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Harmonic Mitigation & EREC G5/5 Sizer (Detuned Reactors)

Non-linear rectifiers in VFDs, EV rapid chargers, and LED arrays inject harmonic currents that distort grid voltage, overheat neutral conductors, and trigger destructive parallel resonance in capacitor banks. This calculator works out $THD_I$, evaluates BS 7671 Table 4Aa neutral derating, checks ENA EREC G5/5 Stage 1 compliance, and sizes anti-resonance detuned reactors.

Supply & Transformer BaselineTransformer Rating ($S_{tx}$)kVA. Common UK ratings: 500, 800, 1000, 1250, 1600, 2000 kVA.Transformer Impedance ($z_{sc}$)% impedance voltage. Typically 4.75% to 5.25% for 1000 kVA.Minimum Fault Level ($S_{sc,min}$)MVA at Point of Common Coupling. EREC G5/5 Stage 1 evaluates against minimum fault level.

Connected Plant LoadTotal Connected Load Current ($I_{b}$)Amperes (design current per phase at 400V 50Hz).Dominant Non-Linear Load CategoryGoverns typical harmonic spectra (3rd, 5th, 7th proportions).Non-Linear Load Share (%)% of total connected current drawn by power electronic rectifiers.

Power Factor Correction (PFC)Existing / Proposed Capacitor Bank ($Q_c$)kVAR at 400V. Enter 0 if no capacitor bank is present.

Engineering Analysis & Results

Harmonic Distortion Summary

Fundamental Current ($I_1$):200.0 A

Total True RMS Current ($I_{rms}$):209.1 A

Current Distortion ():29.6 %

Estimated Voltage Distortion ($THD_V$):3.2 %

G5/5 STAGE 1: COMPLIANT (≤ 5.0% THD)

BS 7671 Neutral Conductor Sizing

3rd Harmonic Content:42.0 A (21.0%)

Triplen Neutral Current ($I_N$):126.0 A

Table 4Aa Sizing Rule ():Line Current × 0.86

Minimum Cable Current Rating ($I_z$):232.6 A

3rd harmonic is between 15% and 33%. Line conductors must be oversized by rating factor 0.86.

PFC Parallel Resonance & Detuning

Undetuned Resonant Order ($h_r$):15.8

Recommended Reactor Detuning ($p$):7.0% ($f_r = 189\text{ Hz}$)

Required Capacitor Voltage ($V_C$):≥ 440 V (Rated 480 V)

Series reactor detuning shifts resonance below the 5th harmonic (250Hz), eliminating capacitor overload. Fundamental voltage on capacitor rises by factor $1/(1-p) = 1.075$, requiring 440V or 480V units.

Transformer Thermal Derating

Calculated K-Factor:3.4

Recommended Maximum Loading:86% of Nameplate

ENA EREC G5/5 & BS 7671 Table 4A3 Harmonic Derating Formulation

ENA EREC G5/5 Issue 1 & BS 7671:2018+A4:2026 Appendix 4 §5.5

AM2 PORTFOLIO & DESIGN AUDIT FORMATTRANSPARENT FIRST-PRINCIPLES MATHS

Point of common coupling evaluation to ENA Engineering Recommendation G5/5 and BS 7671:2018+A4:2026 Appendix 4 (Section 5.5 neutral sizing):

Triplen Neutral Current: I_n = 3 \times I_3. When 3rd harmonic exceeds 33%, cable sizing is governed by neutral current.
PFC Resonance: Capacitor banks cause parallel resonance at harmonic order h_r = √(Ssc / Qc). 7% detuning reactors (fr = 189 Hz) prevent destructive resonance.
Transformer K-Factor: Eddy current losses scale with h^2, requiring transformer derating when supplying high-THDi loads.