Beyond Power Surge

A loss of equipment operation is often attributed to power surges in electric power distribution systems. Whether or not we can see or definitively understand the cause, many conveniently use the broad term power surge to describe the cause of a wide variety of occurrences. This is mostly due to the lack of certainty related to the reason for the loss of operation. Although many experts on the subject have reached a consensus not to use the combination of words power surge, their usage continues as a way to commonly describe problems with electric power quality and associated solutions.

The main problem with the term is that a power surge is indeed not a physical phenomenon. It is a result of the transient over-voltage referred to as the surge voltage and the resultant surge current that can be caused by either a lightning strike or utility switching on the supply utility grid. However, experience has shown that the loss of equipment operation, which has been found to originate with such events, actually occurs due to issues related to the cause such as ground potential rise, flashover and voltage sag.

When lightning strikes

Lightning strikes are environmentally-related events that occur at random in air surrounding the earth. They occur when the negative charge of the clouds and the induced positive charge on the earth below the clouds exceed the dielectric breakdown strength of the air. The result is a ground-to-cloud discharge current referred to as a return strike. This ranges from a few thousand amperes to a few hundred thousand amperes for the duration of a few ten microseconds to a few hundred microseconds.

Most lightning strikes commonly hit high buildings and structures rather than the earth. Direct lightning strikes to overhead power and telecom conductors rarely occur, due to the shielding effect of lightning protection grounding. If properly selected and installed, the lightning protection grounding will intercept and divert lightning strikes to the earth so as to prevent the disruptive damage that commonly occurs when in close proximity to a lightning strike. When a return strike current flows through the earth, a voltage that is equal to the product of the return strike current and ground resistance between the grounding connection and remote ground will increase significantly.  Subsequently, the ground potential rise could appear on the grounded parts of the equipment resulting in a breakdown of its insulation or flashover of air on or near the terminals or components of the affected equipment. When investigating causation, consideration should be given to ground potential rise that can cause electronic equipment to fail.

Lightning strike arresters

It must also be noted, however, that lightning strikes, even at some distance away, can induce the transient over-voltage that can produce a wave of the induced surge current within the power and telecom conductors. While the induced surge current moves away through the power lines, the lightning induced surge voltage will be discharged by the lightning surge arrester installed throughout the power lines. Lightning surge arresters are protective devices that limit the lightning induced surge voltage on the power lines and transformers by conducting the lightning induced surge currents to the ground. As a result, if properly selected and installed, the electric power distribution system connected to these power lines is unlikely to be exposed to the full lightning induced surge voltage.  When the lightning arrester is activated due to the lightning induced surge voltage, a partial or complete loss of the voltage will occur momentarily at the lightning surge arrester. Consequently, the electric power distribution system will encounter voltage sag, referred to as a voltage, that lasts from a few milliseconds to a few seconds. Whether or not a power interruption occurs to the electric power distribution system will depend on the coordination of protective devices used in substations.  Nevertheless, the lightning induced surge current within telecom lines can travel for an extended distance because there are no lightning surge arresters installed throughout the telecom lines.

Utility switching

Utility switching, as opposed to lightning strikes, is frequently and deliberately carried out to sustain the supply of electrical energy over the supply utility grid. Whenever the power lines are closed following tripping and subsequent clearing, the ground fault or the power lines are then connected or disconnected to an alternate supply source, and a transient over-voltage will occur due to inductance and capacitance of the load connected to the power distribution system. Waveforms of the voltage and current through the power lines cannot be changed instantaneously. Although the switching surge over-voltage is of moderate magnitude and duration, from a few milliseconds to a few seconds, lightning surge arresters are designed and approved for the intended use of limiting both lightning and switching surge voltage. Subsequently, either lightning or switching surge voltages will be limited to the level that the power lines and transformers can withstand without failure or damage.

Loss of equipment operation

A loss of equipment operation can be caused by indirect lightning strikes, which frequently cause power disturbances, including predominately momentary voltage sags and interruption. Furthermore, utility switching can result from clearing the surge voltage by the lightning surge arrester. The protective devices of the power distribution system can detect the voltage drop across the lightning surge arrester as abnormal ground fault conditions and isolate or initiate the isolation of the affected power lines. Voltage sags can result in degradation of performance or reduction in the expected service life or even in complete failure of the equipment. The loss of equipment can occur despite the fact that lightning surge arresters for power conductors and surge suppressors for all control and telecom conductors were properly selected and installed at the service entrance of the premises served. While it does matter whether the loss of equipment operation occurs subsequent to the lightning strikes that are environmentally generated or due to the utility switching that are artificially-generated, it is imperative to determine with the highest degree of certainty the cause of loss of the equipment. Due to the complexity of interference between lightning and switching surge voltages, a final determination of the loss of equipment operation should be prepared and stated in engineering terms by a recognized, independent third party. Qualifications should include experience, education and credentials that demonstrate knowledgeable and responsible techniques and methods to determine the cause. Moreover, a final determination should go beyond the term power surge, which is ill defined.     

Mark J. Brys is a senior consultant and project manager with Newtron Engineering Services, a division of Newtron Group.