Be a more valuable generation asset to the balancing authority.
By: Adam Baker
I want to share some control experience with all the Qualifying Facilities (QFs) out there, specifically how to easily implement controls for solar QF sites that help decrease utility curtailment.
Stop saying solar is bad for the grid
There’s concern in the industry that the more generation originating from variable energy sources like wind and solar, the higher grid instability. This is especially a concern in PV-rich areas like North Carolina.
There’s a tiny bit of truth to the issue. A large number of solar sites on the same distribution line powering up instantaneously would cause a shock to the system. Luckily, sites are designed so this shouldn’t happen. Even if this were to happen, controls can govern sites to come and go offline gradually, which would minimize the shock to the distribution system.
This concept that solar is bad for the grid is fundamentally not true. If implemented with the right controls in place, solar can actually be very helpful to the grid.
Why does solar affect grid stiffness?
The issue of grid stiffness occurs based on the design of energy flow - from transmission lines through a substation and out to a distribution feeder to neighborhoods, commercial buildings, and all electricity consumers. It was designed that way 100 years ago. It wasn’t designed with the concept of solar existing down the line of the distribution feeder.
During a good chunk of my time working for First Solar, many sites were built in the Imperial Irrigation District (IID) east of the mountains in southern California. There’s a lot of farmland, wind, and solar generation on the east side of the mountains. It’s a great area for generating power, but there aren’t a lot of consumers of power.
Most of the population lives west of the mountains, so all that energy had to run through transmission lines west towards San Diego and Los Angeles. The problem is, transmission lines leaving the IID area didn’t have enough capacity to take all the energy when it’s both very sunny and very windy.
The result? Some PV sites needed to be curtailed to outputs lower than their design capacity (or shut off altogether) in order to limit the energy flow to the maximum amount the transmission system could handle.
When all solar facilities come online, the wind is at full blast, and congestion is causing a problem with transmission lines, three things can happen:
- Some sites will be left on. Wind sites don’t get significantly curtailed because they don’t run well at a lower output than the wind is trying to push them.
- Some solar sites can be curtailed to a lower output. For example, you can take a 100MW site and command it to 50MW relatively easily…if the site has a controls system.
- If there’s too much generation and sites don’t have the capability to be curtailed, they’re cut off completely. Someone, whether the owner or transmission operator, takes the entire facility offline.
As an owner of a solar power site, you’re most interested in maximizing output all the time. You want to make as much money as you can by keeping the site running at maximum output.
When you have the ability to provide active power curtailment at your site, you’re less likely to be curtailed offline and more likely to be commanded to run at a slightly (or significantly) lower output. If demand goes up or transmission capacity becomes available while you’re in that condition, you’re more dispatchable to increase the active power output from the site on short notice.
It’s better for the utility to have you still running at a lower output than taken offline completely, where you might not be able to start back up quickly.
In short, you’re a more valuable generation asset to the operator.
So how do you avoid curtailment? Have the ability to curtail in an analog manner. It’s not just about turning the site on or off, but the ability to adjust anywhere from 0% output to 100% output.
Let’s revisit IID. When there was an overgeneration situation, solar sites without the ability to do active power curtailment would be shut off first. Then the gas plants would be turned down or shut off.
The solar sites with variable output were curtailed last. They were the most flexible, and therefore most valuable to the balancing authority, so they were chosen to stay on the longest.
How does power curtailment work?
Power meters measure plant output. This number should also correlate with the inverter’s output report.
So, if we want to change a 5MW plant setpoint to 4MW, a control systems integrator can send an updated command to inverters to run perhaps a little higher than 80%. The control process knows how many inverters you have, what the target is, and will command available inverters to give you the desired output. 4MW, 5MW, or 3MW, or any point in between.
As it commands inverters, the control system doesn’t just send a new command at the new value and allow the inverters to step down quickly. It actually ramps the command down over a period of time. 10% of plant capacity per minute is the standard for the large generation industry, but you may want to ramp up faster or slower. Remember, it’s the slow transition of setpoints that allows these solar plants to work well on the grid.
What to use for controls
Historically at solar QF sites, I rarely see a control system unless the interconnection agreement requires it. That doesn’t happen very often.
Typically, QF sites have a recloser controlled by the utility, medium voltage coming into one or multiple transformers, and inverters designed to run at whatever their maximum power output is configured for. This simple setup achieves the output of the plant as designed.
QF sites also have a monitoring system, so there’s infrastructure but no ability to do closed loop, real-time controls.
Fortunately, adding control capability is easy. It’s simply a matter of adding a device that will send the commands to the inverters. It’s not a significant infrastructure change. In fact, it’s a relatively low-cost adder, especially when looking at overall project costs.
RTAC as a controller
The product we recommend for solar QF sites is a product made by Schweitzer Engineering Laboratories called the RTAC (Real-Time Automation Controller). These are often used in large sites as a RTU or data concentrator, which means they’re already well known and well accepted in the industry.
RTAC can do just about all types of control you can do with a PLC. It doesn’t have all the interfaces you use in standard automation where a PLC would be appropriate, but from a functionality standpoint, it can do ladder logic, function block, and structured text programming like a standard PLC.
It’s a data concentrator, but can also be used as a plant controller at the same time.
- The RTAC is PLC-like in its response time. We can do grid voltage control at a 250ms closed loop response time.
- It also does GPS time synchronization and interfaces with external utilities. This means your plant can be tied directly to the balancing authority to take commands and adjust the plant output.
- RTAC talks Modbus, DNP3, Goose protocol, and its own protocol. Any of the standard protocols for devices that exist in your solar power plant, the RTAC can communicate with.
- It’s an extremely inexpensive device (some versions are less than $1,000), much less than many options provided by monitoring companies.
Adding controls to the overall project cost of a solar site is very small. The controls themselves are very inexpensive. The process of reading a meter, taking a command, and distributing commands to devices at the site is very straightforward.
Ultimately, having a system that responds a little slower and is less disruptive to the grid allows solar QFs to play friendlier with the neighbors.
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.