Voltage THD and current THD describe different quantities and should be labelled accordingly. The relationship shown here uses harmonic RMS components relative to the fundamental RMS component. Measurements need a defined location and operating condition so that the result can be interpreted in the context of the installation.
Consider the source and connected equipment together
Non-linear loads and the supply system interact. A harmonic assessment should consider the relevant load data, generator characteristics and operating arrangement. A single percentage from a catalogue or unrelated site does not establish the behaviour of a different combination of equipment.
Why generators care
- alternator heating
- voltage waveform
- neutral current
- power factor
- sensitive equipment
Do not generalise
Harmonics tell you something about the shape of the current it demands.
Specify the THD measurement
Record whether the result is current THD or voltage THD, the measurement point and the operating condition. The denominator is the fundamental RMS value. A change in that reference or in the load condition can change the comparison.
Voltage THD relationship
THDV = [√(Σ Vh²) ÷ V1] × 100%, where the sum covers the measured harmonic RMS voltage components above the fundamental and V1 is the fundamental RMS voltage. State the measurement location, operating condition and harmonic coverage. Current THD uses the corresponding current quantities and must be labelled separately.
Use evidence for any design response
Changes to capacity, equipment or harmonic treatment should follow an assessment of the actual problem and proposed operating modes. Record the measurement basis and applicable manufacturer information. This page explains the metric; it does not prescribe a universal limit or a fixed generator derating factor.
Start Engineering Review
Start Engineering Review. Share the application, location and information available so the next discussion can address your actual requirement.
Related guidance
Engineering intent
Harmonics & THD should be evaluated as part of the complete power system rather than as an isolated catalogue item. Operating duty, connected load, interfaces, protection, controls, installation conditions, testing requirements and lifecycle consequences determine whether the arrangement is appropriate.
Engineering considerations
Motor starting may require separate kVA and voltage-dip review. UPS and sensitive loads may affect current and generator behaviour. UPS loads may require review of UPS input power factor, battery recharge current, harmonics, bypass logic and generator voltage/frequency stability. Alternator, neutral, cable, protection and power-quality review may be required. harmonic currents can cause overloads, vibration, ageing of equipment and disturbances to sensitive equipment, local networks and telephone networks. For rental power and temporary generator installations, current calculation should lead into cable planning. ATS/AMF rating must be selected from actual load, generator current, fault level, transfer philosophy, neutral arrangement and site design. A current value does not automatically select the breaker.
Selection and design considerations
A transformer must be selected for the complete electrical duty—not only the connected load. 10, 16, 25, 40, 63, 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000 and 5000 kVA. The selected standard rating should normally be equal to or greater than the calculated requirement. But the nearest higher rating is not automatically the best choice. The engineer must assess whether the load includes: ⚙️ Large motors 🔥 Welding equipment 🖥️ Data-centre loads 🔋 UPS systems ⚡ Variable-frequency drives 🌡️ High-temperature environments 🏭 Non-linear industrial processes These loads can influence transformer heating, harmonics, voltage regulation and neutral-current requirements. Transformer percentage impedance is one of the most important yet frequently misunderstood design parameters.
Integration and protection considerations
VFDs, rectifiers, LED drivers, battery chargers, UPS systems and switched-mode power supplies can introduce harmonic currents. Modern facilities contain nonlinear loads that can introduce harmonics, causing overheating, neutral loading, voltage distortion, increased losses, relay maloperation and reduced equipment life. AVR diagnostics should therefore consider power quality when voltage complaints repeat. The generator terminal voltage is not always the load terminal voltage. If cable impedance is high, the generator may regulate voltage correctly at its terminals while the load receives poor voltage. Cable-engineering material explains that correct source voltage does not guarantee correct load voltage, because every cable has resistance/reactance and voltage drop depends on current, cable length, conductor resistance, reactance, power factor, conductor temperature and arrangement.
Testing, commissioning and verification
Do not diagnose an AVR before confirming engine speed and frequency. Overexcitation can occur when voltage is too high relative to frequency. Excessive V/Hz can create magnetic-core saturation, overheating and insulation damage. Do not bypass voltage protection to keep the generator running. An AVR replacement will not solve a thermal problem caused by overload or ventilation failure. Voltage issues may appear only when certain loads start or stop: Is the AVR failing, or is the load event exceeding the generator system’s voltage-response capability? If the UPS rejects generator supply, the problem may involve generator voltage stability, frequency stability, harmonics, ramp settings, UPS tolerance or system integration — not necessarily AVR failure alone.
Direct answer
Voltage THD and current THD describe different quantities and should be labelled accordingly. The relationship shown here uses harmonic RMS components relative to the fundamental RMS component.
The practical decision for Harmonics & THD should be based on verified load, equipment, interface and site information. Where equipment-specific data, regulatory applicability or a project study is required, this guidance does not replace that verification.