EEL-5285C and EEL 4930 Lectures 11, 12, 13 & 14 WIND ... 11, 12, 13 & 14 WIND Energy Utilization...
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Transcript of EEL-5285C and EEL 4930 Lectures 11, 12, 13 & 14 WIND ... 11, 12, 13 & 14 WIND Energy Utilization...
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.