What Is Total Harmonic Distortion (THD)? Measurement and Limits

Total harmonic distortion (THD) is the ratio that expresses, as a number, how far the voltage or current waveform in an electrical network deviates from an ideal sine wave. In an ideal AC network, voltage and current are pure sine waves at the fundamental frequency (50 Hz in Türkiye and Europe, 60 Hz in North America). Nonlinear loads, however, add extra frequency components at exact multiples of the fundamental frequency, known as harmonics. THD gives the combined magnitude of these harmonic components relative to the fundamental, as a percentage.

Mathematically, THD is the square root of the sum of the squared RMS values of the harmonic components (2nd, 3rd, 5th, 7th and higher orders), divided by the RMS value of the fundamental (1st order) component. In practice, measuring instruments calculate this ratio by taking harmonics up to the 50th order into account. A THD of 0% indicates a pure sine wave; rising values point to an increasingly distorted waveform with notched peaks and troughs.

In short, THD is one of the most fundamental indicators of power quality. Low THD means a clean network in which transformers, motors, capacitors and sensitive electronics run safely, efficiently and with a long service life. High THD signals problems such as overheating, efficiency losses, nuisance tripping of protection relays and shortened equipment life.

THD-V measures distortion of the voltage waveform, while THD-I measures distortion of the current drawn by a specific load, and confusing the two is a common mistake. THD-V (voltage total harmonic distortion) is the distortion ratio of the supply voltage and is a direct measure of network cleanliness affecting every consumer at that point. THD-I (current total harmonic distortion) is the distortion ratio of the current a given load or facility draws from the network, and it shows how much that load contributes to distortion.

The distinction matters because a load can produce very high THD-I (for example, a variable frequency drive running at light load) while THD-V stays relatively low if the network impedance is low. Conversely, on a weak network even moderate harmonic currents can push voltage distortion above the limits. That is why current distortion is measured order by order at individual loads, while voltage distortion is evaluated for the network as a whole.

To assess current distortion more fairly, engineers use total demand distortion (TDD). TDD relates harmonic currents to the facility's maximum demand current rather than the instantaneous current. This prevents THD-I values that look high during light-load operation from overstating the facility's real impact. For this reason, IEEE 519 defines its current limits in terms of TDD.

Harmonics are caused mainly by nonlinear loads, meaning devices that draw a current that is not proportional to the applied voltage and is therefore not sinusoidal. In industrial facilities the most common sources are variable speed drives (VSDs/VFDs), rectifiers and UPS systems, arc furnaces and welding machines, LED drivers and fluorescent ballasts, and switch-mode power supplies. These loads typically generate 5th, 7th, 11th and 13th order harmonics.

The effects of high THD are not just theoretical; they show up directly in operating costs. Harmonic currents cause additional heating and losses in transformers and conductors, third-order harmonics accumulate in the neutral conductor and overload it, power factor correction capacitors can be driven into resonance and fail violently, and protection relays and meters can measure incorrectly.

Taken together, these effects shorten equipment life, increase unplanned downtime and reduce energy efficiency. Failing to consider harmonics together with reactive power compensation systems is the most common cause of unexpected capacitor failures. Harmonic analysis should therefore be an integral part of every reactive power compensation design.

Harmonic distortion is measured with a power quality analyzer. These instruments record voltage and current waveforms at a high sampling rate and use the FFT (Fast Fourier Transform) algorithm to break the signal down into its fundamental component and individual harmonic orders. The results are reported as a percentage for each order and as an overall THD value.

The method for reliable, standards-compliant measurement is well defined. IEC 61000-4-7 specifies the measurement technique and windowing method for harmonic measurement (a 10-cycle window of about 200 ms on a 50 Hz network). IEC 61000-4-30 defines how power quality parameters are measured and classified (Class A/S). For a meaningful assessment, measurement should typically run for at least one week and cover every load condition of the facility, including production, shift changes and idle periods.

A single spot reading can be misleading, because harmonic levels vary considerably throughout the day depending on the load profile. An accurate diagnosis requires choosing the right measurement points (main switchboard, sub-distribution panels, problem load feeders), monitoring current and voltage distortion simultaneously and evaluating the data as trends. As part of our Power Quality Improvement service, Miratek carries out on-site measurements in line with these standards, maps the harmonic spectrum order by order and reports the results together with recommended solutions.

Under IEEE 519, total voltage distortion (THD-V) should generally not exceed 5% and any individual harmonic should not exceed 3% on systems between 1 kV and 69 kV; IEC 61000 sets compatibility and emission levels alongside it. Harmonic limits are defined within these two main international frameworks, and distribution practice in Türkiye is also based on them. IEEE 519 sets limits for voltage and current distortion at the point of common coupling (PCC), and at higher voltage levels these limits become progressively stricter.

On the current side, IEEE 519 does not use a single THD-I figure; instead it defines TDD limits that vary with the facility's short-circuit ratio to the network (Isc/IL). Consumers that are 'small' relative to the network are allowed more distortion and 'large' consumers less, so each consumer's contribution to the quality of the shared network is limited fairly.

The IEC side has a multi-layered structure. IEC 61000-2-2 and IEC 61000-2-4 set compatibility levels, meaning the acceptable distortion levels for low-voltage public networks and industrial installations; the typical compatibility level for voltage THD on low-voltage public networks is around 8%. IEC 61000-3-2 and IEC 61000-3-12 define the harmonic current emission limits that equipment may inject into the network. In Türkiye, regulations from TEDAŞ (Turkish Electricity Distribution Company) and EMRA (the Energy Market Regulatory Authority, known in Turkish as EPDK) refer to these standards for distribution grid connections and set the power quality conditions that subscribers must meet.

High THD is reduced by choosing a mitigation method that matches the harmonic source and order profile identified through measurement; there is no one-size-fits-all solution. Passive harmonic filters are reactor and capacitor combinations tuned to a specific order, such as the dominant 5th harmonic; they are economical and robust, and suit facilities with a stable, dominant harmonic profile. Adding reactors (line or DC chokes) at drive inputs is also a simple, cost-effective first step to reduce current distortion.

Active harmonic filters inject harmonic currents in antiphase into the network in real time and dynamically suppress a wide range of orders. They are the most flexible solution for facilities with variable load profiles and many different harmonic sources. Multi-pulse (12- or 18-pulse) rectifier configurations and harmonic-rated (K-factor) transformers are preventive options that can be specified at the design stage.

The right solution is determined by evaluating measurement data, the harmonic spectrum, compensation needs and resonance risk together. Miratek Elektrik runs an integrated engineering process within its Power Quality Improvement service, from on-site measurement and filter design to panel manufacturing and commissioning. Our Harmonic Filter Panels and Reactive Power Compensation Panels are among the 340+ panels of all types we have manufactured to date. By addressing harmonic mitigation and reactive power compensation together, we bring THD below standard limits and minimize the risk of reactive power penalties.