As modern power systems rely more on nonlinear loads, Total Harmonic Distortion in Current (THDi) has become a critical factor in ensuring power quality and equipment longevity. In this article, we explore what THDi is, its causes, its impacts on equipment, and how to mitigate it using advanced harmonic filtering techniques.

1. What is THDi?

THDi stands for Total Harmonic Current Distortion. It is used to measure the degree of distortion in the current waveform within a power system, representing the ratio of total harmonic current to fundamental current, typically expressed as a percentage.

According to the national standard GB/T 14549-93:

Public grids: THDi < 5%

High-voltage grids (>10kV): THDi < 4%

Industrial systems: THDi < 5%

Residential systems: THDi < 3%

High-end systems (e.g. energy storage): THDi can be < 3%

2. Main Sources of Harmonic Distortion THDi mainly arises from nonlinear loads — devices where the voltage-current relationship does not follow Ohm's law. These devices draw non-sinusoidal currents even under sinusoidal voltage, creating harmonics at multiples of the fundamental frequency.

Key examples include:

Semiconductor switching devices (diodes, thyristors, IGBTs): cause pulsed or clipped waveforms, leading to higher-order harmonics.

Industrial chargers with rectification: 6-pulse rectifiers produce THDi >30%, with 5th, 7th, 11th, and 13th harmonics. 12-pulse rectifiers reduce THDi to 8–10% but still require filtering.

3. Impacts of High THDi on Equipment

1. Transformer Overheating and Derating Harmonic currents increase eddy current, copper, and stray losses in transformers. The skin effect intensifies, causing temperature rise and requiring derating (typically 15–20%). Long-term overheating leads to insulation aging and reduced lifespan.

2. Capacitor Resonance and Explosion Risk Capacitors have frequency-dependent reactance. High-order harmonics (e.g., 11th) can cause resonance and current surges. Real-world data show harmonic current spikes from 1.3% to 39%, leading to capacitor overvoltage and possible explosions.

3. Cable Insulation Degradation Harmonics intensify the skin effect and double cable heating. Superposed third harmonics in unbalanced systems raise neutral line current up to 3x the phase current, increasing discharge, short-circuit, and fire risks.

4. Voltage Distortion and Equipment Malfunction When THDi exceeds 15%, voltage distortion (THDu) increases, affecting precision control systems and sensitive electronics.

4. Effective Harmonic Mitigation Measures

1. Upgrade to 12-Pulse Rectifiers Replacing 6-pulse rectifiers reduces THDi from 30–40% to 8–12%.

2. Power Factor Correction (PFC) Active PFC circuits in chargers and power supplies can suppress 5th and 7th harmonics, achieving THDi <5%.

3. Active Power Filters (APF) APFs detect harmonics in real time and inject compensation currents. With a <5ms response="" time="" and="">95% suppression efficiency, they cover 2nd to 50th harmonics. Combined with passive LC filters, they can effectively eliminate 11th and 13th harmonics — ideal for transformer-side protection.

5. Real-World Application Scenarios

Our harmonic filtering solutions, when deployed with EverExceed industrial chargers, have significantly improved power quality in key infrastructure:

Transportation hubs (high-speed rail and subway stations)

Municipal facilities and substations

Power plant lighting systems and switching stations

In these deployments, voltage qualification rates increased from 75% to 99%, and component damage due to voltage instability decreased by 90%.

Conclusion: Why THDi Management Matters THDi is more than a technical parameter — it's a critical indicator of power system health. Proactive harmonic mitigation safeguards your equipment, improves efficiency, and ensures operational stability.

EverExceed offers advanced harmonic filtering solutions tailored for industrial applications. Contact us today to learn how we can help you optimize your power systems for performance, safety, and reliability.