Transmission operator limitations shouldn’t stop you from considering solar.
By: Adam Baker
We recently finished an interesting solar application project with a small utility co-op that may help pave the way for other small utilities who desire to get on the solar bandwagon. In all my years in solar, I’d never seen this done before, but it’s quite simple and ingenious.
Here’s what happened: The controls our engineers provided actually helped meet requirements in the co-op’s interconnection agreement with their transmission provider. Ultimately, I think this process can enable co-op utilities to use solar in applications they may not have considered because of limitations they have with their transmission operator.
Including solar is difficult for some smaller utilities
In our recent project, the solar farm in question was just down the street from a substation where the utility buys their power from the transmission authority. That substation has one dedicated circuit for the solar site coming in, and several feeders that go off into their service area.
One of the primary requirements the co-op has with their transmission authority, is that they’re not allowed to export power back onto the transmission system. (The power coming in from transmission can’t be less than zero.)
This presents a challenge for the small utility, because if transmission suddenly drops but the solar site is running strong, the solar site’s power will be forced onto the transmission line, breaking that agreement.
Solving exporting requirements with controls
The small utility installed a meter that measures how much power comes in on the transmission line. Affinity Energy engineers set up a secure wireless connection between the substation and solar site. Through a SEL RTAC, we monitor that meter information to verify what the substation is taking in from transmission.
We agreed with the utility that we had to pick a minimum level of power coming into the meter. We chose 1MW as our reasonable buffer. If we set the buffer too low (say, 100kw) and there was a sudden load change, there might be an instantaneous export of power from the substation back onto the transmission line.
As the solar farm’s controls monitor that meter, any time the incoming power goes below 1MW, the site automatically starts curtailing active power from the site’s inverters.
Overall, it’s a pretty straightforward control loop that trims site output in order to maintain the minimum buffer.
In the middle of summer when solar panels have the most irradiance and the farm is at it’s maximum capability, all the energy it generates is being consumed on the load side of the substation.
But when there is less demand for air conditioning and heating during spring and fall seasons, solar output before and after mid-day would have eaten into that minimum 1mw input.
This is where the new controls system really shines.
How is this control different than large utilities?
When a 20mw, 40mw, or 100mw solar site is tied directly to transmission, this process of curtailment looks a lot different. A utility operation center (operated by the balancing authority) will dispatch a command to the site to curtail it to some lesser output, say 50%.
The balancing authority’s operation center is a luxurious resource a smaller co-op utility doesn’t have. Although they don’t have a way to dispatch commands through an operations center, the controls system we set up takes its corrective actions automatically without anyone making a single phone call.
In summary
Solar is a great option for smaller co-ops, as long as you implement the controls necessary to meet your contract requirements. As a quick recap:
- In the summer and winter when you have a relatively high demand, you have a low-cost source of energy from the solar site that ties in and allows you to reduce consumption from transmission
- During spring and fall when demand is low, the risk of potentially exporting power from the substation onto transmission (and subsequent penalties or trippings) is mitigated via the controls system
These controls we integrated for this solar site are a low-cost approach that might enable small co-op utilities to implement solar power in an area they might not have thought was practical.
Adam Baker
Adam Baker is Senior Sales Executive at Affinity Energy with responsibility for providing subject matter expertise in utility-scale solar plant controls, instrumentation, and data acquisition. With 23 years of experience in automation and control, Adam’s previous companies include Rockwell Automation (Allen-Bradley), First Solar, DEPCOM Power, and GE Fanuc Automation.
Adam was instrumental in the development and deployment of three of the largest PV solar power plants in the United States, including 550 MW Topaz Solar in California, 290 MW Agua Caliente Solar in Arizona, and 550 MW Desert Sunlight in the Mojave Desert.
After a 6-year stint in controls design and architecture for the PV solar market, Adam joined Affinity Energy in 2016 and returned to sales leadership, where he has spent most of his career. Adam has a B.S. in Electrical Engineering from the University of Massachusetts, and has been active in environmental and good food movements for several years.
