Power system glossary

The power that does useful work, measured in watts (W, kW, MW). In a three-phase system P = √3 · U · I · cos φ, with U the line-to-line voltage.

See: Power factor · Reactive power

The short-circuit check for cables, S ≥ I · √t / k, which assumes all the heat of the fault stays in the conductor. The factor k depends on the conductor and insulation (IEC 60364-4-43).

See: Cable sizing calculator

An open-source Python library for power system dynamics, developed at CURENT (University of Tennessee). Electrisim uses it for transient stability and eigenvalue analysis.

See: Transient stability guide · Eigenvalue analysis docs

The product of voltage and current, measured in volt-amperes (VA, kVA, MVA): S² = P² + Q². Transformers, cables and generators are rated in apparent power because their heating depends on current.

See: Power factor

The release of heat and light from an electric arc between conductors or to earth. An arc-flash study estimates the incident energy at the working distance to choose protective clothing and mark boundaries.

See: Arc flash guide · Arc flash docs

The distance from the arc at which the incident energy falls to 1.2 cal/cm² (5.0 J/cm²), the level at which bare skin can receive a second-degree burn.

See: Arc flash guide

Batteries with a power conversion system (inverter) and usually a transformer, able to absorb or inject active and reactive power at the point of connection.

See: BESS sizing guide · BESS docs

A point in the network model where elements connect and the voltage is calculated. In a load flow every bus is a slack, PV or PQ bus.

See: Slack bus · Bus docs

Reference networks published by CIGRE (the International Council on Large Electric Systems) to test methods and software, such as the medium-voltage network with distributed energy resources.

See: CIGRE MV model

Repeating the load flow with one element (line, transformer or generator) out of service at a time, to check that the network stays within thermal and voltage limits after any single outage.

See: N-1 guide

The highest continuous current a cable can carry without its conductor exceeding the temperature limit of its insulation (70 °C for PVC, 90 °C for XLPE). It depends on the installation method, ambient temperature and grouping.

See: Cable sizing calculator

Linearizes the dynamic model around an operating point. The eigenvalues give the oscillation modes with their frequency and damping; modes with a negative real part decay.

See: Eigenvalue analysis docs

The network beyond the study boundary, modelled as a voltage source behind an impedance set by its short-circuit power. In a load flow it usually serves as the slack bus.

See: Slack bus · External grid docs

S″_k = √3 · U_n · I″_k at a point in the network. A high fault level means a strong network with a low source impedance, where load changes move the voltage only a little.

See: Short-circuit calculator

The grid-code requirement that generators stay connected during voltage dips caused by network faults, defined as a voltage-against-time profile.

See: Grid code

The voltage rise at the open or lightly loaded end of a long line or cable, caused by its charging current flowing through the series inductance. Shunt reactors limit it.

See: Shunt reactor · pandapower vs OpenDSS

An iterative load-flow method that updates the bus voltages one at a time. It is simple but converges slowly on large networks, so Newton-Raphson is usually preferred.

See: Newton-Raphson · Load flow guide

The technical rules a network operator sets for connecting generation and demand: reactive power capability, voltage and frequency ranges, fault ride-through and more. In the EU, the Requirements for Generators (RfG) network code sets the common framework.

See: Grid code P-Q docs · Wind farm reactive power guide

Voltage and current components at whole multiples of the fundamental frequency (50 or 60 Hz), produced by non-linear loads such as drives, rectifiers and inverters.

See: Harmonic analysis guide · THD

Transmission with direct current, used for long cables and for links between networks that are not synchronized. Modern links use voltage source converters.

See: VSC · Italian Hypergrid case study

The RMS value of the AC part of the fault current at the moment the fault occurs (IEC 60909). Used to check breaking capacity. pandapower calls it ikss.

See: Short-circuit calculator · Short circuit guide

The highest instantaneous value of the fault current, including the DC component: i_p = κ · √2 · I″_k. Used to check the peak withstand and making capacity of switchgear.

See: Short-circuit calculator · Peak factor κ

The RMS current that would produce the same heat as the actual fault current over the fault duration (IEC 60909). Used to check the thermal withstand of cables and equipment.

See: Cable sizing calculator

The international standard for calculating short-circuit currents in three-phase AC systems, using an equivalent voltage source c · U_n / √3 at the fault location.

See: Short circuit guide · Voltage factor c

The IEEE guide for arc-flash hazard calculations. The 2018 edition gives empirical equations for arcing current and incident energy for systems from 208 V to 15 kV.

See: Arc flash guide

The IEEE recommended practice for harmonic control: limits for voltage distortion at the point of common coupling and for the harmonic current a customer may inject.

See: Harmonic analysis guide

The heat energy per unit area at the working distance during an arc flash, in cal/cm² or J/cm². It decides the arc rating of the protective clothing.

See: Arc flash guide · PPE category

The factor in i_p = κ · √2 · I″_k. It depends on the R/X ratio of the fault loop, κ = 1.02 + 0.98 · e^−3R/X, from about 1.02 for a resistive loop to 2.0 for a purely inductive one.

See: R/X ratio · Short-circuit calculator

The calculation of steady-state bus voltages, branch flows and losses for given loads and generation. It is the starting point for most other studies.

See: Load flow guide

Many load flows with loads and generation drawn at random from their expected ranges, to estimate how likely voltage or loading violations are.

See: Monte Carlo docs

The standard iterative method for load flow. It linearizes the power-balance equations with the Jacobian matrix and usually converges in a few iterations.

See: Load flow guide

A switch on a transformer winding that changes the turns ratio while the transformer carries load, used to keep the voltage within limits.

See: Transformer docs

EPRI’s open-source distribution system simulator, strong in unbalanced multiphase networks, time series and harmonics. Electrisim uses it alongside pandapower.

See: pandapower vs OpenDSS

A load flow that also chooses generator outputs and other controls to minimize cost or losses while keeping voltages and loadings within limits.

See: OPF tutorial

An open-source Python tool for power system modelling and analysis, developed by Fraunhofer IEE and the University of Kassel. Electrisim uses it for load flow, IEC 60909 short circuit, OPF and more.

See: pandapower vs OpenDSS

The point where a plant or customer connects to the public network. Grid-code and power-quality requirements are usually checked there.

See: Grid code

Expressing quantities as fractions of chosen base values for power and voltage. It keeps numbers in similar ranges and removes ideal transformers from the equivalent circuit.

See: Per-unit calculator

The ratio of active to apparent power, P / S. A low power factor means more current, and more losses, for the same useful power.

See: Power factor correction calculator

A level of arc-rated protective clothing. NFPA 70E defines categories 1 to 4 with minimum arc ratings of 4, 8, 25 and 40 cal/cm².

See: Arc flash guide

A bus where active and reactive power are specified, and the voltage magnitude and angle are calculated. Most buses in a network are PQ buses.

See: Slack bus · PV bus

The range of active and reactive power a generator or plant can deliver. Studies compare it at the point of connection with the grid-code requirement.

See: Wind farm reactive power guide · Grid code P-Q docs

A bus where active power and voltage magnitude are specified, and reactive power and voltage angle are calculated, within the generator’s reactive power limits.

See: Slack bus · PQ bus

The ratio of resistance to reactance of an impedance or fault loop. It sets how fast the DC component of a fault current decays and the peak factor κ. Transformers and high-voltage lines have a low R/X (a high X/R), low-voltage cables a high one.

See: Peak factor κ

Power that flows back and forth between source and load because of inductance and capacitance, measured in var. It does no net work but sets the voltages and adds to the current in cables and transformers.

See: Power factor correction calculator · P-Q capability

An inductor connected from phase to earth that absorbs reactive power, used to compensate the charging current of long cables and lines.

See: Shunt docs · Ferranti effect

A simplified drawing of a three-phase network in which each circuit is shown as a single line. Electrisim models are drawn as single-line diagrams.

See: Diagram templates

The bus whose voltage magnitude and angle are fixed in a load flow. It supplies whatever power balances the network, including the losses. Usually the external grid.

See: Load flow guide · External grid

A voltage source converter that injects or absorbs reactive power to support the voltage, with a fast response that holds up well at low voltage.

See: STATCOM docs

Thyristor-controlled reactors and switched capacitors that provide variable reactive power for voltage control.

See: SVC docs

Harmonic current distortion relative to the maximum demand load current rather than the fundamental current. IEEE 519 sets its current limits in TDD.

See: Harmonic analysis guide

The RMS value of all harmonic components divided by the fundamental, in percent. VTHD refers to voltage and ITHD to current.

See: Harmonic analysis guide

A sequence of load flows with loads and generation that change over time, for example 24 hourly steps for a day or 8760 for a year.

See: Time series docs

The ability of synchronous machines to stay in step after a large disturbance such as a fault or a line trip, studied with time-domain simulation of rotor angles and voltages.

See: Transient stability guide

The voltage, in percent of rated voltage, that drives rated current through a transformer winding with the other winding short-circuited. It equals the transformer’s impedance in percent of its rating.

See: Short-circuit calculator · Per-unit calculator

The code for a transformer’s winding connections and phase shift. Dyn11, for example, means a delta high-voltage winding and a star low-voltage winding with the neutral brought out, with the low-voltage side leading by 30°. It decides the zero-sequence path and so the earth-fault current.

See: 33 MW wind farm short circuit · Zero-sequence impedance

A short drop in RMS voltage, for example while a large motor starts or a fault elsewhere in the network is cleared.

See: Motor starting guide

The difference between the voltage at the start and at the end of a cable or line, caused by the load current flowing through its impedance.

See: Voltage drop calculator

An IEC 60909 factor, typically between 0.90 and 1.10, applied to the nominal voltage to cover voltage variations when calculating maximum or minimum short-circuit currents.

See: Short-circuit calculator

A power-electronic converter with turn-off switches (usually IGBTs) that controls active and reactive power independently. Used in HVDC links, BESS and STATCOMs.

See: VSC docs

A simple model of part of a network at a boundary bus: a constant-power load plus a constant-impedance shunt. The extended ward adds a voltage source behind an impedance.

See: Ward docs · Extended ward docs

The impedance to currents that are equal and in phase in all three conductors. It governs earth-fault currents and depends on earthing and transformer connections.

See: Vector group