Doble Testing

[aymo]

What is the recommended standard for how often Doble testing should be done on transformers. Also, there is concern in my plant about causing problem hot spots if we disconnect the secondary connections to perform the Doble test. Has anyone done a Doble test leaving the scondary connections in tact and performing the test with the secondary breaker open? Did it skew the results notably from the original base line Doble tests?

 

[sngpl]

Doble testing is recommended once per year for a power transformer. Its recommended to disconnect the secondary connections otherwise you'll get all the equipment up to the secondary side breaker doble tested, which may include secondary side bushing, lightning / surge arresters, secondary side insulators and even the secondary breaker. Normally it’s recommended to get all this equipment doble tested separately so that its test data could be trended and compared later.

I have never attempted to doble test everything together but it definitely should get the results skewed because every piece of equipment has a totally different Capacitance and Doble testing is all about testing Capacitance and Dissipation Factor. So if you have all the Capacitances in parallel you’d get results that are not even near the values for the transformer alone. Why is the capacitance different for the different equipment... because of the different types of insulation or rather dielectric used in the different equipment.

As regards to the HOT SPOTS... I am not sure why do you fear it would hot spots due to testing… is it due to loose connections??? Well first of all, all bolts should be torqued to the specified torque settings. Even it’s highly recommended to check the bolts torque once a year and you’ll be surprised how many loose connections you may find.

Doble Testing (Capacitance and Dissipation Factor Test) if properly conducted by properly trained personnel won’t do any harm to your equipment.

 

[busbar]

Take the case of advanced age or environmental deterioration. If the gear would be rendered inoperable by breaking a busbar or cable joint, it may be in imminent failure mode for any action or inaction, and likely visually bear clear evidence of poor condition. Isn’t it better to find that during a scheduled outage than during full production or routine facility operation?