intermediate Power Systems

Harmonic Analysis & G5/5 Compliance

Synthesise a distorted waveform from individual harmonic components, run an FFT, and check the spectrum against ENA EREC G5/5 voltage planning limits — the standard a UK harmonic distortion study has to demonstrate compliance with.

preset:

Synthesised waveform (1 cycle, peak = 1.00 pu)

Harmonic spectrum (% of fundamental, h2–h25). Orange dashes = G5/5 EHV limits.

THDv0.00%

G5/5 EHV limit: 3% — within limit

Harmonic content (% of fundamental)

100.0%

0.0%≤2%

0.0%≤1%

h11

0.0%≤1.5%

h13

0.0%≤1.5%

Limits shown are ENA G5/5 EHV planning levels (UK). LV / MV systems have higher allowances.

Start with the 'Pure 50 Hz sinusoid' preset. The waveform is clean, the spectrum is empty above the fundamental, and THD = 0%. This is the goal of every PCC: deliver close-to-sinusoidal voltage to other connected customers.

Show hint

PCC = Point of Common Coupling. The harmonic limits in G5/5 apply at the PCC, not deep inside the customer's installation.
Switch to '6-pulse VFD'. This is the canonical spectrum from a 6-pulse variable-speed drive — 5th, 7th, 11th, 13th, with magnitudes ≈ 1/h. Watch the waveform develop notches and shoulders, and the THD jumps to ~25%. Several individual harmonics already breach the orange G5/5 markers.

Show hint

Real 6-pulse drive harmonics = 6k ± 1 (5, 7, 11, 13, 17, 19, ...). The 1/h amplitude rule is an idealised lower bound — actual drives can be worse, especially on partial load.
Manually pull h5 down to 2% and h7 to 1.5%. Most other orders also need attenuation. This is what a 5/7 harmonic filter (or a 12-pulse rectifier replacing the 6-pulse) physically does to your spectrum. Note how rapidly the THD falls when the dominant harmonics are dealt with first.

Show hint

5th-harmonic filters are typically tuned slightly below 5·50 = 250 Hz (e.g. 245 Hz) so they don't shift into a parallel resonance under detuning.
Try the 'Square wave' preset. Odd-only harmonics with amplitudes 1/h give the classic square wave reconstruction. THD = 48.3% (the well-known theoretical value). This is what a switching-mode load looks like if it has zero filtering.

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Even harmonics are absent because the waveform has half-wave symmetry: f(t + T/2) = −f(t). Real loads with DC offset (like single-phase rectifiers) do introduce even harmonics.
Use the Spectrum CSV button to download a per-harmonic table including the G5/5 limit and pass/fail flag. This is the table you'd paste into a connection-study Word report under 'Harmonic compliance assessment'.

Show hint

Real studies also need background distortion levels (typically ~1.5% THDv before your customer connects) and an emission allocation per IEC 61000-3-6. This lab covers Stage 1 / Stage 2 only.