Renewable Energy 43 (2012) 90e100. Contents lists available at SciVerse ScienceDirect. Renewable Energy

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1 Renewable Energy 43 (2012) 90e100 Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: Improvements in the grid connection of renewable generators with full power converters Dionisio Ramirez *, Sergio Martinez, Carmelo Carrero, Carlos A. Platero Department of Electrical Engineering, ETSI Industriales, Universidad Politecnica de Madrid, C/Jose Gutierrez Abascal, 2, Madrid, Spain article info abstract Article history: Received 17 March 2011 Accepted 29 November 2011 Available online 23 December 2011 Keywords: Full power converter Short circuit current Voltage sag Wind generator The increasing penetration of electricity generation from renewable energy is posing new technical challenges to the operation of power systems. Some of them are related to the different electrical behavior of the generators when compared with traditional directly coupled synchronous generators. This work is focused on the case of renewable generators connected to the grid through full power electronic converters and the improvement of its response to voltage sags. It is proposed a system based on the use of an electronic on load tap changer to increase the generator contribution to the short circuit current during grid voltage depressions, to cooperate in the post-fault voltage recovery of the power system. It is also presented a specific control system on the converter, based on a nonlinear current source, to secure the necessary fast response. An experimental validation of the developed system has been carried out by building and testing a laboratory prototype. The results of the more relevant tests are also presented. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Electric generators are one of the key elements within a power system. Traditionally, these generators are, for the most part, large synchronous rotating machines. Thus, the operation of power systems has been well defined by the performance characteristics of these machines when they are interconnected forming a network [1]. Particularly, as it relates to the problem addressed in this paper, the philosophy of protection of power systems, facilities, and associated equipment is based on how these generators behave when facing to faults or disturbances in the grid [2]. In recent years, mainly due to increased use of renewable energy for electricity generation, this situation is changing. Different types of electric generators are being installed, whose operating characteristics are different from those of synchronous generators. When the degree of penetration of these new generators in power systems was not significant, the performance characteristics of the systems were not substantially altered. However, the increase in its level of penetration has unveiled alterations in the functioning of the systems that have forced the operators of electric power systems to impose increasingly stringent requirements for the connection of these new generators [3e7]. These requirements are * Corresponding author. Tel.: þ ; fax: þ address: dionisio.ramirez@upm.es (D. Ramirez). diverse, but, in general, it can be said that they are aimed at forcing the new generators to include supplementary systems to modify their performance characteristics so that they resemble those of synchronous generators. For example, the ability to withstand voltage dips has been the subject of extensive research, e.g. [8],itis already a requirement included in many grid codes, and, consequently, new renewable generators do include it among their features. Currently, in electric power systems with high penetration of renewable generators, there is a concern about their low contribution to short circuit current in case of fault, compared with that of synchronous generators [9]. This capability is particularly low in generators in which all of the electrical energy is injected into the grid through an electronic converter, due to the fact that the maximum current in the converter is limited to the rated one, or slightly higher. This is the case, for example, of photovoltaic solar plants or wind plants with synchronous generators and full power electronic converters. Although a low short circuit current in a power system has advantages, mainly related to the dimensioning of the breaking capacity of switching devices, it is important to note that much of the system relaying practice is based on detecting the faults by means of the high currents associated to them, at least in conventional systems. In other words, the decrease in the contribution of these new generators to the short circuit current constitutes a problem for the protection of the power system /$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi: /j.renene

2 D. Ramirez et al. / Renewable Energy 43 (2012) 90e Fig. 1. Grid connection diagram of a wind generator with a full power converter as a CCVS. An additional problem related to the grid integration of these renewable generators, as the current injected by an electronic converter is practically limited to its rated current, is that the supply of reactive power to the grid during a fault and, especially, the post-fault period is also limited to the rated one [10e12], degrading the voltage stability of the system. The lower the value of the voltage dip, more important is a fast voltage control. In this work, the authors propose a set of measures to improve the response of this type of renewable generators against voltage sags. The two main contributions of the paper are: first, a solution for increasing the current injection during voltage dips, and second, a control system for the converter to operate as a nonlinear current source in combination with the first. 2. Description of the system In renewable generators with full power converters, all the generated electric power flows to the grid through the electronic converter. For example, Fig. 1 shows a common topology for a wind generator. Usually, the converter is operated as a Current Controlled Voltage Source (CCVS), also known as Linear Current Source [13]. As stated in Section 1, from the point of view of the grid, the behavior of such a converter and that of a conventional synchronous machine during network disturbances are quite dissimilar in several aspects. The objective of the system presented in this paper is focused on increasing the short circuit current capability of the full converter plant, resembling the response of a synchronous generator, i.e., we propose a system for increasing the current well beyond the rated one, not only during the fault period, but also during the undervoltage situation in the post-fault period, thus contributing to the power system voltage restoring. The system is based on the use of a tapped transformer with an electronic On Load Tap Changer (OLTC). The tapped transformer is similar to those currently used in the grid connection of wind farms, but with an extended tap range in the side of grid current increase, up to the desired extent in current boost. The electronic OLTC provides the necessary speed in the tap change. Fig. 2 shows a schematic representation of the system. It is important to note that a change in the tapping voltage ratio to increase the current in the grid side of the transformer also imposes a voltage increase in the converter side. So, to avoid damage to the converter, the tap change must only be allowed when the grid side voltage is depressed, and limited by the admissible voltage value at the converter. Therefore, the maximum change in the tapping voltage ratio depends on the voltage value during the dip. In addition, the voltage change may also disturb the operation of the converter, which is suddenly subjected to a higher voltage. This effect is particularly dangerous because, during a voltage sag, the converter is operated at its whole rated current, in order to contribute to the grid voltage restoration with all the available reactive power. At this operating point, a disturbance in the current can damage the converter. To avoid this, a rapid response to changes in the conditions of the connection is needed. In this work, we propose a Nonlinear Current Source control on the converter, where the sinusoidal current in the inverter side should remain constant throughout the voltage dip. 3. Short circuit current increase with an electronic OLTC As previously stated, the increase of the short-circuit current is based on the use of a tapped transformer, a device extensively used Fig. 2. Schematic representation of the proposed system.

3 92 D. Ramirez et al. / Renewable Energy 43 (2012) 90e100 Fig. 3. Detail of the tapped transformer and the OLTC. Fig. 5. OLTC switching sequence following the voltage restoration. in power systems for many decades. It is commonly employed in combination with an OLTC for voltage control in quasi-steady state, in arrangements specialized to solve such problem. It is well known that the voltage ratio of a transformer (r t ) is the inverse of the current ratio, so that a change in the tapping voltage ratio is also a change in the opposite direction of the current ratio. In this work we propose the use of the tapped transformer and an electronic OLTC for the purpose of current multiplication. As it can be seen in the schematic representation of Fig. 2, and in the detail of Fig. 3, the system includes: - A power transformer with taps in one of its windings. The taps are distributed to obtain several current ratios, including values significantly higher than the rated one. Figs. 2 and 3 show only two taps per phase, for the sake of simplicity. - An electronic OLTC for the rapid change to the desired tap. The taps can be selected by activating or deactivating electronic switches built of antiparallel thyristors (see Fig. 3), with instant firing, and forced or natural turn-off. - A measuring system for currents and voltages. It is represented in Fig. 2 by the incoming bus arrows to the control system. Although a very basic way of implementing the system requires only the measurement of all phase voltages in one side of the power transformer, a more precise control requires voltage and current measurements in both sides. - A control system that implements the algorithm governing the tap changing, as a function of the above measures. The output bus in the right side of the control system of Fig. 2 carries the firing signals for the electronic switches in the OLTC. The switching between taps must be done without interruption of the instantaneous currents to mitigate the undesirable transients. Fig. 4 shows the detail of the switching of one phase of the OLTC after the voltage dip detection in the power system. The figure shows, from top to bottom, the states in three consecutive instants, illustrating the evolution of the voltage ratio of the transformer from r t 0 to r t. To provide the adequate timing of operations, the control system requires instantaneous measurements of the threephase currents. To remove turns in the grid side of the transformer, once the current in each phase becomes zero, K1 automatically switches off, and the control system fires K2. Following the zero crossing of the first phase, a transient unbalance takes place because the neutral is isolated, and the currents in the two remaining phases must be equal but in opposite directions. Finally, the two remaining switches turn-off at a time in the zero crossing of their currents. At this point, it is important to recall that, in addition to the current boost in the grid side of the transformer (what constitutes the goal of this work), the converter side is also subject to a voltage increase. Apart from monitoring the voltage to avoid converter damage, the voltage increase has a positive side effect in the Low Voltage Ride-Through (LVRT) capability. So the extra cost associated to this system can be compensated with the savings in LVRT equipment. Similarly to the previous case, Fig. 5 shows the detail of the switching of one of the phases, after the detection of the voltage recovery in the grid. From an initial state with K1 off, and K2 on, following the zero crossing of the phase current, K2 automatically switches off, and the control system fires K1. Particular attention has to be put to wait for the zero crossing of the current (that is monitored by the control system). If not, as it is the case in mechanical OLTCs, and K1 is fired before K2 is off, a short circuit is imposed to the added turns while the electromotive force due to Fig. 4. OLTC switching sequence following a voltage dip. Fig. 6. Simplified single-phase equivalent circuit for the connection of the converter to the grid.

4 D. Ramirez et al. / Renewable Energy 43 (2012) 90e Fig. 10. Determination of the straight line from the tip of the grid phase space vector with the direction of the derivative of the change in the current space phasor. Fig. 7. Space phasors in the connection of the converter to the grid. Fig. 8. Graphical representation of the change in the current space phasor. the common flux is not null. More details on electronic OLTCs can be found in [14]. The current transients associated with the tap changing can be mitigated with the control of the converter as a Nonlinear Current Source, given its fast dynamic response. 4. Nonlinear current source To ensure that the current at the inverter does not exceed the design value, particularly during and in the instants immediately after the tap changes, a hysteresis controlled nonlinear current source has been used [13,15]. However, as many of these types of sources use only six space vectors, plus the null one, a new specific procedure for network connection is proposed that enables higher current control. It is based on the use of the well-known Space Vector Modulation (SVM) technique, which generates a variety of intermediate space phasors [16,17]. The developed control procedure is performed entirely in space phasors. Unlike conventional hysteresis controllers, it does not use a hysteresis band for each phase current. Instead, it uses a single circular space area at the end of the current space phasor. It combines the advantages of using hysteresis band (simplicity, speed of response, and independence from changes in parameters) with the use of SVM (increased capacity in voltage generation, fewer losses and EMI, and constant switching frequency). In each cycle, the control system calculates what particular voltage space phasor ðu! cþ has to be applied by the converter, with! the SVM technique, to make the actual current space phasor ð I gþ evolve in the right direction, to get closer to the position of the!* reference current space phasor ð I gþ. From the hardware point of view, the source uses a three-phase inverter with six IGBTs, and it measures the three-phase currents (or two of them, if the neutral is not connected) and the voltage at the inverter DC bus (U dc ). For the grid connection, the synchronization angle is obtained from a Phase-Locked Loop (PLL). The voltage to be generated by the converter is calculated from the simple relationship (see Fig. 6):! U c ¼ L d! I g þ! U g (1) dt where L is the series inductance. Fig. 7 shows the above variables in terms of space phasors. Without modulation, the converter is able to generate the six vectors shown in the figure as U1eU6, whose tips form a regular hexagon centered in the aeb plane origin. When the SVM technique is used, the converter is able to generate a whole family of vectors whose tips form the circumference inscribed in the Fig. 9. Graphical representation of the hysteresis space area (shaded circle) with respect to the change in the current space phasor for three different cases. Fig. 11. Determination of the two possible solutions.

5 94 D. Ramirez et al. / Renewable Energy 43 (2012) 90e100 Fig. 12. Diagram of the prototype without electrical generator. hexagon (solid line). The circumference described by the tip of the grid voltage space phasor is also represented, with a dashed line. Lets express the reference current space phasor and the actual current space phasor in terms of their real and imaginary parts in the aeb plane of Fig. 7 (stationary two-axis reference frame):!* I g ¼ I* a þ j$i* b! I g ¼ I a þ j$i b From them, the control system computes the change in the current space phasor needed to reach the reference phasor (see Fig. 8 for a graphical representation):! D I g ¼ C þ j$d with: C ¼ I * a I a D ¼ I * b I b The algorithm, implemented in the control system, then checks if the change in the current space vector goes outside the boundaries of the defined hysteresis space area, regardless of their orientation in space. This condition can be expressed as: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi C 2 þ D 2 > ; 3 where is the radius of the hysteresis space area. Fig. 9 represents three different situations of the change in the current space phasor with respect to the hysteresis space area. 3 If the change is greater than the area radius, the algorithm recalculates the voltage space phasor to be applied by the converter according to Eq. (1). In this equation, the grid voltage phasor can be easily computed from the measured phase voltages. The derivative term, however, is determined through more calculations, as shown below. In the first step, the algorithm computes the two possible solutions given by the intersection of: (1) the straight line defined by a point at the tip of the grid phase space vector and the direction of the derivative of the change in the current space phasor; and (2) the circumference described by the tips of all the possible converter voltage space phasors (see Fig. 10). Note that the radius of this circumference is times the dc bus voltage. The characteristic equation of that straight line, given by the slopeeintercept form, is: y ¼ d$x þ q where the slope d is given by the quotient D/C. The b-intercept, q, is computed taking into account the aeb components of grid phase space vector:! Ug ¼ U ga þ j$u gb : Thus: q ¼ U gb U ga $d: As the equation of the circumference described by all the possible converter voltage space phasors is: x 2 þ y 2 ¼ð0:866$U dc Þ 2 ; Fig. 13. Pretesting. Waveform and space phasor magnitude of the converter voltage following a tap change cycle with constant grid voltage.

6 D. Ramirez et al. / Renewable Energy 43 (2012) 90e Fig. 14. Block diagram of the linear current source used in the test. the intersections with the straight line are given by the two possible solutions of the equation: x 2 þðd$x þ qþ 2 ð0:866$u dc Þ 2 ¼ 0: The algorithm then computes one of the solutions (see Fig. 11): rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2$d$q þ ð2$d$qþ þ d 2 $ q 2 0:750$U dc 2 U ca1 ¼ 2$ 1 þ d 2 If the following condition is satisfied Uca1 U ga $C>0; then U ca1 is the correct solution, because it is a space phasor with the same direction than the change in the current space phasor. Otherwise, the other solution U ca2 is the correct one, and it has to be calculated: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2$d$q ð2$d$qþ þ d 2 $ q 2 0:750$U dc 2 U ca2 ¼ 2$ 1 þ d 2 Finally, the vertical coordinate of the voltage space phasor to be generated with the converter is determined by: U cb ¼ d$u ca þ q; where U ca is the correct solution between U ca1 and U ca2. Once the voltage space phasor has been calculated (U ca þ j$u cb ), it is transformed into the IGBTs switching periods by means of the SVM technique [16,17]. 5. Experimental results To validate the proposed improvements in the grid connection of renewable generators, i.e. the system for current increase during voltage dips and the converter control with the nonlinear current source technique, a laboratory prototype of a wind generator has been built and tested. This section shows the experimental results of the most relevant tests carried out. In a first stage, to avoid interactions with the more complex control system of a whole wind generator, a prototype of the proposed system was tested without a generator, i.e., the electrical generator of Fig. 2 was substituted by an independent threephase source fed from the laboratory grid. Fig. 12 shows a more detailed representation of the system. Thus, the tests were focused on the operation of the proposed electronic OLTC and nonlinear current source. in addition, the performance of the nonlinear current source was compared with that of a linear current source commonly used in the grid connection of Fig. 15. Linear current source. Waveform and space phasor magnitude of the converter current following a tap change cycle.

7 96 D. Ramirez et al. / Renewable Energy 43 (2012) 90e100 Fig. 16. Block diagram of the nonlinear current source used in the test. renewable generators. In a second stage, the system was integrated in a laboratory scaled emulator of a wind generator, similar to [18], to test the behavior of the whole system when facing grid voltage dips (see Fig. 22). The built prototype uses a 5 kva, 400/230e127 V, Yy0 transformer with two taps per phase. The 400 V side is connected to the grid. In normal operation, the converter is connected to the 127 V tap, and it is changed to the 240 V tap during voltage dips. Note that, as the tapped side of the transformed is connected to the converter, the operating sequence is opposite to the general one described in Section 3. The electronic OLTC is composed of thyristors with instantaneous turn-on and natural turn-off. The control system was implemented with the Texas Instruments TMS320F28335 DSP, a 150 MHz floating-point processor. A low value (3 khz) was chosen for the sampling frequency of measurements and the switching frequency of the inverter, to better reproduce industrial control systems on wind generators. The data captures and the control interface in the laboratory tests were carried out using the Real Time Data Exchange (RTDX) feature implemented in the DSP with Matlab-Simulink, as described in [19]. The line was represented with three 16 mh linear core inductances. As a pretesting, for reference purposes, the electronic OLTC was forced to a tap change cycle (127 Ve230 Ve127 V) with a constant reduced grid voltage. Note that, if the grid were at its rated voltage, the subsequent 72% voltage increase would damage the converter. Fig. 13 shows the waveform of the inverter voltage and the magnitude of its space vector Testing the system against tap changing with different current sources The first round of tests is devoted to compare the dynamic response of a linear current source commonly used in the grid connection of renewable generators (see Fig. 14) and the proposed nonlinear current source (see Fig. 16), following a tap change in the transformer. The control system is synchronized with the d-axis component of the voltage space phasor of the external balanced three-phase grid (u gd ) by means of a Phase-Locked Loop (PLL). Under these conditions, the active and reactive powers interchanged with the grid at the Point of Common Coupling (PCC) can be expressed as: P PCC ¼ u gd $i gd Q PCC ¼ u gd $i gq Thus, regardless of variations in the grid voltage magnitude, the injection of active and reactive powers is controlled by acting on the deq components of the reference current, i g *. In the case of the linear source, once the tap change takes place, the reaction of PI controllers to the voltage change is to maintain the reference value of the current, introducing a delay determined by the Fig. 17. Nonlinear current source. Waveform and space phasor magnitude of the converter current following a tap change cycle.

8 D. Ramirez et al. / Renewable Energy 43 (2012) 90e Fig. 18. Nonlinear current source. Waveform of the grid current following a tap change cycle. dynamics of regulators. Fig 15 shows the recorded waveform of the inverter current and the magnitude of its space vector during a voltage sag test. If the wind generator was working at the rated current, the resulting current increase could block or even damage the electronic converter. Tuning the PI controllers for a faster dynamic response can lead to instabilities, and the overcurrent is always present. The same test was performed with the proposed nonlinear current source, whose block diagram is depicted in Fig. 16. The recorded results are shown in Fig. 17. It can be seen that, during the tap switching, the converter current changes very little. This is possible thanks to the faster dynamic response of the Nonlinear Current Source that is not affected by the PI dynamics. Note that, in this first round of tests, the DC voltage regulator does not work, because the tap change does not affect the DC voltage. Therefore, the electronic converter could remain generating the maximum allowable current, even during the voltage sag, without risk of overcurrent. Fig. 18 shows the line current measured at the PCC during the test. As the current in the inverter remains constant during the tap change, a 72% increase in the grid current is achieved. Fig. 19 shows the currents measured in the switches of the electronic OLTC during the tap changing. Although small transients can be seen, the nonlinear source maintains the current values under control Testing the system against grid voltage sags with the nonlinear current source The second round of tests, also without an electrical generator, was devoted to check the system performance alone when facing a grid voltage sag, avoiding interactions with the wind generator controls. Tests were performed with a voltage sag to 45%. Fig. 20 shows that the current source is also able to maintain its value under control. It is important to note that, as seen in Fig. 21, the voltage at the inverter terminals does not exceed the rated value, due to the combined effect of the voltage reduction caused by the external grid fault and the voltage increase associated to the tap change. While a tap change when there is no voltage sag causes a dangerous voltage boost at the inverter terminals (see Fig. 13, reproduced in the upper part of Fig. 21 for reference), this does not happen during a voltage dip if the tap being used is suitably selected Testing the wind generation system when facing grid voltage dips The third round of tests was devoted to check the performance of the system integrated in a wind generator when facing a grid voltage sag. Fig. 22 shows the diagram of the system in which a laboratory scaled wind turbine emulator was used. The emulator is similar to that described in [18]: it is realized by replacing the wind, the gearbox and the turbine rotor with a PC, a current controlled acedc converter, and a dc motor. The software model is implemented with LabView, and it is not so detailed than [18]: it only takes into account the turbine aerodynamic characteristics and a variable wind speed to control the dc converter to emulate the driving torque of the simulated wind turbine. Nevertheless, the emulator has a minor influence on the tests, due to the short duration of a typical voltage dip. In this system, the mechanical power from the wind turbine emulator is transformed into electrical power with a permanent Fig. 19. Nonlinear current source. Current waveforms in switches K1 (dashed line) and K2 (solid line) of OLTC s phase a, following a tap change cycle.

9 98 D. Ramirez et al. / Renewable Energy 43 (2012) 90e100 Fig. 20. Waveform and space phasor magnitude of the converter current following a grid voltage sag and the associated tap changing. Fig. 21. Voltage measured at the inverter terminals following a grid voltage sag and the associated tap changing. In gray, the magnitude of the voltage space phasor if the same tap change was performed without a voltage sag in the grid. magnet synchronous generator. A diode rectifier transforms the variable frequency three-phase system into DC, where a current controlled chopper controls the generator torque. The power from the generator is then converted to fixed frequency AC by the inverter and injected into the grid. Any unbalance between the incoming power to the rectifier and the outgoing power from the inverter modifies the capacitor DC voltage (U dc ). As it can be seen in the left part of Fig. 16, this voltage is monitored by the inverter control system to decide on the amount of active power to inject into the grid. Also, and independently, the inverter control can fix a reference for the reactive power. The system was tested against a grid voltage dip. It is important to remember that, as the system is normally operated at the rated voltage, the taps must be changed only during the voltage dip, by selecting the tap suitable for the magnitude of the actual voltage reduction. In addition, to contribute to the voltage restoration following a grid voltage sag, the system has to inject the maximum available reactive power during the voltage depression. To this objective, in the tests performed with the prototype, the wind generator changes its operation point when a voltage dip is detected, as it can be seen in the active and reactive power records of Fig. 23. In this case, when the sag is detected, the control system Fig. 22. Diagram of the prototype with the electrical generator and the wind turbine emulator.

10 D. Ramirez et al. / Renewable Energy 43 (2012) 90e Fig. 23. Active and reactive powers injected at the PCC, as recorded by the DSP during a grid voltage sag. Fig. 24. Waveform and space phasor magnitude of the converter current following a grid voltage sag. blocks the chopper so that no active power is extracted from the synchronous generator. Thus, once the PI regulating the DC voltage has readjusted, no active power is injected into the grid. Simultaneously, the reactive power reference is changed to its maximum value. Fig. 24 shows that the inverter current is almost constant during the sag. The variations observed at the beginning of the sag and during the post-fault period are due to variations in the active power set point sent to the current source by the DC voltage control. Fig. 25 shows the corresponding line current measured at the PCC during the test. Once again, as the current in the inverter is almost constant during the sag, a 72% increase in the grid current is achieved. Finally, it has been previously stated that one of the objectives of the proposed improvements is to cooperate in the grid voltage recovery. Fig. 26 shows a comparison of the voltage at the inverter terminals during the sag, with and without tap changing. As the sag is shallower the low voltage ride-through capability of the generator is enhanced. Fig. 25. Recorded waveform of the grid current during the voltage sag.

11 100 D. Ramirez et al. / Renewable Energy 43 (2012) 90e100 Fig. 26. Voltage at the inverter terminals during the sag, with and without tap changing. 6. Conclusions The paper presents the current problem of full converter renewable generators from the point of view of its performance against short circuits in the power system, and it proposes and tests a number of improvements. The use of a tapped transformer with electronic OLTC has proven effective in increasing the current injected into the grid during voltage dips. Tests have also confirmed that the use of a common linear source for the inverter control is not adequate due to a poor dynamic response. By contrast, the developed nonlinear source has proven to be fast enough to maintain unchanged the current in the inverter when facing sudden voltage changes, thus avoiding the risk of overcurrent. The nonlinear current source developed uses the concept of spatial hysteresis area to improve the dynamic response, in combination with the SVM technique to generate a large family of different spatial vectors and a constant switching frequency. Another particularity of the current source design is the specific use of the equation that describes the converter-grid connection to calculate the voltage to be generated, which provides optimized performance for this application. Acknowledgments This work was supported in part by the Spanish Ministry of Science and Innovation under Grant ENE , and by the Spanish Ministry of Education under Grant PR References [1] Glover JD, Sarma MS, Overbye TJ. Power system analysis and design. 4th ed. Toronto: Thomson; [2] Horowitz SH, Phadke AG. Power system relaying. 3rd ed. Chichester: Wiley; [3] Regulation TF Wind turbines connected to grids with voltages above 100 kv. Denmark: Elkraft System and Eltra; November [4] Grid connection regulation for high and extra high voltage. E.ON Netz GmbH; April [5] The grid code, issue 3, revision 16. Warwick: National Grid Electricity Transmission plc; 30th May [6] Secretaria General de Energia, Procedimiento de Operacion Requisitos de respuesta frente a huecos de tension de las instalaciones eolicas, Spain, BOE No. 254; 24th October 2006 [in Spanish]. [7] Grid code, version 2.0. EirGrid; January [8] Flannery PS, Venkataramanan G. Unbalanced voltage sag ride-through of a doubly fed induction generator wind turbine with series grid-side converter. IEEE Trans Ind Appl 2009;45:1879e87. [9] Luther M, Radtke U, Winter WR. Wind power in the German power system: current status and future challenges of maintaining quality of supply. In: Ackermann T, editor. Wind power in power systems. Chichester: Wiley; p. 233e55. [10] Chinchilla M, Arnalte S, Burgos JC, Rodriguez JL. Power limits of gridconnected modern wind energy systems. Renew Energy 2005;31:1455e70. [11] Fernandez LM, Garcia CA, Jurado F. Operating capability as a PQ/PV node of a direct-drive wind turbine based on a permanent magnet synchronous generator. Renew Energy 2010;35:1308e18. [12] Hansen AD, Michalke G. Multi-pole permanent magnet synchronous generator wind turbines grid support capability in uninterrupted operation during grid faults. IET Renew Power Gener 2009;3:333e48. [13] Kazmierkowski MP, Malesani L. Current control techniques for three-phase voltage-source PWM converters: a survey. IEEE Trans Ind Electron 1998;45: 691e703. [14] Monroy-Berjillos D, Gomez-Exposito A, Bachiller-Soler A. A lab setup illustrating thyristor-assisted under-load tap changers. IEEE Trans Power Syst 2010;25:1203e10. [15] Buso S, Malesani L, Mattavelli P. Comparison of current control techniques for active filter applications. IEEE Trans Ind Electron 1998;45:722e9. [16] Dai J, Xu DD, Wu B. A novel control scheme for current-source-converterbased PMSG wind energy conversion systems. IEEE Trans Power Electron 2009;24:963e72. [17] Wu B, Pontt J, Rodriguez J, Bernet S, Kouro S. Currentesource converter and cycloconverter topologies for industrial medium-voltage drives. IEEE Trans Ind Electron 2008;55:2786e97. [18] Monfared M, Kojabadi HM, Rastegar H. Static and dynamic wind turbine simulator using a converter controlled dc motor. Renew Energy 2008;33: 906e13. [19] Ramirez D, Martinez S, Rodriguez J, Carrero C, Blanco M. Educational tool for the implementation of electric drives control system with real time data exchange. Int J Eng Educ 2009;25:24e32.

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