III. GESTIOADO CO LA ALEATORIEDAD
2. Formas de abordarla, un verdadero malabarismo
Overview
Description of Application
Substation-based storage systems provide utility-controlled energy storage for any or all of the following:
Peak load management
Frequency regulation and area control
Generation capacity
Reactive power support
Critical load support during outage (islanding)
Systems should generally have a maximum power rating of 1–20 MW (charging and
discharging) and the ability to store 2–6 hours of energy for on-demand delivery to the power grid. For frequency regulation and capacity markets, systems may be able to provide energy for shorter time periods. For peak load management to provide substation grid support, the minimum practical storage size is believed be about 1 MW for 2–6 hours, for 15-kV class distribution systems. For 25-kV and 35-kV classes, the minimum practical size of a unit is probably 4 MW. Products can be modular. Systems would connect at distribution voltage at the substation or feeder. Systems can also serve the purpose of renewable integration (see Section 4).
Systems include stationary units and transportable units. Stationary units are physical assets sited at the substation or distribution feeder, with a useful service live of about 15 years. Transportable units are physical assets that also have a service life of about 15 years but that may be easily relocated from site to site as utility needs change, typically on a seasonal basis. They may be installed with minimal site preparation in a period of approximately one day and removed in approximately one day, with standard utility equipment such as cranes and lifts. Scheduled maintenance is expected, including replacement of a storage device (such as energy storage). Replacement schedules and costs must be understood in the purchasing decision. Figure 2-1 illustrates a typical substation-based storage application.
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Disconnect
Utility Substation or Feeder Transformer (Optional)
Aux. Loads (Optional) Contactor
Energy Storage and Power Conditioning
Figure 2-1
Block Diagram of Substation–Based Storage Applications
Use Cases and Operating Modes
Systems may be capable of serving multiple purposes, each represented by a control mode. These modes could all be supported within the system capabilities and self-protection requirements. Modes may include the following:
Load follow mode. The system ideally could discharge at varying levels according to a control signal. The system ideally would calculate the required discharge relative to a remotely set threshold value.
Frequency regulation mode. The system ideally could charge or discharge in response to signals received approximately every second. The system ideally would seek to maintain a target state of charge (such as 50%) over the long run while supporting frequency or area control. (Although the utilities participating in this project considered load following, for peak load management, to be the primary application, systems that provide only frequency regulation, for either transmission or distribution, are also possible.
Constant power charge mode. The system could charge at a fixed kilowatt power level.
Constant power discharge mode. The system could discharge at a fixed kilowatt power level.
Reactive power mode. Systems may be designed to source or sink reactive power. This function could be part of power charge or power discharge mode.
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Self-directed charge mode. The system could charge according to its own optimum method
to reach a defined ready state at a defined time.
Self-maintenance mode. The system is free to perform its own conditioning, as needed.
Standby mode. The system may neither charge nor discharge but only draw necessary
auxiliary load. Contactors are closed.
Shutdown mode. The system may open its contactors to prevent interaction with the grid. (Nominal auxiliary load contactors may continue to serve these loads.)
Islanding mode. Upon sensing a loss of power from the utility, the system will shut down. If an outage persists for a predetermined time (to be determined by the utility), the system may come back to serve loads to be determined by the utility. A block diagram for a system designed to operate in this mode would differ from Figure 2-1.
Performance Ratings
System Definition
Systems interconnect with the utility at distribution voltage (that is, the voltage of the substation or feeder) and include step-down transformers (if necessary), a main disconnect breaker, a contactor, and all power conditioning and auxiliary systems necessary to support their operation.
Auxiliary Loads
All auxiliary loads necessary to operate and protect the system, such as controls, cooling systems, fans, pumps, and heaters, are considered auxiliary loads internal to the system.
System Rating Practices
Systems should be rated, in both power and energy, as measured at the interface of the system and the utility. All system loads and losses, including wiring losses, power conditioning losses, auxiliary loads, and chemical or ionic losses could be considered internal to the system, and ratings are net of these loads and losses.
In cases where auxiliary loads (such as cooling systems) are periodic in nature, ratings could be described for conditions in which these loads are active in the worst-case conditions (or,
alternatively, provide sufficient supplementary information so that ratings under these worst-case conditions may be easily determined).
System net power ratings may be defined in either kWAC or MWAC for a nominal constant discharge of 4 hours. Additional power ratings, such as pulse power capabilities, may also be specified.
System net energy ratings may be defined in either kWhAC, MWhAC, or hours at rated power that is sustainable for a nominal discharge time.
System reactive power capabilities should be specified in kvar, Mvar, or power factor.
When islanded, systems should be able to cope with momentary overload and support in-rush currents at 2.5 to 3 times the normal ratings for 2–3 seconds and also be able to maintain constant voltage under changing customer loads.
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Storage System Effectiveness
Storage Efficiency
Storage efficiency is defined as follows:
in out kWh kWh Equation 2-1 Where:
kWhout is the total ac energy delivered by the storage system to the grid across the standard duration of discharge from fully charged to fully discharged at its rated continuous power capacity.
kWhin is the total energy delivered from the grid to the storage system over a full 24-hour daily cycle (the charging energy).
The measurement of reported storage efficiency should begin and end with the system at a full state of charge. This measurement would be performed using only the remote operating modes available to the system operator, with no manual intervention.
Performance Curve
The system should have an estimated calendar life of at least 15 years and 1500 full energy cycles. If the life of the product is expected to deteriorate with time or use, the system should be designed so that the full energy requirements are met at the end of the expected lifespan under expected usage conditions.
If the product lifetime is sensitive to depth of discharge, the product should be sized so that the energy requirements are met at a given depth of discharge at the end of life and that this depth of discharge is selected to meet the required cycle life according to manufacturer recommendations. For lifetime assessment, it would be beneficial to have a graph that displays the distinction between depth of discharge and required number of cycles.
Physical Characteristics
Size
Systems should be designed to minimize footprint and volume. For example, systems would ideally be less than 500 ft2
/MWh and include space needed to maintain and install the system. Systems would preferably not exceed 2000 ft2
or 15–20 feet in height. Other siting restrictions should be clearly stated.
Transportation Standards
Systems should be transportable at normal speeds over all North American interstate highways and railways, and meet all U.S. Department of Transportation (USDOT) hazardous materials and other regulations. For example, systems should be designed so that turn radii and bridge
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clearances are met when transported on lowboy trailers. System components (such as electrolyte) may be shipped separately, as needed, and assembled at the site.
Rigging and Harnessing
Systems should be installable using standard industry rigging equipment, such as cranes and lifts, and include provisions for installation in the system design.
Status Lights and Alarms
Systems should be equipped with meaningful status lights and light-emitting diode (LED) panels to operate the system and, at minimum, provide easy access to mode and ac power (charging or discharging) information. Audible alarms should be included as necessary to ensure safety, such as for chemical leaks.
Environmental Conditions
Systems should be designed to meet normal utility standards regarding ambient temperature ranges, humidity ranges, air quality, emissions (sulfur oxides [SOx], nitrogen oxides [NOx], and other air emissions if applicable), seismic, audible noise (similar to power transformers),
electromagnetic interference (EMI), fire protection (National Fire Protection Association [NFPA] standards), and flood protection (specified by utilities in the procurement process). Supplier must provide sufficient information specific to their particular product to facilitate utility personnel training and communications with emergency response and environmental agencies. Material safety data sheets (MSDSs) should be provided as applicable. Sample codes and standards are listed Appendix A.
Electrical Interface
Standards
Systems should meet nationally recognized standards for safety, electrical design,
interconnection, harmonics, dc injection, and insulation. Utilities are not required to meet Underwriters Laboratories (UL) standards for equipment on the utility side of the meter. However, UL and other relevant interconnection standards may be required by the utilities for safety or protection of the grid. (For example, standards that address system response to grid disturbances, such as UL 1741, may be required.)
Disconnect Breaker
The disconnect breaker must be capable of breaking the full rated power of the system and operated either manually or remotely. The utility may require that this device accommodate short-circuit and basic impulse level ratings.
Contactor
The system should have a contactor, the operation of which is dependent on the mode. Auxiliary loads ideally would be served internally to the system, so that all power draw ideally would be through the contactor, unless required by the utility.
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Communications, Control, and Data Management
Communications Method
Systems may be communication agnostic and incorporate open systems communication architecture.
Communications Protocol
The system may require any of several communications options, such as cellular, mesh node, Wi-Fi, and WiMAX. Selection between communications options will depend on the utility. Also, the control interface protocol will be specified by the utility. Typical control interface protocols such as the Distributed Network Protocol 3 (DNP3; serial or Internet protocol [IP]) and
International Electrotechnical Commission (IEC) 61850 may be specified by the utility.
Integrated Interface
Systems may be required to be designed in conformity with the Smart Grid Interoperability Standards [3], to the extent applicable at their current level of development. Specifically, systems may be required to be consistent with the energy storage interconnection guidelines and the Energy Storage and Distributed Energy Resources (ES-DER) Use Cases [4]. Utilities will generally require monitoring and control through their operations centers.
Systems may be operated in various utility-defined modes and are preferably capable of responding appropriately to load signals as described in this report. Systems can include the necessary communication and telemetry hardware, and may support communications protocols, to effectively provide the required services. Types of control include the following:
Load-following signals provide an analog of load measured at the location of interest, such as the substation serving the local area loads, in intervals of 1–10 minutes. The system responds by discharging power proportionate to the load that exceeds a specified constant threshold.
Frequency regulation signals are provided by the RTO or ISO, typically in intervals of 1–5 seconds. The system responds by charging or discharging at a power level proportional to the signal level.
Capacity signals are received from a power market (such as an ISO), in intervals of 10–60 minutes, that indicate the system’s discharge power level.
The system could also provide relevant status information, such as state of charge and measured power, for feedback to the utility control system. The system might also have a liquid crystal display (LCD) or similar local control capability, with additional control capabilities and diagnostics.
Operational Data
Systems could have the provision for storing key operational data in a time-sequenced flat data file. At a minimum, systems should store energy received and energy delivered in minute-by- minute, time-stamped data bins.
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Systems could also store events, such as changes in operational mode, received commands, faults, and shutdowns. Each event could be time stamped.
Data Access
All data could be downloadable, either remotely or locally via a standard computer port or wireless connection. All data could be exportable in a nonproprietary format.
Installation and Maintenance
Systems should be designed to allow installation, operation, and maintenance by utility-qualified substation and distribution personnel and contractors. Adequate documentation and training should be provided. Systems should require only standard shop and electrical tools, lifts, and cranes, or specialty tools should be supplied with the systems.
Subsystems, such as power electronics modules or energy storage banks (ac or dc connections), may be classified by the supplier as nonserviceable in the field, provided that these subsystems may be removed and replaced by utility personnel. All consumable or degradable parts, such as air filters, should be classified by replacement interval.
Safety
Systems should be able to protect themselves from internal failures and utility grid disturbances. Therefore, systems should be self protecting for ac or dc component system failures. In addition, systems should be able to protect themselves from various types of grid faults and other
abnormal conditions on the grid.
Systems should reflect the safety standards of the utility. For example, utility and local fire personnel should be notified of particular safety issues and the appropriate response in case of an emergency.
Systems should be designed to minimize risk of injury to the workforce and public during installation, maintenance, and operation.
Systems should be designed to minimize the risk of damage to the environment, including land contamination or disturbance (footprint), water contamination or diversion, and air emissions. Systems should not require that a utility develop a spill containment plan or provide additional safety equipment.
Systems could be designed to be recycled through known processes. Disconnects could be lockable and have a visible break.
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