Fault current limitation without reactors

[Harry7653]

Hello Gentleman,

Thanks for taking the time to read my question.

I work in utility Industry. We are building a 66/4 kV sub station with dual winding banks. Banks are 15 MVA 66/12/4 kV. Planning report indicated to build station to 12 kV but cost conversion were high at the feeder level so it was decided to build station to 4 kV.

Station has three banks and two banks are paralleled at all times. Horizon fault levels show fault level of 40kA at feeder head when two banks are tied. High fault level number was missed by the engineer before he agreed to build the station at 4 kV instead of 12 kV ( At 12 kV fault level is low).

Station is 80 % build and there is no space for reactors. G&W CLIPS were option but it changes the whole station protection.My standards group tells me that distribution equipment (fuse, fuse cutouts ) outside the station at 4 kV can not handle fault more than 12.5 kA. Station egress is all 1000 MCM cable before it becomes over head. Most of the egress outside the station is cable up to few hundred meters.

I have asked my protection group to tell me the exact location at each feeder when the fault level becomes less than 12.5 kA. An example would be feeder 1 fault level drops below 12.5 kA after 400 meters from the station. I have reommended like to upgrade all the equipment to a level between the station eqress and the point when fault level drops below 12.5 to handle fault current of 12.5 kA. I have agreed to use operation constraints with operating no banks in parallel.

I would appreciate if somebody can comment on this possible scenario. I think cable would be subjected to high fault current but that is the risk I have to take.


Does the solution to upgrade all equipment to limit fault current to horizon 20 kA (when no bank is parallel).
Thanks a lot for your input.

 

[cranky108]

Current limiting fuses are a possibility.

 

[pwrtran]

Try as much voltage upgrade to 12kV as you can and use step down transformers for 4kV load

 

[dfdt]

Could you please explain why installation of CLIP devices changes the protection scheme.As fas as I know these pyrotechnic based current limiting fuses operate in 2 to 4 ms time frame whereas inst protection devices operate in 20 ms time frame

 

[jghrist]

The preferred solution IMHO would be to not parallel the transformers if the feeders can be split up without overloading any of the transformers. This prevents having to worry about the substation bus, circuit breaker, and feeder cable fault capacity. When the feeders are ultimately upgraded to 12 kV, the paralleling restriction can be lifted. I assume that since you have dual voltage secondaries, that the station is insulated for 12 kV.

Pwrtran's suggestion of converting parts of the feeders to 12 kV would also work, but you would have to buy the step-down transformers.

 

[Harry7653]

The problem is that protection design is complete and relay settings are installed in the switchgear. CLIP will need to be wired back to the feeder relays for protection reasons as well as for indication on the HMI and station control centre. Real estate in the station is a problem as it is 85% already built. This device has never been used in our utility before so lot of apprehension to install something new that nobody as worked before with. It is all being done because distribution devices can not handle fault current more than 12.5 or 1 kA at 4, 12 and 24 kV voltage level.

12 kV is not possible because of cost prohibitive reasons.

I am thinking of a possible solution. No parallel operation of banks and upgrade all devices to handle 20 ka fault current, even install 24 kV device on a 4kV system as long as it can handle 20 kA fault current.

Could you please also comment on any current limiting fuse that can handle 20 kA.

Thanks

Harry.

 

[dpc]

Standard GE EJO-1 9F62 fuses (now Ferraz Shawmut, I guess) are rated for 50 kA, IIRC.