Smart Inverter Pilot Testing Helps Prepare the Future of the Grid
North Carolina has more than 8,000 megawatts (MW) of installed solar capacity and is projected to add another 1,700 MW over the next five years. As more renewable energy comes online, it is becoming increasingly important to ensure the electric grid can handle it. One technology designed to play a role is the smart inverter.
Smart inverters, when working correctly, can enable more solar energy to be added to a distribution circuit. They do this by helping to manage reactive power, allowing solar plants to absorb or inject it and allowing utilities to better manage the voltage on a distribution feeder.
With traditional inverters, solar power added to a feeder would raise the voltage on the line, limiting the amount of solar that could economically connect to the grid, or forcing the renewable resource to reduce its output or shut down completely. Smart inverters let those feeders accept more full-output solar by helping to manage the feeder voltage.
The most recent version of IEEE 1547, IEEE 1547-2018 — the standard for interconnecting distributed energy resources to the electrical distribution grid — requires that solar sites have the capability to actively regulate voltage. Therefore, as more utilities implement IEEE 1547-2018, it is critical that solar plants are safely and correctly constructed to have this functionality and use smart inverter technology.
Working with Duke Energy, our renewables team is participating in a pilot project to evaluate the performance of smart inverters and their ability to control reactive power. The pilot consists of seven sites, with two completed to date and two underway.
At a high level, the testing procedure explores whether a solar farm can absorb reactive power to limit voltage rise from increased solar power being delivered to a distribution line. Before testing gets underway, our team connects a portable power quality meter and satellite clock to Duke Energy’s meter at the point of interconnection, enabling us to collect high-resolution information on voltage, current, real power, apparent power, reactive power, power factor, etc. Data is obtained every three seconds, showing in real time how the tests are proceeding.
From there, the testing method — which is conducted under normal grid conditions — typically consists of three components.
- Reactive Capability Test: Assesses whether a solar plant is capable of absorbing and injecting the specified reactive power while simultaneously producing rated power of its own.
- Reactive Control Volt-VAR Test: Evaluates how the power plant controller (PPC) — the brains orchestrating the smart inverter processes — responds to voltage changes on Duke Energy’s grid and whether it quickly adjusts its reactive power as required to help maintain Duke’s distribution circuit voltage.
- Reactive Control Volt-Watt Test: Evaluates how the PPC effectively reduces its real power output (in watts) in response to excessive grid voltage.
The site visits and testing we’ve finished so far have produced lessons learned that can benefit solar developers and utilities.
Power Plant Controllers Should Be Designed to Facilitate Testing
In addition to a smart inverter, a PPC is required to be able to manage the reactive power contribution from the solar plant. Not all distribution-connected solar sites have PPC, however, and beyond that, not all PPC are designed with the user interface to run tests (such as being able to override real voltages with test voltages). Without that functionality, a separate test set and meter need to be brought to a site to conduct testing.
Power Plant Controllers Need Tuning
Even if a site has a PPC that can incorporate testing, it may not be sufficiently tuned initially. Specifically, solar plants may have difficulty adjusting to changes in line voltage. On an initial test, a site may be too slow to adjust reactive power in response to changing grid voltage, or it may overshoot or undershoot and increase reactive power too much or too little. An untuned controller may also have difficulty maintaining a stable reactive power operating point. Tuning the controller settings enables the plant to quickly reach a correct and stable reactive power value without substantial overshoots or undershoots.
Smart inverters are poised to play an essential role in the growing penetration of renewable resources and represent just one area our renewables team is exploring. Let us know if you have questions about our pilot, our findings to date or our other projects.
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