SIMULATION STUDIES ON AUTOTRANSFORMER RECTIFIER UNIT FOR AIRCRAFT APPLICATIONS

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1 International Journal of Electrical Engineering & Technology (IJEET) Volume 9, Issue 5, September-October 2018, pp. 1 11, Article ID: IJEET_09_05_001 Available online at ISSN Print: and ISSN Online: Journal Impact Factor (2016): (Calculated by GISI) IAEME Publication SIMULATION STUDIES ON AUTOTRANSFORMER RECTIFIER UNIT FOR AIRCRAFT APPLICATIONS Harshitha G B Lecturer, Department of Electrical and Electronics Engineering, UBDT College of Engineering, Davangere, India Adithya Ballaji Teaching Associate, School of Electrical and Electronics Engineering, REVA University, Bengaluru, India ABSTRACT The need of High Voltage DC (HVDC) in aircraft is increasing due to use of electric power to drive aircraft subsystems such as an actuators which are used in landing gears, flap angle and rudder system of an aircraft. Actuators are mainly operated with a high DC voltage of 270V as an input power supply requirement. But in the conventional aircraft, DC voltage generation is limited to 28V by using transformer rectifier unit (TRU). So to cope up with the requirement, an additional conversion unit of Autotransformer Rectifier Unit (ATRU) is necessary. This paper presents study on ATRU. ATRU is a essential component of aircraft Electrical Power System (EPS). ATRU is fed with three-phase 115V 400 Hz AC voltages and converts to high DC voltage of 270V which is designed to supply high voltage DC bus of aircraft EPS. ATRU generates 270V DC with less harmonic contents. The DC voltage can be generated by using 18-pulse ATRU and 24-pulse ATRU. Hence, an 18-pulse & 24- pulse ATRU was designed modeled and simulated using the MATLAB-Simulink Software to understand the influence of harmonic contents. Key words: Autotransformer Rectifier Unit (ATRU), Electrical Power System (EPS), Radio Technical Commission for Aeronautics (RTCA), Total Harmonics Distortion (THD). Cite this Article: Harshitha G B and Adithya Ballaji, Simulation Studies on Autotransformer Rectifier Unit for Aircraft Applications. International Journal of Electrical Engineering & Technology, 9(5), 2018, pp INTRODUCTION Increasing use of electric power to drive aircraft subsystems such as an actuator has led to generation of HV voltage DC. Actuator is mainly operated with a high DC voltage (270V) as an input power supply requirement. But in the conventional aircraft, DC voltage generation is 1 editor@iaeme.com

2 Simulation Studies on Autotransformer Rectifier Unit for Aircraft Applications limited to 28V by using transformer rectifier unit (TRU). The use of Power-By-Wire (PBW) technology results in a large number of ac/dc converters supplying power for functions such as fuel pumps, cabin pressurization, air conditioning, engine start and flight control [1]. So to cope up with the requirement of PBW, an additional conversion unit of Autotransformer rectifier unit (ATRU) is necessary. Hence, ATRU is designed to supply the DC bus. Also, 18- pulse ATRUs are implemented due to its simplicity, high reliability, low cost and relatively high efficiency [2]. Moreover, there is no need for isolation makes the autotransformers a preferred choice due to their advantages of reduced ratings and lower cost, smaller size and weight [3]. ATRU generates 270V DC with less harmonic contents. There are 18-pulse ATRU and 24-pulse ATRU that can satisfy harmonic requirements set by radio technical for aeronautics (RTCA) standards [4], [5]. 18-pulse ATRUs have characteristics current harmonics of 17 th and 19 th order so as to produce less harmonic distortion at input supply. These ATRUs offers reduced weight, size, cost and efficiency [6] PULSE ATRU & 24-PULSE ATRU To obtain different phase shifts electromagnetic devices such as autotransformers or multiwinding transformer will be used. The windings are then connected together following particular configuration Pulse ATRU Figure 1 shows the block diagram of 18-pulse ATRU which consists of phase-shifting autotransformer fed to diode rectifiers. Figure 1 Block diagram of 18-pulse ATRU In an 18-pulse ATRU, eighteen pulses are generated from combining three six-pulse diode rectifiers since these rectifiers are connected to phase-shifting transformers, the desired phase shift of 40, 0, and -40 are obtained. Figure 2. shows the phasor diagram of 18-pulse ATRU. 2 editor@iaeme.com

3 Harshitha G B and Adithya Ballaji Figure 2 Phasor diagram of 18-pulse ATRU. An 18-pulse ATRU is fed to three diode bridges. The dc outputs of diode bridges are connected in parallel. The coil configuration of the phase shifting auto-transformer is shown Figure.2. Each phase has eight winding's and is marked as 1x, 2x, 8x (x = a, b, c). The autotransformer has its primary winding connected in delta configuration across the main supplies (v a, v b, v c ). The secondary windings are used to generate required three voltage sets (v a1, v b1, v c1 ), (v a2, v b2, v c ) and (v a3, v b3, v c3 ). On the secondary side, the phase shift between adjacent voltages is equal to 40 and the magnitude of secondary voltages is 91.3% of the original primary voltage. v v 3 a k a 1 3( v va ) k b 2 3( v v ) b c v v k b3 b 1 3( vc v ) k b 2 3( vc va ) v v k 3 c c 1 3( va vc ) k 2 3( va v ) b (1) Where k 1=0.347 and k 2 =0.413 are the turn ratio between primary and secondary windings of the transformer. Writing (1) in a matrix form gives k k k k v a3 v k k k k a v v b b v v c3 k k k k c The three sets of the voltages produced from the transformers are directly fed to three sets of diode bridges. Three six-pulse diode bridges are used to convert secondary three-phase voltages to three dc voltages. The dc outputs are connected in parallel to the load, to provide a dc voltage of 270V having 7200 Hz frequency ripple for 18-pulses. According to RTCA standards, an 18-pulse ATRU eliminates harmonics of order less than 18n±1 (where n is integer) i.e., 17 th or 19 th harmonic order thus reduces the total harmonic distortion (THD) of AC mains current that is less than 6%. (2) 3 editor@iaeme.com

4 pulse ATRU Simulation Studies on Autotransformer Rectifier Unit for Aircraft Applications Figure 3 shows the block diagram of 24-pulse ATRU using an input three-phase autotransformer. Figure 3 Block diagram of 24-pulse ATRU In 24-pulse ATRU, eighteen pulses are generated from combining three six-pulse diode rectifiers since these rectifiers are connected to phase-shifting transformers, the desired phase shift of 15,45, 60, and 90 are obtained. Figure 4 Phasor diagram of 24-pulse ATRU. A 24-pulse ATRU is fed to four diode bridges. The dc outputs of diode bridges are connected in parallel. The coil configuration of the zig-zag transformer is shown in the above figure. The autotransformer primary winding is (V A, V B, V C ), and the secondaries are used to generate required four voltage sets are (V A1, V B1, V C1 ), (V A2, V B2, V C2 ), (V A3, V B3, V C3 ) and (V A4, V B4, V C ).On the secondary side, the phase shift between the adjacent voltages is equal to editor@iaeme.com

5 Harshitha G B and Adithya Ballaji V = -K V + K V - K V A1 5 CA 6 AB 7 BC V = -(K + K )V + K V - K V A2 4 5 CA 6 AB 9 BC V = -(K + K )V + K V + K V A3 4 5 CA 6 AB 10 BC (3) V = -K V + K V - K V A4 5 CA 6 AB 10 BC Where K 4 =0.0779, K 5 =0.4894, K 6 =0.3387, K 7 =0.3387, K 9 =0.1355, K 10 = Four six-pulse bridges are used to convert four dc outputs. The four dc outputs are connected in parallel to the load, to provide a dc voltage of 270V having 9600Hz ripple frequency. In 24-pulse ATRU, 24-pulses are generated from combining four six-pulse diode rectifiers since these rectifiers are connected to phase-shifting transformers, the desired phase shift of 15º,30º, 45º, and 60º are obtained. Four six-pulse bridges are used to convert four dc outputs. The four dc outputs are connected in parallel to the load, to provide a dc voltage of 270V having 9600Hz ripple frequency. According to RTCA standards, a 24-pulse ATRU reduces harmonic order less than 24n±1 (n is integer) i.e., 23 rd or 25 th harmonic order thus it reduces THD of AC main current less than 3%. 3. MODELLING OF 18-PULSE AND 24-PULSE ATRU The above mentioned 18-pulse and 24-pulse ATRU were modeled and simulated by using SimPowerSystems, a Simulink's simulation tool, since all the needed models are available and ready to use. Figure 5 shows Simulink diagrams which represent the 18-pulse and 24-pulse ATRU under consideration. (a) Simulink model of 18-pulse ATRU 5 editor@iaeme.com

6 Simulation Studies on Autotransformer Rectifier Unit for Aircraft Applications (b) Simulink model of 24-pulse ATRU Figure 5 Simulink model of (a) 18-pulse ATRU & (b) 24-pulse ATRU. Figure 5 (a) presents an 18-pulse ATRU whose input is a 3-phase 115V, 400Hz AC source. The supply from the AC source is fed to the three transformers which are phase shifted by 40º each. Phase shifted output voltages from the transformer are fed to the three phase uncontrolled rectifiers which transforms AC supply to 18-pulse DC voltage of 270V with ripple frequency of 7200Hz. Figure 5 (b) presents a 24-pulse ATRU whose functionality is similar to 18-pulse ATRU but with phase shifting of 15º each. The phase shifted output voltages from the transformer are fed to the three phase diode bridges which transforms AC supply to 24-pulse DC voltage of 270V with ripple frequency of 9600Hz. 4. SIMULATION RESULTS Simulation of 18-pulse ATRU and 24-pulse ATRU was performed in detail using MATLAB- Simulink Input Current and Input Voltage The input current waveforms for both ATRU are recorded and shown in Figure 6 for comparison purpose. In Figure 6 (a) & (b) the lines blue, green and red represents three phases of AC supply current. These input current waveforms are used in the ATRU system to determine the total harmonic distortion (THD). 6 editor@iaeme.com

7 Harshitha G B and Adithya Ballaji Figure 6 (a) Figure 6 (b) Figure 6 Comparison of AC input current. (a) 18-pulse ATRU. (b) 24-pulse ATRU The input voltage waveforms for both ATRU are recorded and shown in Figure 7 for comparison purpose. In Figure 7 (a) & (b) the lines blue, green and red represents three phases of AC supply voltage. These input voltage waveforms are used in the TRU system to determine the total harmonic distortion (THD). Figure 7 (a) Figure 7 (b) Figure 7 Comparison of AC input voltages. (a) 18-pulse ATRU. (b) 24-pulse ATRU. 7 editor@iaeme.com

8 Simulation Studies on Autotransformer Rectifier Unit for Aircraft Applications 4.2. Output voltage Figure 8 represents the output dc voltage of 18-pulse & 24-pulse ATRU. Figure 8 (a) shows the waveform of output DC voltage for 24-pulse ATRU of 270V consisting of 18pulses in it with ripple frequency of 7200Hz. Figure 8 (b) shows the waveform of output DC voltage for 24-pulse ATRU of 270V consisting of 24pulses in it with ripple frequency of 9600Hz. Figure 8 (a) Figure 8 (b) Figure 8 Comparison of output voltages. (a) 18-pulse ATRU. (b) 24-pulse ATRU 4.3. Total Harmonic Distortion (THD) Figure 9 represents the THD of input current of 18-pulse & 24-pulse ATRU. Figure 9 (a) shows the current harmonic spectra of 18-pulse ATRU where harmonics order of less than 17 th order are reduced which in turn reduces the total harmonic distortion (THD) of AC mains. Figure 9 (b) shows the current harmonic spectra of 24-pulse ATRU where harmonics order of less than 23 rd order are reduced which in turn reduces the total harmonic distortion (THD) of AC mains. 8 editor@iaeme.com

9 Harshitha G B and Adithya Ballaji Figure 9 (a) Figure 9 (b) Figure 9 Comparison of input current harmonic spectra. (a) 18-pulse ATRU. (b) 24-pulse ATRU Figure 10 (a) Figure 10 (b) Figure 10 Comparison of input voltage harmonic spectra. (a) 18-pulse ATRU. (b) 24-pulse ATRU. Figure 10 represents the THD of input voltage of 18-pulse & 24-pulse ATRU. Figure 10 (a) shows the voltage harmonic spectra of 18-pulse ATRU where harmonics order of less than 17th harmonic order are reduced which in turn reduces the total harmonic distortion (THD) of AC mains. Figure 10 (b) shows the voltage harmonic spectra of 24-pulse ATRU where harmonics order of less than 23rd harmonic order are reduced which in turn reduces the total harmonic distortion (THD) of AC mains. 9 editor@iaeme.com

10 Simulation Studies on Autotransformer Rectifier Unit for Aircraft Applications Table 1 Comparison of THD of input current and THD of input voltage of 18-pulse and 24-pulse ATRU 18-pulse ATRU THD(I) (%) THD(V) (%) 24-pulse ATRU THD(I) THD(V) (%) (%) From the above results we can say that an 18-pulse ATRU produces 270volts DC and eliminates harmonics of order less than 17 th harmonic order thus reduces the total harmonic distortion (THD) of AC mains current less than 6% whereas a 24-pulse ATRU reduces harmonic order less than 23 rd order thus it reduces THD of AC main current less than 3% which is as per RTCA standards. Thus we conclude that a 24-pulse ATRU produces much less harmonics than an equivalent 18-pulse ATRU having the same power rating of 150KW. 5. CONCLUSIONS A 24-pulse ATRU model was designed and simulated and the simulation result was compared with 18-pulse ATRU. An 18-pulse ATRU produced a DC voltage of 270V at an output ripple frequency of 7200Hz with total harmonic distortion (THD) of an input current of 4.43% and THD of an input voltage of 3.05% whereas a 24-pulse ATRU produced a voltage of 270V at an output ripple frequency of 9600Hz with THD of an input current of 0.14% and THD of an input voltage of 0.02% which is as per radio technical commission for aeronautics (RTCA) standards. Hence, a 24-pulse ATRU produces less harmonic contents when compared with 18-pulse ATRU at a same power rating of 150KW. These studies further suggest that a 24- pulse ATRU produces 270Vdc to supply the DC bus of aircraft electric system with higher ripple frequencies and it increases the capability and maintainability which reduces the filter size that in turn reduces the weight of aircraft system and also provides high survivability in operations of aircraft. REFERENCES [1] K. J. Karimi, The role of power electronics in more-electric airplanes (MEA), presented at the Workshop on Computer Power Electronics, NY, USA, [2] A. Uan-Zo-Li, R. P. Burgos, F. Lacaux, A. Roshan, F. Wang, and D. Boroyevich, Analysis of new step-up and step-down direct symmetric 18-pulse topologies for aircraft autotransformer-rectifier units, in Proc. IEEE 36 th Power Electron. Spec. Conf., 2005, pp [3] D. A. Paice, Power Electronic Converter Harmonics: Multipulse Methods for Clean Power, IEEE Ind. Appl. Soc., Eds. Piscataway, NJ, USA: IEEE Press, [4] Environmental Conditions and Test Procedures for Airborne Equipment, RTCAlDO- 160G, December [5] Aircraft Electric Power Characteristics, Department of Defense Interface Standard, MIL- STD-704E, May 1991 [6] R.Butgos, Analysis and Experimental Evaluation of symmetric and Asymmetric 18-Pulse Autotransformer Rectifier Topologies. IEEE Power conversion Conference (2007). [7] A. Emadi, M. Ehsani, Aircraft power system: technology, state of the art, and future trends, IEEE Aeropsp. Electron. Syst. Mag. 15(1) (2000) [8] J.A. Rosero, J.A. Ortega, E. Aldabas, L. Romeral, Moving towards a more electric aircraft, IEEE Aerosp. Electron. Syst. Mag. 22 (3) (2007) 3 9. [9] A. Griffo, J. Wang, Modeling and stability analysis of hybrid power systems for the more electric aircraft, Electr. Power Syst. Res. 82 (1) (2012) editor@iaeme.com

11 Harshitha G B and Adithya Ballaji [10] Tao Yang, Serhiyi Bozhko, Member, IEEE, and Greg Asher, Fellow, IEEE. Functional Modeling of Symmetrical Multipulse Autotransformer Rectifier Units for Aerospace Applications (2015). [11] G. Gong et ai., "Comparative Evaluation of Three-Phase High-Power- Factor AC- DC Converter Concepts for Application in Future More Electric Aircraft", IEEE Transactions on Industrial Electronics, Vol. 52, No. 3, June 2005, pp [12] Bhim Singh, Sanjay Gairola, "A Zigzag Connected Auto-Transformer Based 24-Pulse AC-DC Converter"' IEEE Transactions [13] C. T. Rim, N. S. Choi. " A complete DC and AC analysis of three-phase controlledcurrent PWM rectifier using circuit D-Q transformation." Power Electronics, volume 9, pp , (1994). [14] C. T. Rim, N. S. Choi. "Transformers as equivalent circuits for switches: general proofs and transformation-based analyses." Industry Applications, volume 26, pp , (1990) [15] N. Mohan, T.M. U. Undeland, W.P.Robbins " Power Electronics: Converters, Applications and Design." John wiley& Sons, INC editor@iaeme.com