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Comparison of Power Quality Solutions Using Active and Passive Rectification for More Electric Aircraft

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Comparison of Power Quality Solutions Using Active and Passive Rectification for More Electric Aircraft 25TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES COMPARISON OF POWER QUALITY SOLUTIONS USING ACTIVE AND PASSIVE RECTIFICATION FOR MORE ELECTRIC AIRCRAFT Bulent Sarlioglu, Ph.D. Honeywell Aerospace, Electric Power Systems Torrance, California USA [email protected] Abstract The introduction of variable frequency distribution system on commercial aircraft This paper presents various three phase active results in the elimination of a mechanical and passive rectification schemes and compares interface device for variable ratio transmission them for more electric aircraft applications. that is positioned between the main engine and The passive topologies involve multi-pulse the ac electric generator. The ac electric transformer rectification systems while the generator can now be directly coupled to the active topologies look into active rectification. main engine output shaft via a gearbox. The Passive rectification systems are to be used for effect of such direct coupling is an ac bus the ac-to-dc conversion that comprises the first frequency proportional to the engine speed stage of the ac-to-ac conversion for applications (which can vary over a 2:1 range depending on such as motor drives in aircraft systems. application), while the magnitude of the ac bus Particular emphases are given to comparison of voltage is regulated to a constant value via a dc bus regulation, electromagnetic interface generator control unit (GCU) for the generation (EMI) filtering, current harmonic cancellation, system. power factor, size, weight, cost and reliability With such a distribution system, it is still among the schemes. Results are summarized possible to connect induction motors directly to qualitatively in a table. the bus, however, such an approach yields an oversized motor when compared to the fixed 1 Introduction frequency case. Since weight is critical in aircraft applications, such an approach is not One of the radical changes in the commercial feasible. aircraft industry has been the replacement of the To interface these motors with the bus, a traditional constant voltage, constant frequency motor controller, where power is converted (115 V, 400 Hz) alternating current (ac) with from one ac format to one that is compatible constant voltage and variable frequency (380 to with the motor load is needed [1,2,3]. With the 780 Hz) in the power distribution system. In addition of the motor controller the selection of traditional power distribution systems for the electrical motor is no longer limited to aircraft, significant loads are comprised of fans induction motors, but can be other type of and pumps driven by induction motors. When motors including PM motors. Further, the such devices are directly connected to the ac maximum speed of the motor is no longer bus, it is common to have a large inrush current limited by the bus distribution system present at equipment start up. Apart from this frequency, allowing the individual system problem, and the restriction of speed imposed weights to be optimized. Also the large inrush by the distribution system generator, the motor current of an induction machine directly running load has little effect on the distribution and from the constant frequency line can also be generation system and does not require EMI eliminated using motor controllers. filtering. 1 BULENT SARLIOGLU 2 Reason for Current Harmonic terrestrial based systems where even today Requirements substantial damping is provided by resistive loads, aerospace electrical loads are of the A logical method to achieve the conversion of constant power type, further aggravating the power from the constant voltage variable stability issue. Passive solutions can be used to frequency format to one compatible with the achieve some damping. However, additional motor load is, at its front end, to use a simple inductance or resistance is not desirable in six-pulse rectifier followed by an inverter. aerospace applications due to their associated Unfortunately, due to the nonlinear nature of the weight impact. Therefore, the aircraft operation of the 6-pulse rectification scheme, manufacturers develop stringent power quality the input current drawn from the ac distribution requirements [1]. system can become quite distorted. The In order to facilitate the design of frequencies at which characteristic harmonics electrically powered subsystems and to maintain are produced by such an input rectifier can be control of the power quality the aircraft formulated as noted in equation (1) below. manufacturers have developed stringent power quality requirements [1]. f = (k × q ± 1) × f (1) H 1 Because of these facts, more complex rectification systems or active schemes must be In equation (1), f = the characteristic H considered. A typical requirement for future harmonic, H = the number of harmonics, k = an commercial aircraft power quality requirement integer beginning with 1, q = an integer is provided in reference [1]. Table 1 illustrates representing the number of commutations per the current harmonics limit requirements for cycle, and f = the fundamental frequency. 1 loads larger than 5 kW presented in this The characteristic harmonics of a 6-pulse reference. rectification system (such as the 5th, 7th, 11th, 13th, 17th and 19th harmonics) can have Harmonic Order Limits considerable magnitudes. Therefore, the total rd th th harmonic distortion (THD) of current can 3 , 5 , 7 Ih=0.02 I1 become quite high and, in some applications, can exceed 40% of the fundamental current Odd Triplen Harmonics Ih=0.1 I1/h value. These harmonics are not desired in the (h=9, 15, 21,…, 39) distribution system because voltage distortion Odd Non Triplen Harmonics 0.03 I1 results from them, since the generator source 11, 13 impedance is not zero. Further, such undesired Odd Non Triplen Harmonics 0.04 I1 harmonics can result in increased rating for the 17, 19 generation and distribution system, and Odd Non Triplen Harmonics 0.03 I1 increased power dissipation. Additionally, the 23, 25 current harmonics can distort the voltage Odd Non Triplen Harmonics Ih=0.3 I1/h waveform at the point of regulation. 29, 31, 35, 37 As in the dc systems, the ac current Even Harmonics 2 and 4 Ih=0.01 I1/h harmonics from nonlinear loads can excite ac Even Harmonics > 4 Ih=0.0025 I1 resonances due to interaction of the power (h=6, 8, 10, …, 40) generation and distribution impedance and Subharmonics and Ih=0.0025 I1 or source impedance of various power conversion Interharmonics 5mA equipment in the aircraft. Unless sufficient (whichever damping is added at the equipment and/or is greater) system level, this resonance can cause significant concern and possible disruption of Table 1. Current Harmonic Limits for Three-Phase some of the aircraft equipment. Unlike Equipment Greater Than 5 kVA [1] 2 ` COMPARISON OF POWER QUALITY SOLUTIONS USING ACTIVE AND PASSIVE RECTIFICATION FOR MORE ELECTRIC AIRCRAFT 3 Passive Solutions for Power Quality 3.2 120 Degree vs. less than 120 Degree Angle There are passive solutions that incorporate Conduction for Autotransformer Rectifier more than one 6-pulse rectifier to feed the load. Units (ATRU) These solutions rely on canceling some lower When 6-pulse rectifiers are paralleled, characteristic harmonics of input ac current by interphase transformers are normally used to using a multi-pulse transformer or achieve current sharing between the bridges, autotransformer. otherwise each bridge is required to alternately The multi-pulse rectification solutions are carry the full current for 30 degrees. Clearly, it simple, robust, and reliable compared to active is possible to achieve multi-pulse rectification solutions. It should be noted that they achieve systems whereby the conduction angle of diodes harmonic cancellation best at their rated power is not limited to 120 degrees. The advantage of due to the smoothing effect of the source this type of rectification is that they do not need impedance during the commutation. They are to use interphase transformers. For example, a amenable to be used for high ratio of peak 40-degree conduction of diodes can be achieved current to nominal current applications with by using a properly configured 18-pulse proper designs. From the perspective of autotransformer [6]. The advantage of such rectification only this solution may appear to be systems is that although each diode and each heavier in weight compared to active solutions associated transformer leg carries the full load for levels above certain value. However, many current for a reduced conduction angle, the rms factors have to be considered. These factors rating of the current sizes the transformer, and include thermal losses, EMI, and stress the peak current sizes the diode, resulting in a requirements with respect to mounting of the weight competitive solution. parts in a mechanical design. For example, the multi-pulse rectification solutions typically do not require an input ac common mode EMI filter unlike some active solutions. Therefore, a detailed application specific analysis is needed to evaluate the impact on weight of different active and passive solutions. What these systems cannot provide however is active damping of the input filter circuitry, since they are by nature, passive converters. 3.1 Series or Parallel Connection of Diodes Fig.1. Schematic of 18-pulse rectification using To reduce harmonics, two or three 6-pulse diode autotransformer bridges for obtaining 12-pulse and 18-pulse The 5th and 7th harmonics of input ac rectification respectively can be either current are cancelled in a 12-pulse rectification connected in series or parallel depending on the system. Similarly, the 5th, 7th, 11th, and 13th dc voltage magnitude requirement for a given harmonics are canceled in an 18-pulse input ac voltage magnitude. Parallel rectification system. Fig. 1 shows the schematic connections may require interphase of an 18-pulse rectification using an transformers on the dc link to isolate the autotransformer. In reality, these harmonics are operation of the 6-pulse rectifiers if not fully canceled due to the finite source autotransformers are used.
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