Spare Transformer Along with Bank of Single Phase Transformer

Use your common sense and thought process, than asking all small issues in forum.

Welcome, Ahuja.

What is mandatory depends upon how critical you consider the load. You must compare the cost of the spare equipment against the cost of a longer outage (production loss, possible equipment damage, etc). A hospital, for example, will have 100% spares, a complete transformer bank already connected and waiting to be loaded. This is called 2N redundancy, where N is the number of units required for normal operation. A small manufacturing facility may have no spares (0N), or a single spare as you are considering (N+1).

Specific points to consider:

- If you do not get a complete spare, the items for which you do not have spares (cooler and bushing) become single points of failure. If the cooling unit is damaged on one of the in-service transformers, the spare won't help you, since you need those items from an in-service unit to make your spare work.

- If you decide not to get the cooling unit and bushing, those items will have to be removed from the failed transformer and installed on the spare before you can restore power to the load.

- Keeping the spare connected to the circuit reduces the time required for replacement, but it makes the spare vulnerable to damage from line faults. Also, since the spare would be in close proximity to the in-service units, a tank rupture on one of those units may damage the spare as well. I keep all of my spares disconnected and stored away from the units in service. To ensure that the spare is in good condition, I perform the same maintenance and diagnostic tests on the spares, at the same intervals, as the in-service units.

Hi Ahuja4,

The common practice in such cases is to provide the spare transformer fully equipped (i.e. radiators, bushings, cooling fans, OLTC, oil conservator, mechanical protection, temperature sensors & indicators, transformer protection, Buchholz relays, etc.).

The transformer should be Hot either from the Tertiary winding if it is a three windings transformer, or from the LV side if it is two windings one.

The location of the spare transformer is in the proximity of the other three single phase units. Where a protective Fire Wall between each phase should be constructed to protect each unit from damage due to explosion or fire of adjacent phase. The height of the fire wall should cover the highest point of the transformer which is the "Oil Conservator".

Regards,

Dear TITO60

Please tell me one more thing:

How do i charge the transformer (Incase of Generator Transformer Rating 21kV for LV Winding). What is the industry practise whether to make appropiate busduct connection or through cable.

Can you give me some reference standard of book name so that i can give reference to my vendor.

Regards

Manish Ahuja

Dear Manish,

Please note that the IEC standards recommend that the single phase transformer is to have additional delta stabilizing winding (Tertiary winding) to reduce zero sequence impedance of the connected system. Accordingly, the delta winding of the spare unit shall be connected to the delta winding of the in service units (other three phase units), through a suitable bus duct system to keep the spare transformer energized all the time.

Regards,