Though lightning season may be over by now, it is worth reviewing the impact storms can have on the operation of a facility. Some may have experienced significant damage to equipment from the high frequency transient energy of lightning strikes. Though sags are by far the most common type of occurrence experienced at most facilities, some of these sags are caused by lightning strikes coupling into or directly striking distribution and transmission system wires. The flashover between one-phase conductor and ground, or between multiple-phase conductors, results in very large current flows that cause significant voltage drops across the source impedance, leaving less voltage for the downstream loads. But the transients from close-by lightning strikes can cause damage to equipment in the facility itself.
Some power quality monitors will have recorded no event, despite damage in the thousands of dollars, as the transients are often too narrow to be picked up when sampling only once every 130 microseconds or slower. The term “transient” is now being used in nearly every power quality monitoring equipment spec sheet to mean anything from a one-quarter cycle drop-out to a sub-microsecond impulsive transient. In IEEE lingo, RMS variations are events that last longer than one-half cycle, leaving transients to be anything shorter than that. So, a typical instrument that samples voltage at 128 times a cycle will only capture the typical 1.2-by-50 microsecond lightning impulsive transient if it happens to occur when one of the samples occurs (1.2 usec is the rise time, 50 usec is the fall time). This depends, as Dirty Harry said, on how lucky you feel, and obviously, not very lucky if your facility was damaged by lightning. The odds of your monitor capturing such is, at best, 33 percent, not something that I would bet on.
A couple of products on the market will improve your odds. There are two techniques that can reliably capture such high-frequency transients—1MHz or faster sampling, or using high-bandwidth, resettable dual-peak detectors. How the former works is pretty obvious, providing 100-plus times the number of samples that the low-frequency sampling circuitry does. The latter uses high-frequency circuitry into a peak-hold circuit that saves the largest magnitude value until it is reset. Provided that it is reset often, such as 128 times per cycle, and that there are two sets of peak detectors for both negative and positive impulsive transient capture, then you should capture the lightning-generated voltage transient (though you may not be able to discern its actual waveshape).
With all of the surge suppressor devices employed throughout a facility, people become complacent and think that they are immune to such problems. These surge suppressor strips, which contain TVSS devices (transient voltage surge suppressors), have components that are able to divert large amounts of energy into the ground conductor or phase-to-phase, away from the load, by significantly lowering their impedance when the voltage tries to climb above a preset value. However, it requires a good ground conductor path to carry this energy away from the load. Protection is also required on phone lines or cable modem lines that come into the facility, providing another potential path of destruction.
When these TVSS devices work properly, the voltage level is clamped at a safe level, often under 250V peak for a 120V AC line. Many a power quality (PQ) monitor is set to 500V peak or higher for transient triggering, so when the lightning strike occurs and the TVSS works, the PQ monitor will not trigger. However, there will be a significant current transient, as all that energy had to go somewhere. If your PQ monitor triggers on current transients as well as voltage transients, you will still capture the event even when the voltage transient is supressed.
Why should you care about the event if the TVSS worked and the voltage was clamped? Your equipment wasn’t damaged, but the TVSS can be, ever so slightly. Each time it takes that hit, it loses some of its capability to divert the energy and after enough times, the TVSS may fail. Then you have no protection, and you don’t even know it. This further illustrates the need to monitor for both current and voltage transients even with protection systems in place. It will enable you to see that the lightning transient did get into your electrical system, that the TVSS devices did their job and whether they are degrading and need to be replaced before a catastrophic failure.
Figure 1 shows the 1,038Vpk and 92A transient voltage and current waveforms when there is no TVSS in the circuit. Figures 2a and 2b show what happens when the same transient is generated into a surge suppressor strip that is rated at 400V. Voltage A and Current C are upstream, whereas Voltage B and Current D are near the load. Upstream from the device, the voltage is clamped to just over 400V but the current rises to 360A. Downstream, the voltage is under 400V and the current a mere 5A. Clearly, the device did its job, this time. As mentioned before, if the monitor had been set to 500Vpk trigger, the transient into the TVSS circuit would not have been captured without current transient triggering. With a 250Vpk TVSS, the probability of missing such on a voltage-triggered monitor becomes even greater.
Even with lightning season behind us, there are other sources of transients that can continue to stress your surge protection system. Like your PQ monitoring system, your protection system must be running 24/365 to keep your facilities productivity up and your infrastructure intact. Don’t be lulled into the false sense of security that comes from thinking if you can’t see it, everything is going just fine. Lightning can strike more than once, and you’d best be prepared for it. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.