Replacing MR On Load Tap Changer.

Two separate issues. If the tap changer has the same part number and passes all the required tests then swapping should be OK.

The transformer that tripped on differential failed the TTR most likely has failed, so you need to make sure that its tap changer was not contaminated by the now dirty oil.

Thanks RAMConsult for your comment on the swapping of tap changer. For the transformer that tripped on differential here`s what I have done; 1. Selector switch removed from tap changer compartment. 2. Inspection OK and clean switch contacts. 3. Wash down with new oil, selector switch and tap changer compartment. 4. Replace new oil. The TTR test improve but not OK, transformer trips again on differential current.

The transformer has internal damage. Take an oil sample for DGA then empty tank and inspect for exact location of damage.

<...Can I replace the faulty selector switch?...> CR4 is not a permission-giving entity, nor does it assess competence. If in doubt, contact the equipment manufacturer directly for advice, initially over the telephone, as time is valuable. If necessary, arrange for a Service Technician to call and effect the remedies required.

<...what could be the cause?...> Failure to maintain the settings of the circuit protective device(s) and the quality of fluids used throughout the life of the equipment to date in accordance with the equipment manufacturer's instructions.