Power Problems: Oscillating Transients caused by Grid Switching

One of the most common types of Transient in our electrical system today is a high frequency oscillating transient caused by grid-switching. The National Grid is a constantly monitored and maintained infrastructure of transmission lines and transformers where the flow of power is controlled by supply and demand. Every time a load changes or a power station increases its output the grid adapts itself to the new power flow. The inevitable high current switching causes the largest of these osciallting transients but their effect is normally vastly reduced by long distance and power conversion between the switchgear and the consumer.

Of much greater impact are smaller loads much closer to home. As strange as it sounds next doors laser printer can cause more damage than a lightning strike down the street just because while smaller in effect it is frequent and unattenuated by the comparatively short distance.

Further the oscillation can cause nearly as much damage as the initial transient, depending on the level of damping in your circuit.

Protection against both lightning source and local transients is by fitting Transient Voltage Suppression Systems or TVSS

The physics of these high frequency transients works thus: When a flowing electrical current is suddenly stopped there is a resultant voltage spike that is proportional to how fast the current flow is stopped. The equation for this voltage spike is V=Ldi/dt. 'V' is the value of the voltage. 'L' is the inductance of the electrical circuit where the current is flowing (Inductance is defined as the property of an electric circuit that opposes any change in current). 'di/dt' is the rate of change of the current in time. Therefore when the current is turned off, how fast the current is turned off determines the size of the voltage spike. The voltage spike will decay as it oscillates at a frequency that is the resonant frequency of the electrical circuit as seen in the following diagram:

V=Ldi/dt Typical frequency is 25kHz