On the Emulation of an Isolated Wind Energy Conversion System: Experimental Results Roberto Galindo-del-Valle Maria Cotorogea * Universidad Tecnológica de Altamira, Blvd. de los Ríos km 3+100, Puerto Industrial de Altamira. *Instituto Tecnológico de Ciudad Madero, Ave. 1º. de Mayo S/N, Cd. Madero, Tamaulipas, México. E-mail: rogalva@hotmail.com Infineon Technologies AG Am Campeon 1-12 85579 Neubiberg Germany. E-mail: Maria.Cotorogea@infineon.com Balduino Rabelo, Wilfried Hofmann Technische Universität Dresden, Fakultät Elektrotechnik, Elektrotechnisches Institut, Professur Elektrische Maschinen und Antriebe, Helmholtzstr. 9, Görgesbau, Raum 206/207, 01069 Dresden, Deutschland. Abstract—This paper presents the emulation of an isolated wind energy conversion system, which is composed by a doubly-fed induction generator, a back-to-back converter connected to its rotor, a LC filter to minimize the harmonic pollution in the generated voltage and an isolated three-phase load. In first instance, the test bench is described and its operational capabilities are introduced. Afterwards, the control system design is presented. Next, some associated experimental results are shown as well. A special mention must be made to an experimental study which considers the possibility of using the self-excitation of the doubly fed induction generator to achieve the black-start of the isolated wind energy conversion system. Keywords- Induction generators; variable speed drives; wind power generation; voltage control I. INTRODUCTION Nowadays, there exist increasingly research efforts focused on Wind Energy Conversion Systems (WECSs), since wind power is one of the more feasible renewable energies [1]. Currently, it is possible to obtain lower energy costs than the corresponding to the best fossil fuel generation technology in sites with good wind resources [2]. With regard to the operating speed range, WECSs can be classified as constant-speed and variable-speed. Even more, since variable-speed WECSs must use an electronic power converter, they can be classified in full-power-handling WECS (also called direct-in-line) and partial-power-handling WECS, considering both the converter placement and ratings. In a full-power-handling WECS, the power converter is in series with the induction or synchronous generator, in order to transform the variable amplitude/frequency produced voltages into constant amplitude/frequency ones, and it must be able to process the whole generated power. In a partial-powerhandling WECS, the converter to control the electrical machine is in a secondary generator circuit, and it only processes a portion of the total generated power (e.g. slip power), which constitutes an advantage in terms of reduced cost of the converter and increased efficiency of the system [3]. This paper is focused on a partial power handling standalone WECS based on a doubly-fed induction generator (DFIG). Variable speed WECSs supplying an isolated load have already been considered by other researchers. In [4], a stator voltage direct control is proposed using PI regulators. It offers a good dynamic performance, but is load dependent, which causes some practical difficulties. In [5], a system, where the rotor is fed from a battery through a PWM current source inverter, is presented. Additionally, regulation of the rms generated voltages is proposed, which results in considerable voltage errors and load dependency. In [6]-[7], several PI-based indirect stator voltage vector control approaches are presented for a WECS in which a back-toback (B2B) converter is used to manage the power interchange between the stator and rotor circuits. The proposed control strategies produce good dynamic performance with only slight load dependency. In fact, the work presented here is based on the isolated system treated in [6]-[7]. However, some modifications have been made, which include the addition of a LC filter between the B2B converter and the DFIG stator, the use of a PhaseLocked-Loop to obtain the stator voltage amplitude and phase, and the utilization of a PI controller to assure the right orientation of the DFIG (stator-flux-oriented) control system. Moreover, in this study there are several items which are not treated in the mentioned papers, like the start-up and turn-off procedures and the analysis of the black-start capabilities by using the self-excitation of the DFIG. To present the work, this paper is organized as follows. In Section 2, the considered system is described, and the associated test bench is presented. In Section 3, the controller design is treated. In Section 4, experimental results related with several tests are shown and commented. Finally, in Section 5, the conclusions are given. II. SYSTEM AND TEST-BENCH DESCRIPTION The considered WECS is depicted in Fig. 1. In this system, energy is collected by a wind turbine (WT) and transferred to a DFIG by a drive train (DT). The DT also increases the rotational speed by using a gear box (GB). The DFIG supplies an isolated load (both main and auxiliary). The system operation is controlled by the B2B converter, composed by two VSI-PWMs: the front end (FEC) and the machine side (MSC) converters. The FEC manages the power flow between rotor and stator circuits by a cascaded control system, in which the inner loop controls the converter currents and the outer one regulates the dc-link voltage [6]-[7]. In a similar fashion, the MSC is used for regulating the DFIG stator voltage by means of a cascaded control system, whose inner loop controls the rotor currents, whilst the outer one indirectly regulates the stator voltage amplitude. The LC filter is used to damp the switching-generated high frequency harmonics. The test bench was initially built to emulate a grid connected system and presented in great detail in [8]. In order to emulate the previously described isolated system a modification was proposed. Fig. 2 shows the schematic diagram of the test bench after this modification. Now it has three operational modes: two of them are grid-connected and the remaining one is isolated. It can be seen in Fig. 2 that if the points I and II are connected to B and D, respectively, then the system is grid connected with both the stator and the rotor circuits tied to the secondary of the input transformer. On the other hand, if the points I and II are connected to A and D, respectively, then the system is grid connected again, but now it has the stator directly connected to the grid, whilst the rotor circuit (through the B2B) is connected to the secondary of the transformer. Finally, the isolated system operating mode is achieved by connecting the points I and II to B and C, respectively. In the experimental rig, the DFIG has a nominal power of 4 kW, and the B2B is a SEMISTACK-IGBT by SEMIKRON. The pre-charger is an uncontrolled three-phase rectifier which is manually operated. The capacitor bank in the LC filter is able to furnish the entire magnetizing power to the DFIG, via the stator. The WT is emulated by a 7.5 kW dc machine fed by a 3-phase thyristor controlled bridge rectifier with a separated field winding fed by a 1-phase controlled rectifier. This drive can operate in four quadrants and also in field weakening. At this stage, only a main three phase Yconnected resistive load is considered. III. CONTROL SYSTEM DESIGN The controllers design was carried out by using the same models and assumptions considered in [7] and [9]. The system parameters are given in TABLE I. The root locus method was used for tuning all the controllers. TABLE II presents the gains obtained for each system. For the sake of brevity, detailed mathematical treatment is omitted, which can be found in [9]. Only the most important considerations are listed. 1. For the DFIG control system, it has been used the well known dq model of the induction machine. 2. The former control system considers a synchronous reference frame aligned with the stator flux (s), whose  components are obtained by using (1). TABLE II. PARAMETERS USED IN THE CONTROLLERS DESIGN Value 1.54  0.9  139e-3 H 0.4  1.4  6600 F Parameter rs r’r M r1 r2 C0 (dc-link) Parameter Value 148e-3 H 141e-3 H 2.4 20e-3 H 8e-3 H 69F Ls L’r nsr L1 L2 Cf (LC Filter) where: nsr is the stator to rotor turns ratio. Auxiliary Load Main Load LC Filter Stator High Speed Side Low Speed Side DFIG GB Rotor WT Back-to-back Converter Legend WT = Wind Turbine. GB= Gear Box. DFIG = Doubly Fed Induction Generator. MSC= Machine Side Converter. FEC= Front End Converter. MSC FEC Controller 1 Controller 2 Figure 1. Wind Energy Conversion System (WECS) to supply an isolated load. Electrical Grid Transformer A D I Stator II B C Isolated Load DFIG Rotor DC Motor PreCharger LC Filter L1 r1 r2 MSC L2 FEC Cf Back-to-back converter Figure 2. Schematic diagram of the test bench.  s    v s  rs i s   dt (1) where: v• s, i• s, and r• s are the stator voltage, current and resistance, respectively, whilst  = or . 3. The stator voltage is regulated indirectly by controlling the magnetizing current. It is assumed that the stator resistance voltage drop is negligible. 4. The q component of the rotor current is used to force the reference frame orientation. Initially, the approach used by Peña et al. in [6]-[7] was used. It consists in calculating the proper set-point for i'rq by using (2), which ensures that sq=0. TABLE II. Controlled variable GAINS OF THE PI CONTROLLERS OBTAINED BY USING THE ROOT LOCUS METHOD Requirements DFIG rotor currents (ird, irq) 1.Damping =0.7071 Manipulated variable Kp Ki MSC voltages (vrd, vrq) 38.5 Experimentally adjusted to: 50 3465 Experimentally adjusted to: 4500 2. Settling time ts=30 ms 1.Damping =0.7071 Stator voltage (through magnetizing current: ims) d-component of the rotor current (ird) 0.2 8 2. Settling time ts=0.7 s q-component of the stator flux-linkages (sq) q-component of the rotor current (irq) 40 40 2.Settling time ts=4 s FEC voltages (vnd, vnq) 2.5 800 d-component of the FEC current (ind) 0.1 0.5 FEC currents (ind, inq) 1.Damping =0.7071 1.Damping =0.7071 2. Settling time ts=18 ms 1.Damping =0.7071 dc-link voltage (V0) 2. Settling time ts=1 s L irq   S i sq M (2) where: Ls and M are the stator and magnetizing inductances, respectively. However, the parameters uncertainty did not allow achieving a good performance. In order to cope with this, model (3) can be used to tune a controller, which is able to force the stator-flux orientation: d sq dt  M rs irq  vsq  e sd LS (3) where: e is the synchronous angular speed. 5. By using this approach, the reference frame position can be forced to be equal to the angle:  e   e*dt where: e*  2 f e* (4) is the desired angular frequency. This angle is used in all the reference frame transformations concerning the DFIG control system. 6. The FEC control system is considered in a statorvoltage-oriented reference frame, which implies that a decoupled control of the active and the reactive power can be achieved. 7. Originally, for the orientation of the FEC control system, [6]-[7] propose the use of angle (4) to estimate the stator voltage vector position, by adding /2 to e. However, in the laboratory test bench, better results were obtained when a PLL was used to determine the stator voltage vector position, since the stator resistance voltage drop is not negligible in the considered DFIG. Fig. 3 presents the block diagram corresponding to the used PLL [10]. In this PLL, the PI controller drives the stator voltage q-component to zero in order to achieve the right orientation. So, the PI output constitutes the angular frequency, which is integrated to obtain the proper phase angle. Then, this angle is used in the Park Transformation to determine the actual stator voltage dq-components. Next, the current value of the qcomponent is used to calculate the new error. A feedforward term (=314.15 rad/s for 50 Hz systems) is used to improve the PLL tracking capability. 8. The controller of the dc-link voltage produces a balance between the power which is flowing inwards (outwards) the FEC and the one that is flowing outwards (inwards) the MSC in sub-synchronous (super-synchronous) speed. IV. EXPERIMENTAL RESULTS WITH PI CONTROLLERS The designed controllers were discretized by using the trapezoidal method, and implemented in a dSpace DS1103 Board with a sampling frequency of 1.5 kHz. The IGBTs’ gate signals were obtained by considering SV-PWM and a switching frequency of 4050 Hz. The DS1103 contains a master IBM PowerPC processor running at 1 GHz and a slave Texas Instruments TMS320F240 DSP running at 20 MHz. Furthermore, the board has 36 ADC channels (20 with 16-bit resolution and 16 with 10-bit resolution), and 50-bit for digital IO purposes. Additionally, the DS1103 can be programmed by using Real Time Interface (RTI) blocks from the Simulink environment, which makes easier the implementation tasks. v*sq=0 + PI controller - + 1 s + 314.15 vsd Park Tranformation vsq vsa vsb vsc Figure 3. Three Phase Locked Loop. In the following subsections some experimental results are presented. A. Start of the Stator Voltage Controller The stator voltage controller was tested by using a dc-link voltage of 330 V, which is maintained by the pre-charger, whilst the dc-motor was driven the DFIG at 1400 rpm. A three-phase resistive load of 40  was used. The current controllers were started with their references set to 0A. Fig. 4 shows the start of the voltage controller with a ramp from 0 to 120 Vrms in 5 s. This ramp is initiated at t=1s, whilst the set-points of the rotor current controllers are switched to be automatically calculated by the controllers of the stator voltage and of the q-component of the stator flux linkages (which forces the right orientation). It can be seen that the d component of the stator voltage is not zero, because the stator resistance is not negligible, which produces an average stator voltage error of approximately 6% in the steady state. In addition, it can be noticed that both rotor current components increase with the same pattern as vsq. For the stator voltage to increase it is necessary to furnish some additional magnetizing power, which is provided by i'rd. Meanwhile, as the stator voltage increases, the active power consumed by the resistive load is also increasing. This produces an increment in isq which, in accordance with (2), makes i'rq bigger. B. Connection of the Complete System In this test, the stator voltage controller is initially started as described in the previous section. Then, the FEC current controllers were enabled with their references set to 0A. Next, the FEC currents are controlled to produce 120Vrms in the filter capacitors, in phase with the stator voltage. This is achieved by setting the ind reference to a small negative value (to face the copper losses) and the inq reference to –e*· Cf · Vs*, which enables the FEC to “consume” the reactive power associated with the filter capacitors. Once the steady state has been reached in both subsystems, they are connected to each other, whilst the precharger is disabled and the dc-link voltage controller is activated. Figs. 5 and 6 present the results of this experiment. The connection was carried out around t=1s. It can be seen in Fig. 5 that after the connection the dc-link voltage drops because it was disconnected from the pre-charger. In consequence, a decrement in the generated stator voltage and some variations in the system currents are produced, as shown in Fig. 6. In this figure can be observed that the reactive power associated currents (isd, i'rd, and inq) remain almost the same and that their variations are produced only by the transient in the generated voltage. On the other hand, the active power related currents (isq, i'rq, and ind) must increase their magnitudes in order for them to be able to increase the dc-link voltage. In the end, the steady state value of isq is slightly bigger than the previous one, because now the stator is supplying not only the load, but also the FEC. The same can be said of i'rq, whilst ind assumes a positive value because now it must supply the necessary active power to the dc-link. Figure 4. Start of the stator voltage controller. Figure 5. Transient behavior in stator and dc-link voltages during connection. Figure 6. Transient behavior in stator, rotor and FEC currents during connection. The phase voltage was 113 Vrms with 2.1% THD before the connection and 112Vrms with 1.2% THD after it, representing a regulation error of approximately 6%. This decrement on the harmonic pollution is caused by the LC filter and can be observed in Fig. 5. The load current was 2.93 Arms with 6% THD, before the connection, and 2.94 Arms with 9% THD after it. C. Turn off Procedure In order to disconnect the system easily, a turn off procedure was proposed. It consists in an algorithm to decrease the set-points of the generated stator voltage and of the dc-link voltage. The slopes of the decreasing ramps were chosen for the stator voltage to be 0 in 20 s, whilst the dc-link voltage decreases to 0 in 30 s. Figs. 7 and 8 show the experimental data obtained. It can be noticed that both stator and dc-link voltages decrease monotonically until a point is reached in which the required power to maintain them in their set-points is not available any more. Then the currents have to increase to recover the control. However, the dc-link is discharged by their control efforts. D. Black-Start Capability The black-start consists in the system ability to start in a completely independent way, which is very important in an isolated system. Therefore, self-excitation (SE) of the DFIG was considered. SE is a well-known phenomenon that is used by squirrel-cage induction generators in some WECSs. It requires the use of a three-phase capacitor bank connected to the stator terminals, and it consists in a voltage build-up originated either by the residual magnetic flux in the machine or by the initial charge in the capacitor bank [11]-[12]. In the considered tests, SE was initiated both by the sudden connection of the capacitor bank to the stator, whilst the DFIG was running at a constant speed, or by increasing the speed whilst the capacitor bank is already connected to the stator. In addition, two different ways to initially charge the dclink were verified: (a) using a Y-connected capacitor bank to start the SE, and connecting the pre-charger (tied to the stator) once the steady state has been reached, whereas the rotor windings are short-circuited and the FEC is disconnected; or (b) using a -connected capacitor bank, whilst the dc-link is pre-charged via the free-wheeling diodes in the FEC. Fig. 9 shows some of the experimental results. Stator voltage orientation is considered. For this test the DFIG speed was increased from 1300rpm to 1500rpm, with the conditions described above in option (a). After the start transient, the stator voltage amplitude is equal to the rated one (310 Vpeak), and stator currents take the necessary value for the DFIG to be able to receive the magnetizing power and to furnish the copper losses. Then, in t=2.5s, the pre-charger is activated, which constitutes a disturbance for the stator voltage, and produces a spike in the d-component of the stator current in order to supply the dc-link. Once the dc-link is charged, the stator voltages and currents reach the steady state again. Figure 7. Stator voltage and rotor currents behavior during the turn off. Figure 8. DC-link voltage and FEC currents behavior during the turn off. Figure 9. Stator voltage and current during the self-excitation phenomenon and dc-link pre-charge. Stator voltage orientation is considered. V. CONCLUSIONS In this paper it has been considered the experimental emulation of an isolated WECS. The related test bench has been described, and its control system design has been commented. In addition, experimental results concerning the start-up and turn-off procedures have been presented. In all the tests the control system performance has been acceptable, but it may be additionally improved either by modifying the PIs tuning or by using another kind of controllers. The use of the SE to produce the isolated system blackstart has also been explored. It could be used either a Yconnected capacitor bank, plus a pre-charger tied to the DFIG stator, or a -connected capacitor bank, plus the freewheeling diodes in the FEC, in order to initially feed the dclink. Once the dc-link voltage has been fixed by the SE, the FEC and MSC could start the switching. However, SE could be lost. Additional experiments are required to explore this situation. 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