By William G. Higinbotham
It is clearly in the best interest of anyone responsible for the operation and maintenance of electrical assets to detect and remedy problems with insulation before great damage to personnel or property can occur.
Intro to Partial Discharge
One of the first indicators of insulation deterioration is what is called a Partial Discharge (PD). A PD is an electrical discharge or spark that bridges a small portion of the insulation between two conducting electrodes in medium- and high-voltage assets.
The National Fire Protection Association (NFPA) says that the leading cause of electrical failures is breakdown of insulation. According to the IEEE Gold Book, Table 36, cables, switchgear and transformers are likely to endure the greatest losses from insulation failure.
Many Possible Causes
When there is an issue with insulation in a high-voltage field, it doesn’t always flash over immediately. If the issue is across only part of the insulation, the result can be localized discharge that damages the insulation even more. Over time, the damage will grow to the point of flashover. The issue with the insulation can be a design defect, manufacturing defect, installation mistake, contamination, physical damage or just wear and tear.
Classification of PD is based on where it occurs:
Internal Discharge: Discharge inside the insulation with no direct path to air. A classic example is a cable termination with a jagged semiconductor cutback done by a poorly trained jointer. This will start discharging the day it’s energized and could lead to failure within a year.
Surface Discharge: Discharge along the surface of an insulator is very common and can leave characteristic carbon tracking. This can lead to arc flash and is much more likely to occur in high-humidity or contaminated environments.
Corona Discharge: Discharge directly from a conductor to air due to the shape of the conductor causing a concentration of electric field greater than what the air can withstand. Corona doesn’t lead to failure because the damaged air molecules are quickly replaced with new ones.
Ultrasonic Testing for PD
Surface discharge and corona discharge emit high levels of sound waves. Most of that energy is ultrasonic – above the range of human hearing. Ultrasonic energy from PD has a few characteristics that make it easy to detect:
1) It has a distinctive sound much like bacon sizzling.
2) The sound will be concentrated in 120Hz bursts related to the positive and negative rising edges in the power system sine wave.
3) The sound will have a repetition rate in the range of 6-50 pulses per cycle.
Most metal-clad switchgear has louvers, vents, loosely fitting panels and doors. By moving a sensor along these openings, the technician can usually pick up PD from outside a cabinet without opening anything. In the event of well-sealed compartments where there is no air path, contact sensors can pick up the very slight vibrations due to ultrasonic energy inside.
Transient Earth Voltage Phenomenon and Testing
Figure 1 shows how a PD-induced current pulse travels along the internal, then external surface of the switchgear. The pulse travels through the impedance of the ground connection. Because the typical grounding of metal-clad switchgear presents impedance to high-frequency currents, a voltage is developed on the surface of the gear.
Doing an online, non-invasive TEV test involves first pressing the capacitive sensor plate to the metal cladding of each compartment for a few seconds and then taking a reading.
Unchecked PD leads to progressive deterioration of insulation, inevitably causing electrical breakdown and total failure of high-voltage cables and equipment. Non-invasive testing for PD while systems are powered on can help you avoid the downtime and risk to personnel caused by equipment failure. To find out more about PD testing at your facility, contact us!
About William G. Higinbotham: Senior IEEE Member William G. Higinbotham has been president of EA Technology LLC since 2013. His responsibilities involve general management of the company, which is responsible for EA Technology activities in North and South America. William is also responsible for sales, service, support and training on partial discharge instruments and condition-based asset management. He is an author or co-author of several industry papers. Founded in 1966 and headquartered in the UK, EA Technology specializes in asset management solutions for owners and operators of electrical assets. The company’s operations and customers are global, with seven regional offices around the world.
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