What Is Harmonics Suppression?

Harmonics suppression is a collection of design and mitigation techniques applied in power electronics and power systems to reduce the amplitude of harmonic frequency components generated by nonlinear devices, particularly power converters, inverters, and nonlinear loads. When these devices draw non-sinusoidal currents or generate distorted voltages, the resulting harmonics degrade power quality, cause additional heating in transformers and cables, interfere with protective relays, and increase losses throughout the distribution network. Harmonics suppression addresses these effects through converter circuit design, modulation strategies, filtering, and phase-multiplication techniques, aiming to keep harmonic distortion within the limits established by standards such as IEEE 519-2014 and IEC 61000.

The field draws on power electronics, control theory, and circuit analysis. It has grown in importance as the penetration of variable-frequency drives, switch-mode power supplies, and grid-connected inverters has increased the harmonic burden on distribution and industrial power systems.

Pulse-Width Modulation and Selective Harmonic Elimination

Pulse-width modulation (PWM) is the foundational converter control strategy that determines which harmonics appear in the output waveform and at what amplitude. In sinusoidal PWM (SPWM), a high-ratio carrier frequency pushes harmonic energy to multiples of the switching frequency, which are sufficiently far from the fundamental that small LC output filters can suppress them. Selective harmonic elimination (SHE) takes a different approach: the switching angles of a multilevel or medium-voltage converter are pre-computed offline to place zeros of the harmonic spectrum at specific low-order frequencies, typically the 5th, 7th, 11th, and 13th, which are otherwise the most problematic. IEEE research on programmed PWM techniques for harmonic elimination provides a critical evaluation of SHE and comparable modulation approaches, establishing the computational framework that remains the basis of current selective elimination algorithms.

Phase Multiplication and Multi-Level Converter Design

At the hardware level, harmonics suppression is achieved by using multi-pulse rectifier configurations or multilevel inverter topologies that inherently cancel specific harmonic orders through circuit symmetry. A twelve-pulse rectifier, formed by two six-pulse bridges supplied from transformer secondaries phase-shifted by 30 degrees, cancels the 5th and 7th current harmonics, leaving the 11th and 13th as the lowest significant components. Twenty-four-pulse and higher configurations reduce harmonics further, at the cost of added transformer complexity. Multilevel inverter topologies, such as the neutral-point-clamped (NPC) and cascaded H-bridge designs, synthesize output voltages with more steps per cycle, distributing harmonic energy across higher orders where it is easier to filter. A review of PWM techniques for harmonic suppression in renewable energy power converters surveys modulation-based suppression methods and their suitability for photovoltaic and wind converter applications.

Active Control and Feedback-Based Suppression

Active suppression methods use real-time feedback control to cancel harmonics generated by the converter's own switching. Repetitive control and resonant (proportional-resonant, PR) controllers are designed to present high gain precisely at harmonic frequencies, driving the tracked error toward zero at each harmonic order. These controllers are widely implemented in single-phase and three-phase inverters for grid-connected renewable energy systems, where injection of current harmonics above a threshold triggers disconnection requirements. IEEE research on harmonic distortion reduction in power inverters using feedback control evaluates closed-loop suppression depth achievable with PR controllers relative to conventional PI control, finding harmonic current rejection exceeding 30 dB at targeted orders.

Applications

Harmonics suppression has applications in a wide range of fields, including:

• Industrial variable-frequency drive installations supplying motors in manufacturing and HVAC systems
• Grid-connected photovoltaic and wind power inverters meeting utility interconnection standards
• Traction power converters in electric locomotives and urban rail systems
• Data center and commercial building uninterruptible power supply (UPS) systems
• Medium-voltage drive systems for pumps, compressors, and fans in oil, gas, and mining