About the Harmonics & THD

Harmonics analysis assesses the current and voltage distortion introduced into the electrical system by solar inverters and their impact on grid power quality. All grid-tied solar inverters inject some harmonic content — it must stay within IEEE 519 / IS 16612 limits to avoid interference with other equipment and DISCOM penalties.

Formula

How to calculate it

1. 1Measure THD at the point of common coupling (PCC) using a power quality analyser.
2. 2Compare against IS 16612 / IEEE 519 limits: current THD < 5% for systems up to 1 MW at 33 kV and below.
3. 3If THD exceeds limits: check inverter output filter design, consider adding an LCL filter or active harmonic filter.
4. 4Multiple inverters in parallel can partially cancel odd-order harmonics if staggered by 30° (phase-shifted transformers) — a useful mitigation for large plants.

Worked example

A 500 kWp plant with 10 × 50 kW central inverters at a 33 kV grid connection. Each inverter contributes 3.5% THD individually. At the 33 kV PCC, the combined THD measures 2.8% (partial cancellation between units) — within the 5% IS 16612 limit. No additional filtering required.

What you can calculate

• THD %
• IEEE 519
• Filter sizing
• Grid impedance

Standards & references

Frequently asked questions