sizing neutral grounding reactors for dg plant 3

[60cps]

i am currently designing a dg plant utilizing 6 2500kva generators at 15kv. this plant will parallel with the grid at a municipal electric incoming sub. the sub is 15mva 69kv to 5kv. the muni proposes to upgrade the muni feeder to 15kv in the near future and cut the 69kv feed. i am proposing to use a 15kv/5kv transformer on the generation, close transition to the 5kv muni side of the sub to put the system in operation until they up grade to 15kv. i am proposing reusing the 5kv ucb after their up grade. my problem is how do i size reactors for my generator sets so they can be used on the 5kv system near term and reutilized on the 15kv system after the muni upgrade to their distribution?

 

[scottf]

Figure out if there is an impedance value that would work in both cases. If so, use the highest voltage, current, and short-time current rating resulting from either install for the rating of the reactor.

If one impedance value won't work, you can look at getting a reactor that is tapped or buy 2 different reactors. Somtimes getting a tapped reactor adds a good bit of $, so getting 2 different reactors may be cheaper/better technically in the end.

 

[jwerthman]

If I understand correctly, your generators will be 15 kV now and in the future and you are going to tie to the existing 5 kV system with a 15-5 kV transformer. If that is correct and you specify the transformer as a delta-wye with the delta on the 15 kV side, then the generator grounding reactors won't have any impact on the 5 kV system because the transformer will provide zero sequence isolation. Size the reactors as required for the future 15 kV system.

 

[peterb]

A word of caution re the connection to the utility ultimate 15 kV system - the utility system will probably be solidly grounded, which will impact the application of the generators with reactance grounded neutrals.
Talk to the utility re their system grounding plans - the reactors will still limit the ground fault current contributed by the generators, but the level of ground fault current available for generator internal faults will be decided by the utility grounding method. Usually, you would want to minimize the current level for internal faults, to avoid extensive generator iron damage that could require extensive rebuilding rather than rewinding.

 

[busbar]

A useful reference may be: IEEE C62.92.2-1989 Neutral Grounding in Electrical Utility Systems Part II—Grounding of Synchronous Generator Systems

 

[jbartos]

Suggestion to the original posting: Please, would you clarify the purpose of the reactors. Are they for the short circuit current level limitation or for the system grounding by reactors?

 

[peterb]

Is there a subtle clue as to the purpose of the reactors in the heading "sizing neutral grounding reactors ...."?

 

[jbartos]

I beg your pardon; sometimes I do not get the posting title on the screen.
The 5kV side of transformer, if connected in wye, could be grounded by the high resistance system grounding method, depending on the outcome of the grounding calculations.
The 15kV generator is suitable for the medium resistance system grounding method, although the reactor system grounding method might be applied too. It appears that a concentration on the appropriate system grounding methods would be preferable to saving a relative negligible amount of money.