Generator Backfeed

[Cableman]

The project is adding a generator tap box to an existing switchgear.
MDP, PD-1 east and PD-1 west are in one enclosure as the sketches depict and not the drawings
DS-G1 and DS-G2 will be mounted next to each other and almost equal distance from MDP

The design incorporates and Kirk key system that will not allow the generator and the utility to feed the load at the same time

"backfeed concern.pdf" depicts a concern by the engineer that the current derived from the generator will backfeed to the generator (as depicted by his arrows). The reviewing engineer does not see think it will (he did sketches A and B)
"mark-up of sld 1.pdf" shows better what the proposed new equipment is to be installed (in green highligher)
"sld sketches A.pdf" shows the sld with utility powr a bit clearer
"sld sketches B.pdf" shows the sld with generator power a bit clearer

Please let me know your opinion and feedback. thanks for the help

 

[ron]

As long as the kirk key is operated, the two boards will be paralleled and you don't even need the second path (DS-G2) from gen to board. Just DS-G1 will do it.

 

[hillbilly1]

He is correct in his concerns, the Kirk key should be in the 400's, not the main due to the way the generator feed is tied in.

 

[Cableman]

He is correct in his concerns, the Kirk key should be in the 400's, not the main due to the way the generator feed is tied in.

Current should not flow unless there is load. If there is load then, the current should flow to the load. Do you see a problem with sketch B?

 

[templdl]

Unless I am completely missing something the original oneline is OK.

Unless I am completely missing something the original oneline is OK.

I guess I'm a bit confused with the trem "back feeding." What I'm seing here is simply a loop feed which I can't see an issue with. With the MDP laod interupter open the Utility power is disconnected where you now are able to remove the KK locking the switch in the open position, taking that key to the DS-G LI switch which would now allow you to close the LI. The key stays captive until the switch is mobved to the open position at which time is can be removed.

Essentially, with the MDP open and the DS-G closed you are simply feeding both DSG-A and DSG-2 from opposite directions. Think about it. If the line between DS-G And DS-G2 were omitted the entire load to DS-G2 would be feed through DS-G1 being fed through Line 1 and Line 2 to DS-G2.
Are the (3) 500MCM cables from DS-G and the bus of DS-G1 rated to handled the additional load of DS-G2 is you eliminated the (3)500MCM loop that is though to be "back feeding" the generator.

Under normal power with MDP closed and DS-G open Line 1 has been sized to feed DS-G1 w/ (3) 500MCM and line 2 DS-G2 w/ (3) 500MCM.
As such would (3) 500 MCM from DS-G feeding DS-G1 and the Bus in DS-G1 and the capacity to supple the load as required by DS-G2 if you removed the (3) 500MCM that is marked as "Back feed to generator?
Essentially what you are doing is not relying of the (3)500MCM cables between DS-G and DS-G1 and DS-G1's bus to feed DS-G-2. Also, the (3) 500MCM loop as provided allows for some diversity should you elect to isolate DS-G1 or DS-G2 from the generator.

I think it's a sensible design by covering loads as require by both pieces of gear with the capability of isolating either one.

 

[Cableman]

As long as the kirk key is operated, the two boards will be paralleled and you don't even need the second path (DS-G2) from gen to board. Just DS-G1 will do it.

from your comment, do you also think there is a concern that the current will backflow to the genrator when it is under generator power, as hillbill1 is?

 

[hillbilly1]

Because your Kirk key is in the 800 amp main as drawn, either or both 400 amp switches can be closed effectively parralleling the 400 amp generator switches if both are closed. You need to either tie to the 800 amp feed only, or do a dual Kirk key set up on the 400's so that either one cannot backfeed the other, curious how you will Kirk key the single 800 amp, with two 400 amp switches? I think you will find that cannot be done.

 

[hillbilly1]

You need to move your Kirk key lockout from the 800's to the 400's. Match the keys to the corresponding switches, they both do not need to be keyed the same. You do not need to lock out the main 800 and the 800 amp generator breaker, only the 400's.

 

[Cableman]

I guess I'm a bit confused with the trem "back feeding." What I'm seing here is simply a loop feed which I can't see an issue with. With the MDP laod interupter open the Utility power is disconnected where you now are able to remove the KK locking the switch in the open position, taking that key to the DS-G LI switch which would now allow you to close the LI. The key stays captive until the switch is mobved to the open position at which time is can be removed.

Essentially, with the MDP open and the DS-G closed you are simply feeding both DSG-A and DSG-2 from opposite directions. Think about it. If the line between DS-G And DS-G2 were omitted the entire load to DS-G2 would be feed through DS-G1 being fed through Line 1 and Line 2 to DS-G2.
Are the (3) 500MCM cables from DS-G and the bus of DS-G1 rated to handled the additional load of DS-G2 is you eliminated the (3)500MCM loop that is though to be "back feeding" the generator.

Under normal power with MDP closed and DS-G open Line 1 has been sized to feed DS-G1 w/ (3) 500MCM and line 2 DS-G2 w/ (3) 500MCM.
As such would (3) 500 MCM from DS-G feeding DS-G1 and the Bus in DS-G1 and the capacity to supple the load as required by DS-G2 if you removed the (3) 500MCM that is marked as "Back feed to generator?
Essentially what you are doing is not relying of the (3)500MCM cables between DS-G and DS-G1 and DS-G1's bus to feed DS-G-2. Also, the (3) 500MCM loop as provided allows for some diversity should you elect to isolate DS-G1 or DS-G2 from the generator.

I think it's a sensible design by covering loads as require by both pieces of gear with the capability of isolating either one.

So if i understand you correctly, I dont have to make any modification to the kirk key system and single line. What do you think of hillbilly1 concern?

 

[Cableman]

As long as the kirk key is operated, the two boards will be paralleled and you don't even need the second path (DS-G2) from gen to board. Just DS-G1 will do it.

so under the current layout, is it o.k? if I want to leave DS-G2 as is?

 

[hillbilly1]

so under the current layout, is it o.k? if I want to leave DS-G2 as is?

You will be limited to 400 amps as drawn. You would no longer have the parrallel potential though. If you want to keep the 800 amp feed, you will have to tie it straight to the load side of the 800 amp main, then you can leave the kirk key system as is, otherwise kirk key both 400 amp sub mains.

 

[templdl]

So if i understand you correctly, I dont have to make any modification to the kirk key system and single line. What do you think of hillbilly1 concern?

The KK operation in this scheme is quite basic as I have designed some that were much more complicated which involved multiple LI switches PFC capacitor enclosures, transfer schemes, etc.
The important part of a KK scheme is that you walk through the process.
1) The MDP LI sw. can only be removed when the sw. is in the OFF position. The LI sw. can not be closed unless the KK is in the lock. When you have a utility power failure walk over to the MDP and move the LI sw. to the off position and remove the KK.
2) Walk over to the DS-G LI sw. place the key in the lock which now allows you to close the LI sw. The Key should not be able to be removed while the switch is closed. Now you can supply the emergency power with the generator.
3) When the normal source returns open the DS-G LI sw. remove the KK, walk over to the MDP place the key in the lock which allows you to close the LI sw.

Also, I apologize as I did not reference the switchgear correctly. They should have been PD-1 East and PD-1 West
The name of the game is that if you have the key in your hand both the MDP and the DS-G LI switches will be in the disconnected position.. If either switch is closed the KK will be held captive in the switch. Also, there will be no duplicate KKs as they are registered. Only one key is provided for an operation such as yours so don't loose it.

And disregard any thought of something being back fed as you are simply feeding both sets of gear from either end and they are tie together in the middle forming a basic loop feed configuration. As I mentioned before I don't believe that single set of (3) 500mcm cables from DG-G to PD-1 East as well as the bus in PD-1 East would have the capacity would feed PD-1 West also. Thus, that set of (3) 500 MCM that you think would be back feeding the generator is actually feed PD-1 West from the other end at the same time which is a solid design.

Yes, this is to hillbilly1's point as he is spot on for the reason that I have stated in my previous post an reiterated in this one.

Those (3) 500 MCM cables going from DS-G to PD-1 West also allow the facility to isolate either lineup from the generator should you only want to supply one lineup at a time if it may be required.

 

[hillbilly1]

Technically your not back feeding the generator, you are just parralleling the 400 amp breakers if all four 400 amp breakers are left on, thats why you need the Kirk keys on the 400 amp breakers instead of the 800 amps. This would prevent parralleling through the common tie to the 800 amp main.

 

[tallgirl]

Fortunately, electricity is fast enough that the generator cannot be back-fed by itself.

For all of those arrows going backwards, there are other current arrows going forwards. They cancel.

 

[hillbilly1]

Fortunately, electricity is fast enough that the generator cannot be back-fed by itself.

For all of those arrows going backwards, there are other current arrows going forwards. They cancel.

Yeah, that's a misnomer, depending on switch position, it would actually back feed the other section of gear. The problem is parralleling the 400's effectively increasing the oc rating to 800 amp for a 400 amp buss. The right way of doing it is to move the Kirk keys to the 400's if the existing wiring is to remain as drawn.

 

[templdl]

Fortunately, electricity is fast enough that the generator cannot be back-fed by itself.

For all of those arrows going backwards, there are other current arrows going forwards. They cancel.

Yes, and thank goodness that back feeding isn't an issue when paralleling conductors that are of slightly different lengths.
And then there is the loop feed distribution system which is quite commonly used with pad mount transformers.
The generator energizes both current paths equally as it doesn't favor one path over the other.