Backfeed stepdown transformer? 2

[snorkles]

A utility provides a facility with 12470 volt 3 phase power.At the remote end of this huge facility is a delta connected sub-station and it's secondary steps down to 480 volts for distribution.

A 3 phase 4000 KW generator (back-up) producing 480 volts has a tie to the secondary side of that remote sub-station. Scenario: a hurricane wipes out utility. My question. Could this generator be connected to secondary side of this sub station, step-up the voltage to 12470 for transmission to the facility's main substation for vital distribution within the facility?

 

[ScottyUK]

Yes, in principle and subject to the limitations of the rating equipment the principle is fine. HV system grounding could be awkward if the HV system normally relies on a ground from the utility - are your transformers ungrounded delta primary?. You may run into problems with the protection relaying, and the utility may have some requirements in order to protect their equipment and personnel. But the basic premise of using an LV generator with a step-up transformer is perfectly valid. If you aren't 100% sure, get someone experienced to help you. 12kV and four megawatts isn't a good place to make mistakes while you learn!


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I don't suffer from insanity. I enjoy it...

 

[waross]

I would suggest for the sake of safety that you disconnect from the utility in a way that is not liable to be reconnected by utility workers trying to restore power after the hurricane.
If you are feeding wye primary transformer banks with a delta supply you may have problems.
You have probably already done a survey and decided how to curtail usage and supply only critical loads.
What is the configuration (primary and secondary connections) of all the transformers the will be utilized in an emergency?
Do you have a protective relay scheme, or just type "K" fuses on the primaries?
I am guessing that you are planning ahead to be ready for hurricane season, and want to get things ready in case of trouble.

How much power do you want to back feed?
Do you have any spare transformers of any rating available?
If we know what you have to work with, we can probably help with your contingency plans.
respectfully

 

[davidbeach]

I think the two most critical issues have been raised already; the grounding issue that ScottyUK mentions and the utility connection mentioned by waross.

Grounding could be handled by use of a 12470Y-480V delta transformer somewhere on the system. Another option would be to put the 480V delta - 12470Y transformer in parallel with the existing 12470V delta - 480Y/277V transformer and use the new one instead of the old when running on the generator. That would require an open transition between the transformers and would require a larger transformer than one installed only for grounding.

At the point of common coupling (PCC) between your system and the utility, one need an interlock that will prevent closing to a dead utility, or out of sync to a live utility. The utility may require that you not parallel the utility; in which case your interlock should only permit closing the PCC when your system is dead.

If you have any hesitancy about how to implement this, please find a consultant who is familiar with interconnecting generation with the utility and knows the protection issues. Doing a project of this type without all the proper precautions can let a lot of smoke out of a lot of components in the system.

 

[waross]

Hi davidbeach and ScottyUK;
I think that this is a contingency plan rather than a project. Correct me if I'm wrong, please, snorkles.
I imagine that a hurricane recovery contingency plan would not involve purchasing any switching or interlocks. My experience with hurricane recovery is that in the event of a hurricane in the next 10 or 15 years, the plant crews would follow the contingency plan that is being developed and remove the fuses, and fuse holders at the point of common connection. I would hope they would also remove some connecting jumpers so that if the fuse holders and fuses are inadvertently replaced by utility line crews working 20 hour days to restore service they do not energize the private system.
If the main transformer is wye connected on the primary I am concerned with energizing it with a delta system without a neutral. Hence my query about the existing transformer configurations. Hence also my query about any transformers available on site.
I would expect that if the local standby set at the far end of the facility is 4mw that the local load is close to 4 mw and the power available to back feed will be much less than 4 mw. It may be possible for snorkles to find or aquire some 333 KVA single phase transformers to use in place of of a three phase transformer on which we anticipate problems. We can run three single phase transformers with a floating neutral with less problem than a three phase transformer with a floating neutral.
My hurricane experience suggests that if the utility is out, snorkles' crew will be busy repairing their own downed lines. I would imagine that if he can restore service in 24 or 36 hours, he will be the local hero.
Given the severity of last years hurricane season and the continuation of global warming, everyone in the hurricane belt is nervous about next season.
snorkles can probably get by with severely curtailled usage and minimal protection until utility service is restored. Something like depending on the generator main breaker to protect the generator and type "K" fuses for the transformers.
Let's find out what he has to work with and see what we can do to help him with his emergency plan.
One of the things that impressed me with the power of a hurricane was seeing a section of pole about 6 or 7 feet long with both ends broken and a secondary rack bolted to it. (The conductors had been cut off to clear the street.) The force of the wind on the primary conductors had broken the top off the pole. Then the force of the wind increased and broke the pole again below the secondary location.
Respectfully

 

[KJvR]

My experience with LV generators connected to MV system is limited to industrial plants using mostly high impedance earthed MV networks (11kV):
(a) Have a Zig-Zag transformer connected to the 11kV bus at the step-up transformer (switchable to simplify protection). Only connect during black-out.
(b) Size of Zig-Zag Tx must provide same earth fault current than what the utility would provide (again to simplify protection).
(c) MUST disconnect from the utility in event of utility restoring power (as already stated).
(d) Make sure of insulation levels on MV network (design for efective or non-efective earthed system).
(e) Might consider changing IDMT protection settings (or use numerical relay with double settings) during black-out if you still require effective discrimination.

As far as I understood from previous threads on this forum is that Zig-Zag transformers is not commonly use in America. Therefore, you might ignore this if you are in America but it is still a cheap alternative elsewhere.

 

[snorkles]

Thanks all for your time! And yes, my question to the forum, is due to the need for contingency planning.
Regards

 

[tinfoil]

Run this one past your local utility. They will likely have some rules and guidelines for you to follow.

For example, they will probably want you to contact them before you isolate, and may even insist on issueing 'guarantees' that must be surrendered before you or they attempt to reconnect to the normal supply. They may insist that only they can isolate with their 'experts', unless you have proper switchgear that allows 'laymen' to isolate properly.

 

[steelerfan28655]

As a "local utility" I can say that we have peak shaving gensets set up this way. Our generators are 480 volt diesel units that backfeed through a 480 volt pad mounted transformer,this bumps the voltage up to 12,470 then its on through a set of handle operated gang switches, then on to the bypass bus of our station. We also use thes as our contingency generators for our hospital circuits, restaraunt, and hotel circuits.
Every time there is an ice storm here we have to be very conscious of the generators that are on our system. We do have industrial customers that have backup generators, and we require automated disconnects that will not allow the gen to operate with the switch closed in to the utility.

Happiness is a way of travel, not a destination.

 

[tinfoil]

'peak-shaving' and 'black-start' generators require different control equipment, and not all generators are equipped with both. I inferred from the original poster's message that he? only wanted to 'black-start', so I only spoke to the 'isolation' requirements.

At the end of the day, only your local utility or regulator can state what your particular requirements are.

 

[waross]

Hi snorkles;
There is one point of your contingency plan that I am concerned about.
You may have a probem energising the transformer at the generator location. If so, close the breaker to the transformer and bring the generator and the transformer up together. Depending on your exitation scheme you may do this by just starting the gen set with the breaker closed, or you may start the generator and then energise the field.
You may even connect cables directly from the generator to the transformer. You may have to jumper or disable some trips.
This is not my main concern.
My main concern is the other transformers and transformer banks on the 12,470 volt system. If all your transformers have delta primaries there should be no problem. There are however, problems feeding a wye primary transformer from a delta system.
Hence my questions earlier as to the configuration of your system and the availability of spare transformers.
We are glad to help with your plans and you can backfeed in principal but there may be a few technical issues to consider.
Another question, does your 12,470 volt distribution system include a neutral conductor or just three phase conductors?
How much of your 4000 KW capacity would you be backfeeding in an emergency?
In responce to the valid concerns that some posters have expressed as to issues with the utility company:
I was involved with the emergency response and temporary measures following a hurricane that wiped out virtually an entire small utility. (Very small). The very few wood poles that were not broken by the hurricane were suspect and replaced. 100% of the wood poles were replaced.
At one plant, we were backfeeding the plant from the on-board generator of a ship moored alongside. Another plant located a new generator and had it rushed in.
A contractor from a neighbouring island sent in a crew of volunteer electricians and an old gen-set, and they quickly rigged up temporary street lights for the security of the community. After a month or so, the old genset failed. I was going for lunch and one of the community leaders approached me and asked what options were available to restore the security lighting for the community. We found the manager of the local utility. We were heading to the local seafood plant (where we had previously installed the generator) to consult with the manager/owner and met him going for lunch. The need for lighting was accepted. With my assurances that the plant had a transformer with adequate capacity and the proper voltage, the plant owner gave permission to feed the lighting from the sea-food plant generator. This street corner meeting took about 5 minutes. I was given a small crew and before dark we had power back on the street lights. The system was used for 3 or 4 months until the regular utility system was rebuilt.
The point is, in a serious hurricane emergency, the utility is usually very happy to see a large customer cut themselves free and fend for themselves. It takes a lot of pressure off the utility. snorkles may find himself aproached by the utility with a reqest to backfeed a nearby critical load such as a clinic or small hospital.
respectfully

 

[snorkles]

OK. So everyone agrees that in principal , this genset could be a backup to the local utility (meeting local's standards)
Good points have been made and are being well received. This is a good start for sure!

Waross , using OCPD in place, the main station could continuously be provided with 900kW. The feeder has 3 ungrounded conductors and a bond. I suspect all Xfmers are delta primary.

Regards

 

[waross]

Hi snorkles
If all the primaries are delta there should be no problem in principle. It would be well to check the transformer nameplates to be absolutely sure. A delta primary distributes unbalanced loads on the utility system better than a wye primary, but delta primaries are more expensive than wye primaries and or not as common as wye primaries.
The connections to the ungrounded conductors are pretty obvious. A wye point connection to the "bond" cable may not be so obvious.
What size is the transformer at the main station? You may have to have that connected also when you bring up the genset to avoid excessive inrush curent.
Another option is to install three 333 KVA transformers and leave them float on the secondary side of the line. Use a 100 amp load break switch on the primary or three fused cutouts. The main transformer at the main substation would have to be disconected from the 480 volt bus and then the 3 x 333 KVA transformers will happily supply 900kw. I've energised a similar bank off of a 350 kw generator. The 4 MVA set should have no problem.
I should mention that if the idea sounds so good that if there is a desire to use it for all power failures rather than just a hurricane contingency plan, we would be planning a somewhat more elaborate switching scheme. It can still be done, but I wouldn't advise using the hurricane emergency plan to cover a 2 hour power failure.
My recomendations for a one time hurricane contingency plan are to a much lower standard than I would accept for a backup system that is going to be used for every power failure. One instance is the time to implement. For hurricane recovery, I would accept 2 to 8 hours to get back on line and consider you a hero if you did it in under 2 hours.
For power failure standby I would look at a time range of between 20 seconds and 5 minutes.
Either way is no problem as long as we are on "the same page at the same time".
Anyway, please have someone dependable check for sure the transformer primary connections and the KVA ratings, and if possible the percent impedance ratings.
Are there just the two substations or are there other loads along the line?
respectfully