Glossary

A drive input stage that switches under control instead of using a plain diode bridge. It draws a few percent of current distortion rather than tens of percent. It moves its emission up into the switching band instead.

Used on Industrial and Manufacturing and Water and Wastewater.

A power-electronic unit that measures the load current and injects the opposing harmonic currents. Those currents then circulate locally instead of returning through the source. Some units also supply reactive current or balance the phases, as an option.

Used on Harmonics and Power Factor.

The American national standard that establishes nominal voltage ratings and operating tolerances for 60 Hz electric power systems above 100 V. The current designation is ANSI/NEMA C84.1-2020, reaffirmed in 2025. It is where Range A, Range B, service voltage and utilization voltage come from.

Source: ANSI webstore catalog entry for ANSI/NEMA C84.1-2020 (R2025).

Used on Voltage Stability and Phase Balance.

The product of rms voltage and rms current. It is the capacity a transformer, a conductor and a switchboard must carry, whether or not that current does any work.

Used on SDE Summary and Power Factor.

The electromechanical switch that moves a facility from the utility to its standby generator. The switch operates in a fraction of a second, which is still more than one cycle. The generator takes seconds to start and pick up the load, so in that gap the load rides on stored energy or it stops.

Used on Resilience and Data Centers and Compute.

The window an instrument averages into before it reports a number. Measurement practice builds long windows out of short ones, so a ten-minute or two-hour figure is an average of averages. A short event does not survive a long window, which is why two instruments can disagree about the same supply.

Used on Standards Compliance and Harmonics.

Restarting a network from a fully de-energized state, with no stiff grid to absorb inrush, resonance or unbalance. Note the spelling we use: one word.

Used on Blackstart and Resilience.

The conductor, or set of conductors, in a panel, switchboard or substation that several circuits connect to. The loads on it share its voltage: the source voltage minus the load current times the source impedance.

Used on Impedance Matching, Harmonics, Data Centers and Compute, Generation and Water and Wastewater.

Fixed, contactor-switched or thyristor-switched capacitors that supply reactive current in steps. A bank corrects the timing part of power factor and nothing else. It can also form a resonant circuit with the source impedance.

Used on Power Factor and Harmonics.

The higher of the two measurement classes a power-quality instrument can be built to, used where a number has to survive a dispute. The lower class, Class S, is for surveys and troubleshooting. Two instruments of different classes can report the same supply differently. It is not Class A EMC, the emission class our controller carries for industrial and commercial environments.

Used on Standards Compliance and SDE Summary.

The bodies that set and survey construction and equipment rules for ships. The maritime page names three of them, ABS, DNV and Lloyd’s Register, and the Unified Requirement E24 of their association, IACS.

Used on Maritime.

Restoring a block of load that has been off long enough for everything that cycles to want to start at once. Thermostats, compressors and motors all call together, so the restored demand is far above the load that was lost.

Used on Blackstart and Industrial and Manufacturing.

Voltage that appears equally on the conductors with respect to ground, including the neutral-to-ground voltage that triplen harmonics produce.

Used on Industrial and Manufacturing and Data Centers and Compute.

The point at which an electromechanical contactor releases because its coil voltage has fallen too far. It is one of the most common reasons a process stops during a voltage sag.

Used on Industrial and Manufacturing and Blackstart.

Peak divided by rms. A sine has a crest factor of 1.414, and the pulsed current a switching supply draws has a much higher one.

Used on Data Centers and Compute.

The share of a facility’s load that must be energized first and kept energized. On a base it is the mission systems, in a storm the treatment trains, in a data center the IT bus.

Used on Defense and Critical Infrastructure and Water and Wastewater.

A sensor around a conductor that gives a small current in proportion to the current inside it. Our controller comes with three, one on each incoming phase conductor.

Used on Consumption, Water and Wastewater and Products.

The same spread as voltage unbalance, measured on the three phase currents instead. The US Department of Energy puts it at six to ten times the voltage unbalance that caused it. That is why a small voltage figure can hide a large current one.

Source: US Department of Energy, Motor Systems Tip Sheet 7, DOE/GO-102012-3733, November 2012.

Used on Phase Balance and Water and Wastewater.

The transformer or load connection in which the three phases form a closed triangle with no neutral. Triplen currents circulate inside it rather than returning on a wire.

Used on Delivery, Water and Wastewater and Industrial and Manufacturing.

Operating a transformer, motor or conductor below its nameplate rating so it stays inside its temperature limit under distorted or unbalanced supply. Derating buys margin by giving up capacity that was already paid for.

Used on Phase Balance, Standards Compliance, Delivery, Water and Wastewater and Harmonics.

The cosine of the angle between the fundamental voltage and the fundamental current. It is the timing half of power factor, and the only half a capacitor bank can move.

Used on Power Factor and Standards Compliance.

Generation or storage connected at distribution level rather than to the bulk system.

Used on Microgrids and Storage, Maritime, Defense and Critical Infrastructure and Water and Wastewater.

The enclosure that takes power from a feeder and divides it into branch circuits. Each circuit has its own breaker or fuse. Our controller connects to the panel bus in parallel with the loads, through a dedicated breaker of its own.

Used on SDE Summary, Consumption, Water and Wastewater and About.

The deliberate slope in a source’s frequency-against-load or voltage-against-reactive-load characteristic that lets parallel machines share load without fighting each other.

Used on Microgrids and Storage and Blackstart.

A series device that injects a compensating voltage during a sag, as distinct from a shunt device that injects current.

Used on Voltage Stability.

Circulating-current heating in a transformer’s windings and core. It rises with the square of the harmonic order, which is why a small harmonic current can cost a large amount of heat.

Used on Impedance Matching and Harmonics.

The discipline of equipment neither emitting electrical noise beyond stated limits nor being disturbed by it.

Used on Industrial and Manufacturing and Data Centers and Compute.

The European standard describing the voltage characteristics a public network delivers at a user’s supply terminals, for low, medium and high voltage. It excludes industrial networks. The current British adoption is BS EN 50160:2022 with its 2025 amendment.

Source: BSI publication index entry for BS EN 50160:2022+A1:2025.

Used on Standards Compliance and Phase Balance.

The ERCOT nodal operating guide revision titled Large Computational Load Ride-Through Requirements, approved on 9 July 2026 and effective on 1 August 2026. It applies to a large load where at least 50% of the site demand is power-electronic and aggregate peak demand is at least 75 MW.

Source: ERCOT NOGRR282 issue page and the ERCOT stakeholder briefing of February 2026, ercot.com.

Used on Standards Compliance and Data Centers and Compute.

ERCOT is the Electric Reliability Council of Texas, the grid operator whose ride-through rule is NOGRR282. FERC is the Federal Energy Regulatory Commission. NERC is the North American Electric Reliability Corporation, which FERC ordered to file national standards for computational loads by the end of 2026.

Used on About, Data Centers and Compute and Standards Compliance.

One of the four power quality problems our pages name: low power factor, harmonic distortion, phase imbalance and a weak source. The first three ride on the current, and the weak source sets how much voltage distortion they make. It is not a short-circuit or ground fault, which the entries for fault current and ground fault describe.

Used on SDE Summary and Voltage Stability.

The current a source delivers into a short circuit. A synchronous generator delivers many times its rating for several cycles. An inverter limits itself to little more than its rating, which compresses the margin protective devices depend on.

Used on Blackstart, Defense and Critical Infrastructure and Microgrids and Storage.

The conductors that carry power from a substation or switchboard to the next point of distribution, such as a panel. A utility feeder was built to carry power one way from the substation, and export now sends power back along it.

Used on Delivery, Blackstart, Data Centers and Compute, Water and Wastewater, Industrial and Manufacturing and Electric Optimization.

The impression of unsteadiness of visual sensation caused by a light source whose brightness fluctuates with time. In a plant it looks like lighting pulsing at the rhythm of a load.

Used on Industrial and Manufacturing.

Three phases plus a neutral, the usual North American commercial distribution. Zero-sequence current returns on the neutral rather than cancelling.

Used on Phase Balance and Industrial and Manufacturing.

The 60 Hz component of a current or voltage on a North American system, as opposed to the harmonics riding on it.

Used on Harmonics and Power Factor.

A generator set, or genset, is an engine and an alternator coupled and packaged as one unit. The alternator is the synchronous machine that makes the electricity. On a genset the engine sets the kilowatt limit, and the alternator sets the current limit.

Used on Resilience, Blackstart, Microgrids and Storage, Maritime and Defense and Critical Infrastructure.

Protection that trips on a small difference between the current going out and the current coming back, in the milliampere range. Our controller is not compatible with this protection on its own branch.

Used on Industrial and Manufacturing and Water and Wastewater.

The engine control that holds a generator’s speed, and so its frequency, under load. Many governors sense zero crossings, and harmonic content adds crossings that were not there.

Used on Blackstart and Microgrids and Storage.

An inverter that needs an existing voltage reference and injects current into whatever impedance the network presents. Take the reference away and it stops.

Used on Impedance Matching, Blackstart and Microgrids and Storage.

An inverter that sets its own voltage and frequency reference, so it can energize an island. It still limits its own current.

Used on Blackstart and Microgrids and Storage.

Current returning through the equipment grounding conductor or through the earth instead of the path it was meant to take.

Used on Industrial and Manufacturing and Data Centers and Compute.

A half-cycle refreshed rms: an rms computed over one cycle, starting at a zero crossing of the fundamental, and recomputed each half cycle. It is what makes a short sag that still matters visible at all. The power quality measurement standard writes it Urms(1/2).

Used on Voltage Stability and Impedance Matching.

A current or voltage component at a whole multiple of the 60 Hz fundamental, produced by loads that draw their current in pulses rather than smoothly.

Used on Harmonics and SDE Summary.

Which multiple of the fundamental a component sits at. The 5th is 300 Hz on a 60 Hz system. Loss effects climb with the order, so a high-order ampere costs far more heat in a winding than a fundamental one.

Used on Harmonics and Impedance Matching.

The condition where a capacitance and the source inductance are tuned near an order already present on the bus. A modest harmonic current then produces a large harmonic voltage.

Used on Harmonics and Power Factor.

The capacity a transformer, feeder, generator or inverter has left above the load it carries now. Current that does no work takes up part of it, and removing that current gives that part back.

Used on Electric Optimization, Blackstart, Microgrids and Storage and Voltage Stability.

The heat an iron core gives off as the alternating field magnetizes it one way and then the other on each cycle. It grows with frequency, so harmonic content adds to it. Our pages name it beside eddy-current loss and skin effect, the losses harmonic current raises in transformers and generators.

Used on Blackstart, Microgrids and Storage, Maritime, Defense and Critical Infrastructure, Data Centers and Compute and Industrial and Manufacturing.

The compatibility levels for conducted disturbances on public low-voltage networks. Edition 2.2 covers 0 kHz to 9 kHz, with an extension to about 148.5 kHz for mains signalling. Its 2017 and 2018 amendments added levels across the 2 kHz to 150 kHz band.

Source: IEC catalog entry and issuer previews for IEC 61000-2-2 Edition 2.2 and its 2017 and 2018 amendments, webstore.iec.ch.

Used on Standards Compliance.

The immunity test standard for voltage dips, short interruptions and voltage variations. Edition 3.0 was published on 28 January 2020, and its class 3 test points are one of the envelopes we plot equipment tolerance against.

Source: IEC catalog entry and issuer preview for IEC 61000-4-11:2020, Edition 3.0, webstore.iec.ch.

Used on Voltage Stability and Resilience.

The standard that fixes how power quality is measured: the measurement classes, the half-cycle rms, the aggregation windows and the flagging rule. Edition 4.0 was published on 24 October 2025, replacing the 2015 edition. It splits emission measurement into a 2 kHz to 9 kHz annex and a new 9 kHz to 150 kHz annex.

Source: IEC catalog entry for IEC 61000-4-30:2025, Edition 4.0, webstore.iec.ch.

Used on Standards Compliance and SDE Summary.

The electromagnetic compatibility standard for adjustable-speed drives. Under it, a drive’s own emission is the drive’s responsibility to filter.

Used on Industrial and Manufacturing.

The safety standard for power electronic converter systems. It is the standard under which the controller carries its CE conformity assessment.

Used on Standards Compliance and Industrial and Manufacturing.

The standard for utility connections in port, so that a berthed ship can take power from shore. It is issued in numbered parts, one for the high-voltage connection and one for low-voltage connections to smaller vessels.

Used on Maritime.

The recommended practice for shipboard electrical installations. The single document last issued as IEEE 45-2002 has been replaced by numbered parts. IEEE 45.1-2023 is the design part, published on 13 December 2023 and superseding the 2017 edition. Its scope covers the design of electrical power generation, distribution, propulsion and load systems on merchant, commercial and naval vessels. We state none of its limits here, because the current parts are paywalled and we have not read them.

Source: IEEE SA catalog entry for IEEE 45.1-2023, standards.ieee.org.

Used on Maritime.

IEEE Standard for Harmonic Control in Electric Power Systems. Active, published on 5 August 2022, superseding the 2014 edition. It is the document behind the harmonic limits quoted across our site, and the pages that state one of its numbers cite it there.

Source: IEEE SA catalog entry for IEEE 519-2022, standards.ieee.org.

Used on Harmonics and Standards Compliance.

The recommended practice we refer to as the Emerald Book, which treated noise on the grounding system as a cause of data and logic errors. The 2005 edition is now Inactive-Reserved, withdrawn by ANSI in 2016 and inactivated in 2021.

Source: IEEE SA catalog entry for IEEE 1100-2005, standards.ieee.org.

Used on Industrial and Manufacturing and Data Centers and Compute.

Active, published on 13 August 2019, superseding the 2009 edition. It is the document that sorts power-system voltage events into classes, and the sag, swell, interruption and transient categories we use trace to it. Each page that states one of its boundary numbers names the standard and the edition.

Source: IEEE SA catalog entry for IEEE 1159-2019, standards.ieee.org.

Used on Voltage Stability, Resilience and Phase Balance.

The standard that defines the distribution reliability indices a utility reports. See the entry for SAIDI, SAIFI and MAIFI.

Used on Resilience.

Active, published on 16 May 2025, superseding the 2010 edition. Its abstract describes definitions used for the measurement of electric power quantities under sinusoidal, nonsinusoidal, balanced or unbalanced conditions.

Source: IEEE SA catalog entry for IEEE 1459-2025, standards.ieee.org.

Used on Microgrids and Storage.

The interconnection standard for distributed energy resources. It sets how a distributed resource behaves at its point of connection, including what it must do when it finds itself in an unintentional island.

Used on Microgrids and Storage.

Active, published on 27 November 2017, superseding the 2014 edition. Its scope covers all electrical and electronic equipment on low-voltage power systems that can malfunction or shut down when the supply voltage falls for less than one minute.

Source: IEEE SA catalog entry for IEEE 1668-2017, standards.ieee.org.

Used on Standards Compliance, Water and Wastewater and Industrial and Manufacturing.

Active, published on 22 October 2018. The recommended practice for harmonic studies and analysis of industrial and commercial power systems.

Source: IEEE SA catalog entry for IEEE 3002.8-2018, standards.ieee.org.

Used on Water and Wastewater.

Active, published on 31 October 2018, superseding the 2008 edition. It is the transformer-capability document the trade literature points to when a transformer has to supply nonsinusoidal load current. We do not restate its equations here, because we have not read them.

Source: IEEE SA catalog entry for IEEE C57.110-2018, standards.ieee.org.

Used on Delivery.

Inactive-Reserved, published on 11 April 2003. It described the surge environment of low-voltage AC power circuits, covering surges whose amplitude, duration or rate of change is enough to damage or upset equipment.

Source: IEEE SA catalog entry for IEEE C62.41.2-2002, standards.ieee.org.

Used on Standards Compliance.

The insulated-gate bipolar transistor, the switching transistor used in modern converters. A drive built on controlled IGBT switching draws far less current distortion than a diode bridge does.

Used on Data Centers and Compute and Defense and Critical Infrastructure.

The opposition a circuit presents to alternating current, combining resistance and reactance. Voltage distortion is current distortion multiplied by it: V = I x Z.

Used on Impedance Matching and SDE Summary.

In our usage, one of the four corrections the controller makes. It supplies the reactive, harmonic and negative-sequence currents locally, so they cause no drop across the source impedance. It changes the current through the impedance, not the impedance itself.

Used on Impedance Matching and Technology.

Physical inertia is the spinning mass of a generator and the kinetic energy that mass stores. The mass releases that energy at once when the load changes, which holds frequency steady while the governor reacts. An inverter has no rotor. Its synthetic or virtual inertia is a control loop that reads frequency and adjusts power, so it acts after a measurement.

Used on Generation, Microgrids and Storage, Blackstart and Resilience.

The heavy current a motor or a transformer draws at the moment it is energized, which pulls the voltage down until regulation recovers.

Used on Industrial and Manufacturing and Blackstart.

The service life of winding insulation, which falls as the winding runs hotter. It is the mechanism behind most of our equipment-life arguments.

Used on Industrial and Manufacturing and Water and Wastewater.

A complete loss of voltage. Practice sorts interruptions by how long they last, from momentary through temporary to sustained. The pages that state those boundaries name the standard.

Used on Voltage Stability and Resilience.

A converter that makes alternating current from direct current. Everything downstream of a battery, a solar array or a DC bus arrives through one.

Used on Microgrids and Storage and Resilience.

Any generation or storage that reaches the network through an inverter rather than through a rotating machine. The abbreviation is the industry’s and not ours. Our pages write the phrase out and vary the noun: resources, generation, sources.

Used on Blackstart, Data Centers and Compute and Generation.

The ingress-protection rating of the controller enclosure: finger-safe, indoor and not sealed against water.

Used on Industrial and Manufacturing and Data Centers and Compute.

Running a piece of network separated from the utility. A planned island is a design. An unintentional one is a safety problem, because line workers expect a de-energized conductor.

Used on Microgrids and Storage and Water and Wastewater.

The performance part of the standard for generating sets driven by reciprocating internal combustion engines. Edition 6 was published on 1 May 2025, replacing the 2022 edition. Its scope covers the design and performance criteria that arise from combining such an engine with an AC generator as one unit. It is the source of the performance classes we quote for load-step recovery.

Source: ISO catalog entry for ISO 8528-5:2025, Edition 6, iso.org.

Used on Resilience, Water and Wastewater and Defense and Critical Infrastructure.

The voltage tolerance envelope published by the Information Technology Industry Council for equipment on 120 V nominal supplies. Its application note states a steady-state range of plus or minus 10% for an indefinite period. It also states a dropout to zero for up to 20 ms, a sag to 70% of nominal for up to 0.5 s and a sag to 80% for up to 10 s.

Source: ITI (CBEMA) Curve Application Note, Information Technology Industry Council, sections 3.1, 3.5 and 3.6, itic.org.

Used on Voltage Stability and Resilience.

The heat a current makes in the resistance of a conductor or winding: the rms current squared times the resistance. A harmonic or reactive current adds loss in proportion to its square while doing no work.

Used on Electric Optimization, Harmonics, Power Factor and Maritime.

An index of how much harmonic current a transformer can carry without exceeding its temperature-rise rating. It is computed from the harmonic currents weighted by the square of the order. We give no formula for it here, because the defining text is not open to us.

Used on Impedance Matching and Harmonics.

A transformer listed to a K value and built to tolerate the extra heating that harmonic current causes. It survives the current. It does not reduce it.

Used on Impedance Matching and Data Centers and Compute.

Two ways a utility bills reactive current. A kVA demand charge bills the apparent power drawn over a short window, often 15 minutes, so reactive current raises the bill. A power factor penalty charges for the low power factor itself. Tariffs build it differently, some on the reactive energy metered in kVARh over a month.

Used on Power Factor, Electric Optimization, SDE Summary, Water and Wastewater and Solutions.

A lagging power factor means the current trails the voltage, as in an induction motor. A leading power factor means the current runs ahead of the voltage, as in a capacitor or a lightly loaded server supply. Lagging load is also called inductive, and leading load capacitive.

Used on SDE Summary, Data Centers and Compute, Water and Wastewater, Power Factor and Products.

The pumped intermediate point in a sewer network, where a motor control center or a panel sits.

Used on Water and Wastewater.

Series inductance ahead of a drive. It broadens the drive’s current pulse and so lowers the harmonic current the drive draws.

Used on Water and Wastewater, Industrial and Manufacturing and Maritime.

A sudden change in the load a source carries. A motor start is one, and so is each block a restoration picks up. Performance classes grade a generator set by how far its voltage and frequency move after one, and how fast they recover.

Used on Blackstart, Maritime, Microgrids and Storage and Data Centers and Compute.

The current a motor draws at zero speed, several times its full-load current. It is what a starting motor asks of the supply before it turns.

Used on Industrial and Manufacturing and Water and Wastewater.

A local network that can run connected to the utility or as an island. On inverters it presents a weaker source than the service its loads were designed around.

Used on Microgrids and Storage and Impedance Matching.

The US Navy interface standard for shipboard electric power, dated 25 September 2018 and superseding Section 300B of 2008. It defines three power types: type I at 60 Hz and types II and III at 400 Hz, each with its own voltage, frequency, unbalance and distortion characteristics.

Source: MIL-STD-1399 Section 300 Part 1, 25 September 2018, read in full from the DLA ASSIST mirror.

Used on Maritime and Standards Compliance.

A millisecond is one thousandth of a second, a microsecond one millionth and a nanosecond one billionth. One 60 Hz cycle lasts about 16.7 milliseconds, so a response in microseconds acts well inside a cycle.

Used on Technology, SDE Summary and Defense and Critical Infrastructure.

A cabinet that groups the starters, drives and protection for a set of motors on one bus. Pumps, blowers, chillers and fans run from it.

Used on Water and Wastewater, Industrial and Manufacturing, Voltage Stability and Solutions.

The rating stamped on a transformer, a motor or a service, against which loading and headroom are measured.

Used on Impedance Matching and Electric Optimization.

The balanced set of three components that rotates in the reverse order, produced by unbalance. A motor turns negative-sequence voltage into rotor heat rather than into torque.

Used on Phase Balance and Industrial and Manufacturing.

Motors and Generators, the standard covering what a motor or a generator is rated for and what supply it is expected to run on. The current designation is ANSI/NEMA MG 1-2024, active and published on 7 February 2025. Its parts carry the running conditions for polyphase motors and the harmonic derating of a motor, and the pages that state one of those limits cite it there.

Source: NEMA standards page for ANSI/NEMA MG 1-2024, nema.org.

Used on Phase Balance and Standards Compliance.

The North American reliability standard that sets frequency and voltage ride-through for inverter-based resources. PRC-029-1 was approved by FERC Order No. 909 on 29 July 2025 and took effect on 28 August 2025. PRC-029-2 is its successor.

Source: Federal Register notice for FERC Order No. 909, 29 July 2025, and the NERC Project 2025-05 implementation plan, April 2026.

Used on Data Centers and Compute.

The fourth wire of a wye system. Where triplen harmonics are heavy it can carry more current than any single phase, which is why oversized neutrals are specified.

Used on Phase Balance and Data Centers and Compute.

The current returning on the neutral of a four-wire system. Uneven single-phase loading feeds it, and so do the triplen harmonics, which add arithmetically instead of cancelling.

Used on Phase Balance and SDE Summary.

The standard for emergency and standby power systems. It defines the type of an emergency power supply system by the maximum time allowed between the loss of power and its restoration.

Used on Resilience.

The system’s named voltage, such as 120 V, 208Y/120 V or 480Y/277 V, against which every tolerance is measured.

Used on Data Centers and Compute.

A load whose current is not proportional to the voltage applied to it: drives, LED drivers, chargers, servers and switching power supplies. It draws current in pulses, and the pulses are the harmonics.

Used on Harmonics and Power Factor.

A protective device or a drive stopping on a disturbance rather than on a fault.

Used on Voltage Stability and Defense and Critical Infrastructure.

The three directions the faults arrive from. Consumption: pulsed loads distort the current inside the facility. Delivery: feeders built for one direction now carry power both ways. Generation: an inverter has no rotor to absorb the first instant.

Used on SDE Summary, Power Factor, Phase Balance, Voltage Stability, Impedance Matching, Resilience, Standards Compliance and Technology.

Inductors and capacitors tuned to present a low impedance at one harmonic order. It absorbs that order and moves the resonance somewhere else.

Used on Impedance Matching and SDE Summary.

A quantity written as a fraction of its nominal value, so 0.9 pu on a 480 V system is 432 V. Event limits are stated in per unit rather than in volts, so one figure covers every system voltage.

Used on Data Centers and Compute, Blackstart and Microgrids and Storage.

The value that a stated share of the measured intervals stays at or below. A limit written as a percentile judges a supply on how often it exceeds a value, not on whether it ever did.

Used on Standards Compliance.

Our plain-language name for the three legs not carrying equal current. Phase balance is its correction, and the name of our page on it. See voltage unbalance and current unbalance for the measured forms.

Used on SDE Summary and Technology.

The point on the public supply nearest a particular load at which other loads are, or could be, connected. Harmonic limits are written to apply there and not at an individual load, which is why a measurement taken at a drive does not settle compliance.

Used on Harmonics and Standards Compliance.

The balanced set of three components rotating in the normal order. It is the part of the current that does the work.

Used on Phase Balance.

In a data center, the cabinet that takes the output of the uninterruptible supply and divides it into rack circuits.

Used on Data Centers and Compute.

Real power divided by apparent power: the share of the current a load draws that ends up doing work. Under distortion and unbalance it is not the same quantity as displacement power factor.

Used on Power Factor and Technology.

Supplying reactive current locally so the source does not have to. Traditionally a capacitor bank switched in steps.

Used on Power Factor and Industrial and Manufacturing.

The degree to which the voltage and current at a point of supply match an undistorted sine wave at nominal amplitude and frequency. We treat it as four coupled parameters: power factor, harmonic distortion, phase balance and impedance.

Used on Technology and Standards Compliance.

Our own percentage score, from 0 to 100, for how efficiently electrical energy flows in a network. It was introduced by 3DFS. PQR is a company term, not an industry one, and no standard we know of defines it. The figure our PQR Simulator shows is a model result, not a measurement. The Technology hub’s Electric Optimization card says PQR without expanding it, and this entry defines it.

Used on Electric Optimization and Technology.

The data-center ratio of total facility power to IT power. The narrow point we make about it is that a metering error at the rack strip propagates straight into the arithmetic.

Used on Data Centers and Compute.

The two steady-state voltage bands a supply is judged against. Range A is the band a system is designed and operated to hold, and excursions outside it should be infrequent. Range B is wider and is allowed only for limited conditions that are to be corrected within a reasonable time. The pages that state either band’s numbers name the standard.

Used on Voltage Stability and Standards Compliance.

The part of impedance that comes from inductance and capacitance rather than from resistance. Inductive reactance rises with frequency, so a harmonic current meets more of it than the 60 Hz fundamental does.

Used on Impedance Matching, Consumption and Defense and Critical Infrastructure.

Current that moves back and forth between source and load without producing work. It occupies conductor, transformer and switchgear capacity while it does so. Utilities meter its energy in kVARh.

Used on Power Factor and SDE Summary.

The component of the flow that does work and turns the energy meter.

Used on SDE Summary and Electric Optimization.

The front end of a drive, an uninterruptible supply or a power supply, which converts AC to DC. Its pulse number sets its harmonic signature.

Used on Harmonics, Data Centers and Compute, Defense and Critical Infrastructure and Industrial and Manufacturing.

The ability of a piece of equipment, or of a whole system, to keep operating through a sag or a short interruption. In our usage it belongs to stored energy and to the equipment’s own tolerance. We keep it separate from how fast a correction acts.

Used on Resilience and Voltage Stability.

The equivalent steady value of an alternating quantity. Measured over whole cycles the squares add, so a harmonic ampere raises the rms current a conductor has to carry without doing any work.

Used on Defense and Critical Infrastructure, Maritime, Data Centers and Compute and Harmonics.

The three distribution reliability indices. SAIFI counts the sustained interruptions an average customer sees in a year, SAIDI counts the minutes those interruptions last, and MAIFI counts the momentary ones.

Used on Resilience.

How often an instrument or a controller reads the waveform. It sets the shortest event either one can see or answer.

Used on Technology.

The supervisory control and data acquisition system a utility or a plant runs on. Its update rate is measured in seconds, so it does not see a sub-cycle event.

Used on Water and Wastewater and Blackstart.

The semiconductor industry specification for voltage sag immunity, current as SEMI F47-0706 and reapproved in 2012. Its scope sets minimum sag immunity for semiconductor processing equipment, defined by sag depth and duration, and it covers subsystems and components as well as whole tools.

Source: SEMI store catalog entry for SEMI F47-0706 (Reapproved 0812), store-us.semi.org.

Used on Voltage Stability and Standards Compliance.

The voltage at the point where the supplier’s electrical system and the user’s electrical system are connected.

Used on Voltage Stability, Standards Compliance and Water and Wastewater.

Supplying a berthed ship from the port network so the ship’s own generators can be shut down.

Used on Maritime.

The short-circuit apparent power available at a bus divided by the rated power of the load or source connected there. It says how much a facility’s own current disturbs the voltage everyone else sees. On the Data Centers and Compute page, SCR stands for something else: the silicon-controlled rectifier, the solid-state switch in a static transfer switch.

Used on Impedance Matching and Standards Compliance.

The common three-phase rectifier front end, built from six diodes or six thyristors. Its current distortion is concentrated at the 5th, 7th, 11th and 13th orders.

Used on Data Centers and Compute and Defense and Critical Infrastructure.

The tendency of higher-frequency current to crowd toward the surface of a conductor, raising its effective resistance and so its heating.

Used on Data Centers and Compute and Industrial and Manufacturing.

A reduced-voltage starter that limits a motor’s starting current without the full converter of a drive.

Used on Industrial and Manufacturing and Water and Wastewater.

Measurement plus a controlled current source installed in parallel at the distribution panel. It samples three-phase voltage and current continuously. It separates the working current from the reactive, harmonic and unbalanced components and supplies those components locally. SDE is our term for our method.

Used on SDE Summary and Technology.

The impedance looking back from the panel toward the transformer, the feeder and the utility. It is what turns a facility’s own distorted current into a voltage disturbance.

Used on Impedance Matching, Microgrids and Storage, Water and Wastewater and Industrial and Manufacturing.

A static synchronous compensator: a shunt power-electronic unit that supplies or absorbs reactive current to hold a bus voltage.

Used on Power Factor, Delivery and Blackstart.

A solid-state switch that moves a load between two sources in a few milliseconds. That is fast enough for a single-corded load to be served by a second supply.

Used on Data Centers and Compute and Defense and Critical Infrastructure.

A bus with a high short-circuit ratio. The same harmonic current makes less harmonic voltage there, so the bus tolerates more of it.

Used on Impedance Matching and Industrial and Manufacturing.

Shorter than one cycle of the 60 Hz wave, which lasts about 16.7 milliseconds. A sub-cycle correction acts before the wave completes that cycle.

Used on Technology, Consumption, Delivery, Blackstart and Microgrids and Storage.

Switching residue from power electronics in the band above the classical harmonic range and below radio frequency, roughly 2 kHz to 150 kHz.

Used on Data Centers and Compute, Industrial and Manufacturing and Blackstart.

A device that clamps a fast overvoltage before it reaches the equipment behind it. It typically uses metal-oxide varistors (MOVs), which act in nanoseconds.

Used on Defense and Critical Infrastructure, Data Centers and Compute and Technology.

The assembly where the service enters a building and is divided into feeders. It is the measurement point we keep naming, because it is where the whole facility’s current is visible at once.

Used on SDE Summary and Industrial and Manufacturing.

A rotating generator. It carries physical inertia and delivers heavy fault current, and an inverter does neither. The alternator in a generator set is one.

Used on Blackstart and Microgrids and Storage.

Devices that change a transformer’s turns ratio in steps to hold the customer’s voltage inside its limits. Each senses the voltage, waits out a time delay and then moves through its taps, so the sequence takes tens of seconds. A line drop compensator estimates the drop to the load from the line current.

Used on Delivery and Voltage Stability.

A US Army technical manual on electrical installation and inspection for command and control facilities, dated 10 September 2001. It is the single most cited document on our site, standing behind the neutral-sizing, meter-error, generator-sizing and ride-through figures on the sector pages.

Used on Defense and Critical Infrastructure and Data Centers and Compute.

The harmonic content of a current expressed as a percentage of the maximum demand current rather than of the current flowing at that instant. It is why a lightly loaded panel can show high current distortion and still sit inside its limit.

Used on Harmonics and Standards Compliance.

The square root of the sum of the squared harmonic components divided by the fundamental, as a percentage. THDi is the current form and THDv the voltage form, and the two are related through the source impedance.

Used on Harmonics, Standards Compliance and Power Factor.

A sudden, short departure from the steady state. The impulsive kind is a change in a steady-state voltage or current that goes one way only, and the oscillatory kind rings.

Used on Defense and Critical Infrastructure, Maritime, Blackstart and Harmonics.

The 3rd, 9th and 15th orders. They are zero-sequence by construction, so the three phases add arithmetically in the neutral instead of cancelling there.

Used on Phase Balance and Harmonics.

Displacement power factor multiplied by the distortion factor. With THDi as the current total harmonic distortion, the distortion factor is one divided by the square root of (1 plus THDi squared). True power factor is the number a bill is built on.

Used on Power Factor and Industrial and Manufacturing.

An instrument that computes rms from the sampled waveform rather than inferring it from the average. Average-sensing instruments read a distorted or pulsed current low.

Used on Industrial and Manufacturing and Data Centers and Compute.

A passive filter cut for one harmonic order, usually the worst one on a drive panel. How well it works depends on the load, the source and where it sits.

Used on Electric Optimization and Industrial and Manufacturing.

Two six-pulse bridges fed through a phase-shifting arrangement, which cancels the 5th and 7th orders and leaves the 11th and 13th.

Used on Data Centers and Compute, Maritime, Water and Wastewater and Microgrids and Storage.

The Unified Facilities Criteria, the US Department of Defense design criteria for engine-driven generators. It requires a harmonic analysis once power-electronic load is a significant share of the total, and the defense page states the threshold and cites it.

Used on Defense and Critical Infrastructure.

The listing standard for surge protective devices.

Used on Technology.

The listing standard for dry-type general purpose and power transformers, under which a transformer carries a published K rating. We restate none of its values here, because we have not read the standard.

Used on Impedance Matching, Harmonics and Data Centers and Compute.

A supply voltage that stays below its normal band for longer than a sag lasts. Past that point the event stops being counted as a sag and starts being treated as a steady-state condition. The word also names a protective function: a drive that faults on undervoltage, or a breaker with an undervoltage trip.

Used on Water and Wastewater and Maritime.

A converter with stored energy that carries a critical load through an interruption. Its own rectifier is a harmonic source, and its output has a current limit and an output impedance of its own.

Used on Resilience and Data Centers and Compute.

A converter that runs a motor at variable speed by rectifying the supply and inverting it again at a new frequency. It is the largest single harmonic source in most facilities.

Used on Harmonics and Industrial and Manufacturing.

The 3DFS controller product family, connected in parallel at a panel, a motor control center or the output bus of a power distribution unit. Our product name, not an industry term.

Used on Industrial and Manufacturing and Technology.

The utility’s steady-state control of voltage using tap changers and regulators. It works over seconds to tens of seconds, so it does not answer a sag.

Used on Water and Wastewater and Blackstart.

The generator control that holds terminal voltage by adjusting the machine’s excitation. It acts through the field, so it works on the machine’s timescale rather than within a cycle. Our pages also use the name for the utility’s step voltage regulator. See tap changer and step voltage regulator.

Used on Blackstart, Maritime and Microgrids and Storage.

A short drop in rms voltage that recovers by itself, and the everyday event behind most unexplained process stops. The European name for the same event is a dip. The pages that state its depth and duration boundaries name the standard and the edition.

Used on Voltage Stability and Resilience.

A short rise in rms voltage above its normal band, the mirror image of a sag. It often follows a fault on another phase or the loss of a large load.

Used on Data Centers and Compute, Water and Wastewater and Industrial and Manufacturing.

Two definitions are in use, and a report that does not say which one it used cannot be read. One is the ratio of the negative-sequence voltage to the positive-sequence voltage, as a percentage. We name no source for it, because the only wording available to us is second-hand. The other is one hundred times the largest deviation of a line voltage from the average of the three, divided by that average. That second form is the one motor practice uses and the one the source below states.

Source: US Department of Energy, Motor Systems Tip Sheet 7, DOE/GO-102012-3733, November 2012.

Used on Phase Balance and Standards Compliance.

The shape voltage or current traces out over one cycle. An undistorted supply is a sine, and every defect we name is a departure from it.

Used on SDE Summary and Technology.

A bus with a low short-circuit ratio: a generator, a long feeder, an inverter-fed island. A facility’s own current makes up a larger share of the disturbance there.

Used on Microgrids and Storage and Maritime.

Working current is the part of the current that does work. Non-working current is the rest of what the loads draw: the reactive, harmonic and unbalanced parts. It heats the wire and fills the rating of the transformer, feeder and switchgear while doing no work.

Used on Electric Optimization, SDE Summary, Blackstart, Maritime and Solutions.

The transformer or load connection in which the three phases share a common point, and that point becomes the neutral.

Used on Industrial and Manufacturing.

The instant a sine wave passes through zero. Many governors, voltage regulators and paralleling controls count these points. Harmonics add crossings that were not there, and the controls hunt around their setpoint.

Used on Blackstart, Microgrids and Storage, Maritime, Water and Wastewater and Defense and Critical Infrastructure.

The component that is identical in all three phases, so it does not cancel at the neutral point. Triplen harmonics are zero-sequence by construction.

Used on Phase Balance and Delivery.