Transient Analysis of Three Phase High Power Voltage Source Inverter with Nonlinearities in Hybrid E

Abstract

High power three phase voltage source inverter (VSI) is used in the drive line of hybrid electric and electric vehicles mainly to drive the traction motor. VSI is supplied by battery on board which gets charged and discharged under various control strategies in these vehicles. Throughout a drive cycle, the control signal to the VSI keeps changing. The high switching frequency of power electronics devices increase the stress on the VSI. The response of a three phase VSI under these circumstances has been studied in this paper. The transient analysis of VSI including nonlinearities has been done using axis transformation and average and canonical modelling. The three phase PWM signals are also linearized using d-q axis transformation in synchronous reference frame. It has been observed that a VSI is always a very stable system with nonlinearities and with reference to DC input variation but a very unstable inverse system with respect to the control signal.

Existing System

Modular Multilevel Converter.

Proposed System

The d-q axis average model of the three phase VSI is helpful in linear analysis. An ideal VSI is an oscillating and unstable system with respect to any disturbance in DC input or control input. With the increase of switching frequency and load, different nonlinearities get more effective and increase the losses in the VSI. The real VSI is always stable for any change in DC input but not stable for control signal change. The performance of traction motor used in EV and HEV widely depends on the drive cycle, vehicle condition and driver’s attitude. The power electronics switches are continuously controlled at different modulation index to meet the desired drive performance of the motor. The drive line of the more hybrid vehicles consists of many mechanical rotating components such as clutch and gear. The aim of the paper is to recognise the transient due to the changing modulation index and dropping battery terminal voltage on the performance of three phase VSI driving the traction motor as the same may travel to the latter part of the drive line. First step to achieve this target is to simplify the three phase circuit using a-b-c to d-q-0 transformation. The d-q-0 circuit of VSI is developed by considering the buck characteristics of the three phase VSI as the per phase or line output voltage is always less than the DC input voltage. Network approach has been used to obtain the transfer functions with respect to DC input and control input signals. MATLAB control system tools are used for steady state and transient analysis of the inverter. Whole analysis is very useful in designing different elements of the inverter for various applications including the design of battery or ultra-capacitor for vehicular applications.

Architecture

Average d-q model of three phase VSI