Active Power

Active Power is a real component of the apparent power, usually expressed in kilowatts (kW) or megawatts (MW), in contrast to reactive power. (ENTSO-E UCTE Operational Handbook)

Already Allocated Capacity (AAC)

Already Allocated Capacity is the total amount of allocated transmission rights, whether they are capacity or exchange programs depending on the allocation method.(ENTSO-E UCTE Operational Handbook)

Ancillary services

All services procured by the transmission or distribution system operator from system users to enable them to maintain the integrity and stability of the transmission and distribution system as well as power quality (to be able to provide system services – see definition). They include i.e. frequency control, voltage control, blackstart capability, grid loss compensation (EURELECTRIC report ‘Ancillary services’, 2004)

Annual Load Factor

The ratio of the actual energy output of a Generating Unit, CCGT Module or Power Station (as the case may be) to the maximum possible energy output of that Generating Unit, CCGT Module or Power Station (as the case may be) over a year. It is often expressed in percentage terms (National Grid)

Apparent Power

Apparent Power is the product of voltage (in volts) and current (in amperes). It consists of a real component (Active Power) and an imaginary component (Reactive Power), usually expressed in kilovolt-amperes (kVA) or megavolt-amperes (MVA). (ENTSO-E UCTE Operational Handbook)

Area Control Error (ACE)

Area Control Error is the instantaneous difference between the actual and the reference value (measured total power value and scheduled control program) for the power interchange of a control area (unintentional deviation), taking into account the effect of the frequency bias for that control area according to the network power frequency characteristic of that control area and the overall frequency deviation.(ENTSO-E UCTE Operational Handbook)


Availability is a measure of time during which a generating unit, transmission line, ancillary service or another facility is capable of providing service, whether or not it actually is in service. Typically, this measure is expressed as a percentage available for the period under consideration. (ENTSO-E UCTE Operational Handbook)

Available transfer capability (ATC)

“A measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses. ATC is defined as the total transfer capability, less the transmission reliability margin, less the sum of existing transmission commitments (which includes retail customer service) and the capacity benefit margin.” (Energy Risk Glossary) Available transfer capacity is a measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses. Available transmission capacity is the part of NTC that remains available after each phase of the allocation procedure for further commercial activity. ATC is defined by the following equation: ATC = NTC- AAC (ENTSO-E UCTE Operational Handbook)

Black-start Capability

The capability of a generating unit to start up without an external power supply, called on as a means of restoring supplies following a major failure on all or part of the network. (EURELECTRIC report ‘Ancillary services’, 2004)

Compensation program

Compensation of unintentional deviations is performed by exporting to / importing from the interconnected system during the compensation period by means of schedules of constant power within the same tariff periods as when they occurred (COMP).(ENTSO-E UCTE Operational Handbook)


In the context of regulation EC 714/2009, congestion is a situation in which an interconnection linking national transmission networks cannot accommodate all physical flows resulting from international trade requested by market participants, because of a lack of capacity of the interconnectors and/or the national transmission systems concerned.


Contingency means the identified and possible or already occurred fault within the TSO’s control area, including not only the transmission but also the distribution network on lower voltage levels. (ENTSO-E Operational Security NC)

Control Area (CA)

It is a coherent part of the ENTSO-E interconnected system (usually coinciding with the territory of a company, a country or a geographical area, physically demarcated by the position of points for measurement of the interchanged power and energy to the remaining interconnected network), operated by a single TSO, with physical loads and controllable generation units connected within the Control Area. A Control Area may be a coherent part of a control block that has its own subordinate control in the hierarchy of secondary control).(ENTSO-E 2010)

Control area operator

A Control Area Operator is the operator of a control area usually a TSO. (ENTSO-E UCTE Operational Handbook)

Control Program (CP)

A Control Program constitutes the schedule of the total programmed exchange of a control area/block, the sum of all exchange programs and the compensation program, that is used for secondary control.(ENTSO-E UCTE Operational Handbook)


Curtailment means a reduction in the scheduled capacity or energy delivery. (ENTSO-E UCTE Operational Handbook)


Disturbance is an unplanned event that produces an abnormal system condition.(ENTSO-E UCTE Operational Handbook)

Electric System Losses

Electric System Losses are total electric energy losses in the electric system. The losses consist of transmission, transformation, and distribution losses between supply sources and delivery points. Electric energy is lost primarily due to heating of transmission and distribution elements.(ENTSO-E UCTE Operational Handbook)

Electrical Energy

Electrical Energy is a measure of the generation or use of electric power by a device integrated over a period of time; it is expressed in kilowatt-hours (kWh), megawatt-hours (MWh), or gigawatt-hours (GWh).(ENTSO-E UCTE Operational Handbook)

Electronic Highway (EH)

The Electronic Highway represents a secure, fast, reliable and highly available communication infrastructure for real-time and non-real-time data exchanges between TSOs. (ENTSO-E UCTE Operational Handbook)

Emergency state

The situation where Operational Security Limits are not kept with at least one of the operational parameters outside of the respective limits. (ENTSO-E Draft NC Operational Security)

Exchange Schedule (CAS, CBS)

An exchange schedule defines an agreed transaction with regard to its size (megawatts), start and end time, ramp period and type (e.g. firmness); it is required for delivery and receipt of power and energy between the contracting parties and the control area(s) (CAS) or between control areas and control blocks (CBS) involved in the transaction. (ENTSO-E UCTE Operational Handbook)

Frequency Control

Maintaining the frequency within the given margins by continuous modulation of active power. (EURELECTRIC report ‘Ancillary services’, 2004)

Frequency Deviation

Frequency Deviation means a departure of the actual system frequency from the set value frequency. (ENTSO-E UCTE Operational Handbook)

Frequency Stability

Frequency Stability means the ability of the electric power system to maintain steady frequency in N-Situation and after being subjected to a disturbance in N-1 situation (ENTSO-E, 2012)

Frequency support

Services related to the short-term balance of energy and frequency of the power system include: 1. Primary response: the automatic response to frequency changes released increasingly with time over a period of some seconds from the time of frequency. 2. Secondary response: the automatic response to frequency changes which takes over from the primary response and partially recovers system frequency.

Interconnected system

An Interconnected System is a system consisting of two or more individual electric systems that normally operate in synchronism and are physically connected via tie-lines see also: synchronous area (ENTSO-E UCTE Operational Handbook)


An island represents a portion of a power system or of several power systems that is electrically separated from the main interconnected system (separation resulting e.g. from the disconnection / failure of transmission system elements) (ENTSO-E UCTE Operational Handbook)


The K-Factor is a value, usually given in megawatts per Hertz (MW/Hz), which is normally determined for a (single) control area/block ; it defines the frequency bias of that control area for secondary control (especially to assure the functionality of the network characteristic method); it is not to be confused with the network power frequency characteristic. (ENTSO-E UCTE Operational Handbook)


1. The load is the value at a given moment of the electrical power supplied or absorbed at any point of a system as determined by an instantaneous measurement or by the integration of power during a given period of time. Load can refer to a consumer , an appliance, a group of consumers or appliances or a network (UNIPEDE 1991) 2. Load means an end-use device or customer that receives power from the electric system. Load should not be confused with demand, which is the measure of power that a load receives or requires. Load is often wrongly used as a synonym for demand (ENTSO-E UCTE Operational Handbook). 3. The demand on an energy producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amperes or watts in reference to electricity. (US DoE Solar Glossary)

Load shedding

The disconnection of load from the synchronous electric system, usually performed automatically, to control the system frequency in emergency situations. (ENTSO-E UCTE Operational Handbook)

Loop flow

Loop flow is the movement of electric power from generator to load by dividing along multiple parallel paths; it especially refers to power flow along an unintended path that loops away from the most direct geographic path or contract path (EIA).


Metering describes the methods of applying devices that measure and register the amount and direction of electrical quantities with respect to time. (ENTSO-E UCTE Operational Handbook).

N-1 Criterion

The rule according to which elements remaining in operation after a Fault of one element within TSO’s Control Area must be capable of accommodating the new operational situation without exceeding Operational Security Limits. (ENTSO-E Draft NC Operational Security)

Net Transfer Capacity (NTC)

The Net Transfer Capacity is defined as: NTC = TTC-TRM The Net Transfer Capacity is the maximum total exchange program between two adjacent control areas compatible with security standards applicable in all control areas of the synchronous area, and taking into account the technical uncertainties on future network conditions (ENTSO-E UCTE Operational Handbook)

Network Power Frequency Characteristic

The Network Power Frequency Characteristic defines the sensitivity, given in megawatts per Hertz (MW/Hz), usually associated with a (single) control area/block or the entire synchronous area, that relates the difference between scheduled and actual system frequency to the amount of generation required to correct the power imbalance for that control area/block (or, vice versa, the stationary change of the system frequency in case of a disturbance of the generation-load equilibrium in the control area without being connected to other control areas); it is not to be confused with the k-factor. The network power frequency characteristic includes all active primary control and self-regulation of load and changes due to modifications in the generation pattern and the demand. (ENTSO-E UCTE Operational Handbook)

Operational security

The transmission system capability to retain a Normal State or to return to a Normal State as soon and as close as possible, and is characterized by thermal limits, voltage constraints, short-circuit current, frequency reference value and stability limit. (ENTSO-E Draft NC Operational Security)

Operational Security Limits

The acceptable operating boundaries: thermal, voltage, fault levels, frequency and stability limits. (ENTSO-E Draft NC Operational Security)

Parallel Path Flows

Parallel Path Flows describe the difference between the scheduled and actual power flow, assuming zero inadvertent interchange, on a given transmission path in a meshed grid.(ENTSO-E UCTE Operational Handbook)

Power System

The Power System comprises all generation, consumption and network installations interconnected through the network.(ENTSO-E UCTE Operational Handbook)

Primary Control

The global Primary Control behavior of an interconnection partner (control area / block), may be assessed by the calculation of the equivalent droop of the area (basically resulting from the droop of all generators and the self-regulation of the total demand). By the joint action of all interconnected undertakings, Primary Control ensures the operational reliability for the power system of the synchronous area. (ENTSO-E UCTE Operational Handbook)

Primary Control Reserve

Primary Control Reserve is the (positive / negative) part of the primary control range measured from the working point prior to the disturbance up to the maximum primary control power (taking account of a limiter). The concept of the Primary Control Reserve applies to each generator, each control area/block, and the entire synchronous area. (ENTSO-E UCTE Operational Handbook)

Primary Frequency Control

See: primary control (ENTSO-E UCTE Operational Handbook)

Ramp Period

The Ramp Period is the time between ramp start and end times, usually expressed in minutes and applied to schedules. (ENTSO-E UCTE Operational Handbook)

Reactive Power

Reactive Power is an imaginary component of the apparent power. It is usually expressed in kilo-vars (kVAr) or mega-vars (MVAr). Reactive Power is the portion of electricity that establishes and sustains the electric and magnetic fields of alternating-current equipment. Reactive Power must be supplied to most types of magnetic equipment, such as motors and transformers and causes reactive losses on transmission facilities. Reactive Power is provided by generators, synchronous condensers, or electrostatic equipment such as capacitors, and directly influences the electric system voltage. The reactive power is the imaginary part of the complex product of voltage and current. (ENTSO-E UCTE Operational Handbook)


The measures taken by System Operator Employee to alter the generation or demand pattern in order to change physical flows in the grid and relieve congestion. (ENTSO-E UCTE Operational Handbook)


Reliability describes the degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired. Reliability on the transmission level may be measured by the frequency, duration, and magnitude (or the probability) of adverse effects on the electric supply / transport / generation. Electric system reliability can be addressed by considering two basic and functional aspects of the electric system: • Adequacy — The ability of the electric system to supply the aggregate electrical demand and energy requirements of the customers at all times, taking into account scheduled and reasonably expected unscheduled outages of system elements. • Security — the ability of the electric system to withstand sudden disturbances such as electric short circuits or unanticipated loss of system elements. (ENTSO-E UCTE Operational Handbook)

Reliability (in Distribution networks)

DSOs are subject to technical performance requirements for quality of service including continuity of supply (commonly assessed by zonal indexes such as SAIDI and SAIFI or individual indexes like number and duration of interruptions) and power quality laid out in national law, standards and grid codes. They are also responsible for voltage quality in distribution networks (maintaining voltage fluctuations on the system within given limits). (EURELECTRIC report ‘Ancillary services’, 2004)

Remedial action

The measure activated by the TSO manually or automatically to relieve consequences of disturbances and maintain a Normal State or move towards a Normal State, which can be applied pre-fault or post-fault and may involve costs. (ENTSO-E UCTE Operational Handbook)

Secondary Control

(also called Load-Frequency-Control) Secondary Control is a centralized automatic function to regulate the generation in a control are based on Secondary Control reserves in order • to maintain its interchange power flow at the control program with all other control areas (and to correct the loss of capacity in a control area affected by a loss of production) and, at the same time, • (in case of a major frequency deviation originating from the control area, particularly after the loss of a large generation unit) to restore the frequency in case of a frequency deviation originating from the control area to its set value in order to free the capacity engaged by the primary control (and to restore the primary control reserves). In order to fulfill these functions, Secondary Control operates by the Network characteristic method. Secondary control is applied to selected generator sets in the power plants comprising this control loop. Secondary control operates for periods of several minutes, and is therefore dissociated from primary control. This behavior over time is associated with the PI (proportional-integral) characteristic of the secondary controller. (ENTSO-E UCTE Operational Handbook)

Secondary Control Reserve

The positive / negative Secondary Control Reserve is the part of the secondary control range between the working point and the maximum / minimum value. The portion of the secondary control range already activated at the working point is the secondary control power. (ENTSO-E UCTE Operational Handbook)

Secondary Controller

A Secondary Controller is the single centralized TSO-equipment per control area/block for operation of secondary control (ENTSO-E UCTE Operational Handbook)

Security Limits / Stability limits

Security Limits define the acceptable operating boundaries (thermal, voltage and stability limits). The TSO must have defined Security Limits for its own network. The TSO shall ensure adherence to these Security Limits. Violation of Security Limits for prolonged time could cause damage and/or an outage of another element that can cause further deterioration of system operating conditions. (ENTSO-E UCTE Operational Handbook)

Self-Regulation of Load

The Self-Regulation of Load is defined as the sensitivity of consumers’ demand to variations in the system frequency (a decrease of the system frequency results in a decrease of the load), generally expressed in % / Hz. (ENTSO-E UCTE Operational Handbook)

Spinning reserve

Increase or decrease in generation or reduction in consumption that can be provided at short notice, carried out by partially loaded generating units and interruptible customers. (EURELECTRIC report ‘Ancillary services’, 2004)


Stability is the ability of an electric system to maintain a state of equilibrium during normal and abnormal system conditions or disturbances. • Small-Signal Stability — The ability of the electric system to withstand small changes or disturbances without the loss of synchronism among the synchronous machines in the system while having a sufficient damping of system oscillations (sufficient margin to the border of stability). • Transient Stability — The ability of an electric system to maintain synchronism between its parts when subjected to a disturbance of specified severity and to regain a state of equilibrium following that disturbance. (ENTSO-E UCTE Operational Handbook)

Standing reserve

Increase in generation or reduction in consumption that can be provided by those generating units that are not synchronously on-line, or by interruptible customers. (EURELECTRIC report ‘Ancillary services’, 2004)

Supervisory Control and Data Acquisition (SCADA)

Supervisory Control and Data Acquisition is a system of remote control and telemetry used to monitor and control the electric system. (ENTSO-E UCTE Operational Handbook)

Synchronous Area

A Synchronous Area is an area covered by interconnected systems whose control areas are synchronously interconnected with control areas of members of the association. Within a Synchronous Area the system frequency is common on a steady state. A Synchronous Area is a set of synchronously interconnected systems that has no synchronous interconnections to any other interconnected systems. Within ENTSO-E there are 5 synchronous areas: Continental Europe, Ireland, UK, Nordic & Baltic. (ENTSO-E UCTE Operational Handbook)

System Defence Plan

The summary of all technical and organizational measures undertaken to prevent the propagation or deterioration of an incident in the transmission system, in order to avoid a widespread disturbance and Blackout State. (ENTSO-E Draft NC Operational Security)

System Frequency

{also called Frequency} System Frequency is the electric frequency of the system that can be measured in all network areas of the synchronous areas under the assumption of a coherent value for the system in the time frame of seconds (with minor differences between different measurement locations only). (ENTSO-E UCTE Operational Handbook)

System Services

System services are services provided by network operators to users connected to the system in order to ensure required power quality and the stability of the distribution grid. See ancillary services above (EURELECTRIC report ‘Ancillary services’, 2004)

System State

The operational state of the transmission system in relation to the Operational Security Limits. (ENTSO-E Draft NC Operational Security)

Tertiary Control

Tertiary Control is any (automatic or) manual change in the working points of generators (mainly by re-scheduling), in order to restore an adequate secondary control reserve at the right time. (ENTSO-E UCTE Operational Handbook)

Tertiary Control Reserve

{Minute Reserve} The power which can be connected (automatically or) manually under tertiary control, in order to provide an adequate secondary control reserve, is known as the tertiary control reserve or minute reserve. This reserve must be used in such a way that it will contribute to the restoration of the secondary control range when required. The restoration of an adequate secondary control range may take, for example, up to 15 minutes, whereas tertiary control for the optimization of the network and generating system will not necessarily be complete after this time.(ENTSO-E UCTE Operational Handbook)

Tie Line

A Tie-Line is a circuit (e.g. a transmission line) connecting two or more control area or systems of an electric system.(ENTSO-E UCTE Operational Handbook)

Total Transfer Capacity (TTC)

Total Transfer Capacity is the maximum exchange program between two adjacent control areas that is compatible with operational security standards applied in each system (e.g. Grid Codes) if future network conditions, generation and load patterns are perfectly known in advance.(ENTSO-E UCTE Operational Handbook)


Transmission is the transport of electricity on the extra-high or high-voltage network (transmission system) for delivery to final customers or distributors. Operation of Transmission includes as well the tasks of system operation concerning the management of energy flows, reliability of the system and availability of all necessary system services / ancillary services.(ENTSO-E UCTE Operational Handbook)

Transmission Reliability Margin (TRM)

The Transmission Reliability Margin is a security margin that copes with uncertainties on the computed TTC values arising from: • Unintentional deviations of physical flows during operation due to the physical functioning of secondary control • Emergency exchanges between TSOs to cope with unexpected unbalanced situations in real-time • Inaccuracies, e. g. in data collection and measurements (ENTSO-E UCTE Operational Handbook)

Transmission System Operator (TSO)

a natural or legal person responsible for operating, ensuring the maintenance of and, if necessary, developing the transmission system in a given area and, where applicable, its interconnections with other systems, and for ensuring the long-term ability of the system to meet reasonable demands for the transmission of electricity. (2009/75/EC)

Voltage control

Maintaining voltage through injecting or absorbing reactive power by means of synchronous or static compensation. (EURELECTRIC report ‘Ancillary services’, 2004)