What are the causes of current transformer faults and the countermeasures for faults?

07/03/2025

As a front - line maintenance technician, I deal with current transformers (CTs) daily. CTs convert high - magnitude primary current to low - magnitude secondary current for substation/line protection and measurement, operating in series long - term. However, they face faults from external (unbalanced loads, wrong wiring, etc.) and internal (insulation defects) issues. These faults, like secondary open - circuits or insulation breakdown, harm measurement accuracy, protection operation, and grid stability. Below, I share insights from hands - on experience.

1. CT Structure (Maintenance View)

A CT has primary/secondary windings, a core, and insulation (oil - immersed, SF6, solid). The primary winds in series with the circuit, the secondary connects to instruments/relays. Key: Fewer primary turns, more secondary turns, and near - short - circuit normal operation. Critical: Never open the secondary circuit; ground it reliably (I’ve seen dangerous arc flashes from open circuits).

2. Function & Principle (Practical)

CTs reduce large currents for safe protection/measurement via electromagnetic induction, isolating high voltage. During calibrations, I check primary - secondary current ratios to verify CTs.

3. Performance Classification
(1) Optical CTs (OTA)

Based on Faraday magneto - optical effect, used in grid tests. Temperature - sensitive but good for strong magnetic fields.

(2) Low - Power CTs

With microcrystalline alloy cores, they offer wide linear ranges, low losses, and high precision for large currents—ideal for industrial measurements.

(3) Air - Core CTs

No iron core, avoiding magnetic saturation. Popular in relay protection for strong anti - interference, suitable for complex environments.

4. Fault Causes (Field Experience)
(1) Insulation Thermal Breakdown

High - voltage CTs generate heat/dielectric losses. Defective insulation (e.g., uneven wrapping) causes overheating and breakdown—common in old equipment.

(2) Partial Discharge

Normal CT capacitance distributes evenly, but poor manufacturing/structure (e.g., misaligned screens) causes local high fields. Unresolved discharges lead to capacitor failures.

(3) Excessive Secondary Load

Heavy loads in 220 kV systems increase secondary voltage/current, causing errors. Faults may saturate cores, misoperate relays. Open secondary circuits (e.g., loose wires) create high voltages—risky!

5. Fault Response
(1) Follow Operational Rules

Wiring: Strictly series - connect circuits, windings, and instruments; use proper configurations (single - phase, star).
Error Compensation: Add windings/cores to correct errors via capacitance/inductance.
Calibration: Perform demagnetization/polarity tests post - installation/maintenance.

(2) Emergency Handling (Safety First)

Power Off: Immediately cut power for safety.
Inspect Secondary Circuit: Check for open circuits, minimize primary current, use insulation gear, and follow diagrams.

For secondary open circuits:

Assess Impact: Identify affected circuits, report to dispatch.
Reduce Load/Isolate: Transfer loads and de - energize if damaged.
Short - Circuit Secondary: Use approved materials; sparks mean downstream faults, no sparks mean upstream issues.

(3) Detection Techniques

Insulation Testing: Measure dielectric loss, capacitance to spot defects—good for aging assessment.
Infrared Thermography: My key tool! Detects loose connections/thermal issues quickly.

Conclusion

CTs are vital for grid reliability. Mastering their structure, principles, and fault handling ensures stability. Following guidelines, using detection tools, and acting on emergencies minimizes failures—securing a safer grid.

Topics

CT Failure and maintenance

Hey there! I'm an electrical engineer specializing in Failure and Maintenance. I've dedicated my career to ensuring the seamless operation of electrical systems. I excel at diagnosing complex electrical failures, from malfunctioning industrial motors to glitchy power distribution networks. Using state - of - the - art diagnostic tools and my in - depth knowledge, I pinpoint issues quickly. On this platform, I'm eager to share my insights, exchange ideas, and collaborate with fellow experts. Let's work together to enhance the reliability of electrical setups.