aircraft acts as the prime mover for the field-controlled synchronous generator resulting in variable frequency output power at the generator's terminals.
Modeling and Characterization ofVSCF Aircraft Electric Power Systems with Nonlinear Loading
H. EI-Kishkyl, H. Ibrahimi1, M. Abu Dakka2, A. Eid3, and M. Abdel-Akher3 I The University of Texas at Tyler, Tyler, TX 75703 2The University of Tulsa, Tulsa, OK 74lO4 3South Valley University, Aswan, Egypt In this study, a comprehensive model of variable-speed, constant-frequency (VSCF) aircraft electric power system (AEPS) with a large share of nonlinear loads is developed. The model is used to study the performance of the system under different nonlinear loading conditions. The performance of the VSCF AEPS is studied over the entire range of the aircraft electric power system operating frequency. Moreover, the model is extended to study the performance of the AEPS under nonlinear loading along with harmonic cancellation. Both transient and steady-state performance characteristics of the system are obtained and investigated. The effect of nonlinear loading on power quality of the AEPS is also investigated and compared to applicable aircraft electric power system's IEEE and military standards.
Abstract:
I. INTRODUCTION
The electric power system of an advanced aircraft includes an internal combustion engine, electric starters/generators, integrated power units, solid-state power controllers, electric driven flight actuators, an electric anti-icing system, a fault tolerant solid state electrical distribution system, electric aircraft utility functions, and electric-driven environmental and engine controllers. A simplified block diagram of the electric power system of an advanced aircraft channel [1-7], with AC non-linear loading is shown in Fig. 1. 270-VDC Bus
200-VAC 400-Hz Bus
Staticlo.'lds (A,S,C) Dynamicloads (M],M2,M3)
Fig. 1: Block diagram of VSCF aircraft system structure.
978-1-4577-0631-8/12/$26.00 ©2011 IEEE
The Boeing 767 aircraft electric power system consists of two independent channels, according to the number of starters/generators in the aircraft and an auxiliary/emergency power unit (APU) that contains an additional auxiliary starter/generator to provide power engines starting as well as for emergency back-up. The generating system comprises starter/generators, power control units, and a generator and system control unit. A single channel of the aircraft electric power system is studied here and shown schematically in Fig. 1. Although the rated frequency of the aircraft power system is 400-Hz, in a VSCF system the engine speed changes at approximately 1:2 ratio, resulting in the aircraft synchronous generator frequency changing in the range of 400-Hz to 800-Hz. During starting mode, the constant frequency system provides power through the interface power converter to the electric machine which acts as a starter to the aircraft engine. In the generating mode, the variable speed engine in the aircraft acts as the prime mover for the field-controlled synchronous generator resulting in variable frequency output power at the generator's terminals. This power is then delivered via the interface converter (DC-link) to the constant frequency (400-Hz) aircraft electric power distribution system [6,7]. II. STRUCTURE OF THE MODELED VSCF AEPS
The modeled aircraft power system is equivalent to the Boeing 767 electric power system with a generator output of 90-kVA per channel. In this model, different load combinations are studied including passive and dynamic AC loads as well as various DC loads on the aircraft system. Constant power, constant current, and constant voltage loads are also considered. During the starting mode, the APU system, consisting of the battery and Fuel Cell (or the ground power) system provides power, through the interface power converter, to the electric machine which acts as a starter for the aircraft engine. In the generating mode, a variable speed engine provides input power to the generator. The generator output is then delivered via the interface converter to the constant frequency system. The synchronous generator output voltage has a 400- to 800-
Hz frequency range [2]. The power converter used in an aircraft is either a passive 6-pulse [4], an active 6-pulse [3], or a passive 12-pulse [5] configuration. III.
AEPS SIMULATION RESULTS WITHOUT
HARMONIC FILTERS
In the modeled AEPS, three different types of DC loads are connected to the regulated 270-VDC bus through appropriate DC/DC converters. These loads are further classified as; a constant current (CC) load of 75A, 20.2-KW and a constant voltage (CV) load of 28VDC, 9. 8-KW. The DC loads are regulated using PI controllers. Moreover, there are three different types of AC loads in this simulated aircraft power system. The first type is passive 18-KV A AC load consisting of a series RL load with a PF not less than 0. 85 as recommended by aircraft standards [8,9]. The second kind is a 36.4-KV A 12Pulse Converter, which is a controlled voltage non linear load (***there are more descriptions about this load in the attached file***). The other type is a 6.2KVA current constant induction motor as a representative on ac dynamic load. Since One of the important methods to evaluate the power quality of a power system is to calculate the Total Harmonic Distortion (THD) factor [***], in this paper we have measured power quality of the system at different major nodes, before and after filtering. We also have used different types of filters (Passive Power Filters, and Active Power Filters) to see the effectiveness of those elements in removing distortions from the grid. In this study, our simulations are based on different case studies which cover all different types of loads mentioned above separately and their affects on power quality. Since our focus in this study is non-linear loading and the amount of harmonics and distortion it contributes to the aircraft electric power system under study, the THD values for different cases have been determined for the different loading case-studies. We have also studied the AEPS performance with APF connected to mitigate distortion and harmonic contents from non-linear loading.
harmonics cancellation, we have only applied that particular load to the AEPS with all others disabled. Figure 2 shows the %THD of the current profile at the generator terminals for the different case studies. A range of 8 to 18% for the various case studies is reported at the nominal operating frequency of 400Hz. This range drops to 5-17% at the 800Hz operating frequency. This drop in current THD against operating frequency may be attributed to the harmonic impedance effect at higher frequency.
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IV.
AEPS
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To eliminate distortions generated by switching devices and loads, we have designed a second order low pass filter (RLC type). Since harmonics are multiplies of fundamental frequencies (400-800Hz), we have calculated the values for L and C in each branch of shunt passive filter considering corner frequency in each case (400-800Hz), so that it guaranties all harmonics components and even higher orders will be mitigated from the grid. We have connected each one of this type of devices at the grid's critical nodes. One has to be connected all the time at the output of 12-pulse inverter at the 200-VAC main bus (before AC loads) to mitigate amounts of distortions injected to the loads. The other one is connected to the input node of 12-pulse converter in order to smooth the voltage received from generator. We have also connected one to the output terminal of the generator to cancel out harmonics content. A.
Filter Design and Topology
We have designed a second order passive filter for our simulation which is capable of damping all harmonics components generated by 12 pulse inverter and 12 pulse converter as switching power supplier units, and also 12 pulse converter(270-ac to 28-dc) as an AC non-linear load. Our design is based on simplicity and power optimization. This filter consists of a series of resistance, inductance and capacitance per phase. The values for inductances and capacitances have been selected based on the corner frequencies (4400, 5500, 6600, 7700 and 8800-Hz) which are derived from operating frequencies (400, 500, 600, 700 and 800-Hz). For harmonic components with the frequency equal to corner frequency, filter's capacitance and inductance behave as a shorted impedance and prevent distortions flowing through the AC load. The series resistance, on the other hand, controls the current going into the capacitances. we have selected the optimum values for the resistance in each operating frequency so that the power loss has been kept as low as possible. Second order PPF. In our filter design strategies, we have seriously considered the possibility of Resonance Effect, applying PPF to the grid. Since shunt passive filter, at a certain operating frequency may get into resonance along with grid's AC bus parameters (L and R), hence we have carefully selected our filter parameters (R, Land C) so that remove distortions very well, and never go into resonance and instability.
Figure 4 shows the %THD of the current profile at the generator terminals with 2nd order PPF connected for the different case studies. A range of 3.5 to 7.2% for the various case studies is reported at the nominal operating frequency of 400Hz. This range drop to 2-4% at the 800Hz operating frequency. This drop in current THD against operating frequency may be attributed to the harmonic impedance effect at higher frequency. 8
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Although passive filters are capable of removing distortions considerably, however, they suffer from fixed compensation, and also their weakness of damping current harmonics flowing through the grid, mostly generated by switching devices (rectifiers and inverters) and non-linear load. Specifically current distortions cannot be removed by second order passive filter (as shown in the Figs/Tables). The other issue with PPF is its weakness in damping distortion contents at different operating frequencies, and in fact we have to modify our filter parameters to meet the design requirements in every single case, in terms of power optimization and filter's performance in cancelling harmonics. Disregarding the complexity of design and power losses in the transistor bridge elements, APF have enough flexibility in removing distortions for various operating frequency, only by doing some modifications in the operating program stored in the filter's Ie. The other issue with APF is the time delay caused by Reference
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[1] I. Moir and A. Seabridge, Aircraft systems: mechanical, electrical, and avionics; subsystem integration, American Institute of Aeronautics and Astronautics, Inc., 2001.
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Fig. 5: Voltage %THD at the main AC bus with nonlinear loading and APF Figure 5 shows the voltage %THD at the main AC bus with nonlinear loading and the APF connected. A significant drop in the harmonic content is observed and attributed to the connected APF. Figure 6 shows the voltage and current transients at the generator terminals as the hybrid APU is switched on.
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[3] K.W.E. Cheng, "Comparative study of AC/DC converters for more electric aircraft," lEE Power Electronics and Variable Speed drives Conference, pp. 299-304, Sep. 1998. [4] V.V. Vadher, I.R. Smith and S. Williams, "Mathematical modeling of a VSCF aircraft generating system," IEEE Trans. Aerosp. Electron. Syst., Vol. 22, No. 5, pp. 573 - 582, Sep. 1986. [5] G. Guanghai, M. Heldwein, U. Drofenik, J. Minibock, K. Mino and J. Kolar, "Comparative evaluation of three phase high power factor ACDC converter concepts for application in future more electric aircraft," IEEE Trans. Ind. Electron., Vol. 52, No. 3, pp. 727-737, Jun. 2005.
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[6] A Eid, H. EI-Kishky, M. Abdel-Salam, and Th. EI Mohandes, "On Power Quality of VSCF Aircraft Electric Power Systems" IEEE Trans. PWRD, vol. 25, no.I, Jan 2010 [7] A Eid, H. EI-Kishky, M. Abdel-Salam, and Th. EI Mohandes, "Active Power Filter for Power Quality Improvement in Aircraft Electric Power Systems" Proceedings of the IEEE Industrial Electronics Conference, IECON-08, Orlando, Florida, November 2009.
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[8] IEEE recommended practices and requirements for harmonic control in electrical power system", ANSI/IEEE Std. 519- 1992. [9] Military-STD-704F "Aircraft electric power system characteristics" 2004.
VI. SUMMARY
Nonlinear loading has a significant effect on the characteristics of VSCF aircraft electric power system in terms of harmonics content and the voltage/current distortion level as well as poor true power factor at the point of common coupling. A second order passive power filter with a unique topology was designed to eliminate the most significant harmonics
Corresponding author: Hassan El-Kishky The University of Texas at Tyler 3900 University Blvd. Tyler, TX 75799 helkishky(iiJ,uttyler. edu (903) 565-5580
