Rising levels of inverter-based resources (IBRs) create a need for new approaches to modelling for the power grid, for both operational and planning purposes. Examples of such shifts in modelling approaches are occurring for some regions of Australia’s National Electricity Market (NEM) power system, as the need to understand and address unknowns before events occur has led to the development and use of wide-area electromagnetic transient (EMT) models.
Babak Badrzadeh
Situations prompting the need for electromagnetic transient models
The Australian Energy Market Operator (AEMO) has been using EMT simulation models for several years, including for black start studies, sub-synchronous control interactions between series compensated lines and IBRs, and stability analysis of one to two remote and radially connected IBRs under low system strength conditions.
These applications shared two common features: the need to simulate a small part of the power system under consideration, and a clear indication of previously known power system phenomena that had occurred globally and were well understood in the international community. In Australia’s National Electricity Market (NEM) power system, there has been an increasing number of incidents stemming from unknown plant behaviour. Examples include:
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- 2015: extended commutation failures of a line-commutated high-voltage DC (HVDC) link due to interaction with the power system to which it was connected under remote rather than close-in network faults and the impact of protection settings deployed in the HVDC link.
- 2016: protection settings applied to some IBRs which limited the number of network faults they could ride through, despite network voltages and frequency recovering after each fault as they occurred.
- 2017-18: insufficient synchronous machines online and excess system-wide IBR output, which led to inadequate system strength under conditions where secure levels of inertia are available.
- 2019: sub-synchronous control interactions between multiple IBR without a series compensated line or HVDC link.
In applications like these, it is not feasible to predetermine the extent of the power system that must be modelled to produce an accurate outcome. Further, except for the 2016 instance (which was associated with a black system), the phenomena of interest and associated dominant frequencies are such that the use of root-mean square (RMS) modelling is not suitable [1].
There is very high penetration of IBR in some NEM regions (or areas within NEM regions), and new generation connection applications are almost exclusively IBRs, of a range of fast-developing technologies. The resulting increased potential for adverse interactions, together with the need to understand and address unknowns before events occur in practice, are key factors contributing to AEMO’s development and use of wide-area EMT models to the same extent it has used RMS models.
Development of an integrated EMT model
The investigation of the 2016 South Australia black system event prompted AEMO to begin developing wide-area models of the NEM power system in 2017. By September 2019, AEMO had completed wide-area EMT models of all five NEM regions, using learnings from the earlier South Australian studies.
AEMO found that simulation studies required a wide-area model of one region along with a small part of the adjacent region. The need to better understand suspect intra- and inter-area modes of oscillations experienced in some EMT studies demonstrated the importance of developing an integrated EMT model of the whole mainland NEM power system (excluding its smallest region, Tasmania, due to decoupling via an HVDC link).
This integrated EMT model of the whole mainland NEM was completed in June 2020. It comprises approximately 3,000 busbars and 200 detailed EMT dynamic models, and covers a total area of approximately half the area of the United States. Advanced methods applied by AEMO ensured that the increased computational burden is limited to a maximum of 20 percent compared to a wide-area model of one region. This means that an EMT dynamic simulation run of this system lasting 30 seconds can be completed in three hours.
The use of this model has demonstrated very high levels of accuracy compared against measured system responses, giving AEMO confidence in the veracity of studies for making decisions on what-if scenarios.
Expansion of operational as well as planning applications
Regulatory frameworks determined by the Australian Energy Market Commission in 2017, and a range of guidelines and requirements developed by AEMO in 2018, have further expedited the need for wide-area EMT modelling for different applications, by AEMO and other organisations. Applications have included:
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- Determining whether a new or modified generator connection would adversely impact system strength, and assessing the veracity of different solutions if an adverse impact was identified.
- Calculating background levels of system strength and inertia determined by AEMO and maintained by transmission network owners.
- Developing operational advice for real-time power system operators under system intact and outage conditions, including when operating a normally interconnected power system as a sustained island.
- Determining the system operability envelope under non-credible contingency events.
The growing application of EMT tools in the NEM power system has resulted in an increase of approximately 60 percent in the number of Australian users compared to two years ago.
These EMT studies have been predominantly operationally focused. However, a number of mid- to long-term planning applications have emerged recently requiring EMT dynamic analysis. These include determining the system security benefits of a proposed interconnector between two states where intended benefits relate to phenomena which cannot be simulated by RMS modelling, determining whether a large-scale power system can be operated without synchronous generators, and designing system-wide special protection schemes.
These wide-area EMT models cannot be considered as a one-off model development exercise. Several initiatives are currently being undertaken by AEMO to:
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- Improve the total time taken to conduct simulation studies.
- Perform more extensive modelling of the distribution network to account for: (1) increased uptake of MW range IBR in distribution networks often in remote and sparse areas, (2) increased uptake of kW-range distributed photovoltaics with inferior responses to those of MW-range IBRs, and (3) changing load characteristics due to increased use of inverter-based loads.
- Streamline production of any given wide-area EMT model from state estimator data.
In conclusion, with the advent of high penetration of IBRs, we believe that large EMT system models are both necessary and practical, and will increasingly become only more so.
Babak Badrzadeh
Australian Energy Market Operator (AEMO)
References:
[1] B. Badrzadeh, ESIG blog ‘’ Electromagnetic transient simulation models for large-scale system impact studies in power systems having a high penetration of inverter-based resources’’, June 2019, accessed at: https://www.esig.energy/electromagnetic-transient-simulation-models-for-large-scale-system-impact-studies-in-power-systems-having-a-high-penetration-of-inverter-based-resources/
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