Hilbert Transform Based Control Algorithm of the DG ... - IEEE Xplore

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 2, APRIL 2005

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Hilbert Transform Based Control Algorithm of the DG Interface for Voltage Flicker Mitigation Mostafa I. Marei, Tarek K. Abdel-Galil, Ehab F. El-Saadany, Member, IEEE, and Magdy M. A. Salama, Fellow, IEEE

Abstract—Distributed Generation (DG) is used widely in the modern distribution systems. This paper proposes a novel functionality of the interface between DG and the utility network to mitigate the voltage flicker and to regulate the voltage at the Point of Common Coupling (PCC) in addition to its main function of controlling the power flow. A new control algorithm for the DG interface based on the Hilbert transform (HT) is presented. The HT is employed as an effective technique for tracking the voltage flicker levels in distribution systems. The mathematical simplicity of the proposed technique, compared with the commonly used algorithms in the literature, renders them competitive candidates for the on-line tracking of voltage flicker. The accurate tracking of the HT facilitates its implementation for the control of flicker mitigation devices. Simulation results are provided to verify the tracking capabilities of the HT and to evaluate the performance of the proposed DG interface for multifunction operation. Index Terms—Distributed generation interface, envelope tracking, flicker mitigation, Hilbert transform, power quality, voltage control.

I. INTRODUCTION

V

OLTAGE fluctuations are either systematic variations in the voltage envelope or a series of random voltage changes. According to IEC standards [1], the magnitude of these fluctuations is within 10% of the rated voltage. The fluctuation of voltage is considered as one of the most harsh power quality events because of its detrimental effects on most electronic and control circuits, which are generally characterized by their sensitivity to voltage variation. Since voltage fluctuation is notorious for causing lights to flicker, it is called voltage flicker. Besides reducing the life span of electronic, incandescent, fluorescent, and cathode ray tubes, voltage fluctuation can result in the malfunction of phase locked-loops PLLs, electronic controllers, and protection devices. The sources of voltage flicker are numerous, but arc furnaces and arc welders head the list. Other common causes include the starting of motors, fans, pumps, elevators and the switching of power factor capacitors [2], [3]. Cyclic voltage flicker exists when there is a slow change in the voltage magnitude in the frequencies between 0.5 Hz to 25 Hz, which appears as a change in the fundamental signal envelope. The assessment and measurements of voltage flicker involve the derivation of the system RMS voltage variations, and the frequency at which the variations occur. Manuscript received October 28, 2003; revised May 8, 2004. Paper no. TPWRD-00545-2003. The authors are with the Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON, Canada (e-mail: mimmarei@ hivolt.uwaterloo.ca). Digital Object Identifier 10.1109/TPWRD.2004.843461

Characterized by a fast dynamic response, the Distribution STATic COMpensator (DSTATCOM) based Pulse Width Modulated Voltage Source Inverter (PWM-VSI) is an ideal solution for compensating for voltage flicker [4], [5]. Recently, DSTATCOMs with ratings ranging from several hundreds of KVAR to a few hundreds of MVAR have been installed in various industrial facilities [6], [7]. The control algorithms associated with DSTATCOMs for flicker mitigation can be classified into three types. In the first method, p-q orthogonal components are used along with a High Pass Filter (HPF) to detect the fluctuating components and then a control signal for compensating for the flicker is applied [4], [7], [8]. However, this method has too many calculation demands for the p-q conversion and the HPF is inaccurate, which introducing magnitude and phase errors. The second method relies on the utilization of the instantaneous power theory to derive the fluctuating components of the active and reactive power from the measured currents and voltages [9], [10]. This method’s performance is nearly the same as that of the first one, but its calculation demand is higher, and analogue filters are used. The last method is based on calculating the rms value of the measured voltage and feeding it to the voltage regulation loop [5]. This method results in a sluggish response, because of the time delay consumed by the rms calculator. This paper presents a new control technique for regulating and compensating for voltage flicker. This technique is based on the instantaneous tracking of the low frequency envelope of the measured voltage. Then, the estimated envelope is compared to the required voltage level to produce the appropriate control action. Not only is envelope tracking a crucial task for assessing the flicker level and evaluating the flicker severity, but it also can be applied to control flicker mitigation or voltage regulation. Many techniques are proposed in the literature to evaluate flicker levels. The Fast Fourier Transform (FFT) and the FFTprune techniques [11], [12] are utilized for voltage flicker tracking. The main downside of applying either the FFT or FFT- prune techniques is that it is assumed that the signals are stationary. Therefore, the application of such algorithms on nonstationary signals may lead to inaccurate results. To avoid this limitation, methods based on different estimation techniques, for example, Least Absolute Value (LAV) state estimation [13] and the Extended Kalman Filter (EKF) are introduced [14]. Although both the LAV and the EKF are appropriate for dealing with nonstationary signals, they both suffer from major drawbacks. The main disadvantage of the LAV state estimation technique is the assumption of knowing the flicker frequency in advance, which is not a realistic assumption. Moreover, the LAV state estimation undergoes a slow convergence, because it takes at least three cycles to

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settle to the correct value. Consequently, this algorithm is not attractive for on-line tracking and implementation. The EKF necessitates a large computational effort and requires an accurate adjustment to its parameters, which is not an easy task. Recently, The Wavelet Transform has been introduced as a powerful tool to analyze voltage flicker [15], [16]. However, the proposed method involves not only computational complexity and batch processing, but also the difficult process of choosing the candidate wavelet as a result, the Wavelet Transform is suitable only for off-line diagnosis and analysis. Recently, the Teager Energy Operator (TEO) was introduced in [17] for flicker tracking. The advantage of this method is its low computational burden. However, this technique is characterized by its sensitivity to high frequency components in the measured voltage (a result of the presence of nonlinear loads), which may lead to the instability of the TEO. In an effort to override the limitations of the aforementioned techniques, the Hilbert Transform (HT) is employed in this paper as a novel tool for flicker tracking and mitigation. This technique is accurate, fast, and easy to implement for the on-line voltage tracking. The HT is a linear operator, which has been successfully utilized in tracking the Amplitude Modulated (AM) speech signals, and its characteristics are thoroughly discussed in the literature [18]–[20]. The Distributed Generation (DG) is an electric power source connected directly to the distribution systems either from the utility side or from the customer side [21]. A VSI is normally used to interface and control the power flow from various types of DG sources, such as fuel cell or photo voltaic, to the power grid. The power quality is becoming a considerable factor with the restructuring of the power grid. More attention has been given to improve the quality of power in general and particular deliberation has been given to voltage flicker mitigation. This task can be achieved easily if the functionality of the existing VSI, interfacing the DG systems with the grid, is expanded to act as a DSTATCOM in order to mitigate different power quality problems [22]. This paper introduces a novel DG interface control algorithm for multifunction operation. It regulates the terminal voltage at the Point of Common Coupling (PCC) and controls the output power from the DG source emulating the Unified Power Flow Controller (UPFC). Moreover, this interface compensates the voltage flicker saving the cost of DSTATCOM installation for flicker mitigation on sites where DG already exists. The performance of the HT for envelope tracking and then flicker mitigation is tested by two methods. The first method utilizes artificial signals generated in MATLAB, and the second one implements a practical system modeled in the PSCAD/EMTDC simulation package. The results are discussed to demonstrate the potential of the proposed techniques. II. THE HILBERT TRANSFORM FOR ENVELOPE TRACKING Accurate modeling of the voltage flicker is a crucial task before any envelope tracking algorithm can be applied. The disturbed voltage waveform in distribution systems can be modeled as a variable amplitude sinusoidal waveform as follows: (1)

is the magnitude of the voltage under disturbance, where is the supply angular frequency, and is the phase angle of the depends on the load fundamental component. The shape of that produces the disturbance. The shape can be a step function to represent the switching of heavy loads, or it can be a square wave with a variable frequency and duty ratio to model the case of a resistance welder in different operating cycles. Also, the shape can be a sinusoidal function with a frequency lower than the supply frequency as in the ac arc furnaces as follows: (2) is the magnitude of the fundamental voltage, is the where magnitude of the voltage flicker, is the angular frequency of is the phase angle. the voltage flicker, and The ultimate goal of this study is to track and estimate the en, which is convelope of the fundamental voltage flicker sidered a key step in order to mitigate the voltage flicker in the distribution networks using the shunt compensator. A. Hilbert Transform (HT) Theory For a real signal, which has the Fourier transform, HT is defined as: (3) With the equivalent Fourier Transform, (4) where signal

is the Fourier transform of can be defined as:

. Now the analytical (5)

can be used to esThe modulus and phase derivatives of timate the amplitude envelope . Mathematically, this can be written in the following form [19]: (6) B. Implementation A Finite Impulse Response Filter (FIR) is designed to implement the HT. The HT-FIR filter can be realized in analog or digital forms [19], [23]. In this paper, the digital form of HT-FIR filter is adopted. A HT-FIR filter with odd symmetric coefficients is designed by the Parks-McClellan algorithm, which uses the Remez exchange algorithm and the Chebyshev approximation theory [23]. The adopted method of design minimizes the maximum error between the desired frequency response and the actual frequency response. Filters that are designed in this way demonstrate an equiripple behavior in their frequency response; hence, they are also known as equiripple filters [23]. Fig. 1 is a block diagram of the HT for the envelope tracking. The FIR filter transfer function takes the form (7) The filter length, , affects the accuracy of the tracking of the amplitude and the speed of the calculations. The long filter length ensures a minimal tracking error, but requires more calculation time. To study the effect of the filter length on the speed and accuracy of the HT tracking, two versions of this filter are

MAREI et al.: HT BASED CONTROL ALGORITHM OF THE DG INTERFACE FOR VOLTAGE FLICKER MITIGATION

Fig. 1.

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Block diagram of the HT for flicker envelope tracking.

Fig. 2. Block diagram of the proposed DG interface control system.

adopted: one with short filter length of with a relatively long filter length

N

vt

N

vt

Fig. 3. Voltage flicker tracking using HT with = 20. (a) Waveform of ( ), (b) tracked envelope, and (c) instantaneous tracking error.

and the other .

III. ENVELOPE TRACKING-BASED CONTROL ALGORITHM The proposed control for the VSI, to regulate the voltage at the required value and mitigate the voltage flicker, is shown in Fig. 2. A PI controller is used to process the error between the tracked envelope and the required voltage level . The output from the PI controller is the required quadrature current value for the voltage compensation . A simple open loop control is used to regulate the power flow from DG to the distribution network at its setting value . Dividing the command value of the active power over the PCC voltage results in the direct component of the VSI current, . Also, this component is responsible for feeding the switching losses of the VSI. Finally, the inverse Park transform is adopted to construct the reference value of the VSI three-phase currents , , and . Hysteresis Current Control (HCC) is utilized to force the actual current output from the VSI , , and to their command values. HCC offers fast dynamics and a wide bandwidth compared to other current controllers [24]. Based on the Volt-Ampere (VA) rating of the VSI, a limiter is designed for the reactive power through to prevent overloading of the VSI. IV. SIMULATION RESULTS Performance evaluations of the HT for the flicker envelope tracking and mitigation are carried out by conducting several test cases. Primarily, The first case examines the dynamic performance of the proposed flicker tracker with the aid of MATLAB for a given mathematical waveform. The second case deals with the implementation of a real system that has voltage flicker produced by a resistance welder and other fluctuating loads. EMTDC/PSCAD software is used in the latter test to validate the accuracy of the proposed control technique for flicker mitigation and voltage regulation.

Fig. 4. Voltage flicker tracking using HT with = 60. (a) Waveform of ( ), (b) tracked envelope, and (c) instantaneous tracking error.

A. Performance Evaluation The sampling frequency is equal to 1920 sample/sec, and a ensupply nominal frequency of 60 Hz so that the ratio sures a small error value. In realizing the HT, a FIR filter with a length of ‘ ’ is utilized using the design method explained in Section II. In order to show the effect of increasing the filter length on the HT tracking performance, the HT is designed with a long filter length of . To assess the proposed HT for tracking the flicker envelope (represented in Figs. 3(a) and 4(a)), the voltage flicker waveform with the step change introduced at is given by . (8)

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Fig. 6. Flicker mitigation of the resistance welder: (a) pu voltage at PCC, (b) phase a voltage and its envelope, and (c) VSI current of phase a. Fig. 5. System configuration.

Figs. 3 and 4 depict the actual and estimated envelopes of the and , respectively. same voltage signal with A fast response and accurate tracking are obvious. It is evident that increasing the filter length decreases the tracking error for the voltage envelope. However, this comes at the high cost of the computational time required for the calculation. The tracking error of the HT with is 1% while with is 0.3%. B. Flicker Mitigation Based on Envelope Tracking The second task of the simulation in this study is to evaluate the performance of the proposed control algorithm, presented in Section III, of the DG interface for flicker mitigation, voltage regulation and active power control. A simple, yet realistic, distribution system with an interconnected DG is simulated together with a resistance welder, one of the most common flicker generators in the distribution networks, is simulated by PSCAD/EMTDC. Details of the system being studied are shown in Fig. 5, where the DG system is connected to the distribution system through a shunt VSI cascaded with a transformer (T2) at the PCC. The HT is programmed by using FORTRAN in the EMTDC environment. The welder is modeled by a thyristor switched inductor, as illustrated in Fig. 5. An Integral Cycle Control (ICC) is used to control the welder by switching it on and off for any number of half cycles. Fig. 6(a) portrays the pu voltage at the PCC, when the weldcycle is four cycles with a 50% duty. The command value for the output power, , is 50 KW and for the voltage regulation is 1.0 pu. Before switching on the VSI at , the pu voltage varies from 0.77 to 0.91. The DG interface succeeds in regulating the voltage at the desired value of 1.0 pu and compensating for the voltage flicker. The up and down dips in the pu voltage occur at the switching instances of the welder. Each dip has a magnitude of 0.1 pu and a duration of one cycle.

Fig. 7. Flicker mitigation of the resistance welder: (a) quadrature current command i , (b) compensated reactive power Q, and (c) dc side voltage.

Fig. 6(b) shows the phase voltage at the PCC with its tracked envelope. A fast response and accurate tracking is obvious, before and after introducing the HT in the closed loop control. To investigate the internal performance of the DG interface, Fig. 6(c) shows the VSI current which contains the reactive power required to compensate for and regulate the PCC voltage; also the VSI current contains the desired active power flow from DG. Fig. 7(a) shows the control action which is responsible for compensating for the voltage flicker by feeding the required reactive power Q, shown in Fig. 7(b). Fig. 7(c) draws the supplied active power, which is coinciding with its command value. The fluctuations in the active power result from the coupling between the control loops of the quadrature and the direct current components of the VSI.

MAREI et al.: HT BASED CONTROL ALGORITHM OF THE DG INTERFACE FOR VOLTAGE FLICKER MITIGATION

V. CONCLUSIONS This paper presents a novel multi-functional utilization of the existing DG interfaced by VSI to mitigate the voltage flicker and regulate the voltage at the PCC beside its main function of controlling the power flow. These functionalities are achieved by a new control technique based on envelope tracking. The HT is employed to track the voltage envelope. The difference between the estimated envelope and the required voltage level is passed to the controller to obtain the required reactive power to compensate flickers. The fast response and the accurate tracking of the proposed control system are revealed from the simulation results. The HT realized with long filter length provides a minimum error in tracking the voltage envelope but with higher computation cost and a larger delay time than that of the short filter length. The HT can surpass the EKF in voltage tracking in a sense that it requires less computation effort and avoid the pitfalls of the FFT. The proposed multi-function DG interface can be implemented instead of installing DSTATCOM, which may lead to a reduction in the utility cost.

REFERENCES [1] IEC Standard Voltages, IEC Standard 38, 1983. [2] J. Arrillaga, N. R. Watson, and S. Chen, Power System Quality Assessment. New York: Wiley, 2000. [3] G. T. Heydt, Electric Power Quality. New York: Stars in A Circle, 1991. [4] A. V. Zyl, J. Enslin, and R. Spee, “Converter based solution to power quality problems on radial distribution lines,” IEEE Trans. Ind. Appl., vol. 32, no. 6, pp. 1323–1330, Nov./Dec. 1996. [5] C. Schauder, “STATCOM for compensation of large electric arc furnace installations,” in Proc. 1999 IEEE-PES Summer Meeting, vol. 2, pp. 1109–1112. [6] G. F. Reed, M. Takeda, and F. Ojima, “Application of a 5 MVA, 4.16 KV D-STATCOM system for voltage flicker compensation at SEATTLE IRON & METALS,” in Proc. 2000 IEEE-PES Summer Meeting, vol. 2, pp. 1132–1137. [7] M. Takeda and T. Aritsuka, “Practical application of active filters for power conditioning in distribution networks,” in Proc. 1998 Harmonics and Quality of Power Conf., vol. 1, pp. 304–309. [8] S. Chen and G. Joos, “Direct power control of DSTATCOMs for voltage flicker mitigation,” in Proc. 2001 IEEE-IAS Annual Meeting, vol. 4, pp. 2683–2690. [9] F. Z. Peng, G. W. Ott, and D. G. Adams, “Harmonic and reactive power compensation based on the generalized reactive theory for three-phase four-wire systems,” IEEE Trans. Power Electron., vol. 13, no. 6, pp. 1174–1181, Nov. 1998. [10] M. T. Chen, “Digital algorithm for measurement of voltage flicker,” IEE Proc.-Gener., Trans., Distrib., vol. 144, no. 2, pp. 175–180, March 1997. [11] K. Srinivasan, “Digital measurement of the voltage flicker,” IEEE Trans. Power Del., vol. 6, no. 4, pp. 1593–1998, Oct. 1991. [12] L. Toivonen and J. Morsky, “Digital multirate algorithms for measurement of voltage, current, power and flicker,” IEEE Trans. Power Del., vol. 10, no. 1, pp. 116–126, Jan. 1995. [13] S. A. Soliman and M. E. El-Hawary, “Measurements of power system voltage and flicker levels for power quality analysis: A static LAV state estimation based algorithm,” Int. J. Electric Power Energy Syst., vol. 22, pp. 447–450, Aug. 2000. [14] A. A. Girgis, J. W. Stephens, and E. B. Makram, “Measurement and prediction of voltage flicker magnitude and frequency,” IEEE Trans. Power Del., vol. 10, no. 3, pp. 1600–1605, Jul. 1995. [15] M.-T. Chen and A. P. S. Meliopoulos, “Wavelet-based algorithm for voltage flicker analysis,” in Proc. 9th Int. Conf. Harmonics and Quality of Power, vol. 2, 2000, pp. 732–738. [16] T. Zheng and E. B. Makram, “Wavelet representation of voltage flicker,” J. Electric Power Syst. Res., vol. 48, pp. 133–140, 1998. [17] T. K. Abdel-Galil, E. F. El-Saadany, and M. M. A. Salama, “Energy operator for on-line tracking of voltage flicker levels,” in Proc. 2002 PES Winter-Meeting, vol. 3, pp. 1153–1157. [18] S. L. Hahn, Hilbert Transforms in Signal Processing: Artech House, 1996.

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[19] F. Zhong-Sheng and X. Nan, “An analysis of various methods for computing the envelope of a random signal,” J. Appl. Ocean Res., vol. 17, pp. 9–19, 1995. [20] T. W. Parks and J. H. McClellan, “Chebyshev approximation for nonrecursive digital filters with minimum phase,” IEEE Trans. Circuit Theory, vol. 19, no. 2, pp. 189–194, Mar. 1972. [21] T. Ackermann, G. Andersson, and L. Soder, “Distributed generation: A definition,” Electric Power System Research, vol. 57, pp. 195–204, 2001. [22] M. I. Marei, E. F. El-Saadany, and M. M. A. Salama, “A novel control algorithm for the DG interface to mitigate power quality problems,” IEEE Trans. Power Del., vol. 19, no. 3, pp. 1384–1392, Jul. 2004. [23] T. W. Parks and C. S. Burrus, Digital Filter Design. New York: Wiley, 1987. [24] M. I. Marei, E. F. El-Saadany, and M. M. A. Salama, “A new contribution into performance of active power filter utilizing SVM based HCC technique,” in Proc. 2002 IEEE-PES Summer Meeting, vol. 2, pp. 1022–1026.

Mostafa I. Marei was born in Alexandria, Egypt on June 17, 1975. He received the B.Sc. and M.Sc. degrees in electrical engineering from Ain Shams University, Cairo, Egypt, in 1997 and 2000, respectively, and the Ph.D. degree in electrical engineering from the University of Waterloo, ON, Canada, in 2004. Currently, he is a Postdoctoral Fellow at the University of Waterloo. His research interests include power electronics, hybrid electric vehicles, custom power, artificial intelligent applications in power systems, digital control-based microcontrollers and digital signal processors (DSPs), power quality, and distributed generation.

Tarek K. Abdel-Galil received the B.Sc. and M.Sc. degrees in electrical engineering from Ain-Shams University, Egypt, in 1992 and 1998, respectively, and the Ph.D. degree from the University of Waterloo, Waterloo, ON, Canada, in 2003. Currently, he is with the Research Institute-King Fahd University of Petroleum & Minerals, Dhahran, Saudi-Arabia, where he carries out applied research related to power systems studies. In 2004, he was a Postdoctoral Fellow in the Electrical and Computer Engineering Department at the University of Waterloo. His research interests include the operation and control of distribution systems, power quality analysis, application of artificial intelligence algorithms in power systems, and high-voltage and insulation systems.

Ehab F. El-Saadany (M’01) was born in Cairo, Egypt in 1964. He received the B.Sc. and M.Sc.degrees in electrical engineering from Ain Shams University, Cairo, Egypt in 1986 and 1990, respectively, and the Ph.D. degree, also in electrical engineering, in 1998 from the University of Waterloo, Waterloo, ON, Canada. He is currently an Assistant Professor in the Department of Electrical and Computer Engineering at the University of Waterloo. His research interests are distribution system control and operation, power quality, power electronics and DSP applications to power systems.

Magdy M. A. Salama (F’02) received the B.Sc. and M.Sc. degrees in electrical engineering from Cairo University in Egypt in 1971 and 1973, respectively, and the Ph.D. degree, also in electrical engineering, in 1977 from the University of Waterloo, Waterloo, ON, Canada. He is presently a Professor in the Department of Electrical and Computer Engineering at the University of Waterloo. His interests include the operation and control of distribution systems, cables, insulation systems, and electromagnetics. He has consulted widely with government agencies and the electrical industry. He is a registered Professional Engineer in the Province of Ontario.