International Journal of Aerospace Engineering

Volume 2017 (2017), Article ID 1745154, 12 pages

https://doi.org/10.1155/2017/1745154

## Robust Control of Aeronautical Electrical Generators for Energy Management Applications

Department of Industrial and Information Engineering, University of Campania, 81031 Aversa, Italy

Correspondence should be addressed to Alberto Cavallo

Received 27 February 2017; Accepted 15 June 2017; Published 6 August 2017

Academic Editor: Davide Micheli

Copyright © 2017 Giacomo Canciello et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A new strategy for the control of aeronautical electrical generators via sliding manifold selection is proposed, with an associated innovative intelligent energy management strategy used for efficient power transfer between two sources providing energy to aeronautical loads, having different functionalities and priorities. Electric generators used for aeronautical application involve several machines, including a main generator and an exciter. Standard regulators (PI or PID-like) are normally used for the rectification of the generator voltage to be used to supply a high-voltage DC bus. The regulation is obtained by acting on a DC/DC converter that imposes the field voltage of the exciter. In this paper, the field voltage is fed to the generator windings by using a second-order sliding mode controller, resulting into a stable, robust (against disturbances) action and a fast convergence to the desired reference. By using this strategy, an energy management strategy is proposed that dynamically changes the voltage set point, in order to intelligently transfer power between two voltage busses. Detailed simulation results are provided in order to show the effectiveness of the proposed energy management strategy in different scenarios.

#### 1. Introduction

In the frame of More Electric Aircraft [1], a great emphasis is given about the generation stage, referring to the possibility of providing new “greener” generators, that is, generators with reduced weight and consequently fuel consumption reduction. In addition to increased power density and reduced weight of structures, new improvements about the generation stage for more electrical aircraft can involve also the control stage, which can be optimized for a variety of objectives (e.g., to manage innovative devices for noise and vibrations onboard [2]).

Usually, synchronous machines are used as electric generators. The voltage produced by the generator is distributed to the loads on an AC bus. Moreover, a rectifier or an Autotransformer Rectifier Unit (ATRU) is used, and from the AC bus a derived DC bus is obtained. The control strategy, one of the main points of the paper, is devoted to keep the generator voltage (and, in turn, the DC voltage bus) at a prescribed level. Note that the prescribed level, though constant over small time horizons, may vary in the long run; thus the DC bus voltage is required to reach in finite time and to track slowly varying references (set points). The motivations for a change of voltage output set point are different. For example, consider the following scenarios:(i)A three-phase resistive load is directly connected to the generator output, and a power regulation is desired (for resistive load power regulation is equivalent to voltage regulation).(ii)An uncontrolled AC/DC rectifier is used, for example, an ATRU, providing a DC bus voltage that needs to be regulated.(iii)A sudden change of the rotor speed happens; then the overall rectified voltage variation has to be compensated for.

All the above cases can be dealt with by changing the field voltage of the exciter machine (and consequently of the generator).

Assuming that the generator shaft rotates at constant speed, the regulation of the generator output voltage is performed by varying the generator field windings current, by means of a Generator Control Unit (GCU). The GCU is usually based on a standard controller, that is, Proportional [3] (P) or Proportional-Integral (PI) controller [4] or lead-lag compensators [5]. However, it has been shown [6] that standard, and in general* linear* controllers, have a fundamental limitation: the desired set point is reached only asymptotically, theoretically in infinite time. Moreover, being based on linear control theory, most of them produce only* local* results on practical devices, which are usually nonlinear systems. In contrast, Variable Structure Controllers (VSC) [7] are nonlinear switching controllers known for their capabilities to reach set points in finite time. Moreover, VSC are very robust, since their design is model-free; that is, there is neither the need for an estimate of the generator parameters nor the determination of the accurate values of the parameters of the mathematical equations governing the generator behaviour. Finally, VSC works directly in nonlinear settings. VSC techniques have been recently used for controlling switched reluctance generators [8] and for standalone wound rotor synchronous generators [9]. However, the resulting control law switches between two fixed levels with a variable, unpredictable frequency, and this drawback has motivated the researchers to look for a smooth version of the VSC control, obtained by inserting an integrator at the output of the controller, so that the control action, continuous, though with a discontinuous derivative, can be implemented by standard PWM modulators. Following this approach, in [10] a permanent magnet synchronous generator system has been controlled with a second-order sliding mode controller, by using a supertwisting algorithm [11]. However, in all the previous approaches the generator system was considered as a standalone system and was modelled as a dynamic system with relative degree [12] one. On the contrary, in this paper the model of the generator, including also the filter used for rectification, has relative degree two and a twisting control algorithm is used for the first time.

A second topic addressed in this paper is the energy management on board. It is well known [13] that electric generator sizing is based on the so-called “5 seconds’” and “5 minutes’” overload capability, which are a piecewise linear approximation of the true overload curve of the generator. Roughly speaking, the generator is assumed able to withstand a large overload for the first 5 seconds, while the 5 minutes’ overload level simply indicates a maximum level that the generator can supply in steady state. Obviously, if a total load is connected to the generator such that after a transient of maximum duration of 5 s, it requires more power than the rated , generator sizing has to be increased. Dually, if after 5 s a power level below was guaranteed, no oversize related to 5 minutes capabilities would be needed, thus reducing size and weight of the generator. This idea has been exploited in [14] for a configuration with an ideal generator and a single battery. In this paper we choose another topology. We consider a common configuration for aeronautical electrical networks, where two (or more) DC voltage busses fed by separated generators are linked via a contactor and a line, as reported in Figure 1. The contactor between the DC lines is normally open. Usually, in case of loss of a generator, the contactor is closed in order to let the active generator provide power for the loads originally assigned to the faulty generator. The idea in this paper is to extend the power transfer also to the case of generator overload. The proposed energy management strategy is referred to an architecture where two DC busses are introduced: a critical bus and a noncritical bus. The substantial difference between these busses is related to the connected loads: for the critical loads (CLs), the power demand must always be satisfied satisfying the 5 s–5 min criterion discussed above, without tolerating long lasting overload on the CL generator. Following the MEA approach, particularly referring to regional aircraft, a common architecture involves 4 DC busses [15], so it can be easily possible to select a couple of connected busses and concentrate CLs and NCLs, respectively. Given this, the energy management logic is based on the following steps:(i)After the overload detection on the CLs bus, closure of the common contactor with the NCLs bus.(ii)Increase of the NCLs bus voltage and simultaneous decrease of the CLs bus voltage, and in any case preserving the MIL-STD-704F standard [16] prescribed operating voltages for 270 VDC systems (i.e., DC bus voltage in the range [250–280] Volts). This action permits a current transfer from the NCLs to the CLs bus.(iii)Overload management until the condition termination, associated with the regulation of the power delivered by the CLs generator at a value lower than overload. This action is obtained by iteratively looking for an optimal NCLs bus voltage, hence obtaining a proper power transfer.(iv)Detection of the overload condition ending, restoration of the nominal voltage for the NCLs bus, and opening of the contactor between the two busses.