Modern power systems are no longer reliably sinusoidal. As power electronic converters proliferate, interharmonic disturbances — voltage or current components at frequencies that aren't integer multiples of the fundamental — have become a persistent feature of low-voltage networks. IEC 61000-4-13 exists specifically to evaluate whether grid-connected equipment tolerates this disturbance class.

Why Interharmonics Are a Genuinely Different Problem

Interharmonics arise from non-linear loads with non-synchronous switching — switch-mode power supplies among the most common sources — and fall outside frequency ranges traditional harmonic analysis addresses. Equipment affected rarely fails outright; more commonly it exhibits oscillatory control responses, unexpected protection triggering, or increased thermal stress.

The Voltage Synthesis Approach

Every test item is built on a synthesised supply voltage where harmonic and interharmonic components are superimposed onto the fundamental, while RMS voltage is strictly held at nominal throughout. The question isn't "how much distortion does this equipment produce," but "does this equipment keep functioning correctly when the supply it receives is already distorted." The standard applies to equipment rated at no more than 16A per phase, with disturbance frequencies extending to 2kHz for 50Hz systems.

The Three Core Test Items

Single interharmonic voltage injection introduces one interharmonic component, isolating interaction between a single spectral excitation and the device's control or protection mechanisms.

Swept interharmonic frequency testing systematically varies disturbance frequency while magnitude is held constant, revealing frequency-dependent immunity that fixed-frequency excitation can't capture.

Composite interharmonic disturbance applies several components simultaneously, reflecting realistic networks hosting multiple asynchronous non-linear loads, evaluating nonlinear interaction effects single-frequency injection won't reveal.

Test Setup — Single-Phase and Three-Phase

The actual test voltage is synthesised by a waveform generator delivered through a programmable AC source, with voltage and current measured at the interface between source and EUT. Three-phase equipment uses three synchronised source channels against a common reference.

What to Verify in a Source Before Committing to It

IEC 61000-4-13 Table 5 defines specific source requirements — output current capability, frequency accuracy, phase angle resolution, and interharmonic magnitude/frequency range. ActionPower's CORTEX AC series generally exceeds the standard's minimums, though the standard model's interharmonic frequency range (0.001–200Hz) doesn't reach the IEC's upper band alone; the CORTEX HF variant (0.001–5000Hz) covers that upper band, with voltage de-rated above 200Hz.

Where Interharmonic Immunity Genuinely Matters

Grid-tied inverters synchronise through PLL and control loops sensitive to interharmonics near control bandwidth boundaries, potentially triggering oscillatory current injection or loss of synchronisation — see our companion article on building a megawatt-scale grid code compliance testing platform for the hardware side at utility scale.

EV onboard chargers (V2G/V2H/V2L-capable) can produce unexpected current ripple or unstable control responses from upstream supply distortion.

Industrial motor drives and servo systems can translate interharmonic distortion into torque ripple, acoustic noise, or nuisance trips.

Data centres and UPS/PDU ecosystems commonly exhibit genuine spectral congestion, making interharmonic immunity a practical reliability metric, not just a compliance checkbox.