Case Analysis of Abnormal Secondary Circuits in Voltage Transformers

Felix Spark

07/02/2025

1. Fault Situation

In September 2023, as a front - line fault maintenance worker, I detected abnormal voltage on the 10kV Section I bus of a substation during monitoring duty and informed the operation and maintenance team. The monitoring system showed:

U 0 = 0 kV

U a = 6.06 kV

U b = 5.93 kV

Uc = 6.05 kV

U ab = 10.05 kV

U bc = 5.94 kV

My team and I rushed to the site. We suspected the secondary air circuit breaker of the 10kV Section I bus voltage transformer was closed and found the U - phase fuse blown. After disconnecting this breaker, the 900 bus - sectioning circuit breaker auto - activated, tripping the 95A breaker on the No. 1 main transformer’s 10kV side and inter - tripping lines 911–915, then closing 900.

After restoring the secondary circuit, we checked the voltage transformer’s main body and fuse (both normal). Inspecting the secondary circuit, I found a loose A660 terminal in the cabinet. Tightening it restored normal voltage on the 10kV Section I bus.

2. Cause Analysis

The 10kV Section I bus has 6 feeders, 5 (911–915) connected to small hydropower. At full generation, the No. 1 main transformer’s 10kV side load current drops, raising bus voltage.

Operation and maintenance staff, relying on experience, disconnected the voltage transformer’s secondary air circuit breaker without analyzing impacts on protection devices. At this point, the 95A breaker’s current (≈48A) met the auto - backup’s no - voltage/current condition (secondary value: 25V, 0.02A). The auto - backup activated, tripping the 95A breaker and inter - tripping the 5 small - hydropower feeders. The root cause was failing to exit auto - backup during voltage transformer abnormal handling, causing misoperation.

3. Preventive Measures

Capacitive voltage transformers have various faults, with abnormal secondary voltage output being common. Front - line operation and maintenance staff should:

Strengthen department management, collect more data, and monitor alarms to detect abnormalities early.
Analyze danger points in voltage transformer handling, update on - site operation regulations. Before replacing fuses (or similar tasks), check the main transformer’s current and auto - backup settings. If the current is below the no - current threshold, follow procedures to exit auto - backup.
Regularly revise measures for “three - mistake” (mis - operation, mis - wiring, mis - setting) accidents, conduct training. Standardize emergency disposal processes to avoid improper actions.
Enhance on - site risk control; clearly label equipment at risk of misoperation.

Topics

VT 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.