132/33kV YNd Transformers in Petrochem

Dyn is preferred and YNyn is more common at 132/33kV level. YNd will mean higher voltage stress in 33kV system during single phase-to-ground faults necessitating use of cables and surge arresters having hiher specifications.

So why then most of the petrochem installations I have seen recentrly use YNd transformers for these voltage levels? That's the thing I don't understand...

It could be stipulation from grid authority!!

Grid authorities don't allow YNyn transformers in the consumer premises for thefear that an uncleared earth fault in 33kV system within consumer premises could cause trip of 132kV OHL(s).

However, there should not be a problem from their side if the trafo vector group is Dyn. You may like to check.

The Dyn trafos are generally costlier than Ynd trafos, when compared one-to-one.

That's what I thought initially. But the installations I am looking at operate in island mode. They are remote and don't have any connection to the grid so the grid code does not apply...

I realise that Y primary needs less insulation so it's cheaper...

Does the fact that the 33kV system supplies power to three winding transformers of large VSDs have to do anything with it (reduction in harmonics)?

If not, the only thing I can think of is that YNd transformers are used for phase shifting purposes or just because they are cheaper (although a zig zag earthing transformers needs to be added)...

By the way, does the 33kV have to be resistance earthed? Is solidly earthed acceptable?

Does the fact that the 33kV system supplies power to three winding transformers of large VSDs have to do anything with it (reduction in harmonics)?

Yes,you are right.Delta secondary winding is meant for harmonic reduction.

The Zig zag earthing transformer connected to the Delta Secendary side is meant to obtain a STAR point for system earthing.The same can be resistance earthed or solidly earthed .The Earth fault protection of the system may be designed accordingly.

Check to see if they have a resistor in line with each of the Zig Zag transformer grounds. They use the Zig Zags for reference point for the protective relays for the undervoltage, overvoltage, and 50/51's. You also see these similar set ups in Water Treatment Plants. You are correct in that cost is probably a factor. Maybe ferro issues? I don't know if this set up would be more or less beneficial in terms of preventing ferroresonance in terms of loss of a phase and a floating reference point. Search under this topic in the forum. Usually ferroresonance takes place upstream of this setup at the main feed point where the utility comes into the main plant due to capacitance of the long cables and other factors and because they are too cheap to purchase the right type of input transformer for isolation.