Filter Design for Harmonic Reduction in High-Voltage ... - IEEE Xplore

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IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 20, NO. 1, JANUARY 2005

Filter Design for Harmonic Reduction in High-Voltage Booster for Railway Applications Loris Zanotto, Roberto Piovan, Vanni Toigo, Elena Gaio, P. Bordignon, Tito Consani, and M. Fracchia

Abstract—This paper presents the design of the power section of a 25-kV, 50-Hz, 10-MVAr single-phase static var compensator (SVC) for railway applications. It is based on a third harmonic filter, on a low-pass filter for harmonic reduction, and on a saturable reactor connected in series with the thyristor-controlled reactor. Harmonic filtering is the crucial aspect of the system because of the very stringent limitations imposed on the harmonic current generation due to railway signaling requirements. The effectiveness of this innovative solution in reducing the grid harmonic current injection is shown in the paper and the advantage, in terms of system efficiency, is underlined. The system was successfully constructed and installed at the end of 2002, and the commissioning will be completed during 2003.

TABLE I MAIN HVB TARGET PERFORMANCES

TABLE II VOLTAGE HARMONIC LIMITS AT THE SVC CONNECTION POINT TO THE GRID

Index Terms—Harmonic analysis, rail transportation power systems, reactive power control, saturable reactors, static var compensators (SVCs).

I. INTRODUCTION HE high-voltage booster (HVB) project for railways started in 1998, funded by the European community. The aim of the project was to identify and realize devices able to compensate the voltage drop on 25-kV, 50-Hz single-phase existing railway networks, so as to satisfy the increasing traffic demand without upgrading the power-supply infrastructures [1]. Some feasibility analyses [2], [3] have initially led to a single-phase static var compensator (SVC) which contains a thyristor-controlled reactor (TCR) branch for continuous reactive power control and two tuned filter branches for harmonic reduction. The main HVB target performances and design constraints of the project are summarized in Table I [1] for two voltage levels. The European Standard EN50160 imposes very stringent limits to the harmonics generated by the system, such as the following. — Voltage harmonics produced at the connection point to the grid according to Table II. — Harmonic currents injected into the supply line in the railway signaling frequency spectrum (31th–51th harmonic orders) lower than 30 mA. Fig. 1 shows the theoretical maximum harmonic currents produced by a TCR [5], [6]. To guarantee a reduction of these harmonics in

Fig. 1. Maximum harmonic currents produced by an ideal TCR at the grid connection point. Values are normalized to the maximum fundamental current (only odd harmonics are present).

Manuscript received March 17, 2003; revised August 5, 2003. Paper no. TPWRD-00107-2003. L. Zanotto, R. Piovan, V. Toigo, and E. Gaio are with the Consorzio RFX–Associazione EURATOM/ENEA sulla Fusione, Padova 35127, Italy (e-mail: [email protected]). P. Bordignon and T. Consani are with the ASI–ROBICON, Milano 20126, Italy. M. Fracchia is with the Università degli Studi di Genova, Genova 16146, Italy. Digital Object Identifier 10.1109/TPWRD.2004.835054

the specified spectrum to less than 30 mA, an attenuation factor higher than 20 is needed. — High reduction of the third harmonic injected by the HVB in order to increase the transmission line efficiency in supplying the ac/dc converter of the loads [7]. From an economical point of view, one of the targets of the project was to limit the device market price to 100 k/MVA.

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ZANOTTO et al.: FILTER DESIGN FOR HARMONIC REDUCTION IN HIGH-VOLTAGE BOOSTER FOR RAILWAY APPLICATIONS

Fig. 3.

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Simplified rail track line equivalent circuit.

TABLE III POWER-SUPPLY GRID IMPEDANCES

Fig. 2. General simplified scheme of the circuit.

Benefits deriving from the use of the HVB device are summarized in [1] and consist mainly in an increased traffic capability on existing lines (e.g., an increased number of trains per hour on the same line sector). The TCR branch of the device produces harmonics in excess of the allowed limits, thus requiring careful tuning of the filter branches. This paper presents the analyses carried out to design a configuration of the system able to fulfill the limits on the harmonic currents in the railway supply line. A particular and innovative solution, which includes a saturable reactor on the TCR branch, a third harmonic-tuned filter, and a low-pass filter is presented and analyzed. ATP–EMTP computer simulations have been carried out to accurately model the power-supply grid taking also into account the distributed capacitance of the overhead lines. This allows determining the frequency behavior of the grid with accuracy and properly evaluating the harmonic impact of the device. This HVB has been specifically developed and designed to be connected to two railway grids: Rail Track (England) and SNCF (France). Only the analyses for the English Railway Grid (Rail Track) are presented; similar results have been obtained also for the French grid (SNCF). II. GENERAL CIRCUIT SCHEME Fig. 2 shows the general scheme chosen for filtering. It is based on a classical shunt circuit, where the TCR branch is represented by a current generator producing the -order harmonic . is the filter harmonic impedance, current corresponds to the harmonic impedance of the power-supply is the equivalent railway network harmonic grid while impedance. , which includes capacitive elements, also provides the power for the reactive compensation and control. The harmonic currents injected into the grid are given by the following expression: (1) Some strategies to reduce the harmonic current injection into the grid can be derived from (1). First, a suitable choice of the ) filter configuration and parameters (that is, the choice of allows to obtain the desired harmonic mitigation in the signaling frequency range and for the third harmonic,

strongly related with the increase of efficiency [7]. Moreover, , which means a reduction of the TCR a reduction of harmonic production, would be useful to increase the system efficiency by allowing an optimization of the filter losses (see Section IV). III. TRANSMISSION LINE BEHAVIOR The harmonic reduction factor, defined as (2) depends both on the filter harmonic impedance and on the harmonic behavior of the transmission grid. Due to the wide range of the involved frequencies, it is not possible to model the supply transmission line with a lumped-element simplified model as in Fig. 3, where the equivalent circuit for the rail track grid is shown. Table III reports the lumped-element impedance values; distributed capacitances are not taken into account in this simplified model. In order to correctly evaluate the system harmonic generation, the behavior of the transmission line has to be investigated using a more detailed model, based on a distributed parameter representation, taking also into account the line capacitance. Simulations have been performed by means of the ATP–EMTP code in order to evaluate the differences between the two described grid models: the results are presented in Fig. 4, where the transmission line equivalent short-circuit impedance is plotted versus frequency using both the distributed parameter model and the lumped-element model of Fig. 3. The two models give very different results. A resonance (low , line equivalent impedance) is present at 2050 Hz within the frequency range for which the harmonic limitation is required. The importance of an accurate transmission line model to determine the real grid harmonic injection due to the TCR is self-evident.

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Fig. 4. Transmission line impedance as a function of the frequency: (a) Distributed parameter model. (b) Lumped-element model. Fig. 6. Harmonic currents (rms value) injected into the line for different thyristor firing angles (saturable reactor case).

spectrum, the harmonic reduction factor (2) has to be higher Hz . This can be achieved than 20 for by imposing the following filter time constant: (7)

Fig. 5. Filter configuration including a third harmonic-tuned branch and a low-pass filter.

IV. FILTER DESIGN

The choice of the resistance value is important to limit the resistor losses so as to maintain a reasonable HVB efficiency , which is a suitable value level. Assuming to keep becomes 10 . The filter losses to limit the power losses, will be evaluated in Sections IV-B and C, where a more detailed analysis is reported. Under such conditions, the reactive power absorbed by the MVAr. The third harfilter at 25 kV, 50 Hz is then monic filter parameters can now be computed using (5) and (6) MVAr; the considering following values are obtained:

A. Filter Configuration Let us analyze the filter branch of the HVB. The adopted solution for the filter configuration is shown in Fig. 5; it is composed by a third harmonic-tuned branch and by a low-pass RC filter branch. The third harmonic filter parameters are derived from the following equations: (3) (4) where is the fundamental angular frequency , is the nominal voltage, and is the reactive power absorbed by the filter at the fundamental frequency. Equations (3) and (4) give (5) (6) In order to reduce the high-frequency harmonic generation, a low-pass RC filter is connected in parallel to the third harmonic filter; to meet the required harmonic limitations in the signaling

(8) (9) (10) is chosen in order to have a high selectivity of the filter . B. Reactor Design 1) Linear Reactor Case: Let us evaluate the harmonics injection into the grid with the filtering branch solution described in the previous paragraph and considering a distributed model of the railway transmission line. We assume a classical configuration for the TCR branch, based on a linear reactor ( , ) connected in series to the thyristor valves as in Fig. 5. The harmonic currents injected into the railway grid are computed with an ATP–EMTP simulation for different thyristor firing angles; the grid supply voltage is changed as the firing angle changes, keeping the voltage across the HVB constant and equal to 25 kV. The results are shown in Fig. 6, where a Fourier analysis of the current absorbed by the HVB in the signaling spectrum is presented and the root-mean-square (rms) components are plotted.


Fig. 8.

Fig. 7.

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Saturable reactor characteristic.

Saturable reactor solution.

The analysis shows that the restrictions imposed on the HVB harmonic generation are not met as far as the signaling frequency spectrum is concerned. In particular, the maximum expected value is 330 mA, much higher than the limit (30 mA). This result is due to the previously described resonance of the supply line, which produces a low equivalent line impedance . Moreover, the psophometric current is higher around than 10 A, above the imposed limit in this case. 2) Saturable Reactor Case: a) Reduction of the TCR Harmonic Generation: The HVB harmonic generation and the low-pass filter size and losses can be reduced by decreasing the harmonic generation due to the TCR itself; to achieve this, a new technical solution based on a small saturable reactor connected in series to the linear TCR reactor as shown in Fig. 7 is considered. When one thyristor is conducting, the absorbed current is given by

Fig. 9. TCR harmonic current generation without saturable reactor.

(11) where is the differential inductance of the saturable reactor. The basic idea is simple. The TCR current harmonic content in the high-frequency region is mainly related to the sudden variations of the current derivative, which are observed each time a thyristor turns off or turns on. Therefore, a limitation of the harmonic generation can be obtained if a smoothing effect on the current is introduced near the zero crossing, thus reducing the current derivative change. Looking at (11), if a saturable reactor is used, then the total inductance increases below a suitable current threshold because of the saturation so that the function inside the integral becomes lower. This allows reducing the current harmonic production within a particular frequency range, depending on the saturable reactor characteristic. Fig. 8 shows the magnetization characteristic of the saturable reactor which is employed for this application. The harmonic current generation due to the TCR in the case without saturable reactor is shown in Fig. 9 for different thyristor firing angles; the thyristor snubber circuits and the linear inductor resistance are included in the simulation.

Fig. 10. TCR harmonic current generation with saturable reactor connected in series to the linear reactor.

The same analysis, performed connecting the saturable reactor in series to the linear TCR reactor, shows (Fig. 10) that the TCR harmonic generation is strongly reduced. b) Harmonic Injection With Saturable Reactor: The HVB operation is here investigated including the described saturable reactor. The same analysis as in the linear reactor case is performed. Fig. 11 presents the results. The harmonic current produced by the SVC is less than 30 mA almost for each harmonic order of the spectrum; for , where a line resonance is observed (Section III), the harmonic current value is limited to about 130 mA, a factor 2.5 less than in the case without saturable reactor. Even though this value is not totally satisfactory (it is higher than the limit ), a strong reduction of the harmonics is obtained; near

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TABLE IV POWER LOSSES IN THE LOW-PASS FILTER

Fig. 11. Harmonic currents (rms value) injected into the line for different thyristor firing angles; saturable reactor case.

the psophometric current becomes less than 7 A, meeting the limit. Therefore, this design is considered acceptable as far as the harmonic performances are concerned. C. Low-Pass Filter Losses Optimization Since the harmonic injection is strongly reduced by the saturable reactor solution, an optimization of the low-pass filter losses is possible within some margins. At the fundamental frequency, the low-pass filter impedance is mostly capacitive; the filter losses are thus approximately equal to (12) Therefore, if the time constant of the filter (100 s) is maintained, its capacitance has to be decreased to reduce the filter losses. As a consequence, the resistance has to be increased, but in this case, the behavior of the filter at higher frequencies is worse. and , which gives the losses The previous choice of of Table IV, second column, is compared with a couple of new , parameters that are able to reduce the losses . To maintain the same system reactive power (10 MVAr), the third harmonic filter parameters are recalculated as follows:

Fig. 12. Harmonic currents (rms value) injected into the line for different thyristor firing angles–saturable reactor and reduced losses case.

Fig. 12 presents the harmonic currents injected into the line with the new filter. A slight increase in the harmonic content is observed, but the power losses of the low-pass filter are strongly reduced (see Table IV, third column). An improved system efficiency is then obtained; at the same time, the harmonic reduction remains acceptable because the psophometric current is still under the imposed limit. V. STATUS OF THE PROJECT The HVB has been constructed in Milan and installed onsite. The system has been successfully connected first to the rail track

Fig. 13.

TCR linear reactor and saturable reactor.


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[6] T. J. E. Miller, Reactive Power Control in Electric Systems. New York: Wiley, 1982. [7] L. Hu, R. E. Morrison, and D. J. Young, “Reduction of harmonic distortion and improvement of the voltage form factor in compensated railway systems by means of a single arm filter,” in Proc. V Int. Conf. Harmonics in Power Systems, New York, 2001.

Loris Zanotto was born in 1974. He received the electrical engineering degree (Hons.) and Ph.D. degree on very high power inverters for thermonuclear fusion experiments from the University of Padova, Padova, Italy, in 1999 and 2002, respectively. His research interests include the field of power-supply plants based on ac/dc and dc/dc converters and on capacitor banks for fusion experiments. He is also interested in power- quality problems.

Fig. 14.

Capacitor banks of the HVB.

network and then to the SNCF grid in late 2002. The commissioning of the device will be completed during 2003. Some pictures of the system are shown in Figs. 13 and 14. VI. CONCLUSION The design of the power section of a 10-MVAr, 25-kV, 50-Hz single-phase SVC for railway applications has been presented and discussed. The system is based on a third harmonic filter, a low-pass filter for harmonic mitigation and on a saturable reactor connected in series with the TCR linear reactor, and it has been successfully connected to rail track and SNCF lines during 2002. The main conclusions of the paper are shown in the following. 1) The importance of adopting a railway grid model based on distributed parameters, which takes also into account the line capacitance to ground. 2) The new solution presented in the paper and based on a saturable reactor allows to meet the stringent limits on the harmonic generation imposed by the railway signaling system in the frequency range of 1550–2550 Hz. 3) Finally, the design allowed optimization of the power losses in the low-pass filter, thus increasing the SVC efficiency and reducing the system cost.

Roberto Piovan was born in 1955. He received the electrical engineering (Hons.) degree from the University of Padova, Padova, Italy, in 1980. Currently, he is responsible for the engineering developments with Consorzio RFX, Padova, Italy, a laboratory in the Thermonuclear Fusion Research. His experience covers power- supply plants based on ac/dc and dc/dc converters and on capacitor banks, switching and protection devices for high current and voltage systems, and the use of vacuum circuit breakers (CBs) for the interruption of large dc currents.

Vanni Toigo was born in 1958. He received the Electrical Engineering degree (Hons.) from the University of Padua, Padova, Italy, in 1983. Currently, he is with Consorzio RFX, Padova, Italy, working in the research field of power-supply systems for thermonuclear fusion plants. His main interests are in power electronics and control systems.

Elena Gaio was born in 1958. She received the Dr. degree in electronic engineering from the University of Padova, Padova, Italy, in 1983. Currently, she is with Consorzio RFX, Padova, Italy, working in the research field of power supply systems for thermonuclear fusion plants. Her experience is in the area of high-power ac/dc converter systems and switching converters.

P. Bordignon, photograph and biography not available at the time of publication.

REFERENCES [1] M. Fracchia et al.. High voltage booster for railway applications. presented at The World Congress on Railway Research. [Online]. Available: http://www.sncf.com/wcrr/SP/278.PDF [2] M. Crappe et al., “High voltage booster for railways,” in Proc. 7th Eur. Conf. Power Electronics Applications, Lausanne, Suisse, 1999. [3] S. Bacha et al., “Using SVC for voltage regulation in railways network high voltage booster European project,” in Proc. 9th Eur. Conf. Power Electronics Applications, Graz, Austria, August 27–29, 2001. [4] Railway Applications—Electromagnetic Compatibility—Part 3-1, Eur. Std. EN 50121-3-1, 1996. [5] N. Mohan, T. M. Undeland, and P. Robbins, Power Electronics, 2nd ed. New York: Wiley, 1995.

Tito Consani was born in 1941. He received the electronic engineering degree from the University of Genova, Genova, Italy, in 1967. Currently, he is with ASI–ROBICON, Milano, Italy. He was with Ansaldo Group, Milano, where he had been since 1970. His major experience is in the field of high-power ac/dc converters for special applications and large static var compensation systems.

M. Fracchia, photograph and biography not available at the time of publication.