Using y-y generator transformer for step up and step down

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denvar

Member
Location
Nairobi Kenya
Hi, am designing a small hydro plant 800 KVA (two 415V generators using a single step up transformer) to be connected to the distribution system at 11KV. Am facing a challenge deciding which kind of step up transformer (0.415/11 KV) to use since local supply (30 KVA) both single and three phase is required. Am evaluating the following options:

a) (a) Use star-star step up transformer which enables connection of single phase loads easily but will have risk of imbalance, will pass zero sequence faults and will suffer third harmonic problems.

b) (b) Use delta-star step up transformer which is the most convenient for protection purposes but then connect another yyn0 isolation transformer on generator bus bar to supply the 30 KVA load.

c) (c) Use delta-star step up transformer but then use another 11/0.415 KV step down transformer for local supply.


Since cost is a major issue am leaning towards option (a) but am not sure the effects of using a y-y transformer especially if there are unbalanced loads. Kindly assist.
 

Phil Corso

Senior Member
Denvar:

1) I suggest a Ynd step-up xfmr, with 'd' as the 415V winding, and 'Y' the 11kV winding!

2) Undoubtly, the generators are 3ph, 4-wire, units and their neutral connections available for earthing!

3) The local 30kVA load should be connected to a 3-ph, 415V, 4-wire load-bus, with 3 incoming CBs; 2-Gens, and Xfmr!

Notes:

A) The 11kV winding's neutral connection should be earthed (solidly or impedance) depends on the length of the transmission-line,
and whether or not it is already earthed!

B) Hopefully the generators will have some reactive kVAr capability, i.e., 0.80-0.85 Power Factor !

Regards, Phil Corso
 

denvar

Member
Location
Nairobi Kenya
Denvar:

1) I suggest a Ynd step-up xfmr, with 'd' as the 415V winding, and 'Y' the 11kV winding!

2) Undoubtly, the generators are 3ph, 4-wire, units and their neutral connections available for earthing!

3) The local 30kVA load should be connected to a 3-ph, 415V, 4-wire load-bus, with 3 incoming CBs; 2-Gens, and Xfmr!

Notes:

A) The 11kV winding's neutral connection should be earthed (solidly or impedance) depends on the length of the transmission-line,
and whether or not it is already earthed!

B) Hopefully the generators will have some reactive kVAr capability, i.e., 0.80-0.85 Power Factor !

Regards, Phil Corso

Thank you Mr Corso. The generators are synchronous with KVAR capability and the 11KV line is `10KM long.

My challenge with the ynd step up is how to create the neutral line (4 wire bus) for the local load from the generator terminals considering the load should be supplied from the grid when booth generators are off i.e cannot use the generator neutral for the load.
 

Phil Corso

Senior Member
Denvar, reur grounding dilemma when both gensets are out of service:

I suggest the use of a Zig-Zag Xfmr or a Dy distribution Xfmr connected to perform a like function. Do you need help with sizing?

Phil Corso
 

denvar

Member
Location
Nairobi Kenya
Denvar, reur grounding dilemma when both gensets are out of service:

I suggest the use of a Zig-Zag Xfmr or a Dy distribution Xfmr connected to perform a like function. Do you need help with sizing?

Phil Corso

Thank you Mr Corso for your response. Between Zig zag transformer and having an extra small dry type xfmr which is the better option as per your experience. I dont think i have a challenge with sizing but have a challenge with grounding scheme.
 

denvar

Member
Location
Nairobi Kenya
Denvar, how about a sketch of what you propose?

Find attached.
Can you ground the generators at their star point and at the same time ground the zigzag neutral or will neutral earthing contactor be needed to disconnect the generator neutral ground when its connected to the xfmr.
Transmission line is 10KM long will solid HV earthing be enough
thank you for your assistance
 

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Bugman1400

Senior Member
Location
Charlotte, NC
I think you'll need some type of isolation xfmr with a neutral on the load side to serve the single-phase loads. I don't think its typical to have a zigzag on the generator side of the GSU since that could supply extra ground fault current and defeat the purpose of the high-impedance grounding system on the generators. Seems like a 50kva xfmr at that voltage level would be relatively cheap.
 

Phil Corso

Senior Member
Denvar... definitly not what I had in mind!

4) Why are the gen-sets resistance-grounded? Is there a mandated Ground fault-current restriction?

5) If the LV-side of the Step-up Xfmr was Delta-connected, then the Z-Z can be much smaller! After all its only function is to provide a neutral for the local-load when the Ge-sets are both unavailable!

6) Have you gotten prices for a 800kVA Z-Z Xfmr?

7) Yes, the Z-Z can be contactor-controlled!

8) I believe you have the classic "Tail-wagging-the-dog" situation! Why not just provide a D-y, 1:1, xfmr for the local and skip the Z-Z!

Have I misunderstood your intent?

Phil
 

denvar

Member
Location
Nairobi Kenya
Denvar... definitly not what I had in mind!

4) Why are the gen-sets resistance-grounded? Is there a mandated Ground fault-current restriction?

5) If the LV-side of the Step-up Xfmr was Delta-connected, then the Z-Z can be much smaller! After all its only function is to provide a neutral for the local-load when the Ge-sets are both unavailable!

6) Have you gotten prices for a 800kVA Z-Z Xfmr?

7) Yes, the Z-Z can be contactor-controlled!

8) I believe you have the classic "Tail-wagging-the-dog" situation! Why not just provide a D-y, 1:1, xfmr for the local and skip the Z-Z!

Have I misunderstood your intent?

Phil

Drawings are just rough sketches not working designs

4)The resistance grounding is not mandated but may be a good idea for generator rated 0ver 800A
6) They are not available in the market.
8) a D-Y 1:1 isolation xfmr for local supply will create a 30 deg phase shift of generator output,won't that be an issue?

You understand my intent clearly thank you.

Can the two generators share a common ground? i.e link their neutrals to a common grounding point. is it a good practice to do so?

Thank you
 

denvar

Member
Location
Nairobi Kenya
I think you'll need some type of isolation xfmr with a neutral on the load side to serve the single-phase loads. I don't think its typical to have a zigzag on the generator side of the GSU since that could supply extra ground fault current and defeat the purpose of the high-impedance grounding system on the generators. Seems like a 50kva xfmr at that voltage level would be relatively cheap.

Agreed seems like the better option,just have querry on vector group of the local supply xfmr that wont cause a vector shift i.e yyno
 

Phil Corso

Senior Member
Denvar, reur query on common grounding connection! The simple is Yes and No!

Yes... if you never have to perform maintenance on one while the other is in service!

No ... if you have to perform maintenance on one while the other is in service!

Phil
 

denvar

Member
Location
Nairobi Kenya
Denvar, reur query on common grounding connection! The simple is Yes and No!

Yes... if you never have to perform maintenance on one while the other is in service!

No ... if you have to perform maintenance on one while the other is in service!

Phil

Thank you Phil
Lets say i was to use generator neutral for local load and the two generators might not be running at the same time all the time, meaning i will need to have a change over circuit from one generator to the next hence the query.
 

Bugman1400

Senior Member
Location
Charlotte, NC
Denvar... definitly not what I had in mind!

4) Why are the gen-sets resistance-grounded? Is there a mandated Ground fault-current restriction?

5) If the LV-side of the Step-up Xfmr was Delta-connected, then the Z-Z can be much smaller! After all its only function is to provide a neutral for the local-load when the Ge-sets are both unavailable!

6) Have you gotten prices for a 800kVA Z-Z Xfmr?

7) Yes, the Z-Z can be contactor-controlled!

8) I believe you have the classic "Tail-wagging-the-dog" situation! Why not just provide a D-y, 1:1, xfmr for the local and skip the Z-Z!

Have I misunderstood your intent?

Phil

4.) I think its common practice across the pond to use impedance grounding on everything.
7.) I have never seen a contactor controlled Z-Z. That sounds like a one-off application.
8.) I had the same idea. A D-y, 1:1 at that voltage level should be cheap. And, the GSU xfmr would be much cheaper without the Z-Z as well.
 

Phil Corso

Senior Member
Bugman,

I worked across the pond for quite a number years, and I only ran into one 415 V system that had resistance-earthing!

The down side to such practice is the need to install earth-fault detection/protective devices, on all 415V circuits!

Regards, Phil
 

Bugman1400

Senior Member
Location
Charlotte, NC
Drawings are just rough sketches not working designs

4)The resistance grounding is not mandated but may be a good idea for generator rated 0ver 800A
6) They are not available in the market.
8) a D-Y 1:1 isolation xfmr for local supply will create a 30 deg phase shift of generator output,won't that be an issue?

You understand my intent clearly thank you.

Can the two generators share a common ground? i.e link their neutrals to a common grounding point. is it a good practice to do so?

Thank you

8.) The 30 deg shift is not a problem if you are just supplying plant load unless there is an alternate source to the same load. In this case you need to verify the phase shift is the same or use break-before-make ATS for the transfer.

Two different generators should never share a common ground! This would cause I2 circulating current and result in overheating of the generator windings.
 

Phil Corso

Senior Member
Bugman,


I disagree! There will always be a circulating-current even if grounding-connections are separated! Even if the generators are "identical!" Circulating-current magnitude is based on the relative differences in generator parameters and operating conditions!


Regards, Phil Corso
 
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