Modulation, Modelling and Control of Four-leg Active NPC Inverter as an Interface of a Grid-connected Battery Energy Storage System

Abstract

As a result of increased use and due to the intermittency nature of Renewable energy sources (RES), a growth of Battery energy storage system (BESS) in the electrical power system can be noticed. This thesis work proposes a two stage power conversion system consisting of a DC/DC bidirectional boost converter and a three level four leg four wire Active NPC (4L4W ANPC) inverter. Multilevel four leg four wire (4L4W) inverters are getting more and more attraction in high power, medium voltage applications due to the stable performance with unbalanced loads and nonlinearity in the power system. This work intends to carry out a bibliographic study on various Multilevel inverter (MLI) topologies used as power conversion stage in BESS. Four different modulation schemes will be presented and compared against each other for a single phase leg of ANPC. By using the best possible scheme for each leg, a modified switching frequency optimal PWM (SFO-PWM) method will be used for the proposed 4L4W ANPC. The averaged state-space model for the proposed system is developed and the model is verified against a component level time-step simulation. Finally, a two stage control scheme for the proposed power conversion system will be presented. The control scheme uses a cascaded voltage & current control stage in the DC/DC converter side to control the dc-link voltage and a vector PI decoupled current control stage in the inverter side to control the active & reactive power flow. The modulation, state-space model and the control scheme will be developed and simulated in MATLAB SimscapeTM physical modeling environment to check the performance.