Maintenance and Diagnostic Analysis of Current Transformers in GIS Equipment

Oliver Watts

07/07/2025

I'm Oliver, with 8 years of experience in current transformer testing. These are all practical experiences, shared with you
1 Maintenance of Current Transformers

Having been in the business of current transformer testing for 8 years, I know a thing or two about their maintenance! The maintenance of current transformers is divided into minor repairs, intermediate repairs, and major overhauls.

Minor repairs mainly involve opening the cover (if necessary) and changing the oil, usually carried out once a year. Intermediate repairs include inspecting the transformer body, cleaning, drying, and all the items of minor repairs. Typically, the transformer needs to be disassembled and sent back to the manufacturer for inspection, done once every five to ten years. A major overhaul means replacing some or all of the windings, including all the items of minor and intermediate repairs, scheduled once every fifteen to twenty years.

(1) Overview of Minor Repairs

Minor repairs are further divided into two types: without opening the cover and with opening the cover.

Minor repair without opening the cover: Use a clean cloth or cotton to wipe the porcelain bushing. If it's really dirty, you can use alcohol or gasoline. Then check if the porcelain bushing has cracks, damage, or enamel peeling. See if all the bolts are loose; if so, tighten them evenly. Check if the oil level is normal; the oil level shouldn't be lower than the specified minimum. Inspect if the base is rusted; if it is, clean it and apply paint. If the V - type oil level glass tube is cracked or leaking oil, replace it immediately. Check for oil leakage in all parts. Also, check if the connection between the breather and the upper cover is firm and well - sealed, and ensure the wiring is tight and has good contact.
Minor repair with opening the cover is applicable in the following situations:
- The oil level is lower than the minimum, and oil needs to be replenished.
- The oil gauge glass tube is broken and leaking oil.
- The waterproof plug sealing ring is damaged and needs replacement due to oil leakage.
- The upper cover sealing ring of the oil conservator is damaged and needs replacement.
- The oil fails the laboratory test and needs to be changed.

(2) Overview of Intermediate Repairs

It's best to carry out intermediate repairs indoors, and the room should be clean. If the repair takes a long time and can't be finished in a day, the transformer body should be sealed in qualified oil. Additionally, the lifting equipment must be able to support the entire weight of the current transformer. Before disassembling, take an oil sample from the lower drain plug for testing.

(3) Overview of Major Overhauls

A major overhaul is a thorough repair. If your unit can't handle it, hire a professional maintenance team. A major overhaul includes all the items of minor and intermediate repairs. The equipment is completely disassembled and inspected, and unqualified parts are replaced to restore the equipment's performance. If necessary, technical modifications are made to improve the mechanical performance.

2 Fault Analysis and Diagnosis of Current Transformers

When a current transformer fails, analyze the cause and location based on fault status and parameter changes. Amid numerous state parameters, extract characteristic info for diagnosis. The abnormal equipment condition from a fault is a “fault symptom”. Diagnosis, a form of state recognition, judges fault type and location via these symptoms.

Current transformer diagnosis has four steps:

Preventive tests: Follow Regulations for Preventive Tests of Power Equipment to measure winding/end - shield insulation resistance, dielectric loss factor, capacitance, and conduct oil - dissolved gas chromatography. Analyzing results reveals issues like water ingress, moisture, or poor manufacturing.
Partial discharge measurement: Routine methods miss discharge defects, but discharge tests clearly identify defect types with good results.

Online monitoring & infrared thermometry: Combined with internal structure and operating status, it effectively checks for loose joints.
Visual inspection: A running transformer’s “buzzing” often stems from a secondary circuit open/poor terminal contact (with spark discharge). Open secondary circuits cause terminal overvoltage, risking insulation breakdown and harm—so they’re strictly prohibited! If the “buzz” comes from loose iron - core screws, the sound grows with load, overheating the transformer, aging insulation, and causing grounding/breakdown. Surface dirt/dust - caused sounds turn to “cracking” discharges on rainy/foggy days (possibly leading to corona); severe discharges need enhanced monitoring and outage requests.

3 Conclusion

We've detailed the maintenance and diagnosis of GIS current transformers. Current transformers are transformer - type devices based on electromagnetic principles and are commonly used in power systems. Mastering their maintenance and diagnosis methods is crucial for maintaining the normal operation of equipment and ensuring the smooth progress of power work!

Topics

GIS CT Failure and maintenance

Oliver Watts

Hey! I'm Oliver Watts, an electrical engineer in Inspection and Testing. With years of hands - on experience, I ensure electrical systems meet top safety and performance standards. Using advanced gear, I conduct diverse tests, easily spotting issues in both large - scale industrial and small - scale commercial setups. I love teaming up, sharing knowledge, and keeping up with industry regs. Also, I'm skilled at data analysis with software. If you're into electrical inspection or just want to chat engineering, reach out. Let's connect and explore!