What are the possible causes of cracking failure in CT terminal blocks within GIS equipment?

Felix Spark

07/07/2025

Gas - insulated switchgear (GIS) , often called “

SF 6

combined electrical apparatus”, is widely used in power systems for its high reliability, small footprint, low noise, and low loss. It encloses high - voltage devices like circuit breakers, fast grounding switches, current transformers, and busbars in a grounded metal shell filled with

SF 6

gas . Each device sits in a separate gas chamber with different pressures. The CT terminal block divides gas chambers, connects components, and eases maintenance . A converter station found a 750kV GIS CT gas chamber’s pressure dropped ~0.05MPa daily, persisting after gas refilling. Thus, we analyzed the CT terminal block’s failure.

1 Overview and Cracking Analysis of the Terminal Block
1.1 Overview

Put into operation on 2017 - 06 - 23, the terminal block leaked gas on 2021 - 11 - 06 and showed cracks on 2021 - 11 - 08. The flat side is CT - side, convex is non - CT - side, with 12 outer threaded holes. CT - side has three circles of equidistant yellow copper terminal posts (1, 8, 15 per circle from inside); non - CT - side’s outermost circle has 15 posts (A1 - A5, B1 - B5, C1 - C5 counterclockwise), matching CT - side in middle circles.

1.2 Macroscopic Inspection

A ~30cm - long crack was found on the convex side, at the raised edge’s turn, split into two sections: a wide - opened long crack (A1 - B1) and a small - opened short crack (C5 - A1, barely visible). Penetrant testing followed to check for more cracks.

1.3 Penetrant Testing

Penetrant testing was conducted on both sides of the terminal block:

Convex side: Two cracks were found, consistent with macroscopic inspection in morphology and length (240mm and 60mm). The short crack became obvious after testing, and no other cracks were detected.
Flat side: Two cracks of different lengths (approximately 20mm and 8mm) were found at the inner sealing ring. They did not penetrate through, with an end - to - end distance of about 20mm.

1.4 Fracture Surface Inspection

A section cut from A4 showed non - CT - side penetrative cracks and CT - side non - penetrative ones. Square conductive sheets and hexagonal nuts inside had structural abrupt changes，with penetrant back - seepage (gaps between metal inserts and epoxy resin). Fine cracks (30° to the terminal block axis) and uneven, spotted contact surfaces (with 45° - angled cracks) were seen.

1.5 Force Calculation

With the manufacturer’s 25Nm bolt torque, using

T = kFd

(

(k = 0.15

), the single - bolt vertical preload was 13.9kN. Simulating max preload (M12 bolt, 50cm torque wrench) gave 220Nm torque (44Nm via a 10cm - arm wrench), raising preload to 24.4kN (1.76× standard). The 30° - angled, 31.78mm - long fracture had a 10.78mm discontinuous joint (resin stress increase). Excessive preload and stress concentration caused crack initiation and propagation in resin.

2 Causes of Cracking

Excessive bending stress on the discontinuous seat structure (edge bolt hole - terminal post) caused penetrative cracks. Improper tools/over - tightening led to excessive bolt preload. CT - side gas pressure

added to bending stress. Poor metal - resin bonding (gaps) reduced bearing cross - section and caused stress concentration. Combined, these cracked the terminal block, leaking gas.

3 Preventive Measures

Use torque wrenches per manufacturer specs to avoid over - tightening. Follow gas - filling processes to prevent pressure differences. Optimize terminal block design/casting to avoid stress - causing gaps/sharp inserts. Strengthen quality checks to reject faulty products.

4 Conclusion

CT terminal block cracking in

SF 6

apparatus resulted from improper bolt - tightening (excessive preload). The proposed measures guide other power users.

Topics

GIS 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.