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| Electric power systems: secondary networks |
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| AlbertL:
As regards the 20 kW minimum, the wording of the utility's requirement doesn't explicitly limit it to times when the PV system is producing, but since it requires curtailing or tripping the generation in case of a violation, that appears to be the intent: Install controls and relay(s) that monitor the entire load of the spot network on all three phases. The control shall be set up to maintain a minimum power import of 20 kW. If import falls below 20 kW, the control system shall curtail or trip the generation. When this limit is exceeded, response time for curtailment or trip should occur instantaneously. Which of course brings up the question: what bad thing would happen if import falls below 20 kW at night, due to the building simply not consuming much power? A utility engineer did tell me that the reason for the minimum import (as opposed to just zero export) is that if the voltage on one feeder is lower than the other, and there's no load on the collector bus, there can be enough reverse power flow through that feeder's transformer to trip the protector. The minimum import ensures that there's always a forward power flow through the protectors under all normal feeder voltage variations. But again, a feeder voltage differential can just as easily occur at night. I think in the end it comes down to playing the odds. |
| NiHaoMike:
--- Quote from: AlbertL on November 03, 2020, 12:30:28 pm ---Right now I'm working on a project that will put about 70 kW DC (module nameplate capacity) / 57 kW AC (inverter nameplate capacity) on a building served by a spot network. The utility has specified a 20 kW minimum import. Over the past year, the building had a maximum load (or "demand"; the average over a 15-minute interval) of 140 kW and a minimum of 38 kW, so with the solar there's a substantial likelihood of pushing the demand below 20 kW. Furthermore, the building owner plans substantial HVAC energy-efficiency upgrades. The relay requirement is pretty stringent as regards timing: the relay must operate before the network protector (which makes sense of course). I'm having trouble finding out what the protector's "pickup" time setting is, but I've heard it's 9 cycles (150 ms). If by "operate" the utility actually means "clear", then we have to add the opening time of the circuit breaker (or response time of the inverters' external shutdown feature) to our relay's pickup time. Because of the likelihood of minimum-import tripping, we'll install a "curtailment" system that will ramp down the inverter output before net power flow reaches the 20-kW minimum. This has its own set of problems. Solar in DC is a lucrative investment due to a production incentive, currently worth about $0.40/kWh, on top of the purchased electricity savings of about $0.11/kWh, so we don't want to lose any more production than necessary. At the same time, a curtailment algorithm that's not aggressive enough will result in relay tripping that may require manual intervention to reset. --- End quote --- What is the majority of the load? Is there a significant amount that would make sense to try to schedule to run more when solar is available? For example, dedicated freezers or water heaters? HVAC can also be controlled to a more limited extent, perhaps by increasing the fresh air percentage. Could also propose the idea of renting out a little space for cryptocurrency mining or other computing. |
| richard.cs:
--- Quote from: AlbertL on November 04, 2020, 08:26:16 am ---A utility engineer did tell me that the reason for the minimum import (as opposed to just zero export) is that if the voltage on one feeder is lower than the other, and there's no load on the collector bus, there can be enough reverse power flow through that feeder's transformer to trip the protector. The minimum import ensures that there's always a forward power flow through the protectors under all normal feeder voltage variations. But again, a feeder voltage differential can just as easily occur at night. I think in the end it comes down to playing the odds. --- End quote --- Yes, it sounds like they have set an unreasonable "zero reverse allowable, fast trip" setting and a minimum load is something of an ugly workaround. It's not really clear why they have done so, but I guess you're stuck with it. Actually it hints at a bit of a wider issue, the supplier responsibility would appear to end at the output of each separate supply, and the parallel connection is the responsibility of the building owner who then has to work around protection settings outside of their control. Is that the norm in the US? --- Quote from: NiHaoMike on November 04, 2020, 01:42:54 pm ---What is the majority of the load? Is there a significant amount that would make sense to try to schedule to run more when solar is available? For example, dedicated freezers or water heaters? HVAC can also be controlled to a more limited extent, perhaps by increasing the fresh air percentage. Could also propose the idea of renting out a little space for cryptocurrency mining or other computing. --- End quote --- This sounds like a reasonable workaround to me. Do something useful with the energy you're not allowed to export and also helps meet the minimum load. |
| AlbertL:
--- Quote from: NiHaoMike on November 04, 2020, 01:42:54 pm --- --- Quote from: AlbertL on November 03, 2020, 12:30:28 pm ---Right now I'm working on a project that will put about 70 kW DC (module nameplate capacity) / 57 kW AC (inverter nameplate capacity) on a building served by a spot network. The utility has specified a 20 kW minimum import. Over the past year, the building had a maximum load (or "demand"; the average over a 15-minute interval) of 140 kW and a minimum of 38 kW, so with the solar there's a substantial likelihood of pushing the demand below 20 kW. Furthermore, the building owner plans substantial HVAC energy-efficiency upgrades. The relay requirement is pretty stringent as regards timing: the relay must operate before the network protector (which makes sense of course). I'm having trouble finding out what the protector's "pickup" time setting is, but I've heard it's 9 cycles (150 ms). If by "operate" the utility actually means "clear", then we have to add the opening time of the circuit breaker (or response time of the inverters' external shutdown feature) to our relay's pickup time. Because of the likelihood of minimum-import tripping, we'll install a "curtailment" system that will ramp down the inverter output before net power flow reaches the 20-kW minimum. This has its own set of problems. Solar in DC is a lucrative investment due to a production incentive, currently worth about $0.40/kWh, on top of the purchased electricity savings of about $0.11/kWh, so we don't want to lose any more production than necessary. At the same time, a curtailment algorithm that's not aggressive enough will result in relay tripping that may require manual intervention to reset. --- End quote --- What is the majority of the load? Is there a significant amount that would make sense to try to schedule to run more when solar is available? For example, dedicated freezers or water heaters? HVAC can also be controlled to a more limited extent, perhaps by increasing the fresh air percentage. Could also propose the idea of renting out a little space for cryptocurrency mining or other computing. --- End quote --- Those are good ideas. I had proposed something cruder: a resistive load bank, normally used for testing generators and other power sources. As far as the production incentive is concerned, it doesn't matter how the energy is used (or wasted) as long as it's delivered to a building served by the utility. |
| Red Squirrel:
--- Quote from: jc101 on October 22, 2020, 04:37:36 pm ---There is a group who keep an Infrastructure map, like open street maps, but for services which might be of interest to some... https://openinframap.org --- End quote --- Wow that's really cool. I always nerd out at power transmission lines and wonder where they go or come from and sometimes even check google maps. I had no idea we had a 500kv one in my area but I found it when I wanted to get pictures of it and realized it was literally raising my arm hairs. I knew it had to be higher voltage than the other lines I've been around so got on google maps to follow it to the hydro dam and all the way to the GTA. Seeing this map is cool as it does confirm my findings. And if curious this is the 500kv line, I thought the pic turned out pretty cool. https:// i.imgur.com/6VxxQ3K.jpg (need to manually copy and paste url, forum keeps breaking it) I need to go back there one night with some neon tubes to see if they light up. >:D |
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