Today, the power system is entering a new era where variable renewable energy (VRE) sources such as photovoltaics solar (PV-Solar), wind power, and battery energy storage systems (BESS) are increasingly introduced in the power system due to environmental and economic reasons.
Unlike traditional generators that spin synchronously with the grid frequency, PV-Solar, wind, and BESS are connected to the grid using power electronics, which is why they are referred to as Inverter Based Resources (IBRs). The shift from rotating electrical machines to power electronics will reduce the available inertia in the grid. With this transition, a grid with lower inertia may experience higher rates of change of frequency (ROCOF), leading to the potential for larger frequency deviations and decreased grid stability.
The two main utilizations of IBRs fall under two categories - grid-following (GFL) and grid-forming (GFM) controls. Most inverter applications today fall under the GFL configuration.
In GFL applications, the IBRs are tied to the electrical grid (locked), “follow” the measured voltage, and use it as a reference. It provides P (real power) and Q (reactive power) by adjusting their output to track and external voltage reference. Essentially, current is regulated based on setpoints to the Inverters, and GFL IBRs are designed to prevent them from operating a disconnection from the Grid (Islanding).
In a GFM application, inverters operate independently from a grid connection and are allowed to initiate the microgrid voltage and frequency supplying system loads. P and Q commands are not used in microgrid in islanding mode. All reactive power (Q) is driven by microgrid voltage deviations from nominal voltage and all real power (P) is driven by frequency deviations from nominal frequencies. GFM IBRs are capable of islanding and the control references are voltage and frequency setpoints dependent on the load.
Areas with a high concentration of GFL IBRs compared to traditional synchronous generators may experience electrical system instability. The primary cause of this instability is the limited availability of inertia. Inertia represents the stored kinetic energy that is provided by traditional synchronous generators, allowing them to respond to grid frequency events caused by contingencies and imbalances. As the grid transitions to IBRs, the responsibility for establishing and maintaining grid voltage and frequency falls on the inverters themselves. These inverters must possess grid-forming capabilities, enabling them to generate a waveform at a specified frequency. By doing so, they can serve as the foundation for a synchronous AC power system.
According to NREL, GFM inverters are already used in many zero-inertia microgrid systems (typically less than 10 MW) that do not use synchronous generators. These microgrid systems may employ one of the inverters to provide the master voltage and frequency waveform (grid-forming) with others acting in grid-following mode. In a zero-inertia power system, inverters would need to possess the ability to independently establish and maintain grid voltage and frequency, even in the absence of an explicit communication network.
Grid-forming technology has been used for a long time in microgrids and smaller islands. However, recent advancements are making it possible to use multiple GFM IBRs in larger grids to enhance the reliability of system operations. This is particularly beneficial in areas where there is a larger concentration of IBRs and/or where traditional generators are being phased out.
GFM IBRs, supported by firm energy sources like BESS, can effectively deliver services that have traditionally been provided by synchronous generators. These services include inertia, stabilizing weaker grid areas, enabling islanding operations, and facilitating black start capabilities. When GFM IBRs are used, the stabilization of the system is provided by the generation resources themselves as they are added to the system. This also helps mitigating stability related transmission constraints quickly, offering a cost-effective solution.
According to a research white paper published by Australian Energy Market Operator (AEMO) on application of grid-forming IBRs, four specific applications were identified for operating a gigawatt-scale interconnected power system with limited or no synchronous generators online.
Connecting IBRs in weak grid areas: Capability to maintain stable operation in weak grid areas to meet IBR performance obligations and potentially to provide system strength to support the connection of other nearby IBR plants. This application provides localized capability to stabilize nearby IBR generation but does not necessarily support the broader power system.
Supporting system security: Capabilities to maintain system security that are predominantly provided by synchronous generators today, such as inertia and system strength, to support the broader power system as it transitions to operating with fewer synchronous generators online.
Island operation: Capabilities to maintain stability and supply balancing at a high enough level to support areas of the grid that become separated from the main synchronous system when operating under high penetrations of IBR.
System restart: Capability to energize the local network during the challenging conditions of a black system, or to assist with the restoration/black-start process.
The capability to custom program the power plant controller (PPC) is crucial to ensure a phase-locked loop (PLL) that aligns with the grid voltage at the point of common coupling (PCC). This is essential for optimal synchronization of output of GFL IBRs to meet the POI requirements specified in the power purchase agreement (PPA) and achieve a satisfactory the ROI for the project.
Similarly, for GFM IBRs operating in a standalone microgrid or as part of a larger project, more advanced custom PPC programming is necessary to ensure optimal, safe, and stable operation. The PPC in a GFM arrangement should possess the capability to monitor and control the magnitude and angle of voltage at the PCC. GFM inverters require advanced controls for black start, grid forming, and re-synchronization. Depending on the configuration and scale of microgrid services, there may be a need to monitor, control, or bifurcate loads based on the availability of energy from GFM IBRs.
Nor-Cal Controls has been in control and network integration for renewable energy with and without BESS for over twelve years. We fully understand and have expertise in meeting the custom control schemes required for different IBRs technologies to ensure the success of your project. We offer custom built non-proprietary on-site SCADA System including Historian, PPC, EMS/MPC and MET station solutions for PV-Solar/PV-Solar + BESS/BESS only sites. Please get in touch to discuss your existing or upcoming projects.