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Glossary

A

AC distribution board (ACDB): The two-section low-voltage switchboard, joined by a normally open bus coupler, that the auxiliary transformers and the DG set feed.; Topic: AC auxiliary supply
air circuit breaker (ACB): The breaker of an incomer of the ACDB, and of a feeder whose current is above the largest MCCB rating, in IEC 60947-2 rated currents.; Topic: AC auxiliary supply
air-insulated switchgear (AIS): Switchgear in an open yard, with air as the insulation between its live parts.; Topic: Bus schemes
allowable stroke current ( $I_{s}$ ): The smallest stroke current that can flash over the insulation of a protected level.; Topic: Direct stroke shielding by the rolling sphere, Eq. (3)
alternating current (AC): Current that reverses its direction at the frequency of the network, as the auxiliary supply distributes it to the substation's low-voltage loads.; Topic: AC auxiliary supply
altitude correction factor ( $K_{a}$ ): The factor $e^{m H /8150}$ of IEC 60071-2 by which the design multiplies the electrical distances of a site above the standard altitude.; Topic: Electrical clearances, Eq. (1)
aluminium conductor steel-reinforced (ACSR): A stranded conductor of aluminium wires on a steel core, the conductor of the yard's equipment interconnection.; Topic: Direct stroke shielding by the rolling sphere
apparent resistivity ( $ρ_{a}$ ): The resistivity a Wenner reading gives at one spacing, $2 π S R$ for a resistance $R$ read at a spacing $S$ .; Topic: Soil resistivity and Wenner readings, Eq. (1)
automatic mains failure panel (AMF): The panel that starts the DG set when the transformer supply of the auxiliary switchboard is lost.; Topic: AC auxiliary supply
automatic transfer switch (ATS): The switch through which the standby generator of the ANSI / IEEE set's station service starts.; Topic: AC auxiliary supply
availability-based tariff (ABT): The tariff for which the line bay carries the main and check meters of the grid connection in the IEC + CEA / CBIP set.; Topic: Bays and their equipment

B

base impedance ( $Z_{b}$ ): The impedance $U_{r}^{2} / S_{r}$ that the rated voltage and the rated apparent power of a piece of equipment define.; Topic: Per-unit quantities
basic lightning impulse insulation level (BIL): The lightning impulse withstand voltage of a voltage level, a peak value in kVp.; Topic: Insulation levels
basic switching impulse insulation level (BSL): The switching impulse level that the ANSI / IEEE set takes from IEEE C37.04-2018 Table 6 at its highest voltage level.; Topic: Insulation levels
bay: The set of equipment that connects one circuit to the buses of a high-voltage switchyard.; Topic: Bays and their equipment
bill of materials (BOM): The list of the substation's equipment and materials that the design calculates, with a specification, a unit, a quantity and the spares of each line.; Topic: Bill of materials
bus post insulator (BPI): An insulator that supports a tubular bus, one more per tube than the tube's spans.; Topic: Bill of materials
bus reactor: A shunt reactor on an HV bus, in a circuit position of its own, that absorbs reactive power to hold down the bus voltage.; Topic: Reactive power equipment
bus scheme: The way the circuits of a substation connect to its buses, which sets its number of circuit breakers and the arrangement of the yard's bays.; Topic: Bus schemes

C

capacitor voltage transformer (CVT): The voltage transformer at the line entry of a line bay, also the voltage transformer of an HV bus above the 33 kV level.; Topic: Bays and their equipment
capacity rating factor ( $K_{t}$ ): The rated ampere-hours of a cell per ampere of load held for a time $t$ , from the cell's discharge curve to the end-of-discharge voltage.; Topic: DC system, Eq. (2)
Central Board of Irrigation and Power (CBIP): The board whose manuals the IEC + CEA / CBIP set joins with the IEC standards and the CEA regulations.; Topic: The IEC + CEA / CBIP set
Central Electricity Authority (CEA): The authority whose Construction Regulations 2022 and Safety Regulations 2023 the IEC + CEA / CBIP set follows.; Topic: The IEC + CEA / CBIP set
circuit breaker (CB): The switching device of a bay, rated at the level's voltage, its design fault level and the bay current, whose number follows the bus scheme.; Topic: Circuit breakers and isolators
corona radius ( $R_{c}$ ): The radius of the corona envelope that forms around a conductor under a lightning stroke's surge.; Topic: Direct stroke shielding by the rolling sphere, Eq. (1)
coupling capacitor voltage transformer (CCVT): The name of the capacitor voltage transformer in the ANSI / IEEE set's bill of materials.; Topic: Instrument transformers
current transformer (CT): The instrument transformer that measures the current of a bay's circuit, next to its circuit breaker.; Topic: Bays and their equipment

D

DC distribution board (DCDB): The two-section board, joined by a normally open coupler, that the chargers and batteries feed and from which the DC loads are supplied.; Topic: DC system
decrement factor ( $D_{f}$ ): The factor that raises the symmetrical fault current to the effective value of the asymmetric current over the fault's duration.; Topic: Grid current, Eq. (1)
diesel generator set (DG): The standby generator of the IEC + CEA / CBIP set's auxiliary supply, which supplies the full auxiliary load when both auxiliary transformers are out.; Topic: AC auxiliary supply
direct current (DC): Current of one direction, as a battery and its chargers supply it to the protection, control and trip circuits when the AC auxiliary supply is lost.; Topic: DC system
direct stroke lightning protection (DSLP): The shielding of a switchyard's live parts against lightning strokes that would otherwise terminate on them.; Topic: Direct stroke shielding by the rolling sphere
direct-on-line start (DOL): A motor start that draws several times the motor's full-load current at a low power factor, which causes a transient voltage dip on the supply.; Topic: AC auxiliary supply
double main bus: A bus scheme with a circuit breaker per bay and a bus coupler between its two buses.; Topic: Bus schemes
Drawing Exchange Format (DXF): The CAD file format of the drawings, drawn from the same model of a sheet as the screen and the PDF file.; Topic: Drawing conventions
duty cycle: The current a battery must deliver, period by period, over the time it supports the substation without a charger.; Topic: DC system

E

electric overhead travelling crane (EOT): The crane of a GIS hall, which the BOM lists with the hall.; Topic: Gas-insulated switchgear (GIS)
electrical clearance: A least distance in air that the switchyard keeps around its live parts.; Topic: Electrical clearances

G

gas-insulated bus duct (GIB): A gas-insulated duct that carries a circuit out of the GIS hall, one of the exits the SLD marks, with the SF6/air bushing and the cable end.; Topic: Gas-insulated switchgear (GIS)
gas-insulated switchgear (GIS): Switchgear of gas-insulated modules in an indoor hall, the GIS hall.; Topic: Gas-insulated switchgear (GIS)
grid current ( $I_{G}$ ): The part of an earth fault's current that flows from the earth mat into the soil, raised for the DC offset of the fault current.; Topic: Grid current
grid resistance ( $R_{g}$ ): The resistance of the earth mat to remote earth.; Topic: Grid resistance and GPR, Eq. (1)
ground clearance: The equipment level of a yard, the height of its live parts above the ground.; Topic: Electrical clearances

H

harmonic filter: A tuned bank of capacitors and reactors on an MV section, whose rating comes from a harmonic study.; Topic: Reactive power equipment
high-voltage (HV): The voltage level of the substation's switchyard and of its lines to the grid.; Topic: Bays and their equipment
highest system voltage ( $U_{m}$ ): The first insulation rating of a voltage level, by which IEC 60071-1:2019 divides the levels into range I and range II.; Topic: Insulation levels
horizontal centre-break isolator (HCB): The design's three-phase, motor-operated HV isolator, rated at the bay's current and the level's ratings.; Topic: Circuit breakers and isolators

I

initial symmetrical current ( $I_{k}^{''}$ ): The symmetrical short-circuit current at the first instant of a fault.; Topic: Short-circuit currents by IEC 60909-0
Institute of Electrical and Electronics Engineers (IEEE): The institute whose standards, with the ANSI standards and the NESC, make up the ANSI / IEEE set.; Topic: The ANSI / IEEE set
interconnecting transformer (ICT): A power transformer of the substation, connected to the HV buses through its transformer bay.; Topic: Bays and their equipment
irregularity factor ( $K_{i}$ ): The factor $0.644 + 0.148 n$ of IEEE Std 80-2013 in the mesh and step voltages, which grows with the mat's geometry factor $n$ .; Topic: Mesh voltage and touch voltage, Eq. (6)

L

lightning mast: A mast that protects what the shield wires leave exposed, placed by the design one at a time where it protects the most exposed points.; Topic: Masts and shield wires
lightning protection level (LPL): A level of IEC 62305-3, for which Table 2 gives the radius of the rolling sphere.; Topic: Direct stroke shielding by the rolling sphere
line reactor: A shunt reactor at the end of a 765 kV line, switched by its own circuit breaker, with its neutral earthed through a neutral grounding reactor.; Topic: Reactive power equipment
load tap changer (LTC): The ANSI / IEEE set's name for the on-load tap changer.; Topic: Power transformers
low-voltage (LV): The voltage level of the substation's AC auxiliary supply and its switchboard, and of the lower-voltage side of a transformer.; Topic: AC auxiliary supply
low-voltage power circuit breaker (LVPCB): The breaker of the ANSI / IEEE set's station service incomers, coupler and feeders above the molded-case breaker range, in the frame sizes of IEEE C37.13-2015.; Topic: AC auxiliary supply

M

maximum demand: The apparent power the auxiliary supply carries with the loads at their duty factors, the vector sum of the active and reactive demand.; Topic: AC auxiliary supply
medium-voltage (MV): The voltage level of the substation's MV buses, between the transformers' low-voltage windings and the feeders.; Topic: Bus and bay current ratings
mesh voltage ( $E_{m}$ ): The highest touch voltage inside one mesh of the earth mat during a fault.; Topic: Mesh voltage and touch voltage, Eq. (2)
moulded-case circuit breaker (MCCB): The breaker of a load group's feeder on the ACDB, rated at the first standard current at or above the group's full-load current.; Topic: AC auxiliary supply
multi-ratio current transformer (MR): A current transformer with several ratios, the HV CT of the ANSI / IEEE set, whose full ratio is the bay current.; Topic: Instrument transformers

N

National Electrical Safety Code (NESC): The code, 2007 edition, from which the ANSI / IEEE set takes the clearance of live parts above surfaces where people stand and the fence clearance.; Topic: Clearances of the ANSI / IEEE set
negative-sequence impedance ( $Z_{2}$ ): The impedance of the network at the fault location to a balanced set of currents in the reverse phase order, which the design takes equal to $Z_{1}$ .; Topic: Symmetrical components
neutral grounding resistor (NGR): The resistor through which a transformer's MV neutral is earthed, which limits the line-to-earth fault current on the MV section.; Topic: Bays and their equipment

O

on-load tap changer (OLTC): The tap changer of the ICT, which changes the ratio over its range in steps.; Topic: Power transformers
one-and-a-half breaker: A bus scheme with three circuit breakers per diameter of two circuits.; Topic: Bus schemes

P

peak factor ( $κ$ ): The ratio of the peak short-circuit current to $2 I_{k}^{''}$ , which falls from 2 as the R/X ratio at the fault location rises.; Topic: Peak current and thermal equivalent current, Eq. (2)
peak short-circuit current ( $i_{p}$ ): The first peak of a short-circuit current, which sets the current a circuit breaker must make.; Topic: Peak current and thermal equivalent current, Eq. (1)
per unit (pu): A quantity divided by a base of the same kind: for equipment, its rated voltage, its rated apparent power or the base impedance they define.; Topic: Per-unit quantities
phase-to-earth clearance: The distance from the outer phase of a bay to the edge of the bay.; Topic: Electrical clearances
Portable Document Format (PDF): The file format of the drawings and documents, drawn from the same model of a sheet as the screen and the DXF file.; Topic: Drawing conventions
positive-sequence impedance ( $Z_{1}$ ): The impedance of the network at the fault location to a balanced set of currents in the network's phase order.; Topic: Symmetrical components
potential transformer (PT): The voltage transformer of an HV bus or bus section at the 33 kV level, in place of a CVT.; Topic: Bays and their equipment

R

rated short-circuit voltage ( $u_{k r}$ ): A transformer's impedance in percent of its base impedance, from which the design calculates the impedance in ohms.; Topic: Per-unit quantities, Eq. (1)
reflection factor ( $K$ ): The factor $(ρ_{2} - ρ_{1}) / (ρ_{2} + ρ_{1})$ of the interface of a two-layer soil, between −1 and 1.; Topic: Two-layer soil, Eq. (2)
reinforced cement concrete (RCC): The construction of the control building in the IEC + CEA / CBIP set's BOM, counted by the building's floor area.; Topic: Roads, trenches and buildings
relative reactance ( $x_{T}$ ): The reactance of a transformer per unit of its base impedance, which sets the correction factor $K_{T}$ .; Topic: Per-unit quantities, Eq. (2)
ring bus: A bus scheme of the ANSI / IEEE set: a closed loop with a circuit breaker between two bus sections, each section supplying one circuit.; Topic: Bus schemes
rise of earth potential (GPR): The voltage of the earth mat above remote earth during a fault, the grid current times the grid resistance.; Topic: Grid resistance and GPR, Eq. (3)
rolling sphere: A sphere rolled over the ground, the shield wires and the masts: a point the sphere cannot touch is protected from direct strokes.; Topic: Direct stroke shielding by the rolling sphere
Rural Utilities Service (RUS): The service whose Bulletin 1724E-300, a design guide for rural substations, gives the ANSI / IEEE set its yard clearances, its ring bus and its station service.; Topic: The ANSI / IEEE set

S

sectional clearance: The gap between the nearest phases of the two main buses, for maintenance with one bus live.; Topic: Electrical clearances
shield wire: An overhead conductor that the design runs along a column boundary of the HV yard, above the gantry beam, to shield the yard from direct strokes.; Topic: Masts and shield wires
single-line diagram (SLD): A diagram of the substation's electrical connections with one line for the three phases.; Topic: Single-line diagram (SLD)
solidly earthed neutral: A transformer or reactor neutral connected to earth with no added impedance, as the HV neutrals of the design's ICTs are.; Topic: Earthing of the neutral
split factor ( $S_{f}$ ): The share of the earth fault current that flows from the earth mat into the soil.; Topic: Grid current
spreadsheet workbook (XLSX): The file in which the BOM is delivered, with a BOM sheet and a Vendors sheet.; Topic: Bill of materials
static synchronous compensator (STATCOM): The reactive power device of the ANSI / IEEE set, one on each MV section, rated on Auto for the reactive power of the plant at its rated output and a power factor of 0.95.; Topic: Reactive power equipment
static var generator (SVG): The reactive power device of the IEC + CEA / CBIP set, one on each MV section, rated on Auto at a share of the plant capacity.; Topic: Reactive power equipment
step voltage ( $E_{s}$ ): The difference in surface potential that a person bridges with the feet 1 m apart, without touching an earthed object, during an earth fault.; Topic: Step voltage
striking distance ( $S$ ): The radius of the rolling sphere for a protected level, from the allowable stroke current of that level.; Topic: Direct stroke shielding by the rolling sphere, Eq. (4)
subtransient reactance ( $x_{d}^{''}$ ): The reactance of a synchronous generator that its datasheet gives per unit of its own rating, from which the design calculates the generator's impedance in the short-circuit study.; Topic: Per-unit quantities
sulphur hexafluoride (SF6): The insulating gas of the GIS enclosures and of the HV circuit breakers, from which a GIS line leaves through an SF6/air bushing.; Topic: Yard layout
surface layer: A layer of crushed rock of high resistivity, spread over the soil of the switchyard, that raises the tolerable touch and step voltages.; Topic: Surface layer and Cs
surface layer derating factor ( $C_{s}$ ): The factor that corrects the surface layer's resistivity for its finite depth over the soil below.; Topic: Surface layer and Cs, Eq. (1)
surge impedance ( $Z_{s}$ ): The impedance of a conductor to a lightning stroke's surge under corona, from its height, its radius and the corona radius.; Topic: Direct stroke shielding by the rolling sphere, Eq. (2)
switching impulse withstand voltage (SIWL): The switching impulse withstand of a voltage level in kVp, carried at the highest levels only.; Topic: Insulation levels
symmetrical components: The positive-, negative- and zero-sequence sets of balanced currents whose sum gives the currents of an unbalanced fault.; Topic: Symmetrical components

T

temporary overvoltage (TOV): The overvoltage at a surge arrester's location, given by the neutral earthing study, on which the arrester's rating depends.; Topic: Insulation levels
thermal equivalent current ( $I_{th}$ ): The heat of a short-circuit current over the fault's duration, expressed as a current, which sets the short-time rating.; Topic: Peak current and thermal equivalent current, Eq. (4)
tolerable body current ( $I_{B}$ ): The largest current a person of a given weight tolerates for the duration of a shock.; Topic: Tolerable body current
tolerable step voltage: The step voltage that drives the tolerable body current through the body and the two feet in series.; Topic: Tolerable body current, Eq. (4)
tolerable touch voltage: The touch voltage that drives the tolerable body current through the body and the two feet in parallel.; Topic: Tolerable body current, Eq. (3)
touch voltage: The difference between the rise of earth potential of the earth mat and the surface potential where a person stands while touching an earthed structure.; Topic: Mesh voltage and touch voltage
two-layer soil: A soil model of an upper layer of resistivity $ρ_{1}$ and depth $h$ over a lower layer of resistivity $ρ_{2}$ .; Topic: Two-layer soil

V

vacuum circuit breaker (VCB): The withdrawable circuit breaker of a metal-clad MV panel, and the breaker of an outdoor MV bay.; Topic: Circuit breakers and isolators
valve-regulated lead-acid (VRLA): The lead-acid battery type of the IEC + CEA / CBIP set's DC system.; Topic: DC system
voltage factor ( $c$ ): The factor of the equivalent voltage source $c U_{n} / 3$ : $c_{m a x}$ for the maximum currents and $c_{m i n}$ for the minimum currents.; Topic: Short-circuit currents by IEC 60909-0

W

water-bound macadam (WBM): A road construction that the BOM's road line of the IEC + CEA / CBIP set reads, with bituminous roads.; Topic: Roads, trenches and buildings
Wenner method: The measurement of the soil's resistivity with four electrodes in a straight line at equal spacing, current through the outer pair and voltage across the inner pair.; Topic: Soil resistivity and Wenner readings

Z

zero-sequence impedance ( $Z_{0}$ ): The impedance of the network at the fault location to 3 equal currents in phase, which return through earth or a neutral.; Topic: Symmetrical components