Application Power Electronics in Modern Wind Energy Systems

APPLICATION OF POWER APPLICATION OF POWER ELECTRONICS IN MODERN WIND ELECTRONICS IN MODERN WIND

ENERGY SYSTEMSENERGY SYSTEMS

APPLICATION OF POWER APPLICATION OF POWER ELECTRONICS IN MODERN WIND ELECTRONICS IN MODERN WIND

ENERGY SYSTEMSENERGY SYSTEMS

What is Power Plant(PP)What is Power Plant(PP)What is Power Plant(PP)What is Power Plant(PP)A pp may be defined as a m/c (or) assembly of A pp may be defined as a m/c (or) assembly of equipment that generates & deliver a flow of equipment that generates & deliver a flow of

electrical energy.electrical energy.

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.

Why wind energy?

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

Wind mill operation The turbine rotor converts the fluctuating

wind energy into mechanical energy which is converted into electrical power through generator and then transferred into grid through transformer & transmission lines.

Main components of Wind Turbine system.

Blade & Rotor (Wind Energy)

Gear Box (Optional)

GeneratorPower Converter

Power Transformer

Supply grid transmission lines

ROLE OF MAIN COMPONENTS

• wind turbine capture the power from the wind by means of aerodynamically designed blades & converts it to rotating mechanical power.

• The gear box adapts low speed of the turbine rotor to the high speed 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.

The connection of wind turbine to the grid is possible at LV,

MV, HV, EHV system. • For modern wind turbine system, each turbine has

its own transformer to raise voltage from the voltage level of wing turbine (400 or 690 V) to the medium voltage (11kV or 22kV or 33kV).

• The transformer is normally located close to the wind turbine to avoid LV cables.

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

power characteristics of fixed speed wind turbine & controllers employed.

• stall control(fixed pitch controller)At given wind speed ,the wind turbine rotor starts to stall from the blade root and develop gradually across the whole blade length as WS increases.

• Active stall control – When wind speed < rated wind speed optimize the power output by way of lookup table. When wind speed > rated wind speed, 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.

Pitch control

a

b

c

5 10 15 20 25 30

Wind Speed (m/sec)

Power

1

0.75

0.5

0.25

Variable speed wind turbine system

• Variable speed wind turbine system use power electronics system as interfaces.

• Since the wind turbine operates at variable rotational speed , the electric frequency of the generator varies & must therefore be decoupled from the frequency of the grid.

Application of Power Electronics

• Power electronics has changed rapidly during the last 30 years & the no. of applications has been increasing mainly due to the development of semiconductor devices & microprocessor technology.

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

Role of PEC• PEC may allow both the direction of

power flow & can interface b/w the load/generator and grid.

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

• Grid commutated are mainly thyristor converters, 6 or 12 or even more pulse. These produce harmonics so, harmonic filter is required. Also these are not able to control VAR & consume inductive VAR

Role of PEC

• Self commutated are mainly PWM converters, where IGBT are mainly used. These can control both active & reactive power. That means the reactive power demand can be delivered by a PWM converter. The high frequency switching PWM converter may produce harmonics . These harmonics are in the range of some kHZ, so small size filter is required.

Circuit diagram of VSC with IGBTS

Wave forms of bidirectional active & reactive power of a

VSC

Contd…

Generators for wind turbines

• Both Induction & synchronous generators can be used for wind turbine systems.

• Inductive Generator used in a fixed speed (or) a variable speed

• Synchronous generator normally used in PE interfaced variable speed system.

Induction Generator used in wind turbine system

There are 3 types:– Cage rotor

– Wound rotor with slip control by changing rotor resistance

– Doubly fed Induction Generator

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(Danish concept)

Contd…• The scheme consists of a cage rotor IG

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

The power can be limited aero dynamically by stall control, active stall control or by pitch control.

Advantage: cheap, reliable (no synchronization device required)

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

.

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

• Experience mechanical construction to absorb high mechanical stress since wind gust may cause torque pulsations in the drive train & gearbox.

• VAR compensator to reduce (almost) the reactive power demand from the IG to the grid.

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

• High starting current which require the soft starter (current limiter) implemented by thyristor contoller. The ss has a limited thermal capacity & it is short circuited by a contactor which carries the I FL when the connection to the grid has been completed. In addition it reduces the load on the gear box.

Wind turbine without power converter, but with aerodynamic

power system control.• i) Pitch controlled – The blades are turned out of the wind at

higher wind speed.

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

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

Gear Box

Reactive Compensator

IG GridTxr

Fixed speed Wind Turbine System

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 rotor windings , a full rated PE system is connected between the stator of the generator & the grid, where the total power production must be fed through PE system.

Contd…

b) For IGs with rotor windings, the stator of the generator is connected to the grid directly.Only the rotor of the generator is connected through PE system.This gives the advantage that only a part of power production is fed through PE converter.This means the nominal power of the converter system can be less than (normally 30%) the nominal power of the wind turbine. By controlling the active power of the converter, it is possible to vary the rotational speed of the generator & thus of the rotor of wind turbine.

Variable speed WT with partially rated power

converters Dynamic Slip controlled wound rotor IG

(system IV)Wound rotor IG

Gear Box

Reactive Compensator

Grid

Resistance control with PE

Pitch Control

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

be controlled by PE.

• The variation of rotor resistance produces a group of torque speed characteristics, known as dynamic slip control and gives typically a speed range of 2-5%.

• At the same time an extra control freedom is obtained at higher wind speeds in order to keep the output power fixed.

• This system still needs a soft starter & reactive power compensation.

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

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

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

• A speed variation of +30% around syn.speed can be obtained by the 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 power source to the grid .

• Disadvantage: Bit more expensive compared to the classified system.

Pitch Control

Gear Box

AC

DC

PrefQref

DC

AC

Variable speed wind turbine with full 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 achieve full control of the active & reactive power.

b)Synchronous generator with gearbox & full rated PEC System

(System VII)

Gear Box

AC

DC

PrefQref

DC

AC

Pitch

Syn.

Gen

DC

AC Grid

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

c)Multipole synchronous generator w/o gearbox(System VIII) & full scale

PEC

AC

DC

Pref Qref

DC

AC

PitchSyn.

Gen

DC

AC Grid

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

AC

DC

Pref

Qref

DC

AC

Pitch

PMSM Multipole

Grid

• All 4 systems have the same controllable characteristics since the generator is decoupled from the grid by a dc link.

• Advantage: System is fast in controlling active & reactive power.

• Disadvantage: System is more complex with more sensitive electronic parts.

Summary of WT generator system

• Table 3.1 presents a technical comparison of the above system. Each system has benefits & drawbacks.

Conclusion:1) Fixed speed WT is slightly cheaper simple

since the motor speed cannot be varied, these turbines must be more robust than other designs due the high structural loads involved.

Contd…

2)Variable speed WT generator more energy for a given wind speed ,especially at low wind speed. Moreover the active and reactive power can be easily controlled & there is less mechanical stress.

3)Direct drivers (gear less) are expensive because of the large & relatively heavy generator & power converter of full rated power.

Integration of wind turbines into power system

requirements: a) Frequency & active power control

• Frequency is 50Hz or 60Hz(worldwide). Frequency proportional to the rotating speed of synchronous generator operating in the system.

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

• Increasing electrical load in the system tends to slowdown generators & reduce the frequency.

• The task of frequency control of the system is to increase or reduce the generated power.

b) short circuit power level & voltage variations

• Ability of the grid to absorb disturbances is directly 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 power flow causes system voltage drop & also power losses.

• The IG based wind turbines are the consumer of reactive power.

• To minimize the powers losses & to increase

voltage stability.

• These wind turbines are compensated by VAR compensation panel to a level depending on the requirements of load utility.

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

power generation may cause voltage quality problem.

• Fluctuations in the system voltage (rms value) may cause perceptible light flicker depending on the magnitude & frequency of the fluctuation.

• The allowable flicker limits are generally established by individual utility.

• Rapid variations in the power output from wind turbine, such as generators switching & capacitor switching can also result in variations in the rms values of the voltage.

E) Harmonics

• Harmonics are a phenomenon associated with the distortion of the voltage & current waveform.

• All the harmonics cause increased currents & possible destructive over heating in capacitors as the impedance of capacitors goes down in proposition to the increase in frequency.

F) Stability

• The problem of network stability is often associated with different types of faults in the network such as tripping of transmission lines (eg. Overload), loss of production capacity and short circuit.

Questions ?

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