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Wind Energy
Systems
Power Conversion & Control ERIGrid Spring School
5th March 2019
2
Contents
● Introduction
● Reference frame transformation
● AC machines
● Wind energy systems
● Power Converters in wind energy systems
● Power system stability
● RSCAD basic control blocks
● Scaling the power
● Two grids with different SCC
● Scripting example
● Line faults and parameters
3
Introduction
● Why Renewable Energy?
❏ Clean energy
❏ Less contribution to global warming
❏ No risk of radioactive exposure
❏ Minimal operating expenses
However,
❏ Noise (rotor blades) & aesthetic impact
❏ Intermittent power source
❏ No storage
❏ Remote locations
❏ High initial investment
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Introduction
5
Reference frame transformation
● Simplification of analysis of the electric machines
● Control schemes & digital implementation
● Space vector and it’s three phase variables
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abc/dq reference frame transformation
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abc/αβ reference frame transformation
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AC machines ● Synchronous machines
1. DC current is applied to the rotor winding producing a rotor magnetic field.
2. The rotor is turned by external means producing a rotating magnetic field, which induces a 3-phase
voltage within the stator winding.
3. Wound- Rotor SG- salient poles no need for GB
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● Synchronous Machines (continuation)
4. Permanent Magnet SG
Non salient/ surface
mounted PMSG Salient/ Inset
PMSG
AC machines
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● Asynchronous / Induction Machines
1. The rotating magnetic flux from the stator induces currents in the rotor, which also produces a
magnetic field.
2. If the rotor turns slower than the rate of the rotating flux, the machine acts like an induction
motor (the slip is positive).
3.If the rotor is turned faster, it acts like a generator, producing power at the synchronous
frequency (the slip is negative).
4.
5.Induction generators are not self-exciting, meaning they require an electrical supply, at least
initially, to produce the rotating magnetic flux.
AC machines
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● Asynchronous / Induction Machines (continuation)
6.
7.
AC machines
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Wind energy systems - Wind energy characteristics
1.
Where ρ is the air density, β is the pitch angle, Cp(λ, β) is the wind-turbine power coefficient, R is the
blade radius, V is the wind speed (in m/s) and the term λ is the tip–speed ratio, defined as:
2. Power Curves at various Wind Speeds
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Wind Energy Characteristics 3. Maximum Power Point Tracking (MPPT)
Aim: To maximize the wind power capture at different wind speeds, by adjusting the turbine speed so
as the optimal tip speed ratio to be maintained.
Ways:
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Wind Energy Systems - Configurations 1. Fixed Speed Wind Turbines
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PC in wind energy systems - Two- Level Voltage Source
Converter
● Sinusoidal PWM gating
● Amplitude and frequency modulation index
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Three- Level Voltage Source Converter
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Wind energy systems - Type 3 Wind Turbine This WT uses a doubly fed asynchronous generator where,
• The stator is directly connected to the grid.
• The rotor is is connected through a back-to-back power converter.
Some WTs include either a chopper or a crowbar for LVRT without bypassing or disconnecting the
converter.
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These WT’s are connected to the grid through a full scale power converter.
They can use either synchronous or asynchronous generators and some of them use direct drive
synchronous generators. Thus, they do not have a gearbox.
Wind energy systems - Type 4 Wind Turbine
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Power system stability - Transient stability ● Concerns the ability of the synchronous generators to remain in synchronism after being subjected to a large
disturbance
● The factors that influence transient stability of the system are:
(a) How heavily the generator is initially loaded.
(b) The generator output during the fault. This depends on the fault location and type.
(c) The fault clearing time.
(d) The post-fault transmission system reactance.
(e) The generator reactance. A lower reactance increases peak power and reduces initial rotor angle.
(f) The generator inertia. The higher the inertia, the slower the rate of change angle. This reduces the
kinetic energy gained during fault.
(g) The generator internal voltage magnitude. This depends on the field excitation.
(h) The infinite bus voltage magnitude.
● Critical clearing time (CCT), the maximum time during which a disturbance can be applied without
the system losing its stability,
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Type 4 WT - RSCAD basic control
blocks
• Pitch angle controller
• Two-mass mechanical model
• Aerodynamic model
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Pitch angle controller
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Two-mass mechanical model
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Aerodynamic model
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FRT & LVRT logic
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RSCAD LVRT logic
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Scaling the power - Interface transformer
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Scaling the power - Norton equivalent
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Two Grids - Different SCC
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Two Grids - Different SCC
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Scripting Example
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Line faults
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Line parameters
IEEE- 9 BUS SYSTEM LoadA LoadB LoadC
MW 125 90 100
MVar 50 30 35
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G1 G2 G3
72 163 85
35 0.85 -6.5
Op. Scenario I: Only SGs connected & 3-phase Fault 0.2Ω at the middle of line 5-7.
A. FCT=12c=240ms
IEEE- 9 BUS SYSTEM
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B. FCT=13c=260ms
IEEE- 9 BUS SYSTEM
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Op. Scenario II: WT3 connected & 3-phase Fault 0.2Ω at the middle of line 5-7
A. FCT=10c=200ms
IEEE- 9 BUS SYSTEM
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B. FCT=11 cycles=220ms
IEEE 39 BUS SYSTEM
Bus Pload (MW) Qload (Mvar)
3 322 2.4
4 500 184
7 233.8 84
8 522 176
12 7.5 88
15 320 153
16 329 32.3
18 158 30
20 628 103
21 274 115
23 247.5 84.6
24 308.6 92
25 224 47.2
26 139 17
27 281 75.5
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Bus Pg (MW)
Qg (Mvar)
30 31 32 33 34 35 36 37 38 39
250 520 650 632 508 650 560 540 830 1000
219 239 246 165.7 185.1 277.8 131.5 50.32 74.07 180
Contingency applied: 3ph fault , R=0.001Ω, node 13.
IEEE 39 BUS SYSTEM
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Scenario1: Only SGs conn., CCT =260ms Scenario2: WT2 conn., CCT=240ms
Scenario3: WT2& WT6 conn., CCT=220ms Scenario4: WT2& WT6 & WT1 , conn., CCT=200ms.
FCT=
24
0m
s
FCT
= 2
60
ms
IEEE 39 BUS SYSTEM
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FCT=160ms
Scenario2, FCT=260ms
Scenario 4, FCT=220ms
IEEE 39 BUS SYSTEM
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Study case with big CCT: Scenario A- Only SGs & 3-ph Fault at Bus 39: CCT 89 cycles=1.78 seconds
Scenario B- WT2 instead of SG2 & 3-ph Fault at BUS 39: CCT 80 cycles=1.6 seconds
Scenario A
Scenario B
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Thank you!
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