A Multilevel Transformerless Inverter employing Ground Connection between PV Negative Terminal and Grid Neutral point

 

Abstract

For the safe operation of transformerless grid connected PV inverters, the issue of common mode (CM) leakage current needs to be addressed carefully. In this paper, a novel multilevel transformerless inverter topology is proposed which completely eliminates CM leakage current by connecting grid neutral point directly to the PV negative terminal, thereby bypassing the PV stray capacitance . It provides a low-cost solution consisting of only four power switches, two capacitors and a single filter inductor. As compared to half bridge topologies, with this inverter a minimum of 27% and maximum of 100% more output voltage is obtained for the same DC link voltage. The proposed inverter is analyzed in detail and its switching pattern to generate multilevel output while maintaining the capacitor voltage is discussed. Simulations and experiments results confirm the feasibility and good performance of the proposed inverter.

EXISTING  SYSTEM:

To block the CM current path during the freewheeling period, extra switches are inserted into the full bridge inverter either on the DC or AC side. Therefore, these full bridge inverters can be further classified as DC decoupling based inverters and AC decoupling based inverters as described in . However, in these topologies decoupling is incomplete due to the variable junction capacitance of the power switches leading to the flow of CM currents. To further reduce CM currents, some topologies clamp the common mode voltage to half of the DC link voltage during freewheeling period. Although the active clamping based topologies can effectively reduce CM currents, they need two DC link capacitors and some extra switches for clamping purpose, thereby increasing the total cost of the inverter. Another method employed to eliminate CM current is to directly connect PV negative terminal and the grid neutral point. In these topologies the negative output voltage is generated by the virtual DC bus, where the energy is transferred from the real bus to the virtual one. These topologies can generate 3level output voltage when modulated with unipolar SPWM. To further reduce the filter size and to achieve high power transfer capabilities, multilevel inverters can be employed. A selected few existing topologies employing DC/AC decoupling, clamping and ground connection. This paper proposes a novel multilevel transformerless half bridge topology which employ ground connection between PV negative terminal and grid neutral point, thereby completely eliminating CM leakage current. Although being a half bridge based topology, the proposed inverter is devoid from the need of high input voltages. In this way, the advantages of the fullbridge and half-bridge based solutions are combined together. The proposed inverter can also provide reactive power support to the grid acting as a grid edge controller

PROPOSED  SYSTEM:

In this paper, the proposed multilevel topology is analyzed and its various modes of operation are explained in detail for three level operation. Simulation results for these modes are presented, which depicts its maximum power point (MPP) tracking performance and Low voltage ride through (LVRT) capability. The module capacitor size is estimated and control strategy to maintain its desired voltage during these modes is described. The switching pattern to obtain five level output voltage is presented and the inverter operation is ascertained by a simulation exercise. A brief comparison is made with the existing transformerless inverter topologies which highlights its ability to address their shortcomings related to several key issues. Finally, the inverter operation is experimentally validated which substantiate the simulation results thereby confirming the feasibility and good performance of the proposed inverter.  We have discussed various transformerless inverter topologies which address the issue of leakage current. It can be seen that, to fix this issue the inverter topologies should either isolate the PV panel from the grid during freewheeling period or clamp the output to midpoint of the DC bus or employ ground connection to directly connect the PV negative terminal to the grid neutral point. One of the topology that employ ground connection based on the concept of virtual DC bus is proposed. Here the PV negative terminal and the grid neutral are directly connected thus eliminating the leakage current issue. However, to charge the virtual DC bus, two capacitors with unequal voltages are connected in parallel. Large deviation in the capacitor voltages can lead to very high switch currents. Therefore, this inverter topology is incompatible for high power systems. Also, this inverter topology cannot provide multilevel output voltage. Hence, a new multi-level transformerless inverter topology based on the concept of virtual DC bus is proposed in this paper.

CONCLUSION

A multilevel transformerless PV grid connected inverter is presented in this paper. The main characteristics of proposed transformerless inverter are summarized as follows: 1) Complete elimination of CM leakage current by employing ground connection to bypass the PV stray capacitance. 2) The proposed inverter is modular in nature and hence can be easily extended to get n-level output by adding appropriate number of half bridge modules. 3) The proposed 3-level topology is first of its kind inverter which eliminates the CM leakage current with less than five power switches. 4) The proposed 5-level topology is first of its kind multilevel inverter which eliminates the CM leakage current with less than seven power switches. 5) As compared half bridge topologies, a minimum of 27% and maximum of 100% more output voltage is obtained for the same DC link voltage. Therefore, the proposed topology can be a cheaper alternative to NPC inverter based topologies. 6) It can provide reactive power support to the grid, which makes it LVRT capable. 7) It provides an excellent dynamic response. 8) The capacitance requirement of the proposed inverter is larger than that of conventional full bridge transformerless inverters, such as H5, Heric, and FB-DCBP, etc. 9) Multilevel voltage output improves current THD, thereby reducing filter size requirement With excellent performance in eliminating the CM current, multilevel output voltage and lower power electronics cost requirements, the proposed inverter provides an exciting multilevel alternative to the conventional transformerless gridconnected PV inverters.

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