New Approach to a High-Power GTO PWM Inverter for AC Motor Drives

187

This paper describes the effect of interleaved discontinuous space-vector modulation (SVM) in paralleled threephase systems using three-phase pulsewidth modulation (PWM) rectifiers as an example. At the discontinuous point of the SVM, the phase shift between the switching signals of the paralleled modules generates a zero-sequence excitation to the system. Because the conventional control in a balanced three-phase system with only dq channels cannot reject this disturbance, a beatfrequency circulating current will develop on the zero axis. Based on this observation, a SVM without using zero vectors is used to eliminate the cause of pure zero-sequence current for parallel operation. Using this SVM, the circulating current is observable in dq channels. It can be suppressed dynamically by strong current loops of power-factor-correction (PFC) circuits. The concept is verified experimentally on a breadboard system. Index Terms-Circulating current, discontinuous space-vector modulation, interleaved PWM, parallel three-phase system, power electronics building blocks (PEBB's), zero-axis current. I. INTRODUCTION P ARALLEL three-phase power converters/inverters have many advantages, such as higher current capability, less current/voltage ripple, and higher system bandwidth. Previous research exists in applications of UPS, motor drive, and power factor improvements [1]-[6]. As insulated gate bipolar transistors (IGBT's) and other power semiconductor devices are being integrated with gate drives and control intelligence as standardized modules, such as integrated power module (IPM), it becomes quite natural to parallel directly more of these modules together, either to increase the power level and boost the current capability or to design a system with a higher redundancy. Putting modules in parallel, however, is not risk free. One of the major concerns for the parallel operation of a three-phase system is the crosscoupling between modules because when these modules are connected to the same dc bus and a common source/load, extra current conduction paths are formed. Traditionally, in order to avoid this problem, transformers are used to isolate the direct current flow. These transformers are designed with a certain winding turns-ratio and a certain phase shift, so that the concerned harmonics can be canceled in the other side [3], [7]. However, the transformer Manuscript

Section: B the Zero-axis Current Caused By Interleaved Discontinuousmentioning

confidence: 99%

Interleaved PWM with discontinuous space-vector modulation

Xing

Lee²,

Borojevic³

et al. 1999

187

“…The frequency of an individual sinusoid encodes information about the output current of a particular cell, while the rms frequency of the aggregate signal can be used to effect current-balancing control. The weighted rms frequency for all of the sinusoids is defined as (1) where the 's are weighting coefficients for different frequencies. For equal weighting, the 's can be considered equal to a single (arbitrary) constant.…”

Section: Frequency Estimationmentioning

confidence: 99%

“…The current-sharing behavior of the system is largely dependent on the manner in which the individual converters are controlled. Many parallel converter systems use some form of global control in which a single, possibly redundant, controller directly regulates the load balance among the individual converters [1]- [10]. However, to enhance modularity and improve reliability, it is more desirable to have the load-sharing control distributed among the converters.…”

Section: Introductionmentioning

confidence: 99%

Frequency-based current-sharing techniques for paralleled power converters

Perreault

Selders

Kassakian

1998

A new current-sharing technique for paralleled power converters, which is based on frequency encoding of the current-sharing information, is introduced. The approach has significant advantages over existing methods, including the ability to transformer isolate or eliminate current-sharing control connections. Operation of the current-sharing technique is analyzed, and the design and experimental evaluation of a three-cell prototype system are presented.

“…However, these designs are inappropriate for paralleling large numbers of cells because they are difficult to manufacture, and the cells cannot be made autonomous (an important condition for increased system reliability). Another approach is to place an inductor at the output of each bridge leg [4], [12], [13]. This structure results in a converter system which is modular and more manufacturable.…”

Section: Current Sharingmentioning

confidence: 99%

Analysis and control of a cellular converter system with stochastic ripple cancellation and minimal magnetics

Perreault

Kassakian

1997

A parallel converter architecture based on the resonant pole inverter (RPI) topology is presented. It is shown that this architecture minimizes the output magnetics required for current sharing. A new current control scheme is introduced which reduces peak currents, losses, and output voltage ripple for many operating conditions. This new control method is applicable to both the single RPI and the parallel architecture. Additionally, the paper analytically quantifies the degree of passive ripple cancellation between cells of a parallel architecture. It is shown that the rms ripple current of an N-cell paralleled converter system is a factor of 1= p N lower than for an equivalent single converter. These results are verified using a piecewise-linear model. We conclude that the parallel architecture overcomes some of the major disadvantages of the conventional RPI.