WSEAS TRANSACTIONS on POWER SYSTEMS Manuscript received Oct. 7, 2007; revised Dec. 19, 2007
Yang Han, Mansoor, Gang Yao, Li-Dan Zhou, Chen Chen
Harmonic Mitigation of Residential Distribution System using a Novel Hybrid Active Power Filter YANG HAN, MANSOOR, GANG YAO, LI-DAN ZHOU, CHEN CHEN Department of Electrical Engineering Shanghai Jiaotong University 218# XinShangYuan, 1954 Huashan Road, Xuhui District, Shanghai CHINA
[email protected] Abstract: This paper proposes an LCL-filter-based hybrid active power filter for harmonic mitigation of a 10/0.4kV residential distribution system. By using C-LCL-filter based topology, better switching ripples attenuation is achieved, and the phase margin of the power stage at higher frequency is significantly improved. Adaptive linear neural network (ADALINE) is applied for individual harmonic component extraction and the estimated signals are used for selective harmonic elimination (SHE) purposes. A robust deadbeat current control law is derived based on the low frequency model of the presented topology. Owing to the ADALINE based SHE strategy, the controller bandwidth requirement is noticeably diminished thus the stability of whole system is ensured. The feasibility and effectiveness of the proposed system have been substantially confirmed by the laboratory experiments and field tests. Key-Words: -APF, Harmonic Distortion, LCL-Filter, Deadbeat Control, ADALINE phase margin of the hybrid APF has been significantly improved (see Fig.4). Fast and accurate estimation of nonlinear currents is a prerequisite for satisfactory compensation of harmonics. Recently, artificial neural networks have been applied with success in control of APF [6]-[9]. The learning capabilities of ANNs allow online adaptation to every changing parameter of the electrical network. In retrospect, M. Rukonuzzaman et al. [6] applied adaptive neural network (ADALINE) for harmonic estimation of nonlinear load currents in single-phase pure active power filter. However, no experimental results were given. In [7], Nishida et al. applied ADALINE for fundamental component estimation in three-phase three-wire system, and the load harmonic components are obtained by subtracting the estimated fundamental component from the distorted nonlinear current. In [8], Singh et al. decompose the nonlinear load currents into positive and negative sequence fundamental component, reactive component and harmonic components. Abdeslam et al. [9] proposed a unified neural approach with increased efficiency, robustness and improved performance by using ADALINE. A structure of strongly homogeneous processing elements allows an online adaptation to every changing parameter of electrical network. However, the algorithms reported in [8] and [9] are quite complex and computational intensive and they are not suitable for single-phase systems.
1 Introduction With the proliferation of nonlinear loads such as diode/thyristors rectifiers, non-sinusoidal currents degrade power quality in electrical system. In recent years, the use of active filters has become attractive due to the technological progress in power devices and digital signal processors (DSPs), and advanced control algorithms [1]. Conventional shunt APF requires a high value inductance for output filtering of the voltage source inverter, which degrades system dynamics and results in high voltage rating on the dc side capacitor and power devices. In [2] and [3], hybrid APF topology was proposed by using a voltage source inverter with a series connected inductor and capacitor set. The proposed shunt APF has lower voltage rating of dc capacitor and power devices, smaller filter inductor, smaller dimension, light weight and better filter performance. The hybrid APF topology has received considerable attention in recent years, e.g., the solution of optimum capacitor and inductor values of the passive filter used in shunt hybrid APF was discussed in [4], using the simplex search algorithm. This paper further improves the shunt active filter topology, the conventional LC passive filter of the hybrid APF is modified to higher order C-LCLfilter architecture. By replacing the filter inductance to a third order LCL-filter, better switching ripples attenuation can be achieved during the switching operations of power devices [5], simultaneously, the ISSN: 1790-5060
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Fig.1 Single-phase schematic of the proposed APF This paper takes the advantage of ADALINE for fast harmonic estimation, as in [6]-[9]. However, all the harmonic components from 3rd to the 25th order are estimated in a subroutine in DSP program, but only the lower order harmonics are selected for compensation. The ADALINE algorithm and the deadbeat control law are used consecutively for selective harmonic compensation purposes. Thus the proposed APF works in the selective harmonic elimination (SHE) mode. Therefore, the controller bandwidth requirement is noticeably diminished thus the stability of whole system is ensured. This paper is organized as follows. Section 2 discusses the system configuration and the guidelines for parameter design. Three aspects regarding to the design issues of control system are highlighted in section 3, namely, harmonic detection algorithm, current-loop control and dc-bus voltage control. Section 4 describes the experimental results. Both the laboratory and field test results are presented. Finally, section 5 concludes this paper.
The LCL-filter, composed of Lg, Cf and Lconv with possible passive damping resistance Rd, is used as output filter of the VSI, for switching harmonic attenuation. Simultaneously, the phase margin at higher frequency is noticeably improved (see Fig.4). The LCL-section is equivalent to an inductor at low frequency, and together with ac side capacitor Cac, the LC resonant circuit is formed hence the dc-side voltage of the VSI can be reduced to achieve higher efficiency and lower voltage rating of power devices.
2 General description of the proposed hybrid active power filter 2.1 System Configurations Fig.1 shows the circuit diagram of the proposed LCL-filter-based hybrid active filter. Three analogous single-phase topologies are utilized in this system, thus only single-phase representation is illustrated.
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Fig.2 Equivalent circuit of the proposed system (a) low frequency model; (b) high frequency model
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poles placement method is used, and the results are shown in Table 1.
2.2 Parameter Design Generally, the overall design procedure is a trade-off between a high filtering performance and a fast dynamic response. Fig.2 (a) indicates that the lower resonant frequency of the presented system is ω1 =
1 Cac (Lg +Lconv )
3 Control system design The cascaded controller structure is adopted in this study, which contains inner current-loop and outer dc-voltage loop (Fig.3). The inner current loop is responsible for fast harmonic tracking and the outer loop is used for balancing active power flow through regulating the dc-bus capacitor voltage. The APF works in the feedback mode since the grid currents and the converter currents are sampled. However, by adding current transducers (CTs) at the load-side, feed-forward mode or feedback plus feed-forward mode control can also be implemented to further improve the dynamic response.
(1)
From Fig.2(b), by neglecting the inherent resistance Rg and Rconv due to high quality-factor of inductors used in the system, the transfer function from Vconv to iconv in case Rd=0 can be derived as iconv (s) s[s2 +a] = vconv (s) Lconv [s4 +s 2 b+c]
Where a= c=
(2)
1 1 1 1 1 1 1 , ( + ) , b= ( + )+ Lg Cac Cf Cf Lg Lconv Cac Lg
3.1 Harmonic Detection The adaptive linear neural network (ADALINE) [6][9] is used for individual harmonic component estimation (Fig.1). By using ADALINE, the phase and magnitude of each harmonic component, from the 3rd to the 25th order, are estimated for higher convergence of ADALINE, but only the lower-order harmonics from 3rd to 13th are selected in the currentloop control and used for selective harmonic elimination together with deadbeat control law as discussed in the next subsection. Furthermore, in order to guarantee a fast stable convergence of ADALINE, a dynamic learning rate is adopted in this paper. The performance of the weights updating process might be further optimized by using current genetic algorithm (GA) [10], but it will not be addressed here anymore due to space limitations.
1 1 . Lg Cac Lconv Cf
Table 1 Specifications and system parameters APF power rating 75 kVA (3-phase) Nominal voltage 400 V Ac-side capacitor Cac 1000 μF Grid side inductor Lg 300 μH Converter side inductor Lconc 450 μH Ac capacitor of the LCL-filter 10 μF Damping resistance Rd 2 ohm Dc-side voltage of VSI 250 V Inverter switching frequency 10 kHz Hence, the proposed topology has two more zeros and four more poles compared to a simple L filter. The design of the passive components should consider the following practical constraints. Firstly, since the residential load are dominated by 3rd,5th, 7th, 11th and 13th order harmonics, the lower frequency resonant points indicated by (1) should be designed between the 3rd and 5th order harmonic frequency. Secondly, the choice of LCL-filter section parameters should consider power-rating of the inverter, the line frequency and attenuation of the switching frequency harmonics [5]. Further, the high frequency resonant points indicated by (2) should be in a range between ten times the line frequency and one-half of the switching frequency, to avoid resonance problems in lower and upper parts of the harmonic spectrum. Finally, the passive damping must be sufficient but cannot be so high as to degrade efficiency. After concerning the above mentioned constraints, the
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3.2 Current-Loop Control Fig.4 shows the comparison of bode plots from inverter output voltage to inverter-side current (see Fig.2), it can be deduced that for the two plots resembles each other with frequency range of less than 1500Hz. Utilizing this property, Fig.2(a) can be used to drive the deadbeat current control law for the inner current loop if only lower order harmonics are to be compensated, as following v1 -v 2 -v conv =L
di dt
(3)
Where parameters v1, v2 and vconv are PCC voltage, ac-capacitor voltage and converter output voltage, respectively, and L=Lg+Lconv. Using the triangular tracking PWM technique (Fig.5) and assuming the grid voltage and ac-side capacitor voltage are constants during one PWM cycle (quasi-steady-state model), the discrete form of (3) during one PWM period can be derived as
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Fig.3 Control block diagram of the proposed hybrid APF d d v1 -v 2 -vdc i g [(k+ 2 )T]-i g (kT)= 2 T L v1 -v 2 +vdc d d i g [(k+1- )T]-i g [(k+ )T]=(1-d)T 2 2 L d d v1 -v 2 -vdc i g [(k+1)T]-i g [(k+1- 2 )T]= 2 T L
3.3 Dc-Bus Voltage Control A crucial issue in the proposed active filter is the dcvoltage control. During operation, the active filter may absorb an amount of active power into, or release it from, the dc-bus capacitor. Excessive active power absorption may increase the dc-bus voltage drastically, thus may damage the active filter. Here the output of dc-bus voltage regulator is set as the active fundamental frequency reference for the inner current loop (Fig.3). A software phase-locked-loop (PLL) is designed which tracks the instantaneous phase information of the grid voltage. The output of PI regulator is multiplied by a unit sinusoidal function synchronized to grid voltage by PLL. To ensure a smooth transient response and to avoid sudden increments or decrements in the dc-bus voltage, a limiter of 40V peak-to-peak fundamental voltage is included in the control loop. Further, a low-pass-filter (LPF) with a cut-off frequency of 20Hz is adopted to eliminate ripples from the dc-bus.
(4)
Thus the duty ratio of the PWM signal is obtained as d=-
L{ig [(k+1)T]-i g (kT)} vdc +v1 -v2 + 2vdc T 2vdc
(5)
In order to follow the reference signal and achieve deadbeat control, the current at k+1th sampling interval ig[(k+1)T] is replaced by reference signal at the next sampling cycle. Fig.3 shows the sketch of control block of the proposed system, it indicates that the reference signal is composed of three parts, namely, the output of dcvoltage regulator, the estimated harmonics of the grid current, and the possible fundamental reactive current reference.
Fig.5 Triangular tracking PWM Fig.4 Bode plots from inverter output voltage Vconv to inverter-side current iconv (a) without Rd and Cf ; (b) with Rd and Cf
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4 Experimental Results 4.1 Laboratory Test
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The three-phase load currents and neutral-wire current are shown in Fig.7. It shows that the THD of load currents are 85.5%, 92.7%, 95.7%, respectively, and the dominant harmonics are of the orders 3, 5,7,9,11,13. The root-mean-square (rms) value of neutral-phase current is 36.2A. Fig.8 shows the source-side currents after the proposed APF is in operation. It shows that the THD of source side currents are 17.1%, 18.7%, 17.5% respectively. It can be deduced from Figs.7-8 that the proposed APF compensated quite well for the selected harmonics of the orders 3, 5, 7, 9, 11, 13.
Fig.6 Circuit Diagram for laboratory test of the proposed APF Fig.6 shows the circuit diagram for laboratory test, the load is varied through controlling the conduction angle of thyristors, for the toughest nonlinear load scenarios. The experimental results are recorded using HIOKI 8841 oscilloscope.
4.2 Field Test
Fig.9 Long time trend of THD for phase currents The proposed system is installed in the low-voltage side of a 10/0.4kV residential substation. The transformer capacity is 1250kVA with a short circuit impedance of 6.2% and a contract supply capacity of 500kW for residential customers. The Fluke F435 power quality analyzer is used for recording the data. Fig.9 shows the long time trend of currents THD of the 400V bus at the low-voltage side of transformer. It shows that the THD of phases A, B, C currents are 6%, 12% and 8% respectively before the APF is turned on, however, when the APF is applied, the THD of each phase current deceases to below 3%. (The neutral phase is not connected to Fluke F435 during the experiment.) Fig.10 and Fig.11 show the waveforms of the three-phase currents of the 400V bus before and after the APF is in operation. It can be observed that the currents are almost sinusoidal and harmonic free. Thus it demonstrates the excellent performance of the proposed hybrid active power filter.
Fig.7 Source side currents before compensation
Fig.8 Source side currents after compensation
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References [1] T. JAROU, M. CHERKAOUI, M. MAAROUFI, Contribution to the controlling of the shunt active power filter to compensate for the harmonics, unbalanced currents and reactive power, Proc. WSEAS/IASME Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Tenerife, Spain, Dec. 16-18, 2006, pp. 263-269 [2] Sunt Srianthumrong, Hirofumi Akagi, A medium voltage transformerlss AC/DC power conversion system consisting of a diode rectifier and a shunt hybrid filter, IEEE Trans. Ind. Appl., vol.39, no.3, 2003, pp. 874-882. [3] H. L. Jou, J.C. Wu, Y.J. Chang and Y. T. Feng, A novel active power filter for harmonic suppression, IEEE Trans Power Del., vol.20, no.2, 2005, pp. 1507-1513. [4] A. F. ZOBAA, Optimal sizing of the passive filter’s elements in hybrid active filters, Electr. Power Compon. and Syst., vol. 35, 2007, pp.483-488. [5] Marco Liserre, Frede Blaabjerg, Steffan Hansen, Design and control of an LCL-filter-based threephase active rectifier, IEEE Trans. Ind. Appl., vol.41, no.5, 2005, pp. 1281-1291. [6] M. Rukonuzzaman and M. Nakaoka, Single phase shunt active power filter with harmonic detection, IEE Proc.-Electr. Power Appl., vol.49, no. 5, 2002, pp. 343-350. [7] K. Nishida, M. Rukonuzzaman and M. Nakaoka, Digital control three-phase shunt active power filter with a new harmonic-current-extraction process, IEE Proc. –Gener. Transm. Distrib., vol.152, no.4, 2005, pp. 529-538. [8] Bhim Singh, Vishal Verma and J. Solanki, Neural network-based selective compensation of current quality problems in distribution systems, IEEE Trans. Ind. Electron., vol. 54, no. 1, 2007, pp. 53-60. [9] D. O. Abdeslam, P. Wira, J. Merckle, D. Flieller and Y. A. Chapuis, A unified artificial neural network architecture for active power filters, IEEE Trans. Ind. Electron., vol. 54, no. 1, 2007 pp. 61-76. [10] M. A. ZANJANI, GH. SHAHGHOLIAN, M. B. POODEH, S. ESHTEHARDIHA, Adaptive Integral-Proportional Controller in Static Synchronous Compensator Based on Genetic Algorithm, Proc. WSEAS Int. Conf. on Electric Power Systems, High Voltages, Electric Machines, Venice, Italy, Nov. 21-23, 2007, pp. 40-45
Fig.10 Source side currents before compensation
Fig.11 Source side currents after compensation
5 Conclusions This paper proposes a LCL-filter-based hybrid APF topology, using separate three single-phase structures. By using the hybrid topology, the dc-link voltage of the voltage source inverter can be reduced thus the switching losses of IGBTs can be reduced noticeably, and the switching ripples of pulse width modulation are also attenuated, simultaneously, the phase margin of the system is significantly improved owing to the LCL-filter architecture. The adaptive linear neuron network (ADALINE) is utilized for individual harmonic estimation to obtain a fast and accuracy tracking performance. The deadbeat control law is derived based on the low frequency model of the hybrid APF, and the drawbacks of deadbeat control under model uncertainty has been overcome by using ADALINE. The feasibility and effectiveness of the proposed hybrid active power filter is substantially confirmed by the laboratory experiments and field test.
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