wind turbine systemsgrid converter structures and topologiesnd turbine system.pdf

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Wind Turbine Systems

Grid Converter Structures and Topologies

Mandal Nitu, Maharjan Mamta, Shrestha Tusuju NamrataNorwegian University of Science and Technology, Trondheim, Norway

E-mail: [email protected], [email protected], [email protected]

Abstract- The utilization of wind energy is the area which isgrowing rapidly. The increasing share of wind power in theelectric power system makes it necessary to have grid friendlyinterfaces between the wind turbine s ystem and the grid in order

to maintain power quality. This paper discusses the basic

working principle of power electronic components on grid side.A short overview on the interconnection requirements is given.Different topologies for power converters in a wind turbinesystem are described. Finally, a general technology status of the

wind power system is presented demonstrating more efficientand attractive power generation system.

I. INTRODUCTIONThe demand for the electrical power production is increasingglobally. A large number of distributed generation (DG)

units, including both renewable and non-renewable sources

such as wind turbines, wave generators, photovoltaic (PV)generators, small hydro, and fuel cells are utilized for powerproduction. The production, distribution and the use of theenergy should be technically efficient. Wind turbinetechnology is one of the most emerging renewable

technologies. Wind power production in the beginning didnot have any impact on the power system control but now dueto their increasing size they have to play an active part in thegrid.

On the other hand, the increased use of power electronics,

especially on the grid side, in connection with the control ofthe pitch angle of the blades can partially relieve the problem

of power control, allowing the wind turbine power plant tobehave similarly to a conventional power plant. In this sense,it should be noticed that the introduction of power convertersin a variable-speed wind turbine has been mainly associated

with the possibility of controlling the generator and as aconsequence of controlling the active power in order tomaximize the power extraction. Then the active powercontrol has been viewed as a means to exercise the windturbine system in a similar way to a traditional power plant.However, it is the use of a grid converter that gives to themodern wind turbine system (WTS) the capability of

managing the reactive power exchange and allowing itsparticipation in the voltage regulation. [1]

Fig. 1. Power electronic system with grid, renewable sources, powerconverter and control.[1]

II. WINDTURBINESYSTEM:The basic power configuration of a wind turbine system is

made of two parts: a mechanical part and an electrical asshown in Figure 2. The first sub-system extracts the energyfrom the wind and makes the kinetic energy of the windavailable to a rotating shaft; the second sub-system isresponsible for the transformation of the electrical energy,

making it suitable for the electric grid. The two subsystemsare connected via the electric generator, which is anelectromechanical system and hence transforms themechanical energy into electrical energy.

Fig. 2.Basic power conversion wind turbine system [2]

Wind turbines capture the power from the wind by means of

aerodynamically designed blade and convert it to rotatingmechanical power. The most weight efficient way to convertthe low-speed, high-torque power to electrical power is using


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a gear-box but it is optional. Power converter is insertedbetween the grid and the generator.

Fig. 3. Scenario of the power conversion structures for variablespeed wind turbine systems[1]

The flowchart shown in the Fig.3 indicates how the electricalenergy is efficiently generated from wind energy. Amongthese entire stages grid side converter and its varioustopologies are elaborated here. In order to change themechanical power obtained from wind to electrical power,

two types of generators can be used namely synchronousgenerator and induction generator. Apart from these twotypes, the third kind of generator is the most popular in windsector. It is called Permanent Magnet Generator (PMG). ButPMG is synchronous generator with rotor windings replacedby permanent magnets. It needs no separate excitation so

rotor excitation losses about 30% of total generator losses

are eliminated. This results in high power density and smallsize with the highest efficiency at all speeds, offering themaximum annual production of energy with the lowestlifetime cost. [1]

Fig. 4. Permanent magnet generator

III. GRIDCONNECTION REQUIREMENTFORWINDTURBINE:

1. Active power control:Active power control is required in order to limitoverproduction of wind power that can lead to instabilities

due to island conditions. New wind turbine technologies alsoallow its participation in frequency regulation. Frequency inthe power system is an indicator of the balance betweenproduction and consumption of active power. For normalpower system operation, frequency is stable and close to its

nominal value.2. Frequency control:In systems with relatively high wind penetration, there isoften a requirement for frequency response or frequencycontrol. This can take many forms, but the basic principle is

that, when instructed, the wind farm reduces its output powerby a few percent, and then adjusts it in response to the systemfrequency. By increasing power when frequency is low, ordecreasing power when frequency is high, the wind farm cancontribute to controlling the system frequency.The active power is typically controlled based on the system

frequency so that the power delivered to the grid is decreasedwhen the grid frequency rise above 50 Hz.

3. Reactive power control and voltage stability:

Reactive power production and consumption by generatorsallows the network operator to control voltages throughouttheir system. The control of reactive power at the generators

is used in order to keep the voltage within the required limitsand avoid voltage stability problems. Wind generation shouldalso contribute to voltage regulation in the system; therequirements either concern a certain voltage range thatshould be maintained at the point of connection or certainreactive power compensation that should be provided.

IV. WTSCONTROL:The two subsystems (electrical and mechanical) are

characterized by different control goals but interact in view ofthe main aim: the control of the power injected into the grid.The electrical control is in charge of the interconnection withthe grid and active/reactive power control. The mechanical

subsystem is responsible for the power limitation (with pitchadjustment), maximum energy capture, speed limitation andreduction of the acoustical noise. But they are controlledindependently. Here the focus is on the electrical control. Forelectrical control we use power electronics converter. Thereare two types of converter mainly generator side converter

and the grid side converter. The control of the generator-sideconverter is in charge of extracting the maximum power fromthe wind. The control of the grid-side converter is simply just

keeping the DC link voltage fixed. [1]

Fig. 5. Wind turbine control structure.[1]


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V. GRIDCONVERTER TOPOLOGY:Grid power converter needs to perform AC/AC conversion.The AC/AC conversion can be direct or indirect. In theindirect case there is a DC link that connects two converters

performing AC/DC and DC/AC conversions, while in thedirect case the DC link is not present.

The grid converter topologies can be classified into two types-voltage-stiff (voltage-fed or voltage-source) and current-stiff(current-fed or current-source) respectively, indicated with

the acronyms VSC and CSC.

Grid Converter Structures for Wind Turbine Systemsdepending on the main power flow direction they are namedrectifiers or inverters, or in case they can work with bothpower flows they are bidirectional. Then they can be

classified as phase-controlled (typically using thyristors andnatural commutation synchronized with the grid voltage) orPWM using forced commutated devices. Grid converters for

distributed power generation need to work as inverters. Hereindirect conversion using VSC topologies will be discussedfurthermore [1].

In the case of the VSC a relatively large capacitor feeds themain converter circuit. Switches are used in the main circuit,each composed traditionally of a power transistor and a free-wheeling diode to provide bidirectional current flow and aunidirectional voltage blocking capability. The VSC needs

both AC and DC passive elements.

An AC output voltage cannot exceed the DC voltage.Therefore, the VSC is a buck (stepdown) inverter for DC/AC

power conversion and is a boost (step-up) rectifier (or boostconverter) for AC/DC power conversion. In case the available

DC voltage is limited/excited, an additional DC/DCboost/buck converter is needed to obtain the proper DCvoltage that allows the VSC to operate properly with the grid.Focusing on the topologies of the grid converter, below twotypes of topologies namely two level converter and Neutralpoint clamped (NPC) three level converters are discussed in

detail.

A. Two level back to back grid converter:

Fig.6. Two-level back-to-back PWM-VSI[3]

Fig. 7. Output voltage waveform of two level inverter[4]A basic three-phase inverter consists of three single-phase

inverter switches each connected to one of the three loadterminals. For the most basic control scheme, the operation ofthe three switches is coordinated so that one switch operatesat each 60 degree point of the fundamental output waveform.This creates a line-to-line output waveform that has six steps.

The six-step waveform has a zero-voltage step between thepositive and negative sections of the square-wave such thatthe harmonics that are multiples of three are eliminated [3].

Three level back to back PWM VSI

Fig. 7. Three-level back-to-back PWM VSI[1]

Fig. 8. output voltage waveform of 3 level inverter[4]


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In three level ac conveter each leg has four switches ( mostlyIGBT) connected in series. In NPC(neutral point clamped)inverter, The applied voltage on the IGBT is one-half that ofthe conventional two level inverter. The bus voltage is split intwo by the connection of equal series connected bus

capacitors. Each leg is completed by the addition of twoclamp diodes.

The NPC inverter can produce three voltage levels on theoutput: the DC bus plus voltage, zero voltage and DC bus

negative voltage. The two level inverter can only connect theoutput to either the plus bus or the negative bus.

The three level inverter offers several advantages over themore common two level inverter. As compared to two level

inverters, three level inverters have smaller output voltagesteps and output waveform provides an effective switchingfrequency twice that of the actual switching frequency. Andthe most important advantage is the output from three levelinverter has less harmonic components.

In economic point of view three level inverters are less costlybecause the components will be smaller and less costly thanfor an equivalent rated two level inverter. Most often the NPCinverter is used for higher voltage inverters. Because theIGBTs are only subjected to half of the bus voltage, lowervoltage IGBT modules can be used.[4]

According to the fig 5, the DC link voltage is made almostconstant within the given range. There is reference voltagegiven which is compared with the output voltage of therectifier. And according to the need, Pulse width modulation

can be used to switch the chopper in order to step up or stepdown the rectifier output voltage to maintain the voltage ofthe DC link nearly constant. With the assistance of chopperthe DC link would not rise above or fall below the certainrange.

Pulse width modulation is also used in 3 phase inverter togenerate sinusoidal voltage. For removing higher frequencyharmonics which may result in loss, Low pass filter is used

before connecting to the gird. There is feedback control inorder to continuously maintain the amplitude and phase of

sinusoidal voltage.

Converter structures employ a grid converter, which in mostcases is a VSC. Basically the VSC controls the active andreactive power transfers acting on the amplitude and phase ofthe produced voltage.

Fig. 9. Different power transfers achieved by the grid converter in

the different operating conditions (VL is the voltage drop across thegrid filter)[1]

The power transfer between two sections of a short line can

be studied using complex phasors, for a mainly X >> R,showing that the voltage drop VL is perpendicular to theexchanged current.[1]

In the figure 9, E, Ig refers to the grid voltage and currentrespectively, VL represents the grid converter loss and V

represents the voltage output of VSC.

In Figure 9(a) the case is reported when there is no power

produced by the WTS and a small power is absorbed to keepthe DC link voltage at its rated value. The VSC is working asa rectifier and the absorbed active power compensates the

losses in the overall converter. Figure 9(b) the case isreported when the WTS injects only active power, while inFigure 9(c) and (d)the cases in which the grid converter isworking as a STATCOM (STATIC SYNCHRONOUSCOMPENSATOR). A STATCOM is an electronic generatorof reactive power. A STATCOM is a shunt-connected

reactive power compensation device that is capable ofgenerating and/or absorbing reactive power.

Figure 9(e) and (f) reports the working conditions in which

the WTS injects both active and reactive power.If also the power angle is small, then sin is nearly equal to

and cos nearly equal 1:

P

Q()

where E, P, Q denote respectively the voltage, the activepower and the reactive power of the grid and V is the voltageoutput of the VSC, above equations show that the active


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power injection depends predominantly on the power angle,whereas the reactive power injection depends onthe voltage difference E V.[1]

Hence active power can be used to regulate the angle or the

frequency of the grid voltage, whereas the reactive power canbe used to control the amplitude of the grid voltage. Thus byadjusting the active power and the reactive power, frequency,

and amplitude of the grid voltage can be influenced.Moreover VSC controls the active and reactive powertransfers acting on the amplitude and phase of the produced

voltage V.

So now the challenge is to control the amplitude and phase ofthe produced voltage V in order to inject the required activeand reactive demand of the grid.

The waveforms shown in figure 10 can be used to explainhow to control Van in magnitude as well as in phase with agiven (fixed) dc voltage Vd. It is obvious that by controllingthe amplitude of the sinusoidal reference waveform Vcontrol a,Van can be adjusted. Similarly by shifting the phase ofVcontrol a with respect to Ea the phase angle of Van can be

varied. For balanced operation, the control voltage for phase

B and C are equal in magnitude but 120 degree out of phasewith respect to phase A.[3]

Fig. 10. Output waveform of two level back to back converter[3]

VI. EXISTING SYSTEM IN LARGE WTS:The topology for wind turbine generator connected to the grid(shown in Figure 5) involves lots of power electronicconverter and hence it is expenssive topology. The convertersare mainly introduced to acts as an interface between grid and

wind power so that variable voltage and frequency outputfrom wind generating system can be connected to the grid ofconstant voltage and frequency. With proper system design,wind turbine driven induction generator can be directlyconnected to the grid as shown in Figure 11.

Fig. 11. Commonly Practice WTS structure.

This scheme has following advantages:- Induction generator is cheaper than Permanent

magnet generator- Easy to Synchronize with grid- IG is more robust generator

In this scheme, STATCOM is a current controlled PMWSTATCOM. The reference current is evalaued by measuringthe active power injected by IG and STATCOM current iscontrol to track the reference current so that the STATCOM

generates variable amount of reactive power in a fixedproportion of varuable active power injected by the inductiongenerator.

The beauty of this scheme is that the IG is bound to obey thegrid frequency. Hence, even at variable speed of wind, the

speed of IG remains fairly constant injecting variable amountof active power at different values of slip of IG.

VII. CONCLUSION:Wind turbine system, having active and reactive power

control to have maximum power extraction and voltageregulation respectively, can be connected to grid with the helpof power electronics in generator as well as grid side.Different types of converters can be used as grid side (and/orgenerator side ) converter eg; two level back to backconverter, three level back to back converter, thyrister based

phase controlled CSI etc. Converter higher than three leveleg, four level, five level, seven level can also be used. Higherthe level of the converter, more sinusoidal will be the voltageoutput, meaning the output will contain less harmoniccontents. Also relatively lesser voltage will appear across


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power electronic switches in converter with increasing levelof converter decreasing switching loss and device cost.

REFERENCES:

[1] Remus Teodorescu, Marco Liserre and Pedro Rodrguez Grid Converters

for Photovoltaic and Wind Power Systems 978-0-470-05751-3, John

Wiley & Sons, Ltd, 2011,pp.123-142.

[2] Heier, S., Grid Integration of Wind Energy Conversion Systems, John

Wiley & Sons, Ltd, 1998

[3]Mohan, Undeland, Robbins ,Power Electronics Converters, Applicationsand Design , John Wiley & Sons, Ltd. ,2003,pp.200-289

[4] Akira Nabae,A New Neutral-Point-Clamped PWM Inverter IEEE

transactions on industry applications, vol. ia-17, no. 5.

september/october 1981

[5] Michael P. Bahrman, Jan G. Johansson, Bo A. Nilsson, voltage source

converter transmission technologies- the right fit for the application.

wind turbine systemsgrid converter structures and topologiesnd turbine system.pdf

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