Do you suffer from flashing lights(flicker), unexplained control system failures or strange noises in transformers, while standard measurements seem "normal"? Chances are that interharmonics are the cause. Where "normal" harmonics are predictable, these strange frequencies create complex problems in modern installations. HyTEPS helps you identify and eliminate these elusive disruptors.
What is it? Voltage or current frequencies that are not an exact multiple of the fundamental frequency (50Hz), e.g. 75Hz or 185Hz.
The cause: often caused by frequency-controlled drives (VSDs), cycloconverters, welding equipment and arc furnaces.
The risk: Causes annoying light flicker(flicker), electronics failures and unwanted resonances.
The solution: specialised measurements (beyond the standard THD) followed by filtering with an Active Harmonic Filter (AHF).
Knowledge about interharmonics is crucial for Technical Managers and Installation Managers in heavy industry, marine and manufacturing environments. You work with large power electronics, fluctuating loads or precise control processes where failure leads directly to production loss. This topic is also essential for Engineers dealing with misunderstood 'ghost faults' in PLCs or protection relays.
In an ideal 50Hz network, voltage undulates up and down 50 times per second. 'Ordinary' harmonics are exact multiples of this frequency, such as 150Hz (3rd harmonic) or 250Hz (5th harmonic).
Interharmonics are frequencies that are not an integer multiple of 50Hz. Consider a frequency of 75Hz or 210.5Hz.
A comparison: Imagine a piano. The 50Hz fundamental is the central C-key. Harmonics are the other C-keys (octaves) that sound harmonically along. Interharmonics are the false notes between the keys. They do not belong in the scale and create a dissonant sound that is difficult to filter by standard means.
Interharmonics are often overlooked because standard measurement equipment focuses on Total Harmonic Distortion (THD), which mainly looks at the 'whole' harmonics. Yet interharmonics can seriously threaten operational reliability:
Interharmonics rarely arise in the public grid, but almost always from specific loads within one's own installation or that of its neighbours. The main sources are devices that distort the sinusoidal form of the current not periodically, but irregularly or at a different frequency:
Addressing interharmonics requires a structured approach, as standard solutions often do not work.
1. Analysis and Diagnosis
Measuring is knowing, but only with the right equipment. A standard power meter often overlooks interharmonics. A Power Quality Analyzer complying with standard IEC 61000-4-7 is necessary to visualise the full spectrum.
2. Adaptation of the source (Engineering)
Sometimes a small change in the settings of an AC drive or adjusting the switching frequency can reduce the problem. This is the most cost-effective first step.
3. Active Harmonic Filter (AHF)
The most robust solution is often the deployment of an Active Harmonic Filter. Unlike passive filters, which are difficult to tune to varying interharmonics, a properly sized AHF can dynamically inject countercurrents. HyTEPS configures these filters to specifically compensate for interfering frequencies, improving power quality and reducing risk.
Blinded by THD: Total Harmonic Distortion (THD) often does not include interharmonics in the calculation. So a low THD does not automatically mean there is no problem.
Confusion with resonance: Sometimes a fault is wrongly labelled as resonance, when in fact it is an interharmonic source directly causing interference.
Passive filtering: Installing a capacitor bank or passive filter can exacerbate the problem by changing the impedance of the installation and actually amplifying interharmonics.
Interharmonics are complex business. If your own measurements do not provide clarity, or standard solutions fail, in-depth expertise is needed. HyTEPS combines measurements, simulation and engineering to find the source. Especially in situations where production downtime is not an option (such as in data centres, hospitals or the process industry), our 'Power Quality Doctor' approach provides certainty.
Do you suspect interharmonics are disrupting your installation? Don't wait until it leads to costly downtime. Speak to an engineer from HyTEPS for a targeted analysis and an appropriate solution.
HyTEPS
Beemdstraat 3
5653 MA Eindhoven
