COMPARATIVE ANALYSIS OF MULTIPULSE AC-DC CONVERTERS IN VCIMD

Transcription

1 OMPRTIVE NLYSIS OF MULTIPULSE D ONVERTERS IN VIMD M. Karthika, V.V. Vijetha Inti 2 ssistant professor, Dept of EEE, Vishnu Institute of Technology, himavaram, W.G.Dist, P, India ssistant professor, Dept of EEE Vishnu Institute of Technology, himavaram, W.G.Dist, P, India 2 STRT: Power electronic devices are nonlinear s; the non linear nature of these switching devices causes harmonic current injection into the ac mains there by polluting the Power Quality (PQ) at the Point of ommon oupling (P). This PQ improvement is achieved by using multipulse converters in THREEPHSE D onverters (Ds). The proposed multipulse acdc converter is based on autotransformer configurations and is able to eliminate lower order harmonics in the ac supply current. This paper describes the comparative analysis of different types of multipulse converters in vector controlled induction motor drives (VIMD). The proposed multipulse acdc converter is designed and the simulation model is developed in MTL. Keywords: utotransformer, multipulse D converter, passive filter, harmonic mitigator, power quality improvement and Vectorontrolled Induction Motor Drive (VIMD). I.INTRODUTION The use of induction motors has been increased in industrial applications due to their advantages such as improved efficiency, ruggedness, reliability and low cost. So for variable speed drives dc motors have been used because of their flexible characteristics []. To incorporate the flexible characteristics of a dc motor into an induction motor, vector control technique has been used in many industries. Various methods based on the principle of increasing the number of pulses in ac dc converters to mitigate current harmonics [2]. These methods use two or more converters where the harmonics generated by one converter are cancelled by another converter through proper phase shift [3].These autotransformer based schemes considerably reduce the size and weight of the transformer. utotransformerbased multipulse ac dc converters have been introduced for reducing the total harmonic distortion (THD) of the ac mains current [4]. To provide equal power sharing between the diode bridges and to achieve good harmonic cancellation, interphase transformers are needed [5][6]. The proposed multipulse acdc converters those are suitable for retrofit re to feed vector controlled induction motor drive (VIMD) [7].This proposed multipulse acdc converter results in the elimination of 5th, 7th, th, 3th, 7th and 2th harmonics. It results in near unity power factor operation in the wide operating range of the drive with the THD of ac mains current always less than 5%. II. DESIGN OF PROPOSED 2PULSE ND 24PULSE D ONVERTERS The design of the suitable autotransformer for these proposed multipulse acdc converters along with the design of a reduced rating passive tuned filter for effective harmonic filtering.. Design Of the Proposed 2Pulse c Dc onverter: To design the 2pulse acdc converter, we have to select the mainly two conditions. First condition is, Two sets of balanced threephase line voltages are to be produced, which are either or out of phase with respect to each other. Second the magnitude of these line voltages should be equal to each other to result in reduced ripple in output dc voltage. Simulink model of 2pulse ac dc converter based proposed harmonic mitigator fed VIMD is shown in fig opyright to IJREEIE 486

2 onn onn2 onn3 Discrete, s = 5e005 s powergui Vector ontrol i Ii pulses speed step N 45V 50Hz V V V V V V2 2 L IGT based inverter g Torque step Tm Induction Motor 30 HP / 45 V m V2 2 V V2 autotransformer Vab2 v 2 bridge rectifier v Vab <Rotor speed (wm)> <Electromagnetic torque Te (N*m)> tuned filter Scope z Fig. Simulink model of 2pulse ac dc converter based proposed harmonic mitigator fed VIMD The number of turns required for and phase shift are calculated as follows. onsider phase a voltages as () (2) ssume the following set of voltages:,, (3) Similarly,, (4),, (5) Using above equations, and can be calculated. These equations result in and for the desired phase shift in an autotransformer. The phaseshifted voltages for phase a are (6) (7) Thus, the autotransformer uses two auxiliary windings per phase. phaseshifted voltage (e.g. ) is obtained by Tapping a portion (0.0227) of line voltage and onnecting one end of an approximate 0.38 times of line voltage (e.g., ) to this tap. opyright to IJREEIE 487

3 onn onn2 onn3. Design Of the Proposed 24Pulse c Dc onverter: For achieving the 24pulse operation, four sets of 3phase voltages (phase shifted through an angle of 5 ) are produced. The number of turns required for achieving these phase shifts among different phase voltages is calculated as follows. onsider phase 'a' voltages as: (8) (9) (0) () Where, V is the rms value of phase voltage. Using above equations K, K2, K3 and K4 can be calculated. These equations result in K = , K2 = , K3 = and K4 = for the desired phase shift in autotransformer. Simulink model of 24pulse ac dc converter based proposed harmonic mitigator fed VIMD is shown in fig2 Discrete, s = 5e005 s powergui Vector ontrol i Ii pulses speed step Torque step N 45V 50Hz V V V V V2 V2 2 2 L2 IGT based inverter g Tm Induction Motor 30 HP / 45 V m V V2 V3 V3 V v Vab tuned filter V V4 Vab2 UTO T/F v V4 V RIDGE RETIFIER Scope z <Rotor speed (wm)> <Electromagnetic torque Te (N*m)> Fig. 2 Simulink model of 24pulse ac dc converter based proposed harmonic mitigator fed VIMD phase shifted voltage (e.g. Val) is obtained by tapping a portion ( ) of line voltage Vca and connecting one end of an approximately (0.0704) of line voltage (e.g. Vbc) to this tap. Thus the autotransformer can be designed with these known values of winding constants. opyright to IJREEIE 488

4 III. VETOR ONTROLLED INDUTION MOTOR DRIVE Indirect vector controlling technique is very popular in industrial applications.fig.3 shows the MTL diagram of an indirect vector controlled induction motor drive (VIMD).The power circuit consists of a frontend diode rectifier and a PWM inverter with a dynamic brake in the D link. hysteresisband current controller PWM is used. The speed control loop generates the torque component of current. The speed control range in indirect vector control can easily be extended from standstill (Zero speed) to the field weakening region. In this case, closed loop flux control is needed. In the constant torque region, the flux is constant. However in the fieldweakening region, the flux is programmed such that the inverter always operates in PWM mode. In the rotor flux oriented reference frame the reference vector i ds (flux component of the stator current) is obtained. Flux alculation Phir Id Phir Id wm Teta Teta Iq Iq Teta alculation to dq conversion z 0.96 Phir* Id* speed w w* Te* controller Phir* id* alculation Phir Te* Iq* iqs* calculation Teta Id* Iq* * dq to conversion * Pulses urrent Regulator pulses Fig. 3 MTL block diagram of VIMD The closed loop PI speed controller compares the reference speed (ω r * ) with motor speed (ω r ) and generates the reference torque T * (after limiting it to a suitable value). The torque component of the stator current reference vector i qs is obtained from the output of the PI controller. These current components (i ds * and i qs * ) are converted to stationary reference frame using rotor flux angle calculated as sum of the rotor angle and the value of slip angle. These currents (i ds *, i qs * ) in synchronously rotating frame are converted to stationary frame three phase currents (i as *, i bs *, i cs * ). These three phase reference currents (i as *, i bs * and i cs * ) along with the sensed motor currents (i as, i bs and i cs ) are fed to the PWM current controller which provides the gating signals to different switches of VSI to develop necessary voltages. These voltages are being fed to the motor to develop the necessary torque for running the motor at a given speed under given conditions. IV.RESULTS The proposed autotransformer based harmonic mitigators along with the VIMD are simulated to demonstrate the performance of the proposed converter systems. The THD of the ac mains current at full is 30.0%, which decreases to 65.9% at light and the power factor at full is 0.933, which decreases to at light (20% of full ). The supply current waveform at full along with its harmonic spectrum of 6pulse converter is shown in Fig. 5, and shows that the THD of ac mains current is 30.0%, these results show the need for improving the power quality at ac mains using some harmonic mitigators which can easily replace the existing 6pulse converter. Dynamic response of 2pulse converter based proposed harmonic mitigator fed VIMD with perturbation is shown in Fig. 4, these results shows the improvement of characteristics with respect to 6pulse converter system in opyright to IJREEIE 489

5 terms of THD and power factor. The supply current waveform at full along with its harmonic spectrum of 2pulse converter is shown in Fig. 6, and shows that the THD of ac mains current is 4.45%, and the power factor obtained is Fig. 4 Dynamic response of 2pulse ac dc converter based proposed harmonic mitigator fed VIMD with perturbation. Fig. 5 mains current waveform along with its harmonic spectrum at full in a sixpulse diode bridge rectifierfed VIMD. From these results that the proposed 2pulse harmonic mitigator is able to perform satisfactorily on VIMD with power factor always higher than 0.98 and THD of supply current less than 5%. opyright to IJREEIE 490

6 Fig. 6 mains current waveform along with its harmonic spectrum at full with 2pulse ac dc converter on ac side. Fig. 7 mains current waveform along with its harmonic spectrum at full with 8pulse ac dc converter on ac side. Fig. 8 mains current waveform along with its harmonic spectrum at full with 24pulse ac dc converter on ac side. opyright to IJREEIE 49

7 S.No. ZS Topology Tablecomparison of Power Quality Indexes of VIMD Fed from Different onverters IS() THD of IS (%) Distortion Factor (DF) Displacement Factor (DPF) Power Factor(PF) D Link Voltage (V) 3% 6pulse % 2pulse % 8pulse % 24pulse However, under light condition, these power quality indexes start decreasing, which shows that the supply current (I s ) is always less than the converter input current (I c ), thus, showing the effectiveness of the designed passive filter. The supply current waveform at full along with its harmonic spectrum of 8pulse converter is shown in Fig. 7, showing THD of ac mains current as 3.27% and the power factor obtained as So that the proposed harmonic mitigator operates satisfactorily under varying conditions, the on VIMD is varied. The supply current waveform at full along with its harmonic spectrum of 24pulse converter is shown in Fig. 8, showing THD of ac mains current as.2% and the power factor obtained as So that the proposed harmonic mitigator operates satisfactorily under varying conditions, the on VIMD is varied. Table shows the comparative analysis of different power quality indexes of a VIMD is fed from 6, 2, 8, 24pulse converters and it shows the power quality improvement. From these results that the proposed 24pulse converter based harmonic mitigator performs well under variation on VIMD with a nearunity power factor and THD of supply current always less than 5%. IV.ONLUSION The proposed harmonic mitigators have been designed, modeled, and developed with variable frequency induction motor drives operating under varying conditions. The observed performance of the proposed harmonic mitigators has the capability of these converters to improve the power quality indexes at ac Mains in terms of supply current THD, supply voltage THD, power factor and crest factor. Moreover, the2pulsebased harmonic mitigator can be used for retrofit applications where variation is always higher than 50 the proposed 24pulse acdc converter results in almost unity power factor in the wide operating range of the drive. Thus the proposed acdc converter can easily replace the existing 6pulse converters without much alteration in the existing system layout and equipments. REFERENES []. P. Vas, Sensorless Vector and Direct Torque ontrol. Oxford, U.K.: Oxford Univ. Press, 998. [2]. D.. Paice, Power Electronic onverter Harmonics: Multipulse Methods for lean Power. Piscataway, NJ: IEEE Press, 996. [3]. S. hoi, P. N. Enjeti, and I. J. Pitel, Polyphase transformer arrangements with reduced kv capacities for harmonic current reduction in rectifier type [4]. Utility interface IEEE Trans. Power Electron., vol., no. 5, pp , Sep [5]. G. R. Kamath,. Runyan, and R. Wood, compact autotransformer based 2pulse rectifier circuit, in Proc. 200 IEEE IEON, pp [6]. D.. Paice, Multipulse converter system, U.S. Patent , Oct. 24, 989. [7]. S. M. Peeran and. W. P. ascadden, pplications, design, and specifications of harmonic filters for variable frequency drives, IEEE Trans. Ind. ppl, vol. 3, no. 4, pp , Jul. /ug [8]. G. J. Wakileh, Power System Harmonics, Fundamentals, nalysis and Filter Design. New York: Springer, 200, ch. 5, pp opyright to IJREEIE 492

8 IOGRPHY Ms. M. Karthika obtained her achelor of Technology in Electrical and Electronics Engineering from V.R.S &Y.R.N ollege of Engineering and Technology, hirala, Prakasam Dist., ndhra Pradesh, India. She completed her Master of Technology in Power Electronics from Shri Vishnu Engineering ollege for Women, himavaram, West Godavari Dist., ndhra Pradesh, India. Her area of interest includes Power Electronics & Drives, Power Systems, Power Quality and ontrol Systems. She has 2 years of teaching experience. She is currently working as ssistant Professor in Electrical and Electronics Engineering Department at Vishnu institute of Technology, himavaram, ndhra Pradesh, India. Mrs. I.V.V. Vijetha obtained her achelor of Technology in Electrical and Electronics Engineering from NOV ollege of Engineering and Technology, Jangareddygudem, West Godavari Dist., ndhra Pradesh, India. She completed her Master of Technology in Power Electronics in NOV ollege of Engineering and Technology, ndhra Pradesh, India. Her area of interest includes Power Electronics, Power Systems and Machines and FTS. She has 6 years of teaching experience. She is currently working as ssistant Professor in Electrical and Electronics Engineering Department at Vishnu institute of Technology, himavaram, ndhra Pradesh, India. opyright to IJREEIE 493