Modeling and Characterization of an Aircraft Electric Power System ...

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Abstract- A hybrid power source consisting from a fuel cell and a battery is proposed as an auxiliary power unit (APU) for the more electric aircraft (MEA) ...
Modeling and Characterization of an Aircraft Electric Power System with a Fuel Cell-Equipped APU Connected at HVDC Bus A. Eid

H. El-Kishky

M. Abdel-Salam

T. El-Mohandes

Electrical Engineering South Valley University Aswan 81542, Egypt [email protected]

Electrical Engineering University of Texas at Tyler Texas 75799, USA [email protected]

Electrical Engineering Assiut University Assiut 71518, Egypt [email protected]

Electrical Engineering South Valley University Aswan 81542, Egypt [email protected]

Abstract- A hybrid power source consisting from a fuel cell and a battery is proposed as an auxiliary power unit (APU) for the more electric aircraft (MEA) applications. The output of the fuel cell is controlled using a DC/DC boost converter to provide the voltage of the aircraft 270-VDC bus. The fuel cell and the battery are controlled by using DC/DC converters to provide 270-VDC at their outputs. The output of both fuel cell and battery are connected in parallel at the 270-VDC bus of the aircraft system. Connection at the 270-VDC bus provides several merits for the MEA system; it is simple, feeding DC loads and AC loads through the generator channel 12-pulse inverter, light weight and more economical. The fuel cell/battery hybrid system rating is chosen to be 60-kW to provide power for the aircraft loads in case of the main generator failure or maintenance. To comply with the aircraft standard of MIL-STD-704F and the IEEE Std. 519-1992, an active power filter (APF) is connected at the synchronous generator terminals to reduce or eliminate the harmonics generated from the power converters. The studied aircraft electric power system with the proposed fuel cell/battery hybrid system is simulated without and with the presence of the APF and it is found that the APF reduced the voltage and frequency transients of the system and improves the aircraft electric system performance. Keywords-MEA; fuel cell; battery; auxilliary power unit; DC/DC converter; three-phase inverter

I. INTRODUCTION Electric generators, either driven by an aircraft’s main propulsion engines or by a gas turbine Auxiliary Power Unit (APU), supply the electric power needs of the More Electric Aircraft (MEA). In flight, the marginal efficiency of electric power generated by the main engines and their generators is at most 30-40%, whereas on the ground with the engines shut down, the average fuel efficiency of the turbine-powered APU is typically less than 20% and also has undesirable noise and gaseous emissions [1]. To ease environmental concerns, there is a strong interest in developing fuel cell hybrid systems for aerospace applications that will provide a reduction in fuel consumption while simultaneously reducing emissions [2], [3]. Due to the growing demands for electrical capacity on future aircrafts, generators would have to increase in size, which is

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limited in today’s modern engines by the capability of the core and available space. Hence, the engines have to be redesigned to accommodate the large increase in electrical power needs for the MEA. An improvement would be to replace these heavy generators with one or more highly efficient and clean, continuously operating fuel cell APU. This could meet the electric demands of the aircraft on the ground and in flight. Recently, the use of these hybrid systems is being investigated for transportation applications for on-board power generation particularly for aerospace applications [3], [4]. Hybrid systems composed of fuel cells and batteries combine the high energy density of fuel cells with the high power density of batteries in order to meet both of these requirements [5]. Batteries such as lead-acid or Li-ion are options for fuelcell-based energy storage systems because of their fast response time to load changes. Lead-acid batteries are inexpensive and widely available [6]. However, lead-acid batteries suffer from low energy density, sulfating on stand, and relatively heavy weight. Li-ion batteries are more suitable choices for fuel-cell based hybrid systems, due to their higher energy density, high energy efficiency, light weight, and good cycle life. In addition, because of their high power-to-weight ratio, Li-ion batteries have faster response compared to other batteries. In this paper, the proposed auxiliary power unit consists of a fuel cell stack and a battery. The power rating of the APU is 60-kW compared to the synchronous generator power unit of 90-kW in the advanced aircraft system [7]. The run-time of the proposed APU is chosen to supply the most important loads in the aircraft in case of the generator failure or maintenance, and the battery can serve at the other loads. The main APU components (fuel cell stack and battery) are connected in parallel to the generator. The need for the battery here is to supply the load power in case of fast transients or changing load in a reasonable period of time. The battery response is very fast compared to the fuel cell response. The APU is assumed to carry the load in case of the synchronous generator is disconnected for any reason like maintenance or engine failure. The MEA electric power system is simulated under

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transient conditions equipped with a proposed APU. Both of the APU elements are controlled using different DC/DC power converters placed at their outputs. The transient switching times are 1.5 s when the APU is connected to the synchronous generator channel to share the load and 3 s when the generator is fully disconnected and the APU takes the full load. A controlled APF [7] is connected at the generator terminals so that the aircraft power system complies with the standards regarding the THD limit values. II. AIRCRAFT APU CONNECTION CONFIGURATION The studied aircraft electric power system with a fuel cell/battery-equipped APU paralleled at the 270-VDC bus is shown in Fig. 1. The system consists of two channels feeding the AC load; the synchronous generator and the APU units. The synchronous generator channel consists of a 90-kVA three-phase synchronous generator operating at variable speed with an output voltage frequency of 400-800-Hz at 200-V rms [7]. The output of the synchronous generator is rectified by using a 12-pulse passive converter to provide 270-VDC at the main DC bus of the aircraft. The AC load of 200-VAC at 400Hz is supplied from a 12-pulse sinusoidal PWM inverter. The aircraft electric power system operates under variablespeed constant-frequency (VSCF) operating conditions [7]. The fuel cell/battery-based APU channel is connected to the generator channel at the 270-VDC bus. The output of the APU is controlled to provide a 270-VDC to provide power to the DC loads in case of the generator is maintained or disconnected. The modeled synchronous generator is rated at 90-kW. The prime-mover is modeled and simulated by a DC motor. The speed of the DC motor is controlled by a proportional-integral (PI) controller. The voltage output of the synchronous generator is a regulated 115/200-V rms obtained by controlling the field voltage.

The switches SWg and SWa are used to connect/disconnect the generator and the APU unit to the load, respectively. At any time, the total DC current of the 270-VDC bus (ID) is equal to the sum of the generator-side DC current (Idg) and the APUside DC current (Iau ):

I D = I dg + I au

(1)

The APU-side DC current is equal to the sum of the output DC current from the fuel cell stack converter (Ifc) and the output DC current from the battery converter (Ib):

I au = I fc + I b

(2)

The fuel cell stack model of [8] is used here and shown in Fig. 2. The fuel cell stack is followed by a boost DC/DC converter to provide a controlled voltage of 270-V at its output. With the fuel cell stack current Ifi the stack power can be calculated as:

Pfc = Vtf × I fi

(3)

The battery model of [9] is considered as shown in Fig. 2. The output power of the battery is calculated as a function of its terminal voltage Vtb and current Ibi as

Pb = Vtb × I bi

(4)

The battery is used to provide the required load power during the switching because of the slow response of the fuel cell. Under steady-state operating conditions, the fuel cell provides all required load power in case the generator is shut down. In transient load conditions, the battery feeds the load until the fuel cell warms up and takes the total load and hence charges the battery [10].

Fig. 2. APU circuit model with its DC/DC converters. Fig. 1. APU connection at the 270-VDC bus for the aircraft electric system.

The diode converter circuit consists of two bridges connected in parallel forming a 12-pulse converter configuration to smooth out the output generator voltage, Vdg. The APU output voltage, Vau, is controlled to be 270-VDC.

In this case, the DC/DC converter of the battery must be bidirectional. During transients operation, the battery provides the power to the load and the DC/DC converter will be in boost operation mode where switch Sbd will be active while switch Sbc will be disabled. While in normal operating conditions, the

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PLr = Pfc + Pbr

240

(4)

The battery reference power (Pbr) is compared to the battery output power (Pb) and the difference is manipulated by a limited-output PI controller which controls the bidirectional DC/DC converter of the battery. The different PI controller gains are listed in Table I. The duty cycle of the witches (Sbd and Sbc) are complementary, i.e.:

D bc =1 − Dbd

Load power controller

5x103

80x103

Fuel cell controller Battery controller

133x10

-7

104x10-6

125x10

-6

0.2

2

time (s)

4

230 220

6

0

2

0

2

time (s)

4

6

4

6

150

200

Ifc (A)

240

100

150 100

50

50 0

TABLE I. PI CONTROLLERS’ GAINS OF THE APU Integral gain

0

250

0 0

2

4 time (s)

where Dbc is the duty ratio for the switch Sbc, and Dbd is the duty cycle for the other switch Sbd.

Proportional gain

180

140

(5)

Controller

250

220

Vtf (V)

The control method proposed in this study is based on an operating principle that the battery provides the extra power which exceeds the fuel cell capacity. A complete model of the proposed control method is shown in Fig. 3. A control signal from the APU output DC voltage (Vau ) is taken and compared to the reference value (270-V) and the error signal is manipulated by a PI controller with a limiter to provide the reference load power (P Lr). Then PLr is compared to the fuel cell output power (Pfc) and the difference is processed by another a limited-output PI controller which controls the fuel cell DC/DC converter and provides the required duty ratio for the converter switch (Sfc). At any time, the load power is equal to the fuel cell power and the power of the battery, i.e.:

Vtb (V)

III. THE PROPOSED CONTROL METHOD

determined and illustrated. The fuel cell and battery terminal voltages Vtf, Vtb and their DC/DC converter output currents Ifc, Ib at 400-Hz operating frequency are shown in Fig. 4. Due to slow response of the fuel cell, its output current is zero until 1.75s from the starting of the simulation. The fuel cell steadystate terminal voltage is around 160-V, which is equal to designed value at full load. The battery response is significantly fast compared to the fuel cell. Due to the voltage drop in the battery circuit, its terminal voltage decreases with time. When the fuel cell starts to share the load with the battery at about 1.75s, the battery current starts to decrease and its terminal voltage as a result starts to increase. The same response happens at the switching time at 3s when the fuel cell/battery-based APU takes the entire load and the synchronous generator is switched off.

Ib (A)

fuel cell will charge the battery in the buck-mode operation. In this case, the switch Sbc will be active and controlled while the switch Sbd will be disabled.

6

time (s)

Fig. 4. Fuel cell and battery terminal voltages Vtf, Vtb and their DC/DC converter output currents Ifc, Ib at 400-Hz operating frequency.

The aircraft electric power system is simulated at different frequencies and at each time the variation of the parameters are recorded. The peak-to-peak variation of the generator frequency (Fr), generator rms phase voltage (Vg), the voltage at the DC bus of the generator channel (Vdg), the voltage of the DC bus of the APU unit (Vau ) and the rms phase voltage at the AC load bus (VL) are listed in Table II. A maximum frequency variation of 0.9% was shown at 600-Hz operating frequency, while the lowest variation of 0.4% happens at 400-Hz.

Fig. 3. The proposed APU control method.

IV. SIMULATION OF THE AIRCRAFT SYSTEM WITH THE APU The aircraft electric power system with APU is modeled and characterized at different operating frequencies using PSIM8 [11] and the important characteristics of the system are

The variations in the rms phase voltage of the generator are almost equal due to both effective driving motor speed and generator field controls. The change in the voltage Vdg decreases with increasing the frequency from 16.35% at 400Hz to 12.6% at 800-Hz. The variations of the voltage Vau decrease also with increasing the frequency from 2.28% at 400Hz to 1.39% at 800-Hz. The peak-to-peak variations in Vau are lower than that of Vdg. The variation in VL increases with increasing the frequency from 3.0% at 400-Hz to 3.75% at 800Hz. Note that all system parameters’ peak-to-peak variations are within aircraft standard of MIL-STD-704F [12] and in general, the variation decreases with increasing the frequency of operation except for the synchronous generator frequency.

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TABLE III. PEAK- TO-PEAK PARAMETERS’ VARIATIONS WITH APU AND APF

TABLE II. PEAK- TO-PEAK PARAMETERS’ VARIATIONS WITH APU Maximum allowable variation [92] 50 100 130 130 100

Operating frequency

parameter variation

400-Hz

600-Hz

800-Hz

ΔFr (Hz) ΔVg (V) ΔVdg (V) ΔVau (V) ΔVL (V)

1.7 2.93 44.15 6.15 3.50

5.25 2.42 36.2 4.52 3.84

3.55 2.82 34.0 3.75 4.31

Parameter variation ΔFr (Hz) ΔVg (V) ΔVdg (V) ΔVau (V) ΔVL (V)

Operating frequency 400-Hz With APF Change

2.9 3.65 25.0 5.58 3.84

▲1.20 ▲0.72 ▼19.15 ▼0.57 ▲0.34

800-Hz With APF Change

2.0 6.56 29.0 5.9 3.6

▼1.55 ▲3.74 ▼5.0 ▲2.15 ▼0.71

VI. CONCLUSIONS V. SIMULATION OF THE A IRCRAFT SYSTEM WITH APU AND APF

500 400

Vg (V)

F r (Hz)

The active power filter (APF) is controlled using the perfect harmonic cancellation (PHC) method [7]. The aircraft electric power system is simulated at 400 to 800-Hz frequency operation and the main parameters are calculated and shown in this section. Some effective parameters are shown in Fig. 5. The switching times are 1.5 s and 3 s.

200 0

0

1

2

3 4 time (s)

5

6

200

VL (V)

Ig (A)

300

100 0

0

1

2

3 4 time (s)

5

6

100 0

200 150 100 50 0

0

1

2

3 4 time (s)

5

6

0

1

2

3 4 time (s)

5

6

REFERENCES [1]

300

200

Vdg (V)

IL (A)

300

200 150 100 50 0

The more electric aircraft electric power system is modeled and analyzed at variable speed operating condition. The proposed auxiliary power unit is connected at the 270-VDC bus. This connection provides many merits as economical, there is no need for conversion unit at the AC load bus, less switches and losses and hence better efficiency. Another important advantage of APU connection at the 270-VDC bus is that the transient peak-to-peak system parameters’ variations are all within standard limits whether the active power filter is connected or not. It is found that installing the APF at the generator terminals reduces the parameter variation at transient operating condition and keeps the total harmonic distortion of the synchronous generator voltage and current within standard limits of MIL-STD-704F and the IEEE Std. 519-1992.

0

1

2

3 4 time (s)

5

6

[2]

200 100 0

0

1

2

3 4 time (s)

5

6

[3]

Fig. 5. The generator frequency (Fr), phase voltage (Vg), current (Ig) and the AC load bus voltage (VL), current (IL) and the 270-VDC bus voltage (Vdg) when installing the APF at 400-Hz frequency operation.

[4]

[5]

To study the effect of installing the APF on the aircraft system parameters, the system is simulated at the extreme frequencies (400- and 800-Hz). The system parameters variations are compared to the case of the system without APF. The function of the APF is to keep the THD of the synchronous generator within standard limits of IEEE Std. 519 [13] and to enhance the aircraft power system performance during transient and normal operating conditions. The peak-to-peak transient parameter variation with the APF installed at the generator terminals are listed in Table III at 400-Hz and 800Hz. At 400-Hz frequency operation, the generator frequency Fr, the rms phase voltage of the synchronous generator Vg and the rms phase voltage of the AC load bus VL are increased at the transient switching times. While, the DC bus voltages Vdg and Vau are decreased as an effect of adding the APF circuit to the aircraft model. At 800-Hz frequency operation, the Fr, Vdg and VL parameters are decreased, while Vg and Vau parameters are increased. Noting Table III, it is clear that all peak-to-peak variation values are within standard limits [12].

[6]

[7]

[8]

[9]

[10]

[11] [12] [13]

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