The Importance of Coordinated Control Systems in Solar Generation Plants

Solar photovoltaic (PV) power plants are emerging across the United States to meet state and local energy portfolio requirements. Coordination of the PV plant and its intertie with the existing distribution and/or subtransmission electrical system is essential for reliable, practical operations. This paper describes a PV plant control system in the field, its operation, and the practicality of solving challenges associated with interconnecting large utility-scale PV installations with the bulk market. In its most basic form, a plant control system monitors the overall operations of the generation plant and the point of interconnection (POI) and, based on the conditions, adjusts the equipment to meet operational, performance, and local interconnection requirements. It seamlessly adjusts the equipment operational points in response not only to commanded set-point changes but also to unpredictable conditions such as a fault or extreme weather. In addition to providing internal plant monitoring and control functions, the system also serves as a single-point interface with external systems, where it supplies plant data and accepts control commands from the area electric power system. This single-point interface simplifies the communications burden on the electric power system while providing the necessary functionality to maintain critical voltage support features for the bulk electrical system. This paper describes how the control system can be integrated, including both the internal and external PV plant equipment and devices, with many available communications protocols involved for each. Internal equipment and devices include PV inverters, weather stations, sun trackers, protective relays, revenue meters, local generators, and alarm systems. The external equipment includes weather forecast systems, power management systems, and supervisory control and data acquisition (SCADA) interfaces, to name a few. This paper discusses in depth the architecture of the plant control. Based on the implemented architecture, the paper demonstrates several different control schemes, including open‐loop, closed‐loop, sequential step, and time constraint-based controls. These schemes cover a wide range of possible applications, and as such, a section of this paper discusses a few real-world examples. In order to achieve reliable and deterministic controls using a variety of communications protocols and associated media, the status indicators internal to the field equipment are leveraged as part of a more complex scheme. In addition to the communications status provided by the protocols, this paper describes how the latency bit (also known as heartbeat) technique can aid in achieving more reliable controls.