Wednesday, May 8, 2019

What Is Partial Discharge (PD) and Why Is It Important to Detect?

One of your newly hired maintenance team members attended an online webinar on Partial Discharge (PD) theory, its effects and how to detect it.  Marty was so intrigued by the topic, he asked you for permission to share what he learned with the rest of the maintenance team.  Since continuing education is encouraged by the executive management at your company, you arranged the training event. To help prepare for the training, Marty requested the presentation from the webinar’s speaker.   Let’s see how Marty’s training session went.

Twenty-four people attended Marty’s training event inclusive of maintenance personnel, the head safety engineer and the head of the company’s EHS team.  Marty presented the topic in a logical manner starting with the definition of PD and ending with the effects on electrical equipment if PD remains undetected.  Marty recommended that PD detection be added to his company’s Condition Based Maintenance protocols.

• What is PD: Electrical discharge that does not completely bridge the space between two conducting electrodes and can occur in medium and high voltage electrical equipment

                                                                                                         Theory and Effects of Partial Discharge

• PD Occurs: Anywhere there is a junction between two electrical components. Examples are within solid insulation, across the surface of insulation material, within gas bubbles in liquid insulation and around an electrode surrounded in gas.
• How To Detect PD: PD is most often detected by ultrasound testing devices that pick up the sound emissions given off by the discharge.  Alternately, Transient Earth Voltage (TEV) detection may be appropriate on some equipment.
• Effects of PD: Damage caused by PD can be mechanical, thermal or chemical.  If PD goes undetected, catastrophic damage to electrical equipment may occur and may cause serious safety issues in the workplace such as Arc Flash.
•  Classification of PD: There are three distinct types:
  1) Corona – ionization of fluid or air surrounding a conductor.
  2) Tracking – surface tracking over contaminated insulation
  3) Arcing – electrical breakdown of a gas producing a plasma discharge.
• Physical Signs of PD: Odors (ozone, burning, metallic), discolored lines or carbon tracks.

Conclusion:

Partial discharge (PD) can occur in medium and high voltage switchgear. PD represents a breakdown between two conducting electrodes.  If PD is not detected, the damage to the electrical equipment can be catastrophic and cause serious safety events in the workplace.  Partial discharge can easily be detected by ultrasound and should be part of a routine predictive maintenance inspection strategy of electrical components.