High PD activity ...
... lead to uncovering a critical problem - the incorrectly installed wedges
Company: Midwest US Utility
Ratings: 173 MW, 18 kV, Hydrogen Cooled Turbine Generator, 3 years in operation
Manufacturer: Confidential
Related Info: 25 years old
PD Sensors: Two bus couplers per phase
Case Study Details:"... With the high H2 pressure (30-35 psig) the [PD] levels were expected to be very low but instead were very high after only a year of operation. We initially expected to find problems with the couplers as we did not believe the results..."
High PD activity (slot discharge)...
...was a foresign of an upcoming failure
Company: Western Canadian Utility
Ratings: 184 MVA, 13.8 kV, Air Cooled Hydro Generator
Manufacturer: Confidential
Related Info: 25 years old
PD Sensors: Four PDA Couplers per phase
Case Study Details: The PD data for this unit has been trended for over ten years. In 1995, semi-conducting paint and CRTV were injected into selected bars ...
Intelligent on-line signature analysis...
...revealed broken rotor bars
Company: A large utility in the US northeast
Ratings: 3000hp, 6.6kV 1785rpm Motor
Manufacturer: Confidential
Case Study Details: The suspicion arose from a constant hum during motor operation and abnormal vibration levels during start-up and running. The plant vibration monitoring system did not have the ability to zoom...
High level system PD noise...
...uncovered a well-developed tracking
Company: Italian utility
Ratings: 390 MW, 20kV, Hydrogen Cooled Turbine Generator
Manufacturer: Confidential
Related Info: Installed in 1970
PD Sensors: Two Bus Couplers per phase
Case Study Details: During a partial discharge measurement using the TGA-B portable instrument, high levels of PD activity (over 3000mV position, well above the highest range, see Figure 1) were detected on the C phase outside...
Reading Instability...
...uncovered internal bad connection on a new transformer
Company: US North-West utility
Ratings: 45 MVA power transformer
Manufacturer: Confidential
A customer was commissioning a new 45 MVA power transformer delivered by an overseas manufacturer. During the test, ADWEL's Digital Winding Resistance ReporteR, DWR-10, was consistently issuing a warning of current interruptions, and did not complete the test. Another DWR-10 instrument was used with similar outcome. All other standard measurements, including TTR, Tan-Delta and insulation resistance indicated acceptable resistance value.
Nevertheless, the customer drained the oil and inspected the transformer to detect that the windings on all 3 phases were not properly secured to the bushings. Had the transformer been energized, a catastrophic failure could have occurred.
BusGuard (PD)
BusTrac (PD)
TurboGuard (PD)
FluxTrac
- PDF
Typically used for motors and small turbo generators (less than 100 MVA), Bus couplers detect stator winding PD activity and help separate unwanted electrical noise. In the recent years, Epoxy Mica Capacitors (EMCs) became the industry's choice due to sufficient sensitivity and the relative ease of installation. The number and type of couplers required depends on the application and type of machine. Three couplers are typically needed for motors, whereas 6 couplers are typically used on generators. A standard package includes 3 to 6 couplers, mounting hardware, insulating tapes, termination box, and coaxial cables. A typical installation takes 1.5 to 2 days for a set of 3 couplers, and up to 4 days for a set of 6 couplers.
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Bus Couplers
SSC - Stator Slot Couplers (PD)
TGA-B (PD)
TGA-S (PD)
EL CID
Wedge Tightness Detector
Robotic Inspection Vehicle
DCR-50 (DC Ramp Test)
Corona Probe (PD)