GRID INTERFACE OF WIND POWER

WITH LARGE SPLIT WINDING ALTERNATOR

USING CASCADED MULTILEVEL INVERTER

ABSTRACT

• Wind Energy Conversion System (WECS) based on Cascaded H Bridge multilevel inverter.

• Split winding alternator, Direct Bridge Rectifier and Voltage Source Inverter are as main components.

Main Aim of Project

Grid interface, Real power injunction,

Load compensation, Power factor correction, Harmonic compensation,

Reduction of rating of Power Electronic Equipment etc.

INTRODUCTION

Wind energy is sufficiently available. Many Turbine technologies available to harvest the more energy from wind.

As per World Wind Energy Association (WWEA) - over the past five years the average growth in worldwide new installations has been increasing 30.9 % and expected 15% annual growth in 2013.

Wind Turbines of higher rating are developed to extract the wind power more efficient wayTraditional wind turbine output can’t be interfaced to GRID due to many reasonsBy using modern technology, Wind turbines more that 1.3MW can easily interfaced to GRID.Cascaded Multilevel Inverter Technology – Voltage Source Inverters (STATCOM). In this a number of 1- f H-Bridge cells are series connected along with separate DC source.

The CHBMLI, eliminated the multi-purpose invertors, transformers, flying capacitors, diode clamped inverters etc.,In addition to that – voltage control, real power injunction, p.f. correction, reduction of lower order harmonics were made possibleA new structure in alternator design was proposed – Split winding alternator. In this each phase winding is divided into three parts. In turn this reduces the size and rating of VSI.

THE TECHNOLOGY

WIND ENERGY CONVERTION SYSTEM (SYN. GEN)

In conventional system, 3- f Alternator is coupled to large wind turbine.

It’s then connected to Uncontrolled bridge rectifier DBR, followed by DC-DC converter (for DC voltage regulation). The converter output DC is applied to an Inverter to convert it in AC

All the components are used should be fully rated (i.e., higher power handling capacity)

This leads – more thermal stress, more low order harmonics etc.,

THE NEW TECHNOLOGY used in

present project

In proposed system, armature winding of 3- fAlternator is divided into three parts per each phase.

The output of each winding is then connected to Uncontrolled bridge rectifier DBR, followed by DC-DC converter (for DC voltage regulation). The converter output DC is applied to an CHBML Inverter to convert it in AC (the main building block of CHBMLI is VSI – IGBT – STATCOM)

Each component of lesser capacity connected in series to handle the total load from alternator.

The output of CHMBLI can be controlled easily. There is no need of medium voltage distribution transformer to interconnect the WECS to GRID

ADVANTAGES OF NEW SYSTEM

• Less number of lower order harmonics

• Low power rated Power Electronics equipments and other switches.

• Elimination of High Voltage Transformers.

• Improved performance

The main parts are Wind Turbine with Split

Winding Alternator, AC/DC/AC conversion

system, Insulated Gate Bipolar

Transistor

POWER FLOW FOR WECS

Description Of Main Components

VARIABLE SPEED WIND TURBINEThe high power (above 2 MW) rated wind turbines are designed to provide output at different wind speeds. Here rotational speed is controlled based on available wind speed.Parameters:Blade Radius – 40m; Air Density - 1,299kg/m3; Power coefficient Cp – 0.44

Split Winding Alternator

Each phase winding of alternator is equally divided into three equal parts.Total Nine windings are brought out and connected to nine separate rectifiers/DBRs The field winding on rotor is supplied through slip rings. Parameters:Rating of Alternator3-Phase; 2MW, 8.0kV rms

Rectifier Units (DBR) Simple uncontrolled Diode Bridge Rectifiers are used

to convert AC drawn from nine windings of alternator.

They act as isolated voltage sources to CHBMLI. The output voltage is controlled by control of

excitation of alternator. Hence no need of separate control.

SEVEN LEVEL CHBMLI

Method is presented showing that a cascade multilevel inverter can be implemented using only a single DC power source and capacitors.

A standard cascade multilevel inverter requires n DC sources for 2n + 1 levels.

Without requiring transformers, the scheme proposed here allows the use of a single DC power source (e.g., a battery or a fuel cell stack) with the remaining n-1 DC sources being capacitors.

As the number of levels increases, the harmonic content of the output waveform is reduced.

Reactive power flow can be controlled, as this does not cause unbalance in the capacitor voltages.

• Fast dynamic response.

Operation of Multi-level Inverter

The output of DBR is connected to seven level VSI /CHBMLI. A DC link capacitor is used after each DBR to hold the DC output voltage to CHBMLI.

These H-Bridge cells are connected/phase. They convert DC to AC.

This output can be directly fed to medium-voltage distribution grid without the need of any interfacing transformer.

In closed loop multi carrier modulation, PWM is used.

Closed loop Multicarrier Modulation of CHBMLI

CONTROL OF DC LINK VOLTAGES

By slow simultaneous control of input dc link voltages to CHBMLI using excitation ControlBy instantaneous current control of VSIBy Excitation control

Equivalent circuit of Exciter-SWA-DBR-CHBMLI

Consider the system as shown. The three phase currents may unbalanced and consists of both linear and non linear elements. Hence unbalanced distorted PCC voltages

A compensator/ideal current source is connected at each phase to compensate the requirement.

Real Power injunction

Simulation considerations

EFFECT OF VSI ON PCC VOLTAGE, SOURCE, LOAD CURRENTS, AND POWERS.

(a) Cascaded seven-level inverter output for phase A.

(b) Three-phase source and load currents before and after the VSI is switched on.

(c) PCC terminal voltages before and after the connection of VSI.

(d) Average load, source, and wind power before and after the connection of VSI.

(e) PCC voltage and source current before and after switching on the VSI for phase A.

(f) Reference and actual shunt injected current (VSI current tracking).

(g) Split-winding ac voltages (three per phase, equal for same phase).

(h) DC-link voltages for all the nine cells (three per phase, equal for same phase).

Simulations considering CASE 1

Simulation results for Case 2

EFFECT OF DC-LINK VOLTAGE ON THE PERFORMANCEOF THE CURRENT CONTROL LOOP.

(a) Three phase source and load currents as dc-link voltage, and load is varied. (b) Reference and actual shunt injected current, and tracking error for phase A. (c) Cascaded seven-level VSI voltage output. (d) PCC voltage and source current for varying load, and Vdc for phase A. (e) PCC voltage and load current for varying load, and Vdc for phase A.

Simulation results for Case 3

EFFECT OF INCREASED WIND SPEED ON THE SYSTEM.

(a) Power drawn from source, wind, and load power.

(b) PCC voltage and source current for phase A. (c) Three phase source and load currents.

THE CONCLUSIONS1.The Split Winding Alternator with CHBMLI can be

successfully interfaced with GRID2.The present scheme proved the successful

operation of Real Power Injection, Load compensation, Reduction of lower order Harmonics

3. If wind power supply is more than demanded load, the excess power is exported to the grid

4.The change in voltage can be managed changing the excitation to the Alternator

5.This arrangement leads for reduced cost of power electronic equipment.

REFERENCES

1. Grid Interface of Wind Power 2. Reliable Technology for all turbine applications3. Multilevel inverter4. Power-Electronic Systems for the Grid Integration5. Novel power electronic systems for wind power systems6. Multilevel Converters for Large Electric Drives7. General Model for Representing Variable Speed Wind8. Integrated Doubly Fed Electric Alternator9. Control of a DFIG-based wind system10. Wind Turbine Current-Source Converter 11. Performance Enhancement of Synchronous Generator12. Cascaded multilevel inverter and its application in STATCOM13. A Two-Stage Converter based Controller14. Multilevel Voltage-Source-Converter Topologies15. Control policies for wind-energy conversion systems16. Voltage and Power Flow Control of Grid17. Performance Optimization for Doubly-Fed Wind Power Generation Systems18. Wind Speed Estimation Based Variable Speed Wind Power Generation19. Load Compensating DSTATCOM 20. Power Quality Improvement Using DVR21. Control Strategies for Load Compensation22. PSCAD-EMTDC-Based Modelling and Analysis

This presentation prepared under the guidance ofProject Guide Mr. Ch. Siva KumarAsst. Professor, Dept. Electrical Engineering, Osmania University, Hyderabad - 7

ByM Paul Prasad1005-10-748208ME (PS) V semester, PTPG

Thank you