Nowadays, the negative impact of traction load on power system operation poses a serious problem that may lead to false tripping or even failures of relay protection devices and reduce the quality of electrical energy. This situation is typical for the Zabaikal power system, in which in some areas the share of electricity consumption by railway reaches 70% of total consumption. A computing tool, that would allow simultaneous analysis of the modes, both in the utility's grid and in the traction network, do not exist. The inability to carry out such analysis often leads to inconsistency in the actions of control centers and railway transport authorities, especially when maintenance planning. Mathematic modeling in such software systems as "Mustang", "RastrWin3", "MathCAD", “Matlab/Simulink”, “PSCAD”, “Kortes”. The developed model represents Zabaikal power system and contain detailed railway between substation Razmahnino and substation Shilka. Negative-sequence voltage unbalance factors were calculated for the case of train movement between substation Razmahnino and substation Shilka. Also, the necessity of back-up relays tripping values correction is stated. It was shown that for the powerflow calculations taking into account the traction load, it is rational to use a complex mathematical model, which uses compatible software systems with the ability of quick and easy data exchange

Darkhanova, A. V.

Kats, Iljya Markovich

Shestakova, Vera Vasiljevna

English

Electronic archive of Tomsk Polytechnic University

978-1-7281-2896-2/19/$31.00 ©2019 IEEE Powerflow calculations in powersystems considering traction load  V.V. Shestakova, I.M. Kats, A.V.  Darkhanova National Research Tomsk Polytechnic University Tomsk, Russia shestakova@tpu.ru    Abstract— Nowadays, the negative impact of traction load on power system operation poses a serious problem that may lead to false tripping or even failures of relay protection devices and reduce the quality of electrical energy. This situation is typical for the Zabaikal power system, in which in some areas the share of electricity consumption by railway reaches 70% of total consumption. A computing tool, that would allow simultaneous analysis of the modes, both in the utility’s grid and in the traction network, do not exist. The inability to carry out such analysis often leads to inconsistency in the actions of control centers and railway transport authorities, especially when maintenance planning. Mathematic modeling in such software systems as «Mustang», «RastrWin3», «MathCAD», «Matlab/Simulink», «PSCAD», «Kortes». The developed model represents Zabaikal power system and contain detailed railway between substation Razmahnino and substation Shilka. Negative-sequence voltage unbalance factors were calculated for the case of train movement between substation Razmahnino and substation Shilka. Also, the necessity of back-up relays tripping values correction is stated. It was shown that for the powerflow calculations taking into account the traction load, it is rational to use a complex mathematical model, which uses compatible software systems with the ability of quick and easy data exchange. Keywords— traction load, railway transport, utility’s grid, non-symmetrical modes, mathematic model, specialized software systems, Monte Karlo method. I.  INTRODUCTION  Nowadays, a negative impact of traction load on utility grid operation is widely observed in numerous publications [1-5]. This effect is strongly pronounced when railway consume more than 50% of total demand in the powersystem. This is typical for Zabaikal powersystem (ZPS), where separate railway sections consume  up to 70% from total demand. Traffic flow is taking off from year to year and this fact deteriorates the current  situation. According to [6] it is planned to increase freight traffic through Transib railway, hence, through Zabaikal railway too until 2030. Badly coordinated actions both of the control centers and railway authorities may often lead to mistakes when network calculations accounting the railway influence.  As a consequence, equipment failures, supply interruption both of traction and non-traction consumers. For instance, in ZPS trains were de-energized because of 220 kV overhead transmission line maintenance works. Besides, false tripping of back-up relays are also possible.  II. PROBLEM FORMULATION The challenge is there are no any means that would facilitate utility system mode analysis considering traction systems impact.  As it was mentioned before, this fact causes inappropriate responses of control centers and railway autorities, especially while maintainance planning. The aim of this paper is to develop a mathematic model, which is based on various software systems, to estimate mutual impact of supply and traction systems. III. TRACTION SYSTEMS POWER SUPPLY MODELING PRINCIPLES The main  research method  was mathematical modeling in such software systems as Mustang, RastrWin 3, MathCAD, Matlab/Simulink, PSCAD, Kortes. Each one was used in accordance with theirs application area.  RastrWin3 has wide opportunities in powerflow calculations and steady-state modes optimization. Currently, this software is a major tool in all branches of System Operator of the United Power System. The RastrWin 3 database of the Zabaikal control area was a basis for our researchings. Should be noted that Zabakal power system  is a “deadlock” as it operates apart from the Far-East power system.  Mustang was also main software for steady-state calculations and modeling electromechanial transients until recently. Now, it has been replaced by Eurostag, a more pioneering software package. Mustang has instruments to model automation components, for instance, automatic voltage regulation (AVR), automatic generation control (AGC) and so forth. Mustang as well as RastrWin3 (their databases are compatible) uses positive sequence schemes and this fact doesn’t allow  to determine electrical quantities in case of non-symmetric faults, besides, non-symmetric loads modelling is also impossible. In Tomsk Polytechnic University Shemwizard, an add-on for Mustang, was developed [7]. It automatically forms negative (NS) and zero sequence (ZS) schemes provided with positive sequence (PS) scheme. Shemwizard gave an opportunity to create the equilvalent sequence network, which is based on the RastrWin 3 PS scheme. This is achieved by varying the impedance of the branch connecting PS and NS schemes in accordance with the Fortescue method. The total number of nodes and branches in the PS scheme is 238 and 277 respectively. The model contains generators with AVR Power Engineering and Renewable Energy589 АСВI1 I2a b c a b c a b cI4a b cI5a b c a b c a b crailNeutral sectionAerial contact wireOverhead line 220 kVTrain 1Traction substation 1Train 2 Train 3 Train nI3 and AGC, transformers, loads and 110-220 kV transmission lines, the Gusinoozerskaya’s power plant buses are taken as a swing node. A part of the 220 kV traction transit is showed on the Figure 1. Mustang represents results as envelopes of complex PS and NS currents and voltages, which are time-varying. Such representation is not handy for evaluation, so by means of MathCAD an algorithm that returns instantaneous current and voltage values for each phase was developed.  For more accurate modeling of 220 kV supply system between substation Darasun and Kholbon authors used Matlab/Simulink and PSCAD. This segment includes 4 traction substations. Initial voltage levels and generating power were based on steady-state mode parameters calculated by Mustang. By means of  "MatLab / Simulink" power circuits of a locomotive, including reversible conveter and traction motors were created. Modeling were performed for three phases. When performing powerflow calculations in MatLab / Simulink, it is possible to take into account the type of traction motor. Mostly on Zabaikal railway trains travel that driven by pulse control DC motors . Recently,  locomotives with asynchronous traction motors have appeared on tracks of  Zabaikal railway Supply system of the Trans-Siberian railway uses the 1x25 kV scheme  implemented by three-winding transformers. Interfacing is conventional - two phase wires to the catenary, third phase is grounded. To reduce the non-symmetric effects in traction systems phasing combinations of traction transformers along the trajectory of a railway system was provided. In author’s opinion, Matlab/Simulink and PSCAD is not suitable for modeling ZPS, which consists of several hundred nodes, as these software packages implement advanced mathematical apparatus, hence this fact would slow down large schemes  calculation. In addition, it may be problematic to tune AVR and AGC. Kortes, a specialized software system for railway research centres was used to determine the power consumed by non-symmetric traction load. Kortes supposed to estimate modes only for railway systems (27,5 kV), however, it allows to take into consideration plain and profile conditions, train working diagram and mass of trains. It’s admitted that the mass of trains is between 4000 and 8000 tons. Modern locomotives travelling along Zabaikay railway (3ЭС5К, 2ЭС5) consume from 2 to 10 MW of power depending on mass and track grade. Should be stated that primary power supply systems are simly represented in Cortes as infinite buses, where constant voltage level is sustained. To sum up, created model allows to estimate current and voltages in each branch and node in cases of  both unbalanced short-circuit and non-symmetric loading conditions considering protective relaying and remedial action schemes operation.  IV. FINDINGS In lines of the research, situations of simultaneous train moving  were modelled. In such cases, a substantial voltage decreasing takes place in utility grid. When trains work accordingly to the schedule and supply system operates normally. If several sections of 220 kV supply system are voltage becomes below critical level (18 kV) and trains stop. On the other hand, voltage rises up to 110-130% of nominal voltage if there are no locomotives on the track. This also effects on equipment condition. Fig. 1. Connection scheme of traction substations to the longitudinal power supply line  Power Engineering and Renewable Energy590 K2U,%12624810Substation ChitalkmSubstation Mogocha200 4000   I1I2δI1δI2Time, skA, I δ, grad. iA, iB, iCs Fig. 2.  Change of PS and NS currents when connecting a train on a given segment of the railway    Fig. 3. Change of negative voltage unbalance ratio (600 experiments)  The next research area is verification of back-up relays set-point values. Calculations were performed considering electric values changing during electromechanical transient processes, still initial conditions for repeated switchings were taken into account. Experiments showed that it is necessary to calibrate thresholds of back-up transmission lines protection. In some emergency and restorative modes, it is needed to adjust setting values in back-up relays of Kharanorskaya power plant main assets. Below is a calculated example when connecting a train to the tires of traction substations. The train consumes about 3 MW. 35 kV buses of substation Karymskaya and 11 kV buses of substation Urulga were chosen for voltage monitoring. Asymmetry of currents were controlled on lines Darasun-Karymskaya and Darasun-Urulga. In the time ranges 0,04 – Power Engineering and Renewable Energy591 0,12 s and 0,2 – 0,28  s the train receives power from substation Karymsky and substation Razmahnino respectively, which causes an increase in currents of PS and the appearance of the currents of NS. On the Figure 2a, positive and negative sequence currents I1, I2 flowing through the Darasun-Karymskaya and theirs phase angles δ1, δ2  calculated  in “Mustang” are displayed. The phase currents oscillogram was obtained using RMS values and is presented in the Figure 2b. The authors tend to believe that using a probabilistic approach in modeling trains movement is preferable for powerflow calculations considering railway demand. The key difference between the calculations of traction networks from the calculations of networks with static loads is continuous change in train`s location and consumed power (due to plane and profile conditions). For experiments, it was considered that average power of train is 3 MW and traffic interval is 25 minutes. In addition, the number of trains located near the substation zone at the same time may vary. Hence, this fact causes fluctuations of voltage and current asymmetry levels. The results are given in Figure 3. Calculations showed increasing of the unit capacity and reducing of the traffic interval leads to decreasing of voltage levels and has slight influence on negative sequence voltage unbalance (NSVU) factor. According to the simulation results change in the train unit power by 5 times causes factor deviations by less than 1 %. Monte-Carlo method was used. It was shown to achieve confidence coefficient of 0,95 and accuracy of 0,1 it’s needed to perform about 600 iterations. As you can see the negative voltage unbalance ratio ratio increases to the transit end. The value of the coefficient significantly exceeds the permissible level. At present time, we are offered a range of balancing devices needed for current and voltage asymmetry compensation such as shunt compensation devices, series compensation devices, static var compensators (SVC), static synchronous compensator (STATCOM) and booster transformers. According to [6] shunt compensation devices must be operated reliably and have rapid response of control systems. Series compensation devices possess positive regulation effect that obviously is a merit of such devices. However, series compensation condensers are vulnerable against ultraharmonics, which always take place in AC traction systems. SVC and STATCOM are devices of excellent perfomance due to relatively fast control systems, however tuning them properly may be a challenging process. Installing booster transformer would be simpler and more reliable solution. It represents a three-phase transformer, which has its terminals connected in delta scheme to the catenary. Connection scheme of the booster transformer is shown on Figure 4, where 1 – supply winding, 2 – compensating winding, 3 – auxiliary winding.  АСВa b c*1Traction substation 1* *2 3Booster transformer  Fig.4. Connection scheme of the booster transformer  V. CONCLUSION Сomplex mathematical model based on several software systems, that enable fast data exchange, is more advisable for powerflow calculations considering railway systems impact. It also obtains development dynamics of the processes in the integrated three phase - single-phase electric scheme. This model may be applied for calculating parameters of booster transformer, relays set-point values correction and for maintenance planning. References [1] B. Milesevi, Iv. Uglesic, B. Filipovic-Grcic, Power quality analysis in electric traction system with three-phase induction motors. Electric Power Systems Research 138 (2016) 172–179. [2] Zakaryukin V.P., Kryukov A.V. Cherepanov A.V. Traction power sypply system mode simulation at freight train haul. Proceedings of Irkutsk State Technikal University. 2016, vol.20, no. 11, pp. 133–142 (in Russian). [3] Zakaryukin V.P., Kryukov A.V., Avdienko I.M. Modelirovanie sistem tyagovogo elektrosnabzheniya 2kh25 kV s koaksial'- nymi kabelyami i transformatorami Vudbridzha [Modeling of traction power systems of 2х25 kV with coaxial cables and Woodbridge transformers]. Izvestiya Transsiba [Journal of Transsib Railway Studies], No. 2(26), 2016, pp. 70–78. [4] International Telecommunication Union, Directives Concerning the Protection of Telecommunication Lines Against Harmful Effects from Electric Power and Electrified Railway Lines, vol. IV, International Telecommunication Union, Geneva, 1989. [5] Siemens AG Transportation Systems, Führerstand der 1116 Taurus, January 2014, (http://www.z21.eu/content/download/625/4858/file/Z21%20F%C3 %BChrerstand%20der%201116%20Taurus.pdf) [6] Romanova V.V., Hromov S.V. Investigation of stress asymmetry in the nodes of connection of traction substations to the supply network in the sections of the TRANS-Baikal railway  //  Energy of Russia in the XXI century. 2015, no. 11, pp. 14–22 (in Russian). [7] A. Zupan, A. Tomasovic Teklic, B. Filipovic-Grcic. Modeling of 25 kV Electric Railway System for Power Quality Studies, EuroCon, Zagreb, 2013 (2013). [8] M.H. Albadi, A.S. Hinai, A.H. Al-Badi, M.S. Al Riyami, S.M. Hinai, R.S. Al Abri, Measurements and evaluation of flicker in high voltage networks, in: Int. Conference on Renewable Energies and Power Quality, April 2014, Cordoba, Spain. 2014.  Power Engineering and Renewable Energy592

Powerflow calculations in powersystems considering traction load

http://earchive.tpu.ru/bitstream/11683/57510/1/conference_tpu-2019-C127_p589-592.pdf

Powerflow calculations in powersystems considering traction load

Abstract

Similar works

Full text

Available Versions

Electronic archive of Tomsk Polytechnic University