EEL-5285C and EEL 4930

Lectures 11, 12, 13 & 14

WIND Energy Utilization With Simulink Example and Assignment

Professor O. Mohammed

Technology Growth in Wind Turbine Generators

• Wind turbine generators (WTGs) started as fixed-speed wind turbines with conventional induction generators and capacitor banks as static reactive compensators. Capacitors supplied reactive power for the air gap magnetic flux, which the induction generators could not produce.

• Denmark initially standardized on this model, terming it the Danish Concept. These turbines contributed 71.6% of the total WTGs there by 2006.

• Later, squirrel-cage rotors in induction generators were replaced with wound rotors. Variable rotor resistance, variable speed compatibility with gears, and capacitor banks became standard features.

The Machines

• Doubly fed induction generators (DFIGs) followed with partially rated power electronic convertors. The converter helped to provide independent control of active and reactive power outputs of the WTGs. The PE converter rating was generally at 30% of the WTG rating.

• Finally, TWGs with added functions in the PE convertors arrived. The PE portion increased the costs but gave better control and helped in the fault-ride- through facility.

• This category constitutes just 0.2% of the total WTG population.

Nature of Wind

• Wind may blow steadily during certain periods, varying by day, season, location, and so on. Let us say the velocities fall within some zones. The wind may die down, falling almost to nil. Then it may rise from a very low speed.

• There may be a wind lull, when the wind dies out and then raises in short bursts. A wind gust is the opposite phenomenon to a wind lull. A very strong wind is a storm.

• This nature of wind makes it an unreliable source of power due to its variability and uncertainty.

Components of A Wind Turbine Generator

• The rotor blades, whose pitch is adjustable as per wind velocity so as to catch maximum wind energy.

• The gear box which adjusts the rpm of the rotor of the generator as closely as possible to the grid synchronous frequency.

• The generator, which converts mechanical input into an electrical output.

Wind Turbine

OPERATION OF WIND TURBINE GENERATORS--Output of a WTG

• Power captured by a WTG is given by

Operating Conditions

1. For a given wind condition it should produce maximum possible power. This is possible when λ stands at λopt. 2. There is a minimum wind condition below which the WTG becomes unstable.

λis represents the crossover point for the stable condition limit. At λstall, the WTG will stall. two operating conditions for a WTG:

• Note that the ratio of WTG blade tip speed to wind speed, λ, plays an important part.

• The WTG control should perform in such a way that it is at λopt under different conditions of wind load.

• The rate of change in λ is given by another quotient x:

• A WTG is in an unstable condition when becomes positive. The past figure shows power versus speed curves of a wind turbine with wind velocity as a parameter. The dashed line is a boundary between high- and low-speed regions.

WIND POWER

• The figure relates the power coefficient to the tip speed ratio, λ, defined as the relationship between the rotor blade tip speed and the free speed of the wind for several wind power turbines. As stated earlier, in quantitative terms, the tip speed ratio is defined as λ=v/V =wR/V,

• where w is the angular speed of the turbine shaft, v the blade tangent tip speed, and R the length of each blade.

• It emphasizes the importance of knowing the purpose for which the energy will be used, to allow determination of the best selection for wind power extraction.

Performance Objectives

1. Maximize captured power

1 3

P = 2 ρAv C p

Power in Wind Power Coefficient: Function of turbine

design, wind conditions, and control

2. Minimize structural loads

3. Reduce operational downtime

Possible Control Strategy

• Compute desired rotor speed using the optimal tip speed ratio (TSR) and measured wind speed.

• Issues:

ωdes = λopt v

R

• Shadowing effects corrupt wind speed measurement

• Uncertainty in power coefficient model

vmeas

λopt

TSR

Relation

ωdes Error Control Law

βopt

τ g

v

Turbine

ωmeas

λ

Standard Controller

• Control law (Johnson, et al, 2006 Control System Mag.)

τ = Kω 2

where K = 1 ρAR3 C p ,max

g

Comments

2 3 max

• Convergence to optimal power capture (λ converges to λmax) in steady wind. See next slide for proof.

• Only requires rotor speed sensor

• Control law still depends on uncertain power coefficient

model. Adaptive laws have been developed.

Kω2

βopt

τ g

v

Turbine

ωmeas

p g

Optimality of Standard Controller

• Recall the simplified, one-state turbine model: Jω = τ

a (ω, v, β ) − τ

g =

1

2ω ρAv 3C (β , λ ) −τ

• Substitute standard law τ g

= Kω 2

into the simple turbine model

• Assume constant wind (v=constant) and recall:

• If λ>λmax then the term in parentheses

is <0 and hence dω/dt <0. Thus ω will decrease until λ=λmax.

• If λ<λmax then the term in parentheses is >0 as long as Cp(λ)>F(λ) where F is:

λ = ωR v

This condition holds over a wide range (see diagram) and implies dω/dt <0. Thus ω will increase until λ=λmax.

Ref: Johnson, et al, Control System Magazine, 2006

Rated Power Control

• Objective: Maintain rated power and reduce loads

• Strategy: Hold τg = τrated (constant) and use β to track ωrated

• Reference: • J. Laks, L. Pao, and A. Wright, Control of Wind Turbines: Past,

Present and Future, American Control Conference, 2009.

ωrated Error Control Law

β

τ rated

v

Turbine

ωmeas

• Issues:

Control Issues

• Excitation of flexible modes, e.g. tower fore/aft

• Loads on blades due to wind gusts

• Advanced control methods

• Reduce tower fore/aft with notch filters and/or accelerometer measurements in the nacelle

• Use individual blade pitch control to reduce loads

• Reduce drivetrain vibrations by adding a small generator torque ripple computed by filtering the rotor speed measurement.

Fault Detection and Diagnostics

• Reduce downtimes

• Reduce maintenance costs

• Prevent catastrophic failures

Damaged Gear Teeth (Image courtesy of

Mesabi Range Wind Technology Program)

WIND PROJECT CONSTRUCTABILITY

Project model is created – WTG locations then “field checked”

Access Roads

Collection System

Generator Tie Line

Substation Design

Culverts & Drainage

Road & Infrastructure Evaluation

Grade

Crane Routing

Geotechnical Assessment

Hydrological Assessment

ACCESS ROADS

• 125ft construction area

for roads

• Temporary wind turning

radius

• Coordinate

landowner/farmer input

to road layouts

• Finished Access Roads

are approximately low

profile,16ft wide

TURBINE FOUNDATIONS

• Inverted “T” Foundation style

• Top Soil set aside during

excavation

• Foundation poured/rebar/anchor

bolts installed

•2 truckloads of steel per WTG

• Electrical conduit installed

• Concrete poured

• Over 300 yrds per foundation

• Foundation backfilled to leave

just 16 ft pedestal above ground

UNDERGROUND COLLECTION SYSTEM

• Collection line ditches

are trenched “cross

country”

• ~60 miles of cable

• Boring under streams,

roads, highways,

ditches etc.

Turbine Components

Anemometer

it.m;;

Figure from the US DOE

----------------------------------------------------------------

TURBINE DELIVERIES TO SITE

• Close coordination with local jurisdictions

• More than 10 truck loads per tower

• Base section: 15 Base Section (15’ Dia, 73’ Long, 125,000+ lbs)

• Mid Section (14’ Dia, 82’ Long, 83,000+ lbs)

• Top Section (11’ Dia, 98” Long, 65,900+ lbs)

• Blades (132 ft long)

BASE & BASE/MID SECTION ERECTION FOR A

WIND TURBINE

ROTOR ASSEMBLY & COMPLETE

WTG ERECTION

Desirable Rotor Blade Features

Anatomy of a Rotor Blade

Generic blade cross section -

Fiber reinforced material

l Balsa wood/

foam

Material transition zone

Real time Evaluation of Blade Health

Sensor Location

gravity

lift

lift Where do you

expect failure?

Various types of sensors Strain Gages

Acoustic Emission

Accelerometers

Smart Materials (PZT)

Sensor Locations (1)

Strain gages

Sensor Locations (2)

Smart materials

Wind Turbines

161kV

V

0 Time Low-Voltage Ride-Through

0 − 690V 10 − 60 Hz

Generator

690V 34.5 kV

Power Electronics Converters

60 Hz

Types of Wind-Utility Interface

Wound rotor

Induction Generator

Wind

Induction Generator

Utility

Wind

AC DC

Turbine Turbine DC AC

Generator-side

Converter

Grid-side

Converter

Grid-Connected Induction

Generator

Doubly-Fed Induction Generator

Power Electronics Interface

Gen Conv1

Conv 2

AC Generator and a Power

Electronics Interface

Utility

Simple Rigid Body Model

Newton’s second law for rotational systems

Control inputs are the

generator torque (τg)

Jω = τ a (ω, v, β ) − τ g and blade pitch (β)

Rotational inertia

of blades, rotor and

drivetrain

Aerodynamic torque depends on

rotor speed (ω), wind speed (ν), and

blade pitch angles (β).

ω, τa Blade

pitch, β

τg Generator

torque, τg

Wind, ν

Turbine

Rotor

speed, ω

Interface for Wind Generator:

Power Electronics Interface

Converter

Wind Generator

Utility Grid

Controller

POWER TRANSISTORS

• MOSFETs

• IGBTs

• IGCTs

• GTOs

• Niche devices: BJTs, SITs, MCTs

uutiltilityi LLooaadd

Voltage-Link System

conv1 conv2

Controller

δ

uutiltilityi LLooaadd

Power and Reactive Power Control

conv1 conv2

controller

+

Vconv

−

I

jX +

Vs

−

(a )

δ

I

(b)

Vconv

Vs jXI Re

Step-Down (Buck) Converter

Vin

Filter A

+

v A

−

Tup d =

Vo Ts

v A

v A

VA = Vo VA = Vo = dVin

Step-Up(Boost) Converter

Vo C

iL

vL

Vin

q p

Bi-Directional Power Flow

A V1

+ +

v A V2

− −

q q− =

iL

(1 − q)

p

Power Coefficient, Cp

• Cp :=

Pcaptured

Pwind

= C p (β , λ )

Cp for NREL CART 600kW, 21.7m turbine

• β= Collective blade pitch

• λ= Tip speed ratio = ωR v

• Aerodynamic torque

τ a = Pcaptured

ω

ρAv =

3C (β , λ )

2ω

Figure from:

K. Johnson, L. Pao, M. Balas, and L. Fingersh,

Control of Variable Speed Wind Turbines,

IEEE Control Systems Magazine, June 2006

Average Representation of the Switching Power-Pole

With Bi-directional Power Flow:

+ ida

Vd + v

aN

vaN

ida

ia

= daVd

= da ia

Vd

ia

vaN

− − − N 1 : d

a

qa

(a) (b)

Average Representation of the Switching Power-Pole

With Bi-directional Power Flow:

+ ida

Vd + v

aN

vaN

ida

ia

= daVd

= da ia

Vd

ia

vaN

− − − N 1 : d

a

qa

(a) (b)

Ts

i

Synthesizing Sinusoidal AC:

q = 1 L

q

vaN

Vd

0

vaN

0 T

s

vaN

0 v

aN ωt

V

c

q

Three-Phase Inverters

+ a a + b

Vd b

n

d

c

− N

qa qb

c

−

N 1 : d

a 1 : db

1 : dc

(a) (b)

n

Interface for Wind Generator

va (t) + −

ia (t ) i

A

A(t)

+eA

(t ) −

+ B

Vd −

C

N

Vd

vAN

vAn

1 V

2 d

0 ωt

Harnessing of Wind Energy

Variable

Variable speed

Variable speed

generator

Variable Frequency

AC

Power Processing

Unit

frequency AC

Utility

Wind turbine

Role of an Electric Drive

Electric Drive

Fixed form

Electric Source

(utility)

Power Processing Unit (PPU)

Adjustable

Form

Motor Load

speed / position

Controller

Sensor

Power

Signal

Input command

(speed / position)

Role of Electric Drive: Efficient conversion of power

from electrical to mechanical and vice versa

• Role of PPU: Delivers appropriate form of frequency &

• voltage to the machine (as required by the load or the

• prime mover)

Introduction to AC Machines

Primary AC motor drives

Induction motors

Permanent Magnet Brushless (Synchronous Motors)

b − axis

ib

2π / 3

ic

2π / 3

2π / 3 ia

a − axis

c − axis

Basic Principles

Electromagnetic Force:

external B field

fem fem

subtract

f

add

= B i

resultant

em

[ Nm]

[T ] [ A] [m]

Turbine Modeling

• Rigid body model neglects

• Flex modes: drivetrain, blade and tower

• Detailed turbine aerodynamics

• Drivetrain flexibility and tower fore/aft modes are important in the control law design

• Higher fidelity models

• FAST Sim Package: Blade element theory + key flex modes

(http://wind.nrel.gov/designcodes/simulators/fast/)

• State-space linearizations (periodic and time-invariant)

• Fluid-structure interaction models (Stolarski, UMN)

• Turbine interaction models (Sotiropoulos, Chamorro, et al)

Three-Phase Stator Windings:

Present Day Wind Turbines

161kV

See Details Next

Blad@ ' '

MC

System

High frequency transformer

Hu converter system

I High-Frequency

Transformer and

Converter

Utility

. Light Cable at 34.5 kV ...

34.5 kV, 60-Hz Underground

Wind Turbine Control

• Control strategies depend on the wind conditions

• Supervisory control and mode logic

• Yaw control

• Power capture at low wind speeds

• Rated power + load reduction at high wind speeds

• Good Survey References • K. Johnson, L. Pao, M. Balas, and L. Fingersh, Control of Variable Speed Wind

Turbines, IEEE Control Systems Magazine, June 2006.

• T. Burton, D. Sharpe, N. Jenkins, E. Bossanyi, Wind Energy Handbook, Chapter 8: The Controller, 2001.

• J. Laks, L. Pao, and A. Wright, Control of Wind Turbines: Past, Present and Future, American Control Conference, 2009.

Pow

er (k

W)

Typical Operating (“Run”) Modes

3000

2000

1000

Available

Power

Captured

Power

0 0 5 10 15 20 25 30

Wind Speed (m/s)

Cut-in Rated Cut-out

Region 2:

Maximize

power

Region 3:

Rated power

+ load reduction

Plot based on Clipper Liberty C100 2.5MW turbine assuming Cp,max = 0.4

(Theoretical bound for power capture given by Betz Limit: Cp,Betz = 0.59)

Yaw Control

• Noisy wind measurements and slow yaw rate (~1deg/s) make PID control ineffective for yaw control.

• On\off threshold-based yaw control designed by Caleb Carlson.

• During “Run” mode, yaw is activated when the yaw error has averaged 10 degs for 10 mins

Captured Power Control

• Objective: Maximize captured power

• Strategy: Hold β=βopt (constant) and use τg to track λopt

Figure from: K. Johnson, L. Pao, M. Balas, and L. Fingersh, Control of

Variable Speed Wind Turbines, IEEE Control Systems Mag., June 2006

Mechanical Power

• The turbine mechanical power can be given by

• The air density ρ can be corrected by the gas law (ρ = P/RT) for every pressure and temperature with the following expression:

Advantages Disadvantages

Direct drive operation High cost of PMs

Higher efficiency and energy PM demag. at high T

Higher power to weight ratio Manuf. Difficulties

No additional power supply

12

Wind generator technologies

Double Fed Induction Generator Electrical Exc. Synchronous Generator

Gearbox

DFIG Converter

Grid EESG

Converter

Converter

Grid

Advantages Disadvantages Advantages Disadvantages

Smooth grid-connection Multi-stage gear box Flux control for minimizing loss External field excitation

Reactive power compensation Need carful protection PMs is not required (less cost) Heavy weight

Rotor energy can be fed to grid Control complexity Voltage is controllable Expensive solution

Self Exc. Induction Generator Permanent Magnet Synchronous Generator

Gearbox

SEIG

Converter

PMSG

Grid

Converter

Grid

Advantages Disadvantages

flexible control More expensive conv.

Absence of brushes Higher conv. losses

Less cost and maintenance Multi-stage gear box

SEIG Dynamic Modeling

DFIG

• A three-phase wound-rotor induction machine can be set up as a doubly- fed induction motor. In this case, the machine operates like a Synchronous motor whose synchronous speed (i.e., the speed at which the motor shaft rotates) can be varied by adjusting the frequency fRotor of

the ac currents fed into the rotor windings.

• The same wound-rotor induction machine setup can also serve as a doubly-fed induction generator. In this case, mechanical power at the machine shaft is converted into electrical power supplied to the ac power network via both the stator and rotor windings.

• Furthermore, the machine operates like a synchronous generator whose synchronous speed (i.e., the speed at which the generator shaft must rotate to generate power at the ac power network frequency fNetwork can

be varied by adjusting the frequency of the ac currents fed into the rotor windings.

DFIG

• The ac currents produced by the generator are converted into dc current by an AC/DC converter, then converted by another AC/DC converter back to ac currents that are synchronous with the ac power network. It is therefore necessary for the power electronics devices used in such a circuit to have the size and capacity to process 100% of the generator output power.

• The power electronics devices used in doubly-fed induction generators, on the other hand, need only to process a fraction of the generator output power, i.e., the power that is supplied to or from the generator rotor windings, which is typically about 30% of the generator rated power.

• Consequently, the power electronics devices in variable-speed wind turbines using doubly-fed induction generators typically need only to be about 30% of the size of the power electronics devices used for comparatively sized three-phase synchronous generators.

C

B

Lo

ad

L

oa

d

Lo

ad

L

oa

d

dc

AC/DC power Conversion topologies:

• There are several ways for converting the AC alternate (from wind) power to DC, as a first stage of interfacing with the grid.

Advantages: • Simple design A

L D1 D3 D5

Advantages: + • Controlled output L

A

L

+ D1 D3 D5

C

Disadvantages: C C Vdc Disadvantages: B S V

C

• Uncontrolled output • Large harmonics

Advantages: • Controlled output

D4 D6 D2 _

Diode Rectifier

• Poor power factor • Large harmonics

Advantages: • Controlled output

D4 D6 D2 _

Diode with Boost Converter

• Large power (MW) L +

T1 T3 T5 • Less harmonics +

L S1 S3 S5 A

Disadvantages: B

C

• Large harmonics

C Vdc

A

Disadvantages: B

C

Vdc

• Large size & weight T4 T6 T2 _ • Large switching loss

• Complex control Phase-controlled

Thyristor Converter

S4 S6 S2 _

Fully-controlled IGBT Converter

16

or

DC/AC Inverter interface topologies for ac-loads and Utility grid connectivity:

Z-source inverter

L Voltage or Current

source Converter

+ C C _ or

L Inverter

Small power applications such as: • Fuel cell Vehicles

DC • UPSs

AC • ASDs

Voltage source inverter

+ S1 S3 S5 L

Vdc C

S4 S6 S2

AC Grid Utility

R

Vgrid

Small and medium power applications such as: • Medium voltage industrial appl. • Wind farms • VAR compensators • active filters

_ • FACTS

17

Current source inverters (CSIs) can be utilized for new ideas:

Advantages: • Boost design • Large power (MWs) Disadvantages: • Large inductor size

+

Vdc

L idc

S1 S3 S5 AC Grid Utility

L R

S4 S6 S2

_

For Example: • CSIs can be used for Cascaded multilevel

inverters Larger switching frequency

levels with SiC technologies

Vgrid

Large power applications such as: • plug-in HEVs • PM motor control • PVs grid tie inverters • Propulsion drive systems

18

d d q a

q q d

Converter Modeling

• Voltage source PWM converter

The dynamic equations in the synchronous frame directly such as

1

idc iL

ic

id (s)

Ls R .e (s) v (s) .L.i (s) e R L

e a + 1

b C R

iq (s)

Ls R .e (s) v (s) .L.i (s) ec

b Vdc L

c _

For the converter output side

RL

vd c

.id c

1 CRL s

19

Voltage source PWM converter

• The converter plant model in s-domain representation is

20

Performance Improvement through Blade Pitch Control

• At low speeds, the pitch angle is almost zero. Maximum possible energy is scooped up (Maximum power strategy).

• At high speeds, the pitch angle increases. Beyond a certain wind speed, automatic mechanical brakes apply and electrical dumping resistances are used as loads.

Efficiency of a WTG

• Average efficiency of a WTG is defined as the ratio of energy delivered to grid to the energy at the turbine rotor shaft. As the energy is transmitted from one member to the next in the transmission system of a WTG, losses are incurred.

Losses in a WTG

• Average and rated efficiencies for the three different types of WTGs are 82–86% at low wind speeds and 89.7–89.9% at high wind speeds.

• Thus, weather forecast and past statistical data form important requirements for efficiency and reliability when integrating wind farm energies into today’s mega grids.

Flickers in the Output of a WTG

• There are two main causes of flicker in the supply from a WTG: Mechanically Related Causes • Motor turbine imbalance • Rotor blades passing in front of the wind structure • Structural modes due to mechanical Eigen frequencies

(frequencies at which there is mechanical resonance) Rotational sampling

• The flickers caused by these mechanical causes have a regular pattern, low amplitude, and a low-frequency range of 0.65 to 0.71 Hz.

Wind Velocity Related Causes. Wind flow has regular bursts that can cause flicker. This flicker has high amplitude and a range of 0.01 Hz–10 Hz. This flicker is objectionable and has been investigated deeply.

We Need Proper Control of The

Wind Turbine Genera tor System

MODELING OF A WIND TURBINE GENERATOR

• It is desirable first to understand how a vastly spread electricity power system operates physically. The following gives a brief sketch.

• A transmission system operator (TSO) looks after load following and power quality on a very small scale time scale, say on a minute or 10 minute basis.

• For this, he has a schedule of power offers from various generators. The TSO has also a schedule of time-bound requirements from the customers. He matches these and balances the load.

• The TSO also has an updated chart of transmission facilities with all their characteristics in his computer. He selects an optimum route for a load dispatch. • This route has minimum operating losses and costs. Modern

fast-operating computers and accurate data are essential for his work. Supplying of accurate models of WTGs is compulsory for this reason.

Method

• Electrical and mechanical parameters of a WTG are converted into algebraic quantities. These algebraic notations are used to develop algorithms to arrive at characteristic functions.

• Most of the grids in the world require a WTG dynamic model to be submitted to the transmission system operator for permission to join the grid.

• Typical Irish grid requirements are listed below. Any WTG greater than 5 kW must submit a model incorporating the following features:

1. Generator general characteristics 2. Turbine generator and drive train mechanical characteristics 3. Variation of power coefficients and pitch angle to tip speed ratio 4. Blade pitch control 5. Converter controls 6. Reactive components 7. Protection relays

• Time per step for simulation should not to exceed 5 microseconds. Although models for simulations for thermal and hydro generators have long been used and standardized, those for WTGs are still evolving and there are no standards.

Dynamic Scheduling

• Dynamic Scheduling. With accurate modeling of the system and computerized software he can find out what can happen to the system when an electric component is added or subtracted or controlled in power output. All system components must be put in models for accurate simulation. Modeling right to the last details becomes important.

• Since the WTG technology is fast developing, standards for modeling these are not yet in place yet.

Weather Forecasts

• Accurate Hourly Weather Forecasts. Weather is not all that erratic. Weather behavior falls into a pattern— daily, seasonal, periodical, and geography specific. Excursions out of this pattern might be in a band that can be anticipated.

• This, along with daily weather forecast by meteorology departments, can help the system operator on unit commitments from the wind farm on the day-ahead schedule as well as on the daily schedule, balancing them fairly closely.

• The system operator need not commit too much capacity to reserves. In fact, although wind energy costs are next to nil, their operational costs go largely toward unit commitments.

Wind Energy Conversion System to Be Implemented in MATLAB/SIMULINK

Main Grid Tr2 Tr1

Local Load

TL 2 TL 1

Main Load

Plant

Wind Farm

M

Tr3

PF Correction

Resistive Load

Motor Load

120 kV, 60 Hz (Transmission Level)

120 kV/25 kV 47 MVA

20 km 10 km 25 kV/575 V

6*2 MVA

500 kW 9 MW Wind Farm

(6*1.5 MW)

2 MVA

25/2.3 kV 2.5 MVA

800 kvar

200 kW

CB

1.68 MW 0.93 PF 2300 V

Plant

• The wind farm utilizes DFIG for power generation

• The wind farm is rated 9 MW and consists of six 1.5 MW wind turbines

• The turbines are connected to a 25-kV distribution system

MATLAB/SIMULINK Model

Model Description

• The WECS exports power to a 120-kV grid through a 30-km, 25-kV feeder.

• A 2300V, 2-MVA plant consisting of a motor load (1.68 MW induction motor at 0.93 PF) and of a 200-kW resistive load is connected on the same feeder.

• Both the wind turbine and the motor load have a protection system monitoring voltage, current and machine speed. The DC link voltage of the DFIG is also monitored.

DFIG

• Wind turbines, in this model, use a doubly-fed induction generator (DFIG) consisting of a wound rotor induction generator and an AC/DC/AC IGBT-based PWM converter.

• The stator winding is connected directly to the 60 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter.

• The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind.

• The optimum turbine speed producing maximum mechanical energy for a given wind speed is proportional to the wind speed. For wind speeds lower than 10 m/s the rotor is running at subsynchronous speed . At high wind speed it is running at hypersynchronous speed.

• Another advantage of the DFIG technology is the ability for power electronic converters to generate or absorb reactive power, thus eliminating the need for installing capacitor banks as in the case of squirrel-cage induction generators.

System Ratings

• The nominal wind turbine mechanical output: 6*1.5e6 watts.

• The generator rated power: 6*1.5/0.9 MVA (6*1.5 MW at 0.9 PF), specified in the Generator data menu. The nominal DC bus capacitor: 6*10000 microfarads, specified in the Converters data menu.

• Also, notice that the Controller has to maintain "Voltage regulation". The terminal voltage will be controlled to a value imposed by the reference voltage (Vref = 1 pu) and the voltage droop (Xs = 0.02 pu).

Demonstrations—Turbine Response to a Change in Wind Speed

• Initially, wind speed is set at 8 m/s, then at t = 5s, wind speed increases suddenly at 14 m/s.

• Start simulation and observe the signals on the "Wind Turbine" scope monitoring the wind turbine voltage, current, generated active and reactive powers, DC bus voltage and turbine speed.

• At t = 5 s, the generated active power starts increasing smoothly (together with the turbine speed) to reach its rated value of 9 MW in approximately 15 s. Over that time frame the turbine speed will have increased from 0.8 pu to 1.21 pu.

• Initially, the pitch angle of the turbine blades is zero degree and the turbine operating point follows the red curve of the turbine power characteristics up to point D. Then the pitch angle is increased from 0 deg to 0.76 deg in order to limit the mechanical power. Observe also the voltage and the generated reactive power. The reactive power is controlled to maintain a 1 pu voltage.

• At nominal power, the wind turbine absorbs 0.68 Mvar (generated Q = -0.68 Mvar) to control voltage at 1pu. If you change the mode of operation to "Var regulation" with the "Generated reactive power Qref " set to zero, you will observe that voltage increases to 1.021 pu when the wind turbine generates its nominal power at unity power factor.

Demonstrations—Voltage Sag on the 120-kV System

• You will now observe the impact of a voltage sag resulting from a remote fault on the 120-kV system. First, in the wind speed step block, disable the wind speed step by changing the Final value from 14 to 8 m/s.

• Then open the 120-kV voltage source menu. In the parameter "Time variation of", select " Amplitude". A 0.15 pu voltage drop lasting 0.5 s is programmed to occur at t = 5 s.

• Make sure that the control mode is still in Var regulation with Qref = 0. Start simulation and open the "Grid" scope. Observe the plant voltage and current as well as the motor speed. Note that the wind farm produces 1.87 MW.

• At t = 5 s, the voltage falls below 0.9 pu and at t = 5.22 s, the protection system trips the plant because an undervoltage lasting more than 0.2 s has been detected (look at the protection settings and status in the "Plant" subsystem). The plant current falls to zero and motor speed decreases gradually, while the wind farm continues generating at a power level of 1.87 MW. After the plant has tripped, 1.25 MW of power (P_B25 measured at bus B25) is exported to the grid.

• Now, change the wind turbine control mode to "Voltage regulation" and repeat the test. You will notice that the plant does not trip anymore. This is because the voltage support provided by the 5 Mvar reactive power generated by the wind-turbines during the voltage sag keeps the plant voltage above the 0.9 pu protection threshold. The plant voltage during the voltage sag is now 0.93 pu.

Wind Energy Computer Assignment

• Given the MATLAB/SIMULINK model of the wind energy conversion system example explained during the lecture, apply the following changes to the model,

1. Replace the simple step change wind input with a properly- scaled actual wind pattern and comment on the effect of the wind speed variations on the turbine output power and voltage.

2. Change the rating of the 500 kW load to any another rating value in the range of (1-3 MW), and change the rating of the 2 MVA plant to any value in the range of (4-6 MVA), then apply appropriate design changes to the number of wind turbines, their ratings, as well as the converters’ and generators’ ratings in order to successfully supply the local loads without violating the voltage or frequency limits on the grid connection point.