Replacement of Transformer

The supply cables and their circuit protection device(s) require assessment. In the UK, BS7671 contains algorithms that can be used to evaluate what to do. These algorithms are based upon the cable type and length, its method of securing, whether it is thermally insulated anywhere and by how much, etc., etc.

I would prefer to put a new cable even if existing ones are well in size (and they are suit 1800 kVA trans, imho)

- you are to retrays these cables partially and relocation is not good for insulation of used cables (how old them are) ,

- will length of existing cables suit you ?

It is a good electrical engineering practice that when you have multiple runs of cables to or from a feeder/equipment, all the runs must be of the same cross section.

It is a VERY bad idea to parallel cables of different sizes. The conductor resistance and the cable impedance both will act to unbalance the load between the cables.

It does not appear that the cables are sized based on the load of your present installation. A 500KVA load draws 26A at 11KV. The 95 mm² cable is rated for 300A, and the 185 mm² cable is rated for over 400A. In a long cable run, the conductor size may need to be increased to account for voltage drop, but this still seems excessive. If your 500KVA transformer was 5,000 meters from the pole, you'd still have 10.965KV at the transformer.

The new 1800KVA transformer will draw 95A at full load, less than 1/3 of the smaller rating. I would feel comfortable using either existing cable by itself to supply the new 1800KVA transformer.

If you ask these questions, you are not qualified to perform the work.Refer this work to a qualified company for the safety of all concerned.

Selection of HT Cable is based primarily on the Fault Level of the Substation from where you are getting the supply, the required ampacity comes next which more or less gets taken care of when Fault Level is considered.

No Using the Old Cable is not recommended 1. due to different cable sizes, fault current carying capacity is different 2. The age of cable to be considered and 3. No. of Joints in the cable to be checked.