Hindawi Publishing CorporationConference Papers in EnergyVolume 2013, Article ID 437674, 9 pageshttp://dx.doi.org/10.1155/2013/437674
Conference PaperGrid Code Requirements for Wind Power Integration in Europe
Constantinos Sourkounis and Pavlos Tourou
Institute for Power System Technology and Power Mechatronics, Ruhr-University Bochum, Germany
Correspondence should be addressed to Pavlos Tourou; [email protected]
Received 11 December 2012; Accepted 14 March 2013
Academic Editors: Y. Al-Assaf, P. Demokritou, and A. Poullikkas
This Conference Paper is based on a presentation given by Pavlos Tourou at “Power Options for the EasternMediterranean Region”held from 19 November 2012 to 21 November 2012 in Limassol, Cyprus.
As the capacity of wind power continues to increase globally, stricter requirements regarding grid connection of wind generatorsare introduced by system operators. The development of wind turbine technology is inevitably affected by the new grid codes, andwind power plants are expected to support the grid and provide ancillary services much like conventional power plants. The mostdemanding regulations are found in Europe where wind penetration levels are higher. This paper presents the main aspects ofcurrent grid code requirements for the integration of wind power in European countries and suggests performance characteristicsin order to satisfy the most demanding requirements.The dynamic behavior of wind turbines with doubly fed induction generatorsis investigated and a solution for low voltage ride through compliance is presented.
1. Introduction
WIND power installations continue to increase worldwide,with a total installed capacity of 238GW by the end of 2011,which meets about 3% of the global electricity demand, andan expected capacity of 500GW by 2015 [1, 2]. In Europe,wind power generation is expected to contribute to EU’s2020 targets for reduction of carbon dioxide emissions bymore than 30% and to supply at least 14%–16% of Europe’selectricity [3].The penetration of wind power in the electricalgrids increases steadily in many European countries, withthe highest percentage found in Denmark (28%), a countrythat has recently set the ambitious target to produce 50%of its electricity from wind turbines by the end of 2020. Inorder to maintain reliable grid performance with increasingwind penetration, transmission system operators (TSOs)update their grid connection codes with specific require-ments regarding the operation of wind generators and windfarms. In general, wind farms are expected to support thegrid and to provide ancillary services much like conventionalpower plants (e.g., active power control, frequency regulationand dynamic voltage control, and low voltage ride through(LVRT)).
The requirements vary between countries and their sever-ity usually depends on the wind power penetration level aswell as on the robustness of the national or regional powernetwork. Grid code requirements have been a drive for thedevelopment of wind turbine technology. Manufacturers inthe wind energy sector are constantly trying to improve windturbines, mainly in the area of wind turbine control andelectrical system design, in order to meet the new grid coderequirements. This can often imply higher costs, as moreadvancedpower electronic designs andmore complex controlsystems have to be utilized.
This paper discusses the influence of wind power onthe operation of existing power systems and presents themain aspects of the latest grid code requirements for theintegration of wind power in several European countries.The different requirements are analyzed and compared, andthe most demanding are highlighted. The ability of differentwind turbine technologies to meet these requirements isalso discussed. The low voltage ride through, one of themost important requirements for the dynamic performanceof wind turbines during network failures, is considered indetail. Simulation studies are conducted to study the behav-ior of wind turbines equipped with doubly-fed induction
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generators (DFIGs) during severe voltage dips caused bygrid faults. Control strategies and hardware that improve theLVRT capability of the wind turbines during the fault arepresented.
2. Frequency and Voltage Operating Range
2.1. Importance for Power System Operation. The powersystem frequency is an indication of the balance betweenpower generation and load consumption. Any deviation fromthe planned production or consumption moves the systemfrequency away from its nominal value. In the case of asudden increase in the load, the frequency of the producedvoltage decreases and it is restored back to the nominal whenpower production is increased by primary control. Under-frequency can also occur as a result of an unexpected lossof generation units. On the other hand, over-frequency canoccur with a sudden decrease in load or an unexpectedincrease in generation (e.g., wind gusts) [4].
Grid codes require that wind farms must be capableof operating continuously within the voltage and frequencyvariation limits encountered in normal operating conditions.In addition, they should remain in operation in case offrequency deviations outside the normal operating limits fora specified time and in some cases with a specific activepower output. By having the ability to remain connected tothe grid for a wider frequency range, wind farms support thesystem during abnormal operating conditions and allow fora fast system frequency restoration. Wind turbines must bedesigned appropriately, as abnormal frequencies can overheatgenerator windings, degrade insulationmaterial, and damagepower electronic devices.
2.2. Grid Code Requirements. The frequency ranges requiredby the various grid codes are presented in Figure 1. In thegreen frequency ranges, the wind turbines must remainconnected and operate continuously at full power output. Inthe white ranges, they must remain connected at least for theminimum time specified, usually at a lower power output, inorder to support the grid during frequency restoration. Inmany cases the active power reduction must be controlledproportionally with the frequency deviation from the nom-inal. In the extreme grey frequency ranges, wind turbinesare allowed to disconnect from the grid. The active powerrequirements at different frequencies, if specified in the gridcode, are also shown in Figure 1.
2.3. Comparison and Capability to Fulfill All Requirements.Wind turbines are now required to remain connected in thecase of large frequency deviations, with the most extremefrequencies being 47Hz and 53Hz. As the frequency devia-tion increases, the minimum connection time and minimumactive power conditions are relaxed. In the case of under-frequencies wind turbines must remain connected to thegrid for longer periods before they are allowed to trip. Thelargest frequency ranges of mandatory continuous operation,in which the wind turbines must never trip, are seen inthe UK, Romania (47.5Hz–52Hz), and Italy (47.5Hz–51.5).
Large frequency ranges are expected in isolated systems withweak interconnections where the system stability is morevulnerable to disturbances compared to large interconnectedsystems (e.g., UCTE).
The most extreme requirements, taking also into accountthe voltage range level at which the frequency range isrequired, were combined to produce the frequency-voltageprofile shown in Figure 2. If a wind turbine has the capabilityto operate within the area shown in Figure 2, then it canmeetall the different requirements specified in the European gridcodes.
3. Active Power Control
3.1. Importance for Power System Operation. Active powercontrol is the ability of wind power plants to regulate theiractive power output to a defined level and at a defined ramprate (e.g., in the case of active power curtailment requests byTSOs). These requirements aim to ensure a stable frequencyin the system, to prevent overloading of transmission linesand to minimize the effect of the dynamic operation of windturbines on the grid (e.g., during extreme wind conditions, atstartup/shutdown).
The ability of wind turbines to control their active poweris also important for transient stability during faults. If thepower can be controlled effectively as soon as a fault occurs,the turbine can be prevented from overspeeding. Hence, thereactive power needed for remagnetization of the generatorsis less after the fault is cleared, which helps reestablishingthe grid voltage. Often, active power generation is reducedtemporarily by the control system during the low voltageperiod [4]. This allows the increase of reactive power gener-ation without exceeding the rated current of the converters.After the fault period, a fast return to normal active powergeneration is essential to ensure the power balance andstability of the grid.
3.2. Grid Code Requirements. Most grid codes demand activepower curtailment upon request from the network operator,at a specified set-point. This is done either by disconnectingwind turbines or by controlling the pitch angle of the blades inorder to limit the power extracted from the wind. Some gridcodes also impose limitations on the rate of change of activepower, with maximum and minimum ramp-up and ramp-down rates.These limitations aim to suppress large frequencyfluctuations caused by extreme wind conditions and to avoidlarge voltage steps and in-rush currents during wind farmstartup and shutdown.
3.3. Comparison and Capability to Fulfill All Requirements.The most demanding requirements for active power controlare presented in Table 1.
Under normal conditions many grid codes require aramp-down rate of <10% of 𝑃
𝑁per minute. The most
demanding requirements of both ramp-up and ramp-downrates under normal conditions are in Denmark where windfarms must always be able to vary their active power ramp-rates in the range 10%–100% of 𝑃
𝑁per minute upon request.
Conference Papers in Energy 3
England and Wales Scotland Romania Denmark
Disconnection
2% active powerreduction per Hz
Max 5% linearactive power
reduction
20 s
Max 5% linearactive power
reduction
20 s 20 sDisconnection
within 1 s
Continuous with 20%max power losses
Continuous-linear
proportional to
20 s, 80% power
Freq
uenc
y
53.553
52.552
51.551
50.550
49.549
48.548
47.547
46.546
Freq
uenc
y
53.553
52.552
51.551
50.550
49.549
48.548
47.547
46.546
Germany
Fast automaticdisconnection
Fast automaticdisconnection
30min
30min
20min10min
30min at reducedpower output
Nominal power
Nominal power
SpainFrance Cyprus
Time byagreement
Time byagreement
20 s, power set by TSO15min, power set by TSO
60min, 90% power
5hrs, 90% power3min, 90% power3min, 85% power
20 s, 80% power
3 s5 s
60min
60min
30min <5% powerreduction
power reduction <20%
frequency deviation
3min, power reductionpossible at any level
Continuous
Full power
15min 60%–100% of power
IrelandNordic
5hrs 90%–100% power30min 90%–100% power3min, 80%–100% power
20 s, 80%–100% power
Italy Poland SwedenBelgium
Reduced power output
Disconnection within300ms
30min, 90% power20min, 85% power10min, 20% power
60min
60min
30min, small P reduction
30min, linear reduction from100% to 85%
within 1 s
Figure 1: Required frequency range of operation in different grid codes.
Table 1: Active power requirements in different grid codes.
Condition Active power ramp-rateRamp-up rate range 10–100% of 𝑃
𝑁/min
Ramp-down rate range 10–100% of 𝑃𝑁/min
Ramp-up rate after fault >90% of 𝑃av/second
The fastest ramp-up rate of active power output after a faultback to the prefault value is that of Ireland and the UK (>90%of 𝑃av/s), where due to the isolated nature of their electricalgrids the wind farms must provide fast active power supportto assist in the grid voltage recovery.
In general, the most demanding requirements regardingactive power control are found in the Danish grid code wherewind farms must be equipped with and apply active powercontrol functions with set-points and ramp-rates set by thesystem operator as shown in Figure 3. The “Delta” controlfunction is particularly demanding, as the active poweroutput of wind farms with capacity greater than 25MWmustbe constrained to a required constant value in proportion to
the available active power. This reserve power can be used infast grid frequency control, similar to the spinning reservesin conventional power plants.
4. Reactive Power Control
4.1. Importance for Power System Operation. The voltagelevels in a power systemmust bemaintained constant (withina very narrow range) because equipment of the utility andconsumers are designed to operate at specific voltage levels.Recent adaptations to national grid codes demand fromwindfarms to contribute to voltage regulation in the system, asconventional power plants do. They must have the ability togenerate or absorb reactive power in order to influence thevoltage level at the point of common coupling (PCC). Undernormal operation the voltage at the PCC can be increased byinjecting reactive power to the grid and can be decreased byabsorbing reacting power. Wind farms should have reactivepower capabilities in order to support the PCC voltage duringvoltage fluctuations and to assist in balancing the reactivepower demand in the grid.
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53
52
51
50
49
48
47
8590 95
100105110
115115.8
𝑈/𝑈𝑁 (%)
Freq
uenc
y (H
z)
50.349.7
>30 minno 𝑃
restriction
Continuous
operation
𝑃 = 𝑃𝑁
Continuousoperation
𝑃𝑁
for >20 s
1ho
ur𝑃=90
%
P > 80% PN
P > 80%
Figure 2: Most extreme requirements for wind farm operation atdeviations from nominal grid voltage and frequency.
Activ
e pow
er
Possible active power
Activation of active powerproduction constraint
Activation of deltaproduction constraint
Activation of gradientproduction constraint
Activation of delta anddeactivation
gradient production constraint
Deactivation of absoluteproduction constraint
Spinning
Time
reserve
Figure 3: Active power control functions required in Denmark [4].
4.2. Grid Code Requirements. The reactive power require-ments are usually expressed with 𝑃-𝑄 diagrams (availableactive power versus available reactive power). The requiredamount of reactive power compensation varies with differentpower system configurations.The effect of injected/absorbedreactive power on the PCC voltage level depends on thegrid impedance, grid short-circuit capacity, as well as on anyconnected load near the point of connection [4].The differentreactive power requirements are summarized in Figure 4.
4.3. Comparison and Capability to Fulfill All Requirements.The widest ranges are found in Germany where one of thethree variants must be chosen in agreement with the gridoperator. In order to fulfill all grid code requirements in termsof reactive power capability, a wind turbine or farm mustoperate in thewhole area shown in the𝑃/𝑄 profile of Figure 5.At full active power the wind turbine must be capable ofsupplying reactive power in the range 0.41 p.u. inductive
Figure 4: Reactive power requirements of various grid codes.
1.0
0.2
0
Powerfactor = 0.925
Powerfactor = 0.44
Powerfactor = 0.9
Powerfactor = 0.38
0.41 0.1 0 0.1 0.48Q/PN (p.u.)
P/P
N(p
.u.)
Inductive Capacitive
Figure 5: 𝑃-𝑄 profile to satisfy all grid codes.
to 0.48 p.u. capacitive which corresponds to a power factorrange 0.925 lagging to 0.9 leading. This reactive power rangemust be maintained with active power down to 0.2 p.u. andfor lower active power output the reactive power can bedecreased proportionally.
5. Low Voltage Ride Through (LVRT)
5.1. Importance for Power System Operation. The low voltageride through is the most important requirement regardingwind farm operation that has been recently introduced inthe grid codes. It is vital for a stable and reliable operationof power supply networks, especially in regions with highpenetration of wind power generation. Faults in the gridcan cause large voltage dips across wide regions and somegeneration units can be lost as a consequence. In the past,during grid disturbances and low grid voltages the wind
Ireland/RomaniaItalyPolandCyprusUKTurkeyFinland, Sweden
U/U
N(%
)
France 400kVFrance 63–225 kV
Figure 6: LVRT requirements in various grid codes.
turbines and farms were allowed to disconnect from the grid.But if there is a large amount of wind generation in thenetwork, the simultaneous disconnection of wind generatingunits and farms can cause larger voltage depression and even-tually collapse of voltage in the affected region. Furthermore,the additional loss of power generation as a result of thedisconnection can cause a greater generation/consumptionimbalance and thus drop in the system frequency in thewiderregion [5].
5.2. Grid Code Requirements. Recent grid codes require windfarms to remain connected and support the grid during andafter a fault. They must withstand voltage dips of a certainpercentage of the nominal voltage for the specified timedurations, as shown in the LVRT voltage-time profiles ofFigure 6. Disconnection is not allowed above the borderline.Below the borderline wind turbines are not required tocontribute to the grid and they can be tripped by circuitbreakers.
Furthermore, in some countries voltage control isrequired during the low voltage faults as shown in Figure 7.Wind farms must supply reactive current to the grid basedon the depth of the voltage dip, in order to support thelocal voltage and thus limit the geographical low voltage areacaused by the grid fault. During this low voltage period theactive current can be decreased and priority should be givento the reactive current in order to back up the grid voltage.The German grid code asks for a constant of proportionality𝑘 between the voltage deviation and the required reactivecurrent that can be set in the range 𝑘 = 0–10 after an
agreement with the network operator, with a default value𝑘 = 2.
5.3. Comparison and Capability to Fulfill All Requirements.The most severe requirements for LVRT were combined tocreate the LVRT profile shown in Figure 8. Wind turbinesand farms must remain connected to the grid above thesolid line. Below the solid line and until 1.5 seconds after thestart of the fault, wind turbines can disconnect only if theycan resynchronize with the grid within 2 seconds. If voltageremains below 40% of the nominal after 1.5 seconds, windturbine are allowed to disconnect unconditionally.
Regarding the contribution of the wind farms to gridvoltage support, theGerman grid code is themost demandingas it can require rated reactive current at 20% voltage decreasewith very fast step response characteristics (rise time = 30ms,transient time = 60ms). After fault clearance, the steepestincrease of active power is found in the UK, according towhich the active powermust be increased to the prefault valuewith a rate equal to 1 p.u. per second. In the next sections,the response of DFIG-based wind turbines in the case of lowvoltage grid faults is analyzed, and their capability tomeet theLVRT requirements is examined.
6. LVRT Capability of DFIG-BasedWind Turbines with Doubly FedInduction Generators
Fixed-speed wind turbines with squirrel cage inductiongenerators have a very limited LVRT capability. They are
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Germany k = 2, rise time = 30ms, settling time = 60msGermany k = 10, rise time = 30ms, settling time = 60msDenmark, settling time = 100msSpain, settling time = 150ms
1
0.8
0.6
0.4
0.2
00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Reac
tive c
urre
nt d
eliv
ered
to th
e grid
(p.u
.)
Voltage decrease (dU/Unom ) at the PCC (p.u.)
Figure 7: Reactive current requirements with grid voltage decrease.
U/U
N(%
)
100
80
39.5
156.6
0
0 250 375 625 1500 3000Time (ms)
Disconnectionis not permitted
Disconnectionpossible
Short-time disconnection allowed
Figure 8: Fault ride through profile to satisfy all grid codes.
constantly absorbing reactive power from the grid for theirmagnetization. During low voltage faults they tend to over-speed and they can become unstable, suppressing further thegrid voltage. The LVRT capability of wind farms with fixed-speed turbines can be improved by retrofitting reactive powercompensation equipment such as static Var compensators(SVCs) and static synchronous compensators (STACOMs)[6]. Variable-speed wind turbines with full rated converterscan ride through the faults without significant problems andthey can deliver reactive power for voltage support.TheLVRTcapability of variable-speed wind turbines equipped withdoubly-fed induction generators is studied in the followingsections.
6.1. Doubly Fed Induction Generators. The doubly-fed induc-tion generators (DFIGs) are currently the most widely usedtype of electrical generators for wind turbine systems inthe Megawatt range [7]. The DFIG technology has provento be an efficient and cost-effective solution for variable
speed wind turbines. An important disadvantage of this typeof generators is its behavior during significant voltage dipsat their stator terminals. Before the introduction of LVRTrequirements in the national grid codes, wind farms equippedwith DFIG wind turbines were allowed to disconnect fromthe grid in the case of significant grid voltage deviations.In order for the wind turbines to remain connected andsupport the grid during low voltage periods, enhancementsare required in the hardware and control of these windturbines.
6.2. Description of a DFIG-Based Wind Turbine System.The wind rotor is in most cases connected to the rotorshaft of the generator through a gearbox that increases therotational speed at the generator side as shown in Figure 9.The stator windings are directly connected to the grid. Therotor windings are connected to the grid through two voltagesource converters connected back-to-back. This converterconfiguration decouples the rotor electrical frequency fromthe grid frequency, and as a result the rotor can havea variable speed, normally in a range ±30% around thesynchronous speed.Variable-speedwind turbines can harvestmuch more energy compared to fixed-speed wind turbinesbecause depending on the wind speed, they can operate atthe optimum rotational speed at which the aerodynamicefficiency of the wind rotor is maximum [8].
During normal operation the stator power flows fromthe stator to the grid, while the flow of rotor power overthe DC-link is bidirectional; current flows from the grid tothe rotor at undersynchronous speeds (nr < ns) and in theopposite direction at oversynchronous speeds (nr > ns).The maximum rotor power depends on the slip, and sincethe rotational speed range is limited, the rotor power is onlya fraction of the stator power. This allows significant costsavings as the power electronic converters can be partiallyrated to only 25%–30% of the total power of the generator.Furthermore, the power efficiency is higher because thereare lower switching and conduction losses in the powerelectronics due to the partial rating of the back-to-backconverter.
6.3. Control System. The operation management controls therotational speed of the wind turbine in order to capturemaximum wind power [8]. It provides the active powerreference to the rotor side converter (RSC) and the pitch angleto the pitch actuator system. Pitch control is used to limitthe power output of the wind rotor in the case of very highwind speeds, by decreasing the aerodynamic efficiency of therotor blades. Additionally the operation management limitsthe active power reference in the case of grid faults and afterinstructions from the network operator.
TheRSC controller is responsible for providing decoupledcontrol of the active and reactive power at the stator. Thetask of the grid side converter (GSC) controller is to keepthe DC-link voltage constant, irrespective of the rotor powerflow direction while maintaining unity power factor at theGSC terminals. A vector control approach is adopted for theRSC and GSC, and the resulting reference voltages are fed
Conference Papers in Energy 7
Wind
DFIG
RSC GSC
Wind rotor
Gearbox
Filter
TSO
management
RSCcontrol
Gridtransformer
GSCcontrol
Wind speedPitch angle
Control system
Operation
IGSCVRSC∗ VGSC∗VDC
VDC∗QS∗PS∗
∼3
=3∼
=
nr
nr
QGSC∗ Igrid
VgridQgrid
∗Pgrid
∗
IRSC
Figure 9: DFIG-based wind turbine system.
to space vector modulators to create the switching signalfor the respective converters. By applying 3-phase voltageswith the appropriate amplitude, phase, and frequency, theflow of currents and consequently the active and reactivepower exchange with the grid at both the stator and the GSCterminals can be controlled.
7. Investigations on the DFIG Response toSevere Grid Faults
7.1. Response to Severe Voltage Dips. A detailed model ofthe DFIG-based wind turbine system was developed inMatlab/Simulink in order to investigate its behavior in thecase of severe low voltage faults. In this paper, theworst case isconsidered: an instantaneous voltage drop down to zero voltsfor a time duration of 250ms while the generator operatesat maximum rotational speed and power output. The faultoccurs at the grid-side of the transformer, and the voltagedrop experienced by the wind turbine terminals is shown inFigure 10.
At the start of the fault very high transient currents aredeveloped at the rotor and the RSC reaching 3 times therated value of the converter. Furthermore, the voltage onthe DC-link rises to 2.5 p.u. These effects arise due to theinstantaneous collapse of the stator voltage. The stator fluxspace vector, which before the fault rotates synchronouslywith a magnitude proportional to the stator voltage, stopsrotating and its magnitude decays exponentially with time.Similarly, DC stator currents with a decayingmagnitude startto flow in the stator. Due to the electromagnetic couplingbetween the stator and the rotor circuits, the DC stator fluxand currents induce a high frequency component in therotor voltages, that is, superimposed on the normal voltagethat has a low slip frequency. The magnitude of the inducedrotor voltage is higher at the beginning of the fault and itcan be greater than the stator voltage if the rotor speed at
the time of the fault is oversynchronous [9]. The RSC, dueto its partial rating, cannot produce such high voltages tomatch the induced rotor voltages, the control of current islost, and very high currents result in the rotor and the RSC.These high currents flow into theDC-link, increasing theDC-link voltage. The GSC cannot balance the DC-link voltageby dissipating this energy to the grid, because its power islimited due to the low residual voltage and its rated current.As result theDC-link voltage risesmuchhigher than the ratedvoltage of the capacitor. Similarly, high rotor currents are alsoinduced at the instantaneous return of the grid voltage. Inthis case the GSC can operate at its maximum power output,balancing the DC-link voltage faster.
These very high currents and the significant DC-linkovervoltage are unacceptable because they can damage theDC-link capacitor and the power electronic switches of theRSC. In order to protect themselves against these effectsduring severe grid voltage faults, the DFIG-based windturbines must disconnect from the grid, violating the LVRTgrid code regulations. Appropriate countermeasures must beadopted in order to protect the sensitive devices of the windturbine system and to meet the LVRT requirements.
7.2. Response to Voltage Dips with a LVRT-Enhanced System.In order to mitigate the effects of severe grid faults on theDFIG, the rating of the power electronic converters can beincreased, so that it is possible to control the high transientcurrents at the beginning and end of the voltage dip. Thissolution is undesirable, as it eliminates the partial-ratingadvantage of the DFIG concept, increasing significantly theoverall cost of the system. Amore cost-effective solution is touse an active crowbar circuit between the rotor and the RSCas shown in Figure 11 [10]. This consists of a full-wave bridgerectifier, a power resistor, and an IGBT switch.During normaloperation the switch is open. The switch can be activatedon detection of rotor overcurrents or DC-link overvoltage in
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0 0.1 0.2 0.3 0.4 0.5 0.6Time (s)
024 Reactive current (p.u.)
0
1
2 Active power (green), reactive power (p.u.)0
0.51
1.52
2.53
DC-link voltage (p.u.)
024 RSC currents (p.u.)
0
1PCC voltage (p.u.)
−4−2
−1
−4−2
−1
Figure 10: Response of a DFIG-based wind turbine during a 250msgrid voltage dip.
order to redirect the rotor currents in the crowbar circuit,where the energy is dissipated in the resistor.
The behavior of the DFIG wind turbine equipped with acrowbar during a severe voltage dip is shown in Figure 12.Thecrowbar is activated with rotor overcurrent at the beginningand end of the voltage dip, and the high current peaks aresuccessfully redirected away from the RSC. The DC-linkvoltage is limited below 1.3 p.u. during the fault. The crowbarremains connected to the rotor for about 50ms to allow forthe decay of the initial high transient currents. After thisperiod the crowbar is disconnected and the RSC ramps upthe reactive current to the rated value. Amoderate increase inthe local grid voltage is observed during the reactive currentinjection. During the fault the active power reference is keptto zero. The active power is increased back to the prefaultvalue and the reactive power back to zero about 130ms afterclearance of the fault. In the case of longer voltage dipsat higher residual voltage, sufficient active power must bedelivered to the grid in combination with blade pitch controlin order to prevent overspeeding of the wind rotor.
DC-link
DFIGCrowbar
Rotor-sideconverter
Rcb
Figure 11: DFIG with an active crowbar circuit.
Reactive current (p.u.)
Active power (green), reactive power (p.u.)
DC-link voltage (p.u.)
RSC currents (p.u.)
PCC voltage (p.u.)
0123 Crowbar currents (p.u.)
0 0.1 0.2 0.3 0.4 0.5 0.6Time (s)
12
012
00.5
11.5
22.5
3
024
0
1
−4−2
−1
−2−1
−1
0
−3−2−1
Figure 12: Response of a DFIG-based wind turbine with an activecrowbar during a 250ms grid voltage dip.
Conference Papers in Energy 9
7.3. Fulfilling the LVRT Requirements of Grid Codes. TheDFIG wind turbines, enhanced with a crowbar and a ded-icated control during the fault, can meet all the LVRTrequirements found in the European grid codes. Simulationstudies have shown that the wind turbines can “ride through”voltage dips down to zero voltage for 250ms. During thefault, they can provide voltage support by supplying ratedreactive current to the grid and they can restore their activepower very fast back to the prefault active power. In general,the crowbar solution can protect sensitive components ofdoubly fed induction generators andLVRTcompliance can beachieved without the need for oversizing the expensive powerelectronic converters.
8. Conclusion
The main aspects of current grid code requirements regard-ing wind power integration in European grids have beenpresented. Performance characteristics in order to satisfy allthe grid codes examined have also been suggested. As windpenetration increases, wind turbines and wind farms areexpected to be more tolerant to abnormal conditions andto contribute to grid stability during normal operation, aswell as during and after grid faults. The behavior of DFIG-based wind turbines during grid faults was investigated anda low cost solution for fulfilling the LVRT requirements waspresented. Complying with grid code regulations is vital forgrid stability and the improved wind turbine performancewill allow for larger wind power penetration in electricalpower grids.
References
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