The Application of Active Filters Supported by Pulse Width Modulated Inverters in the Harmonic Simulation of the High Power Electric Traction

Transcription

1 The Application o Active Filters Supported by Pulse Width Modulated Inverters in the Harmonic Simulation o the High Power Electric Traction P. Kiss, A. Balogh 2, A. Dán, I. Varjasi 2 Department o Electric Power Engineering, Power Systems and Environment Group Budapest University o Technology and Economics Egry Józse u. 8, 2 nd loor, H- Budapest (Hungary) Phone number: , ax: , kiss2.peter@vet.bme.hu, dan.andras@vet.bme.hu. 2 Department o Automation and Applied Inormatics Budapest University o Technology and Economics Goldmann György square 3, 4 th loor, H- Budapest (Hungary) Phone number: , ax: , balogh@aut.bme.hu, varjasi@aut.bme.hu. Abstract The paper shows the present results o a long term research work. The authors are working on the modelling o active iltering o harmonics caused by the railways. To simulate the harmonic penetration and iltering eects o the power electric traction the combination o requency and time dependent model should be used. This novel method, called double domain simulation is improving the accuracy o the requency domain simulation. To calculate the sophisticated model o the electric locomotive and active harmonic ilter as a non-linear load a time dependent model must be used. The traction supply system together with the equivalent supply network impedance could be calculated in requency domain. An iteration algorithm is developed converting the variables in every iteration step between the time and requency domain. Keywords Power quality, traction supply systems, computer simulation, active harmonic iltering, PWM inverter. Introduction Strict requirements were established on the voltage quality o the electric supply network in the last decade. One o these requirements is in connection with the harmonic distortion o the voltage. [2] The voltage distortion is caused by the non-linear loads connected to the network on dierent voltage levels. This distortion could cause aults both in the high power energy system and the parallel telephone lines. Ater the spreading o locomotives supported by DC engines and rectiier units the disturbance originating rom railway traction systems has increased. Harmonic ilters are used to limit the harmonic currents lowing into the upstream network and to decrease the resonance eect causing current ampliication along the 25kV supply line. Reducing the harmonic currents decreases the psophometric current and voltage. In our paper the application o active ilters are discussed or the Hungarian 25 kv AC traction supply network. In the chapters o the paper the brie summary o the double domain simulation is presented, the detailed introduction o the active iltering, the ideal and the ilter with PWM inverter are reported. Finally some calculation results (harmonic currents, and voltages, calculated psophometric values) have been published. 2. Modelling the electric traction s supply The electric railway system is consisting o our main components (Fig..a): [] the locomotives the contact line system the eeding transormer the high voltage supply network The locomotives are running under the contact line system, dividing it into two parts. At the contact point the locos could be represented by a undamental requency consumer and a harmonic current generator. The contact line system should be considered as a multiconductor system with earth return that is composed o the contact wire(s), suspension wire(s), and the return rails. It can be reduced to a two-wire-system that leads the current. The eeding transormer could also be considered as a quadripole which consists o the magnetizing and the leakage reactance. The driving point impedance used or the system identiication RE&PQJ, Vol., No.6, March 2008

2 Fig.. Simpliied circuit or calculation o harmonic eect a) general traction-current eeding arrangement b) circuit representation o contact line system The parameters o the contact line system and transormer and the driving point impedance could be determined by laboratory and site measurements. The traction supply system model can be made with these elements as it is shown in Fig..b. This model is calculated in the requency domain, because all the necessary parameters are given in the requency domain. The computer simulation model o traction system is given on Fig. 2. [3] Fig. 2. The traction supply system model For studying low requency disturbance, the harmonic orders must be examined till the 50th harmonic. [2] The locomotive model s block diagram and computer model could be seen on Fig. 3. To calculate the sophisticated model o the electric locomotive as a nonlinear load a time dependent model must be used. It determines the current spectrum o the engine in unction o the distorted supply voltage. Because the voltage distortion is caused by the loco itsel, an iteration algorithm was developed to convert the variables between the time and requency domain vice and verse. [4] 3. Harmonic eect The current harmonic components could cause the ollowing problems: resonance eect with overvoltage and overcurrent consequences, additional losses, psophomentic disturbance o the telecommunication systems, disturbance in the remote control systems, malunction o protection devices, misoperation o semiconductor-controllers. The harmonic disturbance basically could be characterized by the individual () and total (2) harmonic distortion actors: k Dk () THD k 2 2 k (2) k : the harmonic order, 50 Hz k : kth harmonic component o I or V : undamental requency component o I or V. Fig. 3. Simpliied locomotive model a) block diagram, b) circuit representation 4. Psophometric intererence The high power lines could inluence the neighbouring telecommunication networks by the ollowing ways: Capacitive coupling: The voltage o the power line causes charging current Inductive coupling: The line current induces longitudinal em. The most dominant part o the psophometric noise is the inducing eect caused by the zero sequence components o the current. The power balance o the three-phase is near symmetrical during normal operation, thus the coupling is measurable only i the distance between the two systems is comparable with the phase distance o one system. However electric traction is a single-phase system with ground return and in consequence it is a natural zero sequence system. That is why it is important to calculate the psophometric noise. [] RE&PQJ, Vol., No.6, March 2008

3 By telecommunication lines the rate o the disturbance could be characterized by the so called psophometric voltage. It could be calculated by this ormula: V p p p 800 V V : voltage component by requency, p : psophometric weight by requency, p The psophometric weight has been determined ater human tests; it could be seen on Fig. 4. It could be concluded that the main part o the noise disturbance is caused by the 800 Hz and surrounding harmonics. The psophometric weighting could be applied or the current components, too, the ormula is the same like in Eq. (3), however, this value is characteristic to the zero sequence current o power line regarding its possible disturbing eect. This is the so called disturbing current. [] 2 (3) used to compensate inductive reactive power as well, because it shows capacitive reactance on the undamental requency. [] B. Active iltering The active harmonic iltering is an electronic method to convert the basically non-sinusoidal current o the consumer into sinusoidal one regarding the resultant supply side network current. The active ilters are controlled current generators controlled by microprocessors or microcontrollers, injecting the reciprocal value o selected requency components or the whole distortion to the network. [3] [5] 6. Applied active ilter Nowadays the spreading o power semiconductors, system technologies and control strategies made possible the wide-ranging utilization o power converters. The voltage source converters even at high power have switching requency high enough to be able to inject harmonic currents into the grid system. In this orm they can be used or harmonic compensation. Depending on the power level, the application and the switching requency there are several types o useable power converters. For harmonic compensation in case o railway applications the best choice is the single phase bridge inverter with alternative PWM controlled current control [6]. The main circuit arrangement o the inverter and the supply system model can be seen on Fig. 5. Fig. 4. The psophometric weight 5. Harmonic iltering Harmonic ilters are used to limit the harmonic currents lowing into the upstream network and to decrease the resonance eect causing current ampliication along the 25kV supply line. The ilter could be located on the loco itsel or on the substation. Because o the dierent type o locos running simultaneously on the same traction section the most eective place or the harmonic ilter location is the 25kV side o the substation. Basically there are two kinds o ilters: passive and active ilters. A. Passive iltering The passive harmonic ilter is a set o series resonance circuits tuned to the requencies to be iltered and connected parallel with the non-linear load to be iltered.. The passive harmonic ilter has low impedance on its tuned requency that is why it shunts the network or the harmonic current o the tuned harmonic order. Harmonic iltering is oten linked with the problem o undamental requency reactive power balance. The passive ilter is Fig. 5. The main circuit arrangement o the inverter In our simulation some simpliications and conditions were applied as ollows: The power semiconductors including power switches and reewheeling diodes are represented with ideal switches The control deadline between the upper and lower semiconductor in each leg, the switching RE&PQJ, Vol., No.6, March 2008

4 on and switching-o times o the semiconductors were neglected The serial R-C snubber circuits parallel with the switches are used to smooth the computed curves. These components not absolutely needed at the real circuit Alternative PWM is used or the less current ripple. A. The current control loop In theoretical view the controlled part is a serial R-L circuit, which can be easily controlled by a simple PI controller. The block diagram o the proposed control method is given on Fig. 6. A p(+ ) st The main goal o this active ilter was to eliminate above all the 3 rd and 5 th harmonic as the highest value harmonics o the traction supply system current (the calculation results could be studied in Chapter 7.A). Because the high power level and the maximal harmonic requency (250 Hz) the optimal switching requency was determined in 0 khz. The maximal harmonic requency is much less than the switching requency, so the inverter can be replaced in the control loop as a proportional gain and the whole system can be handled as continuous system. The right setting method o the PI controller is given in the next subchapter. B. Setting o the current controller i R+sL Fig. 6. The current control loop i hre: reerence harmonic current, V c : converter voltage, limited to the DC link voltage, V g : grid voltage, V RL : voltage o the coupling inductor between the converter and the grid, I L : injected harmonic current, R, L: resistance and inductance o the inductor, A P, T i : proportional gain and integration time o the PI controller, A CH : constant o the inverter. In the case o continuous system without delays the poles dropping method can be used to the setting. The transer unction (see Fig. 6) o the open loop is: il () s Ap + ACH ihre () s sti R+ sl (4) Converting (4), the transer unction is the ollowing: il () s + sti AA p CH i () s R st + st hre i ind T ind L/R is the electrical time-constant o the inductor. It is seen rom (5), i the electrical time-constant is equal to the integration time o the PI controller than (5) will reduce as ollows: il () s AA p CH i () s R st st hre i cont T cont RTi AA From (6) the transer unction o the closed loop is: Y cloop p CH (5) (6) (7) + stcont (8) It can be seen that with this method the original control loop reduced to a simple one degree system. The timeconstant o the whole controlled system (T cont ) was set to 0 µs, rom (5) T i was set to ms and inally the proportional gain rom (7) was set to 3. The proper operation o the active ilter is veriied with simulations (see chapter 7.). 7. Calculation results Some characteristic simulation regarding the harmonic distortion o the railway systems in unction o the locomotive position and locomotive type are discussed in the ormer papers [3] and [4]. Some special calculations are presented here by dierent iltering strategies using the traction system model (Fig. 2) with total length o 30 km, and the locomotive model (Fig. 3.b) o this paper, the locomotive is located 0 km ar rom the substation. A. Without iltering Some resonant eect can be studied along the supply line. This is a parallel resonance which is caused mainly by the inductance o the transormer and the distributed capacitance o the contact line system. [] [3] Using the model it is possible to calculate the voltage and current spectrum along the supply line. As an example on Fig. 7, RE&PQJ, Vol., No.6, March 2008

5 value at the substation. Fig. 7. Substation V and I without iltering It could be concluded that the current and voltage component on the iltered harmonic orders are reduced dramatically and the typical eect o the 3 rd and 5 th harmonics have been neglected, but the signiicant high requency components (mainly 7 th and 9 th harmonics) have remained. Because o the limitations o the switching requency, the PWM active ilters could not be applied eectively or such high requencies. To ilter this eect, a passive ilter or example a broadband one should be used. 8. Psophometric and THD values Using the harmonic components o the currents and voltages the psophometric and THD values can be calculated along the supply line. The Table I shows the value o the psophometric voltages by the substation and psophometric currents by the substation on the contact line and the upstream network side uniltered one. Besides this the THD values are given in the table. TABLE I. Psophometric and THD Values Fig. 8. Spectrum o currents (loco and substation) the voltage and current at the substation are seen. On Fig. 8 the calculated substation and locomotive current spectrum can be studied. The current spectrum o the locomotive is very characteristic or the AC side o DC motors supported by AC/DC converters. [4] Comparing with the substation spectrum, it could be concluded, that the resonance eect is the highest at the 7 th and 9 th harmonics. Over the 25 th harmonic the supply system is decreasing the harmonic current, like a harmonic ilter. The highest values o currents could be measured on the 3 rd and 5 th harmonics, the values o these components are near the same by the locomotive and the substation. It is possible to apply the active ilter o Chapter 6 to reduce these harmonics. B. Using the active ilter o Chapter 6 Ater installing an active ilter in the substation the ollowing results can be calculated. (Fig. 9) This ilter is a PWM controlled current generator injecting the calculated 3 rd and 5 th harmonic current as anti-phase V substation I HV>subst I subst>loco 9. Conclusion UNFILTERED FILTERED V p V V THD V 2. %.88 % I p 2.76 A 2.8 A THD I % 3.28 % I p 2.76 A 2.84 A THD I % 35.6 % It can be concluded that using active harmonic iltering the network side harmonic distortion is reduced very eectively, but the psophometric eect caused by the current along the traction system basically did not change. Quite the contrary, a minimal increase could be seen. In critical cases a broadband passive ilter might be necessary to decrease the harmonic impedance resonance causing the increase o equivalent disturbing current. This kind o calculation is a suitable method to perorm an analysis on both the harmonic distortion and psophometric eect. The double domain simulation method will help to select the most advantageous solution, regarding the composition o passive and active harmonic iltering. [3] 0. Future plans Fig. 9. Substation V and I with iltering In our model the DC link voltage is made by an outer AC/DC converter. This solution is very expensive, because an auxiliary power converter is needed. However, there is a better solution too. With using o capacitors in the DC link the converter s cost decrease, but the converter control will be much more complicated. In case o capacitors two control loops is needed, one outer loop or the DC link voltage control and one inner loop or the harmonic current injection. The capacitors RE&PQJ, Vol., No.6, March 2008

6 are charged with base harmonic currents through the reewheeling diodes and the capacitor is charged out through the switches with harmonic currents. The block diagram o the modiied control can be seen in Fig. 0. The harmonic injection eiciency can be increased with adaptive current control and PLL based synchronization to the harmonics. R+sL Fig. 0. Control structure in case o capacitors in the DC link i hre50 : base harmonic capacitor charging current reerence, V dcre : DC link voltage reerence, V dc : DC link voltage. In our previous work [7] [8] we successully developed a new control strategy or single and three phase converters, with which the converter eiciency can be improved. The so called 3SC (three state current control) method was successully tested on 0kW converters with about % eiciency increase. The using o 3SC method without any hardware changes in the main circuit may increase the converter eiciency. This control has reactive power and harmonic compensation capability up to the 25 th harmonic with high converter eiciency. Acknowledgement The importance o this paper indicates that its topic is it to one o the projects o the Hungarian State Railways. This is the project o planning active harmonic ilters to the railways substations to reduce the harmonic distortion, resulting in reduction o losses, and better voltage quality. Reerences []. A. Dán, J. Kisvölcsey, Gy. Varjú: Filtering o harmonics generated by thyristor controlled AC traction systems. In Proc. II. International Conerence on Harmonics in Power Systems, pp , Winnipeg, 6-7. October, 986. [2]. A. Dán, T. Tersztyánszky, Gy. Varjú: Electric Power Quality (Villamosenergia minőség). Budapest: Invest- Marketing Ltd, [3]. A. Dán, P. Kiss: Eect on Power Quality o the High Power Electric Traction (Double Domain Computer Simulation vs. Site Measurements). In Proc. International Conerence on Renewable Energies and Power Quality, Palma de Mallorca, 5-7. April [4]. A. M. Dán, P. Kiss: Advanced Calculation Method or Modeling o Harmonic Eect o AC High Power Electric Traction. In Proc. 2th International Conerence on Harmonics and Quality o Power, Cascais, -5. October [5]. P. Brogan, R. Yacamini: Measurements and simulation o an active ilter based on voltage eedback. In Proc. 8th. International Conerence on Harmonics and Quality o Power pp , Athens, 4-6. October, 998. [6]. M. Revisnyei, I. Varjasi, A. Kárpáti, I. Hermann: Investigation o Harmonic Currents o Single Phase Bridge Converter with digital PWM, In Proc. PEMC 98, pp , Prague, 8-0. September, 998. [7]. A. Balogh, I. Varjasi: Discontinuous Current Mode o a Grid Connected PV Converter. In Proc. International Youth Conerence on Energetics, Budapest, 3. May-2. June, [8]. A. Balogh, Z. T. Bilau, I. Varjasi: High Eiciency Control o a Grid Connected PV Converter, In Proc. Power and Energy Systems Conerence, Palma de Mallorca, August, RE&PQJ, Vol., No.6, March 2008