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Electromagnetic Transients of a Micro-Turbine Based Distributed Generation System

H. Nikkhajoei, Member, IEEE, R. Iravani, Fellow, IEEE

Abstract— This paper evaluates the electromagnetic transients of a micro-turbine based distributed generation system that in- cludes an AC-DC-AC converter. An outline of modelling the micro-turbine based generation system including the AC-DC-AC converter is presented. A controller for the converter, that consists of a number of single-input single-output sub-controllers, is de- signed based on the developed model of the system. Furthermore, the thermodynamic model of micro-turbine system is presented. The electromagnetic transients of the overall micro-turbine based generation system including the micro-turbine and converter con- trollers are evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The study considers the dynamic models of generator, converter and power system and the thermodynamic model of micro-turbine system.

Index Terms—Electromagnetic Transient, Micro-Turbine Gen- erator, Distributed Generation, Dynamic Model, AC-DC-AC Con- verter, Control Design, Thermodynamics.

I. INTRODUCTION

THE Micro-Turbine Generator (MTG) has emerged as a viable source of electric energy in the context of Dis- tributed Generation (DG) [1]-[3]. It can also provide power de- mand for remote military/commercial applications, as a stand- alone generator unit. An MTG unit is usually a high-speed (up to 120 krpm) rotating machine with the output power of up to a few hundreds of kilowatts, and the output frequency of higher than 50/60 Hz (e.g. 400 Hz and up to several kHz). Thus, it is interfaced through a power electronic con- verter to the load/utility system. The converter provides con- version/control of frequency as well as control of the output voltage and power flow of the MTG-converter module. Differ- ent types of AC-AC conversion systems, e.g. matrix converter [4], [8] or AC-DC-AC converter, can be adopted as the power electronic interface of an MTG unit. However, the AC-DC-AC based configuration is conventionally used for the MTG unit [5]-[7]. In this configuration, the MTG-side converter is either

converter. The model is based on transformation of the sys- tem equations to a Switching Reference Frame (SRF) [4] and then to the dqo reference frame. The dqo reference frame is selected such that the operating parameters of system can be independently controlled. As a result, the multi-input multi- output converter system is decomposed to a number of Single- Input Single-Output (SISO) sub-systems. For each subsystem, a SISO controller is systematically designed.

The thermodynamic behavior of micro-turbine system is ad- dressed in this paper and the corresponding thermodynamic model is presented. Based on this model, a fast response con- troller is designed for the micro-turbine system. The thermody- namic model is incorporated in the dynamic model of the MTG system since thermodynamics of the high-speed micro-turbine has a fairly small time constant which is comparable to that of the electromagnetic transients of a 50/60 Hz power system [4], [8]. The electromagnetic transient behavior of the overall MTG system including the designed controllers is evaluated based on digital time-domain simulation studies.

II. CONVERTER MODEL

Fig. 1 shows schematic diagram of a back-to-back VSC that interfaces two AC systems with nominal frequencies of ωi and ωo. The AC systems are referred to as source-side and network- side systems, and are simplified representations of a micro- turbine generator and a utility distribution system, respectively.

VSC

Rectifier ω

ω Inverter o

1

i

i di id o

Micro-Turbine Generator (Source Side)

Inp

Output

Utility System (Network Side)

ut

S 1 S 2 S 3

v ai

+

v dc

-

i C

C

S 4

S 4

S 5 S 6

ia si R si L si

i bsi ic si

v R so L so ia so ao

vb o ib so vc o ic so

vc so vb so va so

v bi

v ci

a diode-rectifier, a thyristor-rectifier or a Voltage-Sourced Con- v verter (VSC) and the utility side converter is often a VSC unit, Fig. 1.

v

asi bsi csi

v

S 1 S S 2 3

S S 6 5

The main focus of this paper is to study the electromagnetic transient behavior of an AC-DC-AC based MTG system. This paper also presents a detailed dynamic model of the overall system that is used to design a controller for the AC-DC-AC

————————————————————————- ——————— H. Nikkhajoei and R. Iravani are with the Center for Applied Power Electronics (CAPE), Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario M5S 3G4, Canada. E-mails: h.nikkhajoei@utoronto.ca, iravani@ecf.utoronto.ca ————————————————————————- ——————— Presented at the International Conference on Power Systems Transients (IPST’05) in Montreal, Canada on June 19-23, 2005, Paper No. IPST05-070

Fig. 1. back VSC

A. Switching Reference Frame based Model

The theoretical concept of Switching Reference Frame (SRF) is introduced in [4], [8]. For a back-to-back VSC, transforma- tion to the SRF for the rectifier module, Fig. 1, is defined by

s

Fi = SRFsi, (1)

Schematic representation of two AC systems interfaced by a back-to-

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