Guiding the Selection and Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities (2010)

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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 15 of 61 Sustainable Discharge Time Based on Vendor Data 1 10 100 1,000 10,000 100,000 100 1,000 10,000 Power (kW) Ti m e (s ec on ds ) 100%DOD discharge time at rated power (sec) 75%DOD discharge time at rated power (sec) Figure 4-5: Sustainable discharge time for energy storage device 5 Simulation and Modeling Approach From the information provided earlier and additional details on vehicle and system characteristics summarized in this section, a computer system model can be built and simulations performed. The software selected for simulation is SYSTRA’s RAILSIM Load Flow Analyzer. As part of this simulation software, SYSTRA originally developed an energy storage device model as part of the RAILSIM package to assess the suitability of flywheel applications for rail and transit agencies in support of a rail traction power system study[4] . After consultations with the ESD vendors, the consortium determined that the ESD model in RAILSIM is suitable for general modeling applications including other types of storage media, such as batteries, electrochemical capacitors and hybrid batteries, where power control devices are used. The input data for the load flow model requires parametric data defining characteristics of the rail electrified track system, train operation schedule, vehicle propulsion design including the performance of regenerative braking, electrified line network and the energy storage device. These are summarized in the following sections. Details on the organization, reporting and post- processing of simulation results, and software validation are contained in Appendix C. The simulation models and the assumptions used for selecting and sizing energy storage devices are based fundamentally on the further assumption that using energy storage devices principally for energy saving may not provide the expected return on investment given current costs of electrical energy and energy storage devices. From interviews and discussions with transit agencies in the United States and from simplified calculations of energy saving potential,

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 16 of 61 payback periods were exceedingly long. Simulation results validate this assumption. However, it was apparent that energy storage devices could serve to address other needs of transit such as voltage protection or substation replacement as the primary function, but not necessarily the only function. As a starting point for simulations, component and ESD sizing were based on the primary motivation for eliminating voltage sag problems, and from the resulting ESD design given this function, determine the additional benefit of energy saving. This added benefit shown by simulation is discussed in Section 6. 5.1 Track alignment data Track alignment refers to the mechanical design of the rail track system. It includes track elevation gradients, curve and tangent lengths, varying speed limits and station stops. This track data is spatially mapped as data into the simulation model for each rail mode considered. 5.2 Train operations data Train operation refers to the scheduling of trains and number of trains operating per hour in a designated section of the track alignment. For light rail and heavy (subway) rail systems, train operations are usually based on headways or simply the time between trains passing a segment of track alignment. Light rail and subways are normally scheduled using identical train consists (number of cars) dispatched at regular time intervals according to the time of the day. For commuter rail and mainline train operations, general operating timetables are applied directly as part of the simulation. Because of the nature of the commuter and mainline railroad operations, most trains are unique in length, operational frequency and station stop patterns, and as a result each train must be modeled individually as part the simulation input data. 5.3 Vehicle characteristics data Rolling stock parameters and characteristics determine the interaction between the trains’ movement and their interaction with the traction power supply system. Characteristic curves include propulsion tractive effort, braking resistance (friction, dynamic, regenerative, or appropriate combinations) and motor/generator propulsion system efficiency. Power available for train operation is controlled by the voltage level at the train connection with the traction power supply line or third rail. If the voltage seen by the train is less than a specified minimum, power will not flow to the train because of circuitry control algorithms that protect circuit devices from low voltage conditions. . Similarly, power provided by the train to the power supply line when regenerating from braking is also controlled by the voltage level at the power supply line, but in this case there is a maximum voltage limiting regenerated power rather than a minimum. Voltages controlling train propulsion operation and regenerative braking capability are clearly depicted in Figure 5–1.

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 17 of 61 Figure 5-1: Power control diagram for vehicles In the motoring mode, • If the train voltage is above Vmt (referred to as motoring taper voltage), the train’s power demand can be fully met by the traction power system. • If the train’s voltage is at or below Vmin (minimum system voltage), no traction power is available. The under-voltage protection relay normally stops current supply to the motors at Vmin.Between Vmin and Vmt, the train traction power demand is partially met In the regenerative braking mode, • If the train voltage is below Vrt (referred to as regenerative taper voltage), the train’s regenerative power can be fully accepted by the traction power system (100% receptivity). • If the train’s voltage is at or above Vmax (maximum system voltage), no regenerative power is accepted by the traction power system (0% receptivity). The over-voltage protection relay will ensure that this voltage limit is not exceeded. • Between Vrt and Vmax, the train regenerative power is partially accepted by the traction power system (partially receptivity). 5.4 Electrical network data For the electrical network data input, the start point is the electrical single line circuit diagram. Individual components of the network include: substations (rectifiers or inverters), circuit breaker houses, energy storage devices (ESD), third rail (or OCS) conductors, running rails; feeder connections (both positive and negative), negative reactors (where installed), cross- track bonding connections, etc. Parameters for these components are the constituents of the load flow model. The electrical network simulation and the train movement simulation are carried out in discrete time steps. The time step is a user defined input parameter (with a resolution of 0.1 second). For a given instant of time, the locations and power demands (or back-feeding powers) of trains are known from the train movement simulation module. The electrical network is formed by nodes and branches. Fixed plants (substations, feeder connection points, etc.) form fixed nodes, whose locations do not change over time. Trains are

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 18 of 61 moveable nodes, whose locations change with time. From the locations for all the nodes in the circuit, resistances between nodes and ground and resistances for branches between nodes are calculated for the given time instant. A set of linear equations are then formed and solved. The solution process is an iterative process for the following reasons: • The number of linear equations is equal to the number of electrical nodes in the network. This number changes with system and with time for a given system. As a general solution algorithm, it is not feasible to have a closed-form solution for the equations • There are non-linear elements in the electrical network. For example, the diodes can only allow the current to flow in one direction. • The amount of a train’s power demand or feedback is dependent on the voltage level, as illustrated by the power control diagram in the last section. • Where an energy storage device is used, the state of the ESD (charging, discharging, or idle) is dependent on the voltage level, as illustrated by the ESD power control diagram in the next section. Firstly, a set of voltage values are assumed. Secondly, based on this set of values, all elements in the equations are defined. Thirdly, the equations are then solved and a new set of values are obtained for the voltages. Fourthly, the new set of voltages is compared against the last set of voltages. If the maximum difference exceeds a predefined voltage tolerance by the user, a new solution process starts The process is repeated until the user-defined convergence criterion is satisfied. Then the simulation advances to the next time step. 5.5 Energy storage device (ESD) model ESD is treated as a special type of substation with a finite amount of energy that can be stored or available. Control of charging and discharging cycles in the energy storage device is based on the ESD terminal voltage levels, as shown in the following figure. Figure 5-2: Power Control Diagram for Energy Storage Device

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 19 of 61 The main parameters that define the energy storage device model are: • Energy storage capacity (kWh) • Power rating (kW) • Power conversion efficiency • Maximum current (charging or discharging) (amps) • Control voltage (Vc) • Charging start voltage (Vch) • Discharging start voltage (Vdch) The interaction between an energy storage device and the traction power system is controlled by the terminal voltage of the ESD. • When the ESD terminal voltage is at or below Vdch, the ESD is in discharging mode. • When the ESD terminal voltage is at or above Vch, the ESD is in charging mode. 5.6 Simulation results Three systems are selected to reflect the broad range of rail/transit systems: light rail, metro rail and commuter rail. The parameters in the following table illustrate the differences and similarities between the systems modeled. Furthermore, the variation introduced by considering the different rail modes was intentionally established to best examine how such diversity might affect the potential benefit of energy storage. Table 5-1 System Characteristics Summary System Parameters Light Rail Heavy Rail Commuter Rail Miles of Track 7 5 (part of a large system) 5 (part of a large system) Number of Stations 12 4 NA Nominal DC Voltage (V) 750 700 685 Number of Traction Power Substations 7 each equipped with 1.5 MW rectifier unit 4 3 Number of Circuit Breaker Houses 1 2 1 Number of Cars per Train 2 8 Cars run without regenerative braking Headway – Peak Time Morning (min.) 5 2 General Timetable Headway – Peak Time Mid-day (min) 5 5 General Timetable Headway – Off Peak (min) 15 15 General Timetable

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 20 of 61 5.7 Light rail 5.7.1 System parameters The main operating parameters of the light rail transit (LRT) system being simulated shown in Table 5–1 and repeated here are as follows: • 7 Miles of track (double track system) • 12 Stations • 750V nominal voltage DC traction power system, • 7 Traction power substations (TPSS); each equipped with 1.5MW rectifier unit • 1 Circuit breaker house (CBH) • 2 car trains in operation with regenerative braking • 5 Minute headway in peak hours • 15 Minute headway in off peak hours and weekends 5.7.2 Train voltage support requirement Train voltage is a critical performance parameter for the traction power system. For this particular system, when a train’s voltage falls below 575V corresponding to a voltage sag condition, the train’s power demand cannot be fully met by the traction power system, which will have an adverse impact on the performance of the train. At or below 500V, the train’s traction power motor will be shut down in order to avoid damage to the equipment. Under normal conditions (when all substations are in service), the simulated train voltages are all above 575V, which are adequate for trains to achieve their on-time performance. We intend to model a case in which one substation is out of service or consideration of replacing a substation with an energy storage device. In this case consider failure or removal of substation at position A4 or A5 TPSS. (Light rail systems usually have single unit rectifier substations. Consideration for rectifier outage is normally required in design specifications). In such an instance, the minimum train voltage can fall to 504V and 559V respectively, both below the required minimum for this system. These sags are shown in Figures 5–3 and 5–4, where it is noted that the data points represent solutions at time steps in the simulation and that a single point could represent more than one simulation appearance. In order to avoid the excessively low voltage conditions with the removal of rectifier in A4 or A5 TPSS, addition of an ESD can be considered. Addition of an ESD at a sufficient size rating will help support the voltage sag plus provide a potential energy saving benefit by improving capture of regenerative braking. Section 6 discusses the potential energy saving benefit given ESD sizing sufficient for low voltage protection. This simulation and analysis process, by which we begin with a look at voltage support first and energy saving second is carried throughout the various mode analyses.

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 21 of 61 Simulated Train Voltages (Case 64 - A4 Outage, 5-Minute Headway) 100 200 300 400 500 600 700 800 900 1,000 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 30.5 31.0 31.5 32.0 Location (miles) Tr ai n Vo lta ge (V ) Train Voltage Substations Stations A 1- TP SS ST 0 1 A 4- O U T ST 0 7 A 6- TP SS A 5- TP SS A 7- TP SS A 2- TP SS A 3- TP SS ST 0 2 ST 0 3 ST 0 4 ST 0 5 ST 0 6 ST 0 8 ST 0 9 ST 1 2 ST 1 0 ST 1 1 A 4X -C B H Figure 5-3: Train voltages under A4-TPSS outage condition Simulated Train Voltages (Case 65 - A5 Outage, 5-Minute Headway) 100 200 300 400 500 600 700 800 900 1,000 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 30.5 31.0 31.5 32.0 Location (miles) Tr ai n Vo lta ge (V ) Train Voltage Substations Stations A 1- TP SS ST 0 1 A 4- TP SS ST 0 7 A 6- TP SS A 5- O ut A 7- TP SS A 2- TP SS A 3- TP SS ST 0 2 ST 0 3 ST 0 4 ST 0 5 ST 0 6 ST 0 8 ST 0 9 ST 1 2 ST 1 0 ST 1 1 A 4X -C B H Figure 5-4: Train voltages under A5-TPSS outage condition

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 22 of 61 5.7.3 ESD option Returning to the example above, if an appropriately sized ESD is installed in location A4X, the resulting train voltage improvements can be shown in Figures 5–5 and 5–6, given that rectifiers at positions A4 or A5 TPSS are removed. From simulation results, the above figures indicate that the new ESD installation in A4X location will be adequate for train voltage support with either A4 or A5 TPSS rectifier removed. The minimum train voltages for a system with energy storage are summarized in Table 5–2. Simulated Train Voltages (Case 74b - A4 Outage, 5-Minute Headway) 100 200 300 400 500 600 700 800 900 1,000 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 30.5 31.0 31.5 32.0 Location (miles) Tr ai n Vo lta ge (V ) Train Voltage Substations Stations A 1- TP SS ST 0 1 A 4- O U T ST 0 7 A 6- TP SS A 5- TP SS A 7- TP SS A 2- TP SS A 3- TP SS ST 0 2 ST 0 3 ST 0 4 ST 0 5 ST 0 6 ST 0 8 ST 0 9 ST 1 2 ST 1 0 ST 1 1 A 4X -E SD Figure 5-5: Train voltages under A4-TPSS outage condition with storage

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 23 of 61 Simulated Train Voltages (Case 75b-ESD760V - A5 Outage, 5-Minute Headway) 100 200 300 400 500 600 700 800 900 1,000 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 28.5 29.0 29.5 30.0 30.5 31.0 31.5 32.0 Location (miles) Tr ai n Vo lta ge (V ) Train Voltage Substations Stations A 1- TP SS ST 0 1 A 4- TP SS ST 0 7 A 6- TP SS A 5- O ut A 7- TP SS A 2- TP SS A 3- TP SS ST 0 2 ST 0 3 ST 0 4 ST 0 5 ST 0 6 ST 0 8 ST 0 9 ST 1 2 ST 1 0 ST 1 1 A 4X -E SD Figure 5-6: Train voltages under A5-TPSS outage condition with storage Table 5-2 Minimum train voltage and ESD energy summary Case # Scenario A4X Type Minimum Train Voltage (V) Voltage Improveme nt (V) ESD Energy (kWh) 64 A4 outage CBH 504 n/a n/a 74b A4 outage ESD (Vc=760V) 588 84 3.1 84 A4 outage TPSS 605 101 n/a 65 A5 outage CBH 559 n/a n/a 75b A5 outage ESD (Vc=760V) 630 71 3.7 85 A5 outage TPSS 632 73 n/a Note - ESD power rating at 1500 kW 5.7.4 ESD parameters Table 5–3 shows the minimum energy and power rating requirements for the optimized ESD under voltage support mode as presented above. In this table, variations in voltage set point for the ESD shown as cases 74a and 75a are also included.

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 24 of 61 Table 5-3 Energy and power rating summary in voltage support mode Case # Scenario ESD Mode ESD Energy (kWh) ESD Power (kW) 74a A4 outage, A4X- ESD Vc=720V Voltage support 1.90 1,500 74b A4 outage, A4X- ESD Vc=760V Voltage support 3.10 1,500 75a A5 outage, A4X- ESD Vc=720V Voltage support 1.30 1,500 75b A5 outage, A4X- ESD Vc=760V Voltage support 3.70 1,500 5.8 Metro rail 5.8.1 System parameters A similar simulation analysis, again looking at the conditions of low voltage, primarily and regenerative braking energy, is performed for part of a heavy rail (subway) system using the simulation parameters shown in Table 5-1 and repeated here. • 5 Miles Metro System; 4 Stations • 700V DC traction power system • 4 Traction substations • 2 Circuit breaker houses (CBH) • 8 Car trains with regenerative braking • 2 Minute in peak hours (AM & PM) • 5 Minute headway in midday hours • 15 minute headway in off peak and weekend operations Simulation results indicate for the system considered that there will be low voltage occurrences at the east end of the track, as shown in Figure 5–7. Computer simulations show that if a minimum 3MW sized ESD is installed at location G05B, the low voltage occurrences at the east end of the track will be eliminated, as shown in Figure 5–8. Following sections discuss the potential energy saving benefit given this size of ESD.

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 25 of 61 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 - C B H 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 430-2 Min Headway; 750-840V Regen Taper; No ESD Figure 5-7: Train voltages with CBH at G05B (metro rail) 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 5-8: Train voltages with CBH at G05B (metro rail) with storage

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 26 of 61 5.9 Commuter rail 5.9.1 System parameters A similar simulation analysis, again looking at the conditions of low voltage, is performed for part of a commuter rail system using the simulation parameters shown in Table 5- 1and repeated here. • 5 Miles of track in a large commuter rail network; • 685V DC traction power system • 3 Traction substations • 1 Circuit breaker houses (CBH) • Trains without regenerative braking • Operation schedule according to timetable Train voltage plots under different options at location MP-35 are shown in Figures 5–9 through 5–11. Train Voltages - MP32 to MP37 7-8 AM Peak Hours M P3 5- C B H S ub -B 34 S ub -B 36 S ub -B 32 0 100 200 300 400 500 600 700 800 32 33 34 35 36 37 Location (milepost) Vo lta ge (V ) Voltage (v) Substations Minimum Voltage Figure 5-9: Train voltages under CBH option

Guiding the Selection & Application of Wayside Energy Storage Technologies for Rail Transit and Electric Utilities Transit Cooperative Research Program Transportation Research Board Page 27 of 61 Train Voltages - MP32 to MP37 7-8 AM Peak Hours Su b- 35 (N ew ) S ub -3 4 S ub -3 6 S ub -3 2 0 100 200 300 400 500 600 700 800 32 33 34 35 36 37 Location (milepost) Vo lta ge (V ) Voltage (v) Substations Minimum Voltage Figure 5-10: Train voltages under substation option Train Voltages - MP32 to MP37 7-8 AM Peak Hours ES D -3 5 S ub -3 4 S ub -3 6 S ub -3 2 0 100 200 300 400 500 600 700 800 32 33 34 35 36 37 Location (milepost) Vo lta ge (V ) Voltage (v) Substations Minimum Voltage Figure 5-11: Train voltages under ESD option (commuter rail, 4MW ESD)