Resonance is one of the most destructive and complex phenomena within Power Quality. It occurs when the inductive and capacitive properties in an electrical installation conflict. This leads to dangerous amplifications of currents and voltages, often with catastrophic consequences for equipment such as capacitor banks, transformers and sensitive electronics.
For engineers and plant managers, resonance is often difficult to detect without sophisticated measurements. The problem often lurks undetected until a specific load or circuit activates the resonance point. In this article, we analyse the physics behind resonance, the symptoms in practice and the steps needed to make your installation resonance-free.
What is it? A physical phenomenon in which coils (inductance) and capacitors (capacitance) amplify each other at a specific frequency.
The danger: Extreme voltage or current spikes leading to overheating, insulation faults and components exploding.
The cause: often a combination of non-linear loads (pollutants) and 'ordinary' capacitor banks without barrier filters.
The solution: measuring is knowing. Analyse resonance points and apply detuning (tuning) or active filtering.
Resonance is not a theoretical concept, but an everyday risk in modern industries and utilities. This article was written specifically for:
Once you are working with power electronics (variable speed drives, LED, EV chargers) in combination with reactive current compensation or long cables, understanding resonance is necessary to ensure safety.
Every electrical installation contains components with inductive properties (coils in motors, transformers, cables) and capacitive properties (capacitors, long cables, electronics).
There is always a specific frequency at which these two values (Xl and Xc) are equal to each other. We call this the resonance frequency.
In itself, this is not a problem. It only becomes dangerous if there is a source in the installation that produces harmonic currents or voltages that are exactly (or close to) this resonant frequency. In modern installations, variable speed drives and inverters are the sources of these harmonics (e.g. the 5th, 7th or 11th harmonics).
If the resonant frequency of your installation coincides with a harmonic present, resonance occurs. The electrical system starts to behave like a swing that gets a push every time at the right moment: energy accumulates to destructive levels.
Xc: Inductive reactance [Ω]
ω: angular velocity [rad/s]
C: Inductance [H]
F: Frequency [Hz]
Xc: Capacitive reactance [Ω]
ω: angular velocity [rad/s]
C: Inductance [H]
F: Frequency [Hz]
In the electrical network, inductive and capacitive loads are connected in parallel. For example, a motor (L) and a capacitor bank (C) to supply reactive power. When the wrong combination is made, a small harmonic current can lead to large harmonic voltages that damage all components in the entire installation.
It is crucial to distinguish between the two forms, as they present different symptoms and risks.
1. Parallel Resonance (High Impedance) This is the most common form in industry. Here, the capacitor bank and the grid inductance (transformer) are in parallel as seen from the harmonic source (the load).
2. Series Resonance (Low Impedance) Here the inductance and capacitance are in series. We often see this at the end of long cables or in specific filter setups.
In the past, installations mainly consisted of linear loads (motors directly on-line). Nowadays, the composition of installations changes rapidly, increasing the likelihood of resonance:
Nuance: It is a misconception that only faulty equipment causes resonance. Resonance is a physical consequence of an unfortunate composition of perfectly functioning components.
Resonance is sometimes audible, but often invisible until it is too late. Pay attention to the following signals:
Resolving resonance requires a structural approach. Simply replacing a fuse is symptomatic.
1. Diagnosis and Analysis The first step is always a Power Quality measurement and grid analysis. We need to determine where the resonance point is and which harmonics are present. In complex issues (especially in new construction or major modifications), a simulation study is necessary to predict resonance in advance.
2. Detuning (Tuning) Do you have capacitor banks? Then the most effective measure is to apply coils (reactors) in series with the capacitors. We call this a 'tuned' or 'detuned' capacitor bank.
3. Active Filtering If resonance is caused by excessive harmonic contamination, an Active Harmonic Filter (AHF) can provide the solution. The filter measures the contamination and returns current in counterphase.
4. Grid changes In some cases, changing the transformer tap or regrouping loads can help, although this is often less structural than filtering or detuning.
Symptom relief: replacing a broken capacitor with the exact same type. Without modification, the new capacitor will also quickly fail or explode.
Blind addition: Placing additional capacitors to improve the cos phi without calculating what this does to the resonant frequency. This can exacerbate the problem.
Looking only at current: Many mechanics only measure Amps. However, resonance often causes voltage distortion that cannot be interpreted properly with a standard current clamp.
Underestimating heat: Thinking that "a little heat" is normal. With resonance, the temperature in components can rise so high as to create a fire hazard.
Do you suspect resonance in your installation? Follow these steps:
When capacitors fail repeatedly.
When purchasing new machines in combination with existing capacitor banks.
If you have unexplained outages that shut down production.
When you want certainty about the safety of your installation after an expansion.
Resonance is a complex problem that you should not solve at random. Are you in doubt about the stability of your installation or experiencing unexplained failures? Speak to one of our engineers for a targeted analysis and a lasting solution.
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5653 MA Eindhoven
