Reactive Power Control in Utility-Scale PV
Inside this Article
To facilitate higher levels of distributed PV penetration and meet transmission interconnection requirements, inverter-based generators need to participate in utility-voltage regulation by absorbing or injecting reactive power.
Reactive power control may be one of the more confusing topics in the PV industry, but it is an area of significant promise. In this article, I provide a high-level overview of how voltage is controlled in transmission and distribution systems. I also summarize the effect of inverter-based distributed energy resources on electrical power system voltage levels and describe how reactive power control schemes can be used to mitigate those impacts. I discuss the evolving technical and jurisdictional requirements for reactive power control, which account for much of the confusion about this topic. I present some useful information for PV plant designers related to qualifying inverters for use in projects with reactive power control requirements and quantifying the system performance implications associated with meeting these requirements. I compare inverter-level versus plant-level reactive power control schemes as well as reactive power control requirements for transmission-level versus distribution-level interconnections, and then conclude with a brief discussion of compensation for reactive power capabilities. (For an overview of reactive power in ac circuits, see “Reactive Power Primer,”)
The American National Standards Institute (ANSI) defines acceptable (Range A) and tolerable (Range B) service and utilization voltage levels in ANSI C84.1-2011, “American National Standard for Electric Power Systems—Voltage Ratings (60 Hertz).” Electrical equipment manufacturers design their products to operate within the voltage tolerance boundaries described in this standard. Electric power system operators design and operate the utility grid to maintain those voltage levels under normal operating conditions.
Reactive power management is an essential part of how voltage levels are controlled in the electric power system. In effect, reactive power can be generated as a means of raising voltage levels or absorbed as a means of lowering voltage levels. System designers and operators coordinate reactive power compensation devices with voltage regulators—tap-changing autotransformers located at the source of or along distribution feeders—and on-load tap changers, which allow for stepped voltage regulation by automatically adjusting the turns ratio of substation transformers.
In the article “Ancillary Service Details: Voltage Control” (see Resources), Brendan Kirby and Eric Hirst summarize some of the challenges associated with voltage control and reactive power management at the transmission level: “At very light loading the [transmission] system generates reactive power that must be absorbed, while at heavy loading the system consumes a large amount of reactive power that must be replaced. The system’s reactive power requirements also depend on the generation and transmission configuration. Consequently, system reactive requirements vary in time as load levels and generation patterns change.”
Though traditional synchronous generators are capable of injecting reactive power into the system or absorbing excess reactive power—and can do so quickly—the operating and opportunity costs associated with this method of voltage control are relatively high. Therefore utility operators may seek to minimize the exchange of reactive power between the transmission system and generation units, which operate according to a voltage schedule provided by the transmission system operator. In the event of a disruption, such as the loss of a generator or of a transmission line, the synchronous generator’s excitation system will dynamically inject or absorb reactive power as needed until system voltage stabilizes.