What Causes High THDi and How Can You Reduce It

High THDi — total harmonic distortion of current — is the ratio of the RMS value of all current harmonics to the fundamental current, expressed as a percentage. It rises when nonlinear loads such as variable frequency drives (VFDs), rectifiers, UPS units and chargers draw current in pulses instead of a clean sine wave. A high reading must be explained, not simply cleared: measure the spectrum at the point of common coupling (PCC) and size mitigation from the measured harmonic current.

This guide explains what causes high THDi, how to measure it, and how to choose between line reactors, passive filters, an active harmonic filter (AHF) and a Static Var Generator (SVG).

What Causes High THDi

The root cause is nonlinear loads, which control power electronically so the current they draw is not proportional to the applied voltage. Common sources include:

• VFDs for motors, pumps, fans and compressors
• UPS systems and switch-mode power supplies
• AC and DC electric vehicle chargers
• Rectifiers, welding machines and industrial power supplies
• LED lighting drivers and office electronics
• Solar inverters, battery chargers and energy-storage converters

A six-pulse rectifier produces mainly the 5th, 7th, 11th and 13th odd orders; single-phase equipment adds triplen orders. Distortion worsens when many devices share a transformer or busbar, or when standby generation changes the source impedance.

The Consequence: Heat, Lost Capacity and Resonance

Harmonic current does no useful work but still produces heat: transformers see higher winding and eddy-current losses, and cables, busbars and neutrals run hotter than the fundamental alone suggests. Triplen harmonics add in the neutral instead of cancelling, so it can carry a zero-sequence current approaching the phase current.

The extra RMS current also consumes capacity that would otherwise serve load, and fixed capacitor banks can resonate with the supply inductance, amplifying one harmonic order and overheating capacitors or tripping protection.

How to Measure and Diagnose High THDi

Use a Class A power-quality analyser to log the spectrum at the load, feeder and PCC: THDi and harmonic amps, individual harmonic orders, power factor, THDv and zero-sequence current.

CT placement is decisive: a high reading at one machine may be harmless at the PCC, while several moderate sources combine upstream. Install CTs to capture the load, not the filter’s own output; a reversed CT feeds a false reference and can make a correct filter look worse.

How to Reduce High THDi

Selection depends on the spectrum, load variation, network impedance and the required PCC result.

Identify the Main Harmonic Sources

Determine which equipment creates the most harmonic current, and when, so a filter is not fitted to the wrong feeder or harmonic range.

Review the Distribution Design

Separate sensitive equipment from nonlinear loads and check cable sizing, neutral capacity and phase balance before adding hardware, because a network problem cannot always be filtered away.

Use Line Reactors or DC Chokes

Reactors and chokes reduce distortion from individual drives and rectifiers, but they are usually insufficient where many changing loads share a busbar.

Apply Detuned Passive Filters Where Conditions Are Stable

A detuned passive filter targets selected frequencies when the spectrum is predictable, but a filter tuned near the network resonance point can amplify the harmonic it is meant to remove.

Install an Active Harmonic Filter

An active harmonic filter measures load current, extracts the harmonic components and injects an opposing compensation current. Correction is order-by-order rather than tuned to one frequency, so an AHF does not introduce the resonance of fixed capacitor banks. YT Electric builds the YTPQC-AHF active harmonic filter on a three-level converter topology with a 25.6 kHz switching and control frequency, in modules from 15 A to 150 A.

Add a Static Var Generator Where Reactive Power Dominates

Where a low power factor and fluctuating reactive power dominate rather than harmonic current, a Static Var Generator (SVG) is the better tool: it synthesises inductive or capacitive current to hold the target power factor and can correct three-phase imbalance.

Comparing Harmonic Mitigation Technologies

The table below summarises how the options differ, what each corrects and where each fits.

Table 1 — Comparison of the main harmonic mitigation technologies (YT Electric product data).

How to Choose and Size Harmonic Mitigation

Size a filter from the measured spectrum, not from connected kilowatts or a single THDi reading. The harmonic current to cancel is the RMS sum of the individual orders, I_h = √(I₅² + I₇² + I₁₁² + …). With only a THDi reading, estimate harmonic current ≈ load current × THDi, plus 20–30 % headroom for load growth. Modular units parallel easily, so a design can start with one module and expand later.

Check the installation point, voltage, panel space, cooling and compatibility with existing capacitor banks or generators, and confirm rated current at 45 °C ambient. The three-level topology halves output dv/dt and lowers EMI, and a high IGBT switching frequency such as the 25.6 kHz used in the YTPQC-AHF extends bandwidth, so higher orders are compensated with a smaller output reactor. For the highest orders, a SiC MOSFET active harmonic filter extends the range further.

Table 2 — Engineering selection and parameter matrix for the YTPQC-AHF (confirm the final rating against a site survey).

Limitations and When These Solutions Are Not Enough

• An AHF corrects distortion only from loads inside its measurement loop; it cannot see harmonics from another feeder or generator.
• Compensation capacity is finite — load growth beyond the design needs a new survey and extra modules.
• A passive filter is fixed once installed and loses effectiveness when the spectrum or load profile changes.
• No device removes the need for correct neutral and cable sizing, and none can compensate distortion from the utility side of the PCC.

These limits are why a harmonic study precedes equipment selection and why commissioning measurements are repeated across the operating range.

Frequently Asked Questions

What is considered a high THDi?

Acceptable THDi depends on the network, load, standard and location. IEEE 519 limits current distortion to roughly 5–15 % by short-circuit ratio at the PCC; always read it with the harmonic current in amperes.

How do I size an active harmonic filter for procurement?

Size from the measured harmonic current, not the transformer rating: use the RMS sum of individual orders, or load current × THDi, plus 20–30 % headroom. Specify rated voltage and current, mounting and target THDi. YT Electric supplies the YTPQC-AHF from 15 A to 150 A.

Does installation require a shutdown, and what maintenance is needed?

The filter connects in parallel, so installation needs only a short planned outage for CT fitting and cabling. Inspect cooling fans and air filters annually (quarterly in dusty sites) and check busbar torque, terminal temperature and CT polarity. Planned SVG and AHF maintenance protects the investment.

What is the return on investment of harmonic mitigation?

The case rests on measurable items: lower reactive and distortion-related utility charges, distribution capacity released when harmonic current is removed, and avoided failures of plant equipment. Payback depends on the local tariff, load profile and failure history, so calculate it from the plant’s own meter data.

Conclusion

High THDi is a symptom, not a diagnosis: it points to nonlinear loads, to conditions that amplify distortion, and to the measurement that identifies the source. If your facility sees high THDi, overheating or nuisance trips, YT Electric can review your power-quality data and recommend a suitable solution. Contact our team, or explore the Active Harmonic Filter and Static Var Generator ranges to match a rating to your measured load.