NEW TOPOLOGY OF RESONANT CONVERTER TO IMPROVE POWER QUALITY IN THE DFIG

ISSN: 0976-2876 (Print) ISSN: 2250-0138(Online) NEW TOPOLOGY OF RESONANT CONVERTER TO IMPROVE POWER QUALITY IN THE DFIG SHAHRAM AHMADI a1 AND YOSEF NESHATI b a Department of Power Engineering, Sirjan Science and Research Branch, Islamic Azad University, Sirjan, Iran b Engineering Department of Guilan University, Rasht, Iran ABSTRACT This paper presents a new control method for a doubly fed induction generator (DFIG) and a new resonant converter (RC) for grid side converter (GSC). By using this method, the separated control of active and reactive power will be possible. Also by using the RC, the DC link capacitor in conventional two stage AC-DC-AC conversion will be omitted. To make conventional two stage AC-DC-AC output current and voltage more useful, a low pass filter should be added between the output and the grid or rotor to smooth out the ac portion. These filters need to be lossless, and was constructed from only inductors and capacitors. Capacitors and inductors that were being used in low pass filters structure and also DC link capacitor, in the conventional converters will be used in the resonant circuit of RC. The value of capacitors and nominal voltage of them will be decreased. The reactive power capability is subject to several limitations resulting from the voltage, current, and speed, which change with the operating point. In this paper the current limit of reactive power capability will be reclaimed. The simulation results show the new control method and resonant converter has good and lossless performance. KEYWORDS: Resonant Converter(RC); Doubly Fed Induction Generator (DFIG); Reactive Power Capability; Separated active and reactive power Control In recent years, the doubly fed induction generators are widely used in wind power plants. For many wind farms, wind turbines based on the DFIG technology with converter rated at about 25%-30% of generator rating are used. Modern wind turbines by virtue of the modern power electronic converters that come with them are able to control active and reactive power. But the reactive power capability requires extended design features that come at a high initial cost [1]. As a convention, the two-stage AC-DC-AC conversion system is applied for DFIG rotor side converter. However, the generator side can be used as a diode-rectifier, a thyristor-rectifier or a voltage-sourced converter (VSC). The main defects of this convention are 1) the low reliability of DC capacitor and 2) the poor line power factor and harmonic distortion in line and machine currents [2]. In the 1970s, conventional pulse width modulation (PWM) power converters were operated in switched mode. Power switches have to operate under the hard switching conditions. Hard switching refers to the stressful switching behavior of the power electronic devices. During the turn-on and turn-off processes, the power device has to withstand high voltage and current simultaneously, which results in high switching losses and stress. Also, switching loss is proportional to switching frequency. In the 1980s, much research was focused on the use of resonant converter. The concept was to incorporate resonant tanks in the converters to create oscillatory. In this paper a new resonant (ZCS and ZVS) converter is designed. Switching frequency of this converter is very low. Due to structure of this converter the rated power of IGBT is according to conventional converters (rated at 0.25 p.u). But this converter can supply fairly more reactive power at this rated power. In this paper the symmetrical sequences of stator voltage and current are used. The idea of applying symmetrical sequences in DFIGs control loop was obtained from references that examined unbalanced grid voltage [3]. By finding symmetrical sequences of stator current during rotor speed changes, the negative sequence in this current is observable. Because rotor current is produced by induction, it is expected that this current would also have the negative sequence during rotor speed changes. Regarding the changes in rotor current frequency, traditional calculations for obtaining symmetrical sequences of rotor current would be complicated. Thus introducing a method that can calculate symmetrical sequences of rotor current by using similar stator current and voltage symmetrical sequences will be useful. By expansion of electromagnetic torque and active and reactive power equations into d-axis and q-axis and also by considering symmetrical sequences in these axes, the constant modes of aforementioned equations can be 1 Corresponding author

extracted. In these equations, the oscillated modes induced by rotor speed changes can be separated. In order to prevent oscillated modes to enter control loop of DFIG, constant modes of active and reactive power of stator and GSC and that of electromagnetic torque are used as references of RSC and GSC control loop. THE CONSTANT TERMS OF ELECTROMAGNETIC TORQUE, ACTIVE AND REACTIVE POWER The positive-sequence, negative-sequence and zerosequence components of three phase systems can be according to the expression x x 1 a a x = 1 a ax (1) x 1 1 1 x x,x and x represent the tree-phase unbalance system; x, x and x are positive, negative and zero sequence; π a=e being the time operator. The influence of speed change will be analyzed in the stator current. The positive, negative and zero sequence of stator current can be calculated by using expression (1). Note that stator voltage is balance so the negative sequence of stator voltage is zero (v =0) but the stator current may be unbalanced by the changes in rotor speed. There does not exist any neutral point connection, thus sum of the currents will always be zero. Therefore zero sequence of the current is zero and also due to this reason the zero sequence of the voltage will be zero. The apparent power can be expressed using the following equation: St=PtQt= Rev.ı (2) Substituting positive, negative and zero sequence of stator current and voltage into (2), we get P=P P (3) P = v i v i (4) P = v i v i (5) Q=Q Q (6) Q = v i v i (7) Q = v i v i (8) The terms that consist of negative sequence voltage are zero, therefore these terms have been omitted. For simplicity, the time dependence of each term has been omitted. The terms P and Q are constant at steady state and terms P and Q are oscillated with changes in rotor speed. With the motor applied, the stator positive sequence d-axis and q-axis voltage in arbitrary reference frame can be expressed as: v =r i (9) v =r i (10) i i X i X i i i X i X i From expressions (9) and (10) in the stationary reference frame ( ω=0 ) the positive sequence of rotor current can be calculated as = i = i V V r i r i dt (11) dt (12) On the other hand, the electromagnetic torque can be calculated using the following equation [3]: T = P P R ı ı (13) In expression (13) the term P will be oscillated by changes in rotor speed. Thus the constant terms of active and reactive power can be calculated according to the following expression P = Q v v v v i i (14) GRID SIDE RESONANT CONVERTER(GSRC) In this paper for optimization performance of Conventional two-stage AC DC AC conversion, the resonant converter is designed. The DFIG system and GSRC are shown in Fig1. Some advantages of this resonant converter are elimination of dc link capacitor, low frequency switching, reactive power supply at vast range, resolving of conventional converters switching problems, zero current and zero voltage switching.

Figure 1: Schematic diagram of DFIG with Resonant Converter The switching frequency of GSRC is very low. Switching will be started at zero current and zero voltage. The GSRC are shown in Fig2. Note that the value of series resistance is very low. With this resistance, the current source will not be parallel with the voltage source. To avoid voltage spike at the instant switching a capacitor ( ) is used as snubber. With low value capacitor when the switch is off, the derivative of voltage in the switch will be increase. Also with high value capacitor the derivative of current in the switch will be increase when the switch is on. Thus tradeoff must be made between voltage derivative value versus value of current derivative. The resonant converter will operate at three modes. Equivalent circuits of operating these modes are shown in Fig.3. Mode.I (= ): Diod and IGBT are turn-off. Assuming that the DC link voltage has been created and applying Kirchhoff s voltage law in Fig.2 gives the following equation for GSRC parallel capacitor (1 ) voltage: = (15) Neglecting the losses and assuming an ideal IGBT and diod the inductor current can be calculated as = (16) The inductor voltage can be calculated as = (17) The initial condition of inductor current is zero and voltage of inductor is equal to parallel capacitor voltage ( ). For this initial condition and can be obtained such as: = (18) = (19) Mode.II (= ): Diod is turn-off and IGBT is turn-on. = (20) = (21) Mode.III(= ): Diod is turn-on and IGBT is turn-off. = (22) = (23) The Diod will turn-off when the inductor current becomes zero. The relationship between exchanged active power ( ) and DC link voltage and current will be as follow = (24) The DC component of parallel capacitors current is zero. By applying Kirchhoff s current law The DC link current can be calculated as = (25)

Figure 2: Grid side resonant converter Figure 3: Equivalent circuits of operating The amount of this current is dependent on the switching start up time. The current of GSRC for phase a can be calculated as = (26) On the other hand with assuming fundamental component of inductor current this current given as = (27) = = (28) Note that the operation mode is mode.ii. Thus the expression (27) must be equal to (26). With corresponding the two side of equations, we get 2 = (29) By using expression (29) for starting time of switching (t ) we get

(30) t = tan Also the amplitude in two side of expression (29) must be equal. Thus we get 2V C ω= (31) On the other hand the relationship between grid voltage frequency with inductor and parallel capacitor of ZCS can be calculated as ω= (32) In the DFIG the Dc link voltage ripple will be zero when the GSRC and RSC active power be equal. Thus the active power that flows from GSRC can be obtained as follow P =P (33) Where P is rotor active power. Neglecting the losses of induction machine, we get P = sp (34) As regards the active and reactive power that flow from GSRC are defined. The starting time switching can be obtained. Also the inductor and capacitor of the ZCS can be calculated from (31) and (32). The relationship between the Dc current and DC voltage of DC link from (24) and (25) will be nonlinear that depended on zero-crossing time of inductors current. Depended on grid voltage position, the waveform of inductor current and parallel capacitor voltage will be different. Consequently the DC link voltage and current will be different at the different operation states. The first leg of GSRC switch current and parallel capacitor voltage with grid voltage for phase a at the different states are shown in Fig.4. The grid line to line voltage is 690 V and frequency is 60 HZ. The GSRC are connected to RSC as Fig.1. The simulated generator is a 2-MW DFIG. According to the required rotor active power the starting time t of switching will be changed. This is due to the active power of GSRC and RSC must be equal. Accordingly, by changing the starting time of switching, the value and direction of DC link current and consequently the value of DC link voltage will be changed. According to the direction of DC link current (I ), the turning on of IGBT may be unnecessary. An example for this case is shown in Fig.4(c). With starting time of switching and pulse width control the GSRC will be controlled. Therefore the starting time and pulse width must be provided by GSRC controller. GRID SIDE RESONANT CONVERTER CONTROL The relationship between GSRC active power with RSC active power can be calculated as: CV =P P (35) The dc link voltage ripple will be zero when GSRC and RSC active power be equal. For this purpose the reference of active power in GSRC can be considered as P _ =P P =P T ω P (36) Reactive power reference in GSRC will be constant. With constant reference the amount of reactive power that injected from GSRC to grid, can be controlled. Schematic diagram of the grid side resonant converter controller are shown in Fig.5. Two reason of utilizing purposed control method are (i) Creating an appropriate phase shift of reference current for switching start time control (ii) Creating an appropriate amplitude of reference current for pulse width control. During rotor speed changing the reactive power reference must be changed. Therefore reference current amplitude will be changed. Figure 4: Simulated results of resonant converter at the different states a)at the rotor speed of 1.2 p.u b)at the rotor speed of 1.1 p.u c)at the rotor speed of 8 p.u a

b c Figure 5: Schematic diagram of GSRC controller T e P g Pg ref Q g V ds V qs I dg I qg ref I ag ref I bg ref I cg ω r P s Qg ref I ag K g ROTOR SIDE CONVERTER CONTROL (37): The total mechanical power in DFIG is as equation T ω =P P (37) The reference of active power of RSC can be calculated as: P _ = P T ω (38) Also for reference of reactive power of RSC we can use constant reference depend on rotor speed. By using equation (14) the dq reference of stator current will be obtained. Then by using equation (11) and (12) the rotor current reference will be calculated. By using this control method the amplitude of rotor current reference will be controlled. When the rotor speed changes, the gains and reactive power reference will change in order to make appropriate reference current. Schematic diagram of the rotor current controller are shown in Fig.6.

Figure 6: Schematic diagram of proposed current controller for the RSC T e P s K p Pr ref Qr ref V ds V qs I ds ref ref I qs rs Xss V ds X m V qs I dr I qr ref I ar ref I br ref I cr ω r Ksr Kr θ r SIMULATION RESULTS In this section the simulation results are presented in order to validate the GSRC and the proposed control strategies performance at the super-synchronously condition of DFIG operation. The DFIG consist of the stator windings directly connected to three-phase grid and the rotor windings connected to power converter. The power convereter consist of RSC and GSRC. Fig.1 shows the schematic diagram of the system implemented. The required rotor active power by changes the speed will be changed. Accordingly the Dc link current and voltage can be changed relatively. But the DC link and rotor active power must be equal. Since the required DC link current can be supplied by the GSRC, the DC link Voltage changes should not be a problem. Thus the DC link current I multiplied by the DC link voltage V will be equal with rotor active power. The characteriestics of induction machine and GSRC are shown in table.i. The inductive and capacitive filters that were using in conventional converters are omitted in GSRC and RSC. The switching frequency for RSC is 2000 Hz and for GSRC is 60 Hz. The simulated generator is a 2-MW DFIG. At the super-synchronous operation mode of DFIG the rotor will be supplied active power. The DC link current direction and value of it are depended on this active power. Thus the inductor currents of GSRC will be supplied the DC link current. Due to this current value, required DC link voltage will be created. By assuming the direction that shown in Fig.1 and Fig.2 the stator, rotor, total and GSRC output current are shown in Fig.7. By using the proposed control strategies the stator active and reactive power will be controlled by RSC. The GSRC can be supplied 1.4 MVAR rective power while the switch current rated at about 30% of generator rating. Thus by using RC, the current limit of reactive power capability limitation will be resolved. The simulated Dc link voltge, active and reactive powers and electromanetic torque are shown in Fig.8.

Figure 7: Simulated results of DFIG at the rotor speed of 1.2 p.u. a)three phase stator current, b)three phase total current, c)zooming of three phase stator current, d)zooming of three phase total current, e)three phase rotor current, f)three phase GSRC current, g)zooming of three phase rotor current, h)zooming of three phase GSRC current Figure 8: Simulated results of DFIG at the rotor speed of 1.2 p.u. a)stator active power, b) stator reactive power, c)gsrc active power, d)gsrc reactive power, e)total active power, f)total reactive power, g)dc link voltage, h) Electromagnetic torque

The results of GSRC are shown in Fig.9. The switching frequency for this converter is 60 Hz. Switch K current and operation modes of it are shown in Fig.9(c). The parallel capacitors C i=1,2,3,4,5,6 voltages are shown between them. Also the DC shifting in positive direct of amplitude axis are obvious. The DC link voltage is obtained by summation of these voltages. Note that the IGBT current rated at about 30% of generator rating. in Fig.9(m). With the same amplitude there is 60 phase shift Figure 9: Simulated results of resonant converter at the rotor speed of 1.2 p.u. a)switch K1 current, b)switch K1 voltage, c)zooming of switch K1 current, d)zooming of switch K1 voltage, e) current, f) current, g)zooming of current, h)zooming of current, k) current, l) current, m)parallel capacitors voltage CONCLUSION A new resonant converter and a new control strategy for a DFIG have been proposed in this paper. By using resonant converter the reactive power capability has been reclaimed. The soft switching has been improved conventional converters shortcomings on the grid side. Also the DC link high value with high nominal voltage capacitor has been omitted. The rotor current positive sequence depended on stator current and voltage has been obtained. By using new control strategy active and reactive power has been controlled separately.

Table I: Prameters of the DFIG simulated and GSRC Rated power Stator voltage R R X X X 2MW 690V 5mΩ 5.3mΩ 25mΩ 28mΩ 1 Ω X =X X 1.025Ω Lumped inertia constant.48 Number of pole pairs 2 C 100nF C 5mF L 1.4mH R 1uΩ REFERENCES Stephan Engelhardt, Istvan Erlich, Christian Feltes, Jörg Kretschmann, and Fekadu Shewarega, Klaassens, Reactive Power Capability of Wind Turbines Based on Doubly Fed Induction Generators, IEEE Trans. on Energy Conversion, Vol. 26, No. 1, pp. 364-372, March. 2011. M. Tazil, V. Kumar, R.C. Bansal, S. Kong, Z.Y. Dong, W. Freitas, H.D. Mathur, Three-phase doubly fed induction generators: an overview, IET Electr. Power Appl., 2010, Vol. 4, Iss. 2, pp. 75 89. Gonzalo Abad, Miguel A ngel Rodr ıguez, Grzegorz Iwanski, and Javier Poza, Direct Power Control of Doubly Fed-Induction-Generator-Based Wind Turbines Under Unbalanced Grid Voltage, IEEE Trans. on Power Electronics, Vol. 25, No. 2, pp. 442-452, February. 2011. D.M.Deven and G.Skibinski, Zero switching loss inverters for high power applications, IEEE Transactions on Industry Applications, Vol.IA 25, No.4, 1989, pp. 634 643. C.P.Henze, H.C.Martin, and D.W.Parsley, Zero voltage switching in High Frequency power converters Using Pulse width Modulation, IEEE Applied power Electronics Conference, 1988, pp. 33 40. M.K.Kazimierczuk and I.Jozwik, Class E zero voltage switching and zero current switching rectifiers, IEEE Transactions on Circuits and Systems, Vol. CS 37, No. 3, 1990, pp. 436 444.