Design and Implementation of a High Performance Aeronautical ...

Design and Implementation of a High Performance Aeronautical Active Power Filter Zhong Chen, Yingpeng Luo, Miao Chen, Lei Shi and Jianxia Li Nanjing University of Aeronautics and Astronautics, Jiangsu Province, P. R. China [email protected]

Abstract- With the progress of “More Electric Aircraft”, power capacity of aircraft electric power system is increased. It is not easy for the power-quality characteristics of the system to be in compliance with the harmonic standards by using traditional method to compensate the harmonics introduced by the nonlinear load. Active power filter (APF) gets increasing usage in 50Hz power system for its high performance compensation behavior. In this paper, APF technology is introduced to solve the harmonic problem of aircraft electric power system. Based on analysis and updating two core function modules of APF: current reference generation module and current tracking module, source current direct control strategy and cascaded inverter topology is applied in the proposed aeronautical active power filter (AAPF). Global framework, key technology, and operation principle of the proposed AAPF are presented. In order to verify the compensation performance of the proposed AAPF, a prototype with 1kVA power rating is built and test in the laboratory. Experimental results show that good compensation results are achieved in various kinds of nonlinear load condition.

I.

INTRODUCTION

With the progress of the “More Electric Aircraft” [1], more and more electrical equipments are used in aircrafts. Large number of harmonic currents produced by the non-linear load brings hidden troubles to the safe operation of the aircraft electric power system. Variable frequency AC power system is the trend of the future aircraft power system for its larger power capability and higher reliability. In Airbus A380, power capability is as high as 4 ×150kVA; in Boeing 787, power capability reaches as high as 4 ×250kVA. With the increasing of the reliability and power capability, the electric power system suffers from a higher and variable fundamental frequency. It is not easy to filter the frequencyvariable harmonics with traditional methods, especially with the LC tuned filter method. The active power filter (APF) catches increasing investigations and research works since its emergency in 1976 [2]. It behaves as a practical approach to improve power quality of 50/60 Hz power systems [3-5]. Introducing the APF technology into the aircraft power system to improve the quality and reliability of the aircraft power system becomes a hot spot [6]-[7]. In the classical control scheme, the shunt APF acts as a controlled harmonic current source, injecting current which is inverse equivalent to the load harmonic current [8]. Under the

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action of current controller, practical compensation currents will follow the current reference detected from the load currents. A Good compensation performance is achieved by using this widely used method. In the APF classical theory, compensation performance of APF mainly relies on two core function modules: the current reference generation module and the current tracking module. The current reference generation module usually consists of the harmonic detection algorithm and dc link voltage controller. A high performance harmonic detection algorithm suited for every condition is a hot issue in the field [9]-[10]. The current tracking module usually consists of power stage architecture of the APF, current controller and corresponding PWM modulation, which is another research hot spot [11]. In this paper, source current direct control strategy is used for the proposed aeronautical active power filter (AAPF) to avoid complicated harmonic detection. The shunt AAPF works in feedback control approach, which suits for the high frequency aircraft electric power system. A cascaded inverter is applied as the power stage of the AAPF. By using the carrier phase shift (CPS) control, the bandwidth of the AAPF is increased in a low switching frequency. In order to verify the feasibility of the proposed AAPF, a prototype with 1kVA compensation power rating is built and test in our laboratory. II.

TWO CORE FUNCTION MODULES

A. Current Reference Generation Module In the classical control, the current reference is usually the harmonic and reactive components of the load currents. But the approach, essentially based on feedforward open-loop compensation, is sensitive to the parameter mismatches and relies on the ability to accurately predict the voltage-source inverter (VSI) current control performance. In the aircraft electric power system, fundamental frequency is much higher than 50Hz power system; measure errors, AD conversion time, digital delay and other non-ideal factors will deteriorate the open-loop compensation effect to a worse degree. In the close loop control, detection and control target is the source current. As we known, feedback control has following merits: it could reduce the transfer function from disturbances to the output; it causes the transfer function from the reference input to the output to be insensitive to variations in the gains in the forward path. In the aircraft electric power system, which is harsher than 50Hz power system, close loop control is more suitable than feedforward compensation. Furthermore, the

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complicated harmonic detection algorithm is eliminated in the close loop control. 1. Operation Principle In this paper, source current direct control which is proposed in [12] by J. C. Wu is adopted. The basic system diagram of the close loop control scheme is given in Fig. 1. This simplified control operates as follows: APF’s dc-link voltage is sent to the voltage regulator, output of the regulator is sent to the multiplier as well as a synchronous sine wave which is detected from the phase voltage. Output of the multiplier is sent to the current regulator, being the source current reference. Output of the current regulator will be sent to the PWM modulation to generate the PWM waveforms.

1 (3) ΔPC (t ) = ΔPL (t ) − VS ΔI S (t ) 2 The average voltage of the dc capacitor at this condition can be represented by t 1 1 (4) C (Vdc + ΔVdc (t )) 2 − CVdc 2 = − ∫ ΔPC (t )dt 0 2 2 The magnitude of ΔVdc(t) is much smaller than that of Vdc. Using the Laplace transformation for (4) and simplifying it gives: ΔVdc ( s ) −1 = (5) ΔPC ( s ) CVdc s From the above analysis, the model for active power analysis can be represented as Fig. 2. Here G(s) is the transfer function of the voltage regulator, F(s) is the transfer function of current tracking module, Kf is the transfer function of voltage measurement.

Fig. 2. Model for active power analysis

B. Current Tracking Module

Fig. 1. Control diagram of source current direct control

2. Model for active power control In the control scheme, phase angle of the current reference comes from detected phase voltage. But the value of the current reference comes from the voltage regulator, not from the detection of the load current. Because variations of the dclink voltage reflect the active power variation of the system, the current reference of the close loop control could be obtained from the system’s active power model. For simplicity, the phase voltage is supposed to be sinusoidal. If the source current is a sinewave with an amplitude of IS, then the active power flow can be represented by 1 (1) VS I S = PL − PC 2 Where VS is the amplitude of the phase voltage, PL is the active power consumed by load, PC is the active power supplied from APF. Considering the load variation, (1) can be rewritten as 1 (2) VS ( I S + ΔI S (t )) = PL + ΔPL (t ) − PC (t ) − ΔPC (t ) 2 By subtracting (1) from (2), the variation term ΔPC(t) is represented as

1. Design Consideration A shunt APF acts as a controlled harmonic current source, injecting current which is inverse equivalent to the load harmonic. In the 400Hz aircraft electric power system, frequencies of 11th and 13th harmonics reach as high as 4.4 and 5.2 kHz. How to draw a high frequency harmonic current accurately is a key issue of developing AAPF. Cascaded connection of power converter is a practical approach to increase the bandwidth of the power equipment in a lower switching frequency. In this paper, cascaded inverter technology is introduced into the AAPF system to achieve good compensation performance. Two voltage-source inverters with cascaded connection are applied as the topology of AAPF’s power stage. Equivalent switching frequency reaches as high as four times of the switching frequency of the power devices. On the other hand, dc-link voltage could be decreased to a lower value in the cascaded topology. Power switches with lower voltage rating could be used in the topology, leading to a better performance. Meanwhile, it is quite easy to implement an AAPF system in a modularized approach, making the system more practical. 2. H Bridge Cascaded Inverter The 2-H bridge cascaded inverter is represented by Fig. 3. The upper cascade unit consists of the power Mosfets named Q1, Q2, Q3, Q4 and the dc capacitor C1, the lower cascade unit consists of the power Mosfets named Q5, Q6, Q7, Q8 and the dc capacitor C2. In this paper, CPS PWM modulation is applied in the cascaded topology. Four triangle waveforms with same

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vm

2

1

3

4

vc1, vc3 vc2, vc4

Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 uAB uBC

Fig. 3. 2-H bridge cascaded inverter

amplitude and frequency but with a different phase angle are adopted as the carrier waveforms of four bridge legs. Here, the bridge leg composed of Q1 and Q2 is defined as leg 1, carrier waveform of leg 1 is defined as vC1; similarly, carrier waveforms for Q3 and Q4, Q5 and Q6, Q7 and Q8 are defined as vC3, vC2 and vC4. The Operation principle of the cascaded inverter under CPS control is given in Fig. 4. vm is the modulation wave, dc link voltage of each cascaded unit vdc1=vdc2=Vdc. Here, Vdc is defined as the dc link voltage reference of every cascaded unites. Fig. 4 gives the key waveforms of cascaded inverter under CPS control. The output voltage of the 2-H cascaded inverter varies in different stages. Operation principle of the cascaded inverter in following four stages is depicted as follows: Stage 1 [0, t1]: In the upper cascaded unit, Q1 and Q4 conduct, uAB=Vdc; in the lower cascaded unit, Q5 and Q7 conduct, uBC=0; the output voltage of the cascade inverter is uAC= uAB+uBC = Vdc. Stage 2 [t1, t2]: In the upper cascaded unit, Q1 turns off, Q2 turns on, Q4 keeps conducting, uAB=0; in the lower cascaded unit, Q5 and Q7 keep conducting, uBC=0; output voltage of the cascade inverter is uAC=0. In the following five stages [t2, t7], the cascaded inverter repeats the above two stages, and the output voltage uAC varies between 0 and Vdc. Stage 3 [t7, t8]: In the upper cascaded unit, Q3 turns off, Q4 turns on, Q1 keeps conducting, uAB= Vdc; in the lower cascaded unit, Q5 and Q7 conduct, uBC= 0; the output voltage of the cascade inverter is uAC= uAB+uBC = Vdc. Stage 4 [t8, t9]: In the upper cascaded unit, Q1and Q4 keep conducting, uAB= Vdc; in the lower cascaded unit, Q7 turns off, Q8 turns on, Q5 keeps conducting uBC= Vdc; the output voltage of the cascade inverter is uAC= uAB+uBC =2Vdc. From above analysis, we can find that, in every switching period, the output voltage of the cascaded inverter varies four times. Meanwhile, the maximum value of the cascaded inverter’s output voltage is two times of the dc link voltage. III.

PROPOSED AERONAUTICAL ACTIVE POWER FILTER

Vdc

uAC t1

t2

t3 t4

t5 t6

t7

t8 t9

t10 t11

t12

t13

2Vdc t14

t15 t16

Fig. 4. Operation principle of cascaded inverter under CPS control

From the analysis provided in the previous section, the basic function of the two core function modules is presented. Configuration and operation principle of the proposed AAPF based on this two module will be given in this section. The complete active power filter configuration is illustrated by Fig. 5. In the power stage, AAPF consists of a 2-H bridge cascaded inverter and a boost inductor. Nonlinear loads are selected as the capacitive and inductive load. The control stage consists of the following three control loops: dc link voltage control loop, source current control loop and dc link voltage balance control loop. The operation principle of these three control loops will be given as follow. 1. Dc Link Voltage Control With the voltage control loop, the dc link voltage of AAPF follows its reference, to ensure the proper operation and good compensation performance. In the proposed AAPF’s voltage control loop, the dc link voltages of every cascaded inverter (vdc1 and vdc2) are detected and average value of them is obtained. After comparison with the voltage reference vref, the voltage error verro is sent to the voltage regulator. The output of the voltage regulator AS is sent to the multiplier together with the synchronous sinewave eS. The detailed voltage control scheme is given by Fig. 6. It should be noticed that, in this control scheme, feedback control target is the sum of all inverter’s dc link voltage, which is different with the traditional source current direct control. Furthermore, this voltage control scheme could be expanded to N-H bridge cascaded inverter topology. So this control scheme actually control the maximum value of the cascaded converter VCmax=N ×Vdc. Here, N corresponds to the number of cascaded converter units. One obvious advantage of this control scheme is that final compensation performance would not get worse when one or more cascaded units stop working. Remaining cascaded units would share the dc link voltage of the fault one. This voltage

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Fig. 5. System diagram of aircraft active power filter

+

_+ Fig. 6. Operation principle of dc link voltage control

control scheme can increase the fault toleration and reliability of the AAPF system. 2. Source Current Control In the source current control loop, the current error iSerro is amplified by the current controller and sent to the PWM modulation. It is the same with the traditional current control. 3. Dc Link Voltage Balance Control In order to make the cascaded inverter operate properly, dc link voltage of every cascaded unit needs to keep balanced. Unbalanced dc link voltage will make some units suffer from higher voltage stress, will decrease the reliability of AAPF system, or even will cause power switches damaged. The detailed voltage balance control scheme is given by Fig. 7. An offset voltage ε is introduced to the PWM modulation to make every cascaded unit’s dc link voltage balanced. As Fig. 7(a) illustrates, differential values of the two

Voltage Balance Controller

+

'

_+

'

_+

'

++

'

(a) Operation principle of voltage balance control

(b) Regulation procedure of voltage balance control Fig. 7. Operation principle and regulation procedure of voltage balance control loop

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cascaded units’ dc link voltage is sent to the voltage balance controller, and the voltage balance controller outputs an offset voltage ε. New carrier wave is the sum of triangle wave and ε. Take bridge leg 1 for example to show the regulation procedure of voltage balance control (as shown in Fig. 7(b)). In the steady-state, carrier wave of bridge 1 is vc1, and the conduct times of Q1 and Q2 are ta1 and ta2 respectively. When the situation vdc1 > vdc2 happens, a positive offset voltage ε is obtained under the regulator’s action. Fig. 7(a) shows that the final carrier wave is the sum of vc1 and ε, which becomes vc1’ after regulation. So the conduction time of Q1 and Q2 turn to be ta1’ and ta2’. As Fig. 7(b) illustrates, we could find that ta1’ < ta1, ta2’ > ta2, which means that Q1 conducts less but Q2 conducts more. At the same time, for the leg 3, Q4 conducts less but Q3 conducts more. In upper cascaded unit, dc capacitor charges when Q1 and Q4 conduct; the dc capacitor discharges when Q2 and Q3 conduct. Unbalanced changing and discharging time will make the dc link voltage get decreased. Similarly, dc link voltage of the lower cascaded unit will get increased. The voltage balance is therefore achieved. IV.

TABLE I SPECIFICATIONS OF THE AERONAUTICAL ACTIVE POWER FILTER PROTOTYPE AC supply voltage APF Boost inductor Rectifier AC side inductor APF dc link voltage APF dc link capacitor

115V (rms) / 400Hz

Capacitive load power

400μH 100μH 110V 1000μF FQ24N60 24A, 600V 2kVA

Inductive load power

3kVA

Power switch

From up to down, waveforms of the load current iL, the compensation current iC, the source current iS and the phase voltage vS are given. As Fig. 9 shows, in the inductive steady-state, the waveform of the compensation voltage vC, which is a fivelevel voltage, is given as well. The load current iL is a square

PROTOTYPE AND EXPERIMENTAL RESULTS

A single-phase AAPF with the compensation power rating of 1kVA is built and test in the laboratory. The specifications of the prototype (as Fig. 8 shown) are given in Tab.1. The dc link voltage is selected as 110V, and the switching frequency of the power devices is about 24 kHz. The experimental setup of the prototype consists of four circuit boards and a boost inductor. All control functions and protection are embedded in the control board via analog control chips. A drive board is in charge of the gate drive of the power devices and isolation. Phase shift triangle waves and synchronous sine waves are generated by the signal board. Fig. 9 and 10 illustrate the experimental waveforms of the AAPF prototype under steady and transient load condition.

(a) Full load conditions

Fig. 8. Prototype of proposed AAPF

(b) 1/3 load conditions

Fig. 9. Experimental waveforms under inductive steady load condition

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(a) From empty load to full load

(b) From full load to empty load

Fig.10. Experimental waveforms under capacitive transient load condition [3]

wave with 50% duty cycle, the compensation current iC is inverse equivalent to the harmonic components of iL, and the source current is near a sinusoidal wave which keeps same phase with the phase voltage vS. Compared with the 1/3 load, 2vdc appears in compensation voltage vC more frequently in full load. As Fig. 10 shows, in the transient capacitive load condition, iL is a pulsing wave, which contains mass of harmonics. After the load changes, compensation current tracks the variation of load current in less than one period of the power system. The source current becomes near sinusoidal after compensation. THD of the source current is decreased from 167% to 6%. V.

CONCLUSION

With the increase of power capacity of the aircraft electric power system, it is very important to keep it in a good quality. APF is an advanced approach to compensate nonlinear load. In this paper, source current direct control and cascaded inverter are applied in the proposed aeronautical APF. Good compensation performance from the experimental results shows that the proposed AAPF is a practical method for the aircraft electric power system. ACKNOWLEDGMENT This work was supported by the National Nature Science of China under Award 51007037, Research Fund for the Doctoral Program of Higher Education of China under Award 200802871033, Aeronautical Science Foundation of China under Award 2009ZC52030 and the NUAA Research Funding under Award NS2010062 and NJ2010015.

Varschavsky, A. Dixon, J. Rotella, M. Moran, L., “Cascaded NineLevel Inverter for Hybrid-Series Active Power Filter, Using Industrial Controller, ” IEEE Trans. Ind. Electron., vol. 57, no. 8, pp.2761 - 2767, August 2010. [4] Hamadi, A. Rahmani, S. Al-Haddad, K., “A Hybrid Passive Filter Configuration for VAR Control and Harmonic Compensation, ” IEEE Trans. Ind. Electron., vol. 57, no. 7, pp.2419 - 2434, July 2010 [5] A. Luo, C. Tang, Z. K. Shuai, W. Zhao, F. Rong, K. Zhou, “A Novel Three-Phase Hybrid Active Power Filter With a Series Resonance Circuit Tuned at the Fundamental Frequency, ” IEEE Trans. Ind. Electron., vol. 56, no. 7, pp. 2431-2440, July 2009. [6] A. Eid, H. El-Kishky, M. Abdel-Salam, and T. El-Mohandes, “On power quality of variable-speed constant-frequency aircraft electric power system” IEEE Transactions on Power Delivery, vol. 25, pp. 5567, January 2010. [7] A. Eid, M. Abdel-Salam, H. El-Kishky, and T. El-Mohandes, “Active power filters for harmonic cancellation in conventional and advanced aircraft electric power systems,” Elect. Power Syst. Res., vol. 79, no. 1, pp. 80–88, Jan. 2009. [8] H. Akagi, E. H. Watanabe, and M. Aredes, Instantaneous Power Theory and Applications to Power Conditioning. Piscataway, NJ: IEEE Press, 2007. [9] H. Akagi, Y. Kannazawa, and A. Nabae, “Instantaneous reactive power compensators comprising switching devices without energy storage components” IEEE Trans. Ind. Appl., vol. IA-20, no. 3, pp. 625-630, May 1984. [10] S. Bhattacharya and D. M. Divan, “Hybrid series active/parallel passive power line conditioner with controlled harmonic injection,” U.S. Patent 5 465 203, Nov. 1995. [11] S. Buso, L. Malesani, and P. Mattavelli, “Comparison of current control techniques for active filter applications” IEEE Trans. Ind. Electron., vol. 45, no. 5, pp. 722-729, October 1998. [12] J. C. Wu and H. L. Jou, “Simplified control method for the single-phase active power filter,” IEE Proceedings-Electric Power Applications, vol. 143, no. 3, pp. 219-224, May 1996.

REFERENCES [1] [2]

J. A. Rosero, J. A. Ortega, E. Aldabas, and L. Romeral, “Moving towards a more electric aircraft,” IEEE Aerospace and Electronic Systems Magazine, vol. 22, pp. 3-9, March 2007. L. Gyugyi and E. C. Strycula, “Active ac power filters,” in Conf. Rec. IEEE-IAS Annu. Meeting 1976, pp. 529-535.

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