Understanding and Mitigating Harmonics with AC Drives

Introduction

Harmonic distortion is a growing concern in modern electrical installations. As facilities become increasingly reliant on non-linear loads—such as variable speed drives, computers, electronic lighting, and UPS systems—the quality of the power supply can be significantly degraded. This "electrical pollution" can lead to a host of problems, from overheating equipment and nuisance tripping to inaccurate metering and equipment failure.

Many facility managers and engineers are aware of the symptoms—unexplained transformer heating, capacitor failures, or intermittent data corruption—but may not have investigated the root cause. The source is often harmonic currents generated by non-linear loads connected to the power distribution system. Understanding these harmonics, their sources, and their effects is the first step towards ensuring a reliable and efficient power supply.

What Are Harmonics?

In an ideal power system, the current and voltage are pure sine waves at the fundamental frequency (50 or 60 Hz). However, with the prevalence of non-linear loads, the current waveform deviates from this ideal shape. This distorted waveform can be mathematically broken down into a sum of sinusoidal components: the fundamental frequency plus multiples of it, known as harmonics. For a standard 6-pulse rectifier found in many AC drives, the dominant harmonic frequencies are the 5th (250 Hz), 7th (350 Hz), 11th (550 Hz), and 13th (650 Hz).

The total amount of harmonic distortion in a system is expressed as a percentage known as Total Harmonic Distortion (THD). This value is calculated from the RMS current of the fundamental and all harmonic components. High THD indicates a significant level of distortion that can cause a range of operational issues.

The Consequences of Harmonics

Unchecked harmonic distortion is not merely a technical nuisance; it has tangible financial and operational costs. The effects can be observed throughout a facility:

• Overheating of Equipment: Harmonics increase the RMS current flowing through transformers, cables, motors, and generators, leading to excessive heating and reduced lifespan.
• Capacitor Failures: Capacitors are particularly sensitive to harmonics. The increased heat and voltage stress can cause them to fail prematurely.
• Nuisance Tripping: Circuit breakers and protective relays may trip unexpectedly due to the distorted current waveforms, leading to unplanned downtime.
• Electronic Malfunctions: Sensitive electronic equipment, such as computers, programmable logic controllers, and electronic displays, can malfunction, display errors, or fail entirely.
• Inaccurate Metering: Harmonic currents can cause energy meters to give false readings, leading to billing inaccuracies and making it difficult to track true energy consumption.

By identifying these symptoms and investigating the harmonic distortion levels at your facility, you can take proactive steps to mitigate these risks and improve overall system reliability.

Standards and Evaluation

To manage harmonic distortion, several national and international standards have been established. The most prominent are from the IEC and IEEE. These standards define limits for harmonic emissions from equipment and the allowable voltage distortion levels in public and industrial networks.

A key concept in evaluating harmonics is the Point of Common Coupling (PCC) —the point in the power system where a facility connects to the utility grid. Standards like IEEE 519 base their harmonic limits on the ratio of the available short-circuit current to the maximum demand load current at the PCC. This approach ensures that larger facilities with higher short-circuit capacities are allowed proportionally more harmonic current injection, while smaller facilities with weaker grids are held to stricter limits.

How to Reduce Harmonics

Fortunately, there are several effective methods to reduce harmonic distortion. These solutions can be implemented either through modifications within the drive system itself or by using external filtering equipment.

Structural Modifications to the Drive System

One of the most straightforward ways to reduce harmonics is to increase the size of the DC or AC choke within the drive. A choke, or inductor, smooths the current waveform and significantly reduces the magnitude of low-order harmonics. A larger choke can bring the THD down from over 100% to around 40%.

For even lower harmonic levels, multi-pulse rectifiers are an excellent solution. By using two or more 6-pulse rectifiers with phase-shifted transformer secondaries, certain harmonics are cancelled out. A 12-pulse rectifier effectively eliminates the 5th and 7th harmonics, while a 24-pulse rectifier practically eliminates all low-frequency harmonics.

The most advanced solution is the IGBT supply unit (ISU), also known as an active front end. This technology uses self-commutated IGBTs to generate a nearly sinusoidal supply current with extremely low harmonic content, a true power factor of 1.0 under all load conditions, and the capability to regenerate energy back to the grid.

External Filtering Solutions

For existing installations, external filters can be added to mitigate harmonics. Passive filters are tuned circuits consisting of capacitors and reactors that provide a low-impedance path for specific harmonic frequencies. They can be effective but come with risks, such as causing resonance that can amplify other harmonics or leading to overvoltage under light load conditions.

A more modern and flexible approach is the active filter, which uses power electronics to inject a compensation current that is exactly opposite in phase to the harmonic currents. This effectively cancels out the distortion, leaving a clean sine wave for the rest of the facility. Active filters are well-suited to applications with multiple small drives and variable loading conditions.

Conclusion: Choosing the Right Solution

There is no single "best" solution for harmonics reduction. The optimal choice depends on the specific characteristics of your installation, including the total load, the type of drives used, the available short-circuit capacity, and your budget.

Comparison of mitigation approaches (typical THD at nominal load / drive system efficiency / true power factor / installation footprint / equipment cost, relative to a baseline 6-pulse drive without a choke):

• 6-pulse without choke: THD >100%, efficiency ~98%, power factor ~0.7, footprint 100%, cost 100%
• 6-pulse with large choke: THD 40%, efficiency ~97%, power factor ~0.98, footprint 110%, cost 120%
• 6-pulse with active filter: THD <5%, efficiency ~96.5%, power factor ~0.99, footprint 250%, cost 230%
• 12-pulse drive: THD 6-10%, efficiency ~96%, power factor ~0.98, footprint 300%, cost 200%
• IGBT supplied drive: THD <5%, efficiency ~96.5%, power factor 1.0, footprint 120%, cost 190%

For new installations, specifying a drive with an IGBT supply unit or a large internal choke is a proactive and space-efficient strategy. For existing systems, retrofitting an active filter can be a cost-effective way to manage existing harmonic levels.

By understanding the fundamentals of harmonics and the available mitigation techniques, you can design, specify, and maintain a more reliable, efficient, and safe electrical distribution system, avoiding the hidden costs of this often-overlooked form of electrical pollution.

This article summarises key concepts from 'Technical Guide No. 6 – Guide to Harmonics with AC Drives' by ABB.

Download the full free document here: Technical Guide No. 6 – Guide to Harmonics with AC Drives (PDF)

Disclaimer: This article provides a summary of the 'Technical Guide No. 6 – Guide to Harmonics with AC Drives' technical publication by ABB. While efforts have been made to accurately represent the content, please refer to the official document for complete technical details. Final design, specification, and implementation decisions should be made by qualified electrical engineers and competent professionals in accordance with all applicable regulations and standards.