This paper presents a novel open-end winding induction generation system supplying CVVF AC power for MEA. As a subsystem of an AC/DC hybrid generation.
1
An Open-end Winding Induction Generation System for Frequency Insensitive AC Loads in More Electric Aircraft Y. Jia, Udupi R. Prasanna, K. Rajashekara Erik Jonsson School of Engineering & Computer Science University of Texas at Dallas Richardson, Texas, USA (e.g. wing de-icing system, galleys, etc.) still has potential for further optimization. Since the terminal voltage of the PM generator varies with the engine shaft speed, the constant voltage variable frequency (CVVF) AC power demanded by those loads is first converted to DC power by the active rectifier of the generator, and inverted back to AC power through a dedicated inverter. This two-stage AC-DC-AC conversion adds extra losses and additional hardware to the system.
Abstract - In more electric aircraft (MEA), supplying constant voltage variable frequency (CVVF) AC power to frequency insensitive loads (e.g. wing de-icing system, galleys, etc.) in DC primary distribution system requires an AC-DC-AC conversion which adds losses and additional hardware installment to the system. This paper presents a novel open-end winding induction generation system supplying CVVF AC power for MEA. As a subsystem of an AC/DC hybrid generation system, this CVVF induction generation system uses a series connected inverter-load topology without any energy storage device. A current oriented control scheme is used to regulate both AC and DC side voltages of the system without battery compensation. The proposed series connected induction generation system presents reduced hardware footprint compared to conventional shunt connected and battery compensated series connected induction generation system. The performance of the proposed system is demonstrated by simulation in MATLAB/SIMULINK.
To eliminate the AC-DC-AC conversion and to further improve the system reliability, an induction generator based AC/DC hybrid generation system is proposed to generate AC and DC power independently from the same generator. In this system, a dual stator winding induction machine with dissimilar number of poles, as proposed in [11-13], is used as the starter/generator. This machine, first proposed in [11], has a standard squirrel-cage rotor and a slotted stator with two independent windings to obtain different number of poles. By properly selecting the pole ratio between the two sets of windings, this dual stator winding induction machine behaves as two independent induction machines coupled mechanically on the same shaft. The CVVF AC power can be generated from one set of the stator winding with a series connected voltage-ampere reactive (VAR) compensator, while the constant DC voltage power can be generated from the other set of stator winding using an active rectifier. The total volt-ampere inverter rating of the machine drive is similar to that of single stator machine, yet it enables hybrid AC/DC generation from a single generator and offers better system reliability. In case of a converter failure in the DC generation subsystem during flight mission, power supply for the critical loads on DC bus can be secured by using the inverter/rectifier unit in AC generation subsystem as a back-up. Correspondingly, the voltage-ampere (VA) rating of the inverter/rectifier unit in AC generation subsystem should be above the VA rating of the DC generation converter. Since the two sets of machine winding are electrically and magnetically decoupled, the power circuit topology and control scheme of the two generation subsystem can be designed separately.
Keywords— open-end winding induction generator, frequency insensitive loads, hybrid AC/DC generation, more electric aircraft
I.
INTRODUCTION
The emerging trend towards more electric architecture for both civil and military airplanes is intended to replace mechanical and pneumatic systems with electrical systems as much as possible. It is generally believed that the more electric aircraft (MEA) would lead to the potential for lower fuel consumption and emissions, reduced maintenance, and possibly lower costs [1-3]. However, the electrical power system of MEA is required to have both AC and DC electrical power with various voltage levels and high fault tolerant capability. It is very complex for either AC or DC primary generation system to meet all the power requirements with optimized performance in terms of efficiency, reliability and costs. In [4-10], the high voltage (+/-270 V) DC primary generation system with multi-phase permanent magnet (PM) generators is presented as a favorable candidate for the new electrical system architecture. This type of systems presents high power factor and high efficiency, but the design of generation system for the frequency insensitive AC loads
k,(((
2 In this paper, an open-end winding induction generation system without any energy storage device is proposed to supply CVVF AC power for MEA. The set of machine winding responsible for AC power generation will be modeled as an independent open-end winding induction generator. Compared to conventional shunt connected induction generation systems [14-16], the proposed system eliminates the current filter at converter output terminals by using an inverter-load topology [17]. Compared to battery compensated series connected induction generation system in [17], the proposed system is able to regulate both AC and DC side voltages of the system without the need for battery on DC side. Therefore, the proposed open-end winding induction generation system for MEA presents a reduced hardware footprint.
DC side is used to compensate fluctuation in mechanical input power from the alternate energy source. Unlike this battery compensated generation system, the mechanical input power of the CVVF AC generation system for MEA is extracted from the main engine shaft of the aircraft. Besides the high inertia of the jet engine shaft, the power extracted for electric power generation is only a small portion compared to the propulsion power demand of the aircraft. Hence, for the CVVF AC generation system in MEA, the input power variation is relatively rare and moderate, while the load variation from galley inserts and wind de-icing system is much more frequent and significant. Consequently, instead of a battery, a DC capacitor is used in the proposed system to further reduce the hardware footprint, and both AC voltage and DC side voltage need to be regulated under severe load variation.
The configuration of the proposed series connected induction generation system for MEA is discussed in Section II. The operation principle of the proposed system is explained based on p-q instantaneous power theory in Section III. A current oriented control method originally used for PM generator [18] is introduced to regulate both DC and AC side voltage of the proposed induction generation system in Section IV. In Section V, simulation results obtained from MATLAB/SIMULINK are presented to demonstrate the effectiveness and performance of the proposed system.
Fig. 2 Inverter-load Topology in the Battery Compensated Series Connected Induction Generation System [17]
II. INDUCTION GENERATION SYSTEM FOR FREQUENCY INSENSITIVE LOADS
The proposed open-end winding induction generator system to regulate both AC voltage and DC side voltage in the series connected CVVF AC generation system, without any energy storage device, is shown in Figure 3. The threephase inverter and the frequency insensitive AC loads are connected to each end of the open-end winding induction generator terminals and the DC side of the inverter/rectifier unit is connected to a capacitor.
The circuit diagram of a conventional shunt connected induction generation system [14-16] is shown in Figure 1. The current filter at converter output terminals in this configuration requires additional hardware footprint compared to the series connected inverter-load topology [17].
Open-End Winding Induction Generator
ea
Ls
Rs
ia
uab
eb
Ls
Rs
ib
vab
ubc
ec
Ls
Rs
ic
vbc
AC Loads
C
Inverter/Rectifier Unit
AC Voltage Feedback
Fig. 1 Circuit Diagram of Conventional Shunt Connected Induction Generation System [14-16]
vdc
Controller
DC Voltage Feedback
Fig. 3 System Configuration of the Proposed Open-end Winding Induction Generation System
The inverter-load topology in [17] for a battery compensated generation system is shown in Figure 2. This generation system is used for alternate power generation (e.g. wind energy systems) in isolated areas. The battery at
In most of the MEA applications, besides generating electric power, the main engine generator is also used as a starter for starting the aircraft engine [1-7]. A DC ground power supply is usually available for this process. While
3 starting, the AC loads in the generation system are disconnected, and the AC load terminals are shorted to transform the open-end induction generator into a wyeconnected induction motor. Once the engine shaft reaches its idle speed, induction machine will be connected to form the configuration as shown in Fig. 2, and the DC capacitor will be fully charged. The frequency insensitive loads in MEA such as wing de-icing system and galleys are generally resistive heaters symmetrically distributed at the generator terminals. As a result, they can be modeled as a three phase balanced resistor in series with the stator winding resistance. Two voltage sensors are needed at the load terminals of the generator to provide feedback of the load voltage magnitude. An additional voltage sensor is required to measure the DC link voltage. Neither rotational speed of rotor nor rotor position feedback is essential to control the proposed generation system.
and the variables and parameters are defined in a similar manner as in the stator circuit. As a dual to the voltage-oriented reference frame [15-16], a reference frame in quadrature with stator current vector [18] is utilized to decouple the active and reactive voltage components from the output voltages of series connected converter. Both AC and DC side voltages of the generation system can be regulated independently by controlling the instantaneous active and reactive power output of the series connected converter. In the current oriented reference frame, the direct and quadrature components of the stator currents in the series connected induction generation system are:
Since the frequency insensitive AC loads are modeled as three phase balanced wye-connected resistor in series with stator winding resistance, the open-end winding induction generator in the proposed system behaves as a wyeconnected induction generator with an increased stator resistance. In the synchronously rotating reference frame, assuming balanced impedance in both stator and rotor circuit and neglecting the saturation effects, the voltage equation for three-phase squirrel-cage open-end winding induction generator with series connected resistive load can be expressed as [19]: ݒ௦ ൌ ܴ௦ ݅௦ ߱ ߣௗ௦ ݒௗ௦ ൌ ܴ௦ ݅ௗ௦ െ ߱ ߣ௦
ௗఒೞ
Ͳ ൌ ܴ ݅ ሺ߱ െ ߱ ሻߣௗ Ͳ ൌ ܴ ݅ௗ െ ሺ߱ െ ߱ ሻߣ
ௗఒೝ ௗ௧ ௗఒೝ ௗ௧
(3) (4)
where ߣ௦ ൌ ܮ௦ ݅௦ ܮ ሺ݅௦ ݅ ሻ
(5)
ߣௗ௦ ൌ ܮ௦ ݅ௗ௦ ܮ ሺ݅ௗ௦ ݅ௗ ሻ
(6)
ߣ ൌ ܮ ݅ ܮ ሺ݅௦ ݅ ሻ
(7)
ߣௗ ൌ ܮ ݅ௗ ܮ ሺ݅ௗ௦ ݅ௗ ሻ
(8)
(10)
ܸ௦ ൌ ܴ௦ ܫ௦ ߱ ܮ ܫௗ
(11)
ܸௗ௦ ൌ െ߱ ሺܮ௦ ௦ ܮ ܫ ሻ
(12)
Ͳ ൌ ܴ ߱௦ ܮ ௗ
(13)
Ͳ ൌ ܴ ௗ െ ߱௦ ሺܮ ܮ ௦ ሻ
(14)
ܮ௦ ൌ ܮ௦ ܮ
(15)
ܮ ൌ ܮ ܮ
(16)
߱௦ ൌ ߱ െ ߱
(17)
Equations (11), (12), (13) and (14) describe the steadystate behavior of the open-end winding induction generator for series connected CVVF AC generation system, where ܸ௦ is the active voltage component which is in phase with the stator current vector, and ܸௗ௦ is the reactive voltage component that leads the stator current vector by 90 嘙 . Substituting equations (13), (14) into (11), (12), the active and reactive voltage components of the induction generator terminal voltages can be expressed as:
(2)
ௗ௧
݅ௗ௦ ൌ Ͳ
where
(1)
ௗ௧
(9)
Moreover, for steady-state analysis, the derivative terms in eq. (1) to eq. (4) are equal to zero. Hence, the voltage equation can be re-written as:
III. SYSTEM MODEL AND OPERATION PRINCIPLE
ௗఒೞ
݅௦ ൌ ȁ݅ȁ
ܸ௦ ൌ ሺܴ௦ ߱
ோೝ మ ାఠೞ మ ೝ మ
ܸௗ௦ ൌ െ߱ ሺܮ௦ െ
In the above equations, the s subscript denotes variables or parameters in the stator circuit.ݒ௦ , ݒௗ௦ , ݅௦ , ݅ௗ௦ , ߣ௦ , ߣௗ௦ are the q and d axis stator voltages, currents, flux linkages respectively. ܴ௦ is the total stator resistance, which include the stator winding resistance and AC load resistance. ܮ௦ stands for the stator leakage inductance whereas ܮ is the magnetizing inductance of the induction machine. The r subscript denotes variables or parameters in the rotor circuit,
ఠೞ ோೝ మ
ሻܫ௦
ೝ ఠೞ మ మ
ோೝ మ ାఠೞ మ ೝ మ
ሻܫ௦
(18) (19)
In order to maintain the capacitor voltage constant at a certain DC level, the ideal steady-state instantaneous active power output of the converter (neglecting converter losses) is required to be zero. Therefore, the theoretical steadystate converter output active and reactive voltage commands can be set as: ܸ௦ כൌ Ͳ
(20)
4 ܸௗ௦ כൌ ȁܸȁ
and lower rotor induced voltage. Operating at ௦ଶ results in a lower slip frequency, which requires excessively high voltages at converter terminals, but may reduce the rotor losses and enhance the system efficiency in other applications.
(21)
By applying constraint in eq. (20) in steady-state expression for active voltage component given in eq. (18), the range of operation for the proposed induction generation system can be found by solving the following equation: ൫ܴ௦ ܮ ଶ ܴ ܮ ଶ ൯߱௦ ଶ ߱ ܴ ܮ ଶ ߱௦ ܴ௦ ܴ ଶ ൌ Ͳ (22) The boundaries for mechanical shaft speed and load variations can be obtained by the discriminant of formula (22). When the discriminant is larger than zero, two real solutions of slip frequency can be found. One of the solutions for slip frequency can potentially reduce the rotor losses and enhance the system efficiency; however, it requires excessively high voltages from the converter. This solution may be useful for applications in which generator efficiency is the utmost concern and high converter voltage is tolerable. For MEA applications, since the DC voltage in the aircraft is strictly constrained to avoid the corona discharge [1-2], such an operating point would not be practical. To illustrate the dual slip frequency solutions, a phasor diagram is carried out in rotor flux reference frame, where the voltages and currents in (1) and (2) can be interpreted into vectors as: ࢜௦ ൌ ௦ ሾܴ௦ ݆ܺ௦ ᇱ ሿ ࢋ
(23)
࢜௦ ൌ หݒ௦ ห െ ݆ȁݒௗ௦ ȁ
(24)
௦ ൌ ห݅௦ ห െ ݆ȁ݅ௗ௦ ȁ
(25)
Fig. 4 Phasor Diagram of Operation Principle for the Proposed Open-end Winding Induction Generation System
IV. CONTROL SCHEME The closed-loop control scheme for the proposed openend winding induction generation system is shown in Figure 5.
where
ࢋ ൌ െ݆ȁ݅ௗ௦ ȁ ܺ௦ ᇱ ൌ ሺܮ௦ െ
మ ೝ
మ ೝ
ሻ߱
(26) (27)
ܺ ൌ ܮ ߱
(28)
ܺ ൌ ܮ ߱
(29)
According to equation (23), the generator terminal voltage phasor ࢜௦ is the sum of the voltage drop of stator resistance/reactance and rotor induced voltage. This terminal behavior is illustrated in Figure 4. In Figure 4, the stator current phasors ௦ଵ and ௦ଶ are two theoretically possible operating points of the induction generator for a given load and speed condition. The two current vectors have same magnitude but different phase angles. Since the steady-state instantaneous active power output is nearly zero, the terminal voltage phasors ࢜௦ଵ and ࢜௦ଶ are 90 ahead of the stator current phasors ௦ଵ and ௦ଶ respectively. Compared to the operating point at current phasor ௦ଶ , operating at ௦ଵ results in a larger slip frequency, which leads to a smaller magnitude of rotor flux linkage
Fig. 5 Closed-loop Control Scheme for the Proposed Open-end Winding Induction Generation System
In order to independently control the AC load voltage and DC capacitor voltage of the converter, the proposed system is operated under stator current oriented synchronous reference frame. This current oriented voltage control method is originally used for PM generator in [18]. In the current oriented control for induction generator, the d
5 axis voltage (reactive voltage component) regulates the instantaneous reactive power output of the converter which is supplying the necessary excitation for the induction generator, while the q axis voltage (active voltage component) controls the instantaneous active power output of the converter that provides real power to the DC link and AC induction generator/load.
balanced resistive load. The mechanical rotational speed of the generator shaft is 2000 rpm. The preliminary simulation results are shown as below.
A PI controller is used to regulate the DC side voltage of the converter vdc. By controlling the converter instantaneous active power output to charge and discharge the DC capacitor, the DC link voltage can be maintained at the reference vdc*. The output of this controller is the q axis voltage (active voltage) command vqs*. Since neither any energy storage device nor DC load is connected at the DC side of the converter, vqs* is around zero during steady-state. Assuming resistive load condition, the AC load voltage magnitude vac is proportional to the stator current magnitude ȁܫȁ. According to equation (9), (10) and (18), for a given steady-state operating point, the current magnitude (ȁܫȁ ൌ ܫ௦ ) can be determined solely by the d axis voltage (reactive voltage) command. Therefore, another PI controller is used to regulate vac by controlling the reactive voltage component of the converter output vds*.
DC Link Voltage (V)
510
505
500
495 0.3
0.35
0.4
0.45
0.5 Time (s)
0.55
0.6
0.65
0.7
Fig. 7 The DC Link Voltage Regulation Characteristics of the Proposed Open-end Winding Induction Generation System 290
AC Load Voltage Magnitude (V, RMS)
280 270 260 250 240 230 220 210 200 190 0.3
0.35
0.4
0.45
0.5 Time (s)
0.55
0.6
0.65
0.7
Fig. 8 The AC Load Voltage Regulation Characteristics of the Proposed Open-end Winding Induction Generation System Fig. 6 Basic Configuration of Conventional Phase Locked Loop [20]
80
In the current oriented control, a conventional phase locked loop (PLL) is used to obtain the stator current vector phase angle ߠ . The basic configuration of the conventional PLL is shown in Figure 6 [20]. In order to make the current oriented reference frame in quadrature with stator current vector, the direct current component ݅ௗ is commanded to be zero. The current vector phase angle ߠ is obtained by integrating the frequency command ߱ כgenerated by the ݅ௗ controller. As shown in Figure 5, the output of this PLL is used in the inverse Park's transformation to provide the three phase reference voltage signals to space vector PWM modulator.
Phase A Current (ia) Phase B Current (ib) Phase C Current (ic)
Three Phase Output Current (A)
60 40 20 0 -20 -40 -60 -80 0.3
0.35
0.4
0.45
0.5 Time (s)
0.55
0.6
0.65
0.7
Fig. 9 The three Phase Output Current Characteristics of the Proposed Open-end Winding Induction Generation System
V. SIMULATION RESULTS A closed-loop simulation for the proposed open-end winding induction generation system is simulated in MATLAB/SIMULINK. In the simulation, a 50 HP, 1800 rpm open-end winding induction generator [21] is controlled to supply a 30 kW three phase 230 VAC
6 indicating the effectiveness of the PLL, whereas the steadystate q axis voltage is kept to be roughly zero demonstrating that no instantaneous active power is transferred to the DC side of the series connected converter.
-30 -40
Electro-magnetic Torque (Nm)
-50 -60
VI. CONCLUSION
-70 -80
In this paper, an open-end winding induction generation system for obtaining CVVF AC power for MEA loads is presented. The series connected induction generation system can be recognized as a part of an AC/DC hybrid generation system, in which the CVVF AC power is generated separately from constant voltage DC power. Both the AC and DC side voltages of the system can be regulated without battery compensation. Compared to conventional shunt connected and battery compensated series connected induction generation system, the proposed generation system presents a reduced hardware footprint. The proposed system has good steady-state and dynamic performance and has been verified through simulation results.
-90 -100 -110 -120 -130 0.3
0.35
0.4
0.45
0.5 Time (s)
0.55
0.6
0.65
0.7
Fig. 10 The Electro-magnetic Torque Characteristics of the Proposed Open-end Winding Induction Generation System
In the simulation, the load is changed from 30 kW to 15 kW at 0.4 s. Variation in DC and AC side voltages of the system are shown in Figure 7 and Figure 8 respectively, and characteristics of generator stator currents and electromagnetic torque are illustrated separately in Figure 9 and Figure 10. The dynamic performance for both DC capacitor voltage and AC load voltage regulation is satisfactory.
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50 0
[2] Lester Faleiro, "Summary of the European Power Optimized Aircraft (POA) Project," in 25th International Congress of the Aeronautical Sciences, ICAS 2006.
dq-axis Voltage (V)
-50 -100 d-axis Voltage (vd) q-axis Voltage (vq)
-150
[3] B. S. Bhangu, K. Rajashekara, "Electric Starter Generators: Their Integration into Gas Turbine Engines," Industry Applications Magazine, IEEE, vol.20, no.2, pp.14-22, MarchApril 2014.
-200 -250 -300 -350 -400 0.3
0.35
0.4
0.45
0.5 Time (s)
0.55
0.6
0.65
[4] K. Muehlbauer and D. Gerling, "Two-generator-concepts for electric power generation in More Electric Aircraft Engine," 2010 XIX International Conference on Electrical Machines (ICEM), pp. 1-5.
0.7
Fig. 11 The dq-axis Voltage Characteristics of the Proposed Open-end Winding Induction Generation System
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70 d-axis Curent (id) q-axis Current (iq)
60
[6] W. U. N. Fernando, M. Barnes, O. Marjanovic, "Direct drive permanent magnet generator fed AC-DC active rectification and control for more-electric aircraft engines," Electric Power Applications, IET , vol.5, no.1, pp.14,27, January 2011.
dq-axis Current (A)
50 40 30 20
[7] A. Martin, "A review of active rectification in aircraft ac systems," in Proc. of the More Open Electrical Technologies of the More Electric Aircraft Forum, Barcelona, September 2009.
10 0 -10 0.3
0.35
0.4
0.45
0.5 Time (s)
0.55
0.6
0.65
[8] M. Maldonado, "Management and Distribution System for a More Electric Aircraft," IEEE AES Systems Magazine, December 1999.
0.7
Fig. 12 The dq-axis Current Characteristics of the Proposed Open-end Winding Induction Generation System
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The d and q axis currents and voltages are shown in Figure 11 to Figure 12. As it is mentioned in Section IV, the steady-state d axis current is maintained as zero
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