The collapse that changed science and engineering

What the Tacoma Narrows bridge collapse and distribution networks have in common

On November 7, 1940, just 130 days after opening to traffic, a 42 mph (68 km/h) wind led to the collapse of the famous suspension bridge. Now a chapter found in many undergraduate physics text books, the bridge’s destruction has had lasting effects on science and engineering, leading to new thinking in bridge design.

It is presented as an example of elementary forced resonance with the wind providing an external periodic frequency that matched the natural structural frequency, even though the real cause of the bridge’s failure was aeroelastic flutter.

Could you imagine that in case of distribution network?

Ferroresonance is a phenomenon usually characterized by over-voltages and very irregular wave shapes and is associated with the excitation of one or more saturable inductors through capacitance in parallel or in series with a nonlinear inductor. The record of the ferroresonace process within a distribution network is illustrated in the graph below.

The most common case is the ferroresonance of single-pole insulated medium voltage transformers in an unearthed network. In our experience almost two-thirds of voltage transformers failure is caused by this phenomenon.

The undampened ferroresonant oscillations are dangerous to the installed equipment due to large overcurrents and overvoltages, which may lead to permanent damage. The impact of the damage depends on a protection system used within the concrete distribution networks. In the best cases just the voltage transformer is destroyed, in the worst cases undampened ferroresonance could damage the whole switchgear.

Examples of ferroresonance destruction

To protect against ferroresonance oscillations a medium voltage transformer needs proper and prompt dampening action without creating excessive power consumption during normal operation of the network. ABB has the solution – it’s called the VT Guard Pro.

Main image credit: Nick Bramhall under a CC license via Flickr