61829685 application power electronics in modern wind energy systems

8/2/2019 61829685 Application Power Electronics in Modern Wind Energy Systems

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APPLICATION OF POWERELECTRONICS IN MODERN WIND

ENERGY SYSTEMS


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What is Power Plant(PP)

A pp may be defined as a m/c (or) assembly ofequipment that generates & deliver a flow of

electrical energy.


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Classification of pp

a) conventional - based on fossil fuels.Eg. Steam engine, diesel engine, hydro, gas,

nuclear etc.

b) non conventional based on solid waste,

wind, solarEg. Bio gas, Wind , Solar etc.


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Why wind energy?

No fossil fuel, no emission, clean /green power.


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Wind mill operationThe turbine rotor converts the fluctuatingwind energy into mechanical energy which is

converted into electrical power throughgenerator and then transferred into gridthrough transformer & transmission lines.


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Main components ofWind Turbine system.

Blade & Rotor(Wind Energy)

Gear Box(Optional)

GeneratorPowerConverter

PowerTransformer

Supply gridtransmissionlines


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ROLE OF MAIN COMPONENTS

wind turbine capture the power from the wind by means ofaerodynamically designed blades & converts it to rotatingmechanical power.

The gear box adapts low speed of the turbine rotor to the highspeed of the generator.

The generator converts the mech power into electrical energy.

The electrical energy is fed into a grid through possibly a power

electronics converter. A transformer with circuit breaker & energy meter.

Th f d b


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The connection of wind turbineto the grid is possible at LV, MV,

HV, EHV system. For modern wind turbine system, each turbine has itsown transformer to raise voltage from the voltage levelof wing turbine (400 or 690 V) to the medium voltage(11kV or 22kV or 33kV).

The transformer is normally located close to the windturbine to avoid LV cables.

The MV system could be connected as a radiated feederor as a ring feeder.


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power characteristics of fixed speedwind turbine & controllers employed.

stall control(fixed pitch controller)At givenwind speed ,the wind turbine rotor starts tostall from the blade root and develop graduallyacross the whole blade length as WS increases.

Active stall control When wind speed < ratedwind speed optimize the power output by wayof lookup table. When wind speed > rated windspeed, to keep the power output at rated level

(power limiting mode). Pitch control the blades are turns out of the

wind at higher wind speed.


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Pitch controla

b

c

5 10 15 20 25 30

Wind Speed (m/sec)

Power

10.75

0.5

0.25


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Variable speed wind turbine system

Variable speed wind turbine system use powerelectronics system as interfaces.

Since the wind turbine operates at variablerotational speed , the electric frequency of thegenerator varies & must therefore be decoupledfrom the frequency of the grid.


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Application of Power Electronics

Power electronics has changed rapidly during thelast 30 years & the no. of applications has beenincreasing mainly due to the development of

semiconductor devices & microprocessortechnology.

Three important issues are of concern in using PEsystem: a) Reliability b) Efficiency c) Cost


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Roleof PEC PEC may allow both the direction of power

flow & can interface b/w theload/generator and grid.

Two types of PEC system: a) Gridcommutated b)self commutated.

Grid commutated are mainly thyristorconverters, 6 or 12 or even more pulse.These produce harmonics so, harmonic

filter is required. Also these are notable to control VAR & consumeinductive VAR


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Role of PEC

Self commutated are mainly PWMconverters, where IGBT are mainly used.These can control both active & reactivepower. That means the reactive power

demand can be delivered by a PWMconverter. The high frequency switchingPWM converter may produce harmonics .These harmonics are in the range of some

kHZ, so small size filter is required.


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Circuit diagram of VSC withIGBTS


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Wave forms of bidirectionalactive & reactive power of a VSC


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Contd


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Generators for wind turbines

Both Induction & synchronous generators can beused for wind turbine systems.

Inductive Generator used in a fixed speed (or) avariable speed

Synchronous generator normally used in PEinterfaced variable speed system.


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Induction Generator used in

wind turbine systemThere are 3 types: Cage rotor

Wound rotor with slip control by changing

rotor resistance

Doubly fed Induction Generator


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Fixed speed Wind Turbine

System IG is directly connected to the grid.

The frequency of grid determines the rotational

speed of the IG, and thus of the rotor. The generator speed depends on the no. of pole

pairs & the frequency of the grid.

Power rating available/ used up to 2.3MW(Danishconcept)


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Contd The scheme consists of a cage rotor IG

almost operate at fixed speed (variationof 1-2%).

The power can be limited aero dynamically by

stall control, active stall control or by pitchcontrol.Advantage:cheap,reliable (no

synchronization device required)

Disadvantage: not fast enough (within fewmsec) to control the active power


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.

Other disadvantages: It requires stiff power grid to enable stable operation.

Experience mechanical construction to absorb high mechanicalstress since wind gust may cause torque pulsations in the drivetrain & gearbox.

VAR compensator to reduce (almost) the reactive power demandfrom the IG to the grid.

It is usually done by continuously switching capacitorbanksfollowing the production variations(5-25 steps).

High starting current which require the soft starter (current

limiter) implemented by thyristor contoller. The ss has a limitedthermal capacity & it is short circuited by a contactor whichcarries the I FL when the connection to the grid has beencompleted. In addition it reduces the load on the gear box.

W d b h


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Wind turbine without power converter,but with aerodynamic power system

control.

i) Pitch controlledThe blades are turned out of the wind at higherwind speed.

ii) Stall controlled (fixed pitch controlled)- At given wind speed ,thewind turbine motor starts to stall from the blade root and developgradually across the whole blade length as WS increases.

iii) Active stall controlled- When wind speed < rated wind speedoptimize the power output by way of lookup table. When wind speed >rated wind speed, to keep the power output at rated level (powerlimiting mode).


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Gear Box

ReactiveCompensator

IG GridTxr

Fixed speed Wind Turbine System

l


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Variable Speed Wind Turbines The generator is normally connected to

the grid by a power electronic(PE) system.Decision on rating of PE systema) For synchronous generators & for

induction generators with out rotorwindings , a full rated PE system isconnected between the stator of thegenerator & the grid, where the total

power production must be fed through PEsystem.

C td


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Contd

b) For IGs with rotor windings, the stator of the

generator is connected to the griddirectly.Only the rotor of the generator isconnected through PE system.This gives theadvantage that only a part of powerproduction is fed through PE converter.Thismeans the nominal power of the convertersystem can be less than (normally 30%) thenominal power of the wind turbine. Bycontrolling the active power of the converter,

it is possible to vary the rotational speed ofthe generator & thus of the rotor of windturbine.

V i bl d WT ith ti ll


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Variable speed WT with partiallyrated power converters

Dynamic Slip controlled wound rotor IG(system IV)

Woundrotor IG

Gear Box

ReactiveCompensator

Grid

Resistance

control withPE

PitchControl

C td


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Contd An extra resistance is added in the rotor, which can be

controlled by PE.

The variation of rotor resistance produces a group oftorque speed characteristics, known as dynamic slipcontrol and gives typically a speed range of 2-5%.

At the same time an extra control freedom is obtainedat higher wind speeds in order to keep the outputpower fixed.

This system still needs a soft starter & reactive powercompensation.


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Doubly fed IG(DFIG) (System V) Slip rings are making the electrical connection to the rotor.

If the generator is running super synchronously , electrical poweris delivered to through both the rotor & the stator.

If the generator is running sub synchronously, electrical power isdelivered to the rotor from the grid.

A speed variation of +30% around syn.speed can be obtained bythe use of a power converter of 30% of nominal power.

Further more,it is possible to control both active power (Pref) &reative power (Qref),which gives a better grid performance and

the PE enables the wind turbine to act as a more dynamic powersource to the grid . Disadvantage: Bit more expensive compared to the classified system.


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PitchControl

Gear Box

AC

DC

Pref Qref

DC

AC

Variable speed wind turbine with


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Variable speed wind turbine withfull scale PE converter system

There are 4 possible schemes.

a)IG with gearbox & PE converter (System VI)

Grid

Gear Box

AC

DC

PrefQref

DC

AC

Pitch

A back to back voltage source converts is used in order to achievefull control of the active & reactive power.

b)S h t ith b


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b)Synchronous generator with gearbox& full rated PEC System (System VII)

Gear Box

AC

DC

PrefQref

DC

AC

Pitch

Syn.

Gen

DCAC

Grid

The syn. Generator needs a small power converter for field excitation

c)Multipole synchronous generator w/o


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c)Multipole synchronous generator w/ogearbox(System VIII) & full scale PEC

ACDC

Pref Qref

DCAC

PitchSyn.

Gen

DC

ACGrid

d)M ltip l PMSM / b x


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d)Multipole PMSM w/o gearbox&full scale PEC(System IX)

AC

DC

PrefQref

DC

AC

Pitch

PMSMMultipole

Grid


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All 4 systems have the same controllable

characteristics since the generator isdecoupled from the grid by a dc link.

Advantage: System is fast in controllingactive & reactive power.

Disadvantage: System is more complexwith more sensitive electronic parts.

Summary of WT generator


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Summary of WT generatorsystem

Table 3.1 presents a technical comparison of theabove system. Each system has benefits &drawbacks.

Conclusion:1) Fixed speed WT is slightly cheaper simple sincethe motor speed cannot be varied, these turbinesmust be more robust than other designs due thehigh structural loads involved.

Contd


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Contd

2)Variable speed WT generator more energy fora given wind speed ,especially at low windspeed. Moreover the active and reactive powercan be easily controlled & there is lessmechanical stress.

3)Direct drivers (gear less) are expensivebecause of the large & relatively heavygenerator & power converter of full rated

power.


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Integration of wind turbines into


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Integration of wind turbines intopower system requirements:

a) Frequency & active power control

Frequency is 50Hz or 60Hz(worldwide). Frequency proportional tothe rotating speed of synchronous generator operating in thesystem.

The generators in the same AC system are synchronized, running atthe same speed.

Increasing electrical load in the system tends to slowdowngenerators & reduce the frequency.

The task of frequency control of the system is to increase orreduce the generated power.

b) h t i it l l & lt


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b) short circuit power level & voltagevariations

Ability of the grid to absorb disturbances isdirectly related to the short circuit power level.

C) Reactive power control Reactive power concept is associated with the

oscillations of energy stored in capacitive &inductive components in a power system. Reactive power is produced in capacitive

components and consumed in inductive

components.

The current associated with the reactive


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The current associated with the reactivepower flow causes system voltage drop & alsopower losses.

The IG based wind turbines are the consumerof reactive power.

To minimize the powers losses & to increasevoltage stability.

These wind turbines are compensated by VARcompensation panel to a level depending on therequirements of load utility.

D) Flicker(voltage flicker)


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D) Flicker(voltage flicker) Voltage variations caused by fluctuating wind power

generation may cause voltage quality problem.

Fluctuations in the system voltage (rms value) maycause perceptible light flicker depending on themagnitude & frequency of the fluctuation.

The allowable flicker limits are generally establishedby individual utility.

Rapid variations in the power output from windturbine, such as generators switching & capacitor

switching can also result in variations in the rmsvalues of the voltage.


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E) Harmonics

Harmonics are a phenomenonassociated with the distortion of thevoltage & current waveform.

All the harmonics cause increasedcurrents & possible destructive overheating in capacitors as the impedance

of capacitors goes down in propositionto the increase in frequency.


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F) Stability

The problem of network stability isoften associated with different types offaults in the network such as tripping oftransmission lines (eg. Overload), loss ofproduction capacity and short circuit.


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Questions ?


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Thank You

61829685 application power electronics in modern wind energy systems

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