What is in this article?:
Utility-scale photovoltaic (PV) power plants that support grid stability and reliability are becoming available as PV generation grows to the point of making a significant contribution to the grid, and “grid-friendly” features are clearly needed.
Figure 1: 290MW PV Plant with Grid-Friendly Plant Controls
A typical PV solar generation plant is composed of multiple individual “generators” connected to the electrical network via power electronics (inverters). Through sophisticated control functions, the PV plant can contribute actively to grid stability and reliability and operate effectively in the grid.
A task force under the aegis of the North American Electrical Reliability Corporation (NERC) has made several recommendations on specific requirements that such variable generation plants must meet in order to provide their share of grid support. These recommendations address grid requirements such as voltage control and regulation, voltage and frequency fault ride-through, reactive and real power control and frequency response criteria in the context of the technical characteristics and physical capabilities of variable-generation equipment.
Below, we describe our concept of a “grid-friendly” PV plant that incorporates these recommendations. The “grid-friendly” PV plant also includes the ability to ride through specific low and high voltages or low- and high-frequency ranges. A number of plants with these features are in operation and field data from First Solar-developed utilityscale PV plants are used to illustrate the concepts.
Power Plant Controller Architecture
A key component is the plant-level controller. It is designed to regulate real and reactive power output from the PV plant, such that it behaves as a single large generator. While the plant is composed of individual small generators (or, more specifically, inverters), the function of the plant controller is to coordinate the power output in order to provide typical large power plant features such as active power control and voltage regulation (through reactive power regulation).
The plant controller provides the following plant-level control functions:
- Dynamic voltage and/or power factor regulation of the solar plant at the point of interconnection (POI)
- Real power output curtailment of the solar plant when required, so that it does not exceed an operator-specified limit
- Ramp-rate controls to ensure that the plant output does not ramp up or down faster than a specified ramp-rate limit, to the extent possible
- Frequency control to lower plant output in case of over-frequency situation or increase plant output (if possible) in case of under-frequency
- Start-up and shut-down control
The plant controller implements plant-level logic and closed-loop control schemes with real-time commands to the inverters to achieve fast and reliable regulation. Typically there is one controller per plant that is controlling the output at a single high-voltage bus (referred to as POI). The commands to the plant controller can be provided through the SCADA HMI or even through other interface equipment, such as a substation remote terminal unit (RTU).
Figure 2 illustrates a block-diagram overview of the control system and its interfaces to other devices in the plant. The power plant controller monitors system-level measurements and determines the desired operating conditions of various plant devices to meet the specified targets. It manages capacitor banks and/or reactor banks, if present. It manages all the inverters in the plant, ensuring that they are producing the real and reactive power necessary to meet the desired settings at the POI.
When the plant operator sends an active power curtailment command, the controller calculates and distributes active power curtailment to individual inverters. In general, the inverters can be throttled back only to a certain specified level of active power and not any lower without causing the DC voltage to rise beyond its operating range. Therefore, the plant controller dynamically stops and starts inverters as needed to manage the specified active power output limit. It also uses the active power management function to ensure that the plant output does not exceed the desired ramp rates, to the extent possible. It cannot, however, always accommodate rapid reduction in irradiance due to cloud cover.