PICNICS AND PARTIAL DISCHARGE
Partial Discharge (PD) activity creates a sound often described as that of sizzling bacon. The sound is only applicable to surface PD activity, as sounds emitted from internal discharges are hopelessly trapped within the insulation. Other types of PD signals do escape, as we will discuss later. The sound produced by surface PD is not always similar to sizzling bacon, but the analogy is generally a good one for describing these surface PD events.
While barbecuing one recent weekend, steaks sizzling nicely on the grill, my thoughts randomly drifted to partial discharges. It occurred to me that the process of cooking meat over fire — at least when I am acting chef — shares some similarities to the stages of surface insulation damage that partial discharge causes. When meat is cooking, it exhibits physical, chemical, audible, and even aromatic changes. Similarly, when an insulation's local surface electrical field is compromised, partial discharge activity occurs and creates changes like that of cooking meat. The physical changes can include visible signs of surface tracking, chemical alteration of the insulation, and reaction with atmosphere. Audible sounds are more easily detected with ultrasonic detectors than with the unaided human ear. Aromatic changes, such as the generation of ozone and acidic byproducts which create a unique sharp smell, can also be detected.
As meat continues through the cooking process, the amplitude and frequency of the sizzling events increase — similar to partial discharge events during surface insulation breakdown. Typically, when surface PD is initiated, ultrasonic signals are all that can be detected. During the next stages of the cooking process, the meat could create a sudden fire, which is similar to a complete surface flashover of the electrical insulation. Fire or flashover can cause permanent damage to meat or insulation.
$25 Space Heaters
High humidity has a drastic effect on increasing surface partial discharge activity. (I could go into the cold glass of lemonade condensation example from the same picnic, but I won't do that to you.) Outdoor medium voltage switchgear is always somewhat dusty. When atmospheric conditions are conducive to condensation, the moisture accumulation on the insulation surfaces, combined with other contaminants, can create partial discharge activity that leads to tracking and flashover of non-ceramic insulation. Thermostatically controlled space heaters can allow the internal switchgear elements to remain warm so that condensation and PD are prevented. Thus, the health of expensive outdoor switchgear can be dependent upon the health of a $25 space heater!
Internal Discharge
Electrical fields can also be compromised within electrical insulation. Just as a sharp conductor edge can initiate surface PD activity (see Figure 1), so can an internal sharp edge (in the right circumstances,) coupled with an adjacent air void. In the case of cable terminations, poor preparation or lack of attention to cutback dimensions can create localized voltage stresses along the internal termination interface. These local stresses generate PD activity that can lead to failure, often in a similar manner to surface tracking.
Many other types of internal insulation failures may occur due to PD activity. Because PD activity is internal, we seldom see the damage before it's too late. To prevent internal PD failure from occurring, it is necessary to detect the electrical pulses that are created from the sparking discharges, usually by using transient earth voltage (TEV) sensors. Don’t rely on ultrasonic technology alone, or you may miss the opportunity to prevent internal component failure.
Ultrasonic detectors require a pathway for the signal to reach sensors placed outside of switchgear. A vent or bolt-hole opening may be enough to allow an ultrasonic signal to escape, but not always. Figure 2 is an example of advanced surface PD tracking damage occurring deep inside the switchgear at the back of the breaker receiver stabs. Ultrasonic signals could not escape from this location to a point where they could be detected from the outside. TEV sensors do a great job of detecting such potential disasters in time for performing necessary repairs.
Conclusion
Many every-day experiences have parallel scientific similarities — just like picnics and PD. Trying to understand a complex scientific phenomenon in simple terms allows us to better understand these events.
Just as it is necessary to closely monitor the condition of your steaks while barbecuing, it is also necessary to keep a close watch over your medium-voltage insulation. Performing annual PD surveys using handheld multi-sensor instruments or permanent monitors should be a part of that monitoring.
Hey, I just had a thought: What if I monitored my steaks using ultrasonic technology? Hmmm, maybe I'm on to something. I think I'm going to call Weber tomorrow.
