Resonances

In addition to the “classical” low-frequency harmonic problems, our power supply networks often have pronounced current and voltage poles which suddenly – triggered by a parallel voltage phenomenon from a switching action (e.g. a switch-on transient) – oscillate the entire network into a serious imbalance. Such disturbance phenomena – predominantly to be found in the high-frequency, intermediate (inter)harmonic range (> 1,000 Hz) – are described in professional circles as ‘resonances’ or rather as ‘resonance bells’ (lying like a bell over a certain frequency range). Put in plastic terms: The network gets out of balance. This imbalance is often acoustically perceptible by a “buzzing” or “humming” of the resonance-involved and thus more heavily loaded equipment (e.g. transformers, chokes, power supplies). Due to their – often – high-frequency characteristics, we classify these types of voltage phenomena as ‘supra-harmonics’. But exceptions usually prove the rule, because e.g. in generator-driven low load networks with a very low total impedance, resonance bells can also be found in frequency ranges from 50 to 999 Hz. Typical indicators to detect a critically dormant resonant bell in a power system at an early stage are often high current and voltage levels of the inter-harmonics. Resonance detection is only possible through a professional voltage analysis followed by the expertise of our PQ specailists. Many factors such as the mains short-circuit power and the total connected capacity in the respective network influence this distortion. Experience is simply required here … and this shows our specialists on a daily basis that current and voltage interference levels, based on resonance phenomena, are considerably higher than those of low-frequency interference levels. In this respect, special caution is required, since even a not too high resonance-induced interference level – in a suddenly excited state – can usually result in serious “complications” (such as system failures or fires).