With a simple control loop, learn how to automatically avoid exporting power back onto transmission.
By: Adam Baker
98% of the time, solar sites are connected to large utilities like Duke Energy, NRG, NextEra, and Southern Company. These utilities have large in-house capabilities for dispatch management and sending curtailment controls. If you’re interested in how we work in those environments, stop reading now!
Today we’re focusing on the smaller utilities, co-ops, and a solution we put together that allows them to implement solar on their networks at relatively low cost, and still get the reliability required for their interconnection agreement.
Getting small utilities and co-ops on the solar bandwagon
In most cases, small utilities and co-ops don’t have a large, sophisticated central operation center where they monitor conditions of solar sites they own and automatically or manually ramp up and ramp down generation.
They’re really just tied to a transmission network, buying power, and then distributing it through their network to their customers. Without that central operations center, managing generation coming from solar can be a challenge, particularly because of strict interconnection requirements that we’ll talk about it a minute.
Solar is good for small utilities!
The great thing about solar is, the output tends to be in alignment with when demand is highest. Both in summer and winter, heating or A/C loads tend to be highest towards the end of the day. Early in the morning and late in the evening when solar isn’t as effective, the demand tends to go down. Solar is a great source of energy when you want to offset a higher demand.
Solar is also low cost. Once the site is built, the ongoing operational costs of running a site are virtually zero. The cost of the fuel is nonexistent. As long as the sun is shining, you’re making energy.
The reason a co-op would use solar is to reduce consumption from transmission, helping to reduce the overall cost of energy for your customers.
The problem: interconnection agreements
The most impactful and important rule for most co-op purchase agreements is that they are not allowed to export power from the substation back onto the transmission system. Why? The transmission operator doesn’t have any control or visibility over the quantity or quality of power coming back onto the transmission line. This is the most important rule for co-ops.
If it happens, they face fines or a risk that the transmission operator simply disconnects the site from transmission.
Here’s where solar comes in. If solar generates more power than the load consumes, there is the risk that generation could feed back onto the transmission network. This occurs most significantly in the cool spring and fall when the load from heating or A/C is low.
We wanted to come up with a solution that would allow a solar site to curtail automatically based on the conditions that exist in the substation.
Automatic curtailing control solution
We crafted a control solution for an implementation in North Carolina. We used an SEL RTAC 3505 to monitor the values coming from an energy meter inside the substation.
Learn more about why we like SEL RTAC for solar applications.
After looking at those values, we defined a lower threshold of incoming power that would allow us to maintain a minimum amount of headroom/energy coming into the substation from the transmission system.
With a simple control loop, anytime we get below the threshold we’ve defined, we send commands to inverters to reduce from 100% output to whatever level it takes to keep that minimum amount of headroom.
We try to stay as close as we can to the limit to maximize the energy output and reduce the cost of energy the co-op buys from transmission. But at the same time, we keep sufficient headroom so there is no risk of accidentally exporting power onto transmission.
There is no co-op burden to maintain site monitoring, or come up with commands to limit the output. It’s all done automatically, and ensures they never produce more energy than the local network can consume.
Before Affinity Energy was involved, the original plan was to trip the entire site offline anytime there was the slightest risk of feeding power back onto the network. It would remain offline until there was enough demand on the co-op’s network that they felt comfortable with bringing it back online.
This route was certainly problematic for the owner of the solar site who was trying to maximize their revenue.
Low cost approach to solar power
When compared to traditional solar control systems, the Affinity Energy automatic curtailing control solution is extremely low cost. It doesn’t require sophisticated inverter balancing, or even complex functionality. It just requires the maintenance of headroom. Even the components used are relatively low cost.
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.