Wind Energy Conversion Using Shunt Active Power Filter

Transcription

1 Quatrième onférence Internationale sur le Génie Electrique IGE 1, 34 Novembre 21, Université de echar, lgérie Wind Energy onversion Using Shunt ctive Power Filter Samira Dib, rahim Ferdi, hellali enachaiba bstract This paper presents a Shunt ctive Power Filter using Wind Energy to feed linear or nonlinear loads with current perturbations compensation and the excess of the energy is injected into the mains. s a result of using instantaneous pq theory as a control scheme, the multifunction operation such as harmonic elimination, reactive power control and uninterruptible power supply will be achieved. The system consisting of wind turbine coupled to permanent magnet synchronous generator (PMSG), connected to a diode rectifier feeding a shunt active power filter is simulated in MTL/SIMULINK environment. The simulation results prove the efficiency of using the proposed method for wind energy injection and power quality improvement in the grid power system. Key words: Wind energy, PMSG, Wind turbine, Shunt active power filter, current harmonics, pq theory. 1. Introduction In recent years, wind energy is recognized as one of the most mature, costefficient renewable sources of electricity. lso it is the most rapidlygrowing means of electricity generation at the turn of the 21st century. Global installed capacity has raised 2 % in 24 [1]. The majority of the installed wind capacity is arranged in groups of turbines called wind farms or arrays. Developments in gearless, variablespeed generators with power electronics grid interface leads to a generation of quiet, reliable, economical wind turbines. The most usual technology of this directdriven wind turbine is composed by a multipole permanent magnet synchronous generator (PMSG), threephase bridge rectifier with a bulky capacitor, and currentcontrolled voltage source inverter (VSI) [2]. Nonlinear devices produce distorted current waveforms in the power system. The injected harmonics have several impacts on the utilities grid and loads connected to system. To overcome these power quality problems, harmonic active filters are widely used in the system. [38]. In this paper, the analysis are focused on the system configuration with a direct coupling between the wind turbine and the shunt active power filter employed to inject the wind power into the utility grid under fixed wind speed conditions. The proposed design is not only able of delivering the wind power to the grid, but will also act as a shunt active power filter (SPF) to mitigate the current harmonics and regulate reactive power injected by the nonlinear loads. In order to investigate and mitigate the harmonic capabilities of the proposed system; a 1MW wind turbine with shunt active power filter connected to a threephase power grid feeding nonlinear load was simulated in MTL / SIMULINK environment. 2. Shunt ctive Power Filter Shunt active power filter is a power converter utilized in order to compensate current disturbances (harmonics, reactive power and unbalance). In order to meet quality enhancement constraints proper control of its power switches is needed. Several topologies and configuration have been introduced in the literature and in commercial implementations for this filter that highlight different aspects of its compensation tasks. The most common topology of the shunt active power filter is shown in fig. 1. Its main components are voltage source inverter, D bus (in our situation is a capacitor), output passive filter and a control system. The most important objective of the PF is to compensate the current harmonics generated by non linear loads. The reference currents consists of the harmonic components of the load currents which the active filter must supply [9]. These reference currents are fed through a controller to generate switching signals for the power switching devices of the voltage source inverter (VSI). Finally, the supply will only need to provide the fundamental component for the non linear load. Fig. 1: General structure of shunt active power filter 141

2 Quatrième onférence Internationale sur le Génie Electrique IGE 1, 34 Novembre 21, Université de echar, lgérie 3. Wind Turbine Model There are many different generator concepts for windpower applications in use today. The main distinction can be made between fixedspeed and variablespeed windgenerator concepts. fixedspeed windgenerator is usually equipped with a squirrelcage induction generator whose speed variations are only very limited. Power can here only be controlled through pitch angle variations. ecause the efficiency of windturbines depends on the tipspeed ratio, the power of a fixedspeed wind generator varies directly with the wind speed. In contrast to this, variable speed concepts allow operating the wind turbine at the optimum tipspeed ratio and hence at the optimum powercoefficient for a wide wind speed range. Varying the generator s speed requires frequency converters that increase investment costs. In most modern designs, a synchronous generator or a permanent magnet generator is used [1, 11]. In this case the total generated power flows through the converter as shown in fig.2. Fig. 3: Wind Energy onversion System (WES) The VSI is controlled in such a way that it can be used to inject sinusoidal current into the grid for energy extraction from the wind turbine during linear or nonlinear load conditions. During nonlinear load conditions, VSI can be used also as PF for harmonic and reactive compensation. To control the performance and the effectiveness of the WES, the VSI is operated based on the concept of pq theory. The control input is a current error signal which in this application, is the difference between the actual current injected by VSI and the desired or reference current waveform. 5. Reference urrents Generation The reference currents for the control of the SPF are calculated using the active and reactive power analysis in a stationary αβ frame (pq theory). Load currents and phase voltages of the threephase system expressed in αβ frame are given by: Fig. 2: Permanent magnet synchronous generator based wind turbine 4. Wind Energy onversion System The Wind Energy onversion System (WES) considered in this paper consists of a PMSG driven by a fixed pitch wind turbine. The configuration of this WES is illustrated in Fig. 3. Where i a, i b, i c are the load currents and e a, e b, e c are the threephase grid voltages. The instantaneous real power and the instantaneous imaginary power absorbed by the load are, respectively, defined as follows: p l and q l are made up of a D and an component, so that they may be expressed by: Where and are D components due to fundamental currents while and are components due to harmonic currents. In order to generate the reference currents, a balance between instantaneous powers supplied by the grid and the SPF and drained by the load is to be computed. If p g and q g are the real and imaginary instantaneous 142

3 Quatrième onférence Internationale sur le Génie Electrique IGE 1, 34 Novembre 21, Université de echar, lgérie powers supplied by the main, while p f and q f are the real and imaginary instantaneous powers supplied by the SPF, in order to compensate reactive power and eliminate harmonic currents, the grid should supply p g = and q g =. The oscillatory component of p l is to be fed by SPF, while q l must be fully fed by the SPF because it is also possible in this way to achieve reactive power compensation. The oscillatory part of p l is due to harmonic components, so if it is fed to the load by the SPF, grid current remains sinusoidal, while the load keeps on receiving the same amount of harmonic and fundamental current. Power balance yields: SPF. Hence the instantaneous reference powers for the SPF are: transformation from instantaneous powers to currents allows generating proper reference for currents control according to the following equation: 6. Simulation Results and Discussion Previous equations need to be modified in order to consider proper operation of the capacitor on the D side of the inverter. The capacitor stores energy which is utilized as a power supply for the normal operation of the SPF. More in detail, in normal operating conditions the SPF does not feed active power because it should be able to supply p f = and q f = q l and so only reactive power is fed. For this reason, capacitor voltage level is constant during the steady state. In order to regulate D voltage level, it is necessary to control active power balance among the grid, load and SPF. When the load absorbs a precise quantity of power and if p g > excess power is drawn by the SPF, which increases the D side voltage. If p g <, since the load needs a precise amount of power, the SPF feeds the remaining part in order to have = and hence the D voltage level decreases. To control the proper amount of active power fed or drawn by the SPF, it is necessary to introduce a gain factor k [12]. In normal conditions this gain is quite near unity, because the losses in SPF components are negligible. When D capacitor charging is needed, the gain factor is above unity because grid must supply an additional amount of active power to the SPF. When D voltage level is too high, gain factor is regulated to values below unity, so a power less than is required to the grid and the remaining part of is fed to the load by the The proposed WES is not only capable of supplying extracted wind power to the power system, but it also can significantly mitigate harmonic currents which are drawn by nonlinear loads. In order to demonstrate the validity of the concepts discussed previously a simulation using MTL/SIMULINK environment is done as it is shown in Figure 4. The parameters of the system are shown in table I. N a b c Source ontrol 22 V LOD UR R ENT Output Filter g SOUR E VOLTGE V S I R EF V d c a b c _Dlink [I_Load] [V_S ource] [V_D] Rectifier Rectifier Fig. 4: SIMULINK Implementation of WES Table I : parameters of WES Grid Source Voltage V s Load Power P L Frequency f s Wind Turbine Nominal Power P T Turbine Voltage V T Turbine frequency f T Wind Speed 22 V 8 kv 5 Hz 1 MW 22 V 5 Hz 1 m/s Pitch ngle 8.8 Generator SPF PMSG 143 RL Wind Turbine

4 Vdc (V) P (kw) Is () Iinj () IL () Quatrième onférence Internationale sur le Génie Electrique IGE 1, 34 Novembre 21, Université de echar, lgérie Switching Frequency Output Filter D Link apacitor apacitor D Voltage 12 khz 1 mh 8.8 mf 26 V reference current VSI control pq Method PWM PI L o a d u r r e n t 5 5 I n j e c t e d u r r e n t 1 1 S o u r c e u r r e n t W i n d T u r b i n e P o w e r 5 D L i n k V o l t a g e T i m e ( S ) Fig. 5: Simulation results of WES The simulation results of the proposed WES are shown in the figure 5. Under the constant wind speed operation of 1 m/s the WES produces a 5 Hz sinusoidal current with 12 khz harmonic due to the switching frequency in the VSI of the SPF. From time.3 to.5 second the wind turbine power is increasing what makes the absorbed current from the source by the non linear load decreases. t approximately.5 second the wind turbine produces all the power needed by the non linear 144

5 Quatrième onférence Internationale sur le Génie Electrique IGE 1, 34 Novembre 21, Université de echar, lgérie load and the current of the source decreases to zero. Just after.5 second wind turbine produces more power than it is needed by the load and we can see the current flow into the source, so we can say that the wind turbine starts delivering power to the grid after it has finished feeding the non linear load by all the power it needs. Finally, it is clear that the SPF injects appropriate amount of current to mitigate harmonics generated by the non linear load and at the same time deliver the excess active power to the grid. 7. onclusion Wind power seems to be the favorable clean energy source of the future. So, to optimize its use we have proposed a direct coupling of wind turbine with shunt active power filter (SPF). From the results obtained, it is proven that by using the proposed system, wind power can be efficiently extracted by wind turbine and injected into the grid by SPF which has two functions; the first is feeding the linear or non linear load with harmonic current mitigation capability and second injecting the surplus power into the mains. Finally and according to the obtained results we can consider the proposed system to be efficient solution to the growing demand of power at the present and in the future. References [1] GWE, Global Wind Power ontinues Expansion, Global Wind Energy ouncil Release, march 4 th, 25. [2] Z. hen, E. Spooner, Wind Turbine Power onverters comparative study, IEE onference on Power Electronics and Variable Speed Drives, 2123 September [3] G. Y. Jeong, T. J. Park, and. H. Kwon, "Linevoltage sensorless active power filter for reactive power compensation," Electric Power pplications, IEE Proceedings, vol. 147, pp , 2. [4] M. Izhar,. M. Hadzer, M. Syafrudin, S. Taib, and S. Idris, "Performance for passive and active power filter in reducing harmonics in the distribution system," 24. [5] G. Dawei, L. Qingchun, and S. Xlaorui, "Design and performance of an active power filter for unbalanced loads," 22. [6] H. H. Tumbelaka, " Grid urrent ontrolled Shunt ctive Power Filter Using Polarized Ramptime urrent ontrolled" in Electrical and computer Engineering Perth urtin University of Technology 26. [7] H. H. Tumbelaka and L. J. orle, "pplication of a Shunt ctive Power Filter to ompensate Multiple Nonlinear Loads," presented at ustralasian Universities Power Engineering onference (UPE). Melbourne, ustralia, 22. [8] H. H. Tumbelaka,. V. Nayar, K. Tan, and L. J. orle, "ctive filtering applied to a linecommutated inverter fed permanent magnet wind generator," presented at International Power Engineering onference IPE23,Singapore, 23. [9] K. Wada, H. Fujita, H. kagi onsiderations of a Shunt ctive Filter ased on on Voltage Detection for Installation on a Long Distribution Feeder in Proc. onf. IEEEIS nn. Meeting, 21, pp [1] M. Poller, S. chilles, "ggregated Wind Park Models for nalyzing Power System Dynamics", Fourth International Workshop on LargeScale Integration of Wind Power and Transmission Networks for Offshore wind Farms, 23, Denmark [11] S. chilles and M. Poller, " Direct Drive Synchronous Machine Models for Stability ssessment of Wind Farms," Fourth International Workshop on LargeScale Integration of Wind Power and Transmission Networks for Offshore wind Farms, 23, Denmark [12] Dell quila., Lecci., Zanchetta P., Liserre M., Fuzzy active filter performance in transient conditions, EPE (21). 145