VOLTAGE DIP RIDE-THROUGH CONTROL

VOLTAGE DIP RIDE-THROUGH CONTROL OF DIRECT-DRTVE WIND TURBINES Johan Morren”, Jan T.G. Pierik”, Sjoerd W.H. de Haan’’ I ) Electrical Power Processing, Delft University of Technology, The Netherlands Energy research Centre of the Netherlands (ECN), The Netherlands ABSTRACT With an increasing amount of wind energy installed, the behaviour of wind turbines during grid disturbances becomes more important. Grid operators require that wind turbines stay connected to the grid during voltage dips. This contribution presents a controller that can be used to keep direct-drive wind turbines with permanent magnet generator connected to the grid during voltage disturbances. The behaviour of the wind turbine during a grid fault is demonstrated by simulations. Keywords: voltage dip, voltage source converter, wind energy INTRODUCTION World wide there is an ambition to install a large amount of wind power and to increase the fraction of energy that is produced by wind turbines. To enable large-scale application of wind energy without compromising power system stability, the turbines should stay connected and contribute to the grid in case of a disturbance such as a voltage dip. They should similar to conventional power plants supply active and reactive power for frequency and voltage recovery, immediately after the fault has been cleared. A number of grid operators already require voltage-dip ride-through capability, especially in places where wind turbines provide for a significant part of the total power supply. Examples are Denmark [ I ] and parts of Northern Germany [Z]. The requirements concerning immunity to voltage dips as prescribed by E O N Netz, a grid operator in Northern Germany, is shown in Fig. 1. Only when the grid voltage drops below the curve (in duration or voltage level), the turbine is allowed to disconnect. When the voltage is in the shaded area the turbine should also supply reactive power to the grid in order to support grid restoration. 450% 15% I i I > 7w 15w jMKl Tmainms i 150 Time IauM O C D Y I I ~ ~ Fig. I, Voltage dip that wind turbines should be able to handle without disconnection (Eon Netz) Especially variable speed wind turbines, such as wind turbines with doubly-fed induction generators or direct- drive permanent magnet generators, require careful attention. These turbines use power electronic converters, which have IO be protected against over- currents and over-voltages during disturbances. A number of publications have been presented on the voltage dip behaviour of doubly-fed induction generators 131 [ 5 ] , whereas only little information can be found on the voltage dip behaviour of direct-drive permanent magnet wind turbines. In this contribution we will investigate the operation of a permanent magnet wind turbine during voltage disturbances. The paper will give a description of the model of the permanent magnet generator that has been developed. A description of the controllers will be presented and it will be explained how these controllers can be operated to keep the wind turbine connected during a dip, Simulations will be presented to show the behaviour of the controllers and the turbine during a voltage dip. For the wind turbine and grid model realistic parameters have been chosen. MODEL DESCRIPTION Direct-drive wind turbines often use permanent magnet synchronous generators. In this section the basic equations describing the machine behaviour will be given, followed by controllers design. Generator The machine model that has been used is based on the following set of equations: dr vdr =-R , i , -w, q, -- dt 9s vql =-Rj, +w, dr -- dt 934 with v the voltage [VI, R the resistance [O], i the current [A], o,~ the stator electrical angular velocity [rad/s] and the flux linkagc [Vs]. The indices d and q indicate the direct and quadrature axis components. All quantities in (1) are functions of time. Due to the limited space it will not be possible to give a complete description of the generator model and its control. It can be found in [6], [7]. Converter The permanent magnet synchronous machine is connected to the grid by a three-phase back-to-back converter consisting of two Voltage Source Converters (VSCs) and a dc-link. The dc-link separates the two Voltage Source Converters, and therefore they can be controlled independent of each other and only one converter has to be considered. The controller of the converter will be based on a dq0 reference frame linked to the stator of the PM machine. All signals will be constant in steady-state and PI controllers can be used to control the dq values. The controller is based on two control loops. The inner loop is a current controller, which get its reference from the outer loop controller, which can be for examplc a reactive power or torque controller. The switching function concept has been used to model the converter [8]. Using this concept, the power conversion circuits are modelled according to their functions, rather than to their circuit topologies. If the filter is designed well, the higher harmonics that are generated by the switching process will be attenuated. It can be shown that, with a well-designed filter, in the lower frequency range the frequency components of the reference voltage and the practical obtained voltage are equal if the switching frequency is sufficiently large 191, The whole system can then be replaced by a system, creating sinusoidal waveforms, exactly equal to the reference waveforms. One should be aware that this is only valid for frequencies far below the resonance frequency of the filter. In case of a grid-connected converter, with a grid-frequency of 50Hz, this requirement will mostly be met. Current control The current controllers of the VSC will be obtained with reference to the converter shown in Fig. 2. A vector- control approach is used for the grid side converter, with a reference frame oriented along the grid voltage vector. Such a reference frame enables independent control of the active and reactive power flowing between the converter and the grid. Consider the circuit of Fig. 2. The voltage balance across the inductors and resistors is: di, dr V h = Vbn -vbx" = L,, ,-+ R , . lb (2) With the Park transformation this equation can be transformed to the dq refcrence frame: The last tern1 in both equations causes a coupling of the two equations, which makes i t difficult to control both currents independently. The last terms can be considered as a disturbance on the controller. Reference voltages to obtain the desired currents can be written as: (4) (5) The id and i,, errors can be processed by a Pl controller to give 112 and vq' respectively. To ensure good tracking of these currents, the cross-related flux terms are added to vd' and vq' to obtain the refcrence voltages. Treating thc cross-related terms as B disturbance, the transfer function from voltage to current of ( 5 ) can be found as (for both the d- and the q-component): 1 L,s t R f G(s) = - A scheme of the controller is given in Fig. 3. "d PI 'd E+ I ' w e f Fig. 3. Scheme of current controller Fig. 2. Three-phase full-bridge Voltage Source Converter 935 Using the Internal Model Control principle [lo] to design the current controllers yields: (7) k ' s s where <. is the bandwidth of the current control loop, kp is the proportional gain and k, is the integral gain of the controller. The gains become [I I]: The instantaneous active and reactive power delivered by the converter are given by: p = vd i , + v I q = v 'I i d - v rl i y with the d-axis of the reference frame along thc stator- voltage position, q, is zero and as long as the supply voltage is constant, v , ~ is constant. The active and reactive power are proportional to id and i,. K ( s ) = k , + 2 = I. G-' (.y ) k , = , L , ; ki = =R,, (8) (9) 4 Y DC-link controller In this sub-section a description will be given of the dc- link voltage controller that has been used. The dc voltage controller is designed by use of feedback linearisation [IZ]. The capacitor in the dc-link behaves as an energy storage device. Neglecting losses, the time derivative of the stored energy must equal the sum of the instantaneous stator power P., and grid power P,: This equation is nonlinear with respect to vdc. To overcome this problem a new state-variable is introduced: w = v1. (1 1) Substituting this in (1 0) gives: 1 dW 2 dt - C - = P , - P , which is linear with respect to W. The physical interpretation of this state-variable substitution is that the energy is chosen to represent the dc-link characteristics [ 121. With the dq-reference frame of the current controller along the d-axis, (12) is written as: 1 dW -e-= P, -vdid 2 dr and the transfer function from id to W is then found to be: 2v s c C(s)= -L As this transfer function has a pole in the origin it will be difficult to control it. An inner feedback loop for active damping will be introduced [ 121: With G, the active conductance, performing the active damping, and id' the reference current provided by the outer control loop, see Fig. 4. Substituting (1 5) into (1 3) gives: id = i; +G,W (15) 1 dW 2 di - C - = P , - v ,~&-v ,G ,W The transfer function from iq ' to W becomes [ 121: Using the internal model control principle [ I I ] and since (17) is a first-order system, the following controller is proposed: Which is just an P1-controller. A suitable choice will be to make the inner loop as fast as the ciosed-loop system [12]. When the pole of G'(s) is placed at - the following active conductance is obtained: The P1-controller parameters are then given as 1121: The controller is completed by a feed-forward term from P, to iq'. This reed-forward term is needed to improve the dynamic response of the dc-link controller. - Fig. 4. DC-link controller structure SlMULATtON SETUP The simulation set-up is shown in Fig. 5 . From left to right the turbine, the turbine transformer, the 34kV cable, the 34kVil50kV transformer and the l5OkV ideal grid are shown. The layout and the data have been obtained from The Near Shore Wind park (NSW park) that is planned in the North Sea about 12 kifometres from the Dutch coast. In the simulation, only one turbine is considered. Identical controllers and protective devices are installed in all turbine. A balanced three-phase fault will be studied. The fault is assumed to occur in the 150kV transmission grid. This will result in reduced voltage levels at the 34kVilSOkV transformer. In the simulation the grid has been modelled as an ideal voltage source. The turbine operates at nominal power during the simulations that will be described here. The direct-drive variable-speed turbine has a full back- to-back converter connected between the stator and the grid. The voltage dip behaviour of this turbine can thus mainly be considered as the voltage dip behaviour of the converter. A description of mechanical and aerodynamical models that have been used can be found in [ 131. 936 Turbine Transformer Cable Transformer 150kV Fig. 5. Simulation set-up for voltage dip simulations SIMULATION RESULTS A 50% - 0.5 seconds dip in the voltage has been applied to the wind turbine. The rotor average wind speed during the dip was about 15 d s . The wind turbine then operates at nominal power. The rotor average wind speed and the aerodynamic power in the wind arc shown in Fig. 6. The active and reactive power supplied by the stator of the permanent magnet synchronous machine are shown in Fig. 7. Fig. 6. Roter average wind speed and aerodynamic power Due lo the limited thermal capacity of power electronic components, the current should not become too high for longer times. Therefore the current of the converter should be limited when the fault occurs. At the moment the fault occurs, the grid voltage drops and the current of the grid-side converter has to increase when the same power as before the fault should be supplied to the grid. The current will be limited however, to avoid thermal breakdown of the converter. As a result the DC-link voltage wiil increase, as long as the power from the turbine isn't decreased. Therefore also the generator controller will decrease its setpoint. This can be seen form the power curve in Fig. ,7. When the power is decreased also the electrical torque will decrease, see again Fig. 7. As a result of the decreasing electrical torque the turbine will speed up, at least as long as the aerodymical torque remains the same. The increase in rotational speed wm is shown in Fig. 7. At the moment the speed increases the pitch angle controller has to react to limit the speed increase. In Fig. 7 it can be seen that there is first a short peak in the rotational speed at the dip occurs. Afterwards the rotational speed is slightly increasing. A dip of 0.5 second is too short however to get a significant increase in speed. Almost no pitch controller action has been noted. When the dip will hold on for a longer time the speed may increase however, and pitch angle controller is maybe not fast enough to limit the speed increase. 1 1 , q ................... j ................... ................ _. ................. ................... 10 time Is] time [s] , 5~~ ............... j ............ ........... ................... 10 3 10 rime [SI m e Is] Fig. 7. Active power, reactive power, electric torque and speed The dc-link voltage is shown in Fig. 8. Note that the voltage is practically constant because the switching operation of the converters has not been modelled. Otherwise, there would be a high-frequency ripple on the voltage. The dc-link controller reacts fast enough to control the voltage. In reality, it will not always be necessary to keep the dc-link at the pre-fault voltage. A small increase or decrease in voltage will be allowed. 7 U5 7 U5 I u 1 0 - 5 10 lime [5] 095 0 - 5 10 Imp [5] 095' Fig. 8. DC-link voltage 1.5 z/T-l ................... : ................... 1.5 'r .... : ........ .... 1 ..................... , ........... ......... 3 .............. ............. .............. dme [SI Dme [SI Fig, 9. Voltage and current of grid-side converter In Fig. 9 the voltage and current of the grid-side converter are shown. It can be seen that the current is limited (after a controller overshoot) to about 1 pu. At the moment the dip is cleared, the current drops to about half the nominal current. . It takes some time before the 937 converter adjusts the current to the value before the dip occurred. CONCLUSION In this paper the voltage’ dip behaviour of a variable speed pitch controlled wind turbine with permanent magnet generator has been considered. Models of the generator, the converter and its controllers have been presented with a special focus on those parts that are essential for the behaviour of the wind turbine during voltage dips. In the turbine all the essential parameters can be controlled. Therefore good voltage ride-through can be achieved. The power supplied by the generator i s limited by the controllers during the dip. This is required because otherwise the current in the converter or the voltage in the dc-link becomes too high. To avoid overspeeding the pitch controller will be activated. Due to the short duration of the voltage dip, no significant increase in rotational speed can be noted from the simulations that have been presented. ACKNOWLEGDMENT This research is partially funded by Novem within the Program Renewable Energy in The Netherlands 200 1, and by Senter within the Program IOP-EMVT. REFERENCES 1. 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Ottersten, R., On control of Back-to-Back Converters and Sensorless Induciion Machine Drives, Ph.D. thesis, Technical report no. 450, Chalmers University, G teborg, Sweden, 2003. 13. Pienik, J.T.G., Morren, J. , Wiggelinkhuizen, E.J., de Haan, S.H.W., Engelen, T.G. van, Bozelie J., Elecrricul and control aspects of ofJdmi.e wind rurbines I!, Y d . I: Dynamic models of wind farms, Technical Report ECN- C- -04-050, The Netherlands: ECN, 2004. AUTHOR’S ADRESS Johan Morren Electrical Power Processing Delft University of Technology Mekelweg 4 2628 CD Del0 The Netherlands J.Morren@ewi.tudelft.nl 5 . Morren, J., Haan, S.W.H. de, “Ride through of Wind Turbines with Doubly-fed Induction Generator during a voltage dip”, IEEE Trans. Energy Conv., accepted for publication.