Glossary by "D"

  • 202


    Any effect that tends to reduce the amplitude (magnitude) of oscillations in a system.

    Source: ESA Technical Working Group
  • 203

    Day ahead scheduling reserve

    Acronym(s): DASR

    Supplemental reserves procured and scheduled the day before they will be used.

    Source: ESA Technical Working Group
  • 204

    Day-ahead market

    A forward market for electric energy, capacity or ancillary services that will be provided or purchased during the next day.

    Source: ESA Technical Working Group
  • 205

    Day-ahead price

    See: Day-ahead market

    Price for electricity in an day-ahead market. 

    Source: ESA Technical Working Group
  • 206

    Declining block rate

    An energy price structure for high-volume end-users involving prices for energy that decline as the customer’s energy use increases. Typically; the end-user purchases an initial amount of energy for an initial price and additional energy is purchased at prices whose decline is commensurate with increasing energy use. The opposite of inverted block rate.  

    Source: ESA Technical Working Group
  • 207

    Delayed fault clearing

    Fault clearing consistent with correct operation of a breaker failure protection system and its associated breakers, or of a backup protection system with an intentional time delay.

  • 208


    1.  The rate at which electric energy is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2.  The rate at which energy is being used by the customer.  

  • 209

    Demand charges

    The price paid by a retail electricity user for each unit of power draw on the electric grid. (That power draw drives the amount of electricity generation and T&D infrastructure needed by the utility to serve all load.) Typically demand charges are applied to the maximum demand during a given month, hence units are $/kW-month.  

    Source: ESA Technical Working Group
  • 210

    Demand interval

    A period of time in which system, group, or customer demand is measured. This term is most commonly used to refer to periods of measured or expected energy use ranging from 15 minutes to one hour.

  • 211

    Demand response

    Acronym(s): DR

    Reduction of retail electricity end-users’ electric load (power draw) in response to control or price signals. DR resources are deployed and used in lieu of installing/operating peaking generation capacity.  

    Source: ESA Technical Working Group
  • 212

    Demand side management

    Acronym(s): DSM

    1. Measures or programs undertaken by a utility designed to influence the level or timing of customer demands for energy in order to optimize the use of available supply resources, in turn allowing suppliers to defer the purchase of additional generating capacity. 2. The term for all activities or programs undertaken by Load-Serving Entity or its customers to influence the amount or timing of electricity they use.

  • 213


    1. Of or related to end-user electric demand, often said to be “on the customer side of the meter.” 2. Of or pertaining to electricity demand, sometimes referred to as “customer side of the meter.”

    Source: ESA Technical Working Group
  • 214

    Depth of discharge

    Acronym(s): DOD

    The portion of energy discharged from a storage system relative to the amount extractable stored energy.

    Source: ESA Technical Working Group
  • 215


    (Physics) involving a transfer of heat. 

  • 216


    A substance that does not conduct electricity.

    Source: ESA Technical Working Group
  • 217

    Direct access

    1. Affording generators, marketers and large users of electricity the ability to receive transmission services to serve load directly without exercising the traditional method of going through the utility. 2. A retail utility end-user that can purchase electricity from the wholesale market, thus bypassing the default electric service provider (e.g. the retail electric utility). Typically direct access is available or feasible for large-scale or relatively large-scale end-users, in part because the end-user must also arrange for transmission and delivery of the energy.  

  • 218

    Direct control load management

    Acronym(s): DCLM

    Demand-Side Management that is under the direct control of the system operator.  DCLM may control the electric supply to individual appliances or equipment on customer premises.  DCLM as defined here does not include Interruptible Demand.

  • 219

    Direct load control

    See: Direct control load management
    Acronym(s): DLC
    Source: ESA Technical Working Group
  • 220


    The process of extracting stored energy from the storage system.

    Source: ESA Technical Working Group
  • 221

    Discharge duration

    The amount of time that a storage device can be discharged at the nominal power rating.

    Source: ESA Technical Working Group
  • 223


    The process of varying the output from generation on a moment-to-moment basis to meet changing supply requirements.

    Source: ESA Technical Working Group
  • 224

    Distributed energy resource

    Acronym(s): DER

    Relatively small and modular electrotechnologies that are deployed at the subtransmission or distribution level.

    Source: ESA Technical Working Group
  • 225

    Distributed generation

    Acronym(s): DG

    Small, modular electric generation that is interconnected with the electricity grid at the subtransmission or distribution level.

    Source: ESA Technical Working Group
  • 226


    Acronym(s): D

    The practice of and infrastructure for distribution of electricity to end-users by utilities. Typical voltages range from 12 to 138 kiloVolts (kV)

    Source: ESA Technical Working Group
  • 227

    Distribution cooperative

    Acronym(s): D co-op

    A not-for-profit, customer/member owned electric company that purchases electric power at wholesale and distributes it to its customers.  

    Source: ESA Technical Working Group
  • 228

    Distribution provider

    Acronym(s): DP

    Provides and operates the “wires” between the transmission system and the end use customer. For those end-use customers who are served at transmission voltages, the Transmission Owner also serves as the Distribution Provider. Thus, the Distribution Provider is not defined by a specific voltage, but rather as performing the Distribution function at any voltage. 

  • 229


    1.  An unplanned event that produces an abnormal system condition. 2.  Any perturbation to the electric system. 3.  The unexpected change in ACE that is caused by the sudden failure of generation or interruption of load.  

  • 14

    Disturbance control standard

    Acronym(s): DCS

    The reliability standard that sets the time limit following a Disturbance within which a Balancing Authority must return its Area Control Error to within a specified range.

  • 232

    Disturbance monitoring equipment

    Acronym(s): DME

    Devices capable of monitoring and recording system data pertaining to a Disturbance.  Such devices include the following categories of recorders (e.g. PMUs): 1. Sequence of event recorders which record equipment response to the event 2. Fault recorders, which record actual waveform data replicating the system primary voltages and currents. This may include protective relays. 3. Dynamic Disturbance Recorders (DDRs), which record incidents that portray power system behavior during dynamic events such as low- frequency (0.1 Hz  

  • 233

    Disturbance recovery period

    The maximum amount of time allowed for the responsible entity (balancing authority or reserve sharing group) to meet North American Electric Reliability Corporation (NERC)‘s disturbance recovery criteria for all “reportable disturbances.” The disturbance recovery criteria are: 1. A balancing authority shall return its area control error (ACE) to zero if its ACE just prior to the reportable disturbance was positive or equal to zero. For negative initial ACE values just prior to the disturbance, the balancing authority shall return ACE to its pre-disturbance value. 2. The default disturbance recovery period is 15 minutes after the start of a reportable disturbance. This period may be adjusted to better suit the needs of an Interconnection based on analysis approved by the North American Electric Reliability Corporation (NERC) Operating Committee.

    Source: ESA Technical Working Group
  • 234

    Dry cell

    A primary cell in which the electrolyte is absorbed in a porous medium, or is otherwise restrained from flowing.  

  • 235

    Dynamic interchange schedule or dynamic schedule

    A telemetered reading or value that is updated in real time and used as a schedule in the AGC/ACE equation and the integrated value of which is treated as a schedule for interchange accounting purposes. Commonly used for scheduling jointly owned generation to or from another Balancing Authority Area.

  • 237

    Dynamic transfer

    The provision of the real-time monitoring, telemetering, computer software, hardware, communications, engineering, energy accounting (including inadvertent interchange), and administration required to electronically move all or a portion of the real energy services associated with a generator or load out of one Balancing Authority Area into another.