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Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 32 of 61 Table 6-6 Summary of Cost Savings in 10 Years Total Energy Cost Saving Over 10 Years (Assuming 5% Annual Increase) Year 3MW ESD 4MW ESD 1 55,152 70,917 2 57,909 74,463 3 60,805 78,186 4 63,845 82,096 5 67,037 86,200 6 70,389 90,510 7 73,909 95,036 8 77,604 99,788 9 81,484 104,777 10 85,559 110,016 Total 693,694 891,990 7 Guiding the Selection of Energy Storage Application 7.1 Selection of ESD power rating and energy capacity The ESD power rating and energy capacity are dependent on a number of parameters, such as: ⢠Traction power system parameters, including voltage level, substation spacing, third rail (or OCS) resistance, running rail resistance ⢠Location of the proposed installation, ⢠Desired voltage improvement (in voltage support mode) ⢠System-specific requirement on contingency performance ⢠Train characteristics, consists and power rating ⢠Train schedules The selection process is an iterative process. First, initial assumptions are made on power rating, energy capacity and control voltage levels. Second, simulation results are analyzed to check if these assumptions are appropriate. If not, adjustments are made and further simulation results are analyzed. This process is repeated until a satisfactory set of results is found. 7.1.1 Power rating Power rating requirements from the three simulated transit systems are shown in Figure 7â1.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 33 of 61 ESD Rating Requirement 1.5 3.0 4.0 3.2 4.0 0 1 2 3 4 5 Light Rail 1.5MW ESD Metro Rail 3MW ESD Metro Rail 4MW ESD Commuter Rail 3MW ESD Commuter Rail 4MW ESD Po w er (M W ) Required Power Rating (MW) Figure 7-1: ESD power rating versus system type and power rating 7.1.2 Energy capacity Energy capacity requirements from the three simulated transit systems are shown in Figure 7â2. ESD Energy Capacity Requirement 3.7 12.5 14.7 31.7 42.4 0 5 10 15 20 25 30 35 40 45 Light Rail 1.5MW ESD Metro Rail 3MW ESD Metro Rail 4MW ESD Commuter Rail 3MW ESD Commuter Rail 4MW ESD En er gy (k W h) Figure 7-2: ESD energy capacity versus system type and power rating
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 34 of 61 7.2 Load cycles Load cycles from the three simulated transit systems; light rail, heavy rail and commuter rail are shown in Figures 7â3 through 7â8. 7.2.1 Light rail Simulated ESD Load Cycle (Case 70c-5 Minute Headway) -1,500 -1,000 -500 0 500 1,000 1,500 7:30 AM 7:35 AM 7:40 AM 7:45 AM Time Po w er (k W ) Power (kW) Figure 7-3: Peak hour ESD load cycle (light rail, 5-minute headway) Simulated ESD Load Cycle (Case 70by-ESD790V-15 Minute Headway) -1,500 -1,000 -500 0 500 1,000 1,500 7:30 AM 7:35 AM 7:40 AM 7:45 AM Time Po w er (k W ) Power (kW) Figure 7-4: Off hour ESD load cycle (light rail, 15-minute headway)
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 35 of 61 7.2.2 Metro rail (heavy rail) Simulated ESD Load Cycle (Case 31-2 Minute Headway) -3,500 -3,000 -2,500 -2,000 -1,500 -1,000 -500 0 500 1,000 1,500 2,000 2,500 3,000 3,500 7:35 AM 7:36 AM 7:37 AM 7:38 AM 7:39 AM 7:40 AM 7:41 AM 7:42 AM 7:43 AM 7:44 AM 7:45 AM Time Po w er (k W ) Power (kW) Figure 7-5: Peak hour ESD load cycle (metro rail, 2-minute headway) Simulated ESD Load Cycle (Case 21-5 Minute Headway) -3,500 -3,000 -2,500 -2,000 -1,500 -1,000 -500 0 500 1,000 1,500 2,000 2,500 3,000 3,500 7:35 AM 7:36 AM 7:37 AM 7:38 AM 7:39 AM 7:40 AM 7:41 AM 7:42 AM 7:43 AM 7:44 AM 7:45 AM Time Po w er (k W ) Power (kW) Figure 7-6: Midday ESD load cycle (metro rail, 5 minute headway)
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 36 of 61 Simulated ESD Load Cycle -3,500 -3,000 -2,500 -2,000 -1,500 -1,000 -500 0 500 1,000 1,500 2,000 2,500 3,000 3,500 7:30 7:35 7:40 7:45 7:50 7:55 8:00 Time Po w er (k W ) Power (kW) Case 651 - 3MW ESD; 750-840V Regen Taper; 15 Min Headway Figure 7-7: Off hour ESD load cycle (metro rail, 15-minute headway) 7.2.3 Commuter rail Simulated ESD Load Cycle (Commuter Rail) -2,000 -1,500 -1,000 -500 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 7:00 AM 7:10 AM 7:20 AM 7:30 AM 7:40 AM 7:50 AM 8:00 AM Time Po w er (k W ) ESD 4MW; Vc=670 Figure 7-8: Peak hour ESD load cycle (commuter rail)
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 37 of 61 7.2.4 Summary Assuming daily operational headways as shown in Figures 7â9 and 7â10 and based on the simulated load cycles as shown in the above section for the three systems, the daily total charge/discharge cycles can be calculated. The total charge and discharge cycles over 1 year and over 10 years are listed in Table 7â1, together with the minimum cycle time period between successive charge/discharge cycles. Figure 7-9: Weekday Operating Hours Distribution Figure 7-10: Weekend Operating Hours Distribution This type of information makes it possible to select the types of devices from ESD vendors, as different devices have different life cycle limits and response times. In summary, Table 7â1 simply shows the required life cycles and charge/discharge response times for the ESD under different loading conditions associated with the type of rail mode.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 38 of 61 Table 7-1 Summary charge/discharge cycles for the 3 systems Type Cycles /year Cycles /10 years Minimum cycle time period (minutes) Light Rail 1.5MW ESD 442,800 4,428,000 0.71 Metro Rail 3MW ESD 161,120 1,611,200 2.00 Commuter Rail 4MW ESD 28,770 287,700 15.00 Note that in the above table, the light rail transit system shows the highest number of charge/discharge cycles and the shortest cycle time period. This is due to the selection of the control settings for the ESD in the system. The power and energy ratings of the ESD were determined by the heavier duties in voltage support mode under adjacent substation outage conditions. However, for the majority of the operation time all substations are in service and the ESD is set to energy recycle mode in order to recycle the maximum amount of regenerative braking energy. As a result, the cycle time period is short and the number of cycles is large. For the metro rail and the commuter rail systems, the control settings of the ESD installations were set to voltage support mode. 7.3 Sensitivity analysis A number of parameters are inter-related when trying to optimize the selection of an energy storage device. For example, the voltage improvement requirement that may be needed, the device rating (power and energy), the system receptivity and the amount of energy saving, etc. are all sensitive to the system conditions such as system voltage level, the variation in train schedule, etc. System receptivity is a significant system characteristic that especially affects the performance of a system with regenerative braking. Many factors affect system receptivity and energy saving figures, such as track alignment and grade, passenger station locations, electrical parameters of the traction power system, vehicle characteristics including weight, train operational characters (acceleration and braking rates, coasting, offsets in headway dispatches on the two tracks, timing deviation from regular headways), among others. From nominal operating conditions and parameter values, variations from nominal are introduced to understand this sensitivity. The following sections illustrate sensitivities to variation in some of the key parameters.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 39 of 61 7.3.1 The effect of ESD power rating on voltage improvement For the metro rail system, if an ESD is installed at position G05B at the east end of the track, the low voltage conditions will be improved. A 3MW installation is required to achieve 525V or better, as shown in Figure 7â11. Simulated Train Voltages 0 100 200 300 400 500 600 700 800 900 1,000 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 Location (miles) Tr ai n Vo lta ge (V ) Train Voltage Substations Stations Minimum Voltage G 02 A - PS S St op - 2 G 04 - TP SS G 05 B - 3M W ES D G 05 A - TP SS G 02 B - C B H G 03 - TP SS St op - 3 St op - 4 St op - 5 Case 431-2 Min Headway; 750-840V Regen Taper; 3MW ESD Figure 7-11: Train voltages with 3MW ESD at G05B (metro rail) The above figure shows that the minimum train voltage near G05 is actually improved to 547V. A larger installation of 4MW will achieve an even better result, with the minimum train voltage improved to 617V near G05B, as shown in Figure 7â12. So it is easily seen that larger power rated devices directly affect minimum voltage level protection.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 40 of 61 Simulated Train Voltages (Case 33-2 Min Headway, With ESD 4MW) 0 100 200 300 400 500 600 700 800 900 1,000 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 Location (miles) Tr ai n Vo lta ge (V ) Train Voltage Substations Stations Minimum Voltage G 02 A - PS S St op - 2 G 04 - TP SS G 05 B - ES D 4M W G 05 A - TP SS G 02 B - C B H G 03 - TP SS St op - 3 St op - 4 St op - 5 Figure 7-12: Train voltages with 4MW ESD at G05B (metro rail) 7.3.2 The effect of headway offset on voltage improvement and system receptivity Again, selecting the metro rail system example, operating under a peak hour operation (2- minute headway), simulations for different offsets between east- and westbound train dispatch timing (headway offsets) were performed. Such offsets result in the trains meeting at different locations along the alignment, which in turn affect the train voltage and other conditions in the system. Time-distance plots for three different headway offsets on westbound trains are illustrated in Figure 7â13. The resulting minimum train voltages and system receptivity (the ability to accept regenerative power) under different installations at different headway offsets are shown in Figures 7â14 and Figure 7-15. Shown in Figure 7-14 is the minimum voltage level set point. 7.3.3 The effect of voltage limit on system receptivity Receptivity variations versus voltage limits are shown in Figure 7â16. Receptivity in this figure is measured as a percent, where 100 represents full receptivity, i.e. all generated voltage can be absorbed by the electrical system. Higher voltage limits improve system receptivity regardless of headway variations. The figure also shows that for longer headways, the receptivity is lower in general, because fewer trains are present to accept injected voltage.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 41 of 61 7.3.4 The effect of headway on system receptivity Similarly, receptivity variations versus headways are shown in Figure 7â17. As expected, receptivity improves for a larger installed ESD. Simulated Trains 7:40 7:41 7:42 7:43 7:44 7:45 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 Location (miles) Ti m e (h h: m m ) WB Offset by 0s WB Offset by 15s WB Offset by 30s Substations Stations G 02 A - PS S St op - 2 G 04 - TP SS G 05 B - TB D G 05 A - TP SS G 02 B - C B H G 03 - TP SS St op - 3 St op - 4 St op - 5 Time-Distance Plots; 2 Minute Headway Figure 7-13: Time-distance plot for peak hour trains under different headway offsets for westbound trains (metro rail)
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 42 of 61 Minimum Train Voltages by Different Installations (2 Minute Headways; 840V Voltage Limit) 497 604 558 627 655 644 593 547 634 617617 562 637 619 520 651 400 450 500 550 600 650 700 0 15 30 45 WB train headway offset (seconds) Vo lta ge (V ) CBH 3MW Sub 3MW ESD 4MW ESD Figure 7-14: Minimum train voltages versus headway offsets (metro rail) System Receptivity Under Different Installations (840V Voltage Limit) 67.4 86.9 72.2 66.0 58.1 89.7 76.3 69.5 61.7 72.8 59.1 87.6 61.8 69.7 76.7 90.1 50 55 60 65 70 75 80 85 90 95 0 15 30 45 Headway offset for WB trains Sy et em R ec ep tiv ity (% ) CBH 3MW Sub 3MW ESD 4MW ESD Figure 7-15: System receptivity versus headway offsets (metro rail)
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 43 of 61 System Receptivity Under Voltage Limits (2 Minute Headway) 69.5 95.74 82.93 77.46 68.62 76.3 61.7 89.7 72.17 79.24 84.38 97.41 55 60 65 70 75 80 85 90 95 100 0 15 30 45 Headway offset for WB trains Sy et em R ec ep tiv ity (% ) 840V Voltage Limit 900V Voltage Limit 970V Voltage Limit Figure 7-16: System receptivity versus voltage limits (metro rail) System Receptivity Under Different Installations (840V Voltage Limit) 68.6 49.7 86.9 68.4 49.5 89.7 77.3 63.5 90.1 77.6 65.1 87.6 45 50 55 60 65 70 75 80 85 90 95 2 5 15 Headway (minutes) Sy et em R ec ep tiv ity (% ) CBH 3MW Sub 3MW ESD 4MW ESD Figure 7-17: System receptivity versus headway (metro rail) 7.3.5 The effect of ESD control voltage on load cycle Adjusting the control voltage (Vc) of an ESD installation will affect the level of train voltage improvement. For example, a high voltage level setting will improve the low-voltage level margin, but will also demand a higher power rating for the device.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 44 of 61 Also, due to electrical circuit resistance losses associated with long distances between the ESD and nearby electrical substations, the charging rate is limited. A higher Vc setting positively affects train voltage level, but consideration must also be given to specification of charging rate. A sufficient charging rate must be achievable so that the ESD is kept at a desired capacity of charge to meet the next discharging demand cycle. This tradeoff not only affects power rating of the ESD but also rate of charge and energy capacity. Figure 7â18 shows resulting load cycles under different control voltages for the commuter rail system. Notice in this figure the limiting charge rates imposed by the electrical circuits; seen as voltage charge limits between 1.6 and 1.8 MW. Simulated ESD Load Cycle -2,000 -1,500 -1,000 -500 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 7:40 AM 7:41 AM 7:42 AM 7:43 AM 7:44 AM Time Po w er (k W ) Vc=650 Vc=660 Vc=670 Figure 7-18: ESD load cycle under different control voltages (commuter rail) Minimum voltage limits are affected by the ESD Vc set point and the associated power and energy specifications for the ESD. As can be seen in Table 7â2, higher Vc values can improve the minimum train voltage level but there are also needed increases in ESD power and energy ratings.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 45 of 61 Table 7-2 ESD Rating & Capacity vs. Voltage Improvement (Commuter Rail) ESD Control Voltage Vc=650 Vc=660 Vc=670 Max. MW Output 3.2 3.6 3.8 Max. kWh Usage 31.7 37.5 42.4 Minimum Train Voltage 518 529 536 The variations of ESD power ratings and energy capacities versus the desired levels of voltage improvement are shown in Figures 7â19 and 7â20. ESD Rating Requirement 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 30 35 40 45 50 55 Min Train Voltage Improvement (V) Po w er (M W ) Minimum Power Rating (MW) Figure 7-19: ESD power rating vs. voltage improvement (commuter rail)
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 46 of 61 ESD Rating Requirement 30 35 40 45 30 35 40 45 50 55 Min Train Voltage Improvement (V) En er gy (k W h) Minimum Required Energy Capacity (kWh) Figure 7-20: ESD energy capacity vs. voltage improvement (commuter rail) 7.3.6 The effect of ESD rating on load cycle For a given system condition, a larger device has a higher power output, which demands a large energy capacity. For the metro rail system, simulated load and energy cycles for the ESD at 3MW and 4MW ratings are shown in Figures 7â21 and 7â22. As can be seen in these figures, a larger capacity ESD can provide higher power level discharge, and greater energy availability. Energy availability is represented in Figure 7-22 as the area within the curve. Use of a larger ESD simply means that the magnitude of power and energy discharges are larger, producing a more compliant system by which to meet load cycle demands. Understanding how parameter selection affects system performance variability will help guide the selection of appropriate energy storage devices and guide selection of key operating parameters. Knowledge of sensitivities also provides insight into how specifications might be written for system design and operation.
Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 47 of 61 Simulated ESD Load Cycle -4,000 -3,000 -2,000 -1,000 0 1,000 2,000 3,000 4,000 7:40 7:41 7:42 7:43 7:44 7:45 Time Po w er (k W ) 3MW ESD 4MW ESD2 Minute Headway Figure 7-21: Simulated ESD load cycles (metro rail) Simulated ESD Energy Cycle 4 6 8 10 12 14 16 18 20 22 7:40 7:41 7:42 7:43 7:44 7:45 Time En er gy (k W h) 3MW ESD 4MW ESD2 Minute Headway Figure 7-22: Simulated ESD energy cycles (metro rail)
