical energy in a suitable form as demanded by the user. One of the focus area of the solar PV system is on the study of dynamics when it is connected to the grid, by which power is injected from solar to the grid, thereby reducing dependency on the conventional power sources. However, this application requires transformers as a coupling medium between the grid and PV system, which has the responsibility of (1) matching the inverter voltage with the grid, (2) providing galvanic isolation between the grid and PV system, (3) avoiding the ripples in inverter current to flow into the grid (4) avoid leakage current flow from grid to panel. Using transformers, decreases the conversion efficiency and causes increase in size, weight, and cost; Hence, transformerless PV inverters has become more popular. In transformerless grid connected PV inverters, there exists issue of leakage current, which arises because of the two reasons 1) existence of parasitic capacitance and 2) variation of common mode voltage. The leakage current increases the THD in grid current and causes EMI issues. Hence, leakage current must be avoided in grid connected transformerless PV inverter. There are three ways to suppress it 1) proper topology design 2) pulse width modulation improvement and 3) design of filter. In transformerless grid connected PV inverter (TLGCPVI) decoupling of PV from grid is a common practice to avoid the leakage current by providing galvanic isolation by turning off the switch connected to perform this function. They are broadly classified into two categories, on the basis of isolation method applied during freewheeling period. These classifications are (1) DC decoupling type and (2) AC decoupling type. These approaches are to limit the leakage current. If decoupling active switch is placed on DC side of inverter, ie: between DC link and inverter input terminal, such topology is called as DC decoupling-based topology. If the active switch is placed in between inverter output terminal and grid, then that switch added to the H bridge inverter is called as, AC decoupling-based topology. Isolation of PV from grid, during freewheeling stage, does not assure constant CMV. Some fluctuation can take place in CMV voltage around half of PV voltage, because of existence of junction capacitance of

switches, which is responsible for causing resonant circuit, consequently small leakage current may flow from grid to panel. Generally, clamping of both terminals of decoupling-based inverter, with mid-point of two series DC-link capacitor at input side is practised by a bidirectional clamping switch, or diodes in freewheeling period. They are called mid-point clamped inverter topology. The common grounding is another technique, which is popular because it bypasses the stray capacitance directly by its inherent structure to eliminate leakage current completely. H-bridge, half-bridge, and NPC-based inverters are generally buck in nature, i.e., output voltage value gets reduced in comparison to input DC-link voltage. For maintaining this high input voltage requirement is challenging, costly, and not feasible. Hence, a DC/DC boost converter is required to enhance the input voltage level before DC/AC conversion. However, two-stage power conversion reduces efficiency and increases the system’s cost, size, and weight. To mitigate the leakage current problem, avoiding additional DC/DC boost conversion stage and for reducing the filter size, various switched capacitor based multilevel inverters have been designed and investigated. Proper design of passive components and loss analysis has been performed in details in this thesis. Computer simulation studies has been carried out to verify the performance of various transformerless inverter topologies and modulation strategies under different load conditions. The simulation study has been done for both steady-state and transient conditions. To validate the simulation studies of different topologies, downscaled hardware prototypes have been designed, developed and tested. The Real-Time Interface (RTI) of MATLAB® and RT-Lab controller (OP 5600) are used to generate gate pulses for switching devices of the developed prototype. Multilevel inverter helps in reducing filter size, dv/dt stress, di/dt stress, EMI, etc. Based on the existing topologies, novel 3L, 5L, 7L, 9L, and 13L boost switched capacitor multi-level inverter has been developed. These proposed inverters’ operating principles, modulation techniques, design, simulation and experimental validation has been performed (For 13L simulation study has been done). Further, thermal modelling, loss analysis, and efficiency calculation has been performed. Switch’s voltage and current stress also has been calculated alongwith the proper snubber design. Contemporary topologies also has been compared in detail to show the superiority of proposed ones. Design and development of each inverter is unique, however, it could be observed that, there are a few common advantages for all the proposed topologies.