Abstract—A Doubly Fed Induction Generator Wind Turbine (DFIG-WT) with FACTS capabilities is prented. It is suggested to make u of the grid-side converter as a shunt active filter in order to support the grid with power factor correction as well as harmonic compensation. A vector control scheme is ud to control the grid-side converter with a priority level feature to split extra room for compensation. It allows the grid-side converter to be remote-controlled and provide power quality support at a specific point away from the wind turbines plant. No harmonics are injected into the generator and the system is fully operational even at zero output power from the DFIG-WT. Simulation results illustrate good performance of the propod system.
Index Terms-- Wind power generation, active filter, doubly-fed induction generator, power quality, grid-side converter control.
I. I NTRODUCTION
VER the last fifteen years, many articles have been published about today’s well known and well established DFIG-WT. Bad on Scherbius drive topology adapted to today’s advanced mi-conductors and signal processing power, they are now widely commercialized with output power reaching up to 5 MW for off-shore installations [1]. Facing the world’s energy crisis, they are an importa
nt green power ast to all grids and their penetration level is increasing every year in many countries. However, most of the past articles mainly deal with various control strategies of the rotor-side converter for DFIG-WT and readers may refer to the relevant references given in this article for further details.
Recently, there is a new trend focusing on wind turbines grid integration and their drawbacks in distorted grid conditions. This trend aris becau grid owners are worried about the grid’s ability to support a high penetration level of wind power and to maintain overall stability. Especially in the ca of DFIG-WT, A. Perdana et al [2] have reported The authors would like to thank Natural Sciences and Engineering Rearch Council of Canada (NSERC) for providing financial support for this rearch work.
E. Tremblay, A. Chandra and P. J. Lagacé are with the Département de Génie Électrique, École de Technologie Supérieure, Université du Québec, Montréal, QC H3C1K3, Canada (e-mail: blay.smtl.ca, smtl.ca, smtl.ca). problems with voltage sags and ability to ride-through faults when they occur. They suggest to disconnect the WT right after the rotor-converter is blocked to avoid subquent voltage instability due to stator current and active power fluctuations.
Today, other authors propo solutions while keeping in mind the main goal of the wind turbine to support the grid with voltage-frequency regulation and to prevent damage of the rotor-side converter [3]-[4]-[5]-[6]-[7].
Power quality is actually an important aspect in integrating wind power plants to grids. This is even more relevant since grids are now dealing with a continuous increa of non-linear loads such as switching power supplies and large AC drives directly connected to the network. As far as the authors know, only one group of rearchers have addresd the issue of making u of the built-in converters to compensate harmonics from non-linear loads and enhance grid power quality [8]. In [8], the current of a non-linear load connected to the network is measured, and the rotor-side converter is ud to cancel the harmonics injected in the grid. Compensating harmonic currents are injected in the generator by the rotor-side converter as well as extra reactive power to support the grid. It is not clear what are the long term conquences of using the DFIG for harmonic and reactive power compensation.
finallyIn our opinion, the DFIG should be ud only for the purpo for which it has been installed, i.e., supplying active power only. With the help of simulations, this article shows that it is more natural to make u of the grid-side converter rather than the rotor-side converter for harmonic compensation
as a standard shunt active filter. By using an indirect current control strategy [9] of the active filter, in this ca the grid-side converter, is independent of machine parameters, number of non-linear loads, wind turbine state and rotor-side control strategy. Therefore, this article is not concerned about the rotor-side converter control strategy. Furthermore, neither harmonics nor extra reactive power are injected in the generator even when supplying reactive power to the grid; making the DFIG-WT with its two converters a perfect Flexible AC Transmission System (FACTS) candidate.
Section II of this article prents some basic knowledge about DFIG-WT first, review briefly some control strategies and highlight a few practical considerations while modeling the system. Section III discuss grid-side converter control
Grid-Side Converter Control of DFIG Wind
Turbines to Enhance Power Quality of
Distribution Network
E. Tremblay, A. Chandra, Senior Member, IEEE, and P.J. Lagacé, Member, IEEE
O
1-4244-0493-2/06/$20.00 ©2006 IEEE.
strategy in order to achieve harmonic compensation and
power factor correction. Detailed schematics will be given and explained. Section IV prents and discuss relevant simulation results.
II. D FIG -WT DEVELOPMENTS The DFIG-WT is the only variable speed wind turbine that does not require a full-size converter to provide the grid with a constant frequency power with unity power factor capability. Although it does require converters, they are scaled down to about 30% of the nominal generator power in order to allow the wind turbine to operate at variable speed ωr over a fixed speed range (about ± 20-30%) around the synchronous speed ωs . Following (1) we obtain the active power handled by both converters, P c , in terms of slip s and power fed by the stator P s .
s
P s c P ⋅= (1) ()s ωr ωs ωs −= (2)
It should be noticed in (2) that when the machine is driven at supersynchronous speed, the slip sign is negative and the
machine is now generating power from both the stator and北京师范大学附中
rotor. Besides, as slip speed s·ωs can be controlled via the
rotor-side converter, it is possible to keep ωs constant. Fig. 1
shows a typical hardware schematic of a DFIG-WT.
Fig. 1. Typical hardware schematic of a DFIG-WT.
In general, the grid-side converter is ud to regulate the DC bus between the two converters and its power factor is usually t to unity, or in a way to fulfill the command strategy. To do so, various control strategies may be ud. Perhaps, the most popular one is to align the d axis of the grid-side converter currents abc to dq transformation (Park-Clark) along a reference frame link to the wind turbine voltage. A reference current derived from the DC bus voltage and voltage reference is procesd by a PI controller and oriented along this d axis to supply or absorb active power.
On the other hand, the control strategy of the rotor-side converter is far more complicated in order to control the machine’s behavior in both sub. and supersynchronous modes
as well as tracking the maximum power output characteristic of the wind turbine. In 1991, Yamamoto and O. Motoyoshi
[10] prented an air-gap flux oriented control for DFIG-WT
with P-Q control as well as a harmonic analysis with a cycloconverter. In 1996, R. Pena et al [11] prented a fully tested, emulated DFIG-WT, by using a stator-flux oriented control with two PWM co
nverters in order to achieve P-Q control as well as maximum power tracking. In 1998, L. Zhang and C. Watthanasarn [12] simulated a DFIG-WT with a matrix converter bad on stator-flux oriented control. Matrix converters for DFIG-WT have not received much attention in the literature so far. Finally, in 2002, S.A. Gómez and J.L.
Rodríguez [13] prented a Direct Torque Control (DTC) strategy applied to DFIG-WT and included grid synchronization. In 2003, N. W. Miller et al [14] from GE
Power Systems and GE Rearch relead a paper about
modeling a GE 1.5 and 3.6 MW wind turbines. It was the first
attempt to highlight some physical limitations and key features of DFIG-WT models, particularly in:
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Wind turbines operating speed range. Active and reactive power range. Voltage, active-reactive power, frequency, current and speed regulator control strategies. Converters, machine power-currents limitations and components priority. Rotational model of the machine.
Typical DFIG-WT defaults parameters. Initialization of the model.
The grid-side converter control strategy is engineered in agreement with their recommendations and will be studied under MATLAB/Simulink and SPS in ction IV.
III. G RID -SIDE CONVERTER CONTROL STRATEGY As stated previously, the grid-side converter is needed to ensure constant DC bus voltage. In this ca, it is assumed that all the machine magnetizing current is fed by the rotor-side converter. It is understood that regulating the DC bus must always be the first priority and the extra room for active-reactive current must be ud accordingly. The cond priority is to provide harmonic compensation at or even beyond the PCC (e.g., uplink substation) only by measuring the grid’s line current. This feature enables the grid-side converter to compensate harmonics from different loads at a strategic point instead of tracking every source of harmonics independently. In addition, by measuring the voltage at this strategic point, the wind turbine reactive output power may be adjusted in order to satisfy a pret power factor or regulate grid’s line voltage.
Here again, a dq reference frame rotating at the synchronous speed is ud and allows decoupled control of active and reactive power of the grid-side converter. Fig. 2 shows the complete controller scheme ud in the simulations.
Fig. 2. Grid-side converter control bloc studied under MATLAB/Simulink and SPS.
The angle θs of the synchronous reference frame is derived from the wind turbine pha voltage and synchronized with a PLL on Va . Actual value of the DC Bus Vdc is compared with reference voltage Vdc* and error Vdc’ is procesd by a PI gain limited to ±Imax giving Id_dc’. The DC Bus priority bloc calculates the room for harmonic and reactive power compensation with (3) and ts Iqmax level accordingly.
In order to compensate the power factor at the PCC, Vpcc and Ipcc are transformed to Vdq_pcc and Idq_pcc respectively and the instantaneous reactive power Qpcc is calculated with (4). This value is compared with the reference Q*pcc and the error Q’pcc is procesd by a PI gain with output value limited to ± Iqmax .
2Id_dc'-2Imax Iqmax =
(3)
()Id_pcc Vq_pcc Iq_pcc Vd_pcc 23pcc Q ⋅+⋅= (4)
There are various methods to calculate the harmonic
reference currents. Frequency domain compensation is bad on Fourier analysis and requires more real time processing power as well as perfect synchronization of the reference signals. The result is a complicated system compared to other ways such as Time domain compensation which are more attractive in this ca. They are bad on instantaneous derivation of the compensating signals from either distorted voltages or currents measurement and the method is greatly simplified [15]. In this application, as the system is already bad on a synchronous reference frame, a dq refe
rence frame controller is developed in order to extract the harmonic current components. Since the fundamental components are DC values in the synchronous reference frame, a cond order LPF is ud to extract the fundamental and subtract it from the source signal. As the LPF bandwidth is limited to 5 Hz, no
pha shift is introduced in the harmonic components while the filter respon to sudden changes in the network is smoothed off. Two proportional gains with outputs limited by ±Idhmax and ±Iqhmax enhance system stiffness and provide harmonic reference currents Id_pcc_h’ and Iq_pcc_h’ to the Q and H priority bloc. Idhmax and Iqhmax are calculated in a way to satisfy grid’s priority and in this ca, a unity power factor will be preferred over harmonic compensation resulting in Idq_comp’ reference currents (5-6).
2)Iq_Q'(Iq_pcc_h'2)Id_dc'(Id_pcc_h' Imax +++≥ (5)
]Id_pcc_h' []Iq_Q' [Iq_pcc_h' Idq_comp'+= (6)
Signal Idq_comp’ is fed to the DC Bus priority bloc and is added to Id_dc’ resulting in Idq_gc’ total reference currents command. Idq_gc’ is then compared to Idq_gc and the error is procesd by a PI controller in order to obtain the reference voltage Vdq_gc* needed for the voltage source type P
WM converter. Voltage compensation terms are added to Vdq_gc* resulting in Vdq_gc . A dq to abc rever transformation follows prior PWM gating signals generation.
IV. S IMULATIONS AND DISCUSSIONS
Fig. 3 prents the simulated network. A 575 V DFIG-WT with power rating of 3.6 MW is connected to a 23 kV network through a step-up transformer and feeding a 20 kW local load. The IGBT power converters current limit is t to 0.45 p.u. and a ries L filter is installed on the grid-side converter to filter high frequency current ripples resulting from the PWM modulation (not shown). The grid-side converter switching frequency is t reasonably to 4.2 kHz, which is about 10 times the 7th harmonic at 60 Hz for proper harmonics sampling. A 575 V RL type DC load and a purely inductive
load are fed by a step-down transformer from the 23 kV network. This will act as a non-linear load with lagging power factor. A substation with a 23/138 kV step-up transformer links the wind power plant to an infinite grid, this point is referred as the PCC. Four cas are prented. Cas A&B are under normal condition while cas C&D are under saturated controller condition. Cas B&D are long-range performance evaluations under wind speed disturbances to show propod system’s independence to the wind turbine state.
Fig. 3. Grid model studied under MATLAB/Simulink and SPS.
A. Reactive power and harmonic compensation
The first ca verifies the performance of the system when operating under normal condition. A 0.18 MVAR lagging load combined to 1.1 MW non-linear load at 29 % current THD are fed while the wind turbine is operating at maximum power output of 3.6 MW (1 p.u.). Fig. 4 shows the waveforms before and after compensation at t = 1.2 s. Voltage and current at the PCC in Fig. 4 (a) show clearly the performance of the propod system to compensate both the lagging power factor and current distortion from the load in Fig. 4 (b). The wind turbine output current is maintained constant at 1 p.u. while providing current compensation as shown in Fig. 4 (c). Becau of the narrow LPF bandwidth, there is a little delay before proper harmonic compensation takes place, until Iq_Q’reached a steady state value. This is the trade-off to keep harmonic filtering circuit simple and free of any pha shift. B. Reactive power and harmonic compensation under wind speed disturbances
In this ca, wind speed drops from rated value to 0 m/s. Various waveforms from the DFIG-WT and the PCC are prented in Fig. 5. The same loads as in ca A are ud and a 40-cond simulation takes place in order to show system’s independence to the wind turbine state. As the wind speed an
d wind turbine output power in Fig. 5 (b) fall to zero after 30 and 34 s respectively, current harmonic distortion is kept very low in Fig. 5 (d) while maintaining a unity power factor as desired at the PCC in Fig. 5 (c). The current THD waveform prents a singularity around t = 23 s. At this instant the output power of the wind turbine equals the load (not shown), so no current is flowing at the PCC (P = 0) in Fig. 5 (c). Therefore, distortion increa suddenly (fundamental component is zero although some noi remains). In Fig. 5 (a), compensating signal Id_gc’ maintains DC Bus voltage constant and at t = 10 s, harmonic compensation is activated. Self-supporting DC Bus voltage is kept at 1 p.u. even in transient states and compensating signals are free of any saturation. This long-range simulation shows system’s fundamental capability to operate as a stand-alone FACTS although the wind turbine is in stand-by mode and no longer generating power.
Fig. 4. Waveforms before and after the grid-side compensation at t = 1.2s. a) Voltage and current at the PCC. Current at the PCC is in pha with voltage and harmonics are compensated by the grid-side converter. b) Voltage and current of the load. c) Voltage and current of the wind turbine.
Fig. 5. Long-range performance under wind disturbances. Compensation takes place at t = 10 s. a) DC Bus voltage and Idq_gc’ from the controller. b) Wind speed, active and reactive output power of the DFIG-WT. c) Active and reactive power at the PCC. d) PCC line current harmonic distortion (%). It is clear that although the wind turbine is no longer generating power, THD and reactive power at the PCC are well compensated.
C. Reactive power and harmonic compensation under saturated condition
As the compensating controller is designed in a way to give priority for power factor correction over harmonic compensation, Fig. 6 prents waveforms when the controller
is saturated. A 1.2 MVAR load combined to a 1.26 MW non-linear load at 29 % current THD, e Fig. 6 (b), are fed under the scheme as in ca A. Fig. 6 (a) shows the quick power factor correction at the PCC while harmonics cannot be compensated without overloading the grid-side converter. Waveforms in Fig. 6 (c) reveal that the wind turbine is still generating power at 1 p.u. current value and waveform is purely sinusoidal as a result of proper controller respon.
D. Reactive power and harmonic compensation under wind speed disturbances and saturated condition
This last ca prents same waveforms as in ca B but the dynamics of the propod technique can be obrved in Fig. 7 becau the controller is under saturated condition while the
DFIG-WT is going on stand-by mode. First, although compensation takes place at t 10 s, no harmonic
compensation will be provided becau Id_gc’ and Iq_gc’ in Fig. 7 (a) are at their maximum value for proper DC Bus voltage regulation and power factor compensation. As the wind speed decrea, and so the DFIG output power in Fig. 7 (b), some extra room for compensation will be relead and this leads to harmonic compensation starting around t = 15 s. This fact holds even after the wind turbine is shut down at t = 33 s, although it is impossible to bring the THD level down to 5 % as in ca B. Even if the entire grid-side converter compensating capacity is made available, Id_gc’ and Iq_gc’are still saturated.
Fig. 6. Waveforms before and after the grid-side compensation at t = 1.2 s under saturated condition. a) Voltage and current at the PCC (p.u.). Current at the PCC is fully back in pha with voltage although part of harmonics remains. b) Voltage and current of the load (p.u.). c) Voltage and current of the wind turbine (p.u.). DFIG-WT output current is sinusoidal and providing maximum of VARs to the network.
Cas studied in ction IV prove the concept of using the grid-side converter as a shunt active filter
for harmonic compensation and power factor correction. Experimental verification in the laboratory of the propod technique is underway. Fig. 7. Long-range performance under wind disturbances and saturated condition. Compensation takes place at t = 10 s. a) DC Bus voltage and Idq_gc’ from the controller. b) Wind speed, active and reactive output power of the DFIG-WT. c) Active and reactive power at the PCC. d) PCC line current harmonic distortion (%). Dynamics of the controller are obrved while extra room is made available for compensation.
V. C ONCLUSION
This article has prented a strategy to upgrade the conventional grid-side converter command in order to turn the DFIG-WT into a green power source with FACTS capabilities. Compared to other methods where harmonics as well as reactive power injected in the generator, the propod technique compensate for harmonics and power factor, regardless of the wind turbine state, DFIG parameters, number of non-linear loads and without side effects on the wind turbine generator. Proper priority control for reactive power and harmonics is also introduced and tested, all in agreement with the converter limitations and application specifications to support the grid.
VI. A PPENDIX
Direct and rever transformation of the abc reference frame to the rotating dq reference frame when zero quence is neglected.
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