Glossary

One hundred terms from across this site and our technical reports, each defined in a few sentences at most. The primers go deeper.

Measurement

Continuous Point-on-Wave (CPOW): A waveform record with no gaps, keeping every sample of every channel around the clock rather than windows around a trigger. See the CPOW primer.
Point-on-wave: Measurement of the instantaneous voltage or current at points along the AC waveform, as opposed to a value computed over whole cycles.
Waveform Measurement Unit (WMU): An instrument that records time-synchronized voltage and current waveforms, the waveform counterpart of the phasor measurement unit (PMU).
Triggered capture: Recording the waveform only in a window around an event that crosses a preset threshold, typically a few cycles before and after. A disturbance that crosses no threshold, or one nobody set, leaves no waveform.
Cycle-synchronous and fixed-rate sampling: Two ways to time the samples in a waveform record. In a cycle-synchronous record, a phase-locked loop tracks the line and either steers the converter’s clock or sets the grid that fixed-rate samples are resampled to, giving a fixed number of samples per cycle; it suits harmonic analysis but smears anything not locked to the mains. A fixed-rate record keeps the converter’s own samples, so every frequency stays in hertz.
Synchro-waveform: A time-synchronized waveform measurement, the term IEEE and NASPI use; it covers both triggered and continuous records. See the synchro-waveforms primer.
Phasor: A sinusoid summarized as a magnitude and a phase angle at one frequency, usually the fundamental. Phasors time-stamped against a common clock, called synchrophasors, let operators compare phase angles across a grid.
Phasor Measurement Unit (PMU): An instrument that reports synchronized voltage and current phasors (magnitude and phase) at 30 to 60 reports a second, used for grid-wide stability monitoring.
Power quality analyzer: An instrument that measures power quality to a defined method, records waveforms around events, and checks results against standards. It comes as a portable unit for surveys or as a permanently installed meter.
Digital fault recorder (DFR): An instrument at substations and generating plants that records high-rate waveforms around faults and protection operations, to show how the system and its relays behaved.
Analog-to-digital converter (ADC): The circuit that turns a continuously varying voltage into a stream of numbers, one sample at a time. Its sampling rate and bit resolution set how much detail a recording can hold. A digital-to-analog converter (DAC) does the reverse, turning numbers into a voltage to generate test signals or to transmit data over power lines.
Sampling rate: How many samples a converter takes each second on each channel, given in samples per second or in ksps, thousands of samples per second. At 32 ksps, a 60 Hz cycle holds about 533 samples, one every 31.25 µs.
Phase-locked loop (PLL): A circuit or algorithm that tracks a signal’s frequency and phase, in power systems usually the line voltage’s. Instruments use one to measure frequency, time per-cycle calculations, or set sample timing; grid-tied inverters use one to stay synchronized with the grid.
Nyquist frequency: Half the sampling rate, the highest frequency a recording can represent. An anti-aliasing filter removes signal above it before sampling, so that it cannot fold back into the record as false lower frequencies.
Resampling: Computing new samples at different instants from recorded ones, as when a fixed-rate record is converted to a fixed number of samples per cycle. The new samples are estimates, and the process can add artifacts the original signal never contained.
Decimation: Lowering a record’s sampling rate by filtering it and keeping only every few samples, which discards everything above the new, lower Nyquist frequency.
Bit resolution: The number of distinct levels a converter can report. A 24-bit converter resolves about 16.8 million levels across its range.
RMS (root mean square): The effective value of an AC voltage or current, computed over one or more cycles. RMS summarizes the waveform; it cannot show the shape that produced it.
Crest factor: The ratio of a waveform’s peak to its RMS value, about 1.41 for a pure sine wave. Rectifier loads that draw current in short pulses raise it, stressing UPS systems and generators sized for sinusoidal current.
Calibration and traceability: Calibration compares an instrument with a reference of known accuracy and records its error. Traceability is the unbroken chain of such comparisons back to a national standard, which lets a measurement stand up in a dispute.

Disturbances

Power quality: The cleanliness and stability of electric power: how closely voltage and current hold to their ideal of the right magnitude and frequency, a clean sinusoidal shape, and no interruptions. See the power quality primer.
Sag (dip) and swell: A short decrease or increase in RMS voltage, from half a cycle to about a minute. Sags are the most common cause of equipment trips and resets.
Rapid voltage change (RVC): A quick, step-like change in RMS voltage between two steady levels that stays within normal limits, as when a large motor starts or a capacitor bank switches. IEC 61000-4-30 defines it as an event class of its own, separate from sags and swells.
Interruption: A near-total loss of voltage, below 10 percent of nominal. IEEE 1159 classes interruptions as momentary, temporary, or sustained, the last lasting more than a minute.
Transient: A sub-cycle disturbance, an impulse or oscillation, often from lightning or from switching, including capacitor bank energizing.
Flicker: Rapid, repeated changes in voltage that make lighting visibly fluctuate, typically from arc furnaces, welders, or large motors cycling on and off. Standards rate it with short- and long-term severity indices.
Voltage unbalance: Unequal magnitudes or phase angles among the three phases, often from single-phase loads spread unevenly. Even a few percent can overheat three-phase motors.
Notching: Brief dips cut into the voltage waveform every cycle as the power electronics in drives and rectifiers switch current from one phase to the next.
Harmonics: Waveform components at integer multiples of the fundamental frequency (180 Hz, 300 Hz, and so on at 60 Hz), drawn mainly by power electronics.
Triplen harmonics: The odd multiples of the third harmonic (3rd, 9th, 15th, and so on). In a wye system they add in the neutral instead of canceling, which can overheat neutral conductors; in a delta winding they circulate inside it.
Total harmonic distortion (THD): The combined size of all harmonics relative to the fundamental, in percent.
Total demand distortion (TDD): Harmonic current distortion measured against a facility’s maximum demand current instead of the current flowing at the moment. IEEE 519 sets its current limits in TDD, so a lightly loaded feeder with a high current THD can still comply.
K-factor: A measure of how much extra heating a load’s harmonic current causes in a transformer, and the matching transformer rating. A K-rated transformer is built to carry harmonic-rich loads, like rectifiers and server power supplies, without overheating.
Interharmonics: Components at non-integer multiples of the fundamental frequency. Some sit at fixed fractions of it, as from loads switched on and off in whole cycles; others move independently of it, as from drives, arc furnaces, and inverters.
Supraharmonics: Emissions from 2 to 150 kHz, produced by switch-mode power supplies, inverters, and chargers. Most power quality instruments reduce them to statistics or do not record them.
Resonance: The frequency at which a circuit’s inductance and capacitance exchange energy most readily. When a harmonic lands near it, as when a capacitor bank meets a drive’s harmonics, voltage or current at that frequency can be amplified many times over.
Ground fault: An unintended connection between a live conductor and earth or grounded metal. In a solidly grounded system it trips protection; in an ungrounded or impedance-grounded system it can persist, shifting the voltage of every phase relative to earth.

Grid, Energy, and Equipment

Distributed energy resource (DER): A generator, storage system, or controllable load connected to the distribution grid or behind a facility’s meter. Rooftop solar, batteries, and EV chargers are common examples.
Variable renewable energy (VRE): Generation whose output follows the weather rather than demand, mainly solar and wind. Its swings, and the inverters that connect it, are a growing source of voltage variation and harmonics on distribution grids.
Inverter-based resource (IBR): A source that connects through power electronics instead of a rotating machine, as solar, wind, and battery storage do. Its control software, not physics alone, sets how it behaves during a disturbance.
Grid strength and short-circuit ratio (SCR): How firmly a node on the grid holds its voltage as current changes, usually stated as the short-circuit ratio: the fault current available there relative to the rating of the equipment connected. On a weak grid, with a low ratio, inverter-based resources and large loads swing the voltage more and can destabilize one another’s controls. IEEE 519 scales its harmonic current limits by a similar ratio.
Battery energy storage system (BESS): Batteries with their inverters and controls, used to shift energy in time, support the grid, or back up loads.
Microgrid: A group of loads and sources under common control that can run connected to the utility grid or on its own as an island.
Islanding: Part of the grid, or a microgrid, running on its own sources after separating from the utility. Planned islanding keeps critical loads powered; unintentional islanding is a hazard that DER must detect and stop.
Hosting capacity: How much new load or generation a circuit can accept before voltage, thermal, or power quality limits are exceeded. It decides where new equipment can go without upgrades.
Uninterruptible power supply (UPS): Equipment that carries critical loads through an outage or disturbance on stored energy, usually batteries, until the supply returns or a generator takes over.
Automatic transfer switch (ATS): A switch that moves a load from its normal source to a standby source, usually a generator, when the normal source fails, and back again once it recovers. A static transfer switch does the same with semiconductors, within a fraction of a cycle.
Variable frequency drive (VFD): Power electronics that run a motor at variable speed. Drives are a common source of harmonics and often trip on voltage sags.
Inrush current: The brief surge of current, often several times the running current, drawn when a transformer, motor, or power supply is switched on.
Ride-through: Equipment’s ability to keep running through a sag, swell, or brief interruption without tripping. SEMI F47 and the ITIC curve define ride-through envelopes for loads; IEEE 1547 sets them for DER.
Delta and wye: The two ways to connect three-phase windings and loads. In a wye (star) connection each phase runs from a line to a shared neutral point, giving two voltages, as in 480Y/277 V; in a delta connection each phase runs between two lines, with no neutral. The choice shapes grounding, the voltages available, and how faults and harmonics behave: triplen harmonics, for example, circulate inside delta windings.
Switchgear and switchboard: The assemblies of breakers, switches, and protective relays that distribute power from a facility’s service entrance, where installed power quality meters usually sit.
Current transformer (CT): An instrument transformer that scales a conductor’s current down to a level a meter or sensor can measure. Split-core types open to clip around a conductor, so they install without disconnecting it. A Rogowski coil, a flexible, air-cored alternative, wraps around large or awkward busbars and captures fast transients well.
Voltage transformer (VT): An instrument transformer that scales a high voltage down for measurement, also called a potential transformer (PT). Circuits up to about 600 V are usually measured directly, without one.
Point of common coupling (PCC): The point where a facility connects to the utility grid, and where IEEE 519 harmonic limits apply.
Non-export control: Control that keeps on-site generation or storage from pushing power back into the utility grid, often a condition of interconnection. It measures power at the point of common coupling and curtails the inverters fast enough to stay within the utility’s limit.
Real (active) power: The power that does work or becomes heat, in watts (W): the average, over each cycle, of instantaneous voltage times current.
Reactive power: Power that flows back and forth between source and load within each cycle, building the magnetic and electric fields of motors, transformers, and capacitors without doing net work. It is measured in volt-amperes reactive (var).
Apparent power: RMS voltage times RMS current, in volt-amperes (VA): what cables and transformers must be sized to carry. With sinusoidal waveforms it combines real and reactive power; harmonic distortion adds a further share, distortion power.
Power factor: The ratio of real power to apparent power. Both a phase shift between voltage and current and harmonic distortion lower it, and utilities often charge for a low one. Power factor is often mistaken for the whole of power quality, but it is one measure among many: a site with excellent power factor can still suffer sags, transients, and harmonic distortion.
Volt-VAR control: Regulating voltage by adjusting reactive power, with capacitor banks, voltage regulators, or inverters.
Demand charge: The part of a utility bill set by a facility’s highest average power over a short interval in the billing period, often 15 minutes, rather than by total energy. One short peak can set it for the month.
Demand response: Changing when or how much power a facility uses in response to grid conditions or prices, by shifting or curtailing loads, often in exchange for payment.
Energy management system (EMS): Software that monitors and optimizes how a facility uses, generates, and stores energy, for example by scheduling loads or batteries around tariffs. Utilities use the same name for the control-center system that runs the transmission grid.

Standards

IEEE 519: The IEEE recommended practice for harmonic control, setting voltage and current distortion limits at the point of common coupling.
IEEE 1159: The IEEE recommended practice for monitoring power quality, defining the categories of disturbance and how to characterize them.
IEEE 1547: The IEEE standard for connecting DER to the grid, covering ride-through, voltage regulation, and detection of unintentional islanding.
IEC 61000-4-30: The IEC standard defining power quality measurement methods, so that two instruments measure the same quantity the same way. Class A is its most stringent performance class.
IEC 61000-4-7: The IEC method for measuring harmonics and interharmonics, which IEC 61000-4-30 instruments follow.
IEC 61010 and measurement categories: The IEC safety standard for measurement equipment. Its measurement categories, CAT II to CAT IV, rate how close to the utility supply an instrument may connect; CAT III covers distribution inside a building, from switchgear to panelboards.
SEMI F47: The semiconductor industry’s voltage sag ride-through standard, specifying the sag depths and durations fab equipment must tolerate.
ITIC (CBEMA) curve: A voltage tolerance envelope for information-technology equipment, used to judge whether a sag or swell was severe enough to cause upset.

Communications, Networking, and Security

Application programming interface (API): A defined way for one program to request data or actions from another. An API can work within one computer, as a library call or through a foreign function interface (FFI) between programming languages, or across a network, built on protocols and styles like REST, WebSocket, Modbus TCP, and MQTT.
REST: An architectural style for APIs, almost always over HTTP, in which a client asks for a resource by its URL and receives a response, usually JSON. It suits on-demand queries, like fetching an hour of history.
Latency: The delay between something happening and a system acting on it, from measurement through transmission and analysis to a response. Control needs it in milliseconds; reports and dashboards tolerate seconds.
Hard and soft real-time: A hard real-time system must meet every deadline, because one miss can mean a failure, as in protection and control loops. A soft real-time system can miss an occasional deadline with only degraded results, as a live display can.
Bandwidth: Two related meanings. For a network link, the data rate it can carry: one sensor recording seven channels at 32 ksps and 24-bit resolution produces about 0.7 MB a second, roughly 60 GB a day. For an instrument, the range of signal frequencies it captures, which reaches at most half its sampling rate.
Polling: Asking a device for its latest values at a fixed interval. The client sets the pace, so traffic and timing are predictable, but news waits up to one interval, and whatever happens between two requests is lost unless the device keeps it. Modbus TCP, the long-established industrial protocol most building management and SCADA systems read, works this way.
Publish/subscribe: A messaging pattern in which producers publish to named topics and every subscriber to a topic receives the messages, without either side addressing the other directly. Data arrives as soon as it is published, and subscribers can come and go without changing the producer; in exchange, traffic follows the data rather than a fixed schedule. MQTT is the most common example in industrial and IoT telemetry.
SCADA: Supervisory control and data acquisition: the systems utilities and industrial plants use to monitor and control equipment across a site or network, typically by polling devices every few seconds.
Building management system (BMS): The control system that runs a building’s heating, cooling, lighting, and other services, often reading meters over Modbus or BACnet.
Precision Time Protocol (PTP): The IEEE 1588 protocol for synchronizing clocks across a network. With hardware timestamping it holds devices to within a microsecond, so waveforms from different sensors line up sample for sample.
Virtual private network (VPN): An encrypted tunnel across the internet between a device or site and a private network, so remote users reach it as if they were on site. A conventional VPN routes every connection through a central server. An overlay network instead gives each device its own cryptographic identity and connects devices directly wherever they sit, so access rules follow each device rather than its location.
Air gap: A network with no connection to the internet or any outside network, used where data must never leave the site.
Memory safety: A property of software that every memory access stays within valid, live memory, which rules out buffer overflows and use-after-free errors, the source of most serious vulnerabilities in C and C++ code. Most memory-safe languages, like Java, Go, and Python, get it from a garbage collector, which can pause the program at moments it does not choose. Rust gets it by checking ownership and borrowing when the code is compiled, so it stays safe without a collector and its timing stays predictable enough for real-time control. U.S. cybersecurity agencies urge memory-safe languages for critical infrastructure.
Garbage collection: Automatic reclaiming of memory a program no longer uses. It makes software simpler and memory-safe, but the collector can stall the program at unpredictable moments, which hard real-time control cannot tolerate.

Data and Platform

Gateway: A device that connects one network or group of equipment to another, translating between their protocols. At a facility, an edge gateway collects data from meters, sensors, and controllers, processes or stores it locally, and passes it on to people and software elsewhere.
Historian: A database that stores time-series values from industrial systems, typically at intervals of a second or longer. A continuous waveform record holds the samples a historian would summarize.
Data lake: A central store that keeps data in its original form and full resolution for later analysis, instead of summarizing it on the way in.
Apache Parquet: An open, column-oriented file format for large datasets. Storing each column together lets a query read only the columns it needs and skip blocks whose stored minimum and maximum rule them out, so it scans far less than it would in a row-by-row or text file; similar values side by side also compress well. Python, Rust, and most analytics tools read it directly.
Lossless and lossy compression: Lossless compression is fully reversible: the original samples come back exactly, bit for bit. Lossy compression discards detail to save more space and cannot be reversed. Lossy suits audio and video; applied to a measurement record, it removes the small disturbances an investigation may need.

Analysis, AI, and Control

Fast Fourier transform (FFT): The standard algorithm for splitting a waveform into its frequency components, the basis of harmonic and spectral analysis.
Spectrogram: A picture of how a signal’s frequency content changes over time: time along one axis, frequency along the other, and strength as color. A drive ramping up or a harmonic appearing shows at a glance.
Edge analytics: Analysis that runs on a computer at the site, beside the data, instead of after the data is sent to a cloud.
Anomaly detection: Finding behavior that departs from a learned or expected baseline, without a fixed threshold set in advance.
Root cause analysis: Working back from a symptom, like a trip, to the condition that produced it, which often lies in other equipment and earlier in time.
Physics-grounded AI: AI that works from the results of deterministic signal processing and power-system analysis, cites the measurements behind each conclusion, and leaves decisions to engineers.
Language model: An AI model that reads and writes text. In physics-grounded AI it explains results and answers questions from cited measurements, rather than producing the measurements itself.
Human-in-the-loop: A design in which AI informs and recommends while a person makes the decision and takes the action.
Closed-loop control: Equipment acting automatically on what is measured, with the result measured again, as in voltage regulation or load shedding. It needs low, predictable latency.

See the Terms at Work in the Control Room

Watch waveforms, harmonics, and events on a live continuous record from our lab.