Generator Interconnection Transmission Line Grounding

[proznick]

Looking for design philosophy with repect to transmission line grounding. We have two distributed generators feeding 7km and 8km of interconnection transmission line to the substation of a third generator site where they will tied and bumped up to 138kv. The output of this 12MVA 13.8kv/138kv Trans is tied to the 138kv side of anothter 20MVA 13.8kv/138kv Trans being fed from a 3rd generator. Both 13.8kv/138kv transformers are d-y grounded on utillity side, delta on the generator side and reactance grounding on the generators to limit fault currents.

As long as the generators are on line both the transmission lines being fed from the first two generators have ground reference.

If either generator goes off-line, the respective transmission line is floating. Any ground faults exeperienced will produce overvoltages on the un-faulted lines stressing the insulation on the transformer.

My question is, if the transformer isolation can withstand the expected overvoltage, is it better to leave the transmission line on the delta connection or to utillize a ground refence scheme at the substation for these transmission lines providing a ground reference at the substation and at the generator 7km away?

Utillity is happy witht the D-Yground. Is there any benefit to the IPP for using a Yground-Yground with delta teriary which would outweigh increase in cost versus cheaper tansformer D-Yground config.


Any thoughts on this appreciated.

Thanks

RP

 

[davidbeach]

13.8kV generators connected directly to the lines? Ugly bad; don't even think about that configuration. Put transformers at the generators, even if they are 13.8-13.8kV transformers so that you can have high resistance grounding for the generators and a solidly grounded line. Otherwise you are exposing the generators to all sorts of nastiness from the line.

If you do go forward with your proposed design, you have to treat the line as ungrounded, no phase-neutral/ground loads tapped from the line. All insulation for phase-phase voltages and all lightning arrestors need be rated for phase-phase voltages.

I also would not even consider two transformers in the same transformer diff zone, let alone two transformers of different sizes. Add three more 387's, so that you have two per transformer, the relays are really inexpensive compared to the damage they can prevent. You also need line protection for the lines from Gen 1 and Gen 2; with the fiber line I'd look at the 387L with something else as backup. Using the 300G as line protection is asking too much. I'd also through in a second 300G at each machine.

 

[slavag]

Hi Proznick.
Could you please4 send us SLD, it's not so clear.
Regards.
Slava

 

[proznick]

Thanks for the response David,

Not shown on the single line is the resistor on secondary of generator grounding transformers providing high resistance grounding for the generators. Installing extra 13.8kv/13.8kv transformers at G1 and G2 won't fly with clients so will work with two power transfomers in substation as shown in single line.

Your comment on the differential and backup protection is noted, thanks. Had plans to rework diff-prot. As far as distance and gen protection for G1 and G2, I wanted to nail down the transmission line, grounded or ungroounded when generators offline first.

Grounding at both ends versus grounding at generator only (generator hi-z in both scenarios). I have found some information on pro's and con's of each configuration, appears both methods are in use. For grounding at both ends have been looking at PT in a Ygrnd-Ygrnd with delta tertiary config or Ygrnd-Delta with Zig-Zag ground scheme. Trying to research problems of having two ground references on a trasnsmission line separated by several kilometers.

Fresh out of school and in the business for a couple months, looking for some perspective on Tx line grounded versus ungrounded. The protection can be worked out in either case.


cheers,

RP

 

[proznick]

Slavag,

The SL attached as pdf

cheers,

rp

 

[slavag]

Sorry, I miss attached file.
OK, about trafo diff 87T, I fully agree with David.
Preffered two separated 87T protection.
387-1 will be connected to additional CT between 1CID2 and
trafo T1, this CT will be include two core , one for the trafo diff protection and second for the 138kV busbur diff protecton.
Same you have provide for the trafo T2.
Lines to the G3 and G4 you have protect with 87L ( with fiber optics), as David said.
About redandancy for the generator protection, up to your customer, we always prefferd used second protective relay ( for me full duplicated systems are preffered).
As I wrote above, additional diff protection will be protect zone on the 138kV level.
Regards.
Slava

 

[slavag]

Some additional Q.
Please take in account how will provide maintanace of the systems.
From this point , you have decide where will connect diff protection 87L, before CB or after CB.
Regards.\
Slava