wind energy conversion systems

7/28/2019 Wind Energy Conversion Systems

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Wind Energy Conversion Systems (WECS)

WECS convert wind energy into electrical energy: Wind energy ->mechanicalrotational energy -> electrical energy. The principal component of the WECS is the wind

turbine (WT). WT rotor is coupled to the generator throught a multiple-ratio gearbox or,

gearless in small power applications. Usualy induction generators, (squirrel-cage (SCIG)or doubly-fed (DFIG)), or permanent magnet synchrounous generators (PMSG) are used

in WECS.

WT main components

A wind turbine has three major components: the tower, the rotor and nacelle.

Generally, the rotor may have two or three blades.

For MW-range wind turbines, the rotational speed is typically 10-15 rpm,increasing with the power, up to approx. 400rpm for kW range. The usual wind speed

domain is 4 to 14 m/s corresponding to the minimum amd the rated output power. Theyusually operate up to a wind speed of 25 m/s, after that stopping the operation.

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WECS components

The main parts of a WECS include:- Wind power rotor (two, three-blades)

- Gearbox (optional – gearless)

- Generator (SCIG, DFIG, WRIG, PMSG)- Power converter (optional)

- Power transformer (optional)

All the active components are placed in the nacelle

Capacity factor

The capacity factor (CF) of a wind turbine expresses the ratio of the average

power output to rated power and it strongly depends of the site characteristics, varying

between 15 % (low wind speed locations) to 40 % (high wind speed locations). Theglobal average is around 20%. The power plans have a capacity factor that is not an

indicator of the power plant efficiency. The capacity factor of a WT is mainly determind by: operating at less than maximum output, shut down due to excessive or inadequatewind velocity or other shut downs.

Basic of wind energy conversion

Wind turbines extract the energy from the wind by transferring the thrusting force

of the air passing through the turbine rotor into the rotor blades. The rotor blades are

airfoils that act similary to an aircraft wing; this is the so-called principle lift. As an effectof the resulting air flow, the differential pressure creates a thrust force. The lifting force is

perpendicular to the direction of the resulting force (resulting wing speed). As s result,

the lifting force is converted into a mechanical torque.

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Power curve of WT

The output power of a wind turbine is determined by several factors such as windvelocity, size and shape of the turbine. The mechanical power developed by the WT rotor

is given by:

AV Cp P w ⋅⋅⋅= 3

21 ρ

Where:

- P – is the WT output mechanical power

- Cp – is the performance coefficient

- ρ – is the air density (kg/m3)

- wV – is the wind speed (m/s)

- A – is the blades’ swept area (m2)

To achieve maximum power at different wind speeds the rotor rotational speed

has to be modified. From this reason, the WT are mainly divided in fixed-speed andvariable-speed. A maximum power point tracking (MPPT) mechanism characterizes the

operation of variable-speed WTs.

Fixed-speed and variable-speed WT

The extracted mechanical power is higher for variable-speed configuration at allwind speeds. Variable-speed wind turbine yield greater annual power production

compared with similar fixed-speed wind turbines This improvement in efficiency is

obtain at the cost of greater complexity in the construction of the unit and someadditional losses in the power electronic converters.

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WT concepts

The most commonly wind turbine designs can be categorized into four concepts:1. Fixed Speed WT (Type A)

2. Partial Variable Speed WT with Variable Rotor Resistance (Type B)

3. Variable Speed WT with partial-scale frequency converter (Type C)4. Variable Speed WT with Full-scale Power Converter (Type D)

The main differences between these concepts concern the generating systems andthe way in witch the aerodynamics efficiency of the rotor is limited during the above the

rated value in order to prevent overloading.

1. Fixed Speed Wind Turbines (Type A)

This configuration is so called Danish concept that was very popular in 80’s. This

concept needs a reactive power compensator to reduce (almost eliminate) the reactive

power demand from the turbine generators to the grid. Smoother grid connection occurs by incorporating a soft-starter, besed on tyristors. In a fixed speed wind turbine, the wind

fluctuations are converted into mechanical fluctuations and further into electrialpower fluctuations. The main drawbacks are: does not support any speed control, requires a stiff

grid and its mechanicals construction must be able to support high mechanical stress

caused by wind gusts.

2. Partial Variable Speed Wind Turbine with Variable Rotor Resistance (Type

B)

This configuration corresponds to the limited variable speed controlled wind

turbine with rotor resistance. In order to avoid the problems of introducing slip rings,Vestas designed a so called OptiSlip method.

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The rotor winding is connected in series with a controlled resistance, whose size

defines the range of the variable speed (typically 0-10% above synchronous speed). The

energy coming from the external power conversion unit is dumped as heat loss, this beiga major disadvantage.

3. Variable Speed WT with partial-scale frequency converter (Type C)

This configuration, known as doubly-fed induction generator (DFIG) concept,

corresponds to the variable dpeed controlled wind turbine wind with a wound rotor induction generator (WRIG) and partial-scale frequency converter (appromax. ± 30% of

nominal generator power). Speed range: typically ± 30% around synchronous speed. The

converter performs the reactive power compensation and smooth grid connection. The

smaller frequency converter makes this concept attractive from an economical point of view. Its main drawbacks are the use of slip-rings and the protection schemes in the case

of grid faults.

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4. Variable Speed Wind Turbine with Full-scale Power Converter (Type D)

This configuration corresponds to the full variable speed controlled wind turbine,with the generator connected to the grid through a full-scale frequency converter. The

frequency converter converter performs the reactive power compensation and a smooth

grod connection for the entire speed range. The generator can be SCIG or PMSG. Somevariable speed wind turbines systems are gearless. In these cases, a direct driven multi-

pole generator is used.

Type D WT with PMSG

For small-scale applications, PMSG is mostly used. A major cost benefit in using

PMSG is that no external excitation source is required.

Power limitation

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It is important to be able to control and limit the converted mechanical power at

high wind speeds.

The power limitation may be done by:- stall control (the blades position is fixed but stall of the wind appears along the

blades at higher wind speed) – turbulent wind flow

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- active stall (the blades angle is adjusted in order to create stall along the blades)

- pitch control (the blades are turned out of the wind at higher wind speed) – active

power reduction

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Turning the rotor into the wind (Yawind)

For maximum power extraction from the wind, the rotor has to be aligned withthe wind stream direction. Turning the rotor into the wind is called yawing. WT up to

10m diameter may be yawed into the wind passively by using tail vanes. For larger wind

turbines this method is no longer feasible as tail vane wound to be to large. Instead,electronic or hydraulic motors are used to turn the rotor (nacelle) into the wind – active

yaw. These are called yaw drives.

A wind vane whose are attached on the back of the nacelle is used to check thewind direction and the WT controller acts the yaw mechanism.

Turning the rotor out of the wind (Furling)

After a certain wind velocity, (about 25m/s) the wind turbine is turned off. Small

wind turbine (kW range) can still operate at maximum power, up to 40m/s wind speed,

but require some mechanical control systems to reduce their output power and rotational

speed. Changing the angle of the oncoming air stream by turning the nacelle out of thewind is known as furling. kW range WT use passive furling methods to turn blades out of

the wind either horizontally or vertically. They use spring-based mechanisms that ay acertain wind speed triggers and deviate the rotor. Large WT use complex mechanisms to

shut down in storm condition using the yaw drive and breakes.

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wind energy conversion systems

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