Power Quality and Harmonic Survey: Voltage and Current Distortion Measurement

What it is

A harmonic survey records how far the voltage and current waveforms at a chosen point depart from a clean sine wave, and breaks that departure into its harmonic components. Equipment built around power electronics, including the inverters and rectifiers of battery energy storage systems and the rectifiers that feed direct-current systems, draws or injects current that is not sinusoidal, and that current produces distortion in the voltage of the system it is connected to. The survey measures both sides: current distortion, which describes what the installation injects, and voltage distortion, which describes the condition of the supply at the point of connection. Measurements are taken at a defined point, usually the interconnection with the utility or the main distribution, using a power quality analyzer with the current and voltage transducers suited to the system. Within acceptance testing the survey is often optional or specified for large converters, and it may include a record of power quality at start-up, when the equipment is first brought online.

Why it is done

Harmonic distortion is a quiet problem. It heats transformers, cables and neutrals beyond what the fundamental load would explain, it causes capacitors to overheat or resonate, it disturbs protective relays and meters, and it can upset sensitive electronics, drives and communications. Because the voltage distortion at a connection point is shared by every customer fed from it, a large converter can degrade service for neighbors as well as for its owner, which is why utilities and standards place limits on what a connected installation may inject. A survey at commissioning answers the practical question of whether the new equipment complies with those limits in the real system, with the real impedance of the supply, and not only on a manufacturer's datasheet. It also records the baseline: the condition of the supply before and after the equipment operates. When complaints appear later, a baseline is worth far more than a snapshot taken after the problem is already present.

The principle

Any periodic waveform that is not a pure sine wave can be described as a sum of sine waves at the fundamental frequency and at whole-number multiples of it, called harmonics. Nonlinear loads, such as rectifiers and inverters, draw current in pulses, so their current contains harmonics whose orders depend on the converter design. When that current flows through the impedance of the supply, it creates a voltage drop at each harmonic frequency, and that is what distorts the voltage. Distortion is summarized in two ways. Total harmonic distortion compares the combined harmonic content with the fundamental. For current, a more meaningful measure compares the harmonic content with the maximum demand current of the installation, so that a lightly loaded moment does not exaggerate the apparent distortion. Individual harmonic orders are also examined, because limits are usually applied to each order as well as to the total. How much voltage distortion a given current produces depends on the strength of the supply at the connection point, which is why the ratio between the available short-circuit current and the load current governs the limits that apply.

How it is performed

The survey begins with the definition of the measurement point, which is normally the point of common coupling where the installation connects to the system, and with the single-line diagram, the nameplate of the converter and the supply characteristics. A power quality analyzer of a class suited to harmonic measurement is connected to the voltages and, through current transducers, to the currents at that point, and its clock, ranges and settings are confirmed. The measurement is run across a period long enough to capture the operating states that matter, with the equipment at various power levels and, for a storage system, in charging and discharging modes. The instrument records harmonic content, total distortion, and the voltage and current behavior over time, and the start-up behavior can be captured when the equipment is first brought online. Safety rules for working near live conductors apply, and the connections are made so that the instrument cannot open a current transformer circuit. The operating state of the equipment during the measurement, the supply configuration and any other significant loads on the system are noted, since all of them shape the result.

What is recorded

The record identifies the installation and the point of measurement, describes the instrument, its class and calibration status and the transducers used, and states how the data were aggregated. It captures the distortion measured, the operating state of the equipment during each part of the survey and the supply conditions at the time. It also states how the demand and the strength of the supply at the point of connection were established, because the applicable limits depend on them. These details matter because harmonic levels move with load and with the rest of the network, so a result has meaning only alongside the conditions that produced it.

How results are judged

Results are judged against limits that depend on the strength of the supply relative to the installation, the voltage level of the system and the harmonic order. Voltage distortion at the point of connection is compared with limits for the voltage class, for total distortion and for individual harmonics. Current distortion is compared with limits that depend on the ratio between the short-circuit capacity at the connection point and the maximum demand current of the installation, with limits applied to individual orders and to the total demand distortion. Measurements are normally assessed on statistical terms over the measurement period, not on a single peak reading. IEEE 519-2022 is the published standard for harmonic control in power systems and sets the limits and the measurement guidance. ANSI/NETA ATS-2025 lists the survey among the electrical-acceptance items for battery energy storage systems, where it is optional at start-up. For the limit tables and the details of how they apply, use your own licensed copies, which remain the authority.

Variations

At acceptance the survey establishes whether a new converter-based installation meets the limits and creates a baseline for later. Over its life, the same survey may be repeated after changes to the installation or the supply, or in response to a complaint. The equipment type changes what is expected. A battery energy storage system has two modes, charging and discharging, with different harmonic behavior in each, and the system must be observed in both. A rectifier feeding a direct-current system draws current whose harmonic content depends on its pulse arrangement and loading. The measurement point also varies: a utility-scale installation is measured at the medium-voltage interconnection, while a smaller installation may be measured at a main low-voltage bus. Weaker supplies amplify the voltage effect of a given current, so the same equipment can pass on a strong system and fail on a weak one. The method and the limits follow the governing standard for each case.