Glossary by "S"

  • 505

    Scheduled outage

    An interruption of electric generation, transmission or distribution operation that are planned well in advance to, for example, undertake regular or routine maintenance or to inspect equipment.  

    Source: ESA Technical Working Group
  • 506

    Scheduling coordinator

    An entity authorized to submit to the ISO a balanced generation or demand schedule on behalf of one or more generators and one or more end-user customers.  

  • 508

    Scheduling entity

    An entity responsible for approving and implementing Interchange Schedules.

  • 509

    Scheduling path

    The Transmission Service arrangements reserved by the Purchasing-Selling Entity for a Transaction.

  • 510

    Scheduling, system control and dispatch service

    An ancillary service involving scheduling of movement of electricity through, out of, within, or into a Control Area. It is provided the operator of the Control Area in which the transaction occurs. One of six ancillary services specified by the FERC under Order 888.

    Source: ESA Technical Working Group
  • 511


    The structural part of a galvanic cell that restricts the escape of solvent or electrolyte from the cell and limits the ingress of air into the cell.

  • 512

    Secondary battery

    A battery comprised of secondary cells. A battery that can be recharged numerous times.

  • 513

    Secondary cell

    A cell whose an electrochemical reaction is reversible (i.e., the cell can be recharged.)

    Source: ESA Technical Working Group
  • 515


    A device similar to a recloser that serves as a “bypass switch” for sections of the transmission or distribution systems with faults. They remain open (i.e. no current can flow) until there is no voltage on the line, due, for example, to tripping of a breaker or reclosure. When their is a fault in the primary circuit, the sectionalizer switches current flow to another circuit that provides a path around the section with the fault. The use of sectionalizers ensures that service is not interrupted to critical portions of the grid even when serious faults disable the primary transmission path(s).

    Source: ESA Technical Working Group
  • 516

    Self discharge

    Storage discharge that occurs while the battery is in an open-circuit condition.

  • 517

    Sending balancing authority

    The Balancing Authority exporting the Interchange.

  • 518

    Series connection

    Cells or batteries connected as follows: positive terminals are connected to the negative terminal of the next cell/battery. An important effect is that the output current from cells/battery connected in series is the same as the current from one cell/battery whereas the voltage of the cells/batteries is additive.  

    Source: ESA Technical Working Group
  • 519

    Service drop

    Electric service line between the utility’s distribution cables and the customer’s home or business. Typically they are comprised of two 120 Volt lines whose output can be combined to produce a 240 Volt service, and a neutral third line. When these three lines are used in the same cable run, they are referred to as triplex cable.

    Source: ESA Technical Working Group
  • 520

    Shelf life

    For a dry cell or battery comprised of dry cells; the amount of time during which the cell/battery can retain a specified percentage of its original energy content, under specified conditions. For example; shelf life might be specified assuming that cell retains at least 90% of its original energy content if stored within a specified temperature range.  

    Source: ESA Technical Working Group
  • 522

    Shunt capacitor

    A capacitor or group of capacitors which are placed across an Electric Power line or Electric Appliance to provide a voltage increase or to improve the power factor of the circuit. A switchable shunt may be disconnected from the circuit when conditions warrant, while a fixed shunt is permanently connected to the power line.

    Source: ESA Technical Working Group
  • 523

    Silicon controlled rectifier

    Acronym(s): SCR

    A semiconductor device that functions as an electrically controlled switch. The SCR is one device type in the family of semiconductors that includes transistors and diodes. The basic purpose of a SCR is to function as a switch that can “switch” power on and off. SCRs do not have any mechanical parts. SCRs consist of a semiconductive path and what is called a bridge. When a voltage is applied across the bridge, the semiconductive path becomes conductive and carries the current until something interrupts the current ahead of the SCR. Then the path becomes semiconductive again. They are used in many electronic appliances. For grid applications they are used on the secondary side of electricity supply transformers, effectively isolating customers’ electric service from the grid. SCRs are considered to be very reliable and inexpensive.

    Source: ESA Technical Working Group
  • 524

    Sink balancing authority

    The Balancing Authority in which the load (sink) is located for an Interchange Transaction. (This will also be a Receiving Balancing Authority for the resulting Interchange Schedule.)  

  • 525

    Smart grid

    Collectively; devices, practices and protocols that enable rich monitoring and situational awareness and flexible and robust control of various parts of or entire power systems under varying conditions. Among other objectives, Smart Grid is expected to reduce peak demand, increase energy efficiency and increase electric service reliability and power quality.  

    Source: ESA Technical Working Group
  • 526

    Solar energy

    Heat and light generated from sunlight.

  • 527

    Source balancing authority

    The balancing authority in which the generation (source) is located for an Interchange transaction. (This will also be a Sending balancing authority for the resulting Interchange schedule.)

  • 528

    Southern Electric Reliability Council

    Acronym(s): SERC

    A nonprofit corporation responsible for promoting and improving the reliability, adequacy, and critical infrastructure of the bulk power supply systems in all or portions of 16 central and southeastern states. Owners, operators, and users of the bulk power system in these states cover an area of approximately 560,000 square miles and comprise what is known as the SERC Region. See also regional entities.  

  • 529

    Southwest Power Pool

    Acronym(s): SPP

    A not-for-profit organization that serves as both the reliability entity and the independent system operator One of nine Independent System Operators/Regional Transmission Organizations (ISOs/RTOs) and one of eight North American Electric Reliability Corporation (NERC) regional entities. SPP is mandated by FERC to ensure reliable supplies of power, adequate transmission infrastructure, and competitive wholesale prices of electricity. See also regional entity and independent system operator.

  • 530

    Specific energy

    The amount of energy that a storage system can deliver per unit mass of the system.

    Source: ESA Technical Working Group
  • 531

    Specific power

    The level of power that a storage system can generate per unit mass of the system.

    Source: ESA Technical Working Group
  • 532

    Spot market

    See: Real-time market
    Source: ESA Technical Working Group
  • 533


    The ability of an electric system to maintain a state of equilibrium during normal and abnormal conditions or disturbances.  

  • 534

    Stability limit

    The maximum power flow possible through some particular point in the system while maintaining stability in the entire system or the part of the system to which the stability limit refers.  

  • 535

    Standby service

    “Back-up” electric service provided to end-users that normally produce their own electricity. Historically the price for standby service has been prohibitive, reducing the likelihood that end-users would use on-site generation extensively. As a result of deregulation, utility customers in some areas may be able to purchase “supplementary” energy on the spot market at prices significantly lower than those associated with traditional standby service.  

    Source: ESA Technical Working Group
  • 536

    State of charge

    Acronym(s): SOC

    The degree to which storage is charged relative to the maximum possible amount of energy that can be stored by the system, typically expressed as a percentage.

    Source: ESA Technical Working Group
  • 537

    Static VAR compensators

    Acronym(s): SVCs

    A static VAR compensator (or SVC) is an electrical device for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilising the system. Unlike a synchronous condenser which is a rotating electrical machine, a static VAR compensator has no significant moving parts (other than internal switchgear). Prior to the invention of the SVC, power factor compensation was the preserve of large rotating machines such as synchronous condensers or switched capacitor banks.

    Source: Wikipedia
  • 538

    Stored energy

    The energy available in the storage system to perform physical work through the conversion of its chemical or mechanical energy, stated in kWh or MWh.

    Source: ESA Technical Working Group
  • 539

    Substation onsite power

    Power required at substations so there is sufficient power needed to continue operations (e.g. control and communication) during interruptions on the part of the grid where the substation is located.  

    Source: ESA Technical Working Group
  • 540

    Subsynchronous resonance

    An electrical resonant frequency on an alternating-current transmission line that is less than the line frequency, and results from the insertion of series capacitors to cancel out part of the line and system reactance.

  • 541


    Part of an electricity transmission and distribution system whose voltage is lower than that of the transmission system and higher than that of the distribution system. Subtransmission circuits are usually arranged in loops so that a single line failure does not cut off service to a large number of customers for more than a short time. Subtransmission circuits usually involve overhead lines though some circuits are underground. Typical subtransmission voltages range from 69 kV to 115 kV and sometimes as high as 138 kV.

    Source: ESA Technical Working Group
  • 542

    Supervisory control and data acquisition acquisition

    Acronym(s): SCADA

    A system of remote control and telemetry used to monitor and control the transmission system.

  • 543

    Supplemental operating reserve

    The portion of operating reserve consisting of: 1. Generation (synchronized or capable of being synchronized to the system) that is fully available to serve load within the disturbance recovery period following the contingency event; or 2.  Load that is fully removable from the system within the disturbance recovery period following the contingency event. See also operating reserve and synchronized operating reserve.

  • 544

    Supplemental regulation service

    A method of providing regulation service in which the Balancing Authority providing the regulation service receives a signal representing all or a portion of the other Balancing Authority

  • 545

    Supplemental reserve

    The extra generating capability that an electric utility needs, above and beyond the highest demand level it is required to supply to meet its users needs.

    Source: California Energy Commission
  • 546

    Supplemental reserve service

    One of six ancillary services specified by the FERC under Order 888. See operating reserve and supplemental operating reserve.  

    Source: ESA Technical Working Group
  • 547


    A transient variation of current, voltage, or power flow in an electric circuit or across an electric system.  

  • 549

    Sustained outage

    The de-energized condition of a transmission line resulting from a fault or disturbance following an unsuccessful automatic reclosing sequence and/or unsuccessful manual reclosing procedure.

  • 550

    Switching station

    A substation through which energy is routed from different sources and/or to different customers. Consider a switching station located near a load center that allows the load center to switch between various energy providers. Another example is a switching station that is located near a generation facility that switches some or all of the generator’s output among regions.

    Source: ESA Technical Working Group
  • 551

    Synchronized reserve

    Synchronized Reserve service supplies electricity if the grid has an unexpected need for more power on short notice. The power output of generating units supplying synchronized reserve can be increased quickly to supply the needed energy to balance supply and demand; demand resources also can bid to supply synchronized reserve by reducing their energy use on short notice.

  • 552

    Synchronized reserve service

    One of six ancillary services specified by the FERC under Order 888. See operating reserve and synchronized operating reserve.  

    Source: ESA Technical Working Group
  • 553

    Synchronous condenser

    In electrical engineering, a synchronous condenser (sometimes called a synchronous capacitor or synchronous compensator) is a device identical to a synchronous motor, whose shaft is not connected to anything but spins freely.[1] Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid’s voltage, or to improve power factor. 

    Source: Wikipedia
  • 554

    Synchronous generator

    Machines that operate at “synchronous speed” meaning that the rotational frequency (rpms) of the equipment’s rotor matches the frequency of the power system to which it is connected (e.g. 60 cycles per second in the U.S.)  

    Source: ESA Technical Working Group
  • 555


    See: Phasor measurement unit
    Source: ESA Technical Working Group
  • 556

    System operating limit

    The value (such as MW, Mar, Amperes, Frequency or Volts) that satisfies the most limiting of the prescribed operating criteria for a specified system configuration to ensure operation within acceptable reliability criteria. System Operating Limits are based upon certain operating criteria.  These include, but are not limited to: 1. Facility Ratings (Applicable pre- and post- Contingency equipment or facility ratings) 2. Transient Stability Ratings (Applicable pre- and post-Contingency Stability Limits) 3. Voltage Stability Ratings (Applicable pre- and post- Contingency Voltage Stability) 4. System Voltage Limits (Applicable pre- and post- Contingency Voltage Limits)

  • 557

    System operator

    An individual at a control center (Balancing Authority, Transmission Operator, Generator Operator, Reliability Coordinator) whose responsibility it is to monitor and control that electric system in real time.