Regions such as Hawaii, South Australia, Ireland and Texas have recently experienced significant growth of inverter-based renewable generation, mainly wind and solar, and are now operating with very high instantaneous penetration in excess of 50-60%. As this penetration increases globally, several regions have identified limitations and challenges which indicate that it may not be feasible to maintain the desired level of system reliability with extremely high penetration of inverter-based generation using current technology. Among many identified limitations and challenges such as system inertia and ramping, system strength is one major challenge that needs to be addressed to maintain reliable operations with inverter-based generation. System strength refers to the ability to maintain stable voltages throughout the network during normal operations and disturbance conditions. In systems dominated by synchronous generators, system strength is characterized by the available fault current at a given location or by the short circuit ratio. Almost all commercially-available inverter-based generation resources today require sufficient strength from the system to which they are interconnecting for reliable operations. These generation resources are called “grid-following” resources. At present, system strength is maintained by synchronous generators. However, during system conditions with high penetrations of inverter-based generation, the synchronous generators necessary to maintain system strength may not be online.
One topic discussed in last month’s blog was the critical inertia required in ERCOT based on the current system conditions and operational practice, which allows the system operator to start additional synchronous generation to maintain the needed level of inertia. Similar minimum inertia constraints are set, for example, in Ireland and South Australia. Additionally, ERCOT limits inverter-based generation output from the Texas Panhandle to maintain the necessary system strength in that area. Australian Electricity Market Operator (AEMO) requires transmission service providers to maintain pre-defined minimum system strength at designated buses on the system. Keeping synchronous generation online strictly to maintain system strength or inertia while this generation is not required for energy is uneconomic. Installing synchronous condensers to maintain system strength and/or inertia has been used as an alternative solution in some regions, but it also is costly and may result in additional technical challenges if many of these devices are concentrated in parts of a system.
Continuing to increase the amount of inverter-based generation in a system may not be feasible or economical if a certain amount of synchronous generators need to be committed or new synchronous condensers need to be installed only to provide the minimum system strength needed. Additionally, different equipment manufacturers may have different system strength requirements, and the increasing stability constraints and coordination among resources, transmission service providers and system operators could create a significant burden on the planning and operation of an inverter-based generation dominated power system.
In recent years, the concept of grid-forming inverter-based resources was proactively pursued by the research community. A grid-forming resource does not require a certain system strength to be interconnected to a power system, and can precisely control its output voltage amplitude and frequency to create a strong voltage source itself. This innovation has great potential to improve the performance of inverter-based generation and, according to some studies, can make 100% inverter-based operations feasible. Although various research entities have put great effort into studying the 100% renewable scenario and identified requirements to achieve this ambitious goal, it must be recognized that power systems will not become 100% renewable overnight. If grid-forming technologies are to be a viable solution, they will have to operate reliably in parallel with synchronous generation during the extended transition period. Furthermore, not all of the necessary adjustments and new technologies can be easily implemented to reach a 100% renewable scenario. Meanwhile, since electricity is a critical aspect of people’s lives, there will still be extremely high standards for power system reliability regardless of whether electricity is produced by traditional synchronous machines or by inverter-based renewable generation.
To the best of the authors’ knowledge, there is no practical example of parallel grid-forming inverters in a large power system under various penetration levels of inverter-based generation. There also is currently no commercially-available grid-forming inverter product for a large-scale power system application. This could be due to a lack of collaboration between the research community, manufacturers and system operators resulting in two intertwined issues: (1) without clear technical specifications of what grid-forming capabilities should entail, manufacturers have no clear path or incentive to develop this capability; (2) at the same time, without commercially-available products or prospects for such products, system operators cannot require these capabilities or introduce markets for these products. Thus, we continue planning and operating the system with the tools that are currently available.
It is critical to have a road map to get from present levels of inverter-based generation toward a reliable inverter-based generation dominated power system in the future. It will require collaboration among manufacturers, system operators and research organizations from a holistic perspective to reach consensus and develop a road map that would (1) encourage research entities to continue to identify the improvement and innovation of existing and new technologies, (2) provide incentives to manufacturers for product development and commercial application, and (3) assist system operators with more options and suggestions to continue to maintain and operate the power system in a reliable and cost-effective manner.
Given the growth of inverter-based resources globally, the industry needs to quickly figure out how to make this transition from concept to feasibility to commercial availability for implementation. The longer we wait, the more difficult it will become, since existing inverter-based resources will be difficult or not feasible to retrofit. Some of these collaborations have already started. For example, the European Commission-funded project called the Massive Integration of Power Electronic Devices (MIGRATE) has brought together European transmission system operators, universities and manufacturers in an EU-funded project to address challenges with extremely high penetration of inverter-based generation. ENTSO-E (European Network of Transmission System Operators for Electricity) also established a technical group in 2016 on High Penetration (TG HP) and developed an Implementation Guidance Document on High Penetration of Power Electronic Interfaced Power Sources (HPoPEIPS). Similarly, a new task force will start within the Reliability Working Group at ESIG to aim for the same goal, with the expectation to build a consensus and develop a road map of how to get from where we are today to an inverter-based resources dominated system, while operating the system in a reliable and stable manner.
Lead Planning Engineer, Resource Adequacy
Shun-Hsien (Fred) Huang
Manager of Regional Planning,Transmission Planning