Decoupling control for aircraft brushless wound-rotor synchronous starter-generator in the starting mode

The 4th International Symposium on More Electric Aircraft Technology (MEA 2017) 8 9 November 2017 Decoupling control for aircraft brushless wound-rotor synchronous starter-generator in the starting mode Ningfei Jiao 1, Weiguo Liu 1, Tao Meng 1, Chenghao Sun 1, Yu Jiang 2 1 Department of Electrical Engineering, Northwestern Polytechnical University, Xi an, Shaanxi, 710072, People s Republic of China 2 AVIC Shaanxi Aero Electric Co., Ltd, Xi an, Shaanxi, 710065, People s Republic of China E-mail: jiaoningfei@gmail.com Published in The Journal of Engineering; Received on 10th January 2018; Accepted on 2nd February 2018 Abstract: The main generator (MG) and main exciter (ME) of the aircraft wound-rotor synchronous starter-generator (WRSSG) are electromechanically coupled seriously, which makes the start control for the WRSSG complex in the starting mode. A decoupling control method for the MG and ME of the WRSSG is proposed in this study based on the estimation of the MG field current. In the proposed decoupling method, the MG field current is estimated first and used in the closed-loop excitation control for the ME to make the MG field current meet the MG demand. With the desired field current, the MG is decoupled with the ME and is controlled by using a traditional synchronous motor to start the aircraft engine at maximum torque per ampere. The feasibility and effectiveness of the proposed decoupling control method are verified by experimental results. 1 Introduction An integrated starter-generator (ISG) with less volume and weight is drawing more and more attention in aircraft power system applications, especially for a more- electric aircraft [1 3]. A wound-rotor synchronous machine (WRSM) with a brushless exciter is an attractive candidate for aircraft ISG because of advantages of high safety (possibility of cancelling of the field current in the case of short-circuit and over-high voltage in the generation mode), long lifetime (without brush and slip ring), and mature technique in generation mode (widely used as the generator in a traditional aircraft power system) [4 6]. A two-phase brushless exciter and its control methods were newly proposed to solve the weak excitation problem of wound-rotor synchronous starter-generator (WRSSG) in the starting mode [7, 8]. The structure of the WRSSG with a two-phase brushless exciter is shown in Fig. 1. The WRSSG includes three electrical machines in the same shaft: the main generator (MG), the main exciter (ME) and the pre-exciter (PE). The PE is a permanent magnet synchronous generator, and it only works in the generation mode. The ME is a non-salient asynchronous machine with a twophase stator winding and three-phase rotor winding, and the MG is an electrically excited synchronous machine with the field winding in the rotor. The rotor armature windings of the ME are connected to the field winding of MG through the rotating rectifier, which is a three-phase full-bridge diode rectifier. In the starting mode, the twophase ME controller supplies two-phase AC excitation or DC excitation (the DC excitation is only for high-speed starting mode) for the ME, and then the rotor armature winding of the ME provides a DC field current for the MG through the rotating rectifier. Using a particular start control method with the MG controller, the MG works as a motor to start the aircraft engine. In the generation mode, the two-phase field winding of the ME is connected in series into a single winding and connected to the generator control unit (GCU), and the PE provides DC excitation for the ME through the GCU. The MG works as a generator and supplies electric power for the aircraft electrical loads. The operation principle of the WRSSG in the generation mode is the same as the traditional brushless wound-rotor synchronous generator. From the structure and operation principle of the WRSSG, it can be seen that there is a serious electromechanical coupling between the MG and ME in the starting mode: on the one hand, the speed change caused by the torque output of the MG directly influences the rotor output voltage of the ME, and hence the MG field current; on the other hand, the armature reaction of the MG also influences the MG field current, and hence the rotor armature currents of the ME. Meanwhile, the MG and ME are connected to the rotating rectifier, and the strong non-linearity of the rotating rectifier makes it very difficult to express the relationship between the MG and ME analytically with algebraic functions. Hence, this brings many difficulties for decoupling analysis and control for the MG and ME. Besides, because there is no brush and slip ring in the WRSSG, as the coupling point of the MG and ME, the MG field current cannot be measured. The unknowable MG field current also makes the decoupling analysis and control of the MG and ME difficult. The start control method only for the MG was the main research focus in the literature, and excitation for the ME was usually constant and uncontrolled [9, 10]. Constant excitation for the ME makes the MG field current uncontrollable, which may bring a big risk for the successful start of the WRSSG. For example, a too small MG field current may make the WRSSG cannot output enough torque to start the aircraft engine, while excessive MG field current may cause the diode in the rotating rectifier to malfunction, hence failure to start; meanwhile, an excessive MG field current also influences the speed range in the starting mode, and may cause the WRSSG to fail to reach the self-sustain speed of the aircraft engine. Therefore, during the start-up process, the MG and ME of the WRSSG need to be controlled simultaneously. However, under the situation that the MG and ME are coupled so serious, it is

Also, the electromagnetic torque of the MG, denoted as T e can be expressed as T e = 1.5p n [M sr i gf i qs + (L d L q )i qs i ds ], (3) where p n is the number of pole pairs of the MG. 2.2 Model of the two-phase excitation system The two-phase excitation system includes the two-phase ME and the rotating rectifier. A model of the ME is built in the two-phase stationary reference frame (noted as αβ reference frame). A model of the rotating rectifier is built in the formulation of parametric average value model (PAVM). The voltage equations of the ME can be represented as Fig. 1 Structure of the WRSSG with two-phase brushless exciter very hard to obtain a simple and effective start control method for the WRSSG in the starting mode. Separated researches about the excitation control for the ME and start to control for the MG were carried out in the literature [8, 9]. However, decoupling control research for the ME and MG was rarely carried out. A decoupling control method for the MG and ME of the aircraft WRSSG based on the MG field current estimation is proposed in this study. In the proposed decoupling method, the MG field current was estimated online and then used to decouple the coupling relationship between the MG and ME: The MG field current is estimated first and used in the closed-loop excitation control for the ME to make the MG field current meet the MG demand. With the desired field current, the MG is decoupled with the ME and is controlled by a traditional synchronous motor to start the aircraft engine at maximum torque per ampere (MTPA). An experiment platform with a WRSSG prototype was built and experimental results verified the feasibility and effectiveness of the proposed decoupling method. 2 Models of the MG and excitation system 2.1 Model of the MG The MG of the WRSSG is an electrically excited WRSM, so the voltage equations and flux linkage equations of the MG in the rotor reference frame can be expressed as (1) and (2), respectively u ds = R s i ds + pl ds v r l qs, u qs = R s i qs + pl qs + v r l ds, u gf = R gf i gf + pl gf, l ds = L d i ds + M sr i gf, l qs = L q i qs, l gf = 1.5M sr i ds + L gf i gf, where u ds, u qs, and u gf are d-/q-axis stator voltages and field voltage of the MG, respectively. i ds, i qs, and i gf are d-/q-axis stator currents and field current of the MG, respectively. λ ds, λ qs, and λ gf are d-/ q-axis stator flux linkages and field flux linkage of the MG, respectively. R s and R gf are stator and rotor resistances of the MG. L d, L q, and L gf are d-/q-axis stator inductances and rotor self-inductance, respectively. M sr is the maximum value of the mutual inductance between stator and rotor windings. ω r is the rotor electrical angular velocity of the MG. (1) (2) u ase = R re i ase + dl ase, dt u bse = R se i bse + dl bse, dt u are = R re i are + dl are + v dt re l bre, u bre = R re i bre + dl bre v dt re l are. Also, the flux linkage equations of the ME can be represented as l ase = L se i ase + 1.5M sre i are, l bse = L se i bse + 1.5M sre i bre, l are = 1.5M sre i ase + L re i are, l bre = 1.5M sre i bse + L re i bre. where u αse, u βse, u αre and u βre are α-/β-axis stator and rotor voltages of the ME, respectively. i αse, i βse, i αre and i βre are α-/β-axis stator and rotor currents of the ME, respectively. λ αse, λ βse, λ αre and λ βre are α-/ β-axis stator and rotor flux linkages of the ME, respectively. R se and R re are ME stator and rotor resistances. L se and L re are selfinductances of ME stator winding and rotor winding in the αβ reference frame, respectively. M sre is the maximum value of the mutual inductance between ME stator and rotor windings. ω re is the rotor electrical angular velocity of the ME. The PAVM of the rotating rectifier can be expressed as [11 13] (4) (5) (6) (7) u 2 are + u2 bre = a(z)u gf, (8) i gf = b(z) i 2 are + i2 bre, (9) where coefficients α(z) and β(z) are algebraic functions of loading conditions that can be specified in terms of an impedance [11], which can be defined as z = u 2 are + u2 bre/ i 2 are + i2 bre. (10) Generally, coefficients α(z) and β(z) can be obtained by simulation of the detailed switching model of the WRSSG under different operating conditions [11 13]. 3 Decoupling control for MG and ME On the one hand, the ME supplies field current for the MG, and on the other hand, the MG field current affects the rotor armature current of the ME. So the coupling between the MG and ME is mainly realised by the MG field current. Therefore, the MG field

current is selected as the breakthrough point to implement the decoupling control for the MG and ME. However, the MG field current cannot be measured directly as there is no brush and slip ring in the WRSSG. So, the MG field current should be on-line estimated first before the decoupling control for the MG and ME. Once the MG field current is estimated, the decoupling control for the MG and ME can be carried out based on the estimation of the MG field current. 3.1 Estimation of MG field current Considering the magnetic field saturation effect in the MG during the start-up process, it is difficult to perform accurate estimation of the MG field current from the MG side [14]. So, the MG field current estimation method presented in [13], performing the MG field current estimation from the exciter side, is used in this study. For the completeness of the proposed decoupling control method in this study and the convenience for readers, a simple presentation for the MG field current estimation method from the exciter side is given below. The rotor currents of the ME in the αβ reference frame can be obtained by manipulation of (4) and (6) as i are = 1 1.5M sre (l ase L se i ase ), i bre = 1 1.5M sre (l bse L se i bse ), (11) where the stator flux linkages of the ME, λ αβse, can be obtained by manipulation of the measured stator voltages and currents of the ME as l ase = (u ase R se i ase )dt, l bse = (u bse R se i bse )dt. (12) As pure integration in (12) is only marginally stable, stable approximations to ideal integrators, like a second-order approximation, are typically used to calculate the flux linkages in practice instead of the pure integration [15]. With the information of ME rotor currents i αβre, the ME rotor flux linkages, λ αβre, can be calculated using (7). Also the ME rotor voltages, u αβre, can be computed using (5). After the calculation of the ME rotor currents and voltages, the defined impedance in the PAVM of the rotating rectifier is computed using (10) and hence β(z) can be obtained. Finally, the averaged value of the MG field current can be obtained using (11). 3.2 Decoupling control method for WRSSG After the estimation of the MG field current, the decoupling control for the MG and ME can be carried out. The basic idea of the proposed decoupling control method for the WRSSG is as follows: the MG raises the demand for the field current according to the starting demand and starting characteristics, and then the field current demand of the MG is used as the control target and the estimated MG field current is used as the feedback variable in the closed-loop excitation control for the ME. The closed-loop excitation control for the ME makes the MG field current meet the MG demand, and with the desired field current, the MG is decoupled with the ME and is controlled as a motor to start the aircraft engine under MTPA. After the decoupling, the start control for the WRSSG is divided into two parts: excitation control for the ME and MTPA start control for the MG. The excitation control for the ME is actually the generation control for an asynchronous generator with a diode rectifier (the rotating rectifier) and a series of a resistor and an inductor (RL) load (field winding of MG). Also, the start control for the MG is the same as the start control for a traditional electrically-excited synchronous motor. The block diagram of the proposed decoupling control method for the WRSSG system is shown in Fig. 2. The proposed decoupling control method includes two parts: the closed-loop excitation control for the ME and the MTPA start control for the MG. The objective of the closed-loop excitation control for the ME is to make Fig. 2 Block diagram of the proposed decoupling control method for the WRSSG system

the MG field current meet the field current demand proposed by the MG. Also, MTPA control is used in the start control for the MG to minimise the MG armature current. 3.2.1 Closed-loop excitation control for ME: The field current demand of the MG in the starting mode can be divided into two stages: under the base speed, in order to simplify the start control algorithm for the MG and keep the current flowing in the rotating rectifier stable, the MG field current should be basically constant. While above the base speed, the MG field current should decrease (field-weakening) so that the rotor speed can go higher. This study mainly focused on the start control under the base speed, which is the general case for the aircraft WRSSG, and start control research above the base speed will be carried out in the future. Once the MG field current is estimated and the field current demand of the MG is determined, closed-loop excitation control for the ME can be implemented to make the MG field current meet the field current demand of the MG. The block diagram of the closed-loop excitation control method for the ME is shown in the blue dashed box in Fig. 2. The excitation control for the ME includes two parts: one is the closed-loop control for the MG field current, and the other is the excitation frequency adjustment for the ME. In the closed-loop control for the MG field current, the estimated MG field current, denoted as î gf, is compared with the field current demand of the MG, denoted as i gf, and the comparison error is used to determine the magnitude of the ME excitation voltage through a proportional integral (PI) regulator. The main objective of the excitation frequency adjustment for the ME is to keep the relative speed constant between the ME rotor winding and magnetic field in the ME. Constant relative speed, on the one hand, can make sure that the frequency of the ME rotor voltages and currents remains constant, which is helpful to obtain the average values of the ME rotor voltages and currents in the estimation of MG field current. On the other hand, a constant relative speed can make the frequency of the pulsating component in the MG field current constant, which can reduce the effect of the pulsating components of the MG field current on the starting performance of the WRSSG. After the calculation of ME excitation frequency, the vector angle of the ME excitation voltage can be obtained. Also, combining the vector angle and magnitude of the ME excitation voltage, the excitation voltage for the ME can be determined. 3.2.2 MTPA start control for MG: The closed-loop excitation control for the ME makes the MG field current meet the field current demand proposed by the MG. Then with the desired field current, the MG is decoupled with the ME and can be controlled as a traditional synchronous motor to start the aircraft engine. In this study, MTPA is used as the target to perform the start control for the MG in order to minimise the MG armature currents. The magnitude of the MG armature current is denoted as i s. Then the d-/q-axis armature currents of the MG can be represented as i ds = i s cos b I, i qs = i s sin b I, (13) where β I is the MG current angle defined as the angle between the armature current vector and the d-axis in the MG. By manipulation of (3) and (13), the expression of the MG electromagnetic torque can be transformed into T e = 1.5p n [M sr i gf i s sin b I + 0.5(L ds L qs )i 2 s sin 2b I ]. (14) According to the definition of MTPA, one can obtain dt e du = 1.5p n [M sr i gf i s cos b I + (L ds L qs )i2 s cos 2b I ] = 0. (15) Fig. 3 Experiment platform By solving (15), the MG current angle can be obtained as M sr i gf + Msr 2i2 gf + 8(L ds L qs )2 i 2 s b 1 = a cos. (16) 4(L ds L qs )i s Finally, (16) gives the MG current angle under MTPA. When considering the magnetic field saturation effect in the MG, the inductances in (16) may change with the MG field current and armature currents. In this situation, the inductance parameters of the MG were calculated from the finite element analysis model of the MG under different saturation conditions by setting the different field and armature currents. The calculated inductance parameters with different field and armature currents can be stored in lookup tables or curve-fitted. Then in the MTPA start control for the MG, the MG inductances are first decided by the field and armature currents through the lookup tables or fitted functions, and then used in the MTPA calculation algorithm. The block diagram of the MTPA start control for the MG is shown in the red dashed box in Fig. 3. It can be seen that the estimated MG field current is used in the MTPA calculation module and the inductance calculation module. In the MTPA start control for the MG, the feedback control for the rotor speed determines the magnitude of the MG armature current through a PI regulator, and then the optimised current angle is calculated by the MTPA algorithm shown in (16). Thus, d-/q-axis armature currents of the MG can be obtained from the magnitude and angle of the MG armature current vector, and then the traditional vector control method is used to complete the start control for the MG. 4 Experimental results The experiment platform was built, as shown in Fig. 3, to verify the proposed decoupling control method. The WRSSG prototype with two-phase ME was connected to a load platform through a torque transducer, and the load platform was used to output load torque to emulate the aircraft engine. An RT-Lab real-time simulation system was used in the experiment platform to implement the proposed decoupling control method, and a LeCroy oscilloscope was used to display the measured current signals. Experiments about the MTPA control for the MG were first carried out. In these experiments, the MG field currents were controlled to be 10 and 20 A, respectively, by the closed-loop excitation control for the ME, and the rotor speed of the WRSSG was controlled to be 500 r/min by the feedback control for the rotor

Also, the MG current angles obtained by the MTPA calculation module in each set of experiments were designated by red stars in Fig. 4. It can be seen that these red stars are basically located at the valleys of the curves, which means that the MTPA method predicts the optimised MG current angle well. After the verification of the MTPA control for the MG, experiment of the start control for the WRSSG using the proposed decoupling control method was carried out. During the start-up process, the MG field current was estimated and controlled to be 15 A by the excitation control for the ME, and the load platform output the torque curve of an aircraft engine. The measured armature current and estimated field current of the MG are shown in Fig. 5. It can be seen that the WRSSG starts successfully with the emulated engine load, and during the start-up process, the MG field current remained basically constant at 15 A. 5 Conclusion A decoupling control method for the MG and ME of the aircraft WRSSG based on the estimation of MG field current is presented in this study. In the proposed decoupling method, the MG field current is estimated first, and used in the closed-loop excitation control for the ME to make the MG field current meet the MG demand. With the desired field current, the MG is decoupled with the ME and is controlled as a traditional synchronous motor to start the aircraft engine under MTPA. The proposed decoupling method solved the serious coupling problem between the MG and ME in the start control for the aircraft WRSSG system. Experimental results verified the proposed decoupling control method. 6 Acknowledgment This work was supported by the National Natural Science Foundation of China (51677152). Fig. 4 Locations of MTPA results against MG armature current versus current angle under fixed load torque ai gf =10A bi gf =20A speed. The load torques for the MG, denoted as T L, were set at 10, 20, and 30 Nm, respectively, by the load platform. In each set of experiments, the MG current angles were set at different values manually, and the corresponding armature currents of the MG were measured. The measured MG armature currents versus MG current angle plot under fixed load torque are shown in Fig. 4. Fig. 5 Measured armature current and the estimated field current of the MG during the start-up process 7 References [1] Friedrich G., Girardin A.: Integrated starter generator, IEEE Ind. Appl. Mag., 2009, 15, (4), pp. 26 34 [2] Bhangu B.S., Rajashekara K.: Electric starter generators: their integration into gas turbine engines, IEEE Ind. Appl. Mag., 2014, 20, (2), pp. 14 22 [3] Sarlioglu B., Morris C.T.: More electric aircraft: review, challenges, and opportunities for commercial transport aircraft, IEEE Trans. Transp. Electrification, 2015, 1, (1), pp. 54 64 [4] Griffo A., Wrobel R., Mellor P.H., ET AL.: Design and characterization of a three-phase brushless exciter for aircraft starter/generator, IEEE Trans. Ind. Appl., 2013, 49, (5), pp. 2106 2115 [5] Williams R.H., Foster M.P., Stone D.A., ET AL.: Utilizing existing aircraft wound field generators for starter-generators. 2011 IEEE 8th Int. Conf. on Power Electronics and ECCE Asia (ICPE & ECCE), 2011, pp. 691 696 [6] Ningfei J., Weiguo L., Jichang P., ET AL.: Design and control strategy of a two-phase brushless exciter for three-stage starter/generator. 2014 IEEE Energy Conversion Congress and Exposition (ECCE), 2014, pp. 5864 5869 [7] Jiao N., Liu W., Meng T., ET AL.: Design and control of a two-phase brushless exciter for aircraft wound-rotor synchronous starter/generator in the starting mode, IEEE Trans. Power Electron., 2016, 31, (6), pp. 4452 4461 [8] Jiao N., Liu W., Meng T., ET AL.: Detailed excitation control methods for two-phase brushless exciter of the wound-rotor synchronous starter/generator in the starting mode, IEEE Trans. Ind. Appl., 2017, 53, (1), pp. 115 123 [9] Ma P., Liu W.-G., Luo G.-Z., ET AL.: Starting control strategy for three-stage aviation brushless synchronous motor, Electr. Mach. Control, 2012, 16, (11), pp. 29 32 [10] Ma P., Liu W., Mao S., ET AL.: Torque ripple reduction in three-stage brushless synchronous motor based on α-β-γ filter. Int. Conf. on Electrical Machines and Systems, 2015, pp. 815 815 [11] Jatskevich J., Pekarek S.D., Davoudi A.: Parametric average-value model of synchronous machine-rectifier systems, IEEE Trans. Energy Convers., 2006, 21, (1), pp. 9 18

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