QamarZulfiqarMujahid_Seminar1.ps`ptx

8/14/2019 QamarZulfiqarMujahid_Seminar1.ps`ptx

1/23

Qamar Zulfiqar Mujahid

2007-MS-E-035

Supervisor: Dr. Muhammad Asghar Saqib

Department of Electrical Engineering

University of Engineering and Technology,Lahore

Modeling and Simulation of Variable-Speed Wind

Power Generators and their Impacts on Power

Systems


2/23

Manufacturers of large wind turbines: General Electrics and Clipper

Windpower of USA; Enercon, Siemens, Nordex, REpower andFuhrlaender of Germany; Darwind, Lagerwey ofNetherlands; Gamesa of Spain; Vestas of Denmark; Mitsubishi ofJapan and Suzlon of India.

Largest wind turbine: Clipper Windpower of USA, 7.5 MW with 100mtall and a rotor diameter of 150 m. The previous record was with

Enercon of Germany for 7 MW machines.

Fi ure 1. Historical develo ment o wind-turbine enerators.2

Wind Power Technology and Introduction to

Various Generators Used


3/23

Modern wind power technology since 1970s (rapidly since 1990s.)

Various wind turbine concepts and different generators.

Three types of typical generator systems for large wind turbines.

Wind turbines are: Fixed speed

Limited variable speed

Variable speed

Partial-scale and Full-scale power electronic converters

Geared-drive (multi-stage and single stage) and direct-drive windturbines.

Basic configurations and characteristics of wind generator systemsare briefly discussed.

3


4/23

1. Fixed-Speed Wind Generator

Figure 2. Fixed-speed SCIG system. Fixed-speed wind generator with multi-stage gearbox and a SCIG

directly connected to the grid through a transformer (Figure 2).

SCIG operates in a narrow speed range around synchronous speed.

Conventional concept used by many Danish wind-turbine

manufacturers during 1980s and 1990s.

Capacitor for reactive-power compensation.

Smooth grid operation was achieved through soft starter.

Pole-changeable SCIG for two rotational speeds.

Vestas, Siemens and Nordex have products on this design.

4


5/23

Advantages:

Robust

Easy and relatively cheap for mass production

Provides stable-frequency control and stall-regulated machinesoperate at fixed speed when connected to a large grid.

Disadvantages:

Speed not controllable and variable; only small speed range for

generator operation.

Higher slips means higher energy loss in rotor bars.

Speed fluctuations are directly translated into electromechanical

torque variations so high mechanical stresses on the system.

Turbines speed can not be adjusted with wind speed to optimise

the aerodynamic efficiency.

5


6/23

2. Limited Variable-Speed Wind

Generator

Figure 3. Limited variable-speed WRIG system. Limited variable-speed with multi-stage gearbox using wound rotor IG.

Variable rotor resistance by power electronic and pitch control method.

Rotor winding is connected in series with an external resistor.

Variable-speed operation by extracting energy from the rotor, whichmust be dissipated in external resistor.

A typical variable speed range is 10% above synchronous speed.

Reactive power compensation and soft starter required.

Vestas and Sulzon have products on this concept.

6


7/23

3. Variable-Speed IG with Partial-

Scale Power Converter (DFIG)

Figure 4. Variable-speed DFIG system. Variable-speed wind turbine with WRIG and partial-scale power

converter on the rotor circuit; also known as DFIG (Figure 4).

The stator is directly connected to the grid, and the rotor through a

power electronic converter.

The power electronic converter controls the rotor frequency and thus

the rotor speed.

Wide speed operation depending upon the size of the converter.

Typical speed variations are 30% around synchronous speed.

Rating of the converter is around 25-30% of the generator.7


8/23

Popular and economically attractive.

Vestas, Gamesa, REpower and Nordex use this concept on theirmachines.

Largest commercial wind turbine machine is 5 MW.

Advantages:

Rotor energy is fed into the grid (not dissipated).

The converter performs reactive power compensation and smoothgrid operation.

Disadvantages A multi-stage gearbox is still necessary as common turbine speed is

10-25 RPM. Heat dissipation, regular maintenance and audible noiseare associated with the gearbox.

Use of slip rings to transfer rotor power which require frequent

maintenance and power loss. Power electronic converter needs to be protected against high

currents which result from grid fault conditions (large stator current).

Large stator currents may result in high torque loads on the geartrain.

Ride-through capability of the DFIG is required.8


9/23

3. Variable-Speed Direct-Drive

Generator with Full-Scale Power

Converter The direct-drive generator rotates at a low speed as the generator

rotor is directly connected with the hub of the turbine rotor.

Large size of the generator is required to accommodate high torque.

Simplified drive train, high overall efficiency, high reliability andavailability by the absence of the gearbox.

Smooth grid operation over the entire speed range.

Higher cost and higher power losses.

The generators available in the market under this category are of twokinds: electrically excited synchronous generator (EESG) and

permanent magnet synchronous generator (PMSG).

9


10/23

3.1. Electrically Excited Synchronous

Generator (EESG)

Figure 5. A direct-drive EESG. Generally salient pole rotor for low speed operation.

Amplitude and frequency of the voltage are controlled y the power

converter at generator side.

Generator speed is fully controllable to a wide range, even to very low

speeds.

Flux control for minimised losses in different power ranges.

Mostly used generator in this category.

Typical manufacturer is Enercon and the largest capacity is 4.5 MW.

10


11/23

3.2. Permanent Magnet Synchronous

Generator (EESG)

Figure 6. A direct-drive PMSG.

Advantages:

Higher efficiency and energy yield.

No additional power supply for the field excitation.

No field losses; improves thermal characteristics.

Higher reliability as no mechanical components like slip rings.

Higher power to weight ratio.

Disadvantages:

Higher cost of permanent magnet materials.

Difficulties to handle in manufacture.11


12/23

Demagnetisation of the permanent magnet materials at hightemperature.

Performance of permanent magnet materials in recent years hasbeen improving with the reduction in their cost.

The trend will thus make these machines more attractive. Harakosan and Mitsubishi are using this concept to make 2 MW

commercial wind turbines.

12


13/23

4. Variable-Speed Single-Stage

Geared Concept with Full-Scale

Power Converter

Figure 7. A single-stage geared PMSG with full-scale converter.

A variable speed pitch control wind turbine is connected to a single-

stage gearbox which increases the speed by a factor of 10.

Advantage of higher speed than direct-drive and lower mechanical

components that multi-stage geared drives.

Multibrid and Win Wind have products based on this concept

Clipper system, having a single-stage gearbox with multiple output

shafts that drive a number of medium speed medium torque PMSMs,

has also been introduced; 2.5 MW rated power available in market.13


14/23

4. Variable-Speed Multiple-Stage

Geared Concept with Full-Scale

Power Converter

Figure 8. A multiple-stage geared PMSG with full-scale converter.

4.1. PMSG:

A PMSM with multiple gearbox (Figure 8) is used in order to reduce

the generator volume and improve its efficiency.Advantages:

Compared with DFIG system it has following advantages:

The generator has better efficiency.

The generator can be brushless.

The grid fault ride-through capability is less complex.14


15/23

Disadvantages: Large and more expensive power converter.

Losses in the converter are higher as all power is processed by it.

This concept is being used by GE.

4.2. SCIG System:

Compared with Danish concept this machine has advantages offlexible control with full-scale power, such as variable speedoperation, better reactive power compensation and smooth gridconnection.

High cost and losses of the full-scale converter. The efficiency of theoverall system maybe low.

Siemens is making 3.6 MW, 595-1547 RPM (gen. speed) machines.

Figure 9. A multi-stage geared SCIG with full-scale converter.15


16/23

-DFIG

Figure 10. Typical configuration of a DFIG.

Slip rings take current out or in to the rotor windings. Crow bar protects against

over currents.

Variable-speed operation is obtained by injecting a controllable voltage into therotor at slip frequency.

Rotor winding is fed through a variable-frequency power converter, typically

through two AC/DC voltage source converters linked by a DC bus.

The power converter decouples network electrical frequency from the rotor

mechanical speed. Power is delivered to the grid through stator and rotor, and rotor can also

16


17/23

DFIG - Active Power Relationships in Steady-

State Operation

Figure 11. Power relationships in a DFIG.

Pair-gap = Ps+ PSCL

and, Pair-gap= Pm(Pr+ PRCL)

Thus, Ps = PmPr(PSCL+ PRCL)

In terms of generator torque, T,

T s= TrPr- (PSCL+ PRCL)

17


18/23

Pr= -T (sr) - (PSCL+ PRCL)

In terms of slip, s:

Pr= -sT s-(PSCL+ PRCL)

Pr= -s Ps- (PSCL+ PRCL)

Pmcan be expressed as:

Pm= Ps+ Pr+ (PSCL+ PRCL) = PssPs+ (PSCL+ PRCL)

= (1-s) Ps+ (PSCL+ PRCL)

Total power delivered to the grid is:

Pg= Ps+ Pr= (1-s) Ps

Controllable range of s determines the size of the converter for the DFIG.

Mechanical and other restrictions limit the maximum slip, and a practical

speed range may be between 0.7 and 1.2 pu.

18


19/23

Modeling of the DFIGKey Points

The purpose is to have a model that could represent the dynamic behaviour of

the system.

The three-phase winding, connected to a three-phase source, produce a

rotating magnetic field.

This rotating magnetic field can be represented by two coils on the d-axis and

the other on q-axis rotate at the synchronous speed of the supply voltage.

Three-phase voltages of slip frequency, sfs, are induced on the rotor whenstator magnetic field cuts the rotor conductors.

As a result three-phase current at slip frequency flow through rotor

conductors.

These rotor currents also produce a rotating magnetic field which rotates at

slip speed (s- r= s s) with respect to the rotor. A viewer standing on Earth would see the rotor magnetic field also rotating at

the synchronous speed (ss+ r= s).

Therefore, rotor magnetic field can also be represented by d- and q-axes.

In synchronous reference frame all the coils are thus stationery, and

inductances constant. Now the machines volta e and flux e uations can be ex ressed in d

19


20/23

Grid-Connection Requirements

Major challenge will be the interconnection of large wind farms withthe grid.

Transmission system operators are becoming concerned about their

impact on power systems.

Grid codes for wind turbines connection and operation.

The main issues are:

Active power control

Reactive power control

Voltage and frequency control

Power quality

Fault ride-through capability

At the point of common coupling the impacts (local impacts) are:

Circuit power flows and busbar voltages

Protections schemes, fault currents and switchgear rating

Power quality issues such as harmonic voltage distortion and voltageflicker.

20


21/23

Next Tasks

Modeling of the DFIG to be developed in 5thorder considering statorand rotor transients, and damper winding.

Model of the DFIG to be connected to an infinite bus through a

transformer and a transmission line, and to be implemented in

MATLAB SIMULINK.

The effects of small disturbances such as small step change inmechanical input torque and small variations in the torque and voltage

set points will be investigated.

The effects of faults on the network will also be investigated to explore

fault ride through capability of the machine.

The models of a wind turbine and gearbox may also be implemented

to see the dynamic behaviour of a complete system.

21


22/23

REFERENCES[1] LI H, CHEN ZI: Overview of wind generator systems and their comparisons, IET

Renewable Power Generation, 2007.

[2] HANSEN AD, HANSEN LH: Wind turbine concept market penetration over 10 years(19952004), Wind Energy, 2007, 10, (1), pp. 8197

[3] CHEN Z, BLAABJERG F: Wind turbines-a cost effective power source, Przeglad

Elektrotechniczny, 2004, R. 80, (5), pp. 464469

[4] BLAABJERG F, CHEN Z, KJAER SB: Power electronics as efficient interface in

dispersed power generation systems, IEEE Trans. Power Electron., 2004, 19,(5), pp.

11841194

[5] HARRISON R, HAU E, SNEL H: Large wind turbines design and economics (John

Wiley & Sons, 2000)

[6] SIEGFRIEDSEN S, BOHMEKE G: Multibrid technology a significant step to multi-

megawatt wind turbines, Wind Energy, 1998, 1, pp. 89100

[7] POLINDER H, VAN DER PIJL FFA, DE VILDER GJ, ET AL.: Comparison of direct-

drive and geared generator concepts for wind turbines, IEEE Trans. Energy Convers.,

2006, 21, pp. 725733

[8] HANSEN LH, HELLE L, BLAABJERG F, ET AL.: Conceptual survey of generators

and power electronics for wind turbines Riso National Laboratory Technical Report

Riso-R-1205(EN) Roskilde, Denmark, December 2001.

[9] FOX B, FLYNN D et. al., Wind power integration: connect and system operational

aspects, IET Power and Energy Series, Vol. 50, ISBN 10: 0863414494.22


23/23

[10] POLINDER H, MORREN J: Developments in wind turbine generator systems. Electrimacs

2005, Hammamet, Tunisia

[11] DUBOIS MR, POLINDER H, FERREIRA JA:Comparison of generator topologies for direct-

drive wind turbines. www.ietdl.org; Proc. Nordic Countries Power and Industrial Electronics

Conf. (NORPIE), Aalborg, Denmark, June 2000, pp. 2226

[12] CARLSON O, GRAUERS A, SVENSSON J, ET AL.: A comparison of electrical systems forvariable speed operation of wind turbines. European wind energy conf., 1994, pp. 500505

[13] BYWATERS G, JOHN V, LYNCH J, ET AL.: Northern power systems windPACT drive train

alternative design study report. NREL, Golden, Colorado, Report no. NREL/SR-500-35524,

October 2004

[14] SOENS J: Impact of wind energy in a future. PhD dissertation, Wettelijk depot, UDC

621.548, December 2005[15] DUBOIS MR: Optimized permanent magnet generator topologies for direct-drive wind

turbines. PhD dissertation, Delft University Technology Delft, The Netherlands, 2004

[16] GRAUERS A: Design of direct-driven permanent-magnet generators for wind turbines. PhD

dissertation, Chalmers University of Technology, Goteburg, 1996

[17] VERSTEEGH CJA, HASSAN G: Design of the Zephyros Z72 wind turbine with emphasis on

the direct drive PM generator. NORPIE 2004NTNU Trondheim Norway, 1416 June 2004[18] CHEN Y, PILLAY P, KHAN A: PM wind generator topologies, IEEE Trans. Indus. Appl.,

2005, 41, (6), pp. 16191626

[19] CHEN J, NAYAR C, XU L: Design and finite-element analysis of an outer rotor permanent-

magnet generator for directly-coupled wind turbine applications, Proc. IEEE Trans. Magn.,

2000, 36, (5), pp. 38023809
http://www.ietdl.org/http://www.ietdl.org/

Documents

QamarZulfiqarMujahid_Seminar1.ps`ptx