Aninverter-based resource (IBR) is a source of electricity that is asynchronously connected to theelectrical grid via an electronicpower converter ("inverter"). The devices in this category, also known asconverter interfaced generation (CIG) andpower electronic interface source, include thevariable renewable energy generators (wind, solar) andenergy storages such as battery, super capacitors, etc..[1][2] These devices lack the intrinsic behaviors (like theinertial response of asynchronous generator) and their features are almost entirely defined by the control algorithms, presenting specific challenges to system stability as their penetration increases,[1] for example, a single software fault can affect all devices of a certain type in acontingency (cf.section on Blue Cut fire below). IBRs are sometimes callednon-synchronous generators.[3] The design of inverters for the IBR generally follows theIEEE 1547 andNERC PRC-024-2 standards.[4]
The term unconventional sources includes IBRs as well as other generators that behave differently than synchronous generators.[5]
Agrid-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 acurrent source[6]). The GFL inverters are built into an overwhelming majority of installed IBR devices.[1] Due to their following nature, the GFL device will shut down if a large voltage/frequency disturbance is observed.[7] The GFL devices cannot contribute to thegrid strength, dampen active power oscillations, or provideinertia.[8]
Agrid-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synchronous generator: its voltage is controlled by a locally-controlled oscillator that slows down when more energy is withdrawn from the device.[9] Unlike a conventional generator, the GFM device has noovercurrent capacity and thus will react very differently in theshort-circuit situation.[1] 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 higherstandby consumption than GFL.[10] Modeling demonstrates, however, that it is possible to run a power system that almost entirely is based on the GFL devices.[11] A combination of GFMbattery storage power station andsynchronous condensers ("SuperFACTS") is being researched.[12]
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:[13]
Compliance with IEEE 1547 standard makes the IBR to support safety features:[14]
Once an IBR ceases to provide power, it can come back only gradually, ramping its output from zero to full power.[15]
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 thenameplate capacity, while the synchronous machines can briefly tolerate an overload as high as 5-6 times their rated power.[16]
A typical failure of a conventional synchronous generator (like a loss ofprime mover) is slow (seconds), while the IBR has to disconnect quickly due to low margin for overload.[17]
North American Electric Reliability Corporation (NERC) notes that IBR, like conventional generators, can provideessential reliability services, and summarizes the differences as follows:[18]
| IBR | Synchronous generator |
|---|---|
| Behavior determined by power converter and control software | Behaviour driven mostly by the physical properties of the machine |
| Non-existent to small inertia | Large inertia of the rotor |
| Lowfault current | High fault current |
| Based on sensitive electronics | Design can withstand extreme conditions |
| Fast ramping | Ramping limited by mechanical limits |
| Fast frequency control | Inertial response by design |
| Dispatchability limited be environmental conditions | Full dispatchability |
| Very little plant equipment outside the generator | Extensive auxiliary equipment adding to the risk of tripping |
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 astrong 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.[17] The typical protections include:[19]
Once tripped, the IBRs will restart based on a timer or through manual intervention. A typical timer setting is in the seconds to minutes range (theIEEE-1547 default is 300 seconds).[20]
New challenges to the system stability came with the increased penetration of IBRs. Incidences of disconnections during contingency events where thefault ride through was expected, and poor damping ofsubsynchronous oscillations inweak grids were reported.[1]
One of the most studied major power contingencies that involved IBRs is theBlue Cut Fire of 2016 inSouthern California, with a temporary loss of more than agigawatt of photovoltaic power in a very short time.[15]
TheBlue Cut fire in theCajon Pass on August 16, 2016, has affected multiple high-voltage (500 kV and 287 kV) power transmission lines passing through the canyon. Throughout the day thirteen 500 kVline faults and two 287 kV faults were recorded.[21] The faults themselves were transitory and self-cleared in a short time (2-3.5cycles, less than 60milliseconds), but the unexpected features of the algorithms in the photovoltaic inverter software triggered multiple massive losses of power, with the largest one of almost 1,200megawatts[22] at 11:45:16 AM, persisting for multiple minutes.[23]
The analysis performed by theNorth American Electric Reliability Corporation (NERC) had shown that:
As a result of the incident, NERC had issued multiple recommendations, involving the changes in inverter design and amendments to the standards.[4]
