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The unknown part: The unknown part

It seems like a regular transformer, but is it? What's its function? Why is it grounded?

The diagram itself:

enter image description here

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It is a neutral grounding transformer with a secondary loading resistor to effect high-resistance grounding of that big (1220MVA) synchronous generator. It connects from the neutral (star point) to earth. The drawing is incomplete, it doesn't show the neutral side of the primary being connected to earth but it is in reality.

The zero sequence reactance of synchronous generators can be significantly smaller than the positive and negative such that phase-ground faults can be significantly higher than 3-phase faults. So, we restrict the current that can flow in the neutral.

Below is a three-line view of what it looks like. The 59N is a fundamental frequency neutral overvoltage relay that is used to detect phase-ground faults from the terminals (and buswork) down to within about 5-10% of the neutral.

enter image description here

Below is a picture of one of these neutral grounding transformers (distribution class) along with the secondary loading resistor (this is from a nuclear unit about the same rating as your machine).

enter image description here

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This is is a smaller DT than the generator power and is technically called an NGT or neutral grounding transformer with higher impedance stepdown into a burden load of much lower power and voltage than the fault. It serves many purposes for fault conditions.

The voltage generated indicates the distance from the source of a line fault but it cannot protect the transformer from faults near the source so the impedance is much higher (>50x) than the generator neutral so Kirchoff’s Law creates a voltage divider with the line fault distance at much reduced currents.

from @Relayman ‘s link, there is often much more protection added to the burden R, including OVP and 3rd harmonic UVP and subharmonic fault injection current to protect the stator and measure leakage capacitance.

Without this the line phase fault extends into core saturation and the fault currents will increase abruptly as L goes near 0 from saturation and more expensive failures would occur. So it offers some additional fault protection.

many thanks to @relayman357 for correcting my errors, My limited experience exceeded the fault trip threshold. ;)

My claim to experience was only limited to correcting invisible epidemic Partial Discharge manufacturing flaws as a consultant in otherwise perfectly working 5MVA transformers for wind farms that were failing for level3 DGA H2 absorption within 1 year on warranty and performing Hipot to 200kV AC and DC with no clues But then to discover the flaw using a spark plug immersed in a tank of oil with 3 hr latent PD activity that was in contact with the core at one time yet passing 25KV/mm BDV tests. this led to the cause of all the H2 gassing faults. ... “invisible contamination” to the naked eye. from there , the list of solutions was easy.

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    \$\begingroup\$ No it isn't. It is a neutral grounding transformer to effect high-resistance grounding of a synchronous generator. \$\endgroup\$ Aug 23 '20 at 20:13
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    \$\begingroup\$ That too cause it is 3ph neutral ground. But why high impedance for 500A series. Y to Delta 1220 MVA, 25KV. With neutral imbalance current monitored \$\endgroup\$ Aug 23 '20 at 21:10
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    \$\begingroup\$ I don't understand what you are asking. The phase current on this machine is 28kA at full load. The phase CTs are probably 30,000:5A (6000:1). A ground fault in the machine is probably 10-15A. It is bug dust compared to normal load current. You can't measure that with 6000:1 CTs. Hence, we have other methods for detecting ground faults in the machine (that make use of the high-resistance grounding). \$\endgroup\$ Aug 23 '20 at 21:32
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    \$\begingroup\$ What is the problem with sensing 30kA. I have monitored up to 100kA arcs in diffusion bonding welders. You want Neutral earth bonded at lowest impedance so the CT is very low impedance. \$\endgroup\$ Aug 24 '20 at 4:48
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    \$\begingroup\$ The generator is high-resistance grounded. A ground fault is only 10-15A. You are not looking for 30kA. You are looking for 10-15A of zero sequence (a needle) in the midst of full load current of around 30kA (a haystack) of positive sequence. Here is link to good reference. \$\endgroup\$ Aug 24 '20 at 13:11

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