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A
- Anti-pumping
- Breaker control circuit that prevents repeated closing when the close command is held and the breaker trips.
- Automatic reclosing (79)
- Function that recloses the breaker after a trip, because many faults on overhead lines are transient. Read the guide
- Automatic transfer
- Automatic switching of a load between two sources when the main source fails.
- Autotransformer
- Transformer in which part of the winding is common to the primary and secondary circuits.
- Auxiliary contacts
- Breaker or disconnect switch contacts that follow its position for control logic, interlocks and indication.
B
- Bay
- Set of switching, metering and protection equipment that connects one circuit (line, transformer, feeder) to the bus.
- Bay controller
- IED that handles control, interlocking, metering and indication of a bay, locally and towards the SCADA.
- Breaker failure (50BF)
- Function that trips the adjacent breakers if the breaker does not open after a trip command. Read the guide
- Breaker operating time
- Time between the open or close command and the separation or touching of the poles.
- Breaker-and-a-half
- Arrangement with two buses and three breakers for every two circuits.
- Burden (CT load)
- Impedance connected to the secondary of an instrument transformer, expressed in ohms or in volt-amperes at rated current.
- Bus (busbar)
- Common conductor to which the bays of a substation at the same voltage level are connected.
- Bushing
- Insulator that lets a conductor pass through the tank or wall of an apparatus with controlled insulation.
C
- Capacitor bank
- Group of capacitors connected to the system to supply reactive power and improve power factor and voltage.
- Capacitor voltage transformer (CVT)
- Voltage transformer with a capacitive divider, used at high and extra-high voltage and as the power line carrier coupling.
- Circuit breaker
- Switching device able to make, carry and interrupt normal and short-circuit currents.
- Commissioning
- Set of tests and checks that prove an installation works as designed before it is energized.
- Contact resistance
- Resistance of the closed current path of a breaker or disconnect switch, measured with a high DC current.
- Current reversal guard
- Logic that prevents false trips on parallel lines when the current changes direction as a fault is cleared. Read the guide
- Current transformer (CT)
- Instrument transformer whose secondary current is proportional to the primary current, typically 1 A or 5 A.
D
- Dead zone
- Section between the breaker and its CT where a fault is not cleared by the protection of the zone that opened. Read the guide
- Differential protection (87)
- Protection that compares the currents entering and leaving a zone and operates on their difference. Read the guide
- Digital fault recorder (DFR)
- Device that records the waveforms and events of a disturbance for later analysis.
- Digital substation
- Substation in which the switchyard signals are digitized close to the equipment and travel over an IEC 61850 process bus.
- Direct transfer trip (DTT)
- Teleprotection signal that trips the remote breaker without local supervision. Read the guide
- Directional comparison blocking (DCB)
- Teleprotection scheme in which the end that sees the fault in the reverse direction sends a signal that blocks tripping at the other end. Read the guide
- Directional comparison unblocking (DCUB)
- Permissive scheme that allows tripping on loss of the channel guard signal during a fault. Read the guide
- Disconnect switch
- Switching device that opens or closes a circuit without load and provides a visible isolating distance.
- Distance protection (21)
- Protection that measures the impedance to the fault and compares it with set reach zones. Read the guide
- Distance zones
- Stepped reaches of distance protection: underreaching zone 1 without delay, and time-delayed zones 2 and 3 as backup. Read the guide
- DNP3 (IEEE 1815)
- SCADA communication protocol widely used in the Americas between master stations, RTUs and IEDs.
- Double bus
- Arrangement with two buses in which each bay can be connected to either of them through disconnect switches.
E
- End-to-end test
- Test in which fault conditions are injected at both ends of a line at the same time, synchronized by GPS, to verify the protection and the teleprotection.
- Excitation current
- Current drawn by a transformer at no load to magnetize its core; measuring it reveals core and winding problems.
F
- FAT and SAT
- Factory acceptance tests (FAT) and site acceptance tests (SAT) of panels and systems before commissioning.
G
- Gateway
- Device that gathers the information from the IEDs and translates it into the protocols of the control center.
- GIS
- Gas-insulated substation, with the high-voltage equipment inside metal enclosures filled with SF6 or another gas.
- GOOSE
- IEC 61850 multicast message that carries events between IEDs, such as trips and blocking signals, within a few milliseconds.
- Ground grid
- Network of buried conductors that limits step and touch voltages and provides a path for fault currents.
- Grounding switch
- Switch that connects a de-energized circuit to ground so it can be worked on safely.
- Grounding transformer
- Transformer that creates an accessible neutral in an ungrounded system to limit and detect ground faults.
H
- Harmonic restraint
- Blocking of transformer differential protection by second- or fifth-harmonic content during energization or overexcitation. Read the guide
- HMI
- Human-machine interface through which the operator monitors and operates the substation.
- HSR
- High-availability seamless redundancy for ring networks, defined in IEC 62439-3; each frame travels in both directions of the ring.
I
- ICCP (TASE.2)
- Protocol for exchanging real-time data between control centers.
- IEC 60870-5-104
- Telecontrol protocol over TCP/IP between substations and control centers.
- IEC 61850
- Family of standards for the communication and configuration of substation automation systems.
- IED
- Intelligent electronic device: a relay, controller or meter with a processor and communications.
- Insulation power factor (tan δ)
- Ratio between the resistive and the capacitive current of the insulation; an increase indicates moisture, contamination or aging.
- Insulation resistance
- Resistance measured with DC voltage between a conductor and ground or between windings; it indicates the condition of the insulation.
- Interlock
- Logic or mechanical lock that prevents unsafe operations, such as opening a disconnect switch under load.
- Internal and external fault
- Fault inside or outside the protected zone; a unit protection must operate only for internal faults.
- Interrupter (arc chamber)
- Part of the circuit breaker where the contacts separate and the arc is extinguished, in vacuum, SF6 or another medium.
- Inverse-time curve
- Characteristic in which the operating time decreases as the current increases; standardized in IEC 60255-151 and IEEE C37.112. Read the guide
- IRIG-B
- Time code used to synchronize IEDs to a GPS reference, over coaxial cable or fiber.
K
- Knee point
- Point of the excitation curve of a CT beyond which small voltage increases cause large increases in excitation current.
L
- Lightning shielding
- Protection of the substation against direct lightning strokes with shield wires and air terminals (masts).
- Line trap (wave trap)
- Series reactor in the line that passes power-frequency current and blocks the power line carrier signal from entering the substation.
- Load shedding
- Automatic disconnection of load on underfrequency or undervoltage to restore the balance between generation and demand. Read the guide
- Lockout relay (86)
- Hand-reset auxiliary relay that trips the breakers and keeps them open until it is reset. Read the guide
- Logical node
- IEC 61850 group of data and functions with a standard name, such as PDIS for distance or XCBR for a circuit breaker.
- Loss of field (40)
- Condition in which a generator loses its excitation and absorbs reactive power from the system. Read the guide
M
- Medium-voltage switchgear
- Metal enclosure that houses the breaker, disconnect switches, instrument transformers and protection of a medium-voltage circuit.
- Merging unit
- Device that digitizes the CT and VT signals and publishes them as IEC 61850-9-2 sampled values.
- Mho characteristic
- Circular operating zone on the R-X plane used by distance relays; it passes through the origin and is inherently directional. Read the guide
- MMS
- IEC 61850 client-server service for monitoring, control and report reading between IEDs and the SCADA.
- Modbus
- Serial or TCP protocol for reading and writing registers, widely used in industry.
N
- Negative sequence
- Symmetrical component with reverse rotation that appears with unbalanced loads or faults and heats the rotor of machines. Read the guide
O
- On-load tap changer (OLTC)
- Mechanism that changes the transformer ratio in service to regulate voltage without interrupting the load.
- Out-of-step (78)
- Condition in which a generator or an area stops running in synchronism with the rest of the system. Read the guide
- Overexcitation (24)
- Excessive flux in the core caused by a high V/Hz ratio; it heats the core of the transformer or generator. Read the guide
P
- Partial discharge
- Localized electrical discharge that does not completely bridge the insulation; measuring it reveals incipient defects.
- Percentage differential
- Differential protection whose operating current must exceed a percentage of the restraint current, with one or two slopes. Read the guide
- Phase comparison
- Line protection scheme that compares the phase angle of the currents at both ends to decide whether the fault is internal. Read the guide
- Pickup
- Value of the measured quantity above which a protection function starts to operate or to time.
- PMU
- Phasor measurement unit that measures time-synchronized voltage and current phasors.
- Polarity
- Marks that show the relative direction of the primary and secondary currents or voltages of an instrument transformer.
- Polarization index
- Ratio between the insulation resistance measured at 10 minutes and at 1 minute; it indicates moisture or contamination of the insulation.
- POTT
- Permissive overreaching transfer trip: each end trips if it sees the fault in the forward direction and receives permission from the other end. Read the guide
- Power line carrier (PLC)
- Communication that uses the high-voltage line itself as the teleprotection channel.
- Power quality analyzer
- Instrument that records voltage, current, harmonics, flicker, sags and unbalance to assess power quality.
- Power swing
- Slow variation of the apparent impedance after a disturbance; it can make distance protection trip falsely. Read the guide
- Primary and backup protection
- Primary protection clears the fault in its zone as fast as possible; backup protection acts if the primary protection or the breaker fails.
- Process bus
- Ethernet network that carries sampled values and signals from the switchyard equipment to the IEDs, per IEC 61850.
- Protection and control panel
- Cabinet that houses the relays, controllers, terminal blocks and control switches of one or more bays.
- Protection coordination
- Setting of pickups and time delays so that the protection closest to the fault operates first and the others act only as backup.
- Protection settings
- The pickup values, time delays, curves and logic loaded into a relay so that it operates selectively and securely.
- Protective relay
- Device that measures electrical quantities and trips circuit breakers when it detects an abnormal condition.
- PRP
- Parallel redundancy protocol of IEC 62439-3: each frame is sent over two independent networks.
- PTP (IEEE 1588)
- Precision time protocol over Ethernet, with a power profile for substations.
- PUTT
- Permissive underreaching transfer trip: zone 1 sends a permissive signal so the remote end trips with its overreaching zone. Read the guide
R
- Recloser
- Distribution breaker with its own control that trips and recloses automatically according to a programmed sequence.
- Reliability
- Ability of a power or protection system to perform its function; in protection it combines dependability and security.
- Remedial action scheme (SPS/RAS)
- System-wide automation that executes predefined actions, such as tripping generation or load, for grid contingencies. Read the guide
- Restricted earth fault (REF)
- Zero-sequence differential protection of a grounded-wye winding, sensitive to faults close to the neutral. Read the guide
- Revenue metering point
- Point where the energy exchanged between market participants or with the utility is measured, together with its metering system.
- RSTP
- Rapid spanning tree protocol, which reconfigures a ring Ethernet network when a link fails.
- RTU
- Remote terminal unit that acquires substation signals and sends them to the control center.
S
- Sampled values
- Current and voltage samples published over Ethernet according to IEC 61850-9-2.
- SCADA
- Supervisory control and data acquisition system of the substation or the control center.
- SCL
- IEC 61850 configuration language, in ICD, IID, CID, SCD and SSD files.
- Secondary injection
- Relay test that applies currents and voltages from a test set to its secondary inputs.
- Selectivity
- Property of a protection system that isolates only the faulted part of the network.
- SF6
- Sulfur hexafluoride, an insulating and arc-quenching gas used in circuit breakers and GIS substations.
- Short-circuit current
- Current that flows during a fault; its value depends on the voltage, the system impedances and the type of fault.
- Shunt reactor
- Inductance connected to the system to absorb the capacitive reactive power of long lines or to limit currents.
- Single-pole reclosing
- Opening and reclosing of the faulted phase only, while the other two phases keep transmitting power. Read the guide
- SNTP
- Simple version of the network time protocol, used to synchronize IEDs with millisecond accuracy.
- Station battery
- Backup DC source that supplies protection, control and breaker mechanisms when the AC supply is lost.
- Station bus
- Substation Ethernet network that links the IEDs with the SCADA, the gateway and each other for MMS and GOOSE.
- Station service (AC/DC)
- The substation's own AC and DC supply for control, protection, lighting, ventilation and operating mechanisms.
- Step and touch voltage
- Potential differences a person may be exposed to during a ground fault; the ground grid limits them.
- Substation Ethernet switch
- Network switch hardened for the electromagnetic environment of a substation, compliant with IEC 61850-3 and IEEE 1613.
- Surge arrester
- Device that limits lightning and switching overvoltages by diverting them to ground.
- Switch onto fault (SOTF)
- Logic that trips at once if the breaker closes onto a faulted line, without waiting for the normal time delay. Read the guide
- Synchronism check (25)
- Check that voltage, frequency and angle match on both sides of the breaker before closing. Read the guide
T
- Teleprotection
- Use of a communications channel between the ends of a line to make tripping faster and selective. Read the guide
- Time dial (TMS or TD)
- Multiplier that shifts an inverse-time curve vertically; IEC calls it TMS and IEEE calls it time dial. Read the guide
- Trip circuit supervision
- Continuous monitoring of the integrity of the trip coil and trip wiring of the breaker.
- Trip coil
- Coil of the breaker operating mechanism that releases the opening mechanism when energized.
- Turns ratio
- Ratio between the primary and secondary voltages or currents of a transformer.
V
- Vector group
- Designation of the winding connections of a transformer and the phase shift between them, for example Dyn11.
- VLAN
- Virtual network that separates traffic on the same physical network; with IEEE 802.1Q priority it is used for GOOSE and sampled values.
- Voltage transformer (VT)
- Instrument transformer that reduces the primary voltage to a standard value for relays and meters.
W
- Weak infeed
- Logic that allows the end of a line to trip when its contribution to the fault is too low for its own protection to pick up. Read the guide
- Winding resistance
- DC resistance of each winding; it detects loose connections and tap changer defects.
Z
- Zero-sequence current
- Symmetrical component that is equal in the three phases; three times its value (3I0) is the residual current measured by ground protection.
- Zone acceleration
- Teleprotection scheme in which the signal received from the remote end lets distance zone 2 trip without its time delay. Read the guide
- Zone of protection
- Part of the system covered by a protection, bounded by the current transformers that feed it.
From the term to equipment in service
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Related guides
Other tools
COMTRADE generatorCreate a COMTRADE fault record (.cfg/.dat or .cff) for your test set: fault, DC offset and breaker opening.ANSI / IEEE C37.2 device numbersDevice function numbers 1 to 94, common suffixes and their IEC 61850 logical node, with search.Relay coordinationUp to six relays, reclosers and fuses on a log-log plot, with the IEEE C57.109 damage curve, inrush and the coordination margin of each pair.Inverse-time curvesCalculator and log-log plot of the IEC 60255-151 and IEEE C37.112 curves, with the coordination margin.CT burdenLead resistance, relay burden and comparison with the rated burden of the current transformer.Short-circuit currentQuick estimate of the three-phase fault at the secondary of a transformer.COMTRADE viewerOpen .cfg/.dat or .cff fault records in your browser: waveforms, digital channels, cursor and RMS. The file is not uploaded.IEC 61850 SCL viewerOpen .scd, .cid or .icd files in your browser: IEDs, GOOSE and SV addresses, datasets, subscribers and checks. The file is not uploaded.GOOSE and SV addressesCheck IEC 61850 multicast MAC, APPID, VLAN and priority, find duplicates and export the table as CSV.Process bus networkSV and GOOSE bandwidth, worst-case latency, PRP, HSR or RSTP redundancy and PTP time error of an IEC 61850 network.Interlocking table and CILO equationsInterlocking matrix per device, IEC 61850 EnaOpn and EnaCls equations, GOOSE signals between bays and FAT/SAT test cases per configuration.Per-unit base conversionBase current and impedance, conversion between %, pu and ohms, and change of base for transformers, generators and lines.Harmonic distortion THD and TDDVoltage THD and current TDD against the IEEE 519-2022 limits, with the spectrum as a bar chart.Symmetrical componentsPhase quantities to zero, positive and negative sequence and back (Fortescue), with phasor diagrams and what they mean for 46, 67Q and 51N.Vector group compensation (87T)TAP of each winding, phase compensation matrix, zero-sequence removal and the HV-to-LV clock diagram.Primary and secondary impedance (21)Primary to secondary ohms with the CT and VT ratios, line Z1, Z0 and k0, and zone reaches on the R-X plane.CT saturationRequired versus available voltage (5P/10P, PX or C class), time to saturation and the saturated waveform.DGA interpretationIEEE C57.104-2019 DGA status, Duval triangles 1, 4 and 5, IEC 60599 ratios and CO₂/CO of your transformer.Insulation temperature correctionInsulation resistance at 40 °C (IEEE 43) or 20 °C, PI, DAR and power factor of transformers and bushings, with the trend.Capacitor bankkvar to correct the power factor, steps of the automatic bank, capacitance and resonance with the transformer.Cable voltage dropΔV and % with R and X per km, copper or aluminium, mm² or AWG/kcmil, and the short-circuit check of the cable size.Battery sizingAh, number of cells and charger of a DC battery bank from its duty cycle, per IEEE 485 and IEEE 1115.Grounding resistanceGround rods, horizontal conductor and grid (BS 7430, IEEE 80), and soil resistivity from Wenner measurements.