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Passing climate legislation in the 47th Commonwealth parliament is easy compared to the actions required to achieve the contemplated CO2 emission by 2030.
The focus is on clean (no gas, no coal) electricity generation and consumption with AEMO predicting that by 2050 our national demand will be more than 300 gigawatts, a tenfold increase from where we are today. It is assumed that the grid will be stable when all generation, bar hydro, will be inverter-based but that gets us into unknown unknowns because other than very small grids, there are no examples to base Australian designs on. We are wedded to AC networks by way of investment history and the challenge is to provide voltage and frequency stability when nearly all generation is asynchronous. Remnants of automatic generator control (AGC) and excitation/damping control (AVR) might remain but new control schemes will be required landing us in the unknown unknowns area. Battery-supported voltage forming inverters now being trialled in ARENA projects will be required in the grids of the future but they are not the equivalent of synchronous generators, which have 500-600 % over-current and negative sequence capacity.
Voltage Control – only part of the problem
All of the load busbars in the NEM will be extremely dynamic because of solar rooftop generation (by 2050, AEMO predicts it to be the single largest component in the power mix). The various trials in distribution networks are all about voltage control at the MV/LV level—nary a thought about dynamic control that may be required at zone substations including curtailment of solar distributed energy resources (DER). To envisage some grand scheme of overall grid control in the absence of dynamic, synchronised NEM-wide networks monitoring would be wasted time. However, a dynamic information system would, in time allow the nutting out of possible control schemes to provide stability in asynchronous grids.
We have been reassured—however falsely—that the near 100% asynchronous grid will be a breeze. But by the time, synchronous generation is below 40 % given the current technology, we will be in trouble, stability-wise because it has been the synchronous component that has kept the grid-following inverter-based resources (IBR) providing power. In the simplest terms, grid following IBR are happiest in high fault level circuits, and the reverse applies to voltage forming IBR. In actuality it is more nuanced: the impedances measured from the point of connection (PoC) back to the IBR in question and from the PoC into the grid determine the basic stability of the ‘arrangement’. AEMO models all this in effect but the challenge is that the modelling is often overturned by reality. The impedance from a given PoC back into the grid depends on all the other generators already connected. Modelling can at best provide an ‘admission ticket’ to the main game. As to how the participation of a particular IBR affects stability cannot be predicted on the basis of modelling and the Victorian Rhombus of Regret of solar farms exhibiting voltage and power oscillations is a ‘good’ example.
Dynamic Zone Substations
Zone substations with their highly dynamic loads are really no longer loads—they have become generators, so we might start dropping the term ‘reverse power flow’ and maybe call them ‘logerators’. AEMO is envisaging fast frequency control ancillary services (FFCAS) as ‘harvested’ from aggregated rooftop solar. But more is needed: for one, unloading of transmission lines as a result of reverse power flow is causing voltage control problems, necessitating thyristor controlled-reactors. Under frequency load shedding (UFLS) relays unhelpfully trip on low frequency events, cutting off solar inverters and therefore worsening frequency drop. Dynamic restraint would help but that is a multi-factorial business requiring a kind of fuzzy logic based on sampled network conditions throughout a distribution network. Setting up such a form of restraint requires a distribution network-wide monitoring system. There is little or no enthusiasm for spending money on this and short of some national incentive or forced requirement, for example by Governments, don’t expect anything to happen. Conclusion: trying to design NEM-wide control schemes without first of all putting the means in place to control zone substations is pointless.
