In today’s power sensitive environments, it would behoove every facilities or maintenance supervisor to be aware of voltage transients and ways to suppress them. In the world of electronic motor drives, the manufacturers tell us that transients are the number one cause of power semiconductor failures within VFD s. But, it’s not just the drives that are affected. Transients can wreck havoc almost anywhere.
So, what are they? Voltage transients are instantaneous bursts of high voltage, resulting from a rapid release of stored energy. This energy can be stored within the existing circuit, and released by switching action, or it can be external and injected by either capacitive or inductive coupling. Suppressing transients requires that the energy be dissipated at a voltage level low enough to insure survival of the circuit components.
The detrimental effects voltage transients can have vary from the destruction of power semiconductors, to computer glitches. Some transients are predictable, such as SCR commutation spikes, and are more easily suppressed. Some are random, caused by lightning, switching parallel loads, and other power system changes.
Random transients are more difficult to identify and suppress. It’s a standard joke among service engineers that one of the most reliable ways of suppressing a transient, is to have a service engineer connect a scope and look for it!
A sudden change in the electrical conditions of any circuit will create a transient voltage from the energy stored within the circuit inductance and capacitance. The rate of current change (di / dt) within an inductor (L), will generate a voltage (-L di / dt) which will attempt to keep current flowing in the same direction. It doesn’t last long, because the source of the energy is limited to what is stored in the inductance (1/2 Li squared), and is usually released at a high instantaneous power.
With this in mind, consider for a moment, what happens when a common transformer is energized, and then de-energized.
The step function of the energization is coupled to the secondary by the stray capacitance and inductance, and can generate a burst of voltage twice the normal peak secondary voltage. It’s also possible that regardless of of the transformer turns ratio, the secondary can “see” the primary voltage via capacitive coupling even though it may be many times the normal secondary voltage. (Worth remembering if you work around high voltage step down transformers)
Unless suppression is added, this transient will be placed across the load. De-energizing the transformer causes even more severe transients, due to the collapse of the transformer flux.
Other causes of transients are shorts, fuse blowing, and switch or contact arcing.
Several methods of suppressing transients are common. One is to filter or attenuate the transient. The filter captures the relatively high frequency transient, while allowing the lower frequencies to pass unhindered. The simplest and most used filter is a simple capacitor. Oftentimes, a resistor is placed in series to limit inrush, and this is the makeup of the popular RC “Snubber” networks.
Another popular method is to clamp the transient to an acceptable level. The most common device used for this is the MOV or Metal Oxide Varistor.
External transients can be reduced by using shielded signal wiring, isolating power sources, running control and power wiring in separate conduits, and keeping connections tight.
All of which you already knew you were supposed to do, right?