As the summer approaches, so does the lightning season. Lightning is one of the sources of power quality phenomena, which can manifest as high frequency transients and/or rms variations, including sags and interruptions, depending on where the lightning strikes and the “electrical location” of the facility relative to the lightning strike.
According to the National Oceanic and Atmospheric Administration (NOAA), there are, on average, 25 million cloud-to-ground lightning strikes in the continental United States each year, (www.lightningsafety.noaa.gov/overview.htm), with a typical density distribution shown in the map below.
Lightning is one of the few sources of fast transients, measured in microseconds. The “classic” lightning simulation impulse has a 1.2 usec rise time and a 50 usec fall time. The time between strokes ranges from 100 usec to 10 milliseconds.
This very high speed is coupled with very high voltage and current levels. Since energy is power multiplied by time, the result is a tremendous amount of energy potential in a very short time, which will follow the path of least resistance to earth.
The high-voltage potential means that depending on how close your facility is to the strike, it may cause a breakdown of an insulator on the electrical infrastructure (and cause a short phase-to-phase or phase-to-ground able to “leap tall buildings in a single bound”) or a breakdown of the insulation within equipment (and the subsequent damage of the equipment).
With that much current, once the breakdown path is established, the amount of current pushing through the path can be highly destructive and even result in a fire. Even protective devices, such as TVSS, can be damaged if the energy level is above their joule rating.
Another source of fast transients is the rectifier devices inside electronic equipment that converts AC to DC (and sometimes back to AC). An adjustable-speed drive is such a piece of equipment.
The three-phase, full-wave rectifiers in the device’s front end has the classic commutation problem, when the semiconductor devices (such as SCRs or thyristors) of two phases are actually both on for a very short duration. This is due to the physics of how the device works.
Even though the gate or control signal is turned off, conduction does not stop through the device until the current goes through a zero state.
What basically happens is that two phases are shorted together, producing a very large current flow very quickly. This very large current flow results in a voltage drop in microseconds, hence, a high- frequency transient.
Whereas the lightning-based transient is a one-shot, highly destructive event, the commutation-based transient is a continual, slowly eating-away type event.
This can damage the winding insulation of the motor that is powered off the drive. Motors are available that have special windings for the first X number of turns that can better tolerate such abuse.
In addition, reactors can be placed in the feeds to the drives, to prevent the current transients from propagating through the facilities’ electrical distribution system.
It doesn’t take a superhuman effort to prevent such fast transients from wreaking havoc in your facility. It does take a higher-performance power-quality monitor to record such.
If your monitor only samples at 128 times per cycle, which is good for recording up to 3KHz data, you might not be seeing these potentially damaging events. Remember, just because your monitor did not record it, does not mean it is not happening. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.