Wayside Energy Storage System Modeling Todd Hollett, P.Eng. Salwa Fouda, Ph.D. Bombardier Transportation Bombardier Transportation Kingston, Ontario Kingston, Ontario ABSTRACT Increasing environmental awareness and the requirement for lower project costs is forcing transit system suppliers to think more innovatively and engineer more accurately to strengthen their competitive edge. Of late, clients more often desire a system that is optimized to minimize the energy consumed during operation; a requirement that is often imposed upon transit system suppliers through financially binding energy commitments. One of the more recently popularized means of system energy and cost reduction is the Wayside Energy Storage System (WESS). Many types of energy storage systems exist, each having unique benefits and a preferred application depending on the client s requirements. In addition to reducing energy consumption, energy storage systems can be operated to regulate system voltage levels, provide backup power in the event of a utility power outage, and relieve demand on the utility during the more costly operating periods. An accurate estimation of the benefits of an energy storage system begins with a proper energy storage model, and its incorporation into the transit system model. This paper examines the proper modeling of each of the parameters of a wayside energy storage system and their effect on the transit system. INTRODUCTION Nowadays, transit authorities are required to reduce system energy consumption as well as CO 2 emissions during operation. This is mainly due to energy rate increases driven by limited energy reserves and climate change due to CO 2 emissions. Most new transit systems feature dynamic braking. During dynamic braking, a vehicle s control circuitry permits its motors to be operated as generators which convert the mechanical braking energy to electrical energy. This feature allows the energy to be fed back to the system through the third rail or catenary. Therefore, energy produced by a braking train will be consumed by a concurrently accelerating nearby train. However, if there is no energy-demanding train in the vicinity, this energy will be lost as heat through onboard or wayside resistor banks. Receptivity is a measure of how effectively the system can transfer energy from one train to another with minimal loss. Wayside energy storage is capable of storing potentially lost braking energy and recycling it back to the system, thus increasing the system receptivity. Three unique energy storage technologies on the market are: ultracapacitors, batteries, and flywheels. Each technology exhibits unique pros and cons. For example, to store and release the braking energy in an efficient way, an energy storage system must have: 1. A high energy transfer rate to meet the high output power demands of the train, thus enabling it to capture the maximum amount of regenerated energy over the short braking period (i.e. within seconds), and 2. A high duty cycle to allow for frequent operation. Ultracapacitors and flywheels both meet the above requirements; however battery technology cannot achieve the required duty cycle or the energy transfer rate. Wayside Energy Storage System Application Applications for wayside energy storage systems are: Energy saving: energy is captured from braking train and returned back to the system. Voltage regulation: voltage support is provided at locations that experience unacceptable voltage sags. Emergency backup: power is fed to train in case of a power outage so the train can move to the nearest passenger station. Load shifting: the energy storage is charged during the utilities less expensive off-peak period, and the Page 1 of 9
energy is supplied back to the system during peak energy rate periods. The suitability of energy storage technology is based on the application. For example, the battery is the only technology that is suitable for load shifting where the energy density requirement is high. Whereas the ultracapacitor and flywheel are more suitable for highdemand energy saving, voltage regulation, and emergency backup. An understanding of the specifications for the various technologies available in the market, as well as the application requirements, are key factors in selecting the most suitable technology. Wayside Energy Storage System Design Transit system authorities are always looking for a turnkey solution, where the supplier not only provides the energy storage hardware, but also the energy storage system design. Consider a system that is experiencing unacceptable voltage sag at a specific location. Contemplating wayside energy storage as a possible solution leads to the questions; what is the required energy capacity (rating) of the unit to improve the voltage to the required level, what charge and discharge rates are required to support the vehicles power demand, and how should the unit be set to operate in the system? Likewise, if the wayside energy storage system is installed for energy saving, the transit system authority will be interested in the potential annual energy savings, payback period, and life of the unit. Therefore, a detailed simulator that is capable of WESS modeling is essential to be able to simultaneously consider all the parameters and factors required to design and operate the WESS. The simulator is not only needed to design the WESS, but also to help determine the operating settings of the WESS. It has to be noted that the operational settings are as important as the WESS design. For example, the optimal rating and locations of the WESS can be determined during design. However, if the settings are not conducive to the transit system s operations, the unit may not function to its full potential. In the next section, a sensitivity analysis is conducted on a few the key parameters affecting the design and operation of the WESS. WAYSIDE ENERGY STORAGE SYSTEM MODELING Simulation Software Accurate modeling of energy storage systems can only be accomplished using a simulator which simultaneously incorporates all modeled transit system components into one unified dynamically interactive system. The EnerGplan tool is a Bombardier in-house designed operations and load flow simulator comprised of user-friendly, graphical modeling and real-time simulation analytical interfaces. EnerGplan technology offers generic modeling capability which gives the user complete control over the system model allowing for accurate and project specific modifications. It is important to the customer to provide a turnkey system that is optimized to suit their requirements. EnerGplan s load flow capability allows designers to optimize the power supply and distribution network and study energy storage options. Due to the simulator s flexibility, various scenarios can be modeled easily and thorough comparative analyses can be conducted in a very short time frame [1]. EnerGplan includes a generic energy storage model with an interface that permits modeling of any type of energy storage system. Parameter Descriptions The analyst must have control over a detailed set of parameters that can be modified to simulate realistic WESS performance. Table 1 below defines parameters that are important to the WESS model. capacity initial energy Parameter nominal discharge voltage trigger level (maximum discharge voltage) maximum discharge voltage trigger level (discharge voltage threshold) Description total useable kwh that can be stored kwh stored at the beginning of the simulation voltage below which the energy storage will discharge at the nominal discharge power voltage below which the energy storage will discharge at the maximum discharge power Page 2 of 9
Parameter nominal charge voltage trigger level (minimum charge voltage) maximum charge voltage trigger level (charge voltage threshold) discharge efficiency charge efficiency maximum charge power nominal charge power maximum discharge power nominal discharge power discharge current limit charge current limit self-discharge characteristics rest energy level Description voltage above which the energy storage will charge at the nominal charge power voltage above which the energy storage will charge at the maximum discharge power efficiency while discharging efficiency while charging rate of charge when charging above the charge voltage threshold rate of energy capture when charging to the specified rest state rate of energy release when discharging below the discharge voltage threshold rate of energy capture when discharging to the specified rest state maximum line current during energy release maximum line current during energy capture rate of discharge while energy storage is not in use the level of energy at which to charge or discharge while operating at nominal power Table 1. EnerGplan Energy Storage Parameters The application of these parameters is partially illustrated in Figure 1. The charge and discharge rates are not constant, but refer to the maximum rate of energy transfer within the defined voltage ranges. The actual energy transfer model should account for the inherent capability of the capacitors to transfer energy at an increasing rate as they reach their charge capacity. Figure 1. Partial EnerGplan WESS Input Dialog This paper focuses on the sensitivity of the most crucial parameters in the WESS model; voltage trigger level, capacity, and energy transfer rate. Also, the effect of these parameters on energy savings, voltage support, and emergency backup are examined. Voltage Trigger Level The line voltage is constantly monitored by the WESS during operation. Voltage triggers are set to define when the WESS will be actively operating in the charge or discharge states, and act as the on/off switches of the energy storage unit. The WESS becomes receptive to regenerated energy when the line voltage is at or above the charge voltage trigger, and becomes a power supply when the line voltage is at or below the discharge voltage trigger level. Voltage Trigger Level Sensitivity An example of voltage support demonstrates the reaction of a WESS to the line voltage during a dischargecharge cycle (Figure 2). The voltage profile represents the traction voltage at a specific location on a typical 600Vdc system over a two-minute period. The WESS has been set to discharge when it experiences a line voltage below 530Vdc, and charge when the voltage rises above 600Vdc. The simulation commences with the WESS charged to full capacity (4kWh) as indicated by 1 in Figure 2. The first discharge event begins at approximately the 35 Page 3 of 9
second mark and lasts for ~10 seconds. This can be seen by the decreasing energy level, 2, while the voltage is held constant at 530Vdc, 3. Each succeeding discharge event holds the voltage constant at this discharge voltage level until the WESS is empty at ~98 seconds, 4, after which the line voltage resumes its drop, 5. Similarly, the WESS begins to charge when the line voltage reaches the charge voltage trigger level of 600Vdc at ~109 seconds, 6, and holds the voltage at this level until it is full. Raising the charge voltage and/or lowering the discharge voltage trigger will have a twofold effect on the operation of the unit: 1. The number of trigger events will decrease as more of the voltage spikes are not seen by the WESS, and 2. The duration of energy capture and release will be reduced due to the reduced duration at these higher and lower voltages for any given spike. Figure 3 indicates the decrease in energy savings experienced by a single WESS unit as the charge voltage trigger is increased while the discharge voltage trigger is reduced. In this example, a baseline of an 800Vdc charge voltage trigger, and a 780Vdc discharge voltage trigger (indicated as ±0V) saves >8kWh of energy. Succeeding simulations of 20V increments in the charge trigger and decrements in the discharge trigger show a linear reduction in energy savings. This trend can be generalized for any type of system. However, the magnitude of the effect and its sensitivity to trigger level changes are specific to the system being studied. Figure 3. Effect of Trigger Voltage Level on Energy Savings Effect of Voltage Trigger Level on Voltage Support Figure 2. WESS Operation based on Line Voltage Effect of Voltage Trigger Level on Energy Savings The operating objective for energy saving is to activate the WESS as needed to store energy that would otherwise be lost due to low system receptivity. To accomplish this, the charge and discharge voltage trigger levels are set with a deadband that will permit a maximum acceptable operating time without compromising the life of the unit. It is also important to restrict the WESS from competing with other vehicles that are simultaneously demanding energy since natural receptivity is the most efficient means of recycling regenerated energy. This can be predicted by the line voltage profile. Vehicles in a system will experience fluctuations in the line voltage that is proportional to the demands of the vehicle, and the distance from the power supply point(s). When operating in voltage support mode, it may be necessary to set the charge voltage trigger at a level lower than what would be typical for energy saving. This increases the charge aggressiveness of the WESS, thus increasing the likelihood that it will always have energy available to support the line voltage when required. Ideally, the charge trigger level should be set high enough so the WESS is charged only by regenerated energy if this allows it to maintain a consistent charge level sufficient to support the most stringent low voltage cases when needed. However, a lower setting that is within the normal operating range of the system may allow the WESS to be charged by the traction power substation(s) (TPS). Although this requires extra energy, it is acceptable and often necessary for proper voltage support. Figure 4 shows a WESS supporting the train voltage at various discharge voltage trigger levels. The discharge voltage has a direct impact on the magnitude and duration of the voltage correction. In this example, the WESS is Page 4 of 9
rated to meet the maximum demand of the vehicle, and therefore completely supports the train voltage for the duration of the potential voltage sag. intended function. This simple concept determines how the capacity affects the application. Effect of Capacity on Energy Savings The effect of WESS capacity on energy savings illustrates the law of diminishing returns as shown in Figure 5. Energy storage systems are normally designed to allow for a higher rate of energy transfer with increased capacity. The savings would then follow a more linear relationship with capacity. However, if typical demands on the WESS are limited to a value lower than its maximum energy transfer rate, and a cost-benefit analysis is crucial to determine an optimized capacity limit. Figure 4. Effect of WESS Discharge Voltage Trigger on Train Voltage Effect of Voltage Trigger on Emergency Backup A vehicle rescue operation will typically specify a set of operating requirements such as maximum time of rescue, minimum permitted auxiliary loads, etc. which contribute to the determination of the total energy requirement. Hence, the WESS must be maintained at a level of charge which allows it to meet the energy and power demands required to perform the defined train recovery operation. A WESS that has been exclusively designated as an emergency backup device will only operate following a complete power system failure. Therefore, the WESS charge voltage trigger may be set to operate within the normal system operating voltage range. When discharging, the WESS will behave as a constant voltage source at the specified discharge trigger level. Due to the network resistance between the WESS and the vehicle, the WESS discharge voltage should be chosen so that the vehicle will experience an acceptable voltage range that will allow it to meet the vehicle demands as determined by the intended operational rescue requirements. Unless it is oversized, implementation of an energy storage system for the purpose of emergency backup cannot be used for any other purpose. Capacity Figure 5. Effect of Capacity on Energy Savings (at a Constant Power Limit) Effect of Capacity on Voltage Support Capacity is directly proportional to the length of time the voltage can be supported. Once the WESS is empty a voltage drop is observed as the portion of the demand that was met by the nearby WESS is now transferred to a more distant power supply. To demonstrate this concept, Figure 6 assumes a situation similar to that shown in Figure 4 with a 600V discharge voltage trigger, and a WESS having a capacity not sufficient to fully support the intended voltage correction. The train voltage is supported until the WESS energy supply is exhausted, after which it can no longer support the vehicle demand. The voltage then resumes following the profile as established by the remaining power supply. The capacity indicates the total amount of useable energy the WESS can supply. A full or empty WESS can no longer respectively charge or discharge to perform its Page 5 of 9
A 4 kwh WESS is sufficient to supply the energy required to successfully rescue the vehicle. Lower capacity ratings will impact travel distance as the WESS energy supply to exhausted sooner. Sacrificing travel distance will not allow the train to make a complete return to the destination. Alternatively, travel time can be increased if permitted by returning the train at a reduced maximum speed to lower the energy requirement. Figure 8 demonstrates how maximum travel speed affects the vehicle s return energy requirement and travel time. The maximum speed therefore has a major influence on the required capacity of the WESS when complying with a travel time limitations. Figure 6. Effect of WESS Capacity on Voltage Support Effect of Capacity on Emergency Backup Since the capacity of the WESS is a measure of the amount of energy it can store, as a lone energy supply it is proportional to the distance over which a vehicle can travel under given operating parameters on a given route. As an example, a maximum return travel time requirement of 3 minutes permits a return speed as low as 26 km/h for a given train on a specific route. Figure 7 represents the power profile of the vehicle returning from a stranded destination to the nearest passenger station. The energy under the negative portion of this curve represents 3.83 kwh of energy consumed by the vehicle during its return. Therefore, WESS capacity must be sized to account for this vehicle demand along with distribution losses. Figure 8. Vehicle Energy and Time Requirements at Various Speeds Energy Transfer Rate The energy transfer rate, or power limit, defines the maximum rate at which the WESS can capture or release energy. Energy Transfer Rate Sensitivity Figure 7. Vehicle Power Demand Profile during Return to a Station The power profile of a single vehicle during acceleration, cruising, and braking is shown in Figure 9. Train demand peaks at approximately 1,600kW at the 10 second mark, and its braking power peaks at 1,750kW at ~52 seconds. The WESS has been set to supply energy when it experiences voltages at or below 780Vdc, and store energy at voltages of 800Vdc and above. During charge and discharge, WESS power is limited to a maximum energy transfer rate of 500kW as can be seen by the WESS power curve. Page 6 of 9
Raising the charge and discharge power limits will enable the storage and release of energy at an increased rate, thus allowing the WESS to better meet the demands of the vehicle for the application. However, the increased rate of energy transfer means the WESS will fill and/or empty more quickly which may influence the required capacity. Depending on the application, the WESS should be charged and discharged at a rate that will allow it to be full or empty when needed. Figure 10. General Trend in WESS Energy Savings with Increasing Rates of Energy Transfer Effect of Energy Transfer Rate on Voltage Support Figure 9. Operation of a WESS Limited to 500kW Effect of Energy Transfer Rate on Energy Savings Figure 10 shows how increases in the energy transfer rate limits lead to a diminishing increase in energy savings. The main factors which limit the energy saving potential with increasing energy transfer rates are: 1. The WESS more frequently operates over its entire storage range potential, therefore savings become limited by the capacity, and 2. The WESS approaches the maximum potential power demands of the vehicle(s), after which no further energy is available to be stored by the WESS or supplied to the system. Figure 11 illustrates the effect of WESS discharge rate on the train voltage. Without a WESS, as the vehicle begins to accelerate at some distance from the power supply, its power demand causes the line voltage to drop to 468V. The scenario is then re-simulated with a WESS at increasing discharge rates. With its discharge trigger voltage level set at 600Vdc, the WESS can only support the line voltage to this level for a duration equal to the time the vehicle demand is less than or equal to the discharge rate limit. Vehicle demands higher than the power limits of WESS require supplementary support from the more distant power substation(s) which can be seen as voltage drop spikes within the supporting range of the WESS. Increasing the discharge power limit of the nearby WESS enables it to contribute a higher proportion of the total power demanded by the vehicle, thereby allowing it to more effectively maintain the line voltage. This is apparent by the decreasing magnitude and duration of the voltage drop. Page 7 of 9
Although a thorough analysis is required for each specific system, typically a 4 7 km WESS spacing proves to be most economically effective. Figure 11. Effect of Energy Transfer Rate on Train Voltage Support Effect of Energy Transfer Rate on Emergency Backup As an emergency backup device, the WESS must be rated to meet the required power demands of the vehicle and supply a sufficient amount of energy to complete the defined rescue operation. Lower energy transfer rates will impede the performance of the vehicle and increase return travel time, or not provide adequate power to operate the vehicle at all. OTHER CONSIDERATIONS In addition to the voltage trigger level, capacity, and energy transfer rate, operation of a WESS as part of a complete transit system magnifies the complexity of energy storage operation. Among the many factors affecting WESS design and operation, two of the more important factors which impact the operation of the WESS when installed in the field are discussed below. Location As explained in the previous sections, the WESS will operate based on the line voltage level it experiences, which is a function of the distance between it and the vehicle(s). Figure 12 represents a system with eight power substations spaced approximately 1.5 2 km apart, and a single WESS is placed between TPS 6 and TPS 7. The energy savings experienced by each TPS are represented as a proportion of total substation system energy savings. Two-thirds of the total reduction in TPS energy is experienced by the two TPSs adjacent to the WESS. The relief of energy on other TPSs decreases at increasing distance from the WESS. Figure 12. Sensitivity of WESS Location on TPS Energy Savings When locating a WESS for voltage support, a similar effect is experienced by the vehicles. The WESS will contribute a greater portion of a nearby train s demand than a more distant power supply thus maintaining a more acceptable voltage level due to a lower distribution resistance. As an emergency backup device, the WESS should be located at a position that allows the vehicle to operate within an acceptable voltage range and minimizes distribution losses. Since the rescue operation may have defined limitations, the WESS location will depend heavily on the elevation profile of the system. System Receptivity In an ideal system with short headways, energy storage is unnecessary. Perfect coordination of acceleration and braking events between nearby vehicles will ensure complete natural recycling of the regenerated energy and minimal losses through distribution and storage device inefficiency. Systems or operating periods with longer headways prove to be less receptive due to the increased distance between vehicles [2]. Even at short headways, unavoidable fluctuations in system operation impact the coordination of fleet activity. The natural receptiveness of an un-automated system is influenced by driver behavior, delays at passenger stations, train loading, and the fleet size due to operating schedule requirements. When in operation, varying demands are placed upon the WESS, and it should automatically reconfigure its settings to operate at its full potential as required by each daily operating period. The Page 8 of 9
parameter settings predicted by the modeling and simulations will likely require some refinement during testing and commissioning. CONCLUSION This paper presents the parameters necessary to accurately model and properly study the benefits of a wayside energy storage system through simulation. The sensitivities of WESS voltage trigger, capacity, and energy transfer rate are analyzed, and generalizations are formed regarding their effects on energy savings, voltage support, and emergency backup. Voltage charge and discharge trigger levels impact the benefits of the WESS in a near linear relationship in all applications since they determine how often and for what duration the WESS is in active operation. Increases in WESS capacity exhibit diminishing returns in energy savings, but have a more direct relationship to the duration of voltage support and emergency backup. The rate of energy transfer determines to what extent the WESS can support the demands of the system in relation to other power sources. It also has a diminishing benefit on energy savings and voltage support as the rate reaches the maximum demands of the system. As an emergency backup device, the WESS must be completely capable of supporting the demand of the rescue operation. Other factors such as location and system receptivity impact the operation of the WESS. Accurate modeling and analysis of the WESS, along with refinement of its settings after installation are necessary to ensure the WESS operates at its full potential. REFERENCES [1] Fouda, Hollett, & Gao, Energy Management Solutions for Green Transit Systems, paper presented at the 2013 ASCE APM/ATS Conference, Pheonix, Arizona, USA, April 2013 [2] Fouda & Hollett, EnerGstor A New Wayside Energy Storage System, paper presented at the APTA 2011 Rail Conference, Boston, Massachusetts, USA, June 2011 Page 9 of 9