HV Engineering Glossary | Substation & Transmission Terms

A

ACSR: Aluminum Conductor Steel Reinforced — the most common overhead line conductor: aluminum strands for conductivity around a steel core for mechanical strength.
AIS: Air-Insulated Switchgear — substation switchgear that uses ambient air as the insulating medium, with equipment mounted in an open outdoor yard.
Ampacity: The maximum current a conductor can carry continuously without exceeding its temperature rating.
Autotransformer: A transformer whose primary and secondary share part of the same winding — common at 400/230 kV interconnection points for its smaller size and cost.

B

Bay: A complete switching unit within a substation — typically a breaker with its disconnectors, instrument transformers and connections to the busbar.
BESS: Battery Energy Storage System — grid-connected batteries (commonly LFP lithium-ion) used for backup power, peak shaving and renewable integration.
Busbar: The rigid or strung conductor system that distributes current between bays inside a substation.
Bushing: The insulated feed-through that carries a conductor safely through the grounded tank wall of a transformer or breaker.

C

Capacitor Bank: A set of capacitors connected to the grid to supply reactive power, improve voltage profile and power factor.
Catenary: The natural sag curve a conductor forms between two towers under its own weight — central to clearance and tension design.
Circuit Breaker: A switching device that can interrupt fault current — SF6, vacuum or air-blast — and the protective heart of every bay.
Conservator: The expansion tank on top of an oil-filled transformer that accommodates oil volume changes with temperature.
Corona: Ionization of air at a conductor's surface under high electric field — causes losses, audible noise and radio interference; controlled by bundling.
Cross-arm: The horizontal member of a tower that carries the insulator strings and conductors.
CT: Current Transformer — an instrument transformer that scales primary current down to measurable levels for metering and protection relays.
CVT: Capacitor Voltage Transformer — measures high voltage via a capacitive divider; standard at 132 kV and above.

D

Dead-end Tower: A tension tower that anchors the full mechanical tension of the conductors — used at angles, crossings and line ends.
Dead-tank Breaker: A circuit breaker whose interrupting chamber sits inside a grounded tank, with CTs integrated on the bushings.
Disconnector: An off-load isolating switch that provides a visible break for safe maintenance — also called an isolator.
Double Circuit: A transmission line carrying two independent three-phase circuits on the same towers — six phase conductors total.

E

Earthing Switch: A grounding blade integrated with a disconnector that safely earths an isolated section before work begins.
EHV: Extra-High Voltage — transmission voltages of roughly 345 kV to 765 kV; 400 kV lines are EHV class.
EPC: Engineering, Procurement and Construction — a delivery model where one contractor designs, sources and builds the complete facility.

F

FACTS: Flexible AC Transmission Systems — power-electronic devices (SVC, STATCOM, TCSC) that control voltage and power flow dynamically.
Feeder: A circuit that carries power from a substation toward loads or between voltage levels — line feeders, transformer feeders.

G

Gantry: The steel portal structure at a substation that terminates incoming lines and supports the busbar.
GIS: Gas-Insulated Switchgear — switchgear enclosed in grounded metal housings filled with SF6, achieving a fraction of the AIS footprint.
Grid Code: The technical rules a facility must meet to connect to the transmission network — voltage ride-through, frequency response, protection.

H

Hot-dip Galvanizing: Immersing fabricated steel in molten zinc for corrosion protection — standard for towers and substation structures.

I

IEC 61850: The international standard for substation communication and automation — the backbone of modern digital substations.
Insulator String: A chain of disc insulators connecting an energized conductor to a grounded tower — length scales with voltage.

K

kV: Kilovolt — one thousand volts; the unit used to describe voltage classes such as 63 kV, 132 kV, 230 kV, 400 kV.

L

LFP: Lithium Iron Phosphate — the dominant battery chemistry for stationary storage, valued for thermal stability and cycle life.
Line Trap: A tuned inductor at a line's entry to a substation that blocks power-line-carrier communication signals from entering the bus.

M

Monopole: A single-shaft steel pole structure used where lattice tower footprints are impractical — common in urban lines and telecom.

O

OLTC: On-Load Tap Changer — the mechanism that adjusts a transformer's turns ratio under load to regulate voltage.
ONAN / ONAF: Transformer cooling classes — Oil Natural Air Natural / Oil Natural Air Forced (fans); most large units are dual-rated.
OPGW: Optical Ground Wire — a shield wire with optical fibers inside, providing lightning protection and communications on one cable.

P

PC: Procurement and Construction — a delivery model where the contractor sources and builds to a design the owner provides.
Power Factor: The ratio of real power to apparent power — corrected with capacitor banks or FACTS devices.
Protection Relay: The intelligent device that detects faults from CT/VT signals and trips breakers — modern units are IEC 61850 IEDs.

R

Reactive Power: Power that oscillates between source and load sustaining electric and magnetic fields — controls system voltage.

S

Sag: The vertical drop of a conductor below its attachment points — increases with temperature, governs ground clearance.
SCADA: Supervisory Control And Data Acquisition — the system operators use to monitor and control substations remotely.
SF6: Sulfur hexafluoride — the insulating and arc-quenching gas used in GIS and many HV breakers.
Shield Wire: The grounded wire strung at the top of towers to intercept lightning strikes before they hit phase conductors.
Single Line Diagram: The standard one-line schematic representing a three-phase power system — the master drawing of every substation.
Span: The horizontal distance between two adjacent towers — typically 350–450 m on 400 kV lines.
Stringing: The construction operation of pulling conductors and shield wires onto erected towers under controlled tension.
Substation: A facility that switches, transforms and protects power flow between parts of the grid.
Suspension Tower: The standard tangent tower that simply carries conductors on vertical insulator strings — the majority of any line.
Switchyard: The outdoor equipment area of a substation — gantries, breakers, disconnectors and instrument transformers.

T

Tap Changer: See OLTC — the voltage-regulation mechanism of a transformer.
Turnkey: A delivery model where the contractor hands over a fully commissioned, ready-to-operate facility.

V

VT: Voltage Transformer — an instrument transformer that scales primary voltage down for metering and protection.

HV engineering glossary.