Reactive Power Control in Utility-Scale PV: Page 4 of 11
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California’s Rule 21. In January 2014, the Smart Inverter Working Group published “Recommendations for Updating the Technical Requirements for Inverters in Distributed Energy Resources,” its recommendations to the California Public Utilities Commission (CPUC) for updating the technical requirements for inverters in distributed energy resources (see Resources). The intent of the CPUC’s efforts is to proactively implement new inverter functions that have proven technically feasible and beneficial to grid operators in Europe. In so doing, the CPUC hopes to avoid some of the power system emergencies experienced in Europe associated with high-penetration levels of variable energy resources.
The Smart Inverter Working Group recommends that the CPUC require seven autonomous inverter functions, including two specifically related to reactive power: dynamic reactive power control and fixed power factor operation. As shown in Figure 2, the group suggests that inverters be capable of dynamically injecting or absorbing reactive power in response to local voltage measurements to help maintain voltage levels within their normal ranges or improve the efficiency of electric power systems. It further recommends that inverters be capable of operating with a fixed power factor—0.85 lag to lead for larger inverters—to enable utility operators to compensate for other loads that generate reactive power, as well as to improve circuit efficiency.
Though inverter-based reactive power control requirements are still evolving, they have immediate implications for many industry stakeholders. According to Tobin Booth, CEO of Blue Oak Energy, “Utilities have started to require power factor control to compensate for rapid changes in the weather, which can strain the utility grid power quality in certain locations.” As a result, Blue Oak Energy is now configuring projects to control power factor, to gain interconnection approval where utilities would otherwise have denied it.
Ryan LeBlanc, senior application engineer at SMA America, notes: “Our experience shows that most, if not all, major utilities are now addressing power factor in one way or another on PV plants from 500 kW and up. The list of utilities with reactive power requirements that we’ve worked with includes Hawaiian Electric Companies, Los Angeles Department of Water & Power, National Grid, Pacific Gas and Electric Company, San Diego Gas & Electric Company and Southern California Edison. Most of our experience is on the distribution network, and utilities typically require reactive capabilities of 0.95 or 0.90 lag-to-lead power factor at the point of interconnection.”
Today’s utility-scale inverters include many advanced features, including reactive power support, that allow better interaction with the grid. According to SMA America’s LeBlanc:
“Inverters are very sophisticated power electronic devices that can quickly take a command and adjust to the desired power factor. Inverters are also an increasingly economical source of reactive power. The cost of inverters is reducing at three or four times the rate of costs for traditional var compensation devices, like STATCOMs and capacitor banks.” LeBlanc points out that in many cases these capabilities are critical to project success: “By using advanced inverter functions to provide reactive power support, system designers and developers can eliminate the cost and complexity of providing power factor mitigation by other means, which can cause projects to be delayed or canceled.”