What’s on the horizon for PV plant controls?
By: Adam Baker
So far, the demand for advanced controls has been pretty minimal in small utility-scale solar projects. But that’s all about to change.
Utility grid stiffness requirements and increases in PV plant size will bring output limiting, VAR control, volt regulation, ramp rate control, and battery storage to the forefront of solar controls. Ultimately, it means a more robust SCADA control design and implementation.
But, no matter what location, size, or grid you operate on, the biggest problem with high penetration solar is ramping. That’s why the biggest change I predict for 2018 is an increased desire for ramp rate controls.
1. Optimized ramp rate control
When rapid changes to renewable output occur, it’s difficult for traditional generation schemes to deal with.
That’s why all >50 MW PV sites are required by their utility to implement ramp rate control at the site level. Historically this has not applied to ramping related to changes in fuel (i.e. when a cloud blows over the site and departs). Cloud movement was traditionally considered unavoidable, and no requirement existed to limit the ramp back to active power setpoint after cloud departure.
We are moving to a world where this will no longer be the case.
However, there’s a big problem with today’s typical ramp rate control. It actually deoptimizes solar sites. If inverters are limited to ramp no faster than the site’s ramping setpoint, the site control system will not be able to ramp the inverters with extra capacity faster than those losing capacity. This means site output will be slower than ideal and an overall loss in potential energy dollars.
By optimizing ramp rate controls, sites will allow inverters to compensate for underperforming areas of the site (modules covered by clouds, inverters offline, etc.) Technically, some inverters would individually ramp at a faster rate than the site’s ramping setpoint, but as a whole the plant would never ramp beyond its overall ramp rate limit.
The cost? If an average PV site has ramp rate control and average cloud cover, the cost to optimize ramp rate control would pay for itself within one year.
2. Optimized inverters for ramp rate control
Having worked with dozens of different inverter platforms, I know not all inverters behave the same when commanded by the central control system. Small differences in lag time between a change in active power command and inverter response make a one-size-fits-all control plan impossible. The timing of controls for each brand and line of inverter must be tailored to that particular device.
Let’s go back to ramp rate control. If inverters each have a setpoint, but their individual response behaviors are each a little off…how far away from the ramp rate limit does that make your plant each time you ramp up?
A control system integrator with intimate knowledge of solar inverters should be able to answer these questions:
- How can I ensure the control system can command ramping in order to maximize energy based on each inverter’s behavior?
- How should we best manage the inverters to keep site output as close as possible to the ramp rate limit?
Tweaking control systems to each inverter’s idiosyncrasies is necessary to squeeze every penny out of your plant during ramping.
3. Optimized battery storage
Battery storage will start gaining broader acceptance in 2018 for both new and existing solar sites. Battery storage is the ultimate PV energy optimizer. It allows a site to behave like a firm, dispatchable energy source. It can be used to eliminate the variability induced by scattered clouds, and allow for a smooth ramp when shutting the plant down at the end of the day.
Further, with most battery systems’ ability to deliver both active and reactive power, the capability of delivering VARs is a further benefit that increases the value of the systems working together.
Battery storage requires a control system to coordinate the charging and discharging of batteries, and management of inverter output increasing and decreasing. Coordinating the charge/discharge requirements with inverter capabilities to maintain a smooth output at the POI is a challenge.
Right now, when irradiance drops quickly, you can’t do a thing about it. Battery storage will completely change ramp rate control.
How will sites start using battery storage to optimize ramp rate control?
- Use inexpensive energy in the middle of the night to charge batteries.
- Start commanding inverters to ramp up quickly in the morning, and start using batteries to get the site to max output earlier in the day than irradiance would normally allow.
- Take advantage of storing what would otherwise be lost as clipped energy during peak irradiance hours.
- Use stored energy to inject on the grid during windows of reduced irradiance.
- Use stored energy to extend the ramp rate at the end of the generation day for a more orderly shutdown, thus playing nicer on the grid.
Often, energy late in the afternoon is worth more than in the morning. As a site optimization strategy, sites with battery storage may choose to store 100% of their morning energy until the site is fully operational and batteries are full (around 11 am). Once the sun starts its descent around 3 pm and price of energy is up, the site may choose to send the morning’s stored energy to the grid later in the day when the value of energy is higher.
Time shifting of energy towards the periods where it is in higher demand should result in lower energy cost throughout the day, but those that embrace this revenue strategy early will benefit the most. The longer the PPA term, the more you stand to benefit.
As more and more sites take advantage of battery storage technology during peak demand times, demand will go down and the price of energy will level across the day…but we have a lot of work to do before we get there.
Take advantage of solar control resources
The end game of every solar farm in 2018 should be optimizing their site by taking advantage of the solar control resources they already have.
If you have ramp rate control, how can you optimize it? If you have a 2018 plan for technology upgrades, how can you fit in battery storage?
Let us know how we can help optimize your solar site with controls by contacting us!
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.