The underground neutral transmission system is not recommended because of the following reasons:

1. Insulation being overstressed due to overvoltage: In an underground neutral transmission system, the insulation of the cables is subjected to overvoltage stress due to the capacitance between the conductor and the ground. This can lead to insulation breakdown and failures.

2. Insulation overstress may lead to failure and subsequent phase to phase faults: When the insulation of a cable in an underground neutral transmission system fails, it can lead to a phase-to-phase fault, which can cause significant damage to equipment and the power system. This can result in power outages, damage to equipment, and safety hazards.

3. Being inadequately protected against ground fault: In an underground neutral transmission system, it can be challenging to detect and protect against ground faults. This is because the ground fault current can be distributed across multiple cables and may not be detected by conventional protection systems. This can lead to prolonged outages, damage to equipment, and safety hazards.