transformer

No it doesn't. If you want it, you can have it but it costs and adds to the complexity of the transformer (more things to maintain).

Transformers this small are commodity items - relatively inexpensive and commonly available. The technology is quire mature and reliability is good. Buchholz relaying is found more commonly on larger units where down-time is costly, as well as the cost and availability of a replacement is more difficult. On these larger units the faster and more sensitive tripping may allow the unit to be repaired rather than thrown away, but on a 500kVA if it fails, you buy a replacement.

Periodic (annual) dissolved gas analysis (DGA) will give you enough data and history to plan for an orderly outage for investigation and repairs if gases increase. A fault will should trip your other protection such as overcurrent and differential (if you have it).

Vide Indian Electricity Rules, 1956, Rule No. 64A Sub-Rule 2 (c):

64A. Additional provisions for use of energy at high and extra-high voltage.-

The following additional provisions shall be observed where energy at high or extra-high voltage is supplied, converted, transferred or used, namely:-

(2) Protection – All systems and circuits shall be so protected as to automatically disconnect the supply under abnormal conditions.

The following protection shall be provided namely:-

[(c) Gas pressure type and winding and oil temperature protection to give alarm and tripping shall be provided on all transformers of ratings 1000 KVA and above;]

But, it is prudent to provide Buch Holz Protection for a 500kVA Transformer, considering the repair & dwontime costs.