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Sanjeev Kumar Singh

M-Tech (EPES)

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Three types of typical wind generator system for large wind

turbines exist.

The first type is a fixed speed wind turbine system using a

multistage gearbox and a standard squirrel cage inductiongenerator(SCIG).

The second type is a variable speed wind turbine system with a

multistage gearbox and a doubly fed induction generator (DFIG)

.In this case the power electronics converter feeding the rotor

winding has a power rating of 30% of the generator capacity;

stator winding of the DFIG is directly connected to grid.

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The third type is also variable speed wind turbine but it is a

gearless wind turbine system with a direct drive generator,

normally a low speed high torque synchronous generator and a

full scale power electronics converter are used. Additionally a variety innovative concept of wind turbine appear

for ex., a smaller low speed permanent magnet synchronous

generator with a gearbox

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Referring to the rotational speed wind turbine concept can be

classified into fixed speed, limited variable speed and variable

speed

For variable speed wind turbine based on the rating of the powerconverter related to the generator capacity , they can be further

classified in to wind generator system with a partial scale and

full scale power converter.

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In addition considering the drive train component , the wind

turbine concept can be classified in to gear drive or direct drive

wind turbine.

In gear drive wind turbines, one conventional configuration is amultistage gear with a high speed generator; the other one is

multibridge concept which has a single stage gear and a low

speed generator .

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The squirrel cage induction generator operate only in a narrow

range around the synchronous speed ,the wind turbine equipped

with this type of generator is often called the fixed speed wind

generator system . Since the SCIG always draws reactive power from the grid .this

concept was extended with a capacitor bank for reactive power

compensation .

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The well known advantages of SCIG are it is robust easy and

relatively cheap for mass production . In addition it enables stallregulated machines to operate at a constant speed when it is

connected to a large grid which provide a stable control

frequency

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The speed is not controllable and variable over a vary narrow

range in which only speed higher then the synchronous speed are

possible for generator operation because a higher slip means a

higher dissipation of electrical energy in the rotor bars, forexample the slip is normally not higher then 1% for one MW

wind turbine. Additionally the fixed speed concept means that

wind sped fluctuation are directly translated in to

electromechanical torque variation this cause high mechanicaland fatigue stress on the system and may result in swing

oscillation.

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A three stage gearbox in the drive train is necessary for the wind

turbine concept. Gearbox represent a large mess in the nacelle

and also a large fraction of the investment cost .

Starting is necessary to obtain the excitation current from thestator terminal of SCIG this makes it impossible to support the

grid voltage control.

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This wind turbine concept uses a wound rotor induction

generator (WRIG) with variable rotor resistance by means of a

power electronics converter and the pitch control method.

The stator of WRIG is directly connected to the grid where as therotor winding is connected in series with a controlled resister .

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Variable speed operation can be achieved by controlling the

energy extracted from the WRIG rotor ;however this power must

be dissipated in the external resister .With the increase in the

variable speed range a higher slip means a high power extractedby the rotor and the lower generator efficiency , so that the

rating of the resistor must be also higher therefore the dynamic

speed control range depend on the size of the variable rotor

resistance and the energy extracted from the external resistanceis also damped as heat loss in the controllable rotor resistance

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In this configuration (DIFG)the stator is directly connected to

the grid where as the rotor is connected through a power

electronics converter .the power converter control the rotor

frequency and thus the rotor speed .this concept support the wide

speed range operation depending on the size of the frequency

converter .typically the variable speed range is +-30% around

the synchronous speed .the rating of the power electronics

converter is only 25-30%of the generator capacity .which makes

this concept effective an popular from the economics point ofview .the largest capacity for the commercial wind turbine with

DIFG has been up to 5 MW from repower

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Since this case the rotor energy ,instead of being dissipated can

be fed in to grid by the power electronics converter. More ever

the power converter system can perform reactive power

consumption in smooth grid connection , for example ,the gridside converter can control its reactance power in dependently of

the generator operation ;this allow the performance of voltage

support towards the grid

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A multistage gearbox is still necessary in the drive train because

the speed range for DIFG is far from a common turbine of 10-

25 rpm.

The slip ring is use to transfer the rotor power by means of apartial scale converter , which requires a regular maintenance

and may be results in machine failure and electrical losses

Under the grid fault condition ,on the one hand , large stator

current result in large rotor currents so that the power

electronics converter needs to be protected from destroy

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The difference between gear drive wind turbines and direct drive

type is the generator rotor speed .the direct drive generator

rotates at low speed ,because the generator rotor is directly

connected to the hub of the turbines rotor . To deliver a constant power ,the low speed makes it necessary to

produce a higher torque

A higher torque means a large size of the generator.

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Therefore for direct drive generator , the low speed and high

torque operation require multi poles ,which demand a larger

diameter for implementation of large number of poles with a

reasonable pitch .

Direct drive generator can be classified as-:

electrically excited synchronous generator

permanent magnet synchronous generator

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The EESG is usually built with a rotor carrying the field system

provided with the dc excitation .the stator carries three phase

winding quit similar to that of the induction machine .the rotor

may have salient poles or may be cylindrical (low speed ). The amplitude and frequency of the voltage can be fully

controlled by the power electronics at the generator side ,so that

the generator speed can be fully controlled over a wide range

even to vary low speed .

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Higher efficiency.

No additional supply for the field excitation .

Lighter and therefore higher power to weight ratio.

Implement in thermal characteristics of the PM machine drive to

the absence of field losses.

Higher reliability du to absence of mechanical component such

slip rings.

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High cost of PM material .

Difficulties to handles in manufacture .

Demagnetization of PM at high temp.

Types Radial flux PM machine .

Axial flux PM machine.

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