Short Duration Undervoltage Disturbance Ride-Through Test Procedure

Background

AEMO has identified through analysis of recent power system events, that a proportion of distributed solar PV (DPV) disconnect in response to short duration transmission undervoltage disturbances. Given the levels of DPV installed in South Australia, a severe but credible fault near the Adelaide metropolitan area could cause disconnection of up to half the distributed PV in the South Australian region. 

Under specific conditions this would leave AEMO with very few courses of action available for secure operation of South Australia. These findings have been highlighted as part of the recently published: Renewable Integration Study report, the Minimum operational demand thresholds in South Australia Technical report and recognised by the South Australian Government and SA Power Networks (SAPN).

To reduce the potential for disruption and improve power system security, most immediately in South Australia, AEMO: 

to ensure inverters, and DPV by extension, respond appropriately during short duration undervoltage disturbance events to mitigate any further potential risks to power system security.

Development and publication of a VDRT Test Procedure is part of AEMO’s broader strategy to address DPV inverter behaviour, which also includes:

However, AEMO considers there is a pressing need to accelerate implementation of the VDRT Test Procedure in South Australia.

Test Procedure

Following feedback from the consultation, the final test procedure has been developed. The intent of the procedure is to verify the behaviour of an inverter energy system within a distributed energy resource (DER) installation during a short duration undervoltage disturbance.

The test procedure should be applied in conjunction with existing product certification testing for compliance with AS/NZS 4777.2:2015 and has been developed as a supplementary test. 

Implementation 

The following page has been published by the South Australian Office of the Technical Regulator with details on inverter connection requirements, the scope and timing of application, how to submit inverter test results and the list of approved inverters. This information is relevant for inverter manufacturers and installers that must meet South Australia’s inverter standard requirements: 

Frequently Asked Questions

  • Q. I have internal testing to show my inverters comply with the Voltage Ride Through requirements. Can my inverters be sold in South Australia while getting the testing verified by an accredited test lab?

    A. Yes. The internal test results can be sent to the South Australian Office of the Technical Regulator (OTR) via otr.invertertestresults@sa.gov.au. A legal Undertaking is also required to supply the accredited test results by 31st March and committing to remedial action in the event the inverter is not tested and found to be compliant by an accredited test lab. The Undertaking template can be downloaded from the OTR’s Voltage Ride Through page. 

  • Q. If an inverter already meets the requirements of AS/NZS 4777.2:2015, and has completed the test associated with the standard, does it need to complete the VDRT test?

    A. Yes, the VDRT test is an additional test procedure that is not captured in the current AS/NZS 4777.2:2015 test.  

  • Q. Once the VDRT test has been completed at an accredited test laboratory, where do the results need to be sent? Once test results have been reviewed and approved, where will compliant inverters be listed?

    A. Once the test has been completed, results must be sent to the South Australian Office of the Technical Regulator (OTR) via otr.invertertestresults@sa.gov.au. More details of the submission process can be found on the OTR’s Voltage Ride Through page. The full list of inverters can be found on the OTR’s Voltage Ride Through page. 

    From 31 March 2021 the listing of compliant inverters will be taken over by Clean Energy Council through the products listing process. The CEC is currently reviewing and recording inverter VDRT test results. Therefore results should also be submitted to products@cleanenergycouncil.org.au 

  • Q. What will happen when the new AS/NZS 4777.2:202X is published?

    A. The AS/NZS 4777.2:202X will supersede the Voltage Disturbance Ride Through test procedure once published and applied (which includes a 12-month grace period to allow for development and testing of products). However, it is important that inverters to be installed in South Australia complete the VDRT test procedure in order to meet the South Australian requirements in the interim. Once the revised AS/NZS 4777.2:202X is published and applied all inverters will need to meet these requirements in full. 

  • Q. Is type testing for a series of inverter acceptable?

    A. Certification practices for the VDRT test procedure will remain consistent with the current application of AS/NZS 4777.2:2015. So, if the series of inverters are able to receive the same AS/NZS 4777.2:2015 certificate to be listed on the product listing then the same would apply for this test, otherwise they would require two separate certificates. 

  • Q. The AEMO test procedure Section 2.2 Undervoltage (V<) disconnection test in response to event duration exceeding trip delay time is similar to AS/NZS 4777.2 Appendix G2.2 Steps (h) to (l). Is meeting the AS/NZS 4777.2 G2.2 test sufficient?

    A. The difference between the two tests is described below: in AS/NZS 4777.2:2015 Appendix G2.2 and the test described in of the AEMO test procedure is: 

    1. The AEMO test Section 2.2 the intent is to identify the response following a fast duration voltage dip (“step Voltage down to 177.5 V (2.5V below 180V) with the step change completed within 2 ms”) as would be experienced for a transmission level voltage disturbance. As per the Step (b)  
    2. The AS/NZS4777.2:2015 Appendix G2.2 the intent is to identify the response following a slow voltage sag (“the voltage shall then be decreased in steps less than 1 V to 177.5 V. The dwell time at each voltage step shall be 5s”).

    Through field observations and bench testing, AEMO have identified that the response of inverters to voltage events does appear to differ based on the speed of voltage event (i.e. some inverters are able to stay connected for events that appear for less than 1 second where the voltage is stepped down gradually but not when the voltage is rapidly decreased). It is therefore also necessary to also confirm the converse, that is, that the inverter does disconnect for an islanding event even if the voltage dip is rapid (as a worst case, the inverter might remain connected through a fast voltage event greater than 1 second which is not desired either). 

  • Q. Can this update be achieved through inverter firmware upgrades?

    A. The intention of test report is to identify the subset of inverters that already have the voltage ride-through capability and is not intended to trigger inverter redesign or upgrades. If changes to firmware or design are required, the inverter should undertake compliance testing against AS/NZS4777.2:2015 again to ensure that the full suite of requirements are maintained.

  • Q. The standard only applies to newly installed inverters. How will this mitigate power system security risks currently being faced if the vast majority of installed DER/inverters do not meet the requirements?

    AEMO has identified that whilst this is an immediate risk in South Australia, if it is addressed rapidly it would not require retrospective changes to the existing inverter fleet. AEMO is able to undertake other actions such as procuring additional contingency FCAS or applying new constraints in the meantime.  

    Further, retrospective changes applied through an unmanaged firmware upgrade carries risks that could have unintended consequences across the fleet of inverters. 

  • Q. Will this standard apply to replacement inverters? Battery and PV inverters? And off-grid inverters?

    The changes are intended to apply to all new and replacement inverters, other than warranty replacements, for both PV and battery systemsOff-grid inverters do not impact system security and therefore do not form part of the scope of this test procedure.

     

  • Q. Will these changes increase costs for consumers?

    While testing will incur a cost to inverter manufacturers, this should be minimal (approximately $1000-3000/inverter certificate). The intention of this scheme is to identify the subset of inverters with the capability already built in, and not to trigger inverter manufacturers to redesign or rebuild their inverters to achieve this. So, there should be no additional cost to existing inverters on the market, it may, however, impact the number of available inverters in the market. 

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