Single Stage Power Conversion Of Wind Energy Engineering Essay

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Permanent magnet synchronous generator is driven by the wind turbine is connected to the three phase matrix converter. To operate matrix converter, the switching pulses Space Vector Pulse Width Modulation (SVPWM) are given from a microcontroller or Field Programmable Gate Array (FPGA) controller. While providing SVPWM the output AC voltage with less harmonics. At variable wind velocity the output voltage and frequency are maintains constant. The main objective of the project is to maintain constant frequency and load voltage injected to grid at low wind velocity. In India wind velocity is varies from 3.7m/s to 13.7m/s. The proposed design is suitable for the above range of wind velocity. Hence we can generate suitable voltage and frequency even in low and very high wind velocity.

Keywords: Three phase matrix converter, Space vector pulse width modulation(SVPWM), Permanent magnet synchronous generator (PMSG)

1. Introduction

As energy demands around the world increase, the need for a renewable energy source that will not harm the environment has been increased. Some predictions indicate that the global energy demand will almost double by 2025. And oil can only source the world for up to 200 years. Using wind energy is one way to meet the future need. The Permanent Magnet synchronous generators differ from the induction generator in that the magnetization is provided by a Permanent Magnet pole system or a dc supply on the rotor, featuring providing self-excitation property. Self-excitation allows operation at high power factors and high efficiencies for the PM synchronous generators. It is worth mentioning that induction generators are the most common type of generator use in modern wind turbine systems.

Usually, the power electronics devices applied in WECS (wind energy conversion system) include the converters based converter , known as the green frequency converterâ€-, has lots of advantages such as bidirectional energy flow, sinusoidal input and output current, as well as no large energy storage components. It is an alternative which has a chance to replace active front-end converter in the near future. Especially, Single stage matrix converter which has almost the same function with conventional converter.

Matrix converter is a direct AC-AC converter topology that is able to directly convert energy from an AC source to an AC load without the need of a bulky and limited lifetime energy storage element. Due to the significant advantages offered by matrix converter, such as adjustable power factor, capability of regeneration and high quality sinusoidal input/output waveforms, matrix converter has been one of the AC - AC topologies that receive extensive research attention for being an alternative to replace traditional AC-DC-AC converters in the variable voltage and variable frequency AC drive applications.

2. Single Stage Power Conversion Of Wind Energy System Using AC-AC Matrix Converter

Fig 2.1 Block Diagram of Single Stage Power Conversion Of Wind Energy System Using AC-AC Matrix Converter

Permanent magnet synchronous generator is driven by the wind turbine is connected to the matrix converter. The switching pattern for Matrix converter is a SVPWM (Space Vector Pulse Width Modulation) Scheme generated from a controller either microcontroller or FPGA (Field Programmable Gate Array) controller. The main objective is for closed loop system if the wind speed varies the SVPWM pulse width will change and maintains constant frequency and voltage.

3. Permanent Magnet Synchronous Generator

The permanent magnet synchronous machine has permanent magnets instead of field windings. The permanent magnet synchronous machines can be classified into two main groups according to the installation of the permanent magnet materials on the rotor, Surface Mounted Permanent Magnet (SMPM) machines and Interior Permanent Magnet (IPM) machines. The magnets can be mounted on the rotor surface or they can be internal to the rotor. The permanent magnets are located on the outer surface of the rotor core in surface mounted permanent magnet machines. The rotor magnetic axis is called direct axis (d-axis) or field flux axis and the principal path of the flux is through the magnets. The torque axis (quadrature axis or q-axis) of rotor is electrically orthogonal to the direct axis. Regardless of the manner of mounting the permanent magnets on the rotor.

The basic principles of operation of machines are same. An important difference exists between the direct and quadrature axes inductances for different types of permanent magnet synchronous machines. The SMPM machines have many small permanent magnet materials that are placed whole surface of the rotor. Therefore, they have symmetrical reluctance in both axes and they are non-salient pole synchronous machines. In other words, the d and q axes reactance's of SMPM machines are equal (Lq=Ld). The arrangement of the permanent magnets to the rotor surface provides the highest air gap flux density as is directly faces the air gap without the interruption of any other medium such as part of rotor laminations. Drawbacks of such an arrangement are; lower structural integrity and lower mechanical robustness. Besides that, the interior construction relieves the problem of retaining the magnets against centrifugal force. Therefore the interior construction is suitable for high-speed applications.

4. Matrix Converter

The matrix converter is the most general converter-type in the family of AC-AC converters. The ACAC converter is an alternative to AC-DC-AC converter which is called as direct or single stage converter is shown in Fig. 4.1. The matrix converter is a single-stage converter which has an array of mÃ-n bidirectional power switches to connect, directly, an m -phase voltage source to an n-phase load. The AC-DC-AC converter is also called as indirect converter or two stage power conversion as shown in Fig. 4.2. The matrix converter is a forced commutated converter which uses an array of controlled bidirectional switches as the main power element to create a variable output voltage system with unrestricted frequency. It does not have any DC-link circuit and does not need any large energy storage elements. The key element in a matrix converter is the fully controlled four-quadrant bidirectional switch, which allows high-frequency operation. The converter consists of nine bi-directional switches arranged as three sets of three so that any of the three input phases can be connected to any of the three output lines is shown in Fig. 4.3.

Fig 4.1 Single Stage Power Conversion

Fig 4.2 Two Stage Power Conversion

Fig 4.3 Switch Arrangement of Matrix Converter

The switches are then controlled in such a way that the average output voltages are a three phase set of sinusoids of the required frequency and magnitude. The matrix converter can comply with four quadrants of motor operations, while generating no higher harmonics in the three-phase AC power supply. The circuit is inherently capable of bi-directional power flow and also offers virtually sinusoidal input current, without the harmonics usually associated with present commercial inverters. These switches provide to acquire voltages with variable amplitude and frequency at the output side by switching input voltage with various modulation techniques. These modulation techniques are to change the voltage transfer ratio of matrix converter and out of these methods we mainly concentrate on the venturini modulation technique and the space vector pulse width modulation method. One of the main contributions is the development of rigorous mathematical models to describe the low-frequency behavior of the converter, introducing the "low-frequency modulation matrix" concept. The use of space vectors in the analysis and control of matrix converters

in which the principles of Space Vector Pulse Width Modulation (SVPWM) were applied to the matrix converter modulation problem.

5.3. Scope of the research