Statistics indicate that up to 37 percent of rotating equipment failures occur due to stator insulation breakdown. For numerous decades, partial discharge (PD) testing has been a really helpful tool for assessing the stator insulation condition of medium-voltage rotating apparatus. This write-up examines how PD activity happens and why PD testing is special as applied to generators and motors.
What is Partial Discharge?
Partial discharge is a localized partial breakdown within or on the surface of insulation that causes a spark to occur. This partial failure of the insulation is usually triggered by poor design and style, flaws, voids, or contamination that develop a localized anxiety that exceeds the dielectric breakdown strength of the insulation material. The sparking activity then leads to additional insulation decomposition by way of a combination of thermal, chemical, and electrical phenomena and persists until total failure happens. The detection and monitoring of PD activity has been located to be really useful in assessing the condition of electrical gear. Despite the fact that PD can occur at lower voltages, gear operating beneath two,000 volts to ground typically does not discharge frequently and typically fails by other mechanisms.
Rotating Apparatus Insulation
Switchgear, cable, and transformer insulation can’t tolerate PD activity really effectively. Typically, PD activity will lead to further destruction of the insulation and eventual failure. As a result, these sorts of gear should constantly operate PD free of charge.
The design and style of rotating apparatus creates inherent troubles that make generating PD-totally free equipment not possible. The complex configuration of stator coil insulation, troubles with the manufacturing impregnation approach, and the difficulties fitting the coils inside the stator slots can all produce voids which lead to PD activity. Since PD in rotating apparatus is inevitable, the coil insulation employs mica tape which is extremely resistant to partial discharge harm. So, when assessing the condition of rotating apparatus, it becomes very essential to monitor and trend PD activity more than the course of a lot of years as opposed to assessing the situation of other sorts of gear insulation where periodic PD testing may possibly suffice.
Successful on-line PD testing of switchgear, cable, and transformer insulation generally requires taking regular (annual) spot checks utilizing short-term sensors that are safely attached to grounded elements of the apparatus. The primary aim of this procedure is to make certain a PD-totally free apparatus.
In order to ideal monitor rotating apparatus, permanent coupling capacitors are connected to the ma- chine output bus throughout an outage. These sensors supply a standardized strategy to decouple the PD signals. Added data can be obtained by utilizing the RTD’s that are embedded in the insulation as auxiliary PD measurement points. The RTD’s act as antennas that can pick up the high frequency pulses linked with PD.
As previously stated, the mica insulation is fairly im- pervious to most PD activity. Even so, huge amplitude discharges occurring from voids in the insulation or finish winding contamination can lead to direct insulation failure.
This is just 1 category of troubles that can be detected.
Because the rotating equipment PD activity is trended more than numerous years, it is far more economical to set up permanent monitors than conducting typical spot surveys. These moni- tors (see Figure three) accept other sensors for trending load, temperature, and humidity information along with the PD data so that correlations can be produced that offer further infor- mation for a lot more precise diagnostics of the result in, location, and variety of the PD activity occurring.
Insulation can also fail from the thermal or mechanical wear over the course of a lot of years from overheating, ther- mal cycling and loose wedging. In these instances, increases in PD activity will show as a slow rise over time. This sort of PD activity relates directly to the machine’s overall thermal or mechanical condition, as the PD activity is not the pri- mary dilemma but is in fact an indicator of the presence of other age-connected issues.
Monitoring PD activity in rotating apparatus is really im- portant for increasing reliability and prolonging gear life. All generators and crucial motors need to be monitored continuously and consideration need to be offered to carry out- ing annual PD spot testing to the balance of plant medium voltage rotating apparatus.
Mr. Genutis received his BSEE from Carnegie Mellon University, has been a NETA Certified Technician for 15 years, and is a Certified Corona Technician. Don’s technical coaching and education is comple- mented by twenty-five years of practical field and laboratory electrical testing expertise. He is Vice President of the Group CBS Eastern U.S. Operations and is Technical Manager for their subsidiary, Circuit Breaker Sales & Service situated in Central Florida.