Control of AC-DC-AC Converter under Unbalanced and ... - IEEE Xplore

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Abstract- This paper investigates problems of grid voltage distortions and their influence on AC-DC-AC converter. A specific control algorithm is presented, which ...
POWER QUALITY, ALTERNATIVE ENERGY AND DISTRIBUTED SYSTEMS

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Control of AC-DC-AC Converter under Unbalanced and Distorted Input Conditions Marek Jasinski1, Marian P. Kazmierkowski2, Malgorzata Bobrowska3, Piotr Okon4 Warsaw University of Technology (Warsaw, Poland) [email protected], [email protected], [email protected], [email protected] Abstract- This paper investigates problems of grid voltage distortions and their influence on AC-DC-AC converter. A specific control algorithm is presented, which minimizes influence of voltage dips and higher harmonics on the system. Requirements for control of the Line (Grid) Side Converter (LSC) and Machine (Generator) Side Converter (MSC) are discussed.

I.

INTRODUCTION

It is obvious that the use of renewable energy is one of the most important tasks. The remarkable progress in the field of wind energy is a good example for other sources of clean energy. Wind energy market has already been commercialized. However, there are still some issues that have to be solved [1], [2]. These problems are related to grid integration and selection of an electrical machine type [14]. The market of wind turbines is very wide and a lot of publications have been written in the subject. The solutions for wind energy can also be applied to ocean energy conversion, and particularly for Wave Dragon MW [3]. Therefore, it is important to promote ocean energy between renewable energy specialists. Oceans cover roughly 75 percent of the earth surface, and offer a tremendous amount of renewable energy in form of waves. The problem of high grid impedance in remote locations and grid voltage faults exist for wind and for ocean turbines. The disturbed grid voltage in practice is always present and it would make a power electronic converter unstable [5], [7], [8], [9]. Because of unstable operation the converter should be switched-off causing break in production processes. Moreover, during unstable operation power electronics converter would be a source of distortion for the grid and for other receivers [10], [11]. Therefore, it is important to take into account difficult grid voltage condition in control methodology from software side and in topology of the Variable Speed Energy Generation Systems (VSEGS) from hardware side. In the paper short overview of the state-of-the art for the matter is presented. After that a grid integration method and Line (Grid) Side Converter (LSC) control is described. Electric machine parameters and features are briefly taken into account. Selected simulation and experimental results present features of the proposed VSEGS. And finally summary and conclusions are pointed out what should be taken into account in grid

Effectiveness of proposed control scheme is verified with simulations and experimentally on 3kW model. Solution is dedicated to Variable Speed Energy Systems (VSEGS) connected to high impedance grid, particularly wind farms or ocean wave energy converters such as WaveDragon MW.

integration. The results should be interpreted as preliminary one because the problem of grid integration is very wide and for final results a real grid code should be taken into account [12], [13], and [15]. II. GRID VOLTAGE DISTORTIONS TYPES AND ITS APPEARING REASONS A. Higher Harmonics Voltage Distortion Higher harmonics are the components of the basic signal with multiple frequency of the basic signal. To describe amount of higher harmonics in basic signal the Total Harmonic Distortion coefficient (THD) is defined as [13]: 40

THD =

∑ (uh ) h=2

u1

2

(1)

The main sources of the higher harmonics are nonlinear loads connected to the grid by users. AC-DC power electronics converters can be taken as an example. Amount of higher harmonics can be increased or decreased by physical characteristics of the line, cable or transformers, etc... In the Fig. 1 the grid voltage with 6% of 5th harmonics is presented. B. Voltage Dips Voltage dip is a short term reduction of RMS voltage value. It is a phenomenon similar to short circuit. However, the voltage amplitude during dip cannot be less than 1% of nominal value. According to European standard EN 50160 voltage dips can be classified as [15]: -short (up to few seconds) and shallow (no more than 60%), -short and deep (more than 60%), -long (more than few seconds) and shallow (no more than 60%), -long and deep (more than 60%).

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2009 COMPATIBILITY AND POWER ELECTRONICS CPE2009 6TH INTERNATIONAL CONFERENCE-WORKSHOP

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Fig. 3. Positive and negative phasors composition.

SI = Fig. 1. Grid voltage with 6% of 5th harmonic.

Sudden voltage changes can cause problems in power electronics converter control, such as: DC-link voltage oscillations, distorted and asymmetric AC currents and finally active and reactive powers oscillations. In Fig. 2 voltage dip in phase „a” an 100ms duration time is shown. When the voltage is asymmetrical, its space vector is described by two phasors: with negative and positive angular frequency:

u (t ) = u p (t )e jωt + un (t )e − jωt

u p − un rmin = rmaj u p + un

(9)

When, SI=1 – circle, 0