Accurate Assessment for Informed Decision-making

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Client:

A nuclear plant located in the United States.

Introduction

A 1960s vintage 1500 MCM cable driving a large feedwater pump motor failed unexpectedly. Because the customer had other circuits fed by the same type of cable, a decision was made to have Kinectrics perform an analysis to determine the root cause of the failure.

Kinectrics received short sections of three 4160 V, 1500 MCM cables for failure analysis. Each cable section ranged from 1.5 feet to 3.5 feet in length. Six-inch long cable sections were cut from each end of the cable samples, which were then used to conduct physical and chemical tests to determine the cause of failure.

The 5 kV power cable submitted for evaluation consisted of a copper conductor, taped conductor shield (carbon black impregnated cloth), rubber insulation, an insulation shield of semi conductive polymer coated tape. A metal foil over the shield formed the cable's neutral shield. The cable had a PVC jacket.

Description of work performed

Visual Examination

A visual examination showed that the PVC jacket appeared not to be aged. The surface appeared glossy with no evidence of oily substance or greenish deposit between the tin-coated copper shield and PVC jacket. There was evidence that the rubber insulation had softened and exhibited extrusion and flowing at the cable ends. The rubber insulation in two cable samples was soft and sticky. The conductor semi conductive tape shield was cutting into the insulation material.

Material Tests

Several chemical and physical tests were performed on all three cable samples. These tests included:

On the insulation:

• Fournier Transform Infrared Spectroscopy (FTIR) analysis
• Tensile tests
  • Elongation at break
  • Tensile strength
• Gel content

On the jacket:

• Solubility test
• Tensile tests
• Plasticizer content

Testing of the PVC jackets showed minimal again. Only the inner surface of one cable jacket showed the effect of heat exposure from conductor heating.

FTIR analysis indicated that the insulation material was based on a butyl rubber formulation. The insulaton of all three cables was significantly aged. The combination of unaged jacket and significantly aged insulation indicated that the conductor was operating at an elevated temperature for an extended period of time.

As is typical for butyl materials, thermal aging results in progressive softening of the rubber. There was evidence that the conductor tape shield had penetrated the softer butyl insulation material, increasing dramatically the electrical stress in the insulation.

Conclusions

Overheating at the conductor for extended periods of time softened the butyl insulation and allowed the conductor shield to penetrate the insulation. This increased electrical stress dramatically, leading ultimately to the creation of partial discharge activity and failure of the cable.

The customer is investigating the origin of the conductor overheating.

For more information:

Samuel Peralta, 416.207.6000 x5931, samuel.peralta@kinectrics.com