Study about the possibility of flicker effect simulation caused by nonlinear power loads

Transcription

1 8th WSES International Conference on SYSTEMS THEORY and SCIETIIC COMPUTTIO (ISTSC 08) Rhodes, Greece, ugust 20-22, 2008 Study about the possibility of flicker effect simulation caused by nonlinear power loads MUEL POIU, CIUS POIU, IO ŞOR, RLUC RO Electrical Engineering and Industrial Informatics epartment Polytechnic University of Timisoara Revolutiei str. no 5, code ROMI bstract: - The Electric rc urnace (E) is a very large power load, determining the negative effects on the power quality: flicker effect, harmonics currents, unbalanced load, and reactive power. These negative effects are due to the nonlinear characteristic of the electric arc. This paper present a study based on simulation about flicker effect caused by Ultra High Power Electric rc urnace (UHP-E). or simulation it was use an electric arc model, depending on the nonlinearity of the electric arc. The model was validating using measurements made in an industrial plant in Romania. Key-Words: - simulation and modeling, power quality, flicker, harmonics, interharmonics 1 Introduction The electric arc is a nonlinear element. or study the behavior of the systems based on electric arc it must use techniques to model the nonlinearity of the electric arc. ecause the electric arc s nonlinearity, this is a massive generator of harmonic currents and reactive power in electrical power system. The E are also a reactive power source because the electric arc is also a reactive load. The electric arc furnace is also and unbalance load. However, one of the most substantial disadvantages of arc furnace is caused by the variations in the line voltage leading to flicker, which can be observed due to the luminosity fluctuation of incandescent lamps. Electric arc furnaces are a main cause of voltage flicker due to the interaction of the high demand currents of the loads with the supply system impedance. Therefore the main point of analysis focuses on the characteristics of harmonics, and also on the flicker. The effect of these installations was analyzed using simulation program PSC/EMTC [14]. PSC (Power System Computer ided esign) is a multipurpose graphical user interface capable of supporting a variety of power system simulation programs. This release supports only EMTC (Electro-Magnetic Transients in C Systems). or simulation it was use an electric arc model, depending on the nonlinearity of the electric arc. The modeling approach adopted in the paper is graphical, as opposed to mathematical models embedded in code using a high-level computer language. The well-developed graphic facilities available in an industry standard power system package, namely PSC-EMTC, are used to conduct all aspects of model implementation and to carry out extensive simulation studies. 2. Light flicker due to voltage fluctuations One definition of flicker is Impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time [1]. This means the perception of light flicker is a physiological process. Over the past years numerous studies have been conducted in order to understand the mechanisms behind the flicker phenomenon. There are at least three different mechanisms influencing the light flicker perception by a human. These are: - The characteristics of the light source. - The frequency response of the eye-brain of a human - The time constant of the eye-brain Examples of flicker sources The flicker is in reality a statistical calculation, defined by the E IEC standard and obtained from measuring the rapid variations in voltage. These rapid variations in voltage (figure 1) are, generally speaking, caused by variable loads such as arc furnaces, laser printers, micro-wave ovens or air conditioning systems being started up. s mentioned in the previous section the main source of severe voltage fluctuations are industrial loads with fluctuating power demands but also wind turbines and wave power etc. can generate flicker. Theoretically, flicker can also be caused by sub- and ISS: IS:

2 8th WSES International Conference on SYSTEMS THEORY and SCIETIIC COMPUTTIO (ISTSC 08) Rhodes, Greece, ugust 20-22, 2008 interharmonic frequency components giving a beating frequency component placed within the flicker frequency spectrum [2], [3] as well as caused by modulation of the voltage harmonics [4]. However, the dominating flicker sources are heavy fluctuating loads like arc furnaces, welding machines, rolling-mills etc. ig. 1 Rapid variation in voltage n arc furnace is probably the load that produces most flicker [5], [6], [7]. When the arc furnace operates, an unstable arc will appear between the electrodes and the scrap resulting in fluctuating power consumption and thereby a potential flicker problem. s a rule of thumb the ratio between the short-circuit capacity at the point of common coupling (PCC) to the maximum demand of the arc furnace should be greater than 80 in order to limit the risk for severe flicker caused by the arc furnace. The best way to investigate the actual flicker situation is to perform on-site measurements using a flickermeter based on the IEC standard. If the arc furnace is connected to a network with changing loads over time, a good idea is to measure flicker permanently and thereby see the trend of flicker. Common methods to reduce flicker originating from an arc furnace is to increase the short-circuit level by installing a new main transformer with higher capacity, installing active mitigation equipment like a SVC etc. or to improve the control strategies of the arc furnace. The mitigation methods are quite expensive and discussions between the network operator and the owner of the arc furnace regarding cost sharing are common. 3. Model of the Electric rc urnace for flicker simulation In the specific literature, there are many mathematical models of the electric arc. In [8], [9], [10] and [12] was present some models for the electric arc. rom these models in this paper was choose the model based on the empirical relation between the arc current, arc voltage and arc length. This model are considered by the authors the most appropriate model for describe the electric arc behavior. This model considers the characteristic current-voltage described by relation C U = U th. (1) I In this relation U and I are the arc voltage and arc current, and U th are the threshold voltage. The C and constants determine the difference between the current increasing part and current decreasing part of the current voltage characteristic (C a, a irrespective C b, b ). The typical values ([8], [9], [10], [12]) are: U d = 200 V, C a = W, C b = W, a = b = ecause the real values of the model parameters depend on the voltage arc variations, the dynamic arc voltage current characteristic must be an arc length function, given by relation ([3], [4]): U = k U 0( I ). (2) In (2) U 0 are the value of the arc voltage for a reference arc length l 0 and k is the ratio between the threshold voltage value for arc length l, U th (l) and the threshold voltage value for arc length l 0, U th (l 0 ). The dynamic model for electric arc presumes that the relation between the threshold voltage value and the arc length can be expressed by: U th = l. (3) In (3) is a constant equal with the sum of cathode and anodic threshold voltages ( 40V ) and represent the threshold voltage on the unit length, having usual values of 10 V cm ([9], [12]). The dependency of k by the electric arc length is: l k() l =. (4) l 0 Using this model, the correction of the electric arc power can be done within loose limits by modifying the threshold voltage, which corresponds in practice to the modification of the distance between the electrodes and the metal bath. Modeling the electric arc using the variablelength dynamic characteristic The quick variations of the voltage absorbed by the electric furnace arc during the melting process are closely dependent on the variations of the electric arc length, caused by the position changing of the metal pieces and the variation of the electrode positions. t present, two approaches have been developed as to the variation pattern of the electric arc length, the former supposing a determinist approach and the latter, a statistical one. In this paper it was use the sinus variation of the electric arc length. ccording to the determinist variation it is considered that the electric arc length has a time-dependent variation pattern that can be described by a sinus law. The ISS: IS:

3 8th WSES International Conference on SYSTEMS THEORY and SCIETIIC COMPUTTIO (ISTSC 08) Rhodes, Greece, ugust 20-22, 2008 time-dependency of anode voltage can be obtained considering that the arc length is changing according to relation L l() t = l0 ( 1 sinωt ), (5) 2 where L represents the maximal variation of the electric arc length (the electrode movement range) and l 0 is the maximal length of the electric arc (the maximal distance between the electrode and the metal bath). Using relations (2) - (5) one can obtain the time variation of the dependency U (I ) ( I ) ( 1 sin ωt) L 2 = 1 U U 0 l0 ( I ) (6) y using the notation L m =, (7) 2l L 0 one can notice that for L = 0 we obtain m = 0, and for L = l 0 we obtain m = 1, parameter m representing the modulation index of the electric arc length. The model implemented in PSC are depicted in fig.2. ra11 R Caa Cb l0-5.0 a b k1 rc resistance / k1 f1 Phase Coefficient k am Mag Sin req f 1.0 l0 m l0 rc length on phase 1 Thresold voltage Uth1 Cb b I1-1.0 / u1n egative semialternance Positive semialternance l1 u1p Ctrl u1n Ctrl = 1 Uth1 / ra11 / Caa a Uth1 u1p 0.0 I1 Comparator I1 I1 ig. 2. The electric arc model for flicker simulation implement in PSC EMTC It was obtained, by simulation, the arc voltage waveforms for m=0,2 (red) and for m=0.6 (blue), which are depicted in figure 3. or the modulation frequency it was choice the value 10 Hz. or this value is most probably to appear flicker phenomena [8]. It can be observed, in figure 3, that the maximum value of the current envelope corresponds to the minimum value of the voltage magnitude, as result from (2). ISS: IS:

4 8th WSES International Conference on SYSTEMS THEORY and SCIETIIC COMPUTTIO (ISTSC 08) Rhodes, Greece, ugust 20-22, 2008 ig. 3 The waveforms for arc voltage and the signals spectrum for m=0.2 (red) and m=0.6 (blue) y analyze the current and voltage spectrum present in figure 3, it can observe that the voltage spectral characteristic contained the 5 th, 7 th, 11 th, 13 th harmonics as is well know in literature but appear components with frequencies different from 50Hz. Therefore, it can be observe 60 Hz, 140Hz and 160 Hz components. The same phenomena appear in the current spectrum. The magnitude for 5 th, 7 th, 11 th, 13 th harmonics and the neighboring interharmonics are show in figure 4. ecause by using T the frequency step is 2 Hz, in figure 4 are better mark out the interharmonics magnitudes with different frequencies from multiple of modulation frequency. The influence of modulation index can be analyze from fig. 3. It can be observe that form a high value of modulation index (blue), both low frequency harmonics order and interharmonics are significant. or high frequency harmonics order the amplitude of harmonics are more mitigated. The value of modulation frequency is depending from the distance between harmonics and corresponding harmonic. This distance is increasing with the value of modulation frequency. The spectral characteristic for the medium voltage line (in the primary of voltage transformer) are show in fig. 5. It can be observe the presence of the 5 th, 7 th, 11 th, 13 th harmonics and interharmonics. Therefore, the harmonics and interharmonics are transmitted in the medium voltage line. 4. Comparison with measurements items The measurements were made at a 3-phase power supply installation of a 3-phase E of 100t, to which were not connected the harmonics filters. etails about the measurements method are present in [13]. In fig 6 are show the signal spectrum for measure voltage also in secondary voltage transformer and in fig 7 in the primary voltage transformer (for one phase). Comparing the simulating spectral characteristic from fig. 3 with the measured spectrum from fig. 6, and from fig 5 with fig 7 (for the medium voltage line feed) it can be observe the presence of the same harmonics and interharmonics. ISS: IS:

5 8th WSES International Conference on SYSTEMS THEORY and SCIETIIC COMPUTTIO (ISTSC 08) Rhodes, Greece, ugust 20-22, 2008 ig. 4 The 5 th, 7 th, 11 th, 13 th harmonics and the neighboring interharmonics for arc voltage 10 3 ig. 5 The simulated voltage spectral characteristic for medium voltage line feed 10 2 Voltage (V) requency (H z) ig. 6 The voltage spectral characteristic for measure voltage in the secondary voltage transformer ISS: IS:

6 8th WSES International Conference on SYSTEMS THEORY and SCIETIIC COMPUTTIO (ISTSC 08) Rhodes, Greece, ugust 20-22, Voltage (V) requency (Hz) ig. 7 The voltage spectral characteristic for measure voltage in the primary voltage transformer 5. Conclusion The functioning of electric arc furnace can cause power quality problems, especially as voltage flickers, to the power supply system to which it is connected. owadays, most utilities and power customers are facing the need to solve the power quality problem created by E. In this paper the possibility of flicker simulation was analyze using a dedicated simulation program. The results of simulation were comparing with some measurements made on an industrial plant. References: [1] IEEE recommended practice for monitoring electric power quality. Standard IEEE std [2]T. Keppler,. Watson, J. rrillaga and S. Chen. Theoretical ssessment of Ligth licker Caused by Sub- and Interharmonic requencies. IEEE Trans. Power el. vol. 18, pp , Jan [3] M. e Koster, E. e Jaeger and W. Vancoetsem. Light licker Caused by Interharmonics. Technical report, Laborelec, elgium. [4]Z. Wenhui, L. Lili and G. Weikang. Wavelet Transform ased ew Methods for Voltage licker Signal and Harmonic etection. Int. Conf. On Power Electronics and rive Systems, vol 1, pp , ov [5] S. Mendis, M. ishop, and J. Witte. Investigations of Voltage licker in Electric rc urnace Power Systems. IEEE Industry pplications Magazine, Jan./ ebr [6] M. Morcos and J. Gomez. licker Sources and Mitigation. IEEE Power Engineering Review, ov [7] Z. Zhang,. ahmi and W. orris. licker nalysis and Methods for Electric rc urnace licker Mitigation ( Survey) IEEE Porto Power Tech Conference, Portugal, [8] Montanari, G.C., Loggini, M., Cavallini,., Pitti, L., Zaminelli,. (1994), rc-urnace model for the Study of licker Compensation in Electrical etworks, IEEE Transactions on Power elivery, vol. 9, o. 4, pg [9] Tang, L., Kolluri, S., Mark,. Mc-Granaghan, Voltage licker Prediction for two simultaneously operated rc urnaces, IEEE Trans. on Power elivery, vol. 12, o. 2, [10] Panoiu M, Panoiu C, Modeling and simulating the C electric arc using PSC EMTC, Proceedings of the 5 th WSES Int. Conf. on System Science and Simulation in Engineering, Tenerife, Spain, ec , 2006 [11] Panoiu M., Panoiu C., Osaci M, Muscalagiu I., Simulation Result about harmonics filtering for Improving the unctioning Regime of the UHP E, Proceedings of the 7 th WSES Int. Conf. on Signal Processing, Computational Geometry and rtificial Vision, Vouliagmeni each, thens, Greece, ug , 2007, pg [12] Panoiu M., Panoiu C., Osaci M, Muscalagiu I., Simulation Results for Modeling the C Electric rc as onlinear Element using PSC EMTC, WSES Transaction on circuits and systems, pp vol 6, 2007 [13] Panoiu M., Panoiu C., Osaci M, Muscalagiu I., Simulation Result about Harmonics iltering using Measurement of Some Electrical Items in Electrical Installation on UHP E, WSES Transaction on circuits and systems, vol 7, Jan 2008, pp [14] ISS: IS: