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Regulated AC/DC/AC power supply using scott transformer M. Moussa a, H. Hesham, and Yasser G. Dessouky* a Arab Academy for Science and Technology and Maritime Transport, Miami, P.O. Box: 1029, Alexandria, Egypt. Abstract In today's industry, it is necessary to convert power for equipment used in environments where dissimilar voltages and frequencies are the norm. Static frequency converters or industrial power supplies are used for converting either 50Hz or 60Hz utility line power to 400Hz power. They are more efficient than motor-generator sets. In addition, they offer harmonic cancellation, power factor correction, phase conversion, voltage conversion with balanced, smooth, and controlled power output. Many varied applications in power electronics require sinusoidal outputs at frequency 400Hz. This paper describes the design, simulation and implementation of a power converter topology and control techniques for realizing sinusoidal output systems. A 150 KVA 3-phase power supply, whose line voltage and frequency are 440V ase are used to convert the dc voltage to get two phase AC power supply which is converted via a Scott transformer to a three phase, whose line voltage and frequency are 440V and 400 Hz. A resonant filter is used to eliminate harmonics. Feedback signals from load voltage and dc link current are used to control the rectifier so as to maintain constant voltage at variable load conditions. The system is theoretically analyzed and experimentally verified. Keywords: Static converters, Power supplies, Scott transformer, Resonant filter, Center tapped inverter. * Corresponding author. Tel: +2001001234411 E-mail: [email protected] [email protected] 1. Introduction 2.2. DC Link Filter Power supplies are among the most important components of any industrial application. Standard power supply is designed to optimize the power required, resulting in maximized efficiency, power factor and load regulation. Industrial power supplies are used for applications such as: aircraft power supplies, paper mill, laser power supplies, radar/sonar power supplies, battery charger, and marine propulsion systems [1-3]. In this paper, an industrial application is considered where the (6) MVA from the synchronous generator of a ship is used to supply different loads on board. A power converter is designed to supply 150 KVA of this total power to special loads such as Gyro system and other navigation equipments. The converter, shown in Figure 1, employs two stages of power conversion. In the first stage, the fixed frequency ac supply voltage is rectified to create the required dc bus by using thyristor phase controlled rectifier. In the second stage, the dc bus voltage is inverted at the required output frequency by using two half-bridge inverters 90º phases shifted. The Scott-transformer connection allowed 2-φ to 3-φ components to be interconnected, which adds an advantage to this power supply of having a relatively low cost because of using only two center tap inverters switched at power frequency with no PMW on the switches, meaning lower losses and voltage stresses where the DC link voltage is controlled using bridge rectifier. The function of the dc link filter is to attenuate the rectifier output voltage harmonics across the link inductor Lo and to sink the inverter input current harmonics into the link capacitor Co. However, attenuation of the rectifier output voltage harmonics across Lo creates additional ripple current into Co, while the sinking of the inverter input current harmonics into Co gives rise to additional ripple voltage across Lo. Therefore, both filter components (Lo and Co) are affected by both harmonic sources. The size and cost of this dc filter is determined by the rated system power, rated dc bus voltage, and the specified levels of THD in the link input current, and link output voltage. To smooth the dc voltage, a dc link filter is used whose parameters are designed to be (Lo = 5mH and Co= 22000F) [5]. 2. System description This static converter contains controlled rectifier, DC link filter, Scott transformer, single phase-inverter and series-parallel resonant filter. A description of these components is as follows: 2.1. Three phase fully controlled bridge converter The phase controlled rectifier is obtained by six thyristors. Continuous control over the output dc voltage is obtained by controlling the conduction interval of each thyristor. The load harmonic voltage increases considerably as the average value goes down. The input current contains only odds harmonics of the input frequency other than the triplex harmonics. In this system, the three-phase supply, whose line voltage and frequency are 440V and 60Hz, is converted to dc voltage via controlled rectifier where the conduction interval to control the dc voltage from (425 V) to (510 V) from 10% to 120% of the full load respectively [4]. 2.3. Scott transformer A Scott-transformer, shown in Figure 2 is used to drive three phase current from a two phase source. It consists of a center tapped transformer T 1 and an 86.6% tapped transformer T2 on the 3-φ side of the circuit. The primaries of both transformers are connected to the 2-φ voltages. One end of the T2 86.6% secondary winding is a 3-φ output, the other end is connected to the T 1 secondary center tap. Both ends of the T1 secondary are the other two 3-φ connections [6]. To compensate for the voltage drop in the internal impedance of the different parts of the system, the Scott transformer is a step up whose turns ratio is 1: 1.2 [7]. 2.4. Single-phase centre-tapped transformer inverter An alternating load voltage can be generated from a dc source by the use of a centre-tapped transformer as shown in Figure 3 [8]. Basically, by switching the two switches, the dc source is connected in alternative senses to the two halves of the transformer primary, so inducing a square wave voltage across the load in the transformer secondary [9]. For loads whose current is out of phase with the voltage, anti-parallel diodes feedback the stored load energy during those periods when the current reverses relative to the voltage. Two square wave center-tapped-transformer inverters are used whose output voltages are perpendicular (90 separation) which are the two phase voltage sources of the Scott transformer to get three-phase output voltages [10]. Controlled Bridge Rectifier Center Tap Inverter DC Link Filter Scott Transformer Resonant Filter 3-ph power supply Load Gate Drive and Control Circuits DC Current Feed Back Load Voltage Feed Back Fig. 1 AC/DC/AC Power supply. Fig. 2 Scott-transformer converts 2-φ to 3-φ. capacitor bank overloading, additional heating and losses in AC machines, increased probability of relay malfunctions, disturbances in solid-state and microprocessor based systems, interference with telecommunication systems [14]. The resonant arm filter, shown in Figure 4, is more appropriate to attenuate low order harmonics. Both the series arm L1C1 and the parallel arm L2C2 are tuned to the inverter output frequency. The series arm presents zero impedance to the fundamental frequency, but finite increasing impedance to higher frequencies [15]. The parallel arm presents infinite impedance at the fundamental frequency, but reducing impedance to higher frequencies. Taking the fundamental frequency (1) AC Voltage Making C1 = AC2 and L2 = AL1, and setting ω = nω , where n is the order of the harmonic. The filter transfer function is then given by [16]: (2) Fig. 3 Centre-tapped transformer inverter. 2.5. Harmonic and Filters Harmonic distortion of voltages and currents in power systems are caused by the presence of non-linear loads in the system that produce distorted current. Using Fourier analysis, these distorted voltages and currents can be described in terms of harmonics. The harmonics in the lower frequency band are the most significant [11,13]. A few of the major effects of the harmonics are as follows: The output voltage of the inverter, and then through the Scott transformer, is 400Hz, 180 conduction square wave whose major harmonic is the third (n=3) which equals to 33.3% of the fundamental [17]. If this value is to be attenuated to 4%, then the value of the gain (A) of (2) is determined to be 0.76. The value of the reactance (nω L1) is taken to be less than the load impedance (150KVA, 440 Vline, 0.8 PF) to avoid excessive load voltage changes when the load varies. Take nω L1 = 1.29, then the filter parameters are given in Table 1 3. Simulation A prototype system is used to simulate the proposed system using MATLAB software as shown in Figure 5. The SIMULINK model starts at 10% full load till 0.8 sec when the full load is connected. Then at 1.2 sec, the supply is over-loaded by another 20% of full load. The results are shown in Fig. 6 to Fig. 10. Fig. 6 and Fig. 7 show that the load is supplied by almost a sinusoidal current at almost constant and sinusoidal voltage at different load conditions. The resonant filter reduced the third harmonic in the voltage and in the current at almost 4% and 1% of the fundamental respectively, and reduced the THD in the voltage and current to (4.4 0.7)% and (2.2 0.4)% respectively. The third harmonic appeared, despite three-phase load nature, because the impact of two-phase connection in the Scott transformer. Fig. 8 and Fig. 9 show that the system has a good time response to regulate load voltage at sudden load change. However, the disadvantage of power converter is the harmonic input to the incoming source, this is shown in Fig. 10, where the fifth harmonic is more than 20% of the fundamental and the THD is greater than 25%. However, if the supply is critical, a method to improve supply power quality could be implemented. C1 Vo Fig. 4 Parallel-series resonant arm filter. Table 1 Filter parameters C2 = 500F L1 = 0.41mH L2 = 0.31mH 2.6. Feedback control system The system has two PID controllers fed from the two feedback cascaded loop, namely, the outer loop from the load voltage and the inner loop from the current of the dc link filter. These controllers regulate the load voltage at constant level of 440V from almost no load to 120% full load. g g + + A i - E C A Discrete, Ts = 1e-005 s. 1+ +2 + B + v - 1.1 E 2 1 alpha_deg CA Vb C C c C c E Vc 180 20 Voltage FFT window: 3 of 600 cycles of selectedController signal Current PID Controller 1 3 FFT window: 3 of 600 cycles of selected signal 440 node 10 10 200 node 10 -100 -20 -200 0.401 0.402 0.403 0.404 0.405 0.406 0.407 10 0 -10 -20 0.403 0.404 Time (s) 0.405 0.406 0.407 80 100 60 0 40 -100 -200 20 0 0.9 0 500 1000 1500 2000 2500 Frequency (Hz) 3000 3500 4000 0.901 0.902 0.903 0.904 Time (s) 0.905 0.906 0.907 0.904 Time (s) 0.905 0.906 0.907 Max. current = 263.5A , THD= 2.48% 100 80 80 dament al) 100 60 80 60 40 20 0 (b) Max. current = 27.65 A, THD= 1.79% ament al) 0.903 Max. current = 263.5 , THD= 2.48% (a) 60 0.902 100 200 Mag (% of Fundamental) Mag (% of Fundamental) 20 0.402 0.901 Max. current = 27.65 , THD= 1.79% 100 0.401 0.9 Fig. Ti5me (s) Simulink Block Diagram of the overall system. Output current ia(t) at 100%F.L Output current ia(t) at 10%F.L 0.4 node 10 0 -10 0.4 node 10 100 PID 0 A B 2 C 1+ +3 C Synchronized 6-Pulse Generator Va b C 1.1 g + v - a B +2 E C pulses A b A BC a B B + v - g A B C AB A B C + v - A A C B g C -K- - C A B B C1 = 380F C C2 A L2 B Vi C L1 0 500 1000 1500 2000 Frequency (Hz) 2500 3000 3500 4000 200 0 -200 1.4 1.401 1.402 1.403 1.404 Time (s) 1.405 1.406 1.407 Output current ia(t) at 120% F.L Max. current = 313.3 , THD= 2.66% DC Current (A) 100 DC Voltage (volt) 600 Mag (% of Fundamental) 200 1.4 1.401 1.402 1.403 1.404 Time (s) 1.405 1.406 600 40 400 FFT window: 3 of 600 cycles of selected signal 0 0 200 0 (c) 200 200 20 1.407 400 500 1000 1500 2000 Frequency (Hz) 2500 3000 3500 4000 0 0 0.5 80 60 0.403 0.404 Time (s) 0.405 0.406 FFT window: 3 of 600 cycles of selected signal 0 of 500 0.901 1000 0.902 15000.903 20000.904 Frequency Time (s)(Hz) 25000.905 -40 0.5 4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44 0.445 Time (s) 30000.906 3500 0.907 4000 (a) 20 0 Output load voltage at 100% F.L 0 Max. voltage = 355.2 , THD= 4.93% 0.905 0.906 0.907 0 20 -200 01.4 5001.401 10001.402 15001.403 Fundamental (400Hz) = 355.2 , THD= 4.93% 60 1.4 1000 2000 1.401 1.402 3000 80 60 40 4000 5000 6000 7000 8000 20 Frequency (Hz) 0 1.403 1.404 1.405 1.406 1.407 0 Time (s) (c) 40 500 1000 1500 2500 3000 20080 0 1000 2000 3000 4000 5000 Frequency (Hz) 6000 7000 400 100 60 200 50 0 400 0.5 0 1.5 0 1 Time (s) 0.5 1 Time (s) (a) 20 (b) 3000 4000 5000 Frequency (Hz) 6000 7000 8000 20 Fundamental) 10 2000 of 15 1000 g (% of Fundamental) 20 0 15 10 800 700 1.435 800 1.44 900 1.445 1000 700 800 900 1000 900 1000 80 60 FFT window: 3 of 90 cycles of selected signal 40 200 100 1.405 200 1.41 300 1.415 400 1.42 500 600 1.425 1.43 Frequency Time (s)(Hz) 100 Max. current = 332.7A , THD= 29.00% 0 -20080 100 200 300 400 500 600 700 800 900 1000 Frequency (Hz) 601.4 1.405 1.41 1.415 1.42 1.425 1.43 1.435 1.44 1.445 Time (s) 80 60 40 20 0 0 100 200 300 400 500 600 Frequency (Hz) (c) 4. Experimental work 8000 150 1000 40 Fig. 10 Instantaneous input supply current waveform and FFT at: (a) 10% F.L, (b) 100% F.L, and (c) 120% F.L. Max. current = 399.2 A, THD= 28.28% 20 100 RMS output voltage (volt) 600 Fig. 8 (a) RMS /load current, (b) RMS line voltage. (% 4000 Fig. 7 Instantaneous output phase voltage waveform and FFT at: (a) 10% F.L, (b) 100% F.L, and (c) 120% F.L. 20 Fundamental (400Hz) = 358.2 , THD= 5.07% 100 0 ag 3500 Fundament al) Fundament al) of 2000 Frequency (Hz) Fundament al) 0 800 -200 0 700 400 500 600 Frequency (Hz) 900 0.945 Max. current = 399.2 , THD= 28.28% 20 200 100 00 of 200 10020 800 0.94 100 -200 0 20 60 -200 0.9 0.905 0.91 0.915 0.92 0.925 0.93 0.935 0.94 0.945 0 1.40 Time (s) (b) 40 Input supply current ia(t) at 120% F.L Mag Max. voltage = 358.2 , THD= 5.07% 100 700 0.935 Max. current = 332.7 , THD= 29.00% 80 (% Output load voltage at 120% F.L 40 RMS Current (A) (% 2500 1.405 3000 1.406 3500 1.407 4000 (b) 2500 Mag 20001.404 Time (s)(Hz) Frequency 300 0 40200 0 200 500 600 0.925 0.93 Frequency Time (s)(Hz) (% 0.904 Time (s) 100 900 1000 400 0.92 Mag 0.903 800 300 0.915 1.5 0 80 0 100 200 300 400 500 600 700 800 900 1000 The systemFrequencyhas (Hz) been built in the lab as shown in Figure 11, with a scaled down rate of 1.5 kVA to verify the 60 operation, where the 3-phase 44V, 50 Hz input supply is rectified using the CD43-40B Dual SCR Isolated POW-RBLOK Module controlled rectifier. A 2nd order LC filter (L 40 = 5mH, C = 1500µF/470V) smoothes the output DC which is the input to two single-phase perpendicular centre tap 20 inverters (switches IRFP150N) to produce two-phase AC voltages which are converted to 3-phase voltages via the 120% step up Scott transformer, to make up for the voltage 0 of 0.902 300 Input suppl400y current500ia(t) at 100%600F.L 700 Frequency (Hz) 200 0.91 (% 0.901 200 100 0.905 (a)0 0 Mag 60 0.9 100 0 -200 0 0 0.9 Mag (% of Fundamental) -80200 FFT window: 3 of 600 cycles of selected signal 60 FFT window: 3 of 90 cycles of selected signal 60 200 40 5 20 Max. current = 36.08 A, THD= 39.59% 200 100 80 Fundament al) 8000 of 7000 Mag (% of Fundamental) Fundament al) of (% Mag Fundament al) of (% Mag 1000 2000 Fundament3000al (400Hz) =4000356.8 , THD=50003.73% 6000 Frequency (Hz) Mag (% of Fundamental) 100 2000 0 100 0 Max. current = 36.08 , THD= 39.59% 100 10 (% 0 20 0 0.90 0.445 15 80 2015 -2100 20040 0.407 (b) 20 40 Mag 0.402 Input supply current ia(t) at 10% F.L 20 Fundamental) Mag (% of Fundamental) Fundament al) of (% Mag 0.401 -20 1.5 Time (s) -200 0.4 (a) 1 Time (s) 0.41 0.415 (b) 0.42 DC 0.425 voltage. 0.43 0.435 0.44 Fig. 9 Instantaneous wave: (a) DC 0.405 current, Max. voltage = 356.8 , THD= 3.73% 40 20 0.5 -40 0.4 100 -200 FFT window: 3 of 90 cycles of selected signal 040 Time (s) 0 Fig. 6 Instantaneous output current waveform and FFT at: (a) 10% F.L, (b) 0.4 0.401 0.402 0.403 0.404 0.405 0.406 0.407 100% F.L, and (c) 120%F.L. Time (s) Fundamental (400Hz) = 313.3 , THD= 2.66% Output load voltage at 10% F.L 100 0 60 1.5 0 -200 20080 1 Mag (% of Fundamental) Mag (% of Fundamental) -200 800 60 Mag (% of Fundamental) 0 80 0 100 200 300 400 500 600 700 800 900 1000 Frequency (Hz) drop through the circuit. The load voltage harmonics are eliminated using the resonant filter (series branch: L = 11 mH, C = 15 µF/220V, and parallel branch: L = 3 mH, C = 45 µF/220V). The supply is loaded with a (44V/1.5KVA/400Hz) load. To regulate the load output voltage during loading, a three phase uncontrolled bridge with a small smoothing capacitor are used to measure the output load voltage which is fed back to the control circuit of the controlled rectifier to increase the DC average voltage through a PI controller. Also, a current limiter is used in this control circuit to protect the supply from access loading. To protect the MOSFET switches and the thyristor, a soft staring technique is used in the firing and control signals of both circuits. Fig. 12 to Fig. 16 show the experimental results, where Fig. 12 and Fig. 13, show the full load steady state line output voltage and current, respectively, which are sinusoidal. Fig. 14 and Fig. 15 show the voltage across primary of teaser winding of the Scott transformer and the supply current at steady state, respectively. Figure 16 shows the transient response of the DC link voltage when the supply is loaded suddenly from no load to full load, where the DC voltage is increased from 40 V to 51 V to regulate the output voltage at its nominal rated value. The experimental results show the validity of the supply to produce sinusoidal output voltage. Fig. 12 Steady State Load Line Voltage. Fig. 13 Steady State Load Line Current. Rectifier and firing Scott Inverter and control trans Fig. 14 Voltage across Primary of Teaser Transformer. Filter Load Fig. 11 Experimental rig. Fig. 15 Steady state Supply Current. Fig. 16 DC Bus voltage. 5. Conclusion This paper introduced the design, simulation and implementation of static power converter techniques for realizing sinusoidal output system. The converter is used to feed 150KVA, 440V, 400Hz critical loads on a ship from 440V, 60Hz three-phase supply. The controlled rectifier and dc link filter provide a dc voltage, controlled by feedback signals from load voltage and dc link current, which is then converted to three-phase via two centre tap inverters and a step up Scott transformer. A resonant filter is designed to eliminate 3rd harmonics and higher. The system is experimentally verified at 15KVA, 44V. The simulated and experimental results have been presented to prove the validity of the system. Acknowledgements The authors would like to thank Eng Ahmed EL-Shazly of the Fox Power Electronics Company for his help and support during the practical implementation of the rig. References [1] Basile, G.L.; Buso, S.; Fasolo, S.; Tenti, P.; Tomasin, P. “A 400 Hz, 100 kVA, digitally controlled UPS for airport installations”, IEEE Industry Applications Conference, volume 4, pp. 226-2268, (2000). [2] Beiranvand, R.; Rashidian, B.; Zolghadri, M.R.; Alavi, S.M.H. “Designing an Adjustable Wide Range Regulated Current Source”, IEEE Trans. on Power Electronics, volume 25, pp. 197-208, (2010). [3] Badin, A.A.; Barbi, I. “Three-phase series-buck rectifier with split DC-bus based on the Scott transformer”, IEEE Power Electronics Specialists Conference, PESC 2008, pp. 516-522, (2008). [4] Muhammad H. Rashid. “Circuits, Devices, and Applications in Power Electronics”, third edition, Upper Saddle River, NJ: Prentice-Hall, (2004). [5] Barry W. Williams. “Power Electronics Devices, Drivers, Applications, and Passive Components”, second edition, ISBN 978-09553384-0-3, University of Strathclyde, Glasgow, (2006). [6] Badin, A.A.; Barbi, I. “Unity Power Factor Isolated Three-Phase Rectifier With Split DC-Bus Based on the Scott Transformer”, IEEE Trans. on Power Electronics, volume 23, pp. 1278-1287, (2008). [7] G. R. Slemon. “Electric Machines and Drives”, Addison Wesley, (1992). [8] T. Wildi. “Electrical Machines, Drives, and Power Systems”, fifth edition, Prentice-Hall, (2002). [9] Vinatoru, C.S.; Palagniuc, V.; Lupea, E.; Alexa, D. “An analysis and a simulation of static frequency converter using three-phase rectifiers with almost sinusoidal input currents”, IEEE International Symposium on Signals, Circuits and Systems, volume 1, pp. 209-212, (2003). [10] Drubel, O.; Hobelsberger, M. “Static frequency converters with reduced parasitic effects”, IEEE Power Electronics Specialists Conference, PESC 04, volume 6, pp. 4365 – 4370, (2004). [11] Qiu Nan; Fan Yinhai. “DSP Controlled High Power Pulse Power Supply”, IEEE International Symposium on Computer Science and Information Engineering, volume 3, pp. 202-204, (2009). [12] Mihalache, L. “DSP control of 400 Hz inverters for aircraft applications”, IEEE Industry Applications Conference, 37th IAS Annual Meeting, volume 3, pp.1564–1571, (2002). [13] Nielsen, N. “Loss optimizing low power 50 Hz transformers intended for AC/DC standby power supplies”, IEEE Applied Power Electronics Conference and Exposition, APEC '04, volume 1, pp. 420-425, (2004). [14] Ferreres, A.; Carrasco, J.A.; Maset, E.; Ejea, J.B. “ Small-signal modeling of a controlled transformer parallel regulator as a multiple output converter high efficient post-regulator”, IEEE Trans. on Power Electronics, volume 19, pp. 183-191, (2004). [15] Ahmed, T.; Nishida, K.; Nakaoka, M. “MPPT control algorithm for grid integration of variable speed wind energy conversion system”, 35th Annual Conference of IEEE Industrial Electronics, IECON '09, pp.645-650, (2009). [16] Lander,Cyril W. “Power Electronics”, 2nd edition, McGRAW-Hill, (1987). [17] Sun Zhuo; Jiang Xinjian; Zhu Dongqi. “Study of novel traction substation hybrid power quality compensator”, IEEE International Conference on Power System Technology, volume 1, pp. 480-484, (2002).