Consider supplementing the visual inspections with the periodic use of diagnostic test equipment that allows more detailed inspection of the equipment while it is still energized. Some of these techniques are described below. Table 18-1, Substation Equipment Inspection Technology Matrix, lists equipment and what inspection technologies may be of value in identifying the condition of the equipment.
18.3.2.1 Infrared Thermography: Infrared thermography can provide a non-contact means of online evaluation of “hot spots” in an energized system. The infrared viewer provides a television-like image of the natural infrared energy being emitted from the objects in its field of view. The pictures, commonly referred to as thermograms, can be analyzed in real time or stored electronically and analyzed in the office or lab. When a loose or corrosive connection is present under loaded conditions, the infrared viewer can detect the temperature differential between the connection and the surrounding conductors.
When doing infrared scanning, the following test parameters need to be taken into account:
1. Infrared scanning surveys should be done during periods of maximum possible loading, but not less than 40 percent of rated load of the electrical equipment being inspected. Refer to NFPA 70B, Section 18-16, Infrared Inspection.
2. Inspect distribution systems with imaging equipment capable of detecting a minimum temperature difference of 1°C at 30°C.
A general outline for a thermographic survey follows.
Table 18-1: Substation Inspection Technology Matrix Component Description Infrared
Therm.
Lab Oil Analysis
Ultrasonic Flow
Vibration Ultra- sound
Recording Temp. Surveys
On-Site Oil Analysis
Visual
Generator Step-Up Transformer X X X X X X X X
Transmission Transformer X X X X X X X X
Step-Down Dist. Transformer X X X X X X X X
Current Transformer X X X
Potential Transformer X X X
Trans. Blower Motors X X X X X
Trans. Oil Pumps X X X X X
Trans. Coolers X X
Trans. Tap Changers X X X
Trans. Control Cabinets X X
Oil Circuit Breakers X X X X
Air Circuit Breakers X X X
S.F.6 Breakers
DC Control Batteries X
Trans. & Dist. Bus X X X
Disconnects X X X
Enclosed Switchgear X X X
Ground Networks X X
Dist. Regulators X X X X X
Light & Power Trans. X X
D.C. Control Bus X X
Fuse & Fuse Clip X X
Terminal Blocks X X
Enclosed Buswork X X
Cable Splices X X X
Reactors X X X
Compressor Motors X X X X
Nitrogen Systems X
Motors X X X X
Lightning Arrestors X X X
Carrier Current Wave Traps X X X
Insulators X X X
Batteries X X
THERMOGRAPHIC SURVEY
1. Inspection and Preparation of Equipment To Be Surveyed
a. Inspection of physical, electrical, and mechanical condition of equipment
b. Where applicable, removal of all necessary covers prior to thermographic inspection 2. Report Components
a. Load conditions at the time of the inspection b. Hot spots
c. Temperature difference between the area of concern (hot spot) and the reference area d. Cause of temperature difference
e. Areas inspected, with inaccessible and / or unobservable areas and /or equipment identified
f. Photographs and/or thermograms of the hot spot area 3. Test Results
a. Temperature differences of 1°C to 2°C that indicate possible deficiency and warrant investigation
b. Temperature differences of 4°C to 15°C that indicate deficiency and has to be repaired as time permits
c. Temperature differences of 16°C and above that indicate major deficiency and has to be repaired immediately
18.3.2.2 Laboratory Oil Analysis: All oil-filled power transformers and circuit breakers generate gases during normal operation. But when the equipment begins to function abnormally, the rate of gas production increases. Analyzing these gases and their rate of production is another valuable tool for evaluating the condition of an operating transformer or breaker. Other tests can also be performed on the oil to determine the oil’s condition. For additional information on oil analysis, see Chapter 19 of IEEE Std. C57.106, “Guide for Acceptance and Maintenance of Insulating Oil in Equipment.”
18.3.2.3 Ultrasound: Another method for evaluating a system that is energized is the use of an ultrasound detector. Ultrasound is used to detect inaudible noises on equipment that could indicate potential problems. Listed below are some examples of what ultrasound can be used to detect.
1. Bus: When corona is present in medium- and high-voltage circuits, air is ionized. Corona emits a high-frequency pitch that can be heard with the human ear. However, the ultrasound detector can pinpoint low levels or corona not heard by the human ear and also at greater distances. Substations and transmission lines can be audibly inspected by this method to locate sources of corona. The ionized air generated by corona will attack insulation. The early detection of its presence can allow for corrective measures before further insulation
deterioration occurs.
2. Power Transformers: Ultrasound can detect inaudible noises on transformer internals that could indicate arcing, sparking, partial discharge, combustible gases, corona, core looseness, vibration, or loose parts.
3. Cooling Pumps and Fan Motors: Ultrasound can also be used to evaluate the overall condition, and for cavitation and flow.
18.3.2.4 Portable Gas in Oil Analysis: This analysis measures the amount of dissolved combustive gases in transformer oil and/or hot metal gases in tap changer oil, which can be the evidence of arcing, sparking, partial discharge, corona, and/or overheating. This can be done in the field and can be used for trending and historical information. If there is a noticeable increase in the amount of dissolved
combustive gases, a laboratory oil analysis should be done to determine the cause of the increase. See Chapter 19 for additional information on testing.