An inverter-based resource (IBR) is a source of electricity that is asynchronously connected to the electrical grid via an electronic power converter ("inverter"). The devices in this category, also known as converter interfaced generation (CIG) and power electronic interface source, include the variable renewable energy generators (wind, solar) and energy storages such as battery, super capacitors, etc.. These devices lack the intrinsic behaviors (like the inertial response of a synchronous generator) and their features are almost entirely defined by the control algorithms, presenting specific challenges to system stability as their penetration increases, for example, a single software fault can affect all devices of a certain type in a contingency (cf. section on Blue Cut fire below). IBRs are sometimes called non-synchronous generators. The design of inverters for the IBR generally follows the IEEE 1547 and NERC PRC-024-2 standards. The term unconventional sources includes IBRs as well as other generators that behave differently than synchronous generators.
Grid-following A grid-following (GFL) device is synchronized to the local grid voltage and injects an electric current vector aligned with the voltage (in other words, behaves like a current source). The GFL inverters are built into an overwhelming majority of installed IBR devices. Due to their following nature, the GFL device will shut down if a large voltage/frequency disturbance is observed. The GFL devices cannot contribute to the grid strength, dampen active power oscillations, or provide inertia.
Grid-forming A grid-forming (GFM) device partially mimics certain attributes of a synchronous generator: its voltage is controlled by a locally controlled oscillator that slows down when more energy is withdrawn from the device. Unlike a conventional generator, the GFM device has no overcurrent capacity and thus will react very differently in the short-circuit situation. Adding the GFM capability to a GFL device is not expensive in terms of components, but affects the revenues: in order to support the grid stability by providing extra power when needed, the power semiconductors need to be oversized and energy storage added. GFM has higher standby consumption than GFL. Modeling demonstrates, however, that it is possible to run a power system that almost entirely is based on the GFL devices. A combination of GFM battery storage power station and synchronous condensers ("SuperFACTS") is being researched. European Network of Transmission System Operators for Electricity (ENTSO-E) groups the GFM devices into three classes from 1 to 3, with Class 1 being at the lowest level of contribution to the grid stability (the original classification had the numbers in reverse, with class 1 being the highest). Class 2 is further subdivided in to 2A, 2B, 2C, with 2A being the most basic of the three:
Class 1 devices primarily deal with their own survival (full frequency and voltage operating ranges) and have minimal contributions to the grid, including basic reactive power management to maintain the unity power factor and limited frequency sensitive mode (LFSM-O). Class 2 devices provide additional capabilities: 2A supports the fault ride-through and voltage control for the steady state; 2B adds the dynamic voltage control, frequency sensitive mode (FSM) as well as the LFSM-U; 2C also provides voltage control at zero active power, oscillations damping, and fast fault current injection (FFCI) for periods B and C (the AC periods immediately following the "A" one with the fault) Class 3 is capable of fully autonomous operation with no support from the grid. It creates system voltage, capable of handling fault level in the period A, contributes to the total system inertia (TSI) of the grid, can handle low frequency demand disconnection (LFDD), provides a sink for harmonics and interharmonics of the system voltage, and a sink for the voltage unbalance.
Features Compliance with IEEE 1547 standard makes the IBR to support safety features:
if the sensed line voltage significantly deviates from the nominal (usually outside the limits of 0.9 to 1.1 pu), the IBR shall disconnect from the after a delay (ride-through time), the delay is shorter if the voltage deviation is larger. Once the inverter is off, it will stay disconnected for a significant time (minutes); if the voltage magnitude is unexpected, the inverter shall enter the momentary cessation state: while still connected, it will not inject any power into the grid. This state has a short duration (less than a second). Once an IBR ceases to provide power, it can come back only gradually, ramping its output from zero to full power. The electronic nature of IBRs limits their overload capability: the thermal stress causes their components to even temporarily be able to function at no more than 1-2 times the nameplate capacity, while the synchronous machines can briefly tolerate an overload as high as 5-6 times their rated power. A typical failure of a conventional synchronous generator (like a loss of prime mover) is slow (seconds), while the IBR has to disconnect quickly due to low margin for overload. North American Electric Reliability Corporation (NERC) notes that IBR, like conventional generators, can provide essential reliability services, and summarizes the differences as follows:
Protection functions The IBR devices come with many protection functions built into the inverters. Experience of the late 2010s and early 2020s had shown that some of these protections are unnecessary, as they were designed with an expectation of a strong grid with little IBR penetration. NERC 2018 guidelines suggested removing some of these checks in order to avoid unnecessary disconnections ("trips") of the IBRs, and newer devices might not have them. The remaining checks are essential for the self-protection of the inverters that, compared to a synchronous generator, have relatively little tolerance for overvoltage and overcurrent. The typical protections include:
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