三电平逆变器仿真原理及介绍

A Comprehensive Study of Neutral-Point V oltage Balancing Problem in Three-Level Neutral-Point-Clamped V oltage Source PWM

Inverters

Nikola Celanovic,Student Member,IEEE,and Dushan Boroyevich,Member,IEEE

Abstract—This paper explores the fundamental limitations of neutral-point voltage balancing problem for different loading con-ditions of three-level voltage source inverters.A new model in DQ coordinate frame utilizing current switching functions is developed as a means to investigate theoretical limitations and to offer a more intuitive insight into the problem.The low-frequency ripple of the neutral point caused by certain loading conditions is reported and quantified.

Index Terms—Neutral-point voltage balancing,space vector modulation,three-level converter.

I.I NTRODUCTION

S INCE it’s introduction in1981[1],the three-level neutral-point-clamped(NPC)voltage source inverter(VSI),Fig.1, has been shown to provide significant advantages over the con-ventional two-level VSI for high-power applications.

The main advantages are as follows.

1)V oltage across the switches is only half the dc bus voltage.

This feature effectively doubles the power rating of VSI’s for a given power semiconductor device.Moreover,this is achieved without additional,often cumbersome,hard-ware for voltage and current sharing.

2)The first group of voltage harmonics is centered around

twice the switching frequency[1],[7].This feature en-ables further reduction in size,weight,and cost of passive components while at the same time improving the quality of output waveforms.

On the other hand this topology also has its disadvantages.

1)Three-level VSI’s require a high number of devices.

2)The complexity of the controller is significantly in-

creased.

3)The balance of the neutral-point has to be assured.

The three-level VSI was first considered with respect to high-capacity high-performance ac drive applications[1].To this day, it remains the area where this topology is most widely used [2]–[4],[7]–[9],[15],and[16].Other interesting applications of

Manuscript received March10,1999;revised September22,1999.Recom-mended by Associate Editor,F.Z.Peng.

The authors are with the Department of Electrical and Computer Engi-neering,Virginia Polytechnic Institute and State University,Blacksburg,V A, 24061-0111USA.

Publisher Item Identifier S

0885-8993(00)02327-9.

Fig.1.Circuit schematic of a three-level VSI.

this technology include static V AR compensation systems[11],

[12],HVDC transmission systems[18],active filtering applica-

tions,as well as applications in power conditioning systems for

superconductive magnetic energy storage(SMES)[13].

The neutral-point(NP)voltage balancing problem of

three-level NPC VSI’s has been widely recognized in litera-

ture.Various strategies have been presented,and successful

operation has been demonstrated with a dc-link voltage balance

maintained.In addition,some of the proposed algorithms avoid

the narrow pulse problem[5],[9],minimize losses by not

switching the highest current[10],or share the balancing task

with front-end converters as in[2].

NP control for the carrier-based PWM has been studied

in[15]–[17].In[15],the switching frequency optimal PWM

method is introduced.This method controls the NP by,essen-

tially,adding the zero sequence voltage to the inverter output.

This work was extended in[16],where the authors propose an

analytical method for analysis of the NP potential variation,

show some limitations of the NP control,and also deal with the

dc-link capacitors design issues.In[17],the authors analyze

the stability of the NP control based on an insightful dynamic

model of the NP control they developed.

This paper discusses the issues of NP control from the space

vector modulation(SVM)point of view.In addition,the broader

range of inverter operating conditions is addressed,and a new

mathematical formulation of NP balancing problem is given.

Furthermore,low-frequency NP voltage ripple,normalized with

the output current and the size of the dc-link capacitors,is given

for all operating conditions.

0885-8993/00$10.00©2000IEEE