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Template: AEMO System Strength Impact Assessment Guidelines (Appendix B)

Preview of template's gridmo Flow showing Nodes

Last updated: 3 Apr 2025

Country:

AU

Software required:

PSS®E
PSCAD™

Source: AEMO | System Strength Impact Assessment Guidelines | Version 2.2 | 1 July 2024

How to download: Add a new Flow in gridmo and select this template from the list.

Quick start

Before running this template, complete the following:

  • Configure your Global Variables and Scenario Variables using gridmo's Model setup template.
  • Add this template to your project.
  • Configure the additional Global Variables marked as '[Unique to AEMO SSIAG template]' (e.g. $ssiag_fault_duration_1 changes depending on your nominal connection point voltage).
  • If your X/R ratio at the connection point is unknown, for each Loop Start Node, disable all loop rows, then only enable those with X/R=3 or X/R=10 rows.
  • Set $ssiag_withstand_scr to your target Withstand SCR for your generating system.

Background​

Inverter based generators and loads can impact the grid in ways which require network infrastructure upgrades. AEMO introduced the System Strength Impact Assessment Guidelines (SSIAG) to quantify these impacts for individual projects by assessing a plant’s capability to ride through low SCR operating conditions. The calculated Withstand SCR value is then used by NSPs to calculate project-specific network service charges, whereby projects will be charged more if the NSP must provide for a higher SCR for the plant to operate satisfactorily.

AEMO published a methodology used to quantify these impacts in Appendix B of the following document: AEMO | System Strength Impact Assessment Guidelines | Version 2.2 | 1 July 2024. For clarity, as of the time of writing this template, this document is the latest document in force under the National Electricity Rules.

However, AEMO has since published an updated methodology in the following document: - AEMO | System Strength Impact Assessment Guideline Withstand SCR Methodology Review | Version 2.1 | 28 February 2025. This new document has not officially been adopted under the NER but AEMO, "...encourages NSPs to consider adopting the principles of the methodology proposed in the technical note...". Given the likelihood of this new methodology being officially adopted and NSPs adopting this in the interim, we have updated our SSIAG template in accordance with this new methodology in Section 5 of the Technical Note.

note

As stated by AEMO, "It should be noted that this proposal reflects AEMO's current thinking for further consultation and is not an amendment to the SSIAG". If you would like a copy of the old SSIAG methodology, please contact us.

danger

Whilst the SSIAG document explains what is required as demonstration of the Withstand SCR, there is no clear guidance provided on how to identify the Withstand SCR for a particular project.

If, based on your engineering judgement, a given generating system successfully rides through these tests for a given SCR, it does not confirm that it is stable under all modes for this SCR. However this may help early stage projects identify the likely range for the plant's withstand SCR.

Additional studies will likely be required to confirm this SCR is stable for your generating system, such as the AEMO Generator Performance Standards (GPS) template.

Configuration​

The pre-event operating conditions required in the Appendix B tests require particular configuration. These conditions are described in the image below.

SSIAG operating conditions

The image below shows the static configuration required to achieve this initial operating condition. Note that we use the PSS®E Static Node to achieve this operating point. To achieve this using PSCAD™ Nodes only would require custom static solver logic in PSCAD™. Therefore, we strongly recommend having a PSS®E Static case, even if this model is a heavily simplified representation of the generating system.

SSIAG operating conditions 2

Note that for many projects operating, this operating condition may not be realistic. Consider the example below, where Qpoc = Qmax = +0.395 [p.u. on Prated] to achieve Vpoc = 1.05 [p.u.]. For a generating system operating with a 4% droop curve, this would imply that Vref is 1.09 [p.u], as shown below. Nevertheless, due to the explicit description from AEMO, the template has adopted this operating condition.

SSIAG operating conditions 3

Helper 1 - Coarse search for Withstand SCR​

Demonstration of adequate performance for all tests in Appendix B is used to indicate the likely Withstand SCR of your generating system. However, when starting this process it can be useful to run a smaller subset of tests across a large range of SCR values to coarsely understand where the plant may begin to have stability issues.

The Withstand SCR helper has been provided to help you complete this coarse assessment of Withstand SCR prior to running all of the Appendix B tests.

note

By default, the helper is configured to work for positive active powers. For BESS, the l_pcmd Loop Variable can be updated for the charging scenario to ensure the active power command is reduced according to Table 1 of the Technical Note.

Section 7.4.4(p)​

The purpose of Section 7.4.4(p) is to demonstrate your model's ability to run a flat run in a robust manner.

Appendix B - Table 2​

The purpose of Table 2 is to demonstrate your model's ability to ride through faults under very low grid SCR conditions.

Assumption

The test duration is assumed to be 15 seconds. Figures 12, 13 and 14 in the Technical Note show simulations running for 10 seconds after the application of the disturbance.

Appendix B - Table 3​

The purpose of Table 3 is to demonstrate your model's ability to ride through grid voltage disturbances under very low grid SCR conditions.

Appendix B - Table 4​

The purpose of Table 4 is to demonstrate your model's ability to ride through sudden changes in grid SCR conditions due to network events (e.g. a line tripping).

Assumptions​

  • This template helps with Appendix B studies but doesn't cover all SSIAG requirements like calculating '∆AFL' at the connection point.
  • The SSIAG is unclear if both PSS®E and PSCAD™ should be overlaid for the first two tables of tests in Appendix B. It is also unclear if error bands in compliance with AEMO's PSMG are also required. Both have been included for completeness.
  • The SSIAG does not state what generator output channels should be presented as part of Appendix B. We have used the same plot structure as our AEMO DMAT template.
  • PSS®E does not natively support 3PHG faults. Hence for this template, we apply 3PH faults in PSS®E and 3PHG faults in PSCAD™.

Sources​

Revision history​

Version 5 | 3 April 2025​

New
  • Updated tests based on AEMO's System Strength Impact Assessment Guideline Withstand SCR Methodology Review.
Fixed
  • Updated helper run fault duration to 120ms.
  • The test duration for Table 2 tests was reduced from 21 seconds to 15 seconds.
  • Section 7.4.4(p): The chosen flat run simulation time was updated from 45s to 35 to represent a "...a stable simulation initialisation run > 30 s".

Version 4 | 17 Oct 2024​

  • Updated to AEMO SSIAG v2.2 (1 July 2024) - note there were no changes to the tests required (Section 7.4.4 and Appendix B were not updated in this revision). Removed SSIAG version number from plot subtitles.

Version 3 | 9 Apr 2024​

  • Updated to new template structure, standardised template sticky notes.
  • Added low.scr.mode=yes to all PSS®E Static Nodes to improve low SCR convergence performance.
  • Expanded helper run to loop down to SCR of 1.2 (was previously minimum of 2.0).
  • Added additional Advanced Parameters to the helper run to help with low SCR convergence.
  • Disabled POC X/R 3 and X/R 10 tests by default in all Loop Start nodes.

Version 2 | 30 January 2024​

  • Fixed plot sub-titles overlapping customer logos when describing SCR and XR in some Plot Nodes.

Version 1 | 1 Aug 2023​

  • First release.