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VLF on 35kV cable won't start while one phase is connected to MV transfomer

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    VLF on 35kV cable won't start while one phase is connected to MV transfomer

    While conducting MV cable acceptance testing at 44kV using an HVA60, it was found that the test would not start while one of the phases was connected to the MV transformer (B phase) with the transformer load break switch open.
    All cables did fine on DC insulation resistance and the transformer passed all acceptance testing. Still, until the B phase dead breaks were lifted from the transformer, the VLF would not start in this particular instance. This was just one of approximately sixty transformers on site, so it was very unusual. The cables tested fine (VLF) once they were un-landed.
    Has anyone experienced this?

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    Quote Originally Posted by ericnessl View Post
    While conducting MV cable acceptance testing at 44kV using an HVA60, it was found that the test would not start while one of the phases was connected to the MV transformer (B phase) with the transformer load break switch open.
    All cables did fine on DC insulation resistance and the transformer passed all acceptance testing. Still, until the B phase dead breaks were lifted from the transformer, the VLF would not start in this particular instance. This was just one of approximately sixty transformers on site, so it was very unusual. The cables tested fine (VLF) once they were un-landed.
    Has anyone experienced this?
    Have you verified complete proper operation of the transformer load break switch?

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    Quote Originally Posted by TestTech View Post
    Have you verified complete proper operation of the transformer load break switch?
    Yeah we did, and because the cables (unlanded from the B phase of that particular transformer) and the transformers all passed acceptance testing, they ended up landing all of the cables, energizing the circuit with all of the load breaks closed and immediately regretted it.

    I'll be interested to learn what a visual inspection from inside the transformer reveals.
    I suppose the lesson to the EPC is "If the cables test fine unlanded but the VLF test won't start with them landed, find the problem with the transformer before proceeding."

    Click image for larger version. 

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