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Energy Storage Application into a Double DC Electric Railway
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ScienceDirect ScienceDirect Energy Procedia 00 (2018) 000–000 ScienceDirect Energy Procedia 00 (2018) 000–000 ScienceDirect
Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available
Energy Procedia Procedia 00 151(2017) (2018)000–000 12–16 Energy
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC, 3rd Annual Conference in Energy Storage andSheffield, Its Applications, 11–12 September 2018, UK 3rd CDT-ESA-AC, 11–12 September 2018, Sheffield, UK
Energy Storage Application into a Double DC Electric Railway Energy Storage Application into aonDouble DC Electric Railway The 15th International Symposium District Heating and Cooling Hammad Alnuman, Daniel T. Gladwin
Alnuman, Danielthe T. Gladwin Assessing the Hammad feasibility of using heat demand-outdoor University of Sheffield, Sheffield S1 4DE, UK University of Sheffield, Sheffield district S1 4DE, UK heat demand forecast temperature function for a long-term
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
Abstract a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Abstract b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Electric trainsc use regenerative braking to improve the energy efficiency of DC electric railways. The regenerative braking power Département Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler,The Nantes, France Electric trains use regenerative braking the energy of DC of electric railways. regenerative braking power can cause overvoltage in the feeding linetoifimprove it is higher than theefficiency power demand the other trains at44300 that instant. Braking resistors can cause overvoltage in the feeding line if it is higher than the power demand of the other trains at that instant. Braking are activated to burn the excess energy to protect the network from overvoltage that originates from braking. This excessresistors energy are to burn the excess energy to protect network from overvoltage that originates from braking. energy can activated be exploited by using an energy storage systemthe (ESS). However, a stationary ESS may increase energy lossThis in theexcess transmission can be exploited byisusing an energy storage system (ESS).that However, a stationary ESS may increase energy loss in the transmission line if its location not optimal. This paper helps prove the negative effect of a stationary ESS can be eliminated by optimal Abstract line its location is not optimal. This paper helps prove that the negative effect of a stationary ESS can be eliminated by optimal sitingif and voltage control. siting and voltage control. District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the Copyright © gas 2018emissions Elsevier Ltd. All reserved. greenhouse from therights building sector. These systems require high investments which are returned through the heat Copyright © 2018 2018 Elsevier ElsevierLtd. Ltd.All All rights reserved. Copyright © rights reserved. Selection and peer-review under responsibility of the the 3rd Annualrenovation Conferencepolicies, in Energy Energy Storage and Its Its Applications, Applications, sales. Due topeer-review the changed climate conditions and3rd building heat demand in the future could decrease, Selection and under responsibility of Annual Conference in Storage and Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC. 3rd CDT-ESA-AC. prolonging the investment return period. 3rd CDT-ESA-AC. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand Keywords: Braking resistors; electric railways; energy storage system; regenerative power; transmission line losses; forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Keywords: Braking resistors; electric railways; energy storage system; regenerative power; transmission line losses; buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were 1.compared Introduction with results from a dynamic heat demand model, previously developed and validated by the authors. 1. Introduction The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Power in DC electric railways is an way to improve their energy efficiency. The regenerative (the error exchange in annual demand was lower than 20% foreffective all weather scenarios considered). However, after introducing renovation Power in DC is(depending an effective to improve theira energy efficiency. regenerative power of aexchange braking train iselectric passeduprailways totoother trains in power demand. However, power mismatch orThe a long distance scenarios, the error value increased 59.5% on way the weather and renovation scenarios combination considered). power of trains a of braking trainthe is passed to other trainsenergy in power power a heat. long distance The value slope coefficient increased on line’s average within thedemand. range of 3.8% up toaregenerative 8% per mismatch decade, thatoras corresponds to the between reduces transmission utilisation byHowever, wasting power Braking between trains reduces transmission line’s energy utilisation by wasting regenerative power as heat. Braking decrease are in the number hours of threshold 22-139h during the heating season (depending the DC combination of weather and resistors activated atofthe a heating certain voltage to dissipate regenerative power. Inonmost electric railways, this resistors are activated at a certain voltage threshold to dissipate regenerative power. In most DC electric railways, this renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the regenerative power cannot be refed into the grid because of the unidirectional substations [1]. coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and regenerative power cannot be refed into the grid because of the unidirectional substations [1]. To optimise the energy efficiency of electric railways, reversible substations, onboard or stationary energy storage improve the accuracy of heatefficiency demand estimations. To optimise the energy of electric railways, reversible substations, onboard or stationary energy storage
systems (ESSs) can be employed to reuse braking trains’ regenerative energy. The high cost of using power converters systems (ESSs) canbraking be employed reuse trains’ regenerative energy.it The cost oftousing converters to redirect the DC powertointo thebraking AC distribution network makes morehigh enticing storepower the regenerative ©redirect 2017 Thethe Authors. Published by Elsevier Ltd. to DC braking power into the AC distribution network makes it more enticing to store the regenerative energy in ESSs. a major using power is affecting the national grid’s powerand quality Peer-review underMoreover, responsibility of the concern Scientificfor Committee of Theconverters 15th International Symposium on District Heating energy in ESSs. Moreover, areactive major concern for using power converters isbeaffecting the national grid’s power quality by injecting harmonics and power. On the other hand, ESSs can implemented simply and have no impact Cooling. by injecting harmonics and reactive power. On the other hand, ESSs can be implemented simply and have no impact on the power grid [2], [3]. on the power grid [2], [3]. Keywords: Heat demand; Forecast; Climate change 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. Selection and peer-review under responsibility the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC Selection and©peer-review under responsibility the 3rdLtd. Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC 1876-6102 2017 The Authors. Published byofElsevier Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC. 10.1016/j.egypro.2018.09.020
Hammad Alnuman, Daniel T. Gladwin/ Energy Procedia 00 (2018) 000–000 Hammad Alnuman et al. / Energy Procedia 151 (2018) 12–16
13
The position of installing ESSs in electric railways is significant because it can affect the benefits of using them. ESSs can be installed either aboard electric trains or in a stationary position alongside the tracks. An on-board ESS can store all of the regenerative energy of a certain train if the size of the ESS is large enough. On-board ESS installations result in very little regenerative power loss in the railway network’s power line. However, the ESS’s added weight to trains reduces energy savings. Stationary ESSs, on the other hand, are limited by the power losses in the transmission line. When trains brake far from the ESS’s location, regenerative power loss in the transmission line is exacerbated. It is stated in [4], and [5] that stationary ESSs contribute to more energy loss in transmission lines than on-board ESSs. It is also stated that ESSs can reduce the power utilisation in the line by storing braking power that is supposed to be passed to running trains. This paper investigates the energy savings in a double DC electric railway after applying a stationary ESS. It was assumed that the ESS is ideal in capturing and releasing power and has no sizing limits. The specific objective of this study was to investigate whether the negative contribution of ESSs by increasing the losses in electric railways can be avoided by optimal siting. 2. Case study Since the proposed railway system is decoupled, it was decided to analyse the system between just two substations. The decoupling is used to isolate the sections electrically. This separation balances the load over the national grid and avoids shut-downs due to electrical faults or maintenance. The double railway track is illustrated in Fig. 1 and its parameters are displayed in Table 1. The trains receive power from the third rail and the power returns back to the substations through the fourth rail. The alphabets represent the passenger stations that are displaced by 1 km. The trains travel in two different directions and their diagrams are shown in Fig. 2. The modelling approach and the trains characteristics are discussed in detail in [6].
Fig. 1. Double railway track with two substations, five passenger stations, and eight running trains.
Hammad Alnuman, Daniel T. Gladwin/ Energy Procedia 00 (2018) 000–000 Hammad Alnuman et al. / Energy Procedia 151 (2018) 12–16
14
Fig. 2. Train diagrams for the double railway system.
Table 1. Parameters of the electric railway system. Symbol
3. Optimal siting
Quantity
Value
substation dc voltage
600 V
𝑅𝑅𝑠𝑠
substation inner resistance
20 mΩ
rail electrical resistance
15 mΩ / km
𝑉𝑉𝑚𝑚𝑚𝑚𝑚𝑚
voltage threshold
740 V
𝑉𝑉𝑠𝑠
𝑅𝑅𝑑𝑑
The location’s impact on energy storage in the proposed model is investigated in this section. An ideal ESS that has no limitations with power density and energy density was used to collect energy from braking trains and release it back to trains in power demand. It is worth mentioning that the charging voltage threshold in this study was 630 V and the discharging voltage threshold was 585 V. Changing these voltages will certainly change the presented results in this study. Before applying the ESS, the total energy loss in the railway system was 209.2 kWh. The total energy loss consists of energy lost in the internal resistors of the substations, the transmission line, and the braking resistors. After placing the ESS at different locations, it is concluded in Fig. 3 (a) that the best energy saving location is at passenger station C, which is located in the middle between the two substations. The figure also reveals that the least optimal locations for importing and exporting energy are passenger stations A and E, which is where substations 1 and 2 are located. Fig. 3 (b) displays the association between energy saving and the contribution in the losses reduction. The maximum power loss reduction in the system was found when the ESS was placed in midway between two identical substations. Therefore, to achieve optimal energy savings when using an ESS to import and export energy, it is better to place the ESS at a location where it highly reduces system energy losses. Fig. 4 and Fig. 5 illustrate the transmission line’s sensitivity to voltage control in terms of energy loss. The charging and discharging voltages were varied in small steps to study the effect of voltage variation on the transmission line’s energy losses. In Fig. 4 the ESS was placed at interstation A before the charging and discharging voltages were varied in small steps. It is concluded that varying the discharging voltage threshold had no impact on line losses. This result is because the ESS was releasing power at the same location of the substation. On the other hand, charging at lower voltages increased the losses in the line. Charging at lower voltages reduced the line’s receptivity by reducing the power exchange between trains which increased the line losses. In other words, the ESS imported the power that was supposed to go directly from braking trains to trains in power demand. Consequently, the substations had to feed this
Hammad Alnuman, Daniel T. Gladwin/ Energy Procedia 00 (2018) 000–000 Hammad Alnuman et al. / Energy Procedia 151 (2018) 12–16
15
extra demand which required the power to travel for longer distance than before applying the ESS. Charging at higher voltages had no significant impact on increasing the line losses because the ESS was not highly involved in importing power. To summarise, substations are at the least optimal location for the ESS because the power travels for longer distance due to the ESS’s negative contribution to reducing the power utilisation in the railway line. Fig. 5 shows the voltage variation’s impact on reducing the line’s energy losses when the ESS was located between the two substations. The figure illustrates that exporting power in the middle reduced the line’s energy losses significantly. This reduction is because the power travelled for less distance due to discharging at the furthest point from the two substations. For example, if a train is accelerating in the middle of the two substations, it will consume power from the ESS, which is very close to it instead of consuming power from the substations that are far away from it. Decreasing the discharging voltage threshold reduced the ESS’s impact on reducing the transmission line’s energy losses because the ESS was less involved in supporting the trains’ power demand. Similar to Fig. 4, charging at lower voltages increased the transmission line’s losses due to reducing the power utilisation of the line. However, charging at higher voltages reduced the losses because the ESS allowed the trains to exchange power before it got involved lately in importing the excess power.
(a)
(b)
Fig. 3. (a) Energy saving after placing an ESS at different locations; (b) Total losses in the system after placing an ESS at different locations.
Fig. 4. Transmission line losses increase when placing an ESS at interstation A.
Hammad Alnuman, Daniel T. Gladwin/ Energy Procedia 00 (2018) 000–000 16
Hammad Alnuman et al. / Energy Procedia 151 (2018) 12–16
Fig. 5. Transmission line losses increase when placing an ESS at interstation C.
4. Conclusion The work has presented a case study of eight trains running on a four km double railway. The feeding network voltage reaches high values during braking. To protect the network from overvoltage, braking resistors are connected in parallel with the trains to burn the excess energy. Therefore, an ideal ESS was used to exploit the excess energy instead of burning it in the onboard braking resistors. The optimal siting of ESS and voltage sensitivity analysis has been discussed in detail. It has been concluded that installing an ESS at the substations’ locations contributes negatively, notably when importing power at lower voltages. This negative impact occurs due to reducing the railway line’s receptivity. On the other hand, installing an ESS at the furthest point between two substations increases the energy efficiency of electric railways significantly due to the high reduction of energy loss.
References [1] B. Wang, Z. Yang, F. Lin, and W. Zhao, “An improved genetic algorithm for optimal stationary energy storage system locating and sizing,” Energies, vol. 7, no. 10, pp. 6434–6458, 2014. [2] Á. J. López-López, R. R. Pecharromán, A. Fernández-Cardador, and A. P. Cucala, “Improving the Traffic Model to Be Used in the Optimisation of Mass Transit System Electrical Infrastructure,” Energies, vol. 10, no. 8, 2017. [3] H. Ibaiondo and A. Romo, “Kinetic energy recovery on railway systems with feedback to the grid,” Proc. EPE-PEMC 2010 - 14th Int. Power Electron. Motion Control Conf., pp. 94–97, 2010. [4] G. Vitaly, “Energy Storage That May Be Too Good to Be True,” EEE Veh. Technol. Mag., no. 8.4, pp. 70–80, 2013. [5] P. Arboleya, P. Bidaguren, and U. Armendariz, “Energy is on board: Energy storage and other alternatives in modern light railways,” IEEE Electrif. Mag., vol. 4, no. 3, pp. 30–41, 2016. [6] H. Alnuman, D. T. Gladwin, and M. P. Foster, “Development of an Electrical Model for Multiple Trains Running on a DC 4th Rail Track,” IEEE 18th International Conference on Environment and Electrical Engineering and 2nd Industrial and Commercial Power Systems Europe (EEEIC/I&CPS Europe), Palermo, Italy, 2018.
ScienceDirect ScienceDirect Energy Procedia 00 (2018) 000–000 ScienceDirect Energy Procedia 00 (2018) 000–000 ScienceDirect
Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available
Energy Procedia Procedia 00 151(2017) (2018)000–000 12–16 Energy
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC, 3rd Annual Conference in Energy Storage andSheffield, Its Applications, 11–12 September 2018, UK 3rd CDT-ESA-AC, 11–12 September 2018, Sheffield, UK
Energy Storage Application into a Double DC Electric Railway Energy Storage Application into aonDouble DC Electric Railway The 15th International Symposium District Heating and Cooling Hammad Alnuman, Daniel T. Gladwin
Alnuman, Danielthe T. Gladwin Assessing the Hammad feasibility of using heat demand-outdoor University of Sheffield, Sheffield S1 4DE, UK University of Sheffield, Sheffield district S1 4DE, UK heat demand forecast temperature function for a long-term
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
Abstract a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Abstract b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Electric trainsc use regenerative braking to improve the energy efficiency of DC electric railways. The regenerative braking power Département Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler,The Nantes, France Electric trains use regenerative braking the energy of DC of electric railways. regenerative braking power can cause overvoltage in the feeding linetoifimprove it is higher than theefficiency power demand the other trains at44300 that instant. Braking resistors can cause overvoltage in the feeding line if it is higher than the power demand of the other trains at that instant. Braking are activated to burn the excess energy to protect the network from overvoltage that originates from braking. This excessresistors energy are to burn the excess energy to protect network from overvoltage that originates from braking. energy can activated be exploited by using an energy storage systemthe (ESS). However, a stationary ESS may increase energy lossThis in theexcess transmission can be exploited byisusing an energy storage system (ESS).that However, a stationary ESS may increase energy loss in the transmission line if its location not optimal. This paper helps prove the negative effect of a stationary ESS can be eliminated by optimal Abstract line its location is not optimal. This paper helps prove that the negative effect of a stationary ESS can be eliminated by optimal sitingif and voltage control. siting and voltage control. District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the Copyright © gas 2018emissions Elsevier Ltd. All reserved. greenhouse from therights building sector. These systems require high investments which are returned through the heat Copyright © 2018 2018 Elsevier ElsevierLtd. Ltd.All All rights reserved. Copyright © rights reserved. Selection and peer-review under responsibility of the the 3rd Annualrenovation Conferencepolicies, in Energy Energy Storage and Its Its Applications, Applications, sales. Due topeer-review the changed climate conditions and3rd building heat demand in the future could decrease, Selection and under responsibility of Annual Conference in Storage and Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC. 3rd CDT-ESA-AC. prolonging the investment return period. 3rd CDT-ESA-AC. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand Keywords: Braking resistors; electric railways; energy storage system; regenerative power; transmission line losses; forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Keywords: Braking resistors; electric railways; energy storage system; regenerative power; transmission line losses; buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were 1.compared Introduction with results from a dynamic heat demand model, previously developed and validated by the authors. 1. Introduction The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Power in DC electric railways is an way to improve their energy efficiency. The regenerative (the error exchange in annual demand was lower than 20% foreffective all weather scenarios considered). However, after introducing renovation Power in DC is(depending an effective to improve theira energy efficiency. regenerative power of aexchange braking train iselectric passeduprailways totoother trains in power demand. However, power mismatch orThe a long distance scenarios, the error value increased 59.5% on way the weather and renovation scenarios combination considered). power of trains a of braking trainthe is passed to other trainsenergy in power power a heat. long distance The value slope coefficient increased on line’s average within thedemand. range of 3.8% up toaregenerative 8% per mismatch decade, thatoras corresponds to the between reduces transmission utilisation byHowever, wasting power Braking between trains reduces transmission line’s energy utilisation by wasting regenerative power as heat. Braking decrease are in the number hours of threshold 22-139h during the heating season (depending the DC combination of weather and resistors activated atofthe a heating certain voltage to dissipate regenerative power. Inonmost electric railways, this resistors are activated at a certain voltage threshold to dissipate regenerative power. In most DC electric railways, this renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the regenerative power cannot be refed into the grid because of the unidirectional substations [1]. coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and regenerative power cannot be refed into the grid because of the unidirectional substations [1]. To optimise the energy efficiency of electric railways, reversible substations, onboard or stationary energy storage improve the accuracy of heatefficiency demand estimations. To optimise the energy of electric railways, reversible substations, onboard or stationary energy storage
systems (ESSs) can be employed to reuse braking trains’ regenerative energy. The high cost of using power converters systems (ESSs) canbraking be employed reuse trains’ regenerative energy.it The cost oftousing converters to redirect the DC powertointo thebraking AC distribution network makes morehigh enticing storepower the regenerative ©redirect 2017 Thethe Authors. Published by Elsevier Ltd. to DC braking power into the AC distribution network makes it more enticing to store the regenerative energy in ESSs. a major using power is affecting the national grid’s powerand quality Peer-review underMoreover, responsibility of the concern Scientificfor Committee of Theconverters 15th International Symposium on District Heating energy in ESSs. Moreover, areactive major concern for using power converters isbeaffecting the national grid’s power quality by injecting harmonics and power. On the other hand, ESSs can implemented simply and have no impact Cooling. by injecting harmonics and reactive power. On the other hand, ESSs can be implemented simply and have no impact on the power grid [2], [3]. on the power grid [2], [3]. Keywords: Heat demand; Forecast; Climate change 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. Selection and peer-review under responsibility the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC Selection and©peer-review under responsibility the 3rdLtd. Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC 1876-6102 2017 The Authors. Published byofElsevier Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC. 10.1016/j.egypro.2018.09.020
Hammad Alnuman, Daniel T. Gladwin/ Energy Procedia 00 (2018) 000–000 Hammad Alnuman et al. / Energy Procedia 151 (2018) 12–16
13
The position of installing ESSs in electric railways is significant because it can affect the benefits of using them. ESSs can be installed either aboard electric trains or in a stationary position alongside the tracks. An on-board ESS can store all of the regenerative energy of a certain train if the size of the ESS is large enough. On-board ESS installations result in very little regenerative power loss in the railway network’s power line. However, the ESS’s added weight to trains reduces energy savings. Stationary ESSs, on the other hand, are limited by the power losses in the transmission line. When trains brake far from the ESS’s location, regenerative power loss in the transmission line is exacerbated. It is stated in [4], and [5] that stationary ESSs contribute to more energy loss in transmission lines than on-board ESSs. It is also stated that ESSs can reduce the power utilisation in the line by storing braking power that is supposed to be passed to running trains. This paper investigates the energy savings in a double DC electric railway after applying a stationary ESS. It was assumed that the ESS is ideal in capturing and releasing power and has no sizing limits. The specific objective of this study was to investigate whether the negative contribution of ESSs by increasing the losses in electric railways can be avoided by optimal siting. 2. Case study Since the proposed railway system is decoupled, it was decided to analyse the system between just two substations. The decoupling is used to isolate the sections electrically. This separation balances the load over the national grid and avoids shut-downs due to electrical faults or maintenance. The double railway track is illustrated in Fig. 1 and its parameters are displayed in Table 1. The trains receive power from the third rail and the power returns back to the substations through the fourth rail. The alphabets represent the passenger stations that are displaced by 1 km. The trains travel in two different directions and their diagrams are shown in Fig. 2. The modelling approach and the trains characteristics are discussed in detail in [6].
Fig. 1. Double railway track with two substations, five passenger stations, and eight running trains.
Hammad Alnuman, Daniel T. Gladwin/ Energy Procedia 00 (2018) 000–000 Hammad Alnuman et al. / Energy Procedia 151 (2018) 12–16
14
Fig. 2. Train diagrams for the double railway system.
Table 1. Parameters of the electric railway system. Symbol
3. Optimal siting
Quantity
Value
substation dc voltage
600 V
𝑅𝑅𝑠𝑠
substation inner resistance
20 mΩ
rail electrical resistance
15 mΩ / km
𝑉𝑉𝑚𝑚𝑚𝑚𝑚𝑚
voltage threshold
740 V
𝑉𝑉𝑠𝑠
𝑅𝑅𝑑𝑑
The location’s impact on energy storage in the proposed model is investigated in this section. An ideal ESS that has no limitations with power density and energy density was used to collect energy from braking trains and release it back to trains in power demand. It is worth mentioning that the charging voltage threshold in this study was 630 V and the discharging voltage threshold was 585 V. Changing these voltages will certainly change the presented results in this study. Before applying the ESS, the total energy loss in the railway system was 209.2 kWh. The total energy loss consists of energy lost in the internal resistors of the substations, the transmission line, and the braking resistors. After placing the ESS at different locations, it is concluded in Fig. 3 (a) that the best energy saving location is at passenger station C, which is located in the middle between the two substations. The figure also reveals that the least optimal locations for importing and exporting energy are passenger stations A and E, which is where substations 1 and 2 are located. Fig. 3 (b) displays the association between energy saving and the contribution in the losses reduction. The maximum power loss reduction in the system was found when the ESS was placed in midway between two identical substations. Therefore, to achieve optimal energy savings when using an ESS to import and export energy, it is better to place the ESS at a location where it highly reduces system energy losses. Fig. 4 and Fig. 5 illustrate the transmission line’s sensitivity to voltage control in terms of energy loss. The charging and discharging voltages were varied in small steps to study the effect of voltage variation on the transmission line’s energy losses. In Fig. 4 the ESS was placed at interstation A before the charging and discharging voltages were varied in small steps. It is concluded that varying the discharging voltage threshold had no impact on line losses. This result is because the ESS was releasing power at the same location of the substation. On the other hand, charging at lower voltages increased the losses in the line. Charging at lower voltages reduced the line’s receptivity by reducing the power exchange between trains which increased the line losses. In other words, the ESS imported the power that was supposed to go directly from braking trains to trains in power demand. Consequently, the substations had to feed this
Hammad Alnuman, Daniel T. Gladwin/ Energy Procedia 00 (2018) 000–000 Hammad Alnuman et al. / Energy Procedia 151 (2018) 12–16
15
extra demand which required the power to travel for longer distance than before applying the ESS. Charging at higher voltages had no significant impact on increasing the line losses because the ESS was not highly involved in importing power. To summarise, substations are at the least optimal location for the ESS because the power travels for longer distance due to the ESS’s negative contribution to reducing the power utilisation in the railway line. Fig. 5 shows the voltage variation’s impact on reducing the line’s energy losses when the ESS was located between the two substations. The figure illustrates that exporting power in the middle reduced the line’s energy losses significantly. This reduction is because the power travelled for less distance due to discharging at the furthest point from the two substations. For example, if a train is accelerating in the middle of the two substations, it will consume power from the ESS, which is very close to it instead of consuming power from the substations that are far away from it. Decreasing the discharging voltage threshold reduced the ESS’s impact on reducing the transmission line’s energy losses because the ESS was less involved in supporting the trains’ power demand. Similar to Fig. 4, charging at lower voltages increased the transmission line’s losses due to reducing the power utilisation of the line. However, charging at higher voltages reduced the losses because the ESS allowed the trains to exchange power before it got involved lately in importing the excess power.
(a)
(b)
Fig. 3. (a) Energy saving after placing an ESS at different locations; (b) Total losses in the system after placing an ESS at different locations.
Fig. 4. Transmission line losses increase when placing an ESS at interstation A.
Hammad Alnuman, Daniel T. Gladwin/ Energy Procedia 00 (2018) 000–000 16
Hammad Alnuman et al. / Energy Procedia 151 (2018) 12–16
Fig. 5. Transmission line losses increase when placing an ESS at interstation C.
4. Conclusion The work has presented a case study of eight trains running on a four km double railway. The feeding network voltage reaches high values during braking. To protect the network from overvoltage, braking resistors are connected in parallel with the trains to burn the excess energy. Therefore, an ideal ESS was used to exploit the excess energy instead of burning it in the onboard braking resistors. The optimal siting of ESS and voltage sensitivity analysis has been discussed in detail. It has been concluded that installing an ESS at the substations’ locations contributes negatively, notably when importing power at lower voltages. This negative impact occurs due to reducing the railway line’s receptivity. On the other hand, installing an ESS at the furthest point between two substations increases the energy efficiency of electric railways significantly due to the high reduction of energy loss.
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