Tobias Engelbrecht, Andrew Isaacs, Sergey Kynev, Julia Matevosyan, Bernd Niemann, Andrew J. Owens, Biswajit Singh, Andrea Grondona
The urgency of climate change forces many countries and regions to accelerate toward the goal of a carbon-neutral economy. The consequence of replacing fossil-fuel synchronous generation with inverter-based renewables, such as wind and solar, leads to a paradigm shift from power systems dominated by synchronous machines to inverter-dominated power systems. When the share of inverter-based generation increases, the behavioral differences in terms of the grid services provided to the power network by generation become more salient. Services for grid stability that were historically inherently provided by synchronous generators now must be replaced by services from these inverter-based generation resources, other grid devices, or the combination of the two to ensure stable grid operation in the future as well.
This transformation of the generation landscape is already taking place, leading to a deficit of local dynamic reactive power support, which is crucial for stable grid voltage. Reactive power support, which was initially provided by large synchronous machines, is gradually being replaced with equivalent capabilities of inverter-based resources (IBRs), often in combination with supplemental voltage support equipment, such as static synchronous compensators (STATCOMs) or synchronous condensers, depending on local needs.
However, dynamic reactive power capability is just one of the “inherent services” that were historically provided by synchronous machines. Until recently, sufficient inertia and frequency support reserves were taken for granted, especially in large, interconnected areas. It was assumed that there would always exist a sufficient pool of large rotating machines that could oppose a sudden active power imbalance by providing inertia, which in turn reduces the speed of frequency change in the system. Today, power systems are witnessing a growing number of examples where the absence of synchronous machines creates stability concerns due to a lack of inertia and synchronizing torque. This problem is not limited to small islanded networks that quickly reach 100% renewable generation, as in Figure 1, but also for some weakly connected parts of the large synchronous systems, or in the case of network split, after major events. Currently, system operators around the world are addressing this concern by dispatching a number of synchronous generators not for their power generation capability, but rather only to provide grid stabilization services like reactive power support, inertia, and frequency support. This phenomenon, often referred to as reliability must run, leads to increased operating costs and excessive renewable curtailments, which impedes the progress toward a carbon-neutral economy.
Running generators only to provide grid services demonstrates an industry need for devices that can contribute to maintaining sufficient system inertia and provide frequency support. This article presents a device that is optimized to provide both services. But first, it is important to highlight the difference between inertia and frequency support.
Immediately upon the sudden loss of generation, the demand for electricity remains unchanged, creating an imbalance between supply and demand. To reinstate this balance between production and consumption, kinetic energy, which had been stored as a large rotating mass, is extracted from synchronous machines. This natural phenomenon of the synchronous machine is referred to as inertial response. Inertial response is instantaneous, inherent, and a property of synchronous machine design. This response results in a decline of the machine speed, which manifests itself as a reduction in power system frequency. The rate of change of frequency (ROCOF) is defined by the inertia of all online synchronous machines and the size of the generation loss. If system inertia is low and the generation loss is sufficiently large, the ROCOF after a generation trip event will be so high that there is insufficient time for any mitigating frequency control actions to arrest frequency before it reaches a level to trigger involuntary underfrequency load shedding or trigger a generator’s underfrequency protection.
Inertia reduces the speed of frequency decline but may not completely compensate for the gap in active power balance as a result of an unexpected event. A future power system dominated by IBRs will operate with much lower inertia than historically seen in traditional power systems. In addition to the service provided by synchronous generators with speed governors, a need exists for new faster frequency-response services. An increasing number of system operators require or incentivize this service through frequency-response markets. Other operators, e.g., TenneT, want to maintain a minimum of inertia by optimizing their planned own assets as far as possible for an inertia contribution. A provider of this service delivers a controlled contribution to the power system in response to a frequency-change event. This controlled contribution can be sourced according to different time periods, from seconds to minutes. The fastest of these services, fast frequency response (FFR), provides support for the first few milliseconds to seconds immediately following a frequency excursion. The bulk of FFR services today are being provided by IBRs, such as wind, solar, and battery energy storage systems (BESS).
This article introduces an upgraded STATCOM technology, tailored to provide inertia and FFR, the E-STATCOM. In addition to the functions provided by conventional STATCOM, E-STATCOM can rapidly inject or absorb active power through the dynamic control of the stored energy in the supercapacitors. The letter “E” represents energy in the acronym E-STATCOM. Conventional STATCOM technology has long played an important role in the power system, providing dynamic reactive power control. STATCOMs make use of voltage source converter (VSC) technology and are at their core a source of voltage that can be programmed to hold its frequency. This type of control or behavior of a VSC is now often referred to as having grid-forming (GFM) capability. By holding the VSC’s internal frequency constant, an E-STATCOM can instantaneously and inherently support the grid with its stored energy in cases of frequency imbalances, similar to inertial response of synchronous machines. Conventional STATCOM technology lacks sufficient energy available in its power electronics to provide meaningful frequency support. The introduction of supercapacitors allows an E-STATCOM to not only provide reactive power compensation, but also compensates inertia and provides fast frequency response.
STATCOM technology has rapidly evolved in the recent decade. The utilization of so-called modular-multilevel converters (MMC) has resulted in increased rating in a single power converter, with several major players advertising the capability to deliver more than several hundreds of megavolt ampere (MVA) in a single power converter for STATCOM applications. The MMC technology is an evolution from two- or three-level converters that were used widely in STATCOM applications in the 1990s and early 2000s. MMC technology provides a number of benefits:
Unlike conventional STATCOMs, an E-STATCOM requires a medium-voltage dc-busbar to be able to connect the supercapacitor-based energy storage. Thus, the converter topology reflects a typical double-star configuration, as used for example in symmetrical monopole high-voltage dc (HVDC) stations (Figure 2).
Unlike a BESS, which consists of many smaller standardized modules, E-STATCOM is based on a large tailor-made MMC converter to meet the needs of a particular grid in terms of sizing and control performance. In many aspects, the layout reflects the modern conventional STATCOM design. Being a transmission system device, E-STATCOM typically connects to an HV system by means of a step-up transformer. Along with a few pieces of outdoor equipment, the largest part of the installation is placed inside a building (Figure 3):
Adding active power capability to conventional STATCOM functions provides the opportunity for great flexibility in control modes. System studies and operational experience will identify the most suitable control functions for a particular E-STATCOM installation. However, it is clear today that an E-STATCOM will operate in a GFM mode, emulating synchronous machine behavior, and will therefore bring inertia into the grid. Other additional control modes might include:
With the introduction of E-STATCOM technology, it is interesting to compare E-STATCOM to existing technologies currently providing similar services, in this case BESS (Figure 4) and synchronous condensers with or without flywheels (Figure 5). BESS refers to low-voltage two- or three-level converters coupled with batteries of various chemistry types providing power ratings of up to a few megawatts, which can be paralleled into groups to achieve ratings on the scale of 100 s of megawatt in a single energy-storage installation. Synchronous condensers are a well-proven technology with decades of operational experience providing grid stabilization services to the power systems on the scale of 10 s or 100 s of MVA.
This comparison of technology focuses on the application of providing the grid stabilization services previously referred to in this article, namely instantaneous voltage control, inertia, and fast frequency support on the scale of single digits of seconds. The required time scale of response is especially important when comparing the economical merits of these different technologies, as these services necessitate high-power ratings, on the scale of 100 MW or more, for a shorter time duration of hundreds of milliseconds to a few seconds.
Both E-STATCOM and BESS use a power converter to interface to the power system. This interface provides superior control flexibility and functionality, as both active and reactive power can be exchanged with the power system independently and in a controlled manner. The power converter’s control flexibility allows for the shaping of dynamic behavior. The damping factor, inertia constant, and passive frequency range all become adjustable control parameters, whereas these parameters in a synchronous machine (including a synchronous condenser) are determined by physical design parameters and the size of the flywheel. The power converter also can provide improved functionality providing compensation for system imbalance and harmonics, which could be integrated into the control and adjusted to the respective application. Like all power electronic devices, harmonic emissions must be investigated during the interconnection study; however, due to MMC technology, the number of emitting harmonics is relatively very low.
One of the more widely used technologies to mitigate low inertia is a synchronous condenser, sometimes in combination with an extended rotor-weight, a so-called flywheel. A synchronous condenser has similar physical construction to a synchronous generator but does not generate active power for electrical consumption. It stores kinetic energy in its rotating mass in order to provide the same type of inertial response as a synchronous generator. Some existing synchronous machines, which have been decommissioned as power generators, have been given new life as retrofitted synchronous condensers, providing inertia to the power system, but no longer acting as a traditional power generator.
Following a system event that results in a change in frequency, the synchronous condenser provides an instantaneous response as it exchanges its stored kinetic energy as active power with the grid. The nature of the synchronous operation is such that the energy being exchanged with the power system is proportional to the frequency variation. As a result, only a portion of the installed energy is being exchanged, whereas introducing a power converter between the energy storage and the grid enables the use of a higher portion of the installed energy. In addition, the energy injection in the inertial time frame can be parametrized; therefore, the energy contribution from an E-STATCOM or BESS during a frequency excursion can be larger than that of an equally rated synchronous condenser. At the same time, the flexibility of the E-STATCOM is restricted by its maximum power, while the inertia contribution for a synchronous condenser is fixed by its physical design parameters and the size of the flywheel.
The magnitude of power losses of a synchronous condenser is a point of consideration. Typically, the no-load losses have a high evaluation point since the device is expected to operate around zero current output for a significant part of its lifetime. As a synchronous machine is a large rotating mass, which produces windage and friction losses regardless of the MVA reactive (Mvar) output, the no-load point has a proportionally large portion of the total losses. Like a conventional STATCOM, E-STATCOM losses are mostly dependent on current output and thus produce neglectable no-load losses. Improvements in semiconductor technology and power-converter topology in the past decades has enabled a further decrease in active power losses (see “Comparing E-STATCOM and Synchronous Condenser Response”).
The system application is extremely important when making a comparison between lithium-ion (Li-ion) batteries and supercapacitors. Batteries have 10 to 20 times higher energy density than supercapacitors, while supercapacitors have 10 to 50 times higher power density than batteries. The increased power density and significantly lower internal resistance makes supercapacitors ideal for the short-term, high-power pulses requested in FFR applications. The phenomena are pictorially depicted in Figure 6, with the blue tank representing a battery system, the red bucket representing the supercapacitor system, the volume of water inside the tank/bucket representing the total energy available, and the instantaneous amount of water flow representing power capability. Both the blue tank and red bucket contain the volume and instantaneous water flow capable of extinguishing the fire, but the blue tank is overdimensioned, storing a significantly larger volume of water than necessary to extinguish this type of fire.
The charge rate of supercapacitors is hundreds of times higher than typical Li-ion batteries. The discharge time of supercapacitors is on the order of seconds to minutes, compared to hours for most Li-ion batteries. Given that the cost per kilowatt increases with higher energy-to-power (E/P) ratios in an energy storage system, supercapacitors offer a significantly better option for an energy storage system with a low E/P ratio, as seen in inertia and FFR applications.
High cycling capability is an important attribute in frequent, short-term active power injection. The peak power pulses encountered in these applications have a high impact on a battery’s lifetime since batteries age with cycling, but supercapacitors do not age with cycling. The typical estimated life of an Li-ion battery is about 5–10 years or 500–800 charge cycles, whichever occurs first, whereas supercapacitors can be charged and discharged thousands of times. According to current projections and analyses, the service life is expected to be up to about 20 years, depending on application. Thus, an E-STATCOM, used to provide active power injection for inertial response (a few seconds), is likely to have a longer lifetime, compared to a GFM BESS used for the same application.
Additionally, supercapacitors are in general safe, less toxic, and have the lowest risk of fire hazard. Since supercapacitors can be fully discharged like conventional electrical capacitors, the risk for maintenance personnel is significantly reduced compared to maintenance procedures at batteries that need to be executed under charged conditions.
From a physical perspective, comparing a battery and supercapacitor system in a high-power, short-duration application (i.e., 100 MW for 5 s), a supercapacitor-based system could require 9%–15% of the physical footprint of a battery system. The cost of a supercapacitor-based system could result in a cost of 10%–20% of that of a battery-based system with this power and duration requirement.
With many bulk power systems wrestling with increasing penetration of IBRs and declining conventional sources of system strength and inertia, issues, such as control instability, high and low voltage ride-through, and control interactions, emerge. These problems have traditionally surfaced in radially connected or remote parts of the transmission grid but are becoming increasingly evident in distribution grids and larger transmission grids with high shares of IBRs, particularly in regions with massive quantities of inverter-based distributed energy resources (DERs). One such example is within the Independent System Operator of New England (ISO-NE) footprint. In the ISO-NE system, hundreds of megawatts of distributed photovoltaics are connecting to weak parts of the grid. Strategically placing an E-STATCOM in such remote weakly connected areas will strengthen the local transmission system. It will help the local IBRs to ride through faults and avoid instabilities associated with a weak grid. Due to its GFM capability, E-STATCOM can act as a stable voltage source for short periods of time when the network connection becomes extremely weak or even islanded.
In regions with very high penetration of distributed generation, such as Hawaii, there is a corresponding exposure to frequency events that are caused by instantaneous loss of significant power output from aggregated DER due to faults. When a fault occurs, DER may block and delay in recovering, which leads to 50%–70% of the load being unserved for a fraction of a second (e.g., 0.25 s) following fault clearing. Such events are becoming more common in the United States and are well documented by the North American Electric Reliability Corporation. An occurrence in Odessa, TX, USA, on 9 May 2021 resulted in a loss of more than a gigawatt of generation. Such an event could be devastating for a small, islanded grid. The unserved load can lead to frequency diving 1–2 Hz within cycles. During this period, a significant amount of energy is required to arrest frequency decline and prevent load shedding, and this energy needs to be supplied very quickly. Since an E-STATCOM can provide energy instantaneously for a period of more than 1 s, it is an ideal device to assist with frequency stability in high DER scenarios.
Comparing E-STATCOM and Synchronous Condenser Response
The energy stored in the rotating mass of a synchronous machine cannot be directly compared with energy stored in a power electronic converter device, such as E-STATCOM. Although the amount of energy stored might be comparable on the nameplate, the actual amount of active power released to the grid depends on the physical properties of the machine and the control scheme of the converter. To illustrate this, a simplistic test was been conducted and is shown in Figure S1. In a weak low-inertia grid, a loss of a large portion of generation is simulated first without any additional devices (black color); then with an addition of a 100-MVA synchronous condenser with a typical 2-s inertia [Figure S1(b), blue], then with the same synchronous condenser plus a flywheel with total inertia of 8 s [Figure S1(b), green], and 100-MW E-STATCOM with stored 300-MW energy. First, the E-STATCOM is programmed to act as a synchronous machine with an inertia constant of 20 s [Figure S1(b), red] and second has an additional overlapping FFR [Figure S1(b), purple]. A synchronous machine, especially with a large inertia constant, naturally resists any change in grid frequency by releasing a large amount of power. The amount of power released depends on the size of the machine, inertia constant, and grid frequency deviation. Being a synchronous machine, a condenser must be synchronized with the grid frequency; thus, a part of the stored kinetic energy can be injected, following a natural recovery of active power.
How the grid frequency is changing in the example presented has deliberately not been shown in Figure S1, as it is extremely dependent on the specific characteristics of the grid. The example just demonstrates what kind of services synchronous condensers and E-STATCOMs can deliver to the grid.
In contrast, an E-STATCOM can release its stored energy into the grid without losing its internal voltage frequency, thus staying synchronized with the grid. Being programmed to act as a virtual synchronous machine, the initial response is similar to a synchronous condenser with a very large inertia constant. Furthermore, after the initial stage, the control can be supplemented with other services, such as FFR, as shown in the example. This FFR can release the stored energy into the grid depending on the grid frequency deviation. With these two different behaviors of the E-STATCOM, the flexibility due to the control is demonstrated.
While comparing the E-STATCOM and synchronous condenser, it is important that the E-STATCOM is parametrized, taking into consideration the specific grid scenarios and application that are relevant for the specific design, e.g., maximum ROCOF and frequency variation. The example in Figure S1 shows that the E-STATCOM is designed not to reach its capability limit in this specific case.
A number of utilities, particularly island systems or regions approaching near 100% renewable penetration, such as Hawaii, are concerned about their existing protection infrastructure being vulnerable due to low fault currents or to fault currents that are unpredictable or poorly defined. E-STATCOM presents an additional source of controllable fault currents. Compared to a conventional STATCOM, it no longer must cease the current injection during faults with very low residual voltage.
Lowering system inertia has become a growing concern even for the large interconnected grids, such as in continental Europe. In the event of a system split, power imbalances arise in the grid zones. Historically in Germany, conventional power plants have inherently provided the required inertia to the grid. As a result of the changing generation mix and the increasing transits, there exists a significant need for inertia to continuously maintain balance between production and consumption in the future German grid.
The inertia demand is particularly high if there are disturbances that lead to very rapid energy imbalances in the grid. If demand outstrips supply, the frequency drops. If the supply outweighs demand, the frequency rises. The following two key criteria are considered for stable grid operation:
A system split event occurred in the European Network of Transmission System Operators for Electricity (ENTSO-E) grid on 4 November 2006. The separation of the grid into three frequency areas is shown in Figure 7 as an underfrequency blue area, overfrequency green area, and underfrequency orange area.
Studying the consequences of a similar series of events occurring in today’s power grid paints an even more dire picture. The German grid has seen the retirement of several conventional power plants in combination with increased power transits across the country. Future power imbalances in the event of a system split between the green and blue areas in Figure 7 are projected to be up to 39 GW. This imbalance is mainly created by the huge levels of renewable energy generation in the northern and eastern part of Germany, together with relatively low local power consumption. Fast frequency changes are the consequence, which may not be tolerated by the consumers and generators connected to the grid.
Analysis performed by the German transmission system operators (TSO) reveals that without countermeasures, a frequency gradient of 1 Hz/s cannot be maintained for approximately 20% of the year and a frequency gradient of 2 Hz/s cannot be maintained for approximately 10% of the year. This analysis demonstrates that a similar system split event to the one witnessed in 2006, which resulted in a power imbalance of approximately 10 GW, can no longer be safely controlled during a significant portion of the year.
Consequently, several countermeasures to increase inertia must be simultaneously assessed to improve grid stability in a short time. German TSOs have identified E-STATCOM as one of the solutions. TenneT’s first pilot project of E-STATCOM is described later in this article.
Required system services, such as inertia, frequency support, and GFM, can be provided by varying technologies. Several approaches are taken around the world to incentivize the provision of these services provided by different technologies. Whether to utilize a market mechanism or impose rigid grid code rules is decided by the policymakers in each region. This section provides a few examples of successful paths to integrate the new technology, to stimulate innovation, and to maintain overall system reliability.
Great Britain Stability Pathfinder
The National Grid Electricity System Operator (NGESO) in Great Britain launched an initiative called Stability Pathfinder with the objective of finding economic and efficient solutions for the support of localized grid and system-wide inertia levels. This initiative is also exploring and testing the capabilities of new technologies to provide stability services to meet identified system needs. Stability Pathfinder consists of three consecutive tendering processes or phases. The understanding gained from each phase is used to shape the next. Phase 1, initiated in November 2019, was to procure dynamic voltage support, short-circuit contribution, and inertia at 0-MW output across Great Britain to meet national inertia needs. Phase 1 was open only to proven synchronous solutions, favoring solutions with better dynamic voltage support capabilities and higher inertia contribution. The existing grid code requirements, as well as specifications in the Draft Grid Code by the Virtual Synchronous Machine Expert Working Group, were used to establish technical performance requirements for prospective solutions. Phase 1 concluded in January 2020 and 12 contracts were awarded to five providers of high-inertia synchronous condensers with a total inertia of 12.5 GW seconds (GWs).
Phase 2 was seeking to solve insufficient short-circuit levels in various locations around Scotland, as well as to further increase system-wide inertia levels. Phase 2 was open to a broader range of technology types, including GFM inverters; therefore, technical feasibility studies are carried out as part of the tender process to understand how nonsynchronous technologies can meet system needs. Technical requirements for phase 2 are using some elements of the minimum specification required for provision of Great Britain’s GFM capability that were under development in Great Britain at the time. Phase 2 concluded in April 2022 with the award of 10 contracts, five to synchronous condenser solutions and five to GFM battery storage projects, with a total inertia of 6.75 GWs.
Phase 3 launched in January 2022, seeking to procure additional short-circuit capability and 15 GWs of inertia in specific locations within England and Wales. Phase 3 is open to all technologies and technical requirements are fully aligned with the aforementioned GFM specification draft as of November 2021.
Stability Pathfinder tenders are an exploratory temporary solution for broader procurement of new stability services. NGESO has already approved nonmandatory requirements for GFM capability and is currently developing a market framework for procurement of stability services. This framework opens an opportunity for E-STATCOM technology to participate and provide stability services.
Electrical Reliability Council of Texas
The Electrical Reliability Council of Texas (ERCOT) system is not synchronously interconnected with the rest of the United States. This asynchronous connection means that balancing and maintaining system frequency can only be done within the ERCOT system. In the past decade, rapid growth of wind and solar generation has led to instances when more than 69% of the ERCOT load is being served by IBRs.
In 2018, two synchronous condensers were installed in a part of the ERCOT grid, with an extremely high share of IBRs to address weak grid issues. E-STATCOMs were not considered as a technological solution at the time. Going forward, one could argue E-STATCOM should be considered alongside synchronous condenser solutions. From a regulatory perspective, E-STATCOM should not be viewed as a generation resource similar to battery storage, but rather as a transmission asset in the same way as a synchronous condenser. The supercapacitors in this application have a small energy density, more comparable to kinetic energy stored in rotating masses of a synchronous condenser rather than battery energy storage. This kinetic energy is utilized to provide grid stabilization services on the scale of seconds, not to inject megawatt hours of energy into the power system to be used to service electrical loads.
ERCOT recognizes that, in addition to regional weak grid issues, overall system inertia will eventually decline. ERCOT has determined a critical inertia level. Below this minimum level of inertia, the loss of the two largest generating units (a planning contingency criteria for ERCOT) might result in involuntary load shedding. ERCOT monitors inertia levels in real time and will bring additional synchronous generation online if system inertia starts getting close to critical levels. While this situation has not yet been encountered in operations, maintaining system inertia in this manner will likely result in out-of-market actions and additional curtailments of wind and solar generation. If these actions and curtailments happen frequently, ERCOT is likely to introduce a procurement mechanism for additional inertia. This situation would create an excellent opportunity for technologies, such as E-STATCOM, to offer instantaneous active power contribution in an inertial time frame.
TenneT
Power system stability challenges have been identified in Germany under scenarios with the high penetration of nonsynchronous renewable generation. In response, driven by ENTSO-E, TSOs in Europe are motivated to initiate the deployment of technologies with GFM capabilities through the procurement of new transmission assets, such as HVDC systems, flexible ac transmission systems, and synchronous condensers.
The German TSO TenneT sees the need for its own future assets to contribute to the inertia as much as possible. Particularly, besides the synchronous condensers, E-STATCOM is seen as one of the most suitable technologies to install for a significant inertia contribution. In December 2020, the four German TSOs collectively published a position paper titled “Need to Develop Grid-Forming STATCOM Systems.” The position paper communicates a need for between 23,000 and 28,000 Mvar of controllable reactive power capacity and emphasizes the need for GFM technologies in both the German and broader European grids. The TSOs will procure a significant portion of this 23,000–28,000 Mvar using STATCOM technology, and these STATCOMs will be procured in two distinct phases. During phase 1, conventional STATCOMs should be outfitted with GFM control characteristics. During phase 2, a new generation of STATCOMs with active power capability, E-STATCOM, is expected to provide a certain share of this inertia deficit. To stimulate market development, TenneT purchased the first pilot project for phase 2 with a ±300 Mvar E-STATCOM in 2022, with the goal of pushing market development and for getting technology experience as quickly as possible. Some of the essential points for dimensioning the pilot project have been:
New challenges are surfacing related to the operation of a power system with a high penetration of IBRs and this susceptible risk of grid instability. This article focuses on the lack of GFM elements, low inertia, and the risk of frequency instability. E-STATCOM, a new power stability solution, is an extended version of conventional STATCOM technology, using supercapacitors to provide bulk active power in a fast and controllable manner to contribute to the stability of a power grid.
As with other innovative solutions, the industry will require time to gain operational experience and study all potential benefits of its implementation. It is a joint responsibility of suppliers, system operators, and regulatory policymakers to develop a framework that allows the adoption of new technologies that can improve system reliability. To facilitate this deployment, suppliers should make existing operational experiences and models of this technology readily available for transmission planners. The system planners should formulate the requirements in a way that defines the performance of the device without defining the design of the device itself. Ultimately, E-STATCOM will be an important addition to the existing toolbox of system planners to reach the ambitious carbon neutrality goals while ensuring the required grid stability.
L. Meng et al., “Energy storage enhanced STATCOM for secure and stable power grids,” in Proc. CIGRE Session, 2022, Paper 10518.
S. M. Iftekharul Huq et al., “Methods and requirements for the upgrade of HVDC and STATCOM with grid forming for multi-level converter topologies,” in Proc. CIGRE Session, 2022, Paper 11088.
“Need to develop grid-forming STATCOM systems,” Dec. 2020. [Online] . Available: https://www.netztransparenz.de/portals/1/Content/Weitere%20Ver%C3%B6ffentlichungen/4%C3%9CNB_Positionspapier_netzbildende_STATCOM_final_EN.pdf
“Odessa Disturbance. Texas Events: May 9, 2021 and June 26, 2021,” North American Electric Reliability Corp., Atlanta, GA, USA, Sep. 2021. [Online] . Available: https://www.nerc.com/pa/rrm/ea/Pages/May-June-2021-Odessa-Disturbance.aspx
“Inertia: Basic concepts and impacts on the ERCOT grid,” Electric Reliability Council of Texas, Austin, TX, USA, White Paper, Jan. 2018. [Online] . Available: https://www.ferc.gov/media/inertia-basic-concepts-and-impacts-ercot-grid
Hoke, V. Gevorgian, S. Shah, P. Koralewicz, R. Kenyon, and B. Kroposki, “Island power systems with high levels of inverter-based resources: Stability and reliability challenges,” IEEE Electrific. Mag., vol. 9, no. 1, pp. 74–91, Mar. 2021, doi: 10.1109/MELE.2020.3047169.
Tobias Engelbrecht is with TenneT, 95448 Bayreuth, Germany.
Andrew Isaacs is with Electranix, Winnipeg, MB R3Y 1P6, Canada.
Sergey Kynev is with Siemens Energy, Raleigh, NC 27616, USA.
Julia Matevosyan is with Energy Systems Integration Group, Austin, TX 78727 USA.
Bernd Niemann is with Siemens Energy, 91058 Erlangen, Germany.
Andrew J. Owens is with Hitachi Energy, 72212 Vasteras, Sweden.
Biswajit Singh is with Hitachi Energy, 72212 Vasteras, Sweden.
Andrea Grondona is with Hitachi Energy, 72212 Vasteras, Sweden.
Digital Object Identifier 10.1109/MPE.2022.3230969