Non-linear loads - predominantly six-pulse diode rectifiers in Variable Frequency Drives (VFDs), uninterruptible power supplies (UPS), and induction heaters - draw current in discontinuous, non-sinusoidal pulses. These harmonic currents cause transformer overheating, power factor capacitor resonance, nuisance RCD tripping, and voltage distortion across industrial distribution systems.
Governing Standards: ENA EREC G5/5, BS EN 61000-3-12 and BS 7671
Harmonic limits in the UK and internationally are strictly codified by distribution code authorities and national standards:
- BS EN 61000-3-12 & BS EN 61800-3: Electromagnetic compatibility (EMC) - Limits for harmonic currents produced by equipment connected to public low-voltage systems and adjustable speed electrical power drive systems.Establishes harmonic emission thresholds and equipment classification for variable speed drives operating in industrial environments.
- BS 7671:2018+A2:2022 Section 444 & Regulation 331.1: Measures against electromagnetic disturbances and assessment of general characteristics.Requires electrical installations to account for non-linear load compatibility, harmonic heating in neutral conductors, and prospective waveform distortion.
- ENA Engineering Recommendation G5/5 (UK Distribution Code): Harmonic voltage distortion and the connection of harmonic-sources and/or resonant plant to the distribution systems and transmission systems in the UK.Defines a 3-stage connection assessment process (Stage 1 for simple small equipment, Stage 2 for medium industrial connections, Stage 3 for complex HV plant). Sets planning levels for total harmonic voltage distortion (THDv ≤ 5.0% at 400V and ≤ 4.0% at 11kV) and individual harmonic voltage limits at the Point of Common Coupling (PCC).
- IEEE 519-2022: Standard for Harmonic Control in Electric Power Systems.Defines Total Demand Distortion (TDD) limits based on the ratio of maximum short-circuit current to maximum demand load current (Isc / IL).
THDi vs THDv: Understanding the Distinction
Engineering specifications frequently confuse Current Distortion (THDi) and Voltage Distortion (THDv):
- Total Harmonic Current Distortion (THDi): Measures the harmonic content of current drawn by the load:
THDi = √(∑ Ih²) / I1 × 100%(where I1 is the fundamental 50 Hz current). A standard unmitigated 6-pulse VFD exhibits a THDi of 80% to 120%. - Total Harmonic Voltage Distortion (THDv): Measures the distortion of the terminal voltage waveform resulting from harmonic currents flowing across the upstream supply source impedance (Zsource):
Vh = Ih × ZsourceTHDv = √(∑ Vh²) / V1 × 100%. While the load injects THDi, it is the resulting THDv that distorts the grid and damages other plant equipment.
The 6-Pulse Inverter Harmonic Spectrum: h = 6k ± 1
In a standard three-phase six-pulse bridge rectifier, theoretical symmetry cancels all even harmonics (2nd, 4th, 6th) and all triplen harmonics (3rd, 9th, 15th). The resulting characteristic harmonic orders are governed by:
Characteristic Harmonic Formula:
h = 6k ± 1 (where k = 1, 2, 3, 4 ...)
- k = 1 → 5th (250 Hz) and 7th (350 Hz) (Dominant energy, ~35% and ~18% amplitude)
- k = 2 → 11th (550 Hz) and 13th (650 Hz) (~9% and ~7% amplitude)
- k = 3 → 17th (850 Hz) and 19th (950 Hz) (~4% and ~3% amplitude)
Line Reactors: 3% vs 5% Impedance
The most cost-effective first-line defence is installing a three-phase AC line reactor (choke) in series with the VFD incoming supply:
| Specification | 3% AC Line Reactor | 5% AC Line Reactor | DC Link Choke (Internal) |
|---|---|---|---|
| THDi Reduction | Reduces THDi from ~85% down to 40% to 45% | Reduces THDi from ~85% down to 32% to 38% | Reduces THDi down to 40% to 48% |
| Surge & Transient Protection | Absorbs utility voltage spikes and capacitor switching transients (> 1 kV). | Maximum transient attenuation; protects input diode bridge against heavy fault surges. | Protects DC bus capacitors; zero protection for input bridge diodes. |
| Voltage Drop at Full Load | 3% nominal drop (e.g. 12 V on 400 V supply) | 5% nominal drop (e.g. 20 V on 400 V supply) | Zero AC voltage drop |
| Physical Location | External, mounted in MCC cubicle | External, mounted in MCC cubicle | Integrated inside drive chassis between rectifier and bus capacitors |
Active Mitigation: Active Harmonic Filters (AHF) vs Active Front End (AFE)
When site-wide G5/5 compliance demands THDv ≤ 5% or THDi ≤ 5% at the main incoming breaker, passive line reactors are insufficient. Modern industrial engineering deploys active electronic solutions:
| Parameter | Active Harmonic Filter (AHF) | Active Front End (AFE) VFD |
|---|---|---|
| Connection Topology | Shunt-connected to main MCC busbar | Series replacement for VFD diode bridge |
| Operating Principle | Measures total load harmonic current via CTs and injects opposing 180° anti-phase cancellation currents in real time. | Replaces passive diodes with active IGBT bridge switching at high frequency to draw pure sinusoidal current. |
| Performance | THDi < 5% at MCC busbar; corrects displacement power factor to > 0.98. | THDi < 3% to 5% per individual drive; unity power factor (cos φ = 1.0). |
| Regenerative Braking | None (cannot return energy to grid) | Full 4-Quadrant Regeneration: Returns braking energy directly into the AC mains network. |
| Scalability & Cost | Highly Cost-Effective: One centralized AHF can filter 5 to 20 separate VFDs on the same busbar. | Expensive: Every individual motor requires a dedicated AFE drive chassis. |
Transformer De-Rating and K-Factor
Harmonic currents flowing through distribution transformers dramatically increase winding eddy current losses (proportional to frequency squared, Pec ∝ ∑ h² × Ih²) and stray flux heating in the transformer tank:
- Standard Distribution Transformers (K-1): Designed for pure 50 Hz linear loads. If supplying VFDs without filtering, they must be de-rated to 60% to 70% of nameplate kVA to prevent winding burnout.
- K-Factor Rated Transformers: Incorporate subdivided transposed conductors, oversized neutrals (200%), and enhanced cooling ducts (e.g. K-4, K-13, K-20) specifically designed to withstand harmonic heating.