Traction Power Substation Load Analysis with Various Train Operating Styles and Substation Fault Modes

energies

Article Traction Power Substation Load Analysis with Various Train Operating Styles and Substation Fault Modes

Zhongbei Tian 1 , Ning Zhao 1,* , Stuart Hillmansen 1, Shuai Su 2 and Chenglin Wen 3 1 Department of Electronic, Electrical and Systems Engineering, University of Birmingham, Birmingham B15 2TT, UK; [email protected] (Z.T.); [email protected] (S.H.) 2 State Key Laboratory of Rail Traﬃc Control and Safety, Beijing Jiaotong University, Beijing 100044, China; [email protected] 3 School of Automation, Hangzhou Dianzi University, Hangzhou 310018, China; [email protected] * Correspondence: [email protected]

Received: 30 April 2020; Accepted: 27 May 2020; Published: 1 June 2020

Abstract: The simulation of railway systems plays a key role in designing the traction power supply network, managing the train operation, and making changes to timetables. Various simulation technologies have been developed to study the railway traction power network and train operation independently. However, the interactions between the load performance, train operation, and fault conditions are not fully understood. This paper proposes a mathematical modeling method to simulate the railway traction power network with a consideration of a multi-train operation, driving controls, under-voltage traction, and substation fault modes. The network voltage, power load demands, and energy consumption according to the existing operation are studied. The hotspots of the power supply network are identiﬁed based on an evaluation of the train operation and power demand. The impact of traction power substation (TPSS) outage and a short circuit on the power supply network have been simulated and analyzed. The simulation results have been analyzed and compared with those of a normal operation. A case study based on a practical metro line in Singapore is developed to illustrate the power network evaluation performance.

Keywords: railway; traction power systems; energy consumption; load demand; fault analysis

1. Introduction Due to the increasing passenger flow demand and environmental concerns, upgrading the railway traction power supply system and energy-efficient train control is becoming an important aspect of train operations. The traction power supply network system plays a key role in maintaining the sustainability of train operations. A number of studies have proposed and developed different solutions for calculating and simulating the railway power supply network system from different theoretical points of view to understand traction power flow. Chymera proposed a new simulation model to calculate the tram power supply system, in order to allow a detailed drive model to be included in a light rail system [1]. Chang introduced a comprehensive modeling method for the DC railway system with induction motor driven trains [2]. Goodman applied a diakoptics approach to DC railway power supply network solutions [3,4]. Cai developed a complex DC-fed traction power network with a multibranched lines model, which can simulate return circuits to consider stray current [5]. A railway network with controllable power electronic devices was solved by the current injection power flow algorithm and tested in a complex case study [6]. Based on Thévenin’s equivalent, the power flow analysis was improved to achieve fast convergence, allowing the optimized organization of train operations [7].

Energies 2020, 13, 2788; doi:10.3390/en13112788 www.mdpi.com/journal/energies Energies 2020, 13, 2788 2 of 18

With the introduction of regenerative trains in modern metro systems, energy evaluation of the traction power supply network has become more complex, requiring a more accurate model to solve the power flow [8]. The technologies of energy efficiency improvement in DC railway systems have been reviewed in [9]. The energy efficiency of the network with regeneration has been evaluated, and the energy consumption could be reduced by around 30% using a Monte Carlo algorithm [10]. By analysing the power characteristics of regenerative braking, the medium-voltage feedback system was proposed to feed regeneartive energy back to the AC grid [11]. Inverting substations have been implemented in some metro networks to improve the receptivity of regenerative braking power. A hybrid simulation method of an inverting substation has been proposed in [12]. The control strategies of inverters can be optimized according to the train operation characteristics. The power limitation, trigger voltage, and virtual internal resistance of inverting substations have been optimized to reduce the global costs of investment and energy by 16.8% [13]. The integration of reversible substations and photovoltaics (PV) has been illustrated to improve the utilization of regenerative and renewable energy [14]. Ye developed a simualtion model to identify the optimal operation control for improving the network capacaity [15]. Lu implemented different exact algorithms and exhaustive searching methods to identify the optimal single train operation trajectory and power management strategies for minimizing the energy consumption [16,17]. Bocharnikov and Chang developed different novel approaches to calculate the most appropriate train coasting style [18–21]. Howlett implemented Pontryagin theory to identify the optimal train control with a relatively small compuataion time [22,23]. The author developed an approach to help the train driver identify an optimal control strategy for reducing the train’s energy consumption. This approach has been applied in daily services and achieved satisfying results [24,25]. As train services are becoming more frequent, the railway power system is receiving a higher demand and facing ever more pressure to provide stability and a reliable power supply. Chen presented a fault tree analysis to evaluate railway power systems and identified the maintenance impact on the overall system reliability [26]. An improved sequential Monte Carlo method has been used to realize a quantitative reliability evaluation of metro traction power systems [27]. Huh proposed a superconducting fault current limiter to reduce the impact of voltage unbalance on the Scott transformer [28]. Huang from Taiwan Railway Administration carried out a series of experiments and tests to understand the reasons why short-circuit current was present on their railway lines [29,30]. Besides power quality analysis, fault allocation technologies are becoming more important for rail operators to facilitate maintenance progress. Cho proposed a novel fault-location scheme to be used in Korean railway lines with special autotransformer feeding systems [31,32]. Wang developed fault sectionalization and an error compensation solution to locate the catenary fault on the railway power supply system. Moreover, the solution has been verified in site trials and achieved positive results [33]. For DC traction power systems, Jin also implemented a 600 Hz harmonic ripple fault detection approach to identify single pole-to-earth faults in the London Underground fourth-rail power supply system [34]. Park proposed a probe unit to detect the first single ground fault in metro systems [35,36]. However, most of the previous works have only focused on one of the railway subsystems (e.g., transformer, power short circuit, etc.). In practice, the performance of the railway power supply network is affected by several factors, including the train operation status, substation power load, and transmission line status. Therefore, it is necessary to develop a holistic railway network model with both a train kinematic model and power network system to facilitate understanding of the train power supply correctly and accurately. In this paper, train motion modeling is introduced, followed by a power network calculation solution. The paper then proposes a fault identification method to solve four common power supply faults. Finally, a case study based on a Singapore metro line is presented. The calculated fault identification result is compared with practical data, which proves that the proposed method achieves a great accuracy and efficiency. Energies 2020, 13, 2788 3 of 18

2. Modeling Formulation

2.1. Train Modeling Energies 2020, 13, x FOR PEER REVIEW 3 of 18 2.1.1. Train Kinematics 2.1.1. Train Kinematics TrainEnergies kinematics2020, 13, x FOR PEER modeling REVIEW can be developed using Lomonossoﬀ’s equations as the3 of general 18 equationsTrain of vehicle kinematics motion. modeling The equationscan be developed are based using on Lomonossoff’s Newton’s second equations law ofas motion,the general and are equations of vehicle motion. The equations are based on Newton’s second law of motion, and are subject2.1.1. to theTrain train Kinematics and route constraints [10,37]. subject to the train and route constraints [10,37]. Train kinematics modeling can be developed using Lomonossoff’s equations as the general  d2s equations of vehicle motion.M Thee f f equations2 = F(v )are Fbasedgrad onRcu Newton’s(v) Rmo second(v) law of motion, and are  dt =() − −() −() subject to the train and route constraints [10,37].− − − Fgrad = Mrs gsin(θ) =() (1)  A  Rcu(v)= Mrs g (1)  RAD ( ) ( ) ( )  = −2− − Rmo((v))== a +bv+ cv | | =() () =+|| + (1) where s, t, and v represent the train position, time, and speed, respectively; F is the traction/braking where s, t, and v represent the train() position,= time, and speed, respectively; F is the traction/braking force; Rmo is the train resistance (known as the Davis equation) with the constants a, b, and c [38]; force; Rmo is the train resistance (known() =+ as the| Davis| + equation) with the constants a, b, and c [38]; Rcu Rcu is the curve resistance; A is a constant number, which is set at 600 in this study; RAD is the curve is the curve resistance; A is a constant number, which is set at 600 in this study; RAD is the curve radius;whereg is thes, t, gravitationaland v represent acceleration; the train position,Fgrad istime, a component and speed, forcerespectively; of the weight F is the ( θtraction/brakingis the slope angle); radius; g is the gravitational acceleration; Fgrad is a component force of the weight ( is the slope angle); and Mforce;is R themo is train the train mass. resistance Typical (known train traction as the Davis and equation) resistance with characteristics the constants are a, b, shown and c [38]; in Figure Rcu 1. andrs Mrs is the train mass. Typical train traction and resistance characteristics are shown in Figure 1. is the curve resistance; A is a constant number, which is set at 600 in this study; RAD is the curve The dataThe data comes comes from from rolling rolling stock stock manufacturing manufacturing andand will be be used used for for the the case case study study in Section in Section 4. 4. radius; g is the gravitational acceleration; Fgrad is a component force of the weight ( is the slope angle); and Mrs is the train mass. Typical train traction and resistance characteristics are shown in Figure 1. The data comes from rolling stock manufacturing and will be used for the case study in Section 4.

FigureFigure 1. 1.Train Train traction traction and resistance resistance characteristics. characteristics.

The trainThe train motion motion can can be be further further described described basedbased on on the the time time state, state, as asshown shown in Equation in Equation (2): (2): Figure 1. Train traction and resistance characteristics. =  . = = () + () + () + () + () The train motion s canv be further described based on the time state, as shown in Equation (2):(2)  . Mv = u F(v ) + u F ( v ) + R ( v ) + R ( v ) + F ( s) (2)  rs f tr b br mo cu grad  = Fad = uMrs g where Ftr(v) and uf are the traction = effort() + and the( corresp) +onding() + control() + signal,( respectively;) Fbr(v) and(2) ub are the braking effort and= the control signal, respectively; and F ad is the adhesion traction, which is wheretheF trfriction(v) and betweenuf are the the traction steel rail e ffandort andthe train the correspondingdrive wheels. It controlis important signal, to respectively;note that the Ftrainbr(v) and where Ftr(v) and uf are the traction effort and the corresponding control signal, respectively; Fbr(v) and ub aretraction the braking effort applied effort and on the the track control should signal, be smal respectively;ler than the andadhesionFad is traction, the adhesion otherwise traction, the wheel which is ub are the braking effort and the control signal, respectively; and Fad is the adhesion traction, which is the frictionwill start between to slip. theu is steel the railadhesion and the coefficient, train drive which wheels. depends It ison important the train tospeed note and that track the train the friction between the steel rail and the train drive wheels. It is important to note that the train tractionenvironment, effort applied as shown on the in Figure track should2. be smaller than the adhesion traction, otherwise the wheel traction effort applied on the track should be smaller than the adhesion traction, otherwise the wheel will start to slip. u is the adhesion coefficient, which depends on the train speed and track environment, will start to slip. u is the adhesionμ coefficient, which depends on the train speed and track as shown in Figure2. environment, as shown in Figure0.6 2. maximum values in dry conditions 0.5

0.4 μ 0.6 0.3 averagemaximum values in dryvalues conditions in dry conditions 0.5 average values in rain 0.2 0.4 0.1 minimum values in frost

0.3 average values in dry conditions V (km/h) 0 50 100 150 average 200 values 250 in rain 300 0.2 minimum values in frost 0.1Figure 2. Railway adhesion traction. V (km/h) 0 50 100 150 200 250 300 2.1.2. Driving Controls FigureFigure 2. 2.Railway Railway adhesion traction. traction.

2.1.2. Driving Controls

Energies 2020, 13, 2788 4 of 18

2.1.2.Energies Driving 2020, 13, Controlsx FOR PEER REVIEW 4 of 18

BasedBased on the the equation equation discussed discussed in in the the previous previous section, section, four four typical typical train train movement movement modes modes can canbe produced, be produced, as shown as shown in Table in Table 1 1and and Figure Figure 3.3. For For example, when thethe traintrain isis operatingoperating in in the the tractiontraction mode,mode, the the signal signal (u (fu)f is) is 1. 1. The The traction traction power power is thenis then generated generated to increase to increase the train the train speed speed until reachinguntil reaching the speed the speed restrictions. restrictions. When theWhen train the speed train isspeed small, is the small, output the traction output etractionffort is constant.effort is Afterconstant. the outputAfter the power output reaches power its reaches maximum its maximum value, the value, traction the traction effort keeps effort decreasing, keeps decreasing, as shown as inshown Figure in1 .Figure However, 1. However, in practice, in practice, a reduced a tractionreduced etractionffort is usuallyeffort isapplied usually toapplied the train to tothe improve train to passengerimprove passenger comfort, especiallycomfort, especia in off-peaklly in off-peak services. services. In the cruising In the cruising mode, the mode, traction the traction power ispower only reducedis only reduced to overcome to overcome the motion the resistance,motion resistance, in order toin keeporder the to trainkeep runningthe train at running a constant at a speed. constant speed. Table 1. Control signals in the four movement modes. Table 1. Control signals in the four movement modes. Movement Mode uf ub Equations . MovementTraction Mode u 1f ub 0 Mtrv = FtrEquations(v) + Rmo(v ) + Rcu(v) + Fgrad(s) Traction 1 0 =. () +() +() +() Cruising 1 0 Mtrv = Ftr(v) + Rmo(v) + Rcu(v) + Fgrad(s) = 0 Cruising 1 0 =() +. () +() +() =0 Coasting 0 0 M v = R (v) + R (v) + F (s) =tr () +mo ()cu+ (grad) Coasting 0 0 . Braking 0 1 Mtrv = F (v) + Rmo(v) + Rcu(v) + F (s) Braking 0 1 =() +br () +() +grad()

FigureFigure 3.3. Four train movement modes.

2.1.3.2.1.3. Train PowerPower DemandDemand TheThe train mechanical power power demand demand can can be be determined determined by by the the traction traction controls, controls, which which can can be P F becalculated calculated by byEquation Equation (3), (3), where where me is isthe the train train mechanical mechanical power, power, tr is thethe instantaneousinstantaneous v P tractivetractive eeffort,ﬀort, andand is is the the train train speed. speed. The The electric electric traction traction powerpower ofof thethe traintrain ((el)) cancan bebe expressedexpressed byby (4), where ηis is the the e efficiencyﬃciency of of power power chain chain conversion conversion betweenbetween mechanicalmechanical andand electricalelectrical powerpower (assumed(assumed asas 85%85% inin thisthis paper).paper). The mechanical power is positive when the train is in traction mode, whilewhile itit isis negativenegative whenwhen thethe traintrain isis braking.braking.

Pme ==Ftr ×v (3)(3) ×  Pme ≥0  i f Pme 0 P =  η ≥ (4) el=  (4) Pme η i f Pme < 0 × × <0 Besides the electric traction power demand, the air conditioning and lighting on the train require electricBesides auxiliary the electric power. traction The auxiliary power demand, power can the be ai highr conditioning in the summer and lighting or winter on the due train to a require higher demandelectric auxiliary for cooling power. or heating The auxiliary devices, power respectively. can be high The in total the trainsummer power or winter demand due (P totrain a _higherdemand) candemand be calculated for cooling by or summing heating devices, the electric respectively. traction power The total demand train power and auxiliary demand power (_ (Paux) using) can Equationbe calculated (5). The by trainsumming power the demand electric is atraction dynamic power value demand in the time and domain, auxiliary which power is an ( input ) ofusing the modelEquation of the(5). tractionThe train power power network. demand is a dynamic value in the time domain, which is an input of the model of the traction power network. Ptrain_demand = Pel + Paux (5) _ = + (5)

2.2. Traction Power Network Modeling

Energies 2020, 13, 2788 5 of 18

2.2.Energies Traction 2020, Power13, x FOR Network PEER REVIEW Modeling 5 of 18

2.2.1.2.2.1. Traction Traction Power Power Substation Substation InIn the the DC DC railway railway traction traction power power supply supply network, network, a a traction traction power power substation substation (TPSS) (TPSS) is is used used to to powerpower trains. trains. Some Some modern modern DC DC railway railway systems systems implement implement both both rectifiers rectifiers and and inverters inverters in in the the TPSS.TPSS. TheThe rectifier rectifier converts converts AC AC power power to to DC DC power power to to supply supply trains trains requiring requiring traction traction power, power, while while the the inverterinverter converts converts DC DC power power to to AC AC power power and and transmits transmits regenerative regenerative braking braking power power from from trains trains back back toto the the AC AC network. network. TheThe rectifier rectifier output output voltage voltage normally normally decreases decreases when when the the load load power power oror loadload currentcurrent varyvary [[39].39]. InIn this this simulation, simulation, the the output output voltage voltage characteristic characteristic of of the the TPSS TPSS is is simplified simplified by by a a piecewise piecewise function, function, asas shown shown in in Figure Figure4. 4. The The rectifier rectifier output output voltage voltage curve curve shown shown as as ‘A-B’ ‘A-B’ can can be be calculated calculated using using

EquationEquation (6). (6).rrec is ais virtual a virtual resistance, resistance, which which represents represents the decreasing the decreasing characteristics characteristics of the rectifier of the voltage,rectifier butvoltage, does but not does dissipate not dissipate any resistive any resistive power losses. power Thelosses. value The of valuerrec is of derminated is derminated by the ratedby the power rated andpower current and current of the rectifier. of the rectifier. The inverter The inverter used in used a railway in a railway substation substation can be can based be based on a thyristoron a thyristor or IGBT, or IGBT, where where the output the output voltage voltage can be can fully be controlled fully controlled to follow to follow a fixed a slope fixed likeslope ‘C-D’ like in‘C-D’ Figure in4 .Figure The inverter 4. The output inverter voltage output can voltage be derived can usingbe derived Equation using (7), whereEquationrinv (7),represents where the sloperepresents rate, but the does slope not rate, dissipate but do poweres not dissipate losses. rinv powercan be losses. modified bycan the be controlmodified strategies by the control of the inverterstrategies to meetof the the inverter requirement to meet of the power requiremen receptivity.t of The power trigger receptivity. voltage ofThe the trigger inverter voltage is shown of the at pointinverter ‘C’, whichis shown is normally at point higher‘C’, which than is the normally no-load voltagehigher than of the the rectifier no-load at pointvoltage ‘B’ of to avoidthe rectifier current at circulationpoint ‘B’ to between avoid current the rectifier circulation and inverter. between the rectifier and inverter. = − × (6) Vrec = V rrec Irec (6) noload − × = − × (7) V = V r I (7) inv trigger − inv × inv

Figure 4. Traction power substation voltage regulation. Figure 4. Traction power substation voltage regulation. 2.2.2. Power Network Circuit 2.2.3.The Power traction Network power Circuit network consists of substations, the contact line, return rails, and multiple trains. Figure5 illustrates an equivalent circuit of a railway traction power network. A paralleling post The traction power network consists of substations, the contact line, return rails, and multiple is represented by a conductor connecting the contact lines on the up and down tracks. The paralleling trains. Figure 5 illustrates an equivalent circuit of a railway traction power network. A paralleling post can improve the line voltage and reduce transmission power losses. The contact lines are modeled post is represented by a conductor connecting the contact lines on the up and down tracks. The by conductor resistors split by trains, substations, and parallel posts. The return rails on both tracks paralleling post can improve the line voltage and reduce transmission power losses. The contact lines can be lumped for simpliﬁcation, which reduces the computing time with a reasonably low error [40]. are modeled by conductor resistors split by trains, substations, and parallel posts. The return rails on The resistance of the contact line and lumped rail is determined by the length and resistivity of the both tracks can be lumped for simplification, which reduces the computing time with a reasonably conductor given by (8), where ρ and ρ are the resistivity of the contact line and return rail per track, low error [40]. The resistance cof the contactr line and lumped rail is determined by the length and respectively. The resistance is split by trains and substations. When trains move, the resistance of resistivity of the conductor given by (8), where and are the resistivity of the contact line and the network will be changed according to the location of trains. Based on train motion modeling, return rail per track, respectively. The resistance is split by trains and substations. When trains move, the resistance of the network will be changed according to the location of trains. Based on train motion modeling, the location of trains can be obtained and the length of resistance between trains and substations can be calculated. The earthing resistance is connected with every substation and train.

Energies 2020, 13, 2788 6 of 18 the location of trains can be obtained and the length of resistance between trains and substations can Energies 2020, 13, x FOR PEER REVIEW 6 of 18 be calculated.Energies 2020, The13, x FOR earthing PEER REVIEW resistance is connected with every substation and train. 6 of 18 ( R==×L ρ c =×c × ρr (8) Rr = L (8) =×2 (8) =×× 2 2

N1 N2 N3 N4 N5 N6 N7Contact line N8 N1 N2 N3 N4 N5 N6 N7Contact line N8

T1 + T1 + T2 + + - + - T2 + - - - -

Return rail Return rail

T4 T4 T3 T3

Contact line Contact line

Motoring Braking Parallelling Traction power Motoring Conductor Earthing train Brakingtrain Parallelling Tractionsubstation power train post Conductorresistance Earthingresistance + substation train post resistance resistance + - -

Figure 5. An equivalent circuit of the railway power network. FigureFigure 5. 5.An An equivalent equivalent circuit of of the the railway railway power power network. network. 2.3. Energy Flow and Evaluation 2.3. Energy2.3. Energy Flow Flow and and Evaluation Evaluation By integrating the train and traction power network modeling, the energy flow through the By integratingBy integrating the trainthe train and and traction traction power power network network modeling, modeling, the the energy energy ﬂow flow through through the the whole whole network can be evaluated and analyzed. The energy flow of railway systems can be divided networkwhole can network be evaluated can be evaluated and analyzed. and analyzed. The energy The energy ﬂow of flow railway of railway systems systems can be can divided be divided into four into four levels, as shown in Figure 6. levels,into as four shown levels, in as Figure shown6. in Figure 6.

Figure 6. Typical energy flow chart of metros. FigureFigure 6. 6.Typical Typical energy flow ﬂow chart chart of ofmetros. metros.

Energies 2020, 13, 2788 7 of 18

The railway energy is fed from the AC electricity grid. The traction substation energy is rectiﬁed from the AC network and the surplus regenerative braking energy can be inverted back to the AC network. Therefore, the global system energy consumption (Es) can be calculated using Equation (9), where Erec is the energy from the rectiﬁer and Einv is the energy from the inverter.

Es = Erec E (9) − inv

Electricity through rectifiers and inverters can generate substation losses (Esl). The rectifier power efficiency (ηrec) and inverter power efficiency (ηinv) are assumed to be constants in this paper, with values of 97% and 95%, respectively. Therefore, the substation losses can be calculated using Equation (10). E = Erec (1 ηrec) + E (1 η ) (10) sl × − inv × − inv The electricity from substations is transmitted to the DC catenary system to supply moving trains. The current through conductor rails generates transmission loss (Etl), which is determined by the resistance of the conductor rail (rconductor) and the current passing through it using Equation (11). Z E = r i2dt (11) tl conductor ×

The train energy (Etr) from the DC catenary system is used for the traction and auxilary system. Part of the traction energy is dissipated by onboard converters and motion resistance. The train obtains kinetic energy, which is ﬁnally dissipated by braking systems. The train energy can be expressed in (12), where Ecl is the conversion loss, Emr is the energy required to overcome motion resistance, Eeb is energy for the electro-braking system, and Eaux is energy for the auxilary system.

Etr = Ecl + Emr + Eeb + Eaux (12)

Part of the energy for the electro-braking system can be regenerated and reused by the auxiliary system directly or transmitted back to the catenary system. However, too much regenerative power can increase the train voltage. To protect the train from overvoltage, the surplus electro-braking energy is dissipated by the braking rheostat. To evaluate the efficiency of regenerative braking energy, the regeneration efficiency (ηreg) is defined using Equation (13), where Ereg is the useable regenartive braking energy and Eeb_r is the energy dissipated by the braking rheostat.

Ereg Ereg ηreg = = (13) Eeb Eeb_r + Ereg

From the railway system level, the energy flow should be consistent with the energy conservation equation shown in (14), where the global system energy consumption is equal to the sum of substation loss, transmission loss, and train energy deduced by the regenerative braking energy. The system energy flows have complicated relationships with each other. A comprehensive evaluation of the infrastructure parameters and operation controls is normally required to identify the energy flow characteristics of a practical railway line.

Erec E = E + E + Etr Ereg (14) − inv sl tl − Energies 2020, 13, 2788 8 of 18

3. Fault Identiﬁcation

3.1. Under-Voltage Traction When the train power demand is known, the electric circuit of the traction power network can be solved using power ﬂow analysis algorithms. The train voltage and current can be obtained, which is consistent with Equation (15). Energies 2020, 13, x FOR PEER REVIEW P = I V 8 (15)of 18 train train × train low networkTrains normally voltage when collect the enough traction power power from network the traction is weak. power In order network to protect to meet the train the tractiontraction demands.devices and However, power networks, sometimes, the the control traction devices power demandon-board might will adapt not be the fully train supplied power due demand. to the low To networkstudy the voltage train and when network the traction load performance, power network a comprehensive is weak. In order representation to protect the trainof the traction train model devices is andrequired. power networks, the control devices on-board will adapt the train power demand. To study the trainThe and British network Standard load performance, in Railway aApplications-P comprehensiveower representation Supply and of Rolling the train Stock model introduces is required. the maximumThe British allowable Standard train in current Railway against Applications-Power the train voltage, Supply as shown and Rolling in Figure Stock 7 [41]. introduces The train the maximumtraction modes allowable can be train categorized current against into three the train regions voltage, according as shown to the in Figure pantograph7[ 41]. voltage The train levels. traction In modesregion can1, the be train categorized voltage into is lower three regionsthan the according lowest non-permanent to the pantograph voltage voltage Vmin. levels. The In train region only 1, thereceives train the voltage energy is lowerto supply than the the auxiliary lowest non-permanent system, and not voltage the train Vmin. traction The system. train only In region receives 2, the energytrain voltage to supply is between the auxiliary Vmin system,and aVn, and where not the train is the traction nominal system. voltage In of region the system 2, the trainand voltage is the isknee between point Vminfactor, and which aVn, is where lowerV nthanis the 1 (normally nominal voltage between of the0.8 systemand 0.9). and Ina thisis the region, knee pointthe under- factor, whichvoltage is traction lower than mode 1 (normally is activated. between The 0.8train and tracti 0.9).on In current this region, is limited, the under-voltage which means traction that the mode train is activated.cannot receive The trainthe maximum traction current traction is limited, power. whichIn region means 3, the that train the trainvoltage cannot is higher receive than the Vn, maximum where tractionthe train power. is in the In region normal 3, thetraction train voltagemode. The is higher maximum than Vn, traction where power the train of is the in thetrain normal can be traction fully mode.supplied The from maximum the power traction network. power Taken of from the train the British can be Standard fully supplied in Railway from Applications-Supply the power network. TakenVoltages from of theTraction British Systems Standard [42], in Railwaythe voltage Applications-Supply characteristics, including Voltages under-voltage of Traction Systems and over- [42], thevoltage voltage levels characteristics, for DC railway including systems, under-voltage are specifie andd in over-voltage Table 2. In levelsthis paper, for DC the railway under-voltage systems, aretraction speciﬁed mode in is Table considered2. In this in paper, the substation the under-voltage load analysis traction to modeevaluate is considered the traction in thepower substation supply loadcapability. analysis to evaluate the traction power supply capability.

Itrain_max

Imax

Under-voltage Normal tr action traction

No traction Region 1 Region 2 Region 3 Iaux

Vtrain V a×V V min n max Figure 7. Current limitation of a traction train. Table 2. Permissible voltage limits for DC railways [42]. Table 2. Permissible voltage limits for DC railways [42]. DC Railway Voltage Lowest Non-Permanent Highest Non-Permanent Rated Voltage Vn (V) DCLevel Railway (V) LowestVoltage Non-Permanent Vmin (V) Rated Voltage HighestVoltage Non-Permanent Vmax (V) Voltage Level (V) Voltage Vmin (V) Vn (V) Voltage Vmax (V) 600 400 600 800 600750 500400 750600 800 1000 7501500 1000500 1500750 1000 1950 1500 1000 1500 1950

resistance is considered in the simulation. The earthing leakage resistance coefficient () for the railway network is around 10 Ω·km. For the lumped rail network in Figure 5, the earthing resistance is calculated by (16).

2× = (16)

Energies 2020, 13, 2788 9 of 18

3.2. Rail Potential For DC railway power systems, trains collect current from the overhead line or third rail and then return the current by the running rails. The return current will increase the rail potential, which is determined by the train traction power and location. To understand the rail potential, the earthing resistance is considered in the simulation. The earthing leakage resistance coeﬃcient (Ce) for the railway network is around 10 Ω km. For the lumped rail network in Figure5, the earthing resistance is · calculated by (16). 2 Ce Energies 2020, 13, x FOR PEER REVIEW R = × 9 (16)of 18 e L

3.3.3.3. TPSS Outage AA tractiontraction powerpower substationsubstation (TPSS) can be switchedswitched ooffff whenwhen therethere isis aa faultfault currentcurrent oror duringduring maintenance.maintenance. TheThe railwayrailway traction traction power power network network should should supply supply enough enough power power to to trains trains when when one one of theof the substations substations is switched is switched off. To off. switch To switch off one o substationff one substation in the simulation, in the simulation, the rectifier the resistance rectifier (resistancerrec) in (6) ( is set) in to (6) a large is set value, to a large which value, is 1 which MΩ in is this 1 M paper.Ω in this Therefore, paper. Therefore, the output the current output from current the rectifierfrom the substation rectifier substation is limited is to limited zero. to zero.

3.4.3.4. Short Circuit AA short-circuitshort-circuit faultfault hashas aa signiﬁcantsignificant impactimpact onon thethe railrail powerpower network.network. FigureFigure8 8 presents presents the the topologytopology ofof aa short-circuitshort-circuit fault, fault, which which connects connects the the contact contact line line with with the the earth. earth. To To study study the the impact impact of aof short-circuit a short-circuit fault, fault, a small a small resistance resistance is inserted is inserted into into the the network network circuit circuit in Figure in Figure5. The 5. valueThe value of the of short-circuitthe short-circuit resistance resistance will will also also aﬀect affect the performancethe performance of the oftraction the traction power power supply. supply.

AC utility grid

…... …...

Up direction Short circuit

Return rail

Down direction

Figure 8. Topology of a short-circuit fault. Figure 8. Topology of a short-circuit fault. 4. Case Studies 4. Case Studies 4.1. Simulation Parameters 4.1. SimulationA mathematical Parameters simulation was developed in a MATLAB script based on the methods illustrated in theA previous mathematical section. simulation In order was to evaluate developed and in identifya MATLAB the script developed based method,on the methods a case studyillustrated was carriedin the previous out based section. on a metro In order line into Singapore. evaluate an Thed identify line is 51 the km developed long, with method, 32 passenger a case stations study andwas 27carried substations. out based In on total, a metro 26 of line all substationsin Singapore. are The implemented line is 51 km with long, rectiﬁers with 32 forpassenger the traction stations power and supply27 substations. and 10 of In them total, are 26 implemented of all substations with are inverters. implemented TPSS-2 iswith only rectifiers implemented for the with traction an inverter power withoutsupply and a traction 10 of them power are supply. implemented The rated with power inverter for thes. tractionTPSS-2 is substation only implemented is 4 MW, except with an for inverter TPSS-6, whichwithout has a beentraction upgraded power tosupply. 6 MW. The The rated dwell power time andfor the terminal traction time substation are 40 and is 2004 MW, s, respectively. except for TheTPSS-6, rolling which stock has uses been a DC upgraded 750 V third to rail6 MW. power The supply dwell systemtime and with terminal a regenerative time are braking 40 and system. 200 s, Therespectively. train characteristics The rolling are stock shown uses in a TableDC 7503. V third rail power supply system with a regenerative braking system. The train characteristics are shown in Table 3.

Table 3. Train characteristics.

Parameters Value/Equation Train mass with passengers, tonnes 301 Train formation 3M3T Train length, m 148 Rotary allowance 0.08 Train Resistance, N/tonne 3.49 + 0.039 v + 0.00066 v2 (v: km/h) Maximum traction and braking power, kW 2518 Maximum operation speed, km/h 80 Maximum traction effort, kN 351

Energies 2020, 13, 2788 10 of 18

Table 3. Train characteristics.

4.2.4.2. Train Train Motion Motion Si SimulationSimulationmulation ResultsResults BasedBased on on the the train train traction traction characteristics characteristics and and line line data data shown shown above, above, the the traintrain motionmotion simulationsimulation resultsresultsresults are are presented presented inin Figures Figures9 99 and andand 10 10.10.. TheTheThe singlesinglesingle journey journeyjourney times timestimes for forfor thethethe up-directionup-directionup-direction operation operationoperation and andand down-directiondown-direction operation operation are are 4917 4917 and and 4958 4958 s, s, respecti respectively.respectively.vely. ItIt can can be be observed observed that that the the train train does does not not applyapply thethethe coasting coastingcoasting mode modemode throughout throughoutthroughout the thethe journey, journey,journey, in order inin orderorder to achieve toto achieveachieve a smaller aa smallersmaller journey journeyjourney time and timetime greater andand greaterlinegreater capacity. lineline capacity.capacity. Furthermore, Furthermore,Furthermore, the train thethe maximum traintrain mama operationximumximum operationoperation speeds are speedsspeeds always areare 5 alwaysalways km/h slower 55 km/hkm/h than slowerslower the thanlinethan speed thethe lineline restrictions, speedspeed restrictions,restrictions, in order to inin reduce orderorder to theto reducereduce possibility thethe possibilitypossibility of activating ofof activating theactivating automatic thethe trainautomaticautomatic protection traintrain protection(ATP)protection system (ATP)(ATP) by mistake.systemsystem byby mistake.mistake.

FigureFigure 9. 9. TrainTrain operation operation (up-direction (up-direction with with the the station station speed speed limit limit at at 40 40 km/h).kmkm/h)./h).

FigureFigure 10. 10. TrainTrain operation operation (down-direction (down-direction with with the the station station speed speed limit limit atat 4040 km/h).kmkm/h)./h). 4.3. Energy Consumption with a Normal Operation 4.3.4.3. EnergyEnergy ConsumptionConsumption withwith aa NormalNormal OperationOperation The simulation results of energy consumption will be analyzed in this section. In this case study, TheThe simulationsimulation resultsresults ofof energyenergy consumptionconsumption willwill bebe analyzedanalyzed inin thisthis section.section. InIn thisthis casecase study,study, the speed restriction along stations was 40 km/h. Since the multi-train operation was repeated every thethe speedspeed restrictionrestriction alongalong stationsstations waswas 4040 km/h.km/h. SiSincence thethe multi-trainmulti-train operationoperation waswas repeatedrepeated everyevery headwayheadway period,period, thethe energyenergy consumptionconsumption duringduring thethe headwayheadway periodperiod waswas compared,compared, asas shownshown inin TableTable 4.4. TheThe resultsresults showshow aa comparisoncomparison ofof 1010 scenarscenariosios withwith differentdifferent auxiliaryauxiliary powerspowers (0(0 oror 480480 kW)kW) andand headwaysheadways (90,(90, 100,100, 120,120, 300,300, andand 600600 s).s). TheThe enenergyergy consumptionconsumption resultsresults denotedenote thethe following:following: 1.1. WhenWhen thethe auxiliaryauxiliary powerpower isis 00 kW,kW, therethere isis somesome energyenergy invertedinverted byby substations.substations. TheThe invertedinverted energyenergy increasesincreases whenwhen thethe headwayheadway increases.increases. WhenWhen thethe headwayheadway isis short,short, mostmost regenerativeregenerative brakingbraking energyenergy isis usedused byby motoringmotoring trainstrains inin DCDC systems;systems; 2.2. WhenWhen thethe auxiliaryauxiliary powerpower isis 480480 kW,kW, notnot muchmuch enenergyergy isis invertedinverted byby substations.substations. TheThe invertedinverted energyenergy whenwhen thethe headwayheadway isis 90,90, 100,100, andand 120120 ss isis zero,zero, andand thethe invertedinverted energyenergy increasesincreases aa littlelittle whenwhen thethe headwayheadway isis 300300 andand 600600 s.s. ThisThis isis becausbecausee thethe traintrain auxiliaryauxiliary usesuses aa lotlot ofof regenerativeregenerative brakingbraking energy;energy; 3.3. TheThe timetime intervalinterval isis 11 ss inin thethe simulation.simulation. WhenWhen thethe headwayheadway isis 9090 s,s, therethere areare 110110 trainstrains inin thethe network.network. Therefore,Therefore, therethere areare 99009900 traintrain secondsseconds inin thethe simulationsimulation ofof eacheach scenario.scenario. TheThe timetime ofof

Energies 2020, 13, 2788 11 of 18 headway period, the energy consumption during the headway period was compared, as shown in Table4. The results show a comparison of 10 scenarios with di ﬀerent auxiliary powers (0 or 480 kW) and headways (90, 100, 120, 300, and 600 s). The energy consumption results denote the following:

1. When the auxiliary power is 0 kW, there is some energy inverted by substations. The inverted energy increases when the headway increases. When the headway is short, most regenerative braking energy is used by motoring trains in DC systems; 2. When the auxiliary power is 480 kW, not much energy is inverted by substations. The inverted energy when the headway is 90, 100, and 120 s is zero, and the inverted energy increases a little when the headway is 300 and 600 s. This is because the train auxiliary uses a lot of regenerative braking energy; 3. The time interval is 1 s in the simulation. When the headway is 90 s, there are 110 trains in the network. Therefore, there are 9900 train seconds in the simulation of each scenario. The time of under-voltage means the amount of time when the under-voltage traction mode occurs. When the auxiliary power is zero, the time of under-voltage is very low. However, when the auxiliary power is 480 kW, under-voltage happens 312 times during the operation. The time of under-voltage decreases when the headway increases; 4. The regenerative energy eﬃciency is very high for all of the scenarios, and is between 99% and 100%. This denotes that nearly all of the electro-braking energy can be reused; 5. The total system loss is the sum of the substation, feeder, and transmission loss. The loss accounts for around 10% of the total substation energy consumption.

Table 4. Energy consumption of the 40 km/h case study.

Scenario Index 1 2 3 4 5 6 7 8 9 10 Auxiliary Power (kW) 0 0 0 0 0 480 480 480 480 480 Headway (s) 90 100 120 300 600 90 100 120 300 600

Es (kWh) 802 796 825 840 845 2221 2221 2214 2199 2191 Erec (kWh) 816 803 859 957 1066 2221 2221 2215 2209 2224 Einv (kWh) 14 8 34 117 222 0 0 0 10 34 − − − − − − − Esl (kWh) 25 24 28 35 44 67 67 66 67 69 Eﬂ (kWh) 16 13 17 12 10 84 73 67 33 21 Etl (kWh) 48 46 65 74 74 74 72 81 72 74 Etr_demand (kWh) 1507 1507 1507 1507 1507 1507 1507 1507 1507 1507 Etr (kWh) 1507 1507 1506 1507 1507 1476 1489 1479 1507 1507 Eeb (kWh) 796 796 796 796 796 796 796 796 796 796 Ereg (kWh) 794 795 791 788 790 796 796 796 796 796 Eaux (kWh) 0 0 0 0 0 1316 1316 1316 1316 1316 ηreg 100% 100% 99% 99% 99% 100% 100% 100% 100% 100% Time of under-voltage (s) 0 0 8 0 0 312 192 250 3 0 Time of train running (s) 9900 9900 9900 9900 9900 9900 9900 9900 9900 9900

The energy consumption for the one-hour operation was calculated by a simulation. Four cases with diﬀerent headway times (90, 120, 300, and 600 s) were compared and the results are shown in Figure 11. The energy consumption of each TPSS during the 1 h operation results denote the following:

1. The total energy consumption of all TPSS decreases with the headway time; 2. The maximum energy consumption occurs at TPSS-6 for the cases which are shown in red. This denotes that TPSS-6 could consume more energy than others for most cases. This is consistent with the fact that the rated tractive power of TPSS-6 is 3 MW 2, which is higher than other × the TPSS; 3. The second highest energy consumption occurs at TPSS-12 for the cases; 4. The energy consumption of each TPSS varies with diﬀerent timetables. Some TPSS could consume a high amount of energy due to a particular timetable. Energies 2020, 13, x FOR PEER REVIEW 11 of 18

under-voltage means the amount of time when the under-voltage traction mode occurs. When the auxiliary power is zero, the time of under-voltage is very low. However, when the auxiliary power is 480 kW, under-voltage happens 312 times during the operation. The time of under- voltage decreases when the headway increases; 4. The regenerative energy efficiency is very high for all of the scenarios, and is between 99% and 100%. This denotes that nearly all of the electro-braking energy can be reused; 5. The total system loss is the sum of the substation, feeder, and transmission loss. The loss accounts for around 10% of the total substation energy consumption.

Table 4. Energy consumption of the 40 km/h case study.

Scenario Index 1 2 3 4 5 6 7 8 9 10 Auxiliary Power (kW) 0 0 0 0 0 480 480 480 480 480 Headway (s) 90 100 120 300 600 90 100 120 300 600 Es (kWh) 802 796 825 840 845 2221 2221 2214 2199 2191 Erec (kWh) 816 803 859 957 1066 2221 2221 2215 2209 2224 Einv (kWh) −14 −8 −34 −117 −222 0 0 0 −10 −34 Esl (kWh) 25 24 28 35 44 67 67 66 67 69 Efl (kWh) 16 13 17 12 10 84 73 67 33 21 Etl (kWh) 48 46 65 74 74 74 72 81 72 74 Etr_demand (kWh) 1507 1507 1507 1507 1507 1507 1507 1507 1507 1507 Etr (kWh) 1507 1507 1506 1507 1507 1476 1489 1479 1507 1507 Eeb (kWh) 796 796 796 796 796 796 796 796 796 796 Ereg (kWh) 794 795 791 788 790 796 796 796 796 796 Eaux (kWh) 0 0 0 0 0 1316 1316 1316 1316 1316 ηreg 100% 100% 99% 99% 99% 100% 100% 100% 100% 100% Time of under-voltage (s) 0 0 8 0 0 312 192 250 3 0 Time of train running (s) 9900 9900 9900 9900 9900 9900 9900 9900 9900 9900

The energy consumption for the one-hour operation was calculated by a simulation. Four cases with different headway times (90, 120, 300, and 600 s) were compared and the results are shown in Figure 11. The energy consumption of each TPSS during the 1 h operation results denote the following: 1. The total energy consumption of all TPSS decreases with the headway time; 2. The maximum energy consumption occurs at TPSS-6 for the cases which are shown in red. This denotes that TPSS-6 could consume more energy than others for most cases. This is consistent with the fact that the rated tractive power of TPSS-6 is 3MW × 2, which is higher than other the TPSS; 3. The second highest energy consumption occurs at TPSS-12 for the cases; Energies4. 2020The, 13energy, 2788 consumption of each TPSS varies with different timetables. Some TPSS could12 of 18 consume a high amount of energy due to a particular timetable.

Energies 2020, 13, x FOR PEER REVIEW 12 of 18 Figure 11. Energy consumption of substations during the one-hour operation. Energies 2020, 13, x FigureFOR PEER 11. REVIEWEnergy consumption of substations during the one-hour operation. 12 of 18 4.4. Study of Hotspots 4.4. Study of Hotspots 4.4. Study of Hotspots WithWith the the increase increase of trainof train services, services, the the TPSS TPSS power power demand demand will will increase,increase, whichwhich might cause cause trippingtrippingWith at hotspot atthe hotspot increase stations. stations. of train In In this services, this case case study, thestudy, TPSS TPSS-2 TP SS-2power is is a ademand tie tie stationstation will withoutwithout increase, a traction traction which powermight power supply.cause supply. The following study of the hotspot is based on the operation with the headway of 90 s. The substation Thetripping following at hotspot study ofstations. the hotspot In this is case based study, on theTPSS-2 operation is a tie withstation the without headway a traction of 90 s.power The substationsupply. Thetraction following power, study output of the current, hotspot and is based touch on potent the opialeration are used with to theanalyze headway the hotspot of 90 s. stations.The substation traction power, output current, and touch potential are used to analyze the hotspot stations. tractionFigure power, 12 outputshows thecurrent, maximum and touch traction potent power,ial are th eused maximum to analyze moving the hotspot mean traction stations. power over Figure 12 shows the maximum traction power, the maximum moving mean traction power over 10 sections,Figure 12 and shows the the average maximum traction traction power power, of each th esubstation maximum during moving the mean operation. traction It power can be over seen 10 sections, and the average traction power of each substation during the operation. It can be seen that 10that sections, TPSS-4, and TPSS-6, the average TPSS-14, traction TPSS-15, power and of TPSS-18each substation output duringthe highest the operation. peak power It canand be average seen TPSS-4, TPSS-6, TPSS-14, TPSS-15, and TPSS-18 output the highest peak power and average power. thatpower. TPSS-4, The TPSS-6,hotspot stationsTPSS-14, are TPSS-15, around andthe areaTPSS-18 of TPSS-4, output TPSS-5, the highest and TPSS-6, peak powerand the and area average of TPSS- Thepower.14, hotspot TPSS-15, The stations hotspot TPSS-17, stations are and around TPSS-18,are around the areawhich the of area require TPSS-4, of TPSS-4, careful TPSS-5, TPSS-5, monitoring and and TPSS-6, TPSS-6,to avoid and and overload the the area area faults. of of TPSS-14,TPSS- TPSS-15,14, TPSS-15, TPSS-17, TPSS-17, and TPSS-18,and TPSS-18, which which require require careful careful monitoring monitoring to avoidto avoid overload overload faults. faults.

Figure 12. Traction power of substations. FigureFigure 12.12. TractionTraction power of substations. substations. The current and voltage of each substation are compared in Figures 13 and 14. The current of hotspotTheThe current currentstations and and is voltageusually voltage high, of of each each and substation substationthe voltage areare of hotspot compared stations in in Figures Figures is usually 13 13 and andlow. 14. 14The The. Thereason current current for of the of hotspothotspothotspots stations stations is mainly is is usually usually because high, high, of and theand design the the voltagevoltage and locati of hotspoton of substations. stations stations is is usually usuallyIn addition, low. low. The the The reasontrain reason operation for for the the hotspotshotspotshas an is mainlyimpactis mainly becauseon because the hotspots. of of the the design Therefore,design and and location it locati is neoncessary of of substations. substations. to avoid In heavy In addition, addition, acceleration the the train train in operation theoperation hotspot has anhas impactareas. an impact on the on hotspots. the hotspots. Therefore, Therefore, it is necessary it is necessary to avoid to avoid heavy heavy acceleration acceleration in the in hotspotthe hotspot areas. areas.

Figure 13. Current of substations. Figure 13. Current of substations. Figure 13. Current of substations.

Figure 14. Voltage of substations.

Energies 2020, 13, x FOR PEER REVIEW 12 of 18

4.4. Study of Hotspots With the increase of train services, the TPSS power demand will increase, which might cause tripping at hotspot stations. In this case study, TPSS-2 is a tie station without a traction power supply. The following study of the hotspot is based on the operation with the headway of 90 s. The substation traction power, output current, and touch potential are used to analyze the hotspot stations. Figure 12 shows the maximum traction power, the maximum moving mean traction power over 10 sections, and the average traction power of each substation during the operation. It can be seen that TPSS-4, TPSS-6, TPSS-14, TPSS-15, and TPSS-18 output the highest peak power and average power. The hotspot stations are around the area of TPSS-4, TPSS-5, and TPSS-6, and the area of TPSS- 14, TPSS-15, TPSS-17, and TPSS-18, which require careful monitoring to avoid overload faults.

Figure 12. Traction power of substations.

The current and voltage of each substation are compared in Figures 13 and 14. The current of hotspot stations is usually high, and the voltage of hotspot stations is usually low. The reason for the hotspots is mainly because of the design and location of substations. In addition, the train operation has an impact on the hotspots. Therefore, it is necessary to avoid heavy acceleration in the hotspot areas.

Energies 2020, 13, 2788 13 of 18 Figure 13. Current of substations.

Figure 14.14. VoltageVoltage of of substations. substations.

4.5. Results with TPSS Switched oﬀ

4.5.1. Network Operation Results In this scenario, the headway was set to 120 s and 82 trains were running on the network. In a normal operation, TPSS-13 supplies the highest traction power, with 6490 kW. The railway power network was studied when TPSS-13 was switched off. The network working status before and after switching off TPSS-13 is compared in Table5 and Figure 15. It can be seen that the output voltage at TPSS-13 decreases from 690 to 663 V, and the contact line potential decreases around TPSS-13. However, the potential at the TPSS-13 negative feeder increases from 66 to 109 V, and the rail potential increases to over 100 V around TPSS-13. Due to the outage of TPSS-13, the nearby TPSS supply a higher tractive power. The output current of TPSS-12 increases from 6909 to 7716 A, while the output power increases from 4999 to 6019 kW. The output current of the TPSS-14 increases from 4159 to 5596 A, while the output power increases from 3143 to 4365 kW. The train voltage results are compared in Figure 16. Due to one of the TPSS being switched off, the train voltage around TPSS-13 decreased, leading to five trains being in under-voltage mode.

Table 5. Traction power substation (TPSS) result comparison.

TPSS Positive Feeder Negative Feeder Output Current Power Index Potential (V) Potential (V) Voltage (V) (A) (kW) 10 722 3 720 3622 2760 11 728 9 737 2600 2012 − 12 704 39 665 6909 4999 Normal 13 690 66 624 9333 6490 operation 14 719 9 711 4159 3143 15 730 12 742 2270 1765 − 16 728 9 738 2527 1957 − 10 718 0 718 3701 2887 11 723 9 732 2864 2234 − 12 696 45 651 7716 6019 TPSS-13 13 663 109 554 0 0 switched oﬀ 14 708 21 687 5596 4365 15 724 11 734 2747 2143 − 16 724 11 735 2671 2083 − Energies 2020, 13, x FOR PEER REVIEW 13 of 18

4.5. Results with TPSS Switched Off

4.5.1. Network Operation Results In this scenario, the headway was set to 120 s and 82 trains were running on the network. In a normal operation, TPSS-13 supplies the highest traction power, with 6490 kW. The railway power network was studied when TPSS-13 was switched off. The network working status before and after switching off TPSS-13 is compared in Table 5 and Figure 15. It can be seen that the output voltage at TPSS-13 decreases from 690 to 663 V, and the contact line potential decreases around TPSS-13. However, the potential at the TPSS-13 negative feeder increases from 66 to 109 V, and the rail potential increases to over 100 V around TPSS-13. Due to the outage of TPSS-13, the nearby TPSS supply a higher tractive power. The output current of TPSS-12 increases from 6909 to 7716 A, while the output power increases from 4999 to 6019 kW. The output current of the TPSS-14 increases from 4159 to 5596 A, while the output power increases from 3143 to 4365 kW. The train voltage results are compared in Figure 16. Due to one of the TPSS being switched off, the train voltage around TPSS-13 decreased, leading to five trains being in under-voltage mode.

Table 5. Traction power substation (TPSS) result comparison.

TPSS Positive Feeder Negative Feeder Output Current Power

Index Potential (V) Potential (V) Voltage (V) (A) (kW) 10 722 3 720 3622 2760 11 728 −9 737 2600 2012 12 704 39 665 6909 4999 Normal 13 690 66 624 9333 6490 operation 14 719 9 711 4159 3143 15 730 −12 742 2270 1765 16 728 −9 738 2527 1957 10 718 0 718 3701 2887 11 723 −9 732 2864 2234 TPSS-13 12 696 45 651 7716 6019 switched 13 663 109 554 0 0 off 14 708 21 687 5596 4365 15 724 −11 734 2747 2143 Energies 2020, 13, 278816 724 −11 735 2671 142083 of 18

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

[a] [b] Energies 2020, 13, x FOR PEER REVIEW 14 of 18 FigureFigure 15. 15. NetworkNetwork voltage voltage against location:location: (a(a)) Normal Normal operation; operation; (b ()b TPSS) TPSS 13 13 is switchedis switched oﬀ. off.

[a] [b] FigureFigure 16. 16. TrainTrain voltage voltage against location: ((aa)) NormalNormal operation; operation; (b ()b TPSS-13) TPSS-13 is switchedis switched off .off. 4.5.2. Energy Consumption 4.5.2. Energy Consumption The impact of TPSS outage on the network energy consumption is evaluated in this section. The impact of TPSS outage on the network energy consumption is evaluated in this section. The The scenarios when one TPSS is switched off were simulated for a one-hour operation. The headway scenariostime was when 120 s. one Figure TPSS 17 is compares switched the off energywere simulated consumption for a of one-hour six substations operation. with The various headway outage time wasmodes, 120 s. including Figure 17 a nocompares outage modethe energy and one consumpt of the firstion nine of six TPSS substations outages.When with various the TPSS outage is switched modes, includingoff, the energy a no outage consumption mode and for that one TPSS of the is first zero. nine The TPSS results outages. denote thatWhen the the network TPSS is is switched capable of off, thesupplying energy consumption enough tractive for power that TPSS to the is trains zero. when The oneresults of the denote TPSS isthat switched the network off. The is following capable of supplyingcomments enough can be madetractive by power comparing to the the trains energy when consumption one of the when TPSS one is TPSSswitched is switched off. The off following: comments can be made by comparing the energy consumption when one TPSS is switched off: 1. If one of the TPSS is switched off, the energy consumption of this TPSS is zero. The energy 1. If consumptionone of the TPSS of TPSS is switched around this off, faulty the energy TPSS increases; consumption of this TPSS is zero. The energy 2. consumptionThe amount ofof energyTPSS around consumption this faulty change TPSS of the increases; working TPSS depends on the distance from the 2. Theoutage amount TPSS. of Theenergy maximum consumption variation change occurs of on the the working nearest working TPSS depends TPSS, which on the can distance represent from theup outage to 39% ofTPSS. the increase;The maximum variation occurs on the nearest working TPSS, which can 3. representThe substation up to 39% energy of the consumption increase; is unlikely to be affected if there are more than three 3. Thesubstations substation between energy this consumption substation and is theunlikely outage to substation; be affected if there are more than three 4. substationsIf the fault between TPSS supplied this substati a very largeon and amount the outage of energy substation; when it was on, the impact on the nearby 4. If TPSSthe fault will beTPSS more supplied significant a very when large this faultyamount TPSS of isenergy down. when it was on, the impact on the nearby TPSS will be more significant when this faulty TPSS is down.

Figure 17. Energy consumption of substations with various outage modes.

4.6. Results with a Short Circuit In this section, the simulation results are studied when a short-circuit fault occurs at 26 km between TPSS-14 and TPSS-15. The short circuit resistance is 0.1 Ω, which short connects the contact line up and the earth. Figure 18 shows the network voltage when a short circuit occurs. The potential

Energies 2020, 13, x FOR PEER REVIEW 14 of 18

[a] [b] Figure 16. Train voltage against location: (a) Normal operation; (b) TPSS-13 is switched off.

4.5.2. Energy Consumption The impact of TPSS outage on the network energy consumption is evaluated in this section. The scenarios when one TPSS is switched off were simulated for a one-hour operation. The headway time was 120 s. Figure 17 compares the energy consumption of six substations with various outage modes, including a no outage mode and one of the first nine TPSS outages. When the TPSS is switched off, the energy consumption for that TPSS is zero. The results denote that the network is capable of supplying enough tractive power to the trains when one of the TPSS is switched off. The following comments can be made by comparing the energy consumption when one TPSS is switched off: 1. If one of the TPSS is switched off, the energy consumption of this TPSS is zero. The energy consumption of TPSS around this faulty TPSS increases; 2. The amount of energy consumption change of the working TPSS depends on the distance from the outage TPSS. The maximum variation occurs on the nearest working TPSS, which can represent up to 39% of the increase; 3. The substation energy consumption is unlikely to be affected if there are more than three substations between this substation and the outage substation;

Energies4. If 2020the, fault13, 2788 TPSS supplied a very large amount of energy when it was on, the impact on15 ofthe 18 nearby TPSS will be more significant when this faulty TPSS is down.

Figure 17. Energy consumption of substations with various outage modes. 4.6. Results with a Short Circuit 4.6. Results with a Short Circuit In this section, the simulation results are studied when a short-circuit fault occurs at 26 km between EnergiesIn this2020 , section,13, x FOR thePEER simulation REVIEW results are studied when a short-circuit fault occurs at 2615 km of 18 TPSS-14 and TPSS-15. The short circuit resistance is 0.1 Ω, which short connects the contact line up between TPSS-14 and TPSS-15. The short circuit resistance is 0.1 Ω, which short connects the contact and the earth. Figure 18 shows the network voltage when a short circuit occurs. The potential of the lineof theup andcontact the lineearth. and Figure the rail 18 potentialshows the decrease network a voltagelot. Figure when 19 showsa short the circuit train occurs. voltage The when potential a short contactcircuit lineoccurs. and theThe rail train potential voltage decrease is not aparticularly lot. Figure 19 affected. shows the Because train voltage the absolute when a rail short potential circuit occurs. becomes The very train high, voltage the whole is not particularlynetwork will aﬀ beected. switched Because off thefor absoluteprotection. rail potential becomes very high, the whole network will be switched oﬀ for protection.

FigureFigure 18. 18.Network Network voltage voltage against against the the location. location.

Figure 19. Train voltage against the location.

5. Conclusions A holistic modeling method for evaluating the power supply capability for DC railway traction power systems has been illustrated in this paper. The mathematic simulation integrates the train kinematics, driving controls, power supply infrastructure, and multiple fault modes. A case study based on a metro line in Singapore has been developed to compare the load performance with various train operation strategies and fault modes. The results denote that under-voltage traction is activated by reducing the headway time. By evaluating the instantaneous power supply and energy consumption of substations, the hotspot area can be identified. By understanding the hotspot stations, train operators can improve the power network reliability by reducing the acceleration rate or upgrading the traction power substation capacity. The case study also illustrates the load performance when some substations are switched off due to a fault current or maintenance. The substation outage could increase the rail potential and reduce the train voltage. Therefore, it is necessary to understand the impact of substation outage before conducting maintenance. The results

Energies 2020, 13, x FOR PEER REVIEW 15 of 18

of the contact line and the rail potential decrease a lot. Figure 19 shows the train voltage when a short circuit occurs. The train voltage is not particularly affected. Because the absolute rail potential becomes very high, the whole network will be switched off for protection.

Energies 2020, 13, 2788 Figure 18. Network voltage against the location. 16 of 18

FigureFigure 19. 19.Train Train voltage voltage against against the the location. location.

5.5. Conclusions Conclusions AA holistic holistic modeling modeling method method for for evaluating evaluating the the power power supply supply capability capability for for DC DC railway railway traction traction powerpower systems systems has has been been illustrated illustrated in in this this paper. paper. The The mathematic mathematic simulation simulation integrates integrates the the train train kinematics,kinematics, driving driving controls, controls, power power supply supply infrastructure, infrastructure, and and multiple multiple fault fault modes. modes. A A case case study study basedbased on on a a metro metro line line in in Singapore Singapore has has been been developed developed to to compare compare the the load load performance performance with with various various traintrain operation operation strategies strategies and and fault fault modes. modes. The The results result denotes denote that that under-voltage under-voltage traction traction is activated is activated by reducingby reducing the headway the headway time. By evaluatingtime. By evaluating the instantaneous the instantaneous power supply power and energy supply consumption and energy of substations,consumption the of hotspot substations, area can the be identified.hotspot area By understandingcan be identified the. hotspot By understanding stations, train the operators hotspot canstations, improve train the operators power network can improve reliability the bypower reducing network the accelerationreliability by rate reducing or upgrading the acceleration the traction rate poweror upgrading substation the capacity. traction The power case study substation also illustrates capacity. the The load case performance study also when illustrates some substations the load areperformance switched o ffwhendue tosome a fault substations current or are maintenance. switched off The due substation to a fault outage current could or maintenance. increase the rail The potentialsubstation and outage reduce thecould train increase voltage. the Therefore, rail potential it is necessary and reduce to understand the train thevoltage. impact Therefore, of substation it is outagenecessary before to understand conducting the maintenance. impact of substation The results outage also before indicate conducting that the maintenance. outage of a substation The results can increase the energy demand of nearby substations by up to 39%. The impact on the substation power load reduces with the distance from the outage substation. The substation power load is rarely increased when the outage occurs at more than three substations away from it. The short-circuit faults can affect the rail potential significantly, which will trip the network for protection. Based on the simulation and evaluation, the reliability of the traction power network can be analyzed and predicted, which can be studied in the future. The health and usable life of the power support equipment can be evaluated and optimized by improving the train operation and infrastructure design.

Author Contributions: Conceptualization, Z.T., N.Z., and S.H.; methodology, Z.T., C.W., and N.Z.; software, Z.T. and N.Z.; validation, S.H. and S.S.; writing—original draft preparation, Z.T. and N.Z.; writing—review and editing, S.H., C.W., and S.S.; supervision, S.H., C.W., and S.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was partially funded by the State Key Laboratory of Rail Traffic Control and Safety (Contract No. RCS2020K006), Beijing Jiaotong University, and in part by the National Natural Science Foundation of China under Grant 61751304. Acknowledgments: The authors acknowledge the support given by the staff from SMRT TRAINS LTD. Conflicts of Interest: The authors declare no conflicts of interest.

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