•unifing inverters & grids•
what?
the universal interoperability for grid-forming inverters (unifi) Consortium is a U. S. Department of Energy funded effort to advance grid-forming (GFM) inverter technology • we are led by the National Renewable Energy Laboratory (NREL) with Ben Kroposki serving as our Organizational Director • unifi's relevance, value, and impact are summarized below:
relevance: future power systems will feature an arbitrary mix of machines and inverters at any scale necessitating development of interoperable GFM technology
value: articulate the vision, conduct needed R&D, demo concepts at scale, author best practices & standards, & develop workforce training tailored for future grids with high numbers of inverters
impact: addressing fundamental challenges & developing wide-ranging solutions to seamlessly integrate GFM inverters into power systems of the future will yield affordable, secure, reliable, clean, resilient, and sustainable grids
why?
large-scale integration of inverter-based resources (IBRs) with the power grid has sparked several concerns spanning stability, security, and protection • a majority of IBRs interfaced with the grid today are of the grid-following (GFL) type, wherein, the inverter synchronizes to (and follows) a stiff grid • a growing body of work has recognized that power grids with dominantly GFL inverters can face small-signal stability issues • as a solution, consensus is forming towards the adoption of GFM IBRs • in this paradigm, IBRs do not follow the grid; rather, they form it and offer better control across timescales •
several definitions of GFM tech and capability have been proposed by utilities, system operators, and regulatory agencies • while there is no singular definition, we list some of these below and point to the fact that common themes and features on what GFM means start to become evident •
a GFM inverter maintains a constant internal voltage phasor in a short time frame, with magnitude and frequency set locally by the inverter, thereby allowing immediate response to a change in the external grid • on a longer timescale, the internal voltage phasor may vary to achieve desired performance •
within the power park module current limits, the power park module shall be capable of behaving at its connection point as a voltage source behind an internal impedance (Thevenin source), during the normal operating conditions (non-disturbed grid conditions) and quasi immediately after a grid disturbance (including voltage, frequency and voltage phase angle disturbance) • the Thevenin source is characterized by its voltage amplitude, voltage phase angle, frequency and internal impedance •
the primary objective of GFM control for bulk power system-connected IBRs is to maintain an internal voltage phasor that is constant or nearly constant in the sub-transient to transient time frame • this allows the IBR to immediately respond to changes in the external system and maintain IBR control stability during challenging network conditions • the voltage phasor must be controlled to maintain synchronism with other devices in the grid and must also regulate active and reactive power appropriately to support the grid •
a GFM IBR maintains an internal voltage phasor in the transient time frame, with the magnitude and frequency set locally at each inverter •
a GFM unit shall, within its rated power and current, be capable of self-synchronise, stand alone and provide synchronisation services, which includes synchronising power, system strength, fault current and inertial response •
GFM capability is (but not limited to) the capability a Power Generating Module, HVDC Converter (which could form part of an HVDC System), Generating Unit, Power Park Module, DC Converter, OTSDUW Plant and Apparatus, Electricity Storage Module, Dynamic Reactive Compensation Equipment or any Plant and Apparatus (including a smart load) whose supplied Active Power is directly proportional to the difference between the magnitude and phase of its Internal Voltage Source and the magnitude and phase of the voltage at the Grid Entry Point or User System Entry Point and the sine of the Load Angle • as a consequence, Plant and Apparatus which has a Grid Forming Capability has a frequency of rotation of the Internal Voltage Source which is the same as the System Frequency for normal operation, with only the Load Angle defining the relative position between the two •
the notion of grid following and grid forming should be treated as two book-end characteristics with a continuum of control behavior in between scaling from providing no grid services to fully engaged in providing grid services • it may be counter-productive to attempt to define a distinct boundary between these two bookends since these notions are usually placed at the inverter level (or device level) within an entire plant consisting of inverters; and a system operator is almost always only concerned about the performance/behavior of the entire plant that is observed at the point of interconnection • as a result, an entire spectrum of possibilities can result between the two bookends with each progression bringing with it an improvement in performance when observed from the grid side •
how?
the unifi team brings together leading researchers, industry stakeholders, utilities, and system operators to collaboratively pursue advances in a broad range of GFM tech • unifi will cultivate and foster a culture of engagement and innovation to identify and fill gaps in technologies, business models, standards, and human factors • in particular, unifi will conduct and coordinate research, development, and demonstration, as well as create educational- and workforce-training materials focusing on planning, designing, and operating grids with a high level of GFM Inverter-based Resources (IBRs) • activities will be organized around three Thrusts:
research & development (R&D)
demonstration & commercialization (D&C)
outreach & training (O&T)
unifi will stay true to its name and unify timescales of operation and control across spatial scales for IBRs and associated technologies •
