Ground Return Fluctuation management in More Electric Rotorcraft

03/02/2015
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Ground Return Fluctuation management in More Electric Rotorcraft

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application/pdf Ground Return Fluctuation management in More Electric Rotorcraft Marc Meyer, Véronique Nave, Marc Poncon
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Ground Return Fluctuation management in More Electric Rotorcraft

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Ground Return Fluctuation management in More Electric Rotorcraft Marc MEYER (1), Véronique NAVE (2), Marc PONCON (3), 1: Airbus Helicopters, Aéroport Marseille Provence 13700 Marignane marc.meyer@airbus.com 2: Airbus Helicopters, veronique.nave@airbus.com, 3: Airbus Helicopters, marc.poncon@airbus.com Abstract This paper describes Ground Return Fluctuation (GRF) issue in a full composite Rotorcraft. In such a structure, a metallic network called Electrical Structure Network (ESN) may be used to insure return currents of embedded equipment items. In order to maintain GRF levels acceptable for these equipment items, a simulation approach is described for the design optimization of this network and the definition of harness installation rules. The physics is quite different for both the AC and the DC current distributions which lead to different analysis method and design constraints. The last release of the Airbus Helicopter specification describing environment requirements for embedded equipment items includes a new test procedure and applicable levels for checking the withstanding of new-developed equipment items to GRF disturbance. Introduction The awareness of the Ground Return Fluctuation (GRF) issue in Airbus Group is contemporary to first civil Aircrafts and Rotorcrafts with an airframe largely composed of composite materials. This issue is especially reinforced with the context of equipment item reuse which reduces the room for maneuver on installation and forbids the withstanding of high GRF levels thanks to an appropriate equipment design. Therefore a specific interconnected metallic network (ESN) has to be installed in the structure in order to maintain electric performances of the structure similar to the one of a metallic helicopter. Moreover, the ESN is also used for other electric functions as some protection against Lightning indirect effects and an optimized shape enables an optimization of ESN elements efficiency (fig.1). Whatever the shape, the increase of the embedded power requires a rising of ESN section, which is susceptible to significantly impact the Helicopter weight. Consequently, an advanced simulation approach has been developed for the management of the GRF issue. Fig. 1: ESN element called raceway GRF Phenomenology and ESN design The flowing of return currents in the Rotorcraft structure which presents some resistance creates a voltage drop in the structure. This voltage is seen by the equipment interfaces and could lead to malfunctions if the immunity threshold is exceeded. This threshold is generally limited to few volts according to the equipment design and software logic. In rotorcraft DC current distribution, the voltage drop level (VGRF) on an equipment interface is easily expressed from the structure resistance between the equipment and its connected load (Rs) and the total return current (Itot): The total return current represents the total current of all helicopter systems flowing in the considered structure path, either the maximum nominal current in a particular flight phase or the current consumption during the engine starting or the current induced by a failure event as short-circuit. This last case is more severe in term of maximum level but lasts on a shorter time. Faced with this various scenarios, different threat levels have to be distinguished in terms of level and duration. The GRF constraint is much more complex to estimate in an AC distribution network because the current distribution is then also governed by inductive phenomenon. Indeed, the return current is more concentrated close to forward power wiring (feeders). Therefore ESN elements far from power routes are not participating to the reduction of the GRF, which leads to higher GRF levels compared to the same DC current flowing into a multi path ESN. The figure 2 illustrates the voltage induced on equipment item interfaces due to GRF according to the frequency of the electric distribution. Contrary to DC distribution, the threat in AC also produces an induced magnetic field so the total AC GRF threat level depends highly of the location of the highest current consumers and their wiring paths in regard to the avionics ones. Fig. 2: GRF constraint according to the frequency In a full composite Airframe, the ESN is necessarily a complex multipath architecture since safety constraints lead to redundancy and segregation requirements. Then the section of each path has to be sized to drop the structure resistance and maintain GRF levels acceptable for all the DC and AC possible scenarios. Consequently, Airbus Helicopters appeals to the modeling in order to design as tightly as possible this network for a given electrical architecture and define harness installation rules regarding both the electromagnetic and safety constraints. The simulation approach The phenomenology being simpler in a DC distribution, the modeling approach may rely only on a Spice like simulation tools to size the ESN. The network is simply modelled as a resistance network which drives the current distribution (see figure 3). Fig. 3: Spice modelling of ESN In AC distribution architecture, additional 2D simulation and 3D simulation [1] are required in order to model the influence of magnetic couplings between all the the structure elements. This phenomenon could be represented in term of equivalent circuit as inductances and mutual inductances. Presently the assessment of these couplings required specific modelling tools which generally imply longer simulation analysis than for DC distribution. Airbus Helicopters has developed a simulation tool which relies highly on MEFISTO tool developed by Airbus Group Innovation and already used for the A350 ESN design. The main advantage of this tool is its user friendly simulation flow and the fast computation times compared to 3D models. This tool calculates GRF iso-lines in the environment of the feeders (fig.4) and shows the high influence of the harness location on the induced voltage. Then, the segragation needs between power routes and signal ones are able to be define according to the GRF limit target corresponding to the equipment immunity level. . Fig. 4: GRF constraint according to feeder distance at 400Hz Conclusions In order to take into account the GRF constraint, the last release of the Airbus Helicopters specification describing the environmental requirements for embedded equipment items includes a new test procedure and applicable levels for checking the immunity of new-developed equipment versus GRF. In addition, the simulation approach presented in this document also allows assessing overheating risk at ESN junctions, from the assessment of current repartition. References [1] A.Piche and al, Modeling of large Avionics Structures in Electrical network Simulations, EASA workshop on Aerospace EMC, 2012 1 1 1 2 2 2 2 2 2 3 3 3 3 4 4 5 5 6 7 X (mm) Y (mm) GRF - Local Voc map (V) 550 600 650 700 750 800 850 900 950 1000 2500 2600 2700 2800 2900 3000 GRF:AC Value GRF:DC Value Upper Deck Raceway Feeder location