Peak Shaving

[faresos]

Hello everyone..

I'm working on a project that a client has requested to do a peak shaving by using a generator (will be provided in the future). the service is possibly 1200-1600A at 480/277V. our approach is to provide a service rated closed transition transfer switch. My questions:

1. Is the transfer switch the correct approach for this application?
2. I was told by a generator vender that I need to provide a paralleling switchgear. Is there any advantage by using the gear in lieu of the transfer switch?
3. If the client decided to sell back the excess power to the grid, can this be done by the transfer switch or the gear?
4. My understanding that I need to do coordination with the utility to prevent back feeds to the grid? How is that accomplish, is by providing relays at the customer equipment or the utility will take care of that at their lines?
5. In general, does circuit breaker prevent back feeds?

your help is really appreciated

 

[GoldDigger]

Hello everyone..

I'm working on a project that a client has requested to do a peak shaving by using a generator (will be provided in the future). the service is possibly 1200-1600A at 480/277V. our approach is to provide a service rated closed transition transfer switch. My questions:

1. Is the transfer switch the correct approach for this application?

Only if you want to take one part of your load (or your entire load) completely off the grid while the generator is running. It will not allow you to share a load between POCO and generator.

2. I was told by a generator vender that I need to provide a paralleling switchgear. Is there any advantage by using the gear in lieu of the transfer switch?

It will work, that is the main advantage. See #1.

3. If the client decided to sell back the excess power to the grid, can this be done by the transfer switch or the gear?

Not really. That will require more complex equipment installed to the specifications of the NEC and the utility.

4. My understanding that I need to do coordination with the utility to prevent back feeds to the grid? How is that accomplish, is by providing relays at the customer equipment or the utility will take care of that at their lines?

It is usually your responsibility to do this in a way that the utility approves.

5. In general, does circuit breaker prevent back feeds?

No!

 

[faresos]

Thanks for the reply.

For Item 1, what do you mean by sharing load between generator and POCO? as you know, for closed transition we will have both the generator and utility power source paralleling for less than 100 millisecond, does tha means sharing? My self I'm leaning toward the transfer switch because its less expensive and smaller foot print, I just want to make sure is suited for this application. Thanks again

 

[GoldDigger]

Thanks for the reply.

For Item 1, what do you mean by sharing load between generator and POCO? as you know, for closed transition we will have both the generator and utility power source paralleling for less than 100 millisecond, does tha means sharing? My self I'm leaning toward the transfer switch because its less expensive and smaller foot print, I just want to make sure is suited for this application. Thanks again

No, when I mentioned load sharing I meant a full time parallel connection.
But even for a closed transition transfer switch, you may need to have synchronizing equipment on the generator to bring it into phase with the utility power before activating the transfer. In addition to current during the 100 milliseconds, a hot transfer with mismatched phase can cause extreme currents in transformers and some types of motor loads after the transfer is complete.

A standby generator application will typically not have this problem because the original magnetizing currents and motor rotation will have died out before the transfer is made to generator power.

 

[rcwilson]

A transfer switch moves the total load from utility to generator with the risks of the problems already mentioned. The generator must be sized to handle the total facility load including any motor starting or other intermittant duty. That application is not peak shaving, it is more like peak chopping. The system will monitor the load and if theload nears the 15 minute or 30 minute or 60 minute peak, it will dump all the load onto the generator, effectively stopping the utility meter at the peak output reading. When the next demand period starts and the meter resets, the transfer switch dumps all load back on the utility and the generator shuts down. This may have to happen for 1 minute every 15 minutes.

Utility power is usually cheaper than generator power. That is why the system would keep going back and forth. That is a lot of stress on the equipment and a lot of chances for problems.

With paralleling gear, the master controls could monitor the total demand and predict teh peak demandfor the current interval. If the damnd is going to be a peak, the system would start & synchronize the generator through the paralleling gear and have it share the load for the rest of the demand period to shave the peak.
The generator doesn't have to carry the full load, just part of it, so it could be smaller, maybe sized to handle the emergency systems.

Your system is about 1000 kVA. With a transfer switch you will need an 800 kW generator or larger to reliably carry the load. With peak shaving, you might get by with a 250 kw generator.

The transfer switch and generator could be sized to only run part of the load such as HVAC or a process load that is on all the time. The control ssytem would have to anticipate when a new peak demand might occur and transfer part of the load to the generator soon enough to shave the peak usage for the interval.

 

[Hv&Lv]

A transfer switch moves the total load from utility to generator with the risks of the problems already mentioned. The generator must be sized to handle the total facility load including any motor starting or other intermittant duty. That application is not peak shaving, it is more like peak chopping. The system will monitor the load and if theload nears the 15 minute or 30 minute or 60 minute peak, it will dump all the load onto the generator, effectively stopping the utility meter at the peak output reading. When the next demand period starts and the meter resets, the transfer switch dumps all load back on the utility and the generator shuts down. This may have to happen for 1 minute every 15 minutes.

Utility power is usually cheaper than generator power. That is why the system would keep going back and forth. That is a lot of stress on the equipment and a lot of chances for problems.

I won't doubt you since I haven't worked at all utilities, but peak shaving for us and our customers is for reducing the demand so that the higher blocks of generated energy supplied by the peaking plants do not have to be used. A couple of bills at around $8.00 per kWh along with demand charges will make the generator worth it.

Ours never switch back and forth for 1 minute out of 15. We call for generators to run with load control for the entire peak demand time period.

 

[rcwilson]

Ours never switch back and forth for 1 minute out of 15. We call for generators to run with load control for the entire peak demand time period.

I've never seen a system that switched back and forth every 15 minutes either, I was trying to emphasize the advantages of paralleling switchgear versus a transfer switch.

Some utilities pay customers that do load reduction during heavy load times. A transfer switch shifting part of the facility load to the onsite generator for a few hours makes sense in that scenario.

But peak shaving for the customer can also refer to reducing the customer's demand charges by leveling the demand peaks. Some power contracts carry the maximum 15 or 30 minute demand charge for 12 months or more. If the customer load exceeds their previous demand for one 15 minute period, the demand charges are increased for the next year. The only way to control the peak demand is to monitor every 15 minute period and somehow reduce load or shift load when the system predicts the peak demand will be exceeded. In that scenario, a total plant transfer switch might have to switch every 15 minutes. That would be a terrible design. A transfer switch that moved part of the load to the generator could be used to control the peak by running for more of the 15 minute period. With proper programming of the control system and a better understanding of the facility's load profile, the number of starts and stops could be minimized.

With paralleling switchgear, the generator could come on line and share load to maintain the plant demand at the desired level to minimize demand charges. Which method is best depends a lot on the load profile, and the incentives and penalties in the specific utility power contract.

 

[bob]

A couple of bills at around $8.00 per kWh along with demand charges will make the generator worth it.

I hope you did not mean $8.00 per KWH. Perhaps $8.00 per KW demand + KWHs. I would like to know the estimated
saving using this system.