Leakage current in multilevel cascaded transformerless grid-connected inverters for PV systems
Date of Issue2016-05-24
School of Electrical and Electronic Engineering
Recently, the global energy and environmental degradation problems are becoming more and more serious. Therefore, governments around the world are pushing the development of the renewable distributed generation from a strategic perspective. The transformerless multi-level cascaded inverter has become a trend in residual PV generation systems for its low cost and high efficiency. Unfortunately, the parasitical capacitor of the PV panels would form a common-mode circuit with the inverter and the grid. If the common-mode voltage varies in high-frequency, large common-mode leakage current can be generated, which is not allowed by the international standard for the grid-connected devices. So the leakage current paths in PV CMI and the method to reduce the current are needed. In this report, topology of MCI (multilevel cascaded inverter) is given and analyzed. Principles like the PWM modulation and simulation result in MATLAB/ Simulink environment are needed. Then when MCI needs to be connected to the power grid, the close loop system is established. After that, the leakage current paths are analyzed and the unique features in MCI are discussed. Based on all of these analysis, a filter designed suppression solution is given. Simplified analytical models of leakage current are given to illustrate the suppression mechanism and calculation of leakage current. Several figures are used to demonstrate the solution with filter design example and simulation. At the end of this report, simulation results as well as limitations are analyzed. The model and simulation environment need to be improved. And future works need more accurate calculations. Besides, the solutions given in this report effectively reduce the leakage current in each path.
Final Year Project (FYP)
Nanyang Technological University