Integrating Inverter Based Resources in the Bulk Power Grid
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- Oct 29, 2020 1:56 am GMTOct 23, 2020 3:24 pm GMT
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This item is part of the Special Issue - 10/2020 - Distributed Energy Resources, click here for more
The North American Bulk Power System (BPS) is undergoing a rapid change in generation mix with increased penetration of Inverter Based Resources (IBR) like solar, wind, or storage. Just for reference, if we look in the PJM footprint, that coordinates the movement of wholesale electricity in all or parts of thirteen states and the District of Columbia, we see that in it's latest AF2 interconnection queue, as of March 2020, out of a total of 438 generation interconnection requests, solar led the pack with 330, followed by storage (68) and natural gas (18). If we look at this from the power level, then solar constitutes 20,674MW out of the total 28,502MW of planned queue generation. These resources are interfaced with the grid asynchronously using power electronic converters with fast controls that are different than conventional synchronous generators. Though IBRs has presented new opportunities for grid control due to their fast response, their growing penetration is also a concern for utilities across North America on how to make sure to interconnect these resources and maintain grid reliability.
The interconnection process starts once a developer submits an interconnection request and the transmission provider puts them into a queue. The queue number determines the order in which the generators are studied. A three-step process is followed which consists of Feasibility Study, System Impact Study, and Facilities Study. The interconnection customer is responsible for providing project data needed for the studies to proceed. The overall process can be technically quite complex when multiple generators try to interconnect at the same point of interconnection (POI).
It's well known that when multiple inverter-based resources are electrically close to one another
They interact and reduce the system strength at the POI. System strength is generally measured by the Short Circuit ratio (SCR) which in the IBR context can be defined as the short circuit capacity of the grid to the MW power injection from the IBR at the POI. Generally, if the SCR at the POI is more than three, the grid is said to be a strong grid and if it's less than three it's taken to be a weak grid. It's pertinent to mention here that the definition of SCR itself fails to take into account the interconnection of other IBRs that are near to the POI and might be interacting and reducing the system strength. Thus, there is a fair possibility that the SCR is overestimating the system strength when multiple IBRs are electrically close to one another. Several enhanced version of traditional SCR has been proposed like Weighted Short Circuit Ratio (WSCR) by the Electric Reliability Council of Texas (ERCOT) or the Composite Short Circuit Ratio (CSCR) by General Electric Corp. (GE). Both these methods try to evaluate system strength taking into account the interaction of other inverter-based resources in the vicinity though both have certain shortcomings as well.
In a weak grid, there might be several operational problems when interconnecting IBRs, and thorough studies are generally required to understand the interactions better. During the interconnection process, rms positive sequence tools are currently used, however, there are some known limitations in rms positive sequence simulations during high renewable penetration scenario particularly in weak grid conditions and if the transmission provider recognizes a weak grid condition for a particular queue generator, then more detailed Electromagnetic Transient (EMT) simulation might be required.
The data that is supplied by the interconnection customers during the interconnection request process is generally not enough to perform an EMT simulation. Our experience shows that often it's difficult to obtain as-is EMT dynamic simulation data from the interconnection customers though with utility IBR requirements taking shape and getting fairly standardized, things will be changing soon. Moreover, there is not a strong requirement for spec sheets and testing results though several utilities across North America are asking for these documents as good utility practice as well. There is a strong need for more detailed modeling and parameterization and both The North American Electric Reliability Corporation and The Western Electricity Coordinating Council (WECC) has done a great job working collaboratively with all stakeholders and other key industry representatives addressing emerging reliability issues related to inverter-based resources connected to the BPS. The NERC Inverter-Based Resource Performance Task Force (IRPTF), WECC, and the utility industry have been working diligently on modeling and simulation activities to accurately represent inverter-based resources in dynamic stability analyses and explore the impacts of inverter-based resources on BPS reliability and this has led to the development of generic renewable energy models that can be found in all commercial positive sequence simulation platforms. Understanding how the parameters of these second-generation models affect the grid voltage and frequency during specific scenarios would help to provide better recommendations in control settings from the transmission providers to the interconnection customers during the study process. In fact, it might be a good idea to regularly review controller settings in low system strength areas by all the stakeholders concerned. Also, the RTOs and ISOs should possibly try to coordinate with the transmission operators to develop standardized EMT model process and the ability to perform regular wide-area EMT studies.
According to NERC FAC-001-3 – Facility Interconnection Requirements, each Transmission owner (TO) should develop their interconnection requirements and must address any needed issue at their sole discretion. These requirements should be up to date to address the changing resource mix, including ensuring that the requirements cover the required improvements to the modeling and study activities that are performed. Most utilities have their interconnection requirements and have the jurisdiction to enforce them in the area they control. Going forward, with the kind of IBR related queues we are witnessing, there is a very strong push from most utilities in North America to have dedicated IBR requirements which are closely aligned with NERC recommendations and also local grid conditions.
Though challenges remain as we move to incorporate more IBRs in the bulk power system, collaboration among the various stakeholders like the interconnection customers, transmission providers, ISO/RTOs are going to be the key moving forward.