The basis of rating a NGR in electrical system

NGR stands for Neutral Grounding Resistor. When an earth fault current occurs on a plant, assuming that there is no external device presented to limit the earth fault current, the magnitude of the earth fault current is limited only by the earth impedance presented between the point of fault (to earth) and the return path (typically a star point of a transformer). If the earth impedance is low (type of soil being one of the reason amongst others), the fault current magnitude can be significantly high, and if left unchecked could damage the primary equipment. It is therefore mandatory that the earth fault current be limited to a suitable value, which is typically the rated value of the plant as a thumb rule. Why use the rated value? Because the plant has been designed to carry the rated current continuously.

Let's take an example: say you have a transformer 60MVA, 132/33kV Star-Delta transformer. It is required to calculate the value of NGR to be connected to the zig-zag transformer on the 33kV Delta. the value of the resistor required to limit the earth fault current to the transformer's LV rated value is (33 x 33) / 60 = 18.15 Ohms.

(Earth Fault current limited to rated value = (60 x 1000) / (1.732 x 33) = 1050A) When you go to a supplier you might find he supplies only 20 ohms resistor (as you might not get the exact value that you have calculated theoretically). No problem, use the 20 ohms and calculate what your new value of earth fault current would be (33 x 1000 / (1.732 x 20) = 952.6A, which is less than the transformer's rated LV current. So you're safe. This is how I would go about. In fact I would go a step further and introduce a safety factor of 20% i.e. I'll bump up the value of the resistor from 20 ohms by an extra 20% and buy a resistor/ NGR of 1.2 x 20 = 24 ohms. So I am 100% sure that the earth fault current is way below the rated value and my transformer will be safe, even if the fault current goes undetected for any unforeseen reason say my earth fault protection has failed to pick up.

Make sure however that the earth fault setting that you choose is sensitive enough to pick up for the earth fault current calculated. I would generally put two relays a 64 or REF designed to pick up and operate instantly backed by a 51N with a sensitive setting but with a delay of a couple of seconds to pick up in case the 64 has failed to pick up.

So that's it. I have described how I would go about calculating the earth fault current, selection of NGR value and how I would protect it.
Protection and related devices aiding protection don't come cheap. Also I assume by your comment "this method is the most expensive option available since the cost of the transformer shall be astronomical", you are referring to the Zig-Zag transformer and not the actual 132/33kV Star-Delta power transformer, under question.

I have taken a very generic example and tried to focus on how to arrive at a suitable value of an NGR, assuming an Star HV and Delta LV. My aim being to calculate how I could limit the fault current on the Delta LV. Being a Delta winding, I have to use a Zig-Zag transformer, for providing a low zero sequence path for the flow of earth fault current. It is really the Zig-Zag trafo. that bumps up the cost.

Note: If the above transformer is one of a kind, i.e. this is the only transformer in an isolated network, then I simply disregard the Zig-Zag transformer + NGR method and use the 3 PT broken delta method for 3Vo detection to drive a 59N. My cost here would be very low.

If the transformer is a Star-Star type with HV start solidly grounded, and LV star impedance (NGR) grounded, then I don't need a Zig-Zag trafo. on the LV side. My cost is purely for the NGR alone.(Of course this transformer will have a Delta tertiary which may need it's own protection depending on the whether one plans to load the tertiary or not. We could discuss this bit separately).