THOUGHT LEADERSHIP

NEM connections, complexity, risk and tolerance: achieving a balanced approach to grid connection

With the current bow wave in publicly announced emerging and committed storage, wind and solar farms in Australia’s National Electricity Market (NEM), it is now more important than ever that your connection application processes are rigorous and efficient.

grid connection image crop

This article charts a path of least resistance within the framework of the Australian Energy Market Operator (AEMO) for connection, while also pointing out more effective approaches you could consider. The path of least resistance requires more upfront commitment of resources to ensure the application eliminates doubt or uncertainty. Other approaches take a more balanced approach to risk, accuracy and allocation of effort across the project development timeline.

Maintaining flexibility of deliverables – It’s not all about being technical

Plant dynamic and steady state parameters are likely to change and assessment requirements will evolve from the start to the end of your connection application. This is a natural process of optimising plant performance and compliance under various system scenarios, but comes at the potential cost of disordered delivery of the connection application and delays to the project. Streamlining and clear version control of the report will allow all stakeholders to easily trace changes. Furthermore, with significant updates to PSSe and PSCAD modelling requirements, robust automation of re-assessments would minimise processing time but the changing assessment requirements make investing in such automation somewhat fraught.

Finding a balance between fundamental principles, good practice and efficiency

AEMO has gone to great lengths to increase the certainty and magnitude of expectations about power plant modelling and performance standard assessment. There are paths of least resistance and good practice that can help facilitate a connection application within this framework (which we’ve outlined in the left-hand side of the table below). There are also some fundamental concepts that could help to eliminate wasted effort in the connection application process (the right-hand side below). Achieving a balanced approach is key to an effective and efficient solution not just for an individual project but for the industry as a whole. 

Don’t be scared of EMT modelling  
Whilst RMS (root mean square) assessments are still crucial, network service providers and AEMO are requesting significant effort in EMT (electromagnetic transient) benchmarking and analysis of wind and solar farms. Hence, it’s best to benchmark respective EMT and RMS models (using software such as PSCAD and PSS/e) at early stages of the project to identify inaccuracies. Particular focus should be given to plant responses to network and PoC (point of connection) faults with varying grid strength to ensure ride-through requirements (S5.2.5.5) are met and that active power, reactive power and voltage responses are within tolerable ranges between RMS and EMT models.

The industry (proponents and regulators) must consider which aspects of these models need to be accurate for the development of performance standards and the determination of grid compatibility.

When the overall goal is a stable connection of plant, pragmatic engineering judgment is often more beneficial than a slavish pursuit of absolute precision. We should focus more closely on fundamental issues relating to control stability, such as loop gains and time delays, than on the accuracy of switching controller triggers and sensing.
Understand your connection point and surrounding network  

We have previously discussed the issues of congestion and system strength and how they impact grid connection. These issues can result in plant MW constraints and non-compliant fault ride-through.

It is also important to remember that system strength does not equal SCR (short circuit ratio). System strength is a combination of high impedance network, high concentration of asynchronous machines and lack of nearby synchronous generation. It is a location-specific artefact that adds to the importance of assessing your plant’s control (S52.5.13) and contribution to faults (S5.2.5.5) in network models.

The strength or otherwise of the connection point must also be taken into account when considering whether the performance standards are reasonable. Issues such as fault current injection rates, rise and settling times, voltage regulation ranges and the like depend heavily on the surrounding network and often on other nearby generating units. 

 
Ride-through is not enough on its own  
Gone are the days of plant only being required to ride through PoC voltage dips for various periods of time. Recent versions of the National Electricity Rules require rigorous reactive current injection assessment including amount and rate of injection for varying voltage profiles, and for balanced or unbalanced faults. Hence, work is needed to tune the plant and inverter-based controls to meet these requirements (predominantly in clause S5.2.5.5) and maintain post-fault voltages within network tolerances (commonly 0.9-1.1pu). These more technical requirements of the rules take an inordinate amount of work to achieve compliance with a particular performance standard across the range of operation and system events to which the plant will be exposed. A more standard test could be considered to benchmark the performance of the plant against a small and finite range of events.

At Entura, we bring as much judgement and experience to the work of connection applications as we can. The process does not always reward this approach, but we maintain that understanding the intent of analysis, as well as the need and importance of it, allows us to target and emphasise some analyses over others – and this helps to manage the connection risks for our clients with a high level of efficiency.

If you’d like to discuss how we can assist you with your grid connection projects, please contact Donald Vaughan on +61 427 721 558  or Wayne Tucker.

 

About the author

Donald Vaughan is Entura’s Technical Director, Power. He has more than 25 years of experience providing advice on regulatory and technical requirements for generators, substations and transmission systems. He has worked for all areas of the electrical industry, including generators, equipment suppliers, customers, NSPs and market operators. Donald specialises in the performance of power systems. His experience in generating units, governors and excitation systems provides a helpful perspective on how the physical electrical network behaves. 

December 16, 2021