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), include thevariable renewable energy generators (wind, solar) andbattery storage power stations.[1] 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.[2] The design of inverters for the IBR generally follows theIEEE 1547 andNERC PRC-024-2 standards.[3]
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[4]). 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.[5] The GFL devices cannot contribute to thegrid strength, dampen active power oscillations, or provideinertia.[6]
Agrid-forming (GFM) device partially mimics the behavior of a synchronous generator: its voltage is controlled by a free-running oscillator that slows down when more energy is withdrawn from the device. 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. Modeling demonstrates, however, that it is possible to run a power system that almost entirely is based on the GFL devices.[7] A combination of GFMbattery storage power station andsynchronous condensers (SuperFACTS) is being researched.[8]
Compliance with IEEE 1547 standard makes the IBR to support safety features:[9]
Once an IBR ceases to provide power, it can come back only gradually, ramping its output from zero to full power.[10]
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.[11]
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.[10]
The Blue 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.[12] 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[13] at 11:45:16 AM, persisting for multiple minutes.[14]
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.[3]
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