What are the troubleshooting methods for current transformers?

Felix Spark

07/04/2025

As a front - line fault maintenance worker, I deal with current transformers (CTs) daily. A CT’s secondary winding in operation must never open - circuit! Once open - circuited, the secondary current and demagnetizing potential vanish. All primary - side magnetic potential then becomes the iron core’s exciting potential, sharply increasing its magnetic flux density. High voltage on the secondary side will endanger safety anytime.

Despite strict power operation regulations, user electricians have poor management and technical quality. Since the power operation market opened, design units often fail to design to specs, and installation teams don’t follow drawings or safety measures. Thus, CT accidents during operation keep rising. A typical capacity - increase project accident I recently handled illustrates this, and I’ll share it.

1. Fault Discovery: The Scene of a Burnt - out Transformer

On September 27, 2012, our engineering installation company took on a substation capacity - increase project. When we cut off the power to replace the CT in the incoming line cabinet, I removed it and found that — the CT was burned beyond recognition! Having been in maintenance for so many years, I've seen many burnt - out ones, but a CT suddenly burning out must be fishy, and we must dig deep into the reasons.

2. Cause Analysis: Man - made Open Circuit + Management Chaos
(1) On - site Investigation, Locking on "Secondary Wires Cut"

My team and I conducted a thorough investigation: The secondary outgoing wires of the CT in the incoming line cabinet were actually cut off! Tracing back, it turned out that this matter dated back to the commissioning of the substation:

Initially, the transformation ratios of the incoming line cabinet and the metering cabinet were both 75/5;
During the first capacity increase, the CT of the metering cabinet was replaced with one with a transformation ratio of 150/5, but the CT for protection in the incoming line cabinet was not replaced and remained 75/5;
After the capacity increase, the equipment showed no abnormalities, but as the transformer capacity increased, the protection setting value was not adjusted accordingly.

(2) Misjudging the Fault, Man - made Open Circuit

Later, as the electricity consumption increased, the over - current protection tripped frequently. The user electrician couldn't find the problem and mistakenly thought it was a CT fault causing the relay to operate, and actually cut off the secondary wires! After power transmission, the protection didn't trip, but the secondary circuit of the CT was directly open - circuited — this was a disaster!

(3) Hazards of Open Circuit: Iron Core Saturation → Burning

The secondary side of the CT originally has a small impedance and operates close to a short - circuit. Once open - circuited, the magnetic force generated by the primary current cannot be offset by the secondary side, the iron core is severely saturated, the iron loss increases sharply, and the CT itself overheats and burns out directly.

In the final analysis, this incident was a major management loophole: The construction team failed to replace the CT in the incoming line cabinet, the protection setting value was not updated, and the electrician operated blindly, leading to the CT being damaged layer by layer.

3. Preventive Measures: A "Life - saving Checklist" for Maintenance Workers

In our line of work, we must plug the loopholes from the source. Combining this accident, I have sorted out 6 strict measures, and operation and maintenance, design, and installation must all follow the rules:

(1) Design + Drawing Review: Strictly Comply with Specifications

Design units must conduct on - site surveys and produce drawings in accordance with specifications; the owner must supervise the drawing review to prevent "wrong designs" from entering the site.

(2) Equipment Control: Full - process Supervision

The owner must follow national and industry standards for procurement, testing, and acceptance to prevent defective products from entering the power grid.

(3) Operation Qualifications: Electricians Must Be Certified

If operation and maintenance personnel have no qualifications? Don't let them touch the equipment! Moreover, random modification of wiring and disassembly of equipment are strictly prohibited — this is a red line.

(4) Installation and Acceptance: Keep an Eye on the Construction Team

The installation team must follow the procedures, and any wrong installation, corner - cutting, or under - installation must be reworked immediately! The acceptance must be strict to avoid hidden dangers.

(5) Regular Inspections: Strengthen in Special Periods

Daily inspections and panel monitoring are essential, and special periods such as high load, high temperature, and typhoon days must be closely monitored! Check the appearance, smell for peculiar odors, and listen for abnormal noises to detect abnormalities early.

(6) Overhaul and Testing: Don't Let "Sick CTs" Be Put into Use

Overhauls must be carried out in strict accordance with standards, and the process must be in place; electrical tests must be thorough, and CTs with defects must never be put into use.

4. Conclusion: CT Open Circuit is Extremely Dangerous, Maintenance Workers Must Know "Prevention and Rescue"

A secondary open - circuit of a CT is no small matter; it can cause equipment outages, protection maloperations/refusals to operate, and lead to major accidents at any time. As front - line maintenance workers, we must thoroughly understand the hazards and causes of open circuits, conduct more inspections and investigations in daily work, and be able to solve problems immediately when abnormalities occur. Only by implementing these measures can CTs operate stably and the power grid have fewer troubles!

Topics

CT Failure and maintenance

Felix Spark

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.