Full-scale converter for synchronous wind turbine generators

Full Scale Converter for Synchronous Wind Turbine GeneratorsFull Scale Converter

for Synchronous WTG

Presented by: L. Yang & L. Pham

Nov. 2013

CLASS REE527: WIND POWER GENERATORGROUP 7

1. Introduction – The Evolution of modern WT2. Characteristic3. Design 4. Control5. Application6. Conclusion – The Future

Agenda

1. Introduction – The evolution of WT

Wind energy conversion system convert kinetic energy of the wind into electricity or other forms of useful energy

Increased wind turbine size over the years, larger capacity turbines reaching 5-7 MW level

Increased the use of power electronics, allows more control of the power generation

Rapid growth for variable-speed wind turbine system with full-capacity power converter

Wind turbine system evolution

F. Blaabjerg and Z. Chen, “Power Electronics for Modern Wind Turbines,” Morgan & Claypool Publishers, 2006, pp. 30-55

rotation speed:1. fixed-speed turbines2. variable-speed turbines - classification

based on the drive train components:1) Indirect drive (with gearbox)2) Direct drive (without gearbox)

power electronics: 1. WGS with no power converter2. WGS with a partial-capacity power

converter 3. WGS with a full-capacity power converter

wind energy conversion system

Wu B., Lang Y., Zargari N., Kouro S, “Power conversion and control of wind energy systems,” Wiley-I EEE press, 2011, pp. 16-35

2. Characteristic

Achieve maximum efficiency over a wide range of wind speeds compared with fixed speed wind turbines which only reach peak efficiency at a particular wind speed

variable speed systems could lead to maximize the capture of energy during partial load operation Can use either induction generator or a synchronous generator Can operate gearless, lowers the cost

Variable – speed wind turbines

Wu B., Lang Y., Zargari N., Kouro S, “Power conversion and control of wind energy systems,” Wiley-I EEE press, 2011, pp.16-35

In the early 2000s, Enercon and Siemens introduced the concept of full-scale converter (FSC) for Wind Turbine Systems

All power extracted from the wind is managed and transferred to gridThe machine-side of the FSC can provides the generators torque and speed controlThe grid-side can perform reactive power compensation and supply constant DC voltage to

the grid

Full-scale converter for WTS

R. Teodorescu, M. Liserre, P. Rodriguez, “Grid Converter Structures for Wind Turbine Systems” in Grid Converters for Photovoltaic and Wind Power Systems, 1st ed. Chichester, UK. John Wiley & Sons, 2011, pp. 123-143.

Short-circuit behavior comparison shows: DFIG resulting high current surge because it is directly connected to the grid. FSC does not have a current surge because it is decoupled from grid.

• In the WRSG, the rotor flux is generated by the rotor field winding

Advantage: •The WRSG with more numbers of poles and operates at low rotational speeds

can be used for gearless direct-driven wind turbine.

Disadvantages:1) Large numbers of field winding – field loss, heavy weight, more expensive,

large diameter2) DC excitation required - standby power requirement

Variable-Speed WTS using wound-rotor synchronous generators (WRSG)

• The PMSG uses permanent magnets on the rotor to produce the magnetic field.

Advantages:1) high power density as well as high efficiency can be achieved due to no field winding2) Multipole PMSG with a full-capacity converter can also achieve gearless direct-driven wind

turbine3) No additional power supply for the magnet field excitation4) Higher reliability due to the absence of mechanical components such as slip rings.

Disadvantages:1) High cost of rare-earth magnets 2) Demagnetization of permanent magnets at high temperature

Variable-Speed WTS using permanent-magnet synchronous generators (PMSG)

Advantages: 1. High energy conversion efficiency 2. reduced mechanical stress on the wind turbine3. can operate gearless which lowers the cost4. enables full control of the real and reactive power generated

Disadvantages:1. More components - increased equipment capital cost2. Increased complexity of the system

Overall Characteristics

3. Design

Design of a Gearless Wind Turbines (Enercon)

Variable Speed WECS - Configuration70%

Any types of Generator

• The simplest circuit topology.

• Has a diode bride, DC/DC boost & 2 level VSI

• Advantages are low cost and simple control.

• Drawbacks: Stator current is distorted not sinusoidal -> Harmonic losses, torque ripples, etc.

• For Low power

DC/DC BOOST CONVERTER INTERFACED SG WIND ENERGY SYSTEM

•For higher power: ▫2 or 3 channels

interleaved (phase shift) boost converter

▫12-pulses rectifier Increase current reduce harmonic

•Preferable for low- and medium-power WTS from a few kilowatts to ~1MW

DC/DC BOOST CONVERTER INTERFACED SG WIND ENERGY SYSTEM

Harmonic distortion - Comparison

Next topologyBack-to-Back VSI

Two-Level Back-To-Back Voltage-Source Converters•Most popular•2 VSI on each side•Very flexible, lower

harmonics•High switching loss

(hard switching)•Big DC link capacitor

3-Level Back-To-Back Neutral Point Clamped•The desires for MV:▫In a LV 690V/2MW: ~ 1700A each phase transferred from a nacelle to ground!

Multi-Level Converter

Create a sinusoidal high voltage output from several levels of voltage

•Require more switching components (higher cost)

•Complexity in design, control•High conducting loss

Drawbacks

• Lower harmonic distortion• Lower voltage change rate

(dv/dt)•Higher working voltage.• Lower switching loss.• Lower EMI, etc.

Advantages

3-levels & multi-levels

Preferable for WTS with rated power over 2MW

Integrated gate-commutated thyristor (IGCT)

Integrated gate-commutated thyristor (IGCT)

4. Control

Phasor Representation of 3-phase Variables

Phasor = Phase + Vector

Any 3-phase variable can be represented as a phasor rotating with angular velocity w.

http://www.ece.umn.edu/users/riaz/anim/spacevectors.htmlhttp://www.ece.umn.edu/users/riaz/anim/spacevector_viewb.html

Synchronous dq rotating frame

Synchronous dq rotating frame is a rectangular frame rotating at angular velocity w.

Phasor of a 3-phase variable can also be expressed in the synchronous dq rotating frame.

The frame rotating at the same angular velocity with the vector => the component in d- and q- axis are constants (if the vector magnitude = const)

http://www.ece.umn.edu/users/riaz/anim/dq_transformations.html

Full Scale Converter

•Based on instantaneous power theory. For any dq frame we have:

•For a particular dq reference frame that has the d axis aligned with grid voltage phasor e

Control of grid side converter

•Wind speed and decide the optimal torque Tmppt that generator should have to maximize power captured from the wind (MPPT)

•Generator side converter control the generator so that the output torque equal Tmppt

•Electromagnetic torque output Te of the generator expressed in dq synchronous reference frame:

Control of Generator Side

Ld, Lq, id , iq : d- and q- axis synchronous inductances and current of statorΨPM : magnet flux of rotor

Non-salient Generator: Zero d-axis Current Ld = Lq

Salient-pole Generator: MTPA : Ld < Lq

4. Application

•Before: variable speeds were used to smooth out the torque

fluctuations in drive train caused by wind turbulence and to allow more efficient operation in variable and gusty winds

•Now: onshore wind turbine with rated capacity over 2 MW use

variable- speed wind turbine system

Application: onshore wind power

The first offshore wind power plant was built in 1991 in Denmark, consisting of eleven 450 kW wind turbines

Advantages of offshore wind energy1. minimal environmental impact2. large areas available for wind farm development3. wind speed are higher

Variable-speed direct-driven wind turbines using PMSG meets the offshore wind farm requirements:

1. High turbine power capacity2. High reliability 3. Maintenance free

Application: offshore wind power

Mitsubishi Heavy Industries Wind Power Technologies

J. Roney, “Offshore Wind Development Picking Up Pace,” 22 August 2012. [Online]. Available: http://permaculturenews.org/2012/08/22/offshore-wind-development-picking-up-pace/. [Accessed 18 November 2013]

6. Conclusion & Future Directions

R. Wiser, Z. Yang “wind energy,” 2010. [Online]. Available: http://srren.ipcc-wg3.de/report/IPCC_SRREN_Ch07.pdf. [Accessed 18 November 2013]

Development of offshore wind farm Improve the efficiencyIncrease the reliabilitymanage the high level of wind energy penetration to

the utility grid to meet the grid codeDevelopment of power electronics to lower the cost

The future

Full Scale Converter for Synchronous Wind Turbine Generators

Thank you!