options analysis appendix

8/13/2019 Options Analysis Appendix

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International Research Report

for the

Wind Generation Investigation Project

Technical Solutions of WindFarms to meet EmergingGrid Code Requirements

Authors: Chandana SamarasingheSunil Abeyratne

Date: October 2007

Version number: 4.0

Status: Final

Classification: Public


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Table of Contents

1.0 Purpose........................................................................................................................................... 32.0 Introduction .................................................................................................................................... 4

2.1 Background................................................................................................................................. 42.2 This Report ................................................................................................................................. 4

2.3 Performance requirements for generators................................................................................... 53.0 Types of Wind Turbines commonly in use..................................................................................... 6

3.1 Fixed Speed Induction Generator (FSIG)................................................................................... 63.2 Two speed FSIG......................................................................................................................... 93.3 Doubly Fed Induction Generator (DFIG) ................................................................................... 93.4 Full Scale Frequency Converter (FSFC) .................................................................................. 123.5 WINDFLOW 500kW Wind Turbine........................................................................................ 13

4.0 Basics of Power System operation in the context of wind generation.......................................... 144.1 Generator offers and wind generation ...................................................................................... 144.2 Power system frequency control and frequency reserves......................................................... 154.3 Role of Inertial effect................................................................................................................ 164.4 Voltage control and dynamic voltage stability support ............................................................ 164.5 Fault ride through Capability.................................................................................................... 16

4.6 Power Quality........................................................................................................................... 175.0 Turbine technology and grid connections: The Early years ......................................................... 18

5.1 Deliver the stated output within a specified voltage range under steady state conditions ........185.2 Participate in frequency control and contribute to reserve requirements.................................. 185.3 Participate in voltage control and contribute to reactive power requirements .......................... 185.4 Remain connected during specified grid voltage characteristics at the point of connection forspecified times (Fault ride-through capability)............................................................................... 195.5 Comply with specified standards on emission of flicker and harmonics.................................. 19

6.0 Turbine technology and grid connections: Present and future...................................................... 206.1 Deliver the stated output within a specified voltage range under steady state conditions ........206.2 Participate in frequency control and contribute to reserve requirements.................................. 226.3 Participate in voltage control and contribute to reactive power requirements .......................... 266.4 Remain connected during specified grid voltage characteristics at the point of connection for

specified times (FRT capability) .................................................................................................... 266.5 Comply with specified standards on emission of flicker and harmonics.................................. 27

7.0 Summary ...................................................................................................................................... 31


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1.0 Purpose

The purpose of this report is to present an overview of technical solutionsdeveloped for wind farms to comply with revised Rules and Grid Codes in

overseas jurisdictions. The following areas are covered in this report:

Types of wind turbines commonly in use;

Basics of power system operation in the context of wind generation;

Turbine technology and grid connections: The early years;

Turbine technology and grid connections: Present and future;

Other technical solutions such as the development of new controlsystems; and

Management of power quality issues with new designs.


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2.0 Introduction

2.1 Background

High levels of wind energy penetration in a power system raise a number ofchallenging issues. These are caused by:

(1) The variability and the unpredictability of wind; and

(2) The nature of energy conversion devices, which include inductiongenerators and power electronic interfaces.

Historically, in other jurisdictions, there has been no expectation that windgenerators should contribute to power system operation and stability.Conventional generators have provided these services.

However, an increasing penetration of wind generation on the power systemdecreases the proportion of conventional generating stations on the powersystem. If wind generators have low capability in terms of supporting systemoperation and stability, then growth in wind generation will mean that it isessential that minimum levels of conventional generation remain connected atall times to support system operation. It is also likely that this will become alimiting factor for the development of wind farms.

This limiting factor has been recognised, and internationally, Grid Codes1have been revised to reflect the emerging requirements necessitated by highlevels of wind energy penetration. These revisions, which have occurred in

Ireland, South Australia and Denmark, require wind farms to contributepositively to the operation of the power system. Such Grid Code revisions,together with the thrust to develop viable renewable energy alternatives, havebeen driving developments in modern wind turbine technology, including thecontrol capabilities of existing wind turbine designs. Grid Codes are notcovered in this report.

2.2 This Report

This report briefly discusses wind turbine technology and associated gridconnection practices in the early years of wind generation, and identifiesdevelopments in turbine technology that have already occurred or are likely to

occur in future in response to anticipated revisions to existing grid connectionrequirements. Improvements in turbine control systems that enhance windfarm performance to meet the revised Grid Code requirements are alsodiscussed.

Developments in wind turbine technology and turbine control systems haveled to the widespread use of power electronic interfaces with power systems.Widespread use of power electronic devices has the potential to adversely

1

A grid code is a document containing the rules governing the operation, development and use of thepower system. In New Zealand, the Electricity Governance Rules and Regulations and Transpowers

connection codes would together be considered the grid code.


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impact power quality and existing and future solutions to this emerging issueare also discussed.

2.3 Performance requirements for generators

Under most Grid Codes, generators are required to contribute to the

preservation of power system security and stability by meeting the followingrequirements:

1. Deliver the stated output within a specified voltage range under steadystate conditions;

2. Participate in frequency control and contribute to reserve requirements;

3. Participate in voltage control and contribute to reactive powerrequirements; and

4. Remain connected during specified grid voltage characteristics at the pointof connection for specified times (fault ride-through capability).

Conventional generators are designed to ensure that power injected intopower systems does not adversely affect power quality. Wind power has thepotential to introduce harmonics when power electronic interfaces are used.Wind gusts cause rapid variations in power output leading to voltagefluctuations, which manifest as voltage flicker. Such voltage flicker has been adominant factor affecting power quality in the past. As can be seen later,modern variable speed wind turbines have reduced voltage flicker though notentirely eliminated it.

Harmonics and flicker are parameters that affect power quality. To maintain

these parameters within desired levels, a power quality requirement, such asthe following, is necessary to maintain power system security with increasingwind generation.2

5. Comply with specified standards such as IEC 61400-21.

Wind turbines that comply with this standard (most wind turbinemanufacturers are compliant) are fundamental to avoiding adverse impacts onpower quality.

This paper uses this 5-point framework to elaborate on existing and expected

technical solutions arising out of advances in turbine technology andinnovative applications of modern control systems. Table 1 (in appendix 1),which lists Wind Farm performance capabilities in respect to the threecommonly used wind turbines, is based on the same framework.

2The power quality at a GXP or GIP is affected by loads and generators. Rule 2.3 of section III, Part C

of the EGRs sets out what the System Operator is required to do when specific standards are not met at

a point of connection. Wind generation manufacturers faced no standards in this regard, and IEC 61400was developed for this purpose with 61400-21 being a standard for the determination of power qualitycharacteristics of the output of a wind turbine.


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3.0 Types of Wind Turbines commonly in use

This section gives a brief description of the main types of wind turbines thatare currently in use. For completeness, a description of the Windflow 500

turbine design is also included.

3.1 Fixed Speed Induction Generator (FSIG)

This is the simplest arrangement, where a wind turbine is connected to aninduction generator through a gear box as shown in Figure 1. This is also thecheapest wind turbine option.

The gear box is an essential component, required to match the wind turbinerotor speed to the induction generator rotor speed. If an induction generatorwere to be directly coupled to a wind turbine, the induction generator rotor has

to run at the same speed as the wind turbine rotor. Wind turbine rotors run ata speed that is too low for a reasonably sized induction generator to deliverthe power generated. The implication is a huge induction generator that has tobe located in the nacelle.

The induction generator is directly connected to the grid through a step-uptransformer. Since induction generators require external reactive power, acapacitor bank is used for reactive compensation. It is common to use a soft-starter to ensure a smooth grid connection.3

The generator slip4 slightly varies with the level of generation, but this

variation is only about 1% and it is for this reason that this type is referred toas the Fixed Speed Induction Generator. It is common to use a squirrel-cageinduction generator to minimise the cost.

Figure 1: FSIG Configuration

3An induction machine can be connected to a power supply by many methods. Direct connection is onesuch method where starting currents may cause supply voltage to dip below allowed levels. A smoothgrid connection is achieved when starting currents are kept low enough to prevent supply voltage fromdipping below allowed levels.4Induction generators run at a speed that is slightly higher than the synchronous speed. Slip is definedas the difference between the synchronous speed and the generator speed as a fraction of thesynchronous speed.


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Aerodynamic Power Contro lThe speed of a wind turbine tends to rise with wind speed and so, for safetyreasons, it is necessary to be able to control turbine speed. A rise in windturbine speed is generally associated with an increased power production,which is the result of an increased electrical torque. This increased electrical

torque counters the tendency for wind turbine speed to rise. Notwithstandingsuch an intrinsic balance, it is critical that a wind turbine has the capability tocontrol its speed by a reduction in the mechanical torque produced byprevailing wind flow. It is precisely this capability that allows wind turbines tocompletely shutdown under conditions of high winds to avoid any damage.

The mechanical torque produced by wind can be controlled by turning theblades away from the optimum angular position. The blades could be eitherturned out of the wind or turned up against the wind. Turning blades out of thewind is called pitch control whereas turning up against the wind is called stallcontrol.

There are two variations of stall control as discussed below.

Passive Stall ControlThis is the simplest method of speed regulation. It takes advantage of theaerodynamic stall effect. The angle at which the wind hits the blade is one ofthe critical factors that determine the aerodynamic lift to the blade and thusthe mechanical torque. This angle is called the angle of attack, whichdepends on wind speed, turbine rotor speed and the blade angular position.

For a given blade angular position, when the wind speed rises, the angle ofattack also rises unless such wind speed rise is matched by a proportionaterise in the turbine rotor speed.

When the angle of attack is gradually increased, at a certain angle the air flowwill no longer be along the back of the blade but will separate from the blade.This creates large eddy flows on the back side of the blade impairing theaerodynamic lift on the blade.

For a turbine operated at constant speed, as the wind speed increases, theangle at which the wind hits the blade also increases. This increase leads to

a loss in the mechanical torque, which counters the tendency for the turbine toincrease its speed with increasing wind speed.

With this passive stall control of turbines, the blades are designed with a fixedangular position such that the stall effect just starts at the rated wind speed.5The pitch angle6remains fixed because the blades are rigidly fixed.

5Rated wind speed is the lowest wind speed at which the wind turbine delivers the rated power output.

The wind speed can increase beyond the rated speed and still the wind turbine would deliver the ratedpower output.6The blades are circular in cross-section at the hub and with some turbine designs, the blades can beturned about the hub (or pitched) over a given angle, thereby altering the angular position of the blade.The angle between the plane of rotation and the blade is called the pitch angle


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The power output of a stall controlled turbine effectively remains constant fromrated wind speed to high wind cut-out speed.

Active Stall ControlIn the previous paragraph it was noted that blades of Passive Stall technology

wind turbines are rigidly fixed so that the stall effect just starts at the ratedspeed. With passive stall, the turbulence associated with the stall effectcauses a little sag in the power curve for wind speeds between rated powerand high wind cut-out. This means some lost production at those windspeeds.

Active Stall control is only possible when the blades can be pitched asexplained below under Pitch Control. Active Stall Control enables theadjustment of blade angular position to ensure that the power curve remainsflat from rated wind speed to cut-out.

With Active Stall control, it is also possible to start the stall effect before windspeed reaches rated wind speed. For this to happen, the blade angle needs tobe turned into wind so that eddy flows form behind the blades at a wind speedlower than the rated wind speed. This is power limitation through bladeangular position being set for stall to occur at wind speeds lower than ratedwind speed. Similar control allows aerodynamic braking as well. It is commonfor Active Stall turbines to be set up with full (~90 degrees) pitching to providethese benefits.

Some wind turbine manufacturers consider this power regulation method to besimple, efficient and reliable. The dynamic loads on the blades are almostnegligible as a result. Automatic adjustment of the blade pitch enables themaximum power to be maintained at a pre-selected level. The BONUSCombiStall system uses active stall control. The pitch angle of the rotorblades is modified automatically (CombiStall) to ensure that the stall effect isdynamically adjusted to the specific wind conditions on site.

Pitch ControlWith some wind turbines (particularly most FSIG), the blades are rigidly fixedto the hub with a fixed blade angular position. Modern wind turbines providethe facility to change the angular position of blades as explained in footnote 6.

The blades are turned about the hub (pitched) using hydraulic or electricactuators. The angle through which a blade is pitched can be preciselycontrolled using position controllers. The blade angular position is measuredby the pitch angle.

In the previous section on passive stall control, it was noted that the angle ofattack depends on wind speed, turbine rotor speed and the blade angularposition. With variable speed turbines, the blade angular position is changedby pitching the blades, which allows the angle of attack to be controlled toextract the maximum energy from varying wind speeds. Speed regulation

through blade angular position control is referred to as pitch control. When


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pitch control is used to achieve aerodynamic stall effect, it is called active stallcontrol.

With variable speed turbines (DFIG is one such turbine see description insection 3.3), the angle of attack is maintained at the desired level using pitch

control despite wind speed variations. This is possible because both theturbine rotor speed and pitch angle are variable. Thus the angle of attackbecomes independent of wind speed. The power output above the rated windspeed can also be controlled by pitching the blades. The blades are pitchedtowards the vane position by hydraulic actuators, effectively feathering theblades. Note that variable speed turbines are normally operated at ratedspeed when wind speed is above the rated wind speed.

Active stall control limits the power extraction by the formation of eddy flowscaused by aerodynamic stall whereas pitch control regulates energyextraction rates by controlling the angle of attack, limiting power extraction by

pitching out of wind.

3.2 Two speed FSIG

The two-speed FSIG has an induction generator with the facility to select twospeeds. The configuration is identical to that of a FSIG apart from the facilityto change the stator winding connections, which is located externally.

The main advantage is that the two-speed FSIG has limited capability tochange the turbine speed in response to changing wind speeds so that energy

is extracted more efficiently than a FSIG with only a single speed option. Afurther advantage is the reduction in noise.

3.3 Doubly Fed Induction Generator (DFIG)

The DFIG uses a wound-rotor induction generator and its stator is directlyconnected to the AC system, which may be either the grid or a point ofconnection to a medium voltage network that connects many other windturbines in a wind farm. The stator delivers about 70% of the power throughthis direct connection. The rotor is connected to a back-to-back voltage sourceconverter7 and delivers about 30% of the power through this converter.Hence, this type of turbine is referred to as a Doubly Fed InductionGenerator.

The DFIG is more expensive than the FSIG, but often it is more easilyintegrated into a grid (as discussed later in this paper). This is primarilybecause other wind farm infrastructure (for example, STATCOM) may not berequired.

7A back-to-back voltage source converter is a power electronic system comprising three stages, which

are a rectifier, a DC link voltage (a capacitor) and an inverter all connected in series. The systemenables the connection of an AC system to another AC system of a different and variable frequency andallows energy flow in both directions.


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A gear box is required for a DFIG for the same reasons as for an FSIG.

Unlike the FSIG, the DFIG has a power electronic interface with the grid,which can potentially introduce harmonic currents to the grid.

The stator is connected to the grid through a step-up transformer whereas therotor is connected to the converter and then to the grid through a step-uptransformer. It is common to use a three-winding transformer as shown inFigure 2.

DFIG are variable speed turbines where the rotor blades are pitched to controlturbine speed so that they extract the maximum available energy when windspeed is less than the rated wind speed. A speed range of 60-110% of therated wind speed is sufficient to extract the maximum energy available in awind site.

As noted previously, the back-to-back voltage source converter comprises twoindependent converters called the rotor side converter and the stator sideconverter, which are connected to a common DC bus. These two convertersare independently controlled by a rotor side converter controller and a statorside converter controller.

Figure 2: DFIG Configuration

These two controllers and the two converters are shown in Figure 3.

The two converter controllers determine the behaviour of the DFIG throughthe two converters and for this reason; the two converter controllers togetherare considered the DFIG control level. As can be seen from Figure 3, theDFIG control level receives set points from the wind turbine control level andthe Wind Farm Controller.

The rotor side converter controllerThe rotor side converter controller controls the rotor voltage magnitude andits phase angle. Such control capability allows the rotor side converter to


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inject slip frequency currents of varying magnitude, including directionreversals. Hence, the rotor side controller can change the electrical torquethereby allowing variable speed operation regardless of an unchangedmechanical torque developed by wind. It is important to note that the rotor slipfrequency changes with turbine speed and the rotor side converter is capable

of either injecting variable frequency power into the rotor or extracting variablefrequency power from the rotor. Thus the rotor side converter controller canindependently control the active and reactive power at the point of connection.

The stator side converter controllerThe stator side converter controller controls the DC link voltage and ensuresthat the power factor of power flow to the grid through the rotor branch isunity.

The wind turbine controller sits above these two controllers. The wind turbinecontroller comprises power controller and speed controller, which are in the

wind turbine control level. The power controller controls the pitch angle andthe speed controller sets the reference power output to the grid.

The hierarchical control structure of a wind farm is shown in Figure 3. Thewind farm controller sits above all the wind turbine controllers. For eachturbine, the wind turbine controller sits above the DFIG control level.

This is a basic discussion of the DFIG and the reader who wishes tounderstand modelling is referred to Electrical and Control Aspects of OffshoreWind Farms (ERAO II) Volume 1: Dynamic Models of Wind Farms.

Figure 3: Control Structure of a DFIG8

8Reproduced with permission from Wind Farm Models and Control Strategies "Wind Farm Models andControl Strategies" by Poul Sorensen, Anca D. Hansen, Florin Lov, Frede Blaabjerg and Martin H.Donovan.


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3.4 Full Scale Frequency Converter (FSFC)

This type of wind turbine can be connected to (1) a wound-rotor inductiongenerator, (2) a permanent magnet synchronous generator, or (3) an

externally magnetised synchronous generator.

The generator is connected to the grid through a full-scale frequencyconverter. The full-scale frequency converter is a back-to-back voltage sourceconverter. This frequency converter performs the reactive powercompensation and smooth grid connection. This technology is moreexpensive than the DFIG because the frequency converter needs to be sizedto cater for the entire load. Other disadvantages of a FSFC converter incomparison to a DFIG converter of similar size include greater losses andgreater weight. For these reasons, the full scale converter is not as commonlyused as the DFIG.

Figure 4 shows the arrangement for an induction generator where a gear boxhas been used in this instance.

Figure 4: Full Scale Frequency Converter Configuration (Induction Generator)

Figure 5 shows the arrangement for a synchronous generator where there isno gear box. The permanent magnet allows the operation without externalmagnetisation.

Figure 5: Full Scale Frequency Converter Configuration (Permanent Magnet Synchronous

Generator)

Figure 6 shows the arrangement for a synchronous generator with external

magnetisation. A gear box has been used in this instance.


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Figure 6: Full Scale Frequency Converter Configuration (Externally magnetised Synchronous

Generator)

3.5 WINDFLOW 500kW Wind Turbine

The Windflow 5009 is a 2-bladed pitch teeter coupled design. The Windflow

500 incorporates blade pitching for power control and the rotor is designed toinclude a teetering hub, which reduces the overturning load on the structure.

This turbine uses a torque-limiting gearbox (TLG) that provides closeregulation of power, which allows the use of a synchronous generator that isdirectly connected to the grid. Under conditions of wind above the rated windspeed, the TLG holds the torque constant and allows the wind rotor speed tovary while keeping the synchronous generator speed constant. This isachieved by having a mechanical slip introduced via a hydraulic pump withinthe TLG. The torque level at which slip occurs is fully adjustable through arelief valve and under normal conditions would be set at a level equivalent to

the MW rating of the generator.

The wind turbine is designed to operate within a wind speed range of 5.5-30 m/s and active pitch control keeps the rotor speed within operating limits.

The manufacturer refers to this turbine as synchronous and synchronised todifferentiate it from the full scale frequency converter (FSFC) type described insection 3.4 because this type of wind turbine has the ability to run asynchronous generator directly online without any power electronic devices.

The usual grid connection advantages associated with synchronous

generators should be inherently available from this wind turbine

10

.

9Windflow Technology Limited Windflow 500kW Wind Turbine, General specification, standard Scopeof Supply and Options10

Wernher Roding and Geoff Henderson Synchronous and synchronised Wind power generation.


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4.0 Basics of Power System operation in the context of windgeneration

This section can be skipped by readers who are familiar with power systemoperation and the basics of wind power generation.

Recent revisions made to Grid Codes in a number of jurisdictions require thatwind farms contribute positively to the operation of the power system. Windfarms are expected to behave like typical conventional generators andconsequently, deliver the stated megawatt output, as well as meeting voltageand frequency requirements similar to those listed in Section 2.0 (numbered1-5), in order to facilitate smooth power system operations.

Power system operation in New Zealand is the focus of this section.

4.1 Generator of fers and wind generation

Under the New Zealand Electricity Governance Rules (the EGRs) generatorsare required to make generation offers relating to a trading period well inadvance of that trading period. However, any generator can change its offersup until gate closure (two hours prior to the beginning of a trading period) for agiven trading period, and wind generators can also change their offers aftergate closure, though their offers must be based on a persistence forecast.

A fundamental difference between wind generation and conventionalgeneration is that wind is self dispatched based on prevailing wind flow. Wind

farms in many other jurisdictions are expected to behave much more liketypical conventional generators and consequently, deliver the offered MWoutput to facilitate smooth market operations. In contrast to other generatortypes, expected wind generation is entirely dependent on the prevailing windflow during the trading period. Hence site-specific two hour ahead and dayahead wind forecasts are important, and become more so as total connectedwind generation capacity increases.

The available wind power is proportional to wind speed raised to the power ofthree, which leads to greatly varying levels of wind power output underdifferent wind conditions unless generation levels are controlled.

It is possible to curtail the megawatt output of a wind farm by pitch control inorder to meet the generation requirements stipulated by the System Operator.However, it is not possible to maintain a stated output from a wind farm whenexisting wind speeds are either inadequate to support the stated output or areexcessive. Excessive wind speeds lead to a complete shutdown of the windfarm to protect its turbines.

This report focuses on how megawatt output can be controlled, subject to theassumption that sufficient wind exists to achieve such control.


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4.2 Power system frequency control and frequency reserves

The generated power must be equal to the sum of power consumption andlosses in the system for system frequency to remain at its target value. Whenthe power consumption and losses exceed generated power, the rotatingparts (generators and motors) slow down, thereby lowering the system

frequency. This deviation of the frequency causes the prime movers to supplymore energy to their generators, resulting in increased generation, mitigatingthe lowering of the frequency. This control is called primary control andgovernors (devices that respond to frequency variations by changing energysupply rates) carry out this task without intervention from the SystemOperator. Primary control can be blocked within a defined band of frequency(called the dead-band) to allow the designated generator (called the frequencykeeper) to perform this function.

Typically, primary control takes place within a part of a trading period knownas the dispatch period. In New Zealand, this period is five minutes. Every fiveminutes, subject to any constraints, the system demand is allocated amonggenerators to minimise the total cost to consumers. This allocation isimplemented by sending appropriate signals (dispatch instructions) togenerating units, which alter their governor set points. This process is calledsecondary control. Primary control is required to cater for load changes thattake place during the dispatch period because there is no reallocation of loadby the System Operator within the dispatch period. Generally, load changesthat occur during a dispatch period are both slowly varying and well within themagnitudes catered for by the frequency keeper. In summary, the frequencykeeper compensates for load changes that occur within the dispatch period,

whereas secondary control is implemented at the beginning of each dispatchperiod. Secondary control is therefore slower than primary control. There aresituations, for example when a large block of generation is lost, where systemload is reallocated during a dispatch period.

With the variability and the unpredictability of wind, it is inevitable that thepower output of a wind farm will unpredictably change within a dispatchperiod. Such a change will result in an imbalance of power generation anddemand in the system. In order to keep the system frequency reasonablyconstant, such changes in power output will need to be counterbalanced by asuitable amount of frequency regulating reserve. With high wind penetration

levels, greater frequency keeping capability than is currently procured may berequired.

As explained below, primary control plays a critical role when a generatorsupplying part of the system demand is removed for some reason. Theremoval of a generator causes a sudden shortfall in generation, as opposed toa gradually rising load. The MW change could easily exceed the contractedfrequency keeping service. For this reason, in the great majority of cases,such removal causes a drop in frequency that exceeds the lower limit of thefrequency dead-band. When this happens, generators that are connected tothe power system automatically respond to the drop in frequency by raising

energy supply rates, thereby restoring energy supply to match the system


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demand. This is primary control as performed by generators that areconnected to the power system, and is critical to power system operation.

4.3 Role of Inertial effect

It is relevant to note that the rate of fall of frequency following the removal of a

generator that supplies energy is dependent on the total rotating inertia. Thehigher the rotating inertia, the slower will be the rate of fall of frequency.Large thermal units have large rotating masses, which slow the rate of fall offrequency. This characteristic is called the inertial effect. Inertial effecteffectively buys time so that primary control can act to supply the deficit ingeneration caused by the removal of a loaded generating unit.

The proposed changes to Grid Codes in many jurisdictions require wind farmsto provide (1) Primary control, (2) Secondary control, and (3) Inertial effect, inthe same manner as conventional generators do.

4.4 Voltage control and dynamic voltage stability support

Voltage control is the task of keeping system voltages within specified limitsconditions, which include varying power system loads. When power systemloads rise, reactive power is generated due to high currents in branches.Transfer of reactive power is associated with high reactive power losses andfor this reason, reactive power requirements are produced and consumedlocally.

Reactive power sources at various voltage levels are required across thepower system for effective voltage control. Synchronous generators,

STATCOM11

, SVC12

, capacitors and reactors are all reactive power sources.Synchronous generators are the preferred source of reactive power becausethey support dynamic voltage stability. STATCOM and SVC also supportdynamic voltage stability whereas capacitors and reactors support steadystate voltage stability.

4.5 Fault ride through Capability

Generators in most jurisdictions are required to stay connected and generateduring system disturbances unless these disturbances preclude the running ofgenerators or disturbances are so severe that operation of generators maydamage them. Renewable sources, particularly wind generation, have

historically been the exception to this requirement.

Disturbances are typically characterised by a dip in voltage arising from theoccurrence of a fault and the subsequent recovery of the voltage following theclearance of the fault. The recovery of the voltage is not entirely smooth andit is possible to have further transient dips. In order to contribute to power

11The STATCOM (Static Synchronous Compensator) is a shunt device that regulates voltage at its

terminal by controlling the amount of reactive power injected or absorbed from the power system. Thevariation of reactive power is performed by forced-commutated power electronic devices on thesecondary side of a coupling transformer. Generally, STATCOM are faster than SVC.12The SVC (Static Var Compensator) typically consists of a thyristor controlled reactor and a thyristorswitched capacitor along with AC filters. The SVC provides fast and continuous capacitive and inductivereactive power.


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system operation without jeopardising system stability, the connectedgenerators need to ride out this voltage dip. Conventional generators meetthis requirement, as do most new wind turbine designs. However, someexisting wind turbines do not have fault ride through capability, for example,early DFIG designs.

Figure 7 shows the Fault Ride through Requirements of E-On Netz (Germany)where instability or disconnection of generators is not acceptable as long asthe voltage at the connection point remains within and above the shaded area.

Figure 7: Fault Ride through Requirements of E-on Netz

4.6 Power Quality

The ideal voltage source is a perfectly balanced voltage in the three phases, apure sine wave with a constant frequency and magnitude. It is not possible tomeet all these conditions all the time because intermittent loads, starting andstopping of induction motors and power electronic loads cause voltage sags,dips, swells and harmonics. When the characteristics of the ideal voltagesource are not met, the power quality is said to have deteriorated.

The ideal frequency is the stated power system frequency, and Grid Codesspecify an allowed frequency excursion range around this. When frequencyexcursions are not within specified limits, power quality has deteriorated.

Wind farms directly affect power quality by (1) the nature of the energyconversion device, (2) starting of wind turbines, and (3) the variability of wind.

The International Electrotechnical Commission (IEC) developed the standardIEC 61400-21 for Power Quality of Wind Turbines. Most large wind turbinemanufacturers provide power quality characteristic data as specified in thisstandard.
http://powerstandards.com/tutorials/sagsandswells.htmhttp://powerstandards.com/tutorials/sagsandswells.htmhttp://powerstandards.com/tutorials/sagsandswells.htmhttp://powerstandards.com/tutorials/sagsandswells.htm


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5.0 Turbine technology and grid connections: The Early years

In the past, wind generation was essentially embedded in the distributionnetworks and consequently grid integration issues were not evident.

5.1 Deliver the stated output wi thin a specified voltage range understeady state conditions

With the variability and the unpredictability of wind, a wind farm operatingunconstrained will not meet the basic requirement of delivering the statedoutput within a specified voltage range under steady state conditions.However, with existing low levels of wind penetration, the variability and theunpredictability of wind energy would not significantly adversely affect powersystem or market operations.13 The only exceptions occurred when windfarms were located on the periphery of power systems where wind generationlevels were significant in relation to local network capabilities, leading to

network constraints. Hence, owing to insignificant wind penetration levels, thedelivery of a stated output from a wind farm was not a major area of concern.On the contrary, there were instances where this requirement was relaxed toaccommodate wind generation as an incentive for the development ofrenewable energy resources.

5.2 Participate in frequency control and contribute to reserverequirements

Generators contribute to power system frequency regulation by controlling theprimary energy supply rates to generator prime movers. With wind farms, theprimary energy source (wind) cannot be controlled. What is possible is to

control energy extraction rates, though such methods of output control canonly be implemented by limiting the turbine power output to a value that islower than what existing wind flow levels would allow. Such control techniqueslead to the full potential of wind power not being utilised. For this reason, windpower developers often argue that such control methods distort theeconomics of wind power development notwithstanding the associatedproblems with power system operation.

While wind energy penetration levels were low, it was not necessary for windfarms to participate in frequency regulation of power systems. Further, theprovision of features to participate in frequency regulation is not easilyachieved with FSIG wind turbines unless modern advanced controllers areused. FSIG have been the most commonly used wind turbine in the past.

5.3 Participate in voltage control and contribute to reactive powerrequirements

With low wind energy penetration levels, there has historically been norequirement in most jurisdictions for wind farms to contribute to supportvoltage or participate in meeting reactive power demands. FSIG, connectedto stall controlled turbines (the main type of turbine installed in the past) are

13Note: With respect to the Danish system, while the penetration level has been quite high for sometime, it is not considered highly significant when compared with the larger (European) power system withwhich it is interconnected.


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not able to control their voltage. Further, such generators cannot providereactive power and are in fact consumers of reactive power. These drawbackscan be, and have been, mitigated by the installation of appropriate dynamiccompensation systems at the points of connection.

5.4 Remain connected during specified grid voltage characterist icsat the point of connection for specified times (Fault ride-throughcapability)

In the past, wind turbines were mainly treated as embedded generators (andstill are in some countries) and consequently they were not required tocontribute to power system stability. In addition, wind farms were expected tobe disconnected when voltage or frequency limit violations occurred. This isbecause it was considered potentially hazardous for generating plantembedded in distribution systems to continue to operate during or aftersystem disturbances. Hence, past Grid Codes in overseas jurisdictions havenot included any fault ride through requirements.

5.5 Comply with specified standards on emission of flicker andharmonics

Power electronic interfaces are a recent phenomenon and consequentlyharmonic injections arising out of power electronic interfaces were absent inthe past. With variable FSIG outputs, voltage fluctuations occurred when theseries impedance between the wind turbine (or wind farm) and the point ofconnection was significant. However, with low levels of wind penetration,voltage flicker was essentially a local issue.


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6.0 Turbine technology and grid connections: Present andfuture

Wind turbines are connected to form wind farms, which are in turn connectedto the grid. There are many wind farm connection configurations. Thebehaviour of a wind farm is different from the behaviour of an individual windturbine. Factors that affect and improve wind farm behaviour include dynamicreactive compensation devices (STATCOM and SVC) and advanced windfarm controllers.

This report does not cover DC connection of wind farms primarily becausesuch connections are a new technology applied to large offshore wind farmswhere HVDC cables are a better option when compared to AC submarinecables. The report focuses on AC connections with Dynamic Reactive

Compensation. The Te Apiti wind farm is a good example of this.

The discussion on turbine technology is limited to the following three types ofwind turbine: the Fixed Speed Induction Generator with active stall control, theDoubly Fed Induction Generator, and the Full Scale Frequency Converter.The configurations of all these types were discussed in Section 3.

This section discusses how wind turbine technology has developed to addresseach of the identified power system issues, so that wind generators canpositively contribute to the operation of the power system.

6.1 Deliver the stated output wi thin a specified voltage range understeady state conditions

6.1.1 Operation of a wind farm at a specified power outputWhere wind farms are controlled, they can be operated at a specified poweroutput. This is achieved hierarchically through the Wind Farm Controller(WFC) that sits on top of the control structure. The WFC receives commandsfrom the System Operator that specify the required output from the wind farm.Based on such commands, the WFC sets power outputs for all wind turbinesin the farm and communicates these required outputs to each of the wind

turbine controllers. These power outputs set by the WFC are based on thepower capability, the current MW output and the ON/OFF state of each windturbine. Each wind turbine controller limits the power output of the windturbine as communicated by the wind farm controller regardless of wind speedas long as the wind speed is neither too high to protect the turbine nor too lowto deliver the set power output.

A specified power output can be achieved by Balance Control14, where theWFC limits the wind farm output to the values requested by the SystemOperators dispatch tools (refer to central part of Figure 8, which shows thatthe wind farm has a constant output of 60MW despite the potential to

14Balance Control is named so because the wind farm output is lowered to balance power demand in

the grid


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generate in excess of 100MW, based on prevailing wind speeds). This controlmethodology is applicable to both FSIG and DFIG.

Figure 8: Balance Control with Ramp Rate Limitation15

6.1.2 Operation of a wind farm to deliver specified power output at aspecified voltage

With a wind farm, the specified voltage is the voltage at the point ofconnection where the delivery of power takes place.

The FSIG does not have the inherent capability to provide voltage control.For this reason, to control voltage, compensation devices need to be providedat each turbine or at the point of connection of a wind farm comprising FSIG.STATCOM and SVC are commonly employed compensation devices at thepoint of connection of a wind farm. Capacitors are used at each wind turbineduring starting and these can be used for static compensation.

The DFIG has the inherent capability to provide steady state voltage controlusing its back-to-back voltage source converter system connected betweenthe stator and the rotor.

The FSFC has complete flexibility to control voltage and reactive power.

Current technology therefore allows wind farms with any of the three types ofturbines to deliver the specified power at the specified voltage.

15Reproduced with permission from Wind Farm Models and Control Strategies "Wind Farm Modelsand Control Strategies" by Poul Sorensen, Anca D. Hansen, Florin Lov, Frede Blaabjerg and Martin H.Donovan.

____Available

__Controlled


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6.2 Participate in frequency control and contribute to reserverequirements

6.2.1 Primary Control with Wind TurbinesWith conventional generating plant, the two parameters that define primary

control are the droop and the dead-band. The droop determines thechange in generation level associated with a change in frequency. In NewZealand, the EGRs specify that the droop should be adjustable over a rangeof 0-7%.

The power output of a conventional generating station rises when thefrequency falls. Similarly, the power output falls when the frequency rises. Ifwind turbines are required to respond to frequency changes as conventionalgenerators do, they will need to have the capability to raise or lowergeneration when frequency changes. For this to happen, the wind turbineswould always need to have reserve power, i.e. they would need to operate at

a power output level that is lower than the maximum power that can beextracted from prevailing wind conditions. Such operation is called DeltaControl. The specified amount of reserve power (the delta) is alwaysavailable, and the delta requirement can be specified by the SystemOperator. This arrangement occurs in overseas jurisdictions.16

Figure 9: Delta Control with Ramp Rate Limitation17

Conventional generators are used for primary control (i.e. within the dispatchperiod) by allowing changes to the fuel input. The same result can be obtainedfrom wind turbines as described above, by implementing delta control at thewind farm controller level. The difference, however, is that wind farms do not

16Refer to the central part of Figure 9, which shows that the wind farm output is consistently less than

the potential generation based on prevailing wind speeds by a constant value of 40MW.17

Reproduced from Wind Farm Models and Control Strategies "Wind Farm Models and ControlStrategies" by Poul Sorensen, Anca D. Hansen, Florin Lov, Frede Blaabjerg and Martin H. Donovan.


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fully utilise the available wind resource whereas conventional generators savefuel.

The WFC has information on wind speeds at each of the turbines andconsequently has information on the maximum power that can be generated

at each instant. With additional information on wind speed forecasts (despitethe inaccuracies in the forecasts), the WFC has a wealth of information thatcan be used to implement primary control.

Based on the system frequency and the specified power frequencycharacteristic, the WFC determines the required farm output. The WFCdelivers this output by setting the megawatt output levels of each wind turbinebased on their capability. Each WTC regulates the power output to meet theset output levels by pitching the turbine blades.

Pitching of FSIG blades results in less than available wind power being

extracted, but the wind turbine rotor speed remains almost constant. Pitchingof DFIG blades also results in less than available wind power being extracted.Additionally, there will be a reduction of wind turbine rotor speed leading to aless efficient operating point, which diminishes the advantage that DFIG hasover FSIG.

When reserves are required to be held by wind farms, the power frequencycharacteristic for the wind farm is set by the System Operator, and the valuesfor droop and dead-band are set in the WFC by way of the required powerfrequency characteristic. With conventional generating plant, automaticprimary frequency control is implemented in the same way.

The Danish TSO requires that primary control is implemented at wind turbinelevel, and the Horns Rev offshore wind farm conforms to this requirement. 18Horns Rev is the first well-publicised wind farm where these features havebeen incorporated, though it is likely that all large European wind farms willneed to meet this requirement in the future.

6.2.2 Secondary Control wi th Wind Turb inesWith conventional generating plant, the parameter that defines secondarycontrol is the governor set point, which the System Operators dispatch tools

communicate to each generator. Similarly, the System Operators dispatchtools communicate the set point to a WFC. The fundamental differencebetween the energy source of a wind farm and that of a conventionalgenerating plant is the unpredictability and the variability of the former. TheWFC has information on wind speed at each of the turbines andconsequently, the wind power availability at any given time is known. Thisinformation can be communicated to the System Operators dispatch tools.Secondary control is necessarily limited to this level of available power andthe communicated set point has to be always within this available power limit.Under this arrangement, wind turbines need to operate at a power output levelthat is lower than what prevailing wind speeds would allow, the difference

18 Horns Rev offshore windfarm: its main controller and remote control system, by

Jesper R. Kristoffersen and Peter Christiansen.


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between the potential power output and the set point being the capacityavailable for secondary control.

The WFC sets the power output requirements for each wind turbine controllerto meet the dispatch requirement received from the System Operators tools.

At each turbine, the wind turbine controller changes power output by pitchcontrol. If the need to use reserve arises, the System Operators tools thencommunicate with the WFC through dispatch instructions. Hence, secondarycontrol is implemented in a similar manner to primary control except thatsecondary control is initiated by dispatch signals sent by a System Operatorsdispatch tools.

6.2.3 Ramp Rate Limi tation ControlAs previously noted, the available wind power is proportional to wind speedraised to the power of three. The high variability of wind speed leads toaccentuated ramp rates because of this cubed relationship. When wind

penetration levels are high, such ramp rates can exceed the ramp ratecapability of existing conventional generation.

Ramp up rates caused by sudden rises in wind speedThe output of a wind farm can be controlled by pitch control and it is thereforepossible to limit the ramp up rate of a wind farm by pitch control.

Ramp down rates caused by sudden drops in wind speed or a sudden rise ofwind speed above 25 m/sThe output of a wind farm can be controlled by pitch control and consequently,it is possible to limit the ramp down rate of a wind farm by pitch control. Forthis to happen, wind farms would need to be operated at output levels that arelower than what prevailing wind speeds would allow, i.e. there is an unutilisedwind energy buffer (delta control). Delta Control in combination with pitchcontrol is used to limit the ramp down rate of a wind farm.

Implementation of Ramp Rate Limitation ControlRamp rate limitation control is implemented with other types of control whichinclude balance controland delta control.

Figure 8 (on page 21) shows how ramp up and ramp down rate control has

been implemented with balance control. Figure 9 (on page 22) shows howramp up and ramp down rate control has been implemented with delta control.

In New Zealand, changes to generation levels of conventional generatingunits currently compensate for and accommodate ramp rates caused byexisting wind generation, thereby maintaining power system frequency. Suchan approach may not continue to be effective with increased levels of windintegration because a significant block of conventional generators would berequired to accommodate increased ramp rates. Hence, there is likely to be astrong argument to support the view that future wind farms need to have allmodern control capabilities, and Rules would need to be developed regarding

the use of such capabilities.


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6.2.4 Inertial response with Wind TurbinesAs noted earlier, the rate of change of frequency depends on the shortfall orthe surplus of generation and the power system inertia. For a givengeneration shortfall, the higher the system inertia, the lower the rate of changeof frequency. Consequently, this inertial response is a critical factor that allows

enough time for governor primary control to supply sufficient energy tostabilise system frequency. It is relevant to note that interruptible loads areused to arrest the fall in system frequency in New Zealand, in addition togovernor primary control. Such interruptible loads and primary control arecollectively called instantaneous frequency reserves.

With high levels of wind penetration, wind turbines in a cluster that aresimilarly affected by rapid changes in wind can cause high ramp rates in apower system where conventional generation forms a lower than normalproportion of the total generation. These high ramp rates in combination withlow inertia produce larger fluctuations in system frequency than would be the

case with a higher proportion of conventional generation, making it moredifficult to keep the frequency stable. The impact of such a generation mix onsystem inertia depends on the inertia of newly integrated wind generation.

Inertial Response of FSIGStandard fixed speed induction generators contribute to the inertia of thepower system because the stator is directly connected to the power system.Any change in power system frequency manifests as a change in the speed ofstator-led rotating flux. Such speed changes are resisted by the rotating mass(generator rotor and the wind turbine rotor) leading to rotational energytransfer to the power system via the stator.

Inertial response of DFIGStandard variable speed wind turbines have a hierarchical control structurewhere the power output is controlled to harness the maximum energyavailable from wind. Power system frequency is normally not used as an inputvariable for the control of power output. These wind turbines are frequencyfollowers and have almost zero inertia as seen from the power system.

However, with modern wind turbine controllers, it is possible to use the rate ofchange of frequency as an input to deliver energy to the power system. The

energy delivered can be set by specifying a desired value for inertia (calledvirtual inertia). The controller extracts energy from the rotor, thereby slowing itdown, and delivers the extracted energy to the power system. The energyextracted is dependent on the virtual inertia specified.

If a very high value for virtual inertia is specified, the controller extractsenergy commensurate with this high virtual inertia, thereby slowing the windturbine to a speed that is lower than the speed that may have resulted if thecorrect inertia value was specified. Such slowing down is inevitably associatedwith a reduction in the rotor mechanical torque because the operating pointshifts from the best position to a less satisfactory position, assuming that wind

speed remains unaltered.


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Inertial response of FSFCWith the FSFC, the generator torque and grid frequency are decoupled.Hence the generator does not respond to frequency variations and it is afrequency follower. The same control methodology to specify a virtual inertiafor DFIG can be applied to FSFC. There is very little literature is available on

FSFC.

6.3 Participate in voltage control and contribute to reactive powerrequirements

6.3.1 Voltage control & reactive power control in a wind farm with FSIGFSIG are consumers of reactive power and they can cause voltage stabilityproblems on weak local grids to which they are connected. STATCOM andSVC systems are commonly used in FSIG wind farms.

FSIG are entirely reliant on reactive sources outside the induction generatorand such sources include capacitors, STATCOM and SVC. The function ofcapacitors that are connected to FSIG is to ensure that power factor remainsclose to unity rather than to provide dynamic voltage support. A wind farmcomprising FSIG may have STATCOM/SVC to provide dynamic voltagesupport. It is possible to provide sufficient reactive resources to ensure that awind farms reactive capability is similar to that of a conventional generator.

6.3.2 Voltage control & reactive power control in a wind farm with DFIGFigure 3 shows the basic control structure of a DFIG and Figure 2 shows the

configuration of a DFIG. As explained earlier, the rotor side convertercontroller can independently control the reactive power at the point ofcommon coupling. The specific reactive power requirement is set by the WFCthrough the rotor side converter controller.

6.3.3 Voltage control & reactive power control in a wind farm with FSFCThe full converter has the capability to precisely control reactive power andvoltage at the point of connection.

6.4 Remain connected during specified grid voltage characterist ics

at the point of connection for specif ied times (FRT capabili ty)With increased wind penetration, wind turbines have become a significantproportion of total generation and as a consequence, wind turbines in somecountries are now required to contribute to power system stability in the samemanner as conventional power plants do. One such contribution is to stayconnected and sustain generation through transient disturbances, which arecharacterised by voltage and frequency excursions. The requirements of thiscontribution extend to supporting system voltage and frequency, which leadsto active power and reactive power control and support.

The operating ranges for voltage and frequency set the limits within which theturbines must not be disconnected from the grid. Rather, turbines should be


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capable of sustained operation. The active and reactive power controlrequirements determine control capabilities under various fault conditions.

With the early turbines, because of their slow responsiveness to powercontrol, maintaining generator stability during grid faults had been a problem.

For this reason, it was considered appropriate for wind turbines to bedisconnected during a fault on the system. With high wind power penetrationlevels, solutions to this problem have been and are being developed.

Modern wind turbines are equipped with improved power control features (e.g.active pitch control and dynamic voltage support) that reduce the accelerationof these turbines during grid faults. However, in some situations theclearance times associated with the low voltage buses and/or feeders can belarger than the design targets for wind turbines. In such situations, the faultclearing times can be improved by implementing better protection techniques,but at a cost.

Power electronic devices are used in DFIG to enable them to ride-throughfaults, for example a crowbar circuit is used to protect the rotor side converterduring disturbances. Fault ride through capability of DFIG has beendemonstrated through simulations and tests.19 Information on theperformance of DFIG turbines in existing networks under transient and faultconditions is not readily available.

FSIG (induction) generators have no fault ride through capability. A wind farmwith FSIG connected to the grid with an AC connection without dynamic Varcompensation, cannot meet the fault ride through requirements as specified inGrid Codes of many jurisdictions.

6.5 Comply with specified standards on emission of flicker andharmonics

Power quality is affected by the nature of the energy conversion device,starting and stopping of wind turbines, and the variability of wind. Generallywind generation has a negative influence on power quality and there has beenextensive research effort in this area. What is presented here is only a highlevel overview.

6.5.1 Impact of the nature of energy conversion device on power quality

FSIGFSIG do not have power electronic interfaces and directly connected inductiongenerators are not expected to have any emission of harmonic currents.

DFIGWith DFIG, the stator is directly connected to the AC system, which may bethe grid or a point of connection to a medium voltage network that connects

19Refer to Technologies for integrating wind farms to the grid (Interim report) by AREVA T&DTechnology Centre (with University of Manchester) 2006.


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many other wind turbines in a wind farm. The stator delivers about 70% of thepower through a direct connection. The rotor is connected to a voltage sourceback-to-back converter and delivers about 30% of the power through theconverter. Hence a DFIG has a power electronic interface (roughly about30%) with the grid, which can potentially introduce harmonic currents to the

grid.

Full Scale Frequency ConverterThe full-scale converter is a power electronics interface with the grid and ithas a greater potential to introduce harmonic currents to the grid, incomparison to the DFIG.

6.5.2Impact of switching operations of wind turbines on power quality

FSIGTypical switching operations are the starting and stopping of turbines.

Induction machines require a large starting current (to establish the magneticfield) and consequently starting is associated with a drop in voltage dependingon the strength of the connection (fault level). Capacitors connected acrosseach wind turbine supply this current, thereby keeping the voltage dip to aminimum. With soft starting (gradually increasing current during starting), thevoltage variation has been greatly reduced.

Figure 10 shows the measured voltage variation during the starting of a FSIG.This FSIG has a soft starter and shunt capacitor banks to provide reactivecompensation. It can be seen that the initial voltage drop persists for about sixseconds and the voltage rises as power production commences.

The shutting down of a turbine can take place under conditions of low windspeeds (3-4m/s) or under high wind speeds (greater than 25m/s). Under lowwind speed conditions, the active power is almost zero and the stop is softand the impact on voltage at the point of connection is small. Under high windspeed conditions the active power is at its rated value and this power drops tozero within a short time. Clearly such a stop will have an adverse impact onthe voltage at the point of connection. However, all turbines in a wind farm donot shut down at the same time and therefore the impact is not as sharp asthe theoretical impact of all turbines stopping at the same time.


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Figure 10: Measured Voltage during start-up of a FSIG20

DFIG and FSFC (variable speed wind turbines)Generally, all variable speed wind turbines are equipped with pitch control.Pitch control facilitates variable speed, and thereby ensures smooth startingand stopping. When turbines are stopped under high wind speed conditions,pitch control gradually (over about six seconds) reduces the power to zero.Hence variable speed turbines do not contribute to flicker during switchingoperations.

6.5.3 Impact of the variability of wind on power quality

FSIGVariability of wind is seen as the availability of varying levels of wind energyfrom varying directions. This causes variations in the developed mechanicaltorque, which in turn lead to minor changes in the rotor speed of an FSIG.These minor changes in speed alter the developed electrical torque so that itmatches the new mechanical torque. It is the torque speed characteristic of aninduction generator that allows such near constant speed operation. However,the torque variations are translated as power output variations. These poweroutput variations are associated with varying amounts of reactive power drawfrom the grid. Such behaviour is a characteristic of an induction generator.

Reactive power draw variations cause changes to voltage at the point ofconnection, particularly when a wind turbine is connected through a circuit thathas a higher than normal impedance. These changes in voltages lead to highflicker values.

When each turbine blade passes in front of the turbine tower, there is areduction in the energy extracted from the wind, this occurring three timeseach revolution (for a three blade turbine). This torque dip is also reflected asa dip in electrical power output, which in turn leads to flicker. This is referredto as the 3p effect.

20Extracted from Emission of Wind turbines During Continuous Operation by A. Larsson, IEEETransactions on Energy Conversion 17(1) 114-118 2002 IEEE.


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DFIG & FSFCDFIG, being variable speed turbines, are designed to achieve maximumaerodynamic efficiency over a wide range of wind speeds.

It is not possible to extract all the kinetic energy in wind because this wouldmean that the wind speed after leaving the wind turbine would be zero. Thewind speed is only reduced by the wind turbine and consequently, only part ofthe energy is extracted. For highest energy extraction, the wind turbine rotorspeed needs to change with wind speed.

Figure 11 shows how the efficiency varies with the rotational speed of thewind turbine for a given wind speed. To achieve the highest energy extractionrates, the wind turbine needs to be operated on the outer periphery as shown.This clearly shows that the turbine speed needs to rise as wind speed risesand similarly the turbine speed has to fall when the wind speed falls in order to

achieve optimum operation. This is precisely what is achieved with a DFIG.The change in speed is achieved by controllers in a complex control structure.

Figure 11: Variation of efficiency with rotor speed

Variability of wind speed is translated into variability of rotor speeds after atime lag, which allows the extraction of increased levels of energy.

Since the wind turbine speed is allowed to change, some of the energyextracted is used in speeding up the turbine rotor when the wind speed hasincreased. When wind speed has dropped, energy in the turbine rotor isextracted to lower the turbine speed and such energy is delivered to thepower system. Hence, the variation of turbine speed acts as a buffer inmitigating the impact of wind variability. Thus, for DFIG machines, thevariations in power output are not as pronounced as for FSIG when identical

variations in wind speeds occur. The implication is that flicker caused by avariable speed turbine is lower than that caused by a fixed speed turbine.


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The deterioration of power quality arising out of power fluctuations is greatlyreduced when DFIG are used instead of FSIG. It is important to note that bothFSIG and DFIG respond with varying power outputs when wind speedvariations occur.

A FSFC behaves in a similar manner to a DFIG.

7.0 Summary

Table 1 in appendix 1 is a summary of the performance of different types ofwind farms against the revised Grid Code requirements implemented for windgenerators in many jurisdictions.


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Appendix 1

Table 1: Summary of Assessment of Wind Turbine Technology

Performance against the requiremRequirement

FSIG DFIG Deliver the stated output within aspecified voltage range under steadystate conditions

Possible with active stall control Good performance with pitch control

Participate in frequency control andcontribute to reserve requirements

Unable to meet the requirementMeets the requirement by a variety ofcontrols

Participate in voltage control andcontribute to reactive powerrequirements

Unable to meet the requirementMeets the requirement by a variety ofcontrols

Remain connected during specifiedgrid voltage characteristics at thepoint of connection for specifiedtimes (Fault ride-through capability)

Unable to meet the requirement

1. Old designs are unable to meetthis requirement

2. New units meet the requirementby a variety of controls

Comply with specified standards onemission of flicker and harmonics

1. Flicker caused by MW fluctuationsbased on wind speed variations.

2. Flicker caused by starting

1. Moderate potential to introduceharmonics by the partial scaleconverter

2. Flicker caused by wind speedvariations are greatly mitigated

3. Flicker caused by starting iseliminated


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Performance against the requirement

AC Connect ion to Grid(Switched Shunt Elements may be present)

AC CWith Dynami

Requirement

FSIG DFIG, FSFC FSIGDeliver the stated outputwithin a specified voltagerange under steady stateconditions

Possible with active stallcontrol

Good performance with pitchcontrol

Possible with active stcontrol

Participate in frequencycontrol and contribute toreserve requirements

Can be made compliantdepending on engineeringdesign

Meets the requirement by avariety of controls

Meets the requirementvariety of controls

Participate in voltage controland contribute to reactivepower requirements

Case specific Case specificMeets the requirementvariety of controls

Remain connected duringspecified grid voltage

characteristics at the point ofconnection for specifiedtimes (Fault ride-throughcapability)

Unable to meet therequirement Meets the requirement by avariety of controls Meets the requirementvariety of controls

Comply with specifiedstandards on emission offlicker and harmonics

Case specificMeets the requirement by avariety of controls

Meets the requirement

Table 2: Summary of Assessment of Wind Farm Performance with varying Turbine Technology and c

Note: Part of the above information is extracted from Technologies for integrating wind farms to the grid (Interim report) by AREVA T&D Technology


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References

1. "Wind Farm Models and Control Strategies", by Poul Sorensen, Anca D.Hansen, Florin Iov, Frede Blaabjerg and Martin H. Donovan. Riso NationalLaboratory Denmark, August 2005

2. Operation and Control of large wind farms Final Report, by PoulSorensen, Poul Erik Morthorst, Lars Henrik Nielsen, Florin Iov, FredeBlaabjerg, Henrik Aalborg Nielsen, Henrik Madsen and Martin H.Donovan. Riso National Laboratory Denmark, September 2005

3. Horns Rev offshore wind farm: its main controller and remote controlsystem, by Jesper R. Kristoffersen, Peter Christiansen. Wind EngineeringVolume 27 No 5, 2003

4. Technologies for integrating wind farms to the grid (Interim report), byAREVA T&D Technology Centre (with University of Manchester), 2006

5. Emission of Wind turbines During Continuous Operation, by A. Larsson,IEEE Transactions on Energy Conversion 17(1) 114-118 2002 IEEE

6. Wind Power in Power Systems, by Thomas Ackermann, 2005

7. Wind Energy Integration in New Zealand, by Ministry of EconomicDevelopment and Energy Efficiency and Conservation Authority,May 2005

8. Performance of doubly fed induction generator (DFIG) during networkfaults, by Anaya-Lara, O., Wu, X., Cartwright, P., Ekanayake, J.B.,Jenkins, N. Wind Engineering, Volume 29, Number 1, January 2005, pp.

49-66(18)9. The feasibility of Wind Power Production Forecasting in the Australian

Context by CSIRO Atmospheric Research, December 2003

10. Review of Impacts of high wind penetration in electricity networks, byGarrad Hassan Pacific, 2005.

options analysis appendix

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