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`Professional Network.
`
`1
`
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`
`SGRE EX1020.0001
`SGRE v. GE, IPR2022-01279
`
`

`

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`
`SGRE EX1020.0002
`
`

`

`"I
`
`75
`
`FAULT CURRENT CONTRIBUTION OF DFIG
`WIND TURBINES
`
`0 . Anaya-Lara and N. Jenkins
`
`The Manchester Centre for Electrical Energy
`University of Manchester, UK
`email:o.anaya-lara@manchester.ac .uk
`Tel: +44(0)1612004682; Fax:+44(0)1612004820
`
`Keywords: DFIG, voltage control, fault ride through.
`
`Abstract
`
`reduce the DFIG fault current contribution. With this control
`strategy the requirements imposed on the rotor-side converter
`for voltage or power factor control are reduced and hence the
`fault currents in the rotor circuits. Results from simulations
`conducted in PSCAD/EMTDC are presented and discussed
`using typical turbine data for wind farms.
`
`Doubly Fed Induction Generators (DFIGs) are widely used in
`modern high-power wind turbines. As a result of the
`anticipated increase in wind energy penetration, current Grid
`Code proposals require wind farms to have fault ride through..-r2 DFIG based wind turbine
`capabilities to remain connected to the sy~em. This ·paper
`presents a control solution where the DFIG voltage is
`controlled using both rotor- and grid-side converters through
`a coordinated control scheme. It is shown that by adopting
`this scheme the DFIG fault current contribution may be
`reduced enabling the generator to ride through faults with
`deep stator voltage sags.
`
`DFIG wind turbines use a wound rotor induction generator,
`where the rotor winding is fed through a back-to-back
`variable frequency PWM converter as shown in Fig. 1.
`Voltage limits and an over-current "crowbar" circuit protect
`the machine and converters [6].
`
`1 Introduction
`
`It is expected that 4 to 6 GW of wind energy will be added to
`the UK power network to meet 20 IO Government targets on
`renewable energy [1]. Many of the new wind farms will be
`based on Doubly Fed Induction Generator (DFIG) wind
`turbines due to their controllability and superior post-fault
`performance over fixed speed generators [2][3]. This control
`capability is provided by the DFIG back-to-back converters
`where the rotor-side converter is typically used to control
`torque, terminal voltage or power factor, whereas the grid(cid:173)
`side converter is used to control de link voltage. As a result of
`the anticipated increase in wind energy penetration, current
`Grid Code proposals for wind farm connection are more
`demanding on the performance of the wind farm. One of the
`most challenging requirements is that wind farms have to
`provide fault ride through capability and remairi connected
`during network faults [4][5]. During the fault the DFTG
`voltage is severely reduced and high current induced on the
`rotor-side converter if control and protective measures are not
`adopted. Different solutions have been proposed where the
`typical approach is to reinforce the hardware of the rotor
`converter. A practice adopted by a number of manufacturers
`is to use rotor crowbar protection, while other solutions
`propose the use of the crowbar with de damping resistors.
`that relies on
`This paper presents a control strategy
`controlling both rotor- and grid-side converters for the
`provision of DFIG voltage and reactive power control to
`
`Windmill
`
`~. ±JQ,
`
`P,,.t ±JQ,iet
`
`Power
`Network
`
`C2n
`Jt-4
`
`PWM Converters
`
`Fig. I. Typical configuration of a DFIG wind turbine.
`
`The converter system enables the two-way transfer of power.
`Converter 2 (C2) is fed from the generator stator terminals via
`a reactive link and provides a DC supply to Converter 1 (Cl)
`that produces a variable frequency three-phase supply to the
`generator rotor via slip rings. Manipulation of the rotor
`voltage permits control of the generator operating conditions.
`
`3 Control Scheme development
`
`The favoured way of representing a DFIG -tor the purpose of
`analysis, simulation and control is in terms of direct and
`quadrature axes ( dq axes), which form a reference frame that
`rotates
`synchronously with
`the
`stator
`flux vector
`
`SGRE EX1020.0003
`
`

`

`( a>s = 21r fs) [7][8]. In terms of this form of representation,
`the q-axis was assumed to be 90° ahead of the d-axis in the
`direction of rotation and the d-axis was chosen such that it
`coincides with the maximum of the stator flux. Therefore, Vqs
`
`equals the terminal voltage and v ds is equal to zero.
`
`Torque am/ voltage control - rotor-side converter
`
`Fig. 2 illustrates a typical strategy to control the DFIG torque
`the rotor-side
`terminal voltage/power factor using
`and
`converter. In this strategy the torque control loop modifies the
`electrqmechanical torque of the generator to respond to
`variations
`in
`the rotor speeds. Given a
`rotor speed
`measurement, a look-up table representing the wind turbine
`torque-speed characteristic is used to obtain a reference
`torque T,,p , which after some manipulation is imposed upon
`the DFIG rotor as [9]:
`
`considering the total grid (stator) side reactive power given by
`[9]:
`
`(3)
`
`the reference frame adopted the following
`Considering
`rerationship was obtained between the total stator reactive
`power and the rotor current i,. :
`
`{4)
`
`is then subdivided into- a
`The rotor current component idr
`generator magnetising component, and a component for
`controlling generator terminal voltage by means of the control
`gain Kvc . The addition of these two current components
`provides the reference for the rotor cm-rent in the d-axis idr,-ef ,
`
`T=
`
`e
`
`(1)
`
`which is then compared with the actual measured current id,.
`to generate an error signal.
`
`i,
`
`v,. t i,.
`r::::l..,__ Cl
`c:J--,- J~
`
`abc
`
`Voltage/PF control
`
`V.u4 ~ -J: K vc
`
`-
`
`+
`
`V,
`
`+
`+
`
`-
`
`- ~
`I
`
`· (J)s 'Lm
`
`Magnetising current control
`
`Fig. 2. DFIG Torque and Vpltage/PF control using rotor-side converter CI.
`
`From (1) the reference for the d-axis current component to
`obtained the desired optimal torque is given by:
`
`..,.
`w. (Is +I,,,) _
`1qr_ref =
`-
`,- ,
`·'I',µ
`L,,, V,
`
`(2)
`
`Although
`
`iq,
`
`imposes the effect of torque control, the
`
`converter Cl is a controlled voltage source, therefore the
`required rotor voltage vq, to operate at the reference torque
`
`set point is then obtained through a PI controller.
`
`In a similar way to the torque control loop, the error signal is
`processed by a PI controller, which generates the required
`rotor voltage in the d-axis vd, . This control strategy is widely
`used in the open literature, however previous investigations
`have shown that the DFIG fault current contribution is very
`sensitive to voltage control loop gain Kvc settings and tuning
`[10]. As shown in this paper this situation can be avoided by
`using both rotor- and grid side converter to exercise DFIG
`terminal voltage and reactive power control.
`
`Torque and voltage control - rotor- and grid-side converters
`
`The voltage control loop is developed by maintaining the
`reference frame with the stator resistance neglected, and
`
`Fig. 3 illustrates the block diagram of the strategy proposed to
`control DFIG torque and voltage/PF using both converters.
`
`,.
`
`l ~.
`
`I•
`
`.l ,,
`
`i ,·
`
`SGRE EX1020.0004
`
`

`

`77
`
`P, ±JQ,
`~
`v,
`~
`i, ~
`P,,., ± JQ,,.,
`
`t ig
`
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`
`Er CJ~
`
`Vdc
`
`PWM
`
`cj~ vg
`
`~
`±Pg ±}Qg
`
`·I
`
`Fig. 3. DFIG Torque and Voltage/PF control using both rotor- and grid-side converters.
`
`In this implementation the rotor-side converter (Cl) controls
`the torque Te, and the stator reactive power Q, required to
`maintain the terminal voltage V, at the reference value Vsref .
`
`This voltage control loop also incorporates the signal Q111ag,
`which allows for additional control over the stator reactive
`power. According to the dq reference frame selected to
`control the rotor-side converter, the rotor voltage component
`in the q-axis vq,.
`is used to control the torque, and the
`
`.
`
`'
`t
`
`component in the d-axis vd,.
`is used to control the stator
`reactive power and thus the terminal voltage.
`
`I
`
`f
`
`I
`
`The grid-side converter (C2) is used to keep the DC-link
`voltage v de constant, and also to provide the stator reactive
`power Q, at the desired operating power factor. As shown in
`_Fig. 3 the reactive power Qg exchanged by the grid-side
`converter is controlled as a function of both Q, for terminal
`voltage control, and Qnet for power factor control. In order to
`achieve these control objectives a vector-control approach is
`used with a dq reference frame oriented along the stator
`voltage vector position, enabling independent control of the
`active and reactive power exchanged by the grid-side .
`converter. The PWM converter is current regulated, with the
`d-axis current idg used to maintain the DC-link voltage
`
`constant regardless of the magnitude and direction of the rotor
`power. The q-axis current component iqg is used to control
`the reactive power exchange. Standard PI controllers are used
`to generate the appropriate voltage commands vqgc and
`
`vdgc to control the grid-side converter.
`
`4 Simulation studies in PSCAD/EMTDC
`
`To investigate the :_dynamic pe1formance of the DFIG wind
`turbine under system fault conditions and to determine the
`machine fault current contribution with the control strategies
`described in this paper, the network shown in Fig. 4 was
`implemented and simulated in PSCAD/EMTDC. The DFIG is
`rated at 0.69kV/2 MW. The converters CI and C2 were
`modelled as 2-level Voltage Source Converter (VSC) and
`controlled using space-vector PWM. A switching frequency
`fs = 2250 Hz was used for both converters. The rated DC
`voltage was set at 1100V with a DC capacitance of 16mF.
`The grid-side converter is connected to the grid through an
`inductance of Lg = 0.3 ml-I. ·
`
`In the following studies a three-phase fault is applied at the
`low-voltage side terminals of the DFIG coupling transformer
`as shown in Fig. 4. The fault impedance Z 1 -is adjusted to
`control the voltage dip magnitude at t~e terminal of the DFIG.
`
`SGRE EX1020.0005
`
`

`

`For the results illustrated in this section Z I is set to generate
`a stator voltage drop of 100% of its nominal value during
`fau lt. The connection transformer was rated at 2.5 MVA with
`a leakage reactance of 5 %. An infinite busbar was used to
`represent a very large power system. The crowbar protection
`was de.activated to allow the DFIG to remain in operation
`dw-ing the fault in order to observe the wave shape and
`maximum value of the DFIG fault current contribution.
`
`690V/2MW
`
`690V/l l kV
`2.5MVA
`
`Vs ......
`
`i,
`
`igt
`
`0.3ml-l
`
`CJ~
`
`16ml'
`
`Three-phase
`Fault
`
`J~
`
`when Kvc was small the DFIG was able to recover stability
`after the fault. The voltage tracking however was less
`accurate than before when a larger value of K vc was used.
`
`+25
`
`+17
`
`+9
`
`•1
`
`-
`
`-7
`· 1f9.9
`
`Isa lk ,,
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`-
`-
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`- I
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`-
`-~ __ ,_
`A ~
`-- · . - -
`-
`- \ -~- \- -·-·i - u ~
`
`~.- L I-
`
`~-
`- -
`.
`
`19.95
`
`20
`
`,0,05
`
`20.1
`
`20.15
`
`20.2
`
`lnfinite
`bus
`
`+J l----1----11 ,-,;--+---xet--..,._.
`
`.Jt:=::.t::.~-:.-i-=..~t-.-+,..,._ -_ -_ --+_,_-r-,_-_-_-_-+ .... -j:--1
`
`-9 t - -- t - - -+ - - - 1,SM-+---+----+-->,
`
`Fig. 6. Phase a stator and rotor currents using rotor-side converter
`control. Voltage control loop gain set at Kvc = 10 .
`(isa =19.96kA, ira =ll.93kA) .
`
`PWM Converters
`Fig. 4. Network used for the studies considered.
`
`The simulation studies were conducted with the DFIG wind
`turbine operating in steady state with a constant mechanical
`torque of T,n = 0.8 PU at unity power factor. The three-phase
`fault is applied at t = 20 s with a clearance time of 200ms.
`
`In the first set of simulations the DFIG was controlled using
`only the rotor-side converter. Different values of the voltage
`control loop gain K,"' were used in order to observe the effect
`on the DFIG fault current. Fig. 5 shows the terminal voltage
`and Fig. 6 shows the stator and rotor currents in phase a with
`K vc =10 .
`
`., :....•
`
`+12
`•8
`•4
`+O
`-4
`
`18.95
`
`20
`
`20.05
`
`20.1
`
`20.15
`
`20.2
`
`lra
`
`• 10-
`• 6
`• 2 -
`-2
`-6 -·-7----
`•1ru.a
`20.05
`19.95
`20
`lime (sec)
`
`20.1
`
`20.15
`
`20.2
`
`Fig. 7. Stator and rotor currents in phase a using rotor-side converter
`control. Voltage control loop gain set at Kvc = 0.75.
`(i,a = 1 l.5SkA, i,.0 = 7.67kA)
`
`Vs PUl
`
`-
`
`_2
`
`•
`
`-Vs PU)
`•1 .2
`•1
`+0,8
`+0.6
`+0.4
`10.2
`•010
`
`18.5
`
`19
`
`20.5
`
`21
`
`21.5
`
`22
`
`20
`19.5
`lime (sec)
`Fig. 5. DFIG terminal voltage using rotor-side conve1tcr control.
`Voltage control loop gain set at K vc = 10.
`
`It can be observed in Fig. 5 that the DFIG controller operates
`satisfactorily as the terminals voltage tracks the 1.0 PU
`reference before and after the fault, that is, the DFIG exhibits
`fault ride-through capabilities. However, Fig. 6 illustrates that
`the stator and rotor currents are extremely high for this value
`of K vc (isa = 19.96kA, i,.0 = l l.93kA) . Fig. 7 presents the
`stator and rotor currents for Kvc = 0.15 . It can be observed
`that for this smaller value of Kvc both the stator and rotor
`fault
`currents
`are
`considerably
`reduced
`Usa =l l.55kA, i,.0 =7.67kA). It was observed that even
`
`1
`+1
`+O.Bt-----t--i----+-·----1-----• - - - (cid:173)
`+0.6- -+ - - , . .--+-- ,1- 1--+--+--+-__,
`+0.4>-----,.--+--+----+- 1- - - - - - - -
`+0.2 - -+- - l --+ - -11- 1---t----l---+---1
`• 01a
`
`10.5
`
`10
`
`20
`19.5
`lime (sec)
`Fig. 8. DFIG tetminal voltage using both rotor- and grid-side
`converter control.
`
`20.5
`
`21
`
`21 .s
`
`22
`
`In the second set of simulations the DFIG was controlled with
`the strategy proposed in the paper using both converters. Fig.
`8 shows the DFIG terminal voltage, where it can be seen that
`the DFIG exhibits again fault ride-through capability and
`recovers stability after the fault. However, the results in Fig. 9
`
`SGRE EX1020.0006
`
`

`

`79
`
`and Fig. 10 illustrate that with the proposed strategy the stator
`and rotor fault currents are reduced due to the reactive power
`support provided by the grid-side converter. Although the
`stator fault current is not reduced by a large amount, it is
`observed that the rotor fault current is reduced significantly to
`ira = 4.63kA as shown
`even
`in Fig. 10, where the
`magnetising level of the DFIG is controlled by adjusting
`
`that by adopting the proposed strategy it is possible to reduce
`considerably the magnitude of the rotor fault current during
`the fault. This performance is possible due to the reactive
`power support provided by the grid-side converter reducing
`the requirements imposed on the rotor-side converter for
`voltage and power factor control. As the crowbar protection
`was omitted during the simulations the calculated for currents
`are not likely to exist in a physical application, as the rotor
`over current protection is necessarily used. However, these
`results illustrate that it is possible to achieve reduced rotor
`fault currents decreasing in turn the rating of protection and
`converter system components.
`
`References
`
`Qmag ·
`
`+16
`+12
`+8
`
`+4
`+O
`
`Isa
`/kA\
`
`A
`A
`V ~
`
`A
`
`"
`
`~
`
`./ A
`
`A
`
`)
`
`V
`
`"
`
`It J\...r- ~
`
`-~ .--
`
`I
`
`-4
`·-1\i.9 19.94 19.118 20.01 20.05 20.09 20.13 20.16 20.2
`
`+1 0 ;:::::::.l::.::ra-l'l<A'-"-\-.---r---.--.---,---,--,
`
`+6 >----+--+----1•11+-----t-
`
`-+---I---·· - - - (cid:173)
`
`-t
`+21---+--+-.-~'-l·-rrrr -=-/\-- -+ - -+-
`__ ..,___, _ _., 1~1v1--'f---_v,t-v-+ - -+ --t
`V
`-+--+-----+- -t--+---- -'--
`
`-2
`.51---+-
`
`·11\J.0 19.94 19.98 20.01 20.05 20.09 20.13 20.1 6 20,2
`Tlme (secl
`·-0
`Fig. 9. Phase a stator and _rotor currents with both rotor- and grid(cid:173)
`side converter control. (i,0 =10.34kA, i"' =7.15kA).
`
`~
`
`ct.•r.!- -
`
`lsa(kAl
`
`· -
`
`I
`
`I ~
`
`A
`
`IA J
`
`V
`
`I'\__,-. .,.:
`
`" "
`
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`
`-
`+16
`+12
`+8
`+4
`
`+O
`
`-4
`
`·4\J.9 19.94 19.98 20.01 20.05 20.09 20.13 20.16 20.2
`
`Ira kA\
`
`+10
`
`+2
`
`-2
`
`-A-A-A- '
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`
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`~ V V
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`-
`-6
`•1ll9.9 19.94 19.98 20.01 20.05 20.09 20.1 3 20.16 20.2
`lime (sec]
`Fig. 10. Phase a stator and rotor currents using both rotor- and grid(cid:173)
`side c~nverter control. (i,0 = I 0.28kA, ira = 4.63kA) .
`
`5 Conclusions
`
`A DFIG control strategy based on both rotor- and grid-side
`converters has been presented in this paper to reduce DFIG
`current contribution during faults. It was illustrated that the
`DFIG fault current contribution is very sensitive to the
`voltage control loop settings and tuning when only the rotor(cid:173)
`side converter is used for both torque and voltage/power
`factor control. In th.is case it was observed that extremely
`/ large fault currents were generated due to the control action .
`provided by the voltage control loop. However, it was shown
`
`[1] DTI Consultative Document: NEW & RENEWABLE
`· ENERGY - Prospects for the 21s1 Century, 30th March
`1999.
`[2] TANDE, J. 0.: "Grid integration of wind farms," Wind
`Energy Journal, 2003, 6, pp. 281-295.
`[3] MULLER, S., DEICKE, M. and DE DONCKER:
`"Doubly fed induction generator systems for wind
`,/
`turbines,'' IEEE Industry Applications Magazine, 2002,
`pp. 26-33.
`[4] NATIONAL GRID TRANSCO: "Appendix 1, Extracts
`from
`the Grid Code Connection Conditions,"
`www.nationalgrid.com, June 2004.
`[5] BURGES, K.: "Dynamic modelling of wind farms in
`transmission networks," article contracted by Sustainable
`energy Ireland, 2004.
`[6] EKANAY AKE, J.B., HOLDSWORTH, L., WU, X., and
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`SGRE EX1020.0007
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