Wind Power Generation

Power electronics and wind power

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Abstract

The global demand for electricity is

still on the rise hence there is an increase in

the demand for electricity. This fact has made

it necessary that production, distribution and

the consumption of the energy be as

technological efficient as possible. The

deregulation of the energy sector has led to

the lowering of the investments in the

existing power plants hence causing

investment into future power plants to be

relatively expensive. This has made the

following production technologies to be

marked as turning points in the solving of the

problems bedeviling the sector i.e. power

production from conventional fossil fuels and

the generations from renewable energy

resources. The other solution to this problems

will be to use high efficient power electronics

in the power systems. This paper will explore

the energy power generation mechanism

which is wind power and through the

application of power electronics it is

becoming a force to reckon with in electricity

generation. Wind power is now deriving

utility from power electronics.

Introduction

Typical power systems are usually

composed of the major generating plants that

are located in remote geographical areas

where a significant amount of the power

generated is transferred to the consumption

areas using transmission lines. The power is

monitored so as to ensure the quality of the

transferred electricity. The proliferation and

the emergence of the dispersed generation

units (DG) will increase the management of

the power distribution system [1].

The wind turbine technology is a

relatively new technology, and it started with

a few kW in power production. Therefore the

initial production did not impact the power

system control greatly. Current with the

increase in the amount of power being

produced and fed into the grid problems

arise. The initial generation relied on the

squirrel-cage induction generator that was

fed directly to the grid [1]. This means the

pulsation in the power that is associated with

the wind power was transferred to the grid.

This meant the parameters that influence the

frequency of the transmitted power (active

and reactive power) were ni controlled in any

way. But with the increase in the size of the

power generated there is the need for this

regulation hence power electronics have been

applied in the changing of the essential

characteristics of the wind turbines.

The power electronic converters form

the interface between the load (generator)

and the electrical grid. Power, in this case,

can flow in either direction, but this is

dependent on the topology. The main factors

to consider when using these devices is the

reliability, the cost, and the efficiency.

Figure 1: Power electronic system as applied

to the grid

System design

Wind power generation utilizes wind

turbines that harness the power from wind

through aerodynamically modeled blades and

then turn it into rotary mechanical power.

The blades are usually three in number [2].

Multi-MW wind turbines the have a

rotational speed of 10-15 rpm. So as to

convert this low-speed, high torque power

efficiently to electrical power, this turbines

utilize gear-box. A power converter can be

inserted between the generator and the grid

system. Several turbine configurations exist.

Figure 2: Conversion of the wind power to

electrical energy using wind turbine.

Fixed speed wind turbine configuration

The control and the limitation of the

mechanical energy converted at higher wind

speed are important as the power of the wind

is three times its speed [2]. The limitation of

this power can be done through stall control

this when the blade position is fixed, but the

stall of the wind will appear along the blade

when the wind is at a higher speed; active

stall is when the blade angle is adjusted so

as to create stall along the blades; pitch

control the blade, in this case, will be turned

out at higher wind speeds. Below are the

different topologies

Figure 3: Pitch controlled system

Figure 4: Stall controlled system.

Figure 5: Active stall controlled system

Figure 6: A cage-induction-generator which

is an example of a fixed speed wind turbine

with power electronic soft-starter.

This three system require a reactive

power compensator to reduce the demand for

reactive power by the generator to the grid.

Wind gusts can still cause torque pulsation.

Variable speed wind turbine

configuration

These are wind turbines with partially

rated power converters and hence have a

performance control system. These turbines

have power converters hence they can

monitor their speed even when the wind

speed and torque vary. They are

advantageous as they reduce wear and tear on

the tower and the gearbox. They can also

increase the electric power generated and at

the same time minimize the fluctuation of the

power injected to the grid. This system has

the generator connected to the grid via a

power electronic system [3].

In case there are a synchronous

generators and induction generators that do

not have rotor winding a complete power

system that is full rated will be attached to the

stator of the generator and the electrical

power grid. For this instance, the whole

power produced will be transferred to the

power grid via the power electronic system.

In the the case of generators with the rotor

winding their stator is usually connected to

the grid directly [3].

Figure 7: a wound rotor generator with rotor

resistance.

Maximum Wind power control

It is evident from the previous

discussion the variable-speed wind turbine

can function at the optimum rotation speed in

relation to the wind speed [3]. The power

electronic converters can be used to control

the rotation of the turbines so that you can

obtain the maximum attainable power. This

can be achieved through the maximum power

point tracking (MPPT) algorithm. This

practice can be used to avoid surpassing the

nominal power when the wind speed increase

above the rated [4]. This also ensures that the

DC-link capacitor is kept at constant so as to

achieve the decoupling between the grid-side

converter and the turbine-side converter. In

this instance, the grid-connected converter

will function as an inverter so as to generate

a PWM voltage that will be the fundamental

component in the grid frequency while

supplying the active nominal to the grid. The

graph below illustrates the relationship

between the wind speed and the power

generated

Figure 8: output power of the turbine in

relation to the wind speed

Converter control

This is a vector current controller that

is utilized in the determination of the active

and reactive power references. The reference

will result from the voltage and frequency

droop control. The converter controls the

voltage and the frequency dependong on the

instanteneous measurement obtained from

the convereter terminal. The voltage can be

controlled through the sinus modulation. The

method involves the comparing of the

triangular carrier wave with the three voltage

references which can be marked as ua, ub,

and uc. This will cause the logic signals that

definie the switching state of the power

transistor to be generated. The main reference

of the control is a sine wave. The limit is

arrived at when the reference signal of the

carrier wave is m = 1. For the 2-level

converter there are six active and zero vectors

for the switching states. the voltage vectors

are given by √2/3 Udc. The converters output

voltage will always be within the hexagon.

Figure 9: Voltage vectors for the 3-phase 2-

level converter.

Conclusion

This paper has been instrumental in

the review of the application of the power

electronics converters in the field of wind

power generation. The paper explores the

wind power generation system before and

after the application of the power system

converters. This paper illustrates the

capabilities of wind farms when they apply

the power electronic converters. We have

also illustrated that ability of the electronic

converters improving the power system so

that they can meet the grid connection

requirements while allowing for

controllability of the power components.

References

[1] B. F, C. Z, T. R and I. F, "Power

Electronics in Wind Turbine Systems,"

International Power Electronics and Motion

Control Conference, vol. 5, no. 06, pp. 1-11,

2006.

[2] I. Florin and B. Frede, "Power

Electronics Control of Wind Energy in

Distributed Power Systems," T J Hammons,

Aalborg, 2009.

[3] Y. Murthy.N, "A Review on Power

Electronics Application on Wind Turbines,"

International Journal of Research in

Engineering and Technology, vol. 02, no. 11,

pp. 360-376, 2013.

[4] B. F, M. K and Z. D, "Power Electronics

and reliability in renewable energy systems,"

Aalborg University, Aalborg, 2011.

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