Too often, harmonic profiling is approached as a simple box-ticking exercise — a mindset that runs many sites into trouble. Without a proper understanding of the harmonic profile across the whole installation, decisions are often based on incomplete or misunderstood data. Here, John Mitchell, global sales and marketing director at CP Automation, explains why getting the profile right is essential to solving the root cause of any issues, not just the symptoms.
In practice, this misguided approach becomes apparent very quickly in the numbers being used to justify harmonic mitigation. It’s not unusual to hear that a site “needs” a 300-amp harmonic filter, yet when asked where that figure originated, it has often been passed down from those specifying or installing the equipment.
When you dig into the measurements, the data often says very little. Sometimes the calculation is wrong, sometimes it’s incomplete and sometimes it’s correct purely by chance. That’s the risk of not fully understanding the harmonic profile of an asset or piece of equipment.
Measuring doesn’t equal understanding
Power quality meters generate huge amounts of information, including voltage, current and frequency. However, the results aren’t always interpreted correctly. A common error is measuring a single branch of an installation, seeing current distortion rise when a particular machine runs and immediately blaming that load for the wider site issue.
Often, the bigger picture is missed. A common example is measuring harmonics on a new machine while a variable speed drive (VSD) or HVAC system is running elsewhere on site. Without a site-wide assessment, the additional distortion could be wrongly attributed to the machine, and mitigation specified for a problem that isn’t actually present.
When assumptions get designed in
These assumptions frequently begin at the design stage. Consultants may size a transformer and allow space for protective devices that feed power factor correction or harmonic mitigation, without carrying out a harmonic study. Those early protection and space decisions then quietly dictate what is possible later.
By the time someone realises a harmonic filter is required, the conversation becomes, “We need a 300-amp filter because the MCB is rated at 300 amps.” The actual harmonic current present on site rarely drives that conclusion.
Instead, it reflects the fact that the upstream protection and cabling were sized early in the project — often before any harmonic analysis was carried out. Once derating is considered, that protection realistically only supports a 200 to 250-amp filter. The limitation wasn’t technical. It was baked in by assumption.
We’ve also seen sites where the harmonic filter was correctly sized for the load, but upstream protection wasn’t. As a result, we had to deliberately limit the filter output to prevent nuisance tripping until costly retrofits could be made.
Another common misunderstanding is how harmonic currents combine, because Individual harmonic components do not add directly.
Not just a snapshot
That’s why a harmonic profile is a long-term picture. What’s more, getting the full picture involves monitoring a site for around seven days, allowing for typical operation, peak conditions, worst-case scenarios and averages to be captured.
From that, the correct mitigation can be specified to keep the site within relevant standards. Profiling often highlights issues beyond harmonics alone. Failed power factor correction equipment, redundant systems and opportunities to integrate power factor correction directly into a filter can all become clear once the data is properly understood.
Profiling doesn’t have to be a one-off exercise. Fixed power quality analysers act as a continuous safeguard, showing exactly when distortion rises and what changed at that moment. This helps turn reactive troubleshooting into informed decision-making, ensuring investment is spent on the equipment that solves the root cause, not just the symptoms.
For more information on industrial power quality, including measurement and mitigation options, visit the CP Automation website or speak to a member to help you make the right engineering decisions today.
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