Digital radiography for inspection of high-voltage SF6 power circuit breakers
Author: Brian Wahl
04/01/2006 - Volume III - Issue I
Siemens has developed the application of utilizing digital radiography for internal inspection of high-voltage power circuit breakers. This technology applies a "best of both worlds" approach to breaker inspections, combining OEM support with the reduction of outage time and materials.
Digital radiography utilizes a field-applicable radiological source (x-ray, Cesium, Iridium, and/or Cobalt) to pass gamma radiation through the breaker tank assembly onto the opposite where a phosphorous laden type and strength as well as the type of materials and internal medium (SF6, air, oil, or vacuum) affect the required exposure time per image, amount of radiation penetration, size of the required safety zone during the exposure, and image quality. Radiation attenuation by the internal components (contacts, nozzles, shield, etc.) provides the image contrast due to different material densities.
How does DR work on high-voltage circuit breakers?
As described above, DR is method of "internal" inspection on high-voltage breakers. Siemens would note this does NOT replace the need to perform recommended "external" inspections and maintenance as prescribed by the original equipment manufacturer. Siemens performs the "external" preventive maintenance testing and checks as part of the DR inspection, ensuring that the circuit breaker meets acceptable standards for continued safe and reliable operation.
As Siemens began applying DR, Siemens quickly determined that circuit breaker kV class and interrupting medium have significant bearing on the radiological survey results. Through initial testing, Siemens determined that bulk oil type breakers typically require excessive amount of source strength (radiation) due to the properties of oil exposed to radiation. An equivalent ratio of necessary radiological penetration is that every five inches of oil cross-section equates to one inch of steel in thickness, making DR impractical on large bulk oil breakers. However, unlike oil breakers, SF6, air, and vacuum breakers lend themselves well to DR inspection. Based on the quantity and criticality of SF6 breakers, Siemens determined that this would be the most applicable medium and circuit breaker to inspect utilizing DR.
Siemens typically performs several radiological exposures per phase to ensure that critical components are revealed. This varies with the circuit breaker voltage class and if problems are identified during inspection. For a 72-145kV class SF6 dead tank breaker, two to three exposures per phase are required to provide adequate data. For this breaker size, Siemens can typically complete inspections on one to two breakers per day.
Figure 1: Graphical representation - radiography setup
As shown in Figure 1, the radiological source distance and location from the interrupter will vary depending on source strength, breaker size (kV), and breaker style (dead tank, live tank, and dual pressure). Figure 1 represents a dead tank breaker.
With DR inspection, Siemens can examine and measure critical interrupter components and dimensions. The inspection can identify arcing contact and nozzle wear, contact alignment, and mechanical defects (loose hardware, cracks in components, debris in the tank, and overall mechanical interrupter condition).
FIgure 2: Detailed 115kV Interrupter Inspection
The accuracy of the digital image reduces the subjective nature of an internal inspection while eliminating the need to handle SF6 gas and also preventing personnel errors during breaker disassembly and reassembly.
Figure 3: SP breaker contact inspection
Figure 3 shows an image in comparison to the actual breaker. The DR inspection allows for significant savings in outage time and resources which should allow additional breakers to be inspected without additional incremental costs.
In conclusion, Siemens believes that digital radiography combined with regular "external" preventive maintenance will improve breaker reliablility, ensure internal components are acceptable for continued use, and allow for tracking interrupter wear over the lifecycle of the breaker. Depending on breaker type and size, DR inspections could be as much as 50% less costly than traditional internal inspections.