Wind energy conversion into electricity by means of the rectifier with near sinusoidal input current‐1 converter
2013; Institution of Engineering and Technology; Volume: 7; Issue: 5 Linguagem: Inglês
10.1049/iet-rpg.2012.0158
ISSN1752-1424
AutoresD. Alexa, Irinel Valentin Pletea, Adriana Sîrbu, Alexandru Lazar,
Tópico(s)Advanced DC-DC Converters
ResumoIET Renewable Power GenerationVolume 7, Issue 5 p. 475-483 ArticleFree Access Wind energy conversion into electricity by means of the rectifier with near sinusoidal input current-1 converter Dimitrie Alexa, Dimitrie Alexa Faculty of Electronics, Telecommunications and Informations Technology, Technical University 'Gheorghe Asachi' of Iasi, Blv. Carol I, no. 11, Iasi, 700506 RomaniaSearch for more papers by this authorIrinel Valentin Pletea, Corresponding Author Irinel Valentin Pletea ivpletea@etti.tuiasi.ro Faculty of Electronics, Telecommunications and Informations Technology, Technical University 'Gheorghe Asachi' of Iasi, Blv. Carol I, no. 11, Iasi, 700506 RomaniaSearch for more papers by this authorAdriana Sirbu, Adriana Sirbu Faculty of Electronics, Telecommunications and Informations Technology, Technical University 'Gheorghe Asachi' of Iasi, Blv. Carol I, no. 11, Iasi, 700506 RomaniaSearch for more papers by this authorAlexandru Lazar, Alexandru Lazar Faculty of Electronics, Telecommunications and Informations Technology, Technical University 'Gheorghe Asachi' of Iasi, Blv. Carol I, no. 11, Iasi, 700506 RomaniaSearch for more papers by this author Dimitrie Alexa, Dimitrie Alexa Faculty of Electronics, Telecommunications and Informations Technology, Technical University 'Gheorghe Asachi' of Iasi, Blv. Carol I, no. 11, Iasi, 700506 RomaniaSearch for more papers by this authorIrinel Valentin Pletea, Corresponding Author Irinel Valentin Pletea ivpletea@etti.tuiasi.ro Faculty of Electronics, Telecommunications and Informations Technology, Technical University 'Gheorghe Asachi' of Iasi, Blv. Carol I, no. 11, Iasi, 700506 RomaniaSearch for more papers by this authorAdriana Sirbu, Adriana Sirbu Faculty of Electronics, Telecommunications and Informations Technology, Technical University 'Gheorghe Asachi' of Iasi, Blv. Carol I, no. 11, Iasi, 700506 RomaniaSearch for more papers by this authorAlexandru Lazar, Alexandru Lazar Faculty of Electronics, Telecommunications and Informations Technology, Technical University 'Gheorghe Asachi' of Iasi, Blv. Carol I, no. 11, Iasi, 700506 RomaniaSearch for more papers by this author First published: 01 September 2013 https://doi.org/10.1049/iet-rpg.2012.0158Citations: 2AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract The study presents a wind generator system based on a new converter configuration with a rectifier with near sinusoidal input currents (RNSICs-1 converter with DC capacitors connected in parallel with diodes). A detailed analysis of the system for different values of the load current is presented and the advantages of the solution are emphasised. The new converter configuration is characterised by smaller power losses, reduced electromagnetic interference (EMI) problems, low harmonic input currents, high reliability, as well as reduced costs. This original configuration could also be used for small hydro interconnection with squirrel cage induction generator (SCIG) and partial variable-speed wind turbine (typically 60–100% synchronous speed). 1 Introduction In view of the fact that wind energy is green energy, its share in electricity generation is expected to increase in the future. In recent years, we have witnessed remarkable technologic progress, but, at present, the electricity produced from this source is not yet entirely competitive from an economic point of view. A wind system must draw maximum power when the wind speed varies widely [1, 2]. In the beginning, some of the installed wind turbines operated at near constant speed. This implies that regardless of the wind speed, the angular speed of the rotor is fixed and determined by the frequency of the supply grid, the gear ratio and the generator layout. In general, constant speed solutions are characterised by a simple and reliable construction of the electrical parts, while the mechanical parts are subject to higher stresses and additional safety factors must be incorporated in the mechanical design. Most fixed speed turbines use induction generators. Figs. 1a and b present two versions of the wind systems, with fixed speed and with partially variable speed. The first version, with fixed speed wind turbines, according to Fig. 1a, corresponds to the so-called Danish concept that was very popular in the 1980s. This wind turbine has a fixed speed controlled machine, with asynchronous squirrel cage induction generator (SCIG) directly connected to the grid via a transformer. This concept needs a reactive power compensator to reduce (almost eliminate) the reactive power demand from the turbine generators to the grid. This is usually done by continuously switching capacitor banks following the power production variation (5–25 steps). Smoother grid connection occurs by incorporating a soft-starter as shown in Fig. 1a. Regardless of the aerodynamic power control principle in a fixed speed wind turbine, the wind fluctuations are converted into mechanical fluctuations and further into electrical power fluctuations. Thus, the main drawbacks of this variant are: does not support any speed control, requires a stiff grid and its mechanical construction must be able to support a high mechanical stress caused by wind quests. Fig. 1Open in figure viewerPowerPoint Variants of wind systems with constant speed and partial variable speed a Fixed speed wind turbine with directly grid-connected squirrel-cage induction generator b Partial variable-speed wind turbine with variable rotor resistance c Doubly fed induction generator using back-to-back PWM converter The second version has a partially variable speed wind turbine with variable rotor resistance, according to Fig. 1b. It uses a SCIG and it has been used since the mid 90's. The generator is directly connected in series with a controlled resistance, whose size defines the range of the variable speed (typically 0–10% above synchronous speed). A capacitor bank compensates the reactive power. Smooth grid connection occurs by means of a soft-starter. An extra resistance is added in the rotor circuit. Thus, the total rotor resistance is controllable and the slip, and thus the power output in the system are controlled. The dynamic speed control range depends on the size of the variable rotor resistance. The energy coming from the external power conversion unit is dissipated as heat loss activated at full load operation. As the size of wind turbines increases and the penetration of wind energy in certain areas increases, the inherent problems of the constant speed wind turbines become more and more pronounced, especially in areas with relatively weak supply grids. To overcome these problems, the trend in modern wind turbine technology is doubtless towards variable-speed concepts. However, the introduction of variable-speed wind turbines increases the number of applicable generator types and further introduces several degrees of freedom in the combination of generator and power converter types [3-13]. The variable-speed systems offer a number of important advantages: The turbine can be adjusted to local conditions or to imperfections in blade characteristics; Aerodynamic noise at a low wind speed can be reduced by lowering the turbine speed; The useful energy capture on partial load is maximised by the optimal speed operation; Power fluctuations are reduced; and Lengthy stress on the rotor blades and on the transmission system is reduced. The doubly fed induction generator using a back-to-back PWM converter in the rotor circuit (Scerbius drive) has long been a standard drive option for high-power applications involving a limited speed range according to Fig. 1c. The characteristics of such a Scerbius scheme, in which both converters are vector controlled, are as follows: Operation below, above and through the synchronous speed range restricted only by the rotor-voltage rating of the doubly-fed induction generator (DFIG). Operation at synchronous speed, with DC currents injected into the rotor with the inverter working in chopping mode. Low distortion stator, rotor and supply currents. Control of displacement factor between the voltage and the current in the supply converter, and hence control over the system power factor. The stator is directly connected to the grid, while a partial-scale power converter controls the rotor frequency and thus the rotor speed. The power rating of this partial-scale frequency converter defines the speed range (typically ± 30% around synchronous speed). The smaller frequency converter makes these concepts attractive from an economical point of view. In this case, the power electronics is enabling the wind turbine to act as a dynamic power source to the grid. However, its main drawbacks are the use of slip-rings and the protection schemes/controllability in the case of grid faults. 2 Wind generator system with rectifier with near sinusoidal input current (RNSIC)-1 converter Fig. 2 presents a new variable-speed wind system. The electricity produced by the induction generator SCIG is transferred into the network by means of a frequency converter. It is made up of a RNSIC-1 converter, a boost converter and a PWM inverter connected to the supply grid. Fig. 2Open in figure viewerPowerPoint System of wind/hydro generator with a rectifier with near sinusoidal input currents The RNSIC-1 converter has three capacitive levels. The first level is represented by capacitors C1, C3 and C5 that are permanently connected in parallel to diodes D1, D3 and D5. The second level is made up of capacitors C41, C61 and C21, connected in parallel to diodes D4, D6 and D2 by means of switches K41, K61 and K21, respectively. The last capacitive level is obtained by connecting in parallel capacitors C42, C62 and C22 to the corresponding diodes. This system insures the reactive power needed by the induction generator in order to function with partially variable speed in a range of about 60–100% of the synchronism speed. The switch between the capacitive levels is made to insure a practically constant magnetisation current for the induction generator, when its speed varies. The RNSIC-1 converter, according to Fig. 2, represents a resistive–capacitive load for the induction generator. This converter has the following main advantages: It provides practically sinusoidal stator currents iR, iS and iT to the induction generator, according to the functioning principle of RNSIC converters [14-19]. This advantage does not exist for the wind systems in Figs. 1a and b, because of the soft-starter [1]. It determines a practically constant magnetisation current for the induction generator when its speed varies. In order to obtain a variable-speed operation and stable DC bus voltage, a boost DC–DC converter could be inserted in the DC link, as shown in Fig. 2. The RNSIC-1 converter output voltage Vd is amplified to the value Vdc at the input of the PWM converter connected to the grid. 3 Characteristics of the RNSIC converters Two variants of RNSIC converters have been proposed that practically eliminate the current input harmonics [14-19]. The operation of these converters is not influenced by the voltage or current harmonics from the power grid; thus, avoiding resonance phenomena. The RNSIC-1 variant, shown in Fig. 3a, has six capacitors C1, C3, C5, with a capacitance equal to (C + ΔC) and C4, C6, C2, with a capacitance equal to (C − ΔC). Fig. 3Open in figure viewerPowerPoint Variants of RNSIC converters a RNSIC-1 with six DC capacitors connected in parallel with diodes b RNSIC-2 with three AC capacitors connected in the AC side The ΔC value can vary between ( − C) and ( + C), whereas the functioning of the RNSIC-1 converter remains the same [18]. The phase currents iR, iS, iT and the angle ϕ are not modified on the variation of ΔC. Of course, the currents through the capacitors are proportional to the values of the associated capacitors. Also, by choosing ΔC equal to ( − C) or ( + C), one can obtain three capacitors for RNSIC-1 having the capacitance 2C connected in parallel with diodes D4, D6, D2 or D1, D3, D5. The RNSIC-2 converter, shown in Fig. 3b, has three alternating current (AC) capacitors equal to C. The RNSIC converter variants also have three inductors L1. For a single RNSIC-1 or RNSIC-2 converter, the following condition must be fulfiled (1)In order to provide the practically psinusoidal iR, iS, iT phase currents (ω denotes the mains angular frequency), [14, 15]. The variation of the φ angle between the phase voltage (for example vR0) and the fundamental harmonic of the phase current [i.e. iR(1)] is shown in Fig. 4a. Fig. 4Open in figure viewerPowerPoint Characteristics of RNSIC converter a Angle φ as a function of ratio Vd/Vref b Ratio Vd/Vref and I(1)/Imax as a function of RL/RLr The rated regime, indicated by the F point, is considered for φ = 0°, the load resistor of the rated value RLr and the rated current I(1)r. Vd is the rectified average voltage, is the reference voltage specific for three phase rectifiers with classical diodes and Vmax is the AC input voltage maximum magnitude. The Vd voltage can be established at a certain value of the load current Id(2)where I(1) is the magnitude of the phase current fundamental and t1 is the time when the diodes of the RNSIC converter begin to conduct. The ωt1 angle varies between 40 and 50° for the rated regime. The rectifier voltage Vd for RNSIC-1 is equal to Vref/(1 − 2L1Cω2). This voltage exceeds the reference value Vref. Fig. 4b shows the variation of the output voltage rated to the reference value Vref and the magnitude of the phase current I(1) rated to the reference value Imax = Vmax/L1ω as a function of the ratio RL/RLr for the RNSIC-1 converter. The minimum amplitude Imin of the phase current has the following value [15] (3)and is obtained when the load resistance RL has an infinite value. Next, we apply the RNSIC-1 converter to the wind system presented in Fig. 2. The stator angular frequency ωS of the induction generator varies between the minimum value ωmin and the maximum value ωmin, depending on the wind speed. The ratio ωmin/ωmax is approximately 0.6. The maximum value ωmax is equal to 2πfmax, where the maximum stator frequency fmax is 50 or 60 Hz. The maximum amplitude of the phase currents iR, iS, iT is equal to [15] (4)If the load resistance RL is zero and the induction generator is replaced by a three-phase source with the amplitude Vmax of phase voltages vR0, vS0 and vT0. 4 Functioning of the induction generator connected to the wind system with RNSIC-1 Fig. 5 shows the self-excitation process of the induction generator, based on the presence of a remanent flux in the rotor [1, 2]. This remanent flux creates the stator voltage Erem when the magnetisation current is null. For this generator, the RNSIC-1 converter with several capacitive levels, illustrated in Fig. 2, represents a resistive–capacitive load. In order to insure the necessary magnetisation current Img = I(1)sinφ, when ωS has lower values down to the minimum value ωmin, the first level, represented by the permanently connected capacitors C1, C3, C5 must be replaced by the second or third level. The second level consists of connecting capacitors C41, C61, C21 by means of switches K42, K62, K22, respectively. Thus, a practically constant magnetisation current is provided when the angular frequency ωS varies between its maximum (ωmax) and minimum value (ωmin). Fig. 5Open in figure viewerPowerPoint Variations of ratio Vm/Vmax as a function of Img/I(1)r for different values of ωS If a simple battery of capacitors was connected to terminals of the induction generator, the capacitive current delivered by the battery would decrease with the reduction of the voltage Vm and of the angular frequency ωS. If we used a battery of capacitors with more sections, we would find that, when the speed of the generator rotor is low, the excitation capacity must be high, higher that the one needed for realising a RNSIC-1 converter with more capacitive sections, according to Fig. 2. Moreover, the solution does not eliminate the high current harmonics of the phase currents iR, iS, iT. In the present case, we cannot use passive filters for reducing the current harmonics of stator currents because ωS is variable. The switches can be realised with two antiparallel connected thyristors, as for the case of reactive power compensators with thyristors. Owing to the fact that these switches are traversed by low currents, the power losses are negligible. The average currents that circulate through the switches do not exceed (3–5%) I(1)r for the variation range of ωS between ωmin and ωmax. These average values mentioned above are necessary to chose thyristors of the switches. The capacitors of the RNSIC-1 converter play a double role. In the one hand, they ensure near sinusoidal input currents iR, iS, iT, and on the other hand some capacitors connected in parallel with the capacitor C0 at Vd allow a better load supply. For instance, for low ωS, when diodes D1 and D6 are in conduction, capacitors C41, C42 and C3 are connected in parallel to C0. Moreover, the composed element C5 connected in series with (C21 + C22) is connected to C0. Therefore the capacitors of the RNSIC-1 converter contribute by the value ΔC0 to reduce the capacitance needed for C0. The contribution of ΔC0/C0 can be of about 5–10%. One of the advantages of the wind system, illustrated in Fig. 2, is the fact that it requires DC capacitors, which are smaller in size than the AC capacitors used for systems presented in Figs. 1a and b [20]. As for the boost converter made up of capacitors C0 and Cdc, the inductance L2, the diode D and the switch K, it allows the voltage to increase from Vd to Vdc. The voltage Vdc can be used at the input of the PWM inverter. Fig. 6 presents the variation of power P transmitted by the wind turbine to the generator shaft for different values of the wind speed vW and for the three operating steps. This figure illustrates that the wind system has an optimum functioning for a variation of the rotor angular speed ωr of about 40%. Fig. 6Open in figure viewerPowerPoint Power transmitted to the hub shaft of different wind speeds VW The variable-speed wind turbines were designed to achieve the maximum aerodynamic efficiency over a wide range of wind speeds, for instance, on three levels. Seen from the wind turbine system point of view, the most important advantages of the variable-speed operation compared with the conventional fixed speed operation are reduced mechanical stress and the mechanical components such as shaft and gearbox, increased power capture and reduced acoustical noise. 5 Experimental and simulation results Laboratory experiments and simulation results have proved the effectiveness of the proposed wind system with RNSIC-1. The experimental model is made up of the following components: A DC motor that drives a variable-speed induction generator (SCIG). The generator nominal power is Pgr = 10 kW, the value of the maximum phase stator voltages Vmax = 311 V, the nominal stator current Igr = 23.7, the maximum stator frequency fmax = 50 Hz and the minimum stator frequency is fmin = 30 Hz. An RNSIC-1 converter with three capacitive levels, according to Fig. 2. The capacitors for the first level are C1 = C3 = C5 = 2C, having the capacitance 2C = 80 µF. The additional capacitors for the second level are C41 = C61 = C21, having a capacitance of 20 µF, and the ones for the third level are C42 = C62 = C22, with a capacitance equal to 40 µF. The capacitance of the C0 capacitor is 3000 µF and includes the contribution ΔC0 of about 140 µF. The three inductors L1 have an inductance of 20 mH. The parameter is thus equal to 0.079. The boost converter is made up of C0, the Cdc capacitor of 2000 µF, the L2 inductor of 20 mH, the diode D and the switch K, using an insulated-gate bipolar transistor (IGBT) transistor. The voltage applied to Cdc is equal to the maximum value of 647 V applied to C0 when SCIG functions at the maximum stator frequency of 50 Hz. We adopted an adjustable resistance Rreg as a load for the experimental model; its value depends on the ratio Vdc/Vd(5)considering that the boost converter functions with no losses and that the calculation resistance RL adopted is equal to 40 Ω. The nominal resistance is RLr, on which the nominal voltage Vdr is applied. Considering this nominal regime, the angle φ = 0. For (1), the is considered equal to 0.066, for a procentual total harmonic distortion (THD) of 5%. The nominal voltage (Vdr) in this case is The nominal resistance RLr (using a simplified formula deduced from simulation and experimental investigations): . The RNSIC-1 converter has to work on the capacitive side of the φ angle (Fig. 4a). On functional stage switching, the absorbed capacitive current by the SCIG remains practically unchanged, as can be seen in Fig. 7. However, the active current supplied by the SCIG varies from a nominal value to a minimal one and vice versa. Fig. 7Open in figure viewerPowerPoint Characteristics of RNSIC-1 converter as a function of ratio Vm/Vmax Table 1 presents some experimental data depending on the ratio ωS/ωmax, which vary between 1.0 and 0.6. It indicates the values of the total capacitance Ctot corresponding to a phase of the converter RNSIC-1, namely 80, 100 and 140 µF for the three adopted capacitive levels. The values presented next are the Vd voltage at the terminals of the capacitor C0 and the amplitude of the fundamental harmonic current I(1) of the stator current. The angle ϕ shows that stator currents iR, iS, iT have capacitive components. When the generator is idling (the calculation resistance RL and the load resistance Rreg have infinite values and the IGBT transistor is blocked), these currents are purely capacitive [14-16]. The capacitive currents delivered to SCIG by RNSIC-1 remain practically constant when the ratio ωS/ωmax arises and represent the magnetisation currents Img of the induction generator. The THD factor of the stator currents, which varies between 4.25 and 8.6%, points out the advantage of using the RNSIC-1 converter in order to considerably reduce the high harmonics of the stator currents. Among all stator current harmonics, the highest one is of the order n = 5 and is indicated in Table 1. The load resistance Rreg is connected to the terminals M and N at the output of the boost converter. Finally, the last column of the table represents the active power delivered by the induction generator that dissipates on the resistance Rreg. Adopting a constant calculation resistance RL of 40 Ω at the output of the RNSIC-1 converter, according to Fig. 3a, allows the capture of active energy at partially maximised load, by an optimal speed operation, according to Fig. 6. Table 1. RNSIC-1 with three capacitive steps: C1 = C3 = C5 = 2C = 80 μF, C41 = C61 = C21 = 20 μF, C42 = C62 = C22 = 40 μF, L1 = 20 mH and C0 = 3000 μF ωS/ωmax Ctot, μF Vd, V I(1), A φ, ° Img, A THD, % [I(5)/I(1)],% Rreg, Ω P, W 1.00 80 647 25.2 − 24.2 10.33 4.25 3.73 40 10450 0.95 80 602 23.1 − 25.2 9.84 4.71 4.25 46.2 9060 0.90 80 557 21.1 − 26.3 9.35 5.28 4.85 54 7750 0.85 100 538 21.4 − 29.2 10.44 4.80 4.13 58 7200 0.80 100 494 19.3 − 29.8 9.58 5.32 4.81 68.5 6120 0.75 100 452 17.3 − 30.6 8.81 6.05 5.70 82 5120 0.70 140 439 18.3 − 34.3 10.31 5.44 4.26 87 4805 0.65 140 395 16.1 − 34.8 9.19 5.56 5.05 107 3910 0.60 140 355 14.1 − 36 8.29 6.6 6.18 133 3150 Fig. 7 experimentally illustrates the variation of the Img/Igr ratio, depending on the Vm/Vmax ratio. Certainly, if we had adopted an RNSIC-1 with more capacitive levels (for instance, four or five level), the deviation of the magnetisation current Img from the average value would have been even smaller. Figs. 8 and 9 present the variations of the iR stator current and of the Vd voltage at the output of the RNSIC-1 converter for two distinct functioning cases of the proposed wind system. In the first case, according to Figs. 8a and b, the first functioning level, with ωS/ωmax = 0.95 and Ctot = 80 µF, is replaced by the second functioning level, with ωS/ωmax = 0.80 and Ctot = 100 µF. In the second case, according to Figs. 9a and b, the transition is made from the third functioning level, with ωS/ωmax = 0.65 and Ctot = 140 µF, to the first functioning level, with ωS/ωmax = 0.95 and Ctot = 80 µF. Fig. 8Open in figure viewerPowerPoint Wind system with RNSIC-1 converter. Transition from first to second level a Voltage vd b Current iR Fig. 9Open in figure viewerPowerPoint Wind system with RNSIC-1 converter. Transition from third to first level a Voltage vd b Current iR The experimental results illustrated in Figs. 8 and 9 show that the wind system proposed in Fig. 2 can be used as a partially variable-speed system in the range of 60–100% of the value of the ωS/ωmax ratio. This system has all the advantages of variable-speed systems as mentioned in Section 1. For comparison purposes, we present in Fig. 10 a variable-speed wind turbine with SCIG, AC capacitor bank, boost converter and PWM grid inverter. This wind system is considered to be a function at a stator frequency ωS that varies between 60 and 100% of ωmax. When the minimum values ωmin = 0.6ωmax and Vm = 0.6Vmax occur, the required value of the capacitance Cb corresponding to a phase in order to obtain an average amplitude of the magnetisation current Img = 9 A, is given by the formula (6) Fig. 10Open in figure viewerPowerPoint Variable-speed wind turbine with SCIG and AC capacitor bank We note that the value obtained for the Cb capacity is almost twice as high as the total value Ctot = 140 µF, which is necessary for the functioning of RNSIC-1 converter at the third level. Moreover, the wind system, according to Fig. 2, insures a lower content of high current harmonics for currents iR, iS and iT. These two advantages result from the functioning principle of the RNSIC-1 converter that catches increasing attention for its simple configuration, high reliability, as well as the reduced cost. The pulsation of the electromagnetic torque for the proposed wind system is within the normal limits and their effect can be mechanically reduced. The most important stator current harmonic is the fifth. Even if we use the method designed to reduce the L1 inductances, this harmonic does not exceed 7% of the fundamental harmonic. Comparing with different wind turbine topologies is respect to their performances that will reveal a contradiction between cost and performance to the grid. In our research, we have noted that the size and losses of the boost converter result is important. As far as the three L1 inductances are concerned, they can be shrunk by 40–50% using the following method. In Fig. 11 presents the variation of the THD% factor according to the values of the L1Cω2 parameter and of the RL/RLr ratio. If L1Cω2 is given a lower value, for instance 0.05 instead of 0.079 (the value adopted in the paper), the inductance sizes can be reduced by 40–50%. On the other hand, the value of the THD% factor of stator currents iR, iS and iT can increase from 3.7–4 to 7–8%, as shown in Fig. 11. For comparison purposes, we mention that the line current in a three-phase rectifier in the idealised case in which AC-side inductance Ls = 0 and the DC-side current Id is constant, has a THD% factor = 29.66. Fig. 11Open in figure viewerPowerPoint Variations of factor THD as function of RL/RLr and L1Cω2 As to the L2 magnetic core inductance within the boost DC converter, it can have acceptable sizes if the K switch operates with a frequency between 1.5 and 2 kHz. One class of magnetic materials is comprised of alloys principally of iron and small amounts of other elements including chrome and silicon. These alloys have large electrical conductivity compared with ferrites and large values of saturation flux density, near 1.8 T (one T = 1 Wb/m2). If a comparison in size is made between a back-to-back PWM converter and the RNSIC converter system version, the former requires a three-phase transformer in the rotor circuit, whereas the latter does not. This three-phase transformer can have a power of about 30% of the DFIG generator power. It is necessary, even if DFIG is directly connected to the grid, as in Fig. 1c, because voltages on the rotor rings must be limited [21]. Therefore the RNSIC-1 converter wind energy system presented in this paper is adequate for applications involving tenths of kilowatts (kW) to hundreds of kW. This system is seen as more a robust and requires less maintenance as compared with other more modern solutions. 6 Conclusions This paper presents a wind generator system based on a new converter configuration with an RNSIC-1 converter. The new wind generator system is characterised by smaller power losses, reduced EMI problems, low harmonic statoric currents, reduced power fluctuations, high reliability, as well as reduced costs. 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