By Phil Kreveld
The wilful blindness to untrammelled expansion of rooftop solar is stopping sensible engineering to take the advantages it offers—keeping sources of energy generation close to consumption points.
That would dramatically reduce capex in large-scale generation and transmission. The major hurdle is that we have energy market designs that impede a ground-up, rational revisitation of measures to be taken to meet the nation’s aspirational renewables targets. The apocryphal story of the hapless traveller, arriving in Ireland and on seeking directions to Dublin from a local is told, “Well now, I wouldn’t be starting from here” comes to mind when looking at the floundering going on regarding the effective harvesting of Australia’s ballooning rooftop solar resource.
Rooftop solar, according to AEMO’s 2024 Integrated System Plan (ISP) will be 70GW by 2050. We do not marvel at the prediction, nor do we reflect on the fact that nearly all generated and transmitted electrical energy ends up in distribution networks. It’s in Australia’s population centres where dark, gleaming solar panels are an unremarkable feature; they are the fundamental part of any engineering plan for integrating renewables. Distribution networks don’t figure in grid development planning, and that is a gross deficiency because the planning has to start at the main points of energy consumption, and move from there upstream into transmission and large-scale generation.
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Given the uncontrolled growth in solar, rooftop distributed energy resources, often exceeding aggregate demand, networks have to make a basic decision as to whether LV, MV, or sub-transmission reverse power will be permitted, when and to what scale. Decisions on reverse power in sub-transmission in terminal stations are location, and transmission-layout dependent (availability of absorbing load).
Decisions made on maximum reverse power will see to curtailment of domestic inverters and/or limits on the solar panel power size permitted to be installed. The voltage level (MV, or LV) at which reverse power will be permitted will govern where excess power absorbing batteries are to be placed, i.e., community batteries are a misnomer as their main task is voltage control. Alternatively, reverse power could be allowed into sub-transmission (66kV), where BESS-voltage forming converter/inverters could be located, storing energy, capable of maintaining a stable voltage for power to local substations and for stable operation of grid following, PLL-inverters in the LV ‘edge of network’.
We need to take a breather here and ask: inasmuch as distribution networks use over 90% of all generated electrical energy, where would the excess aggregate capacity in distributed energy resources (DER), mainly solar, go to? Technically the difficulty with sending excess power into the sub-transmission and transmission system is that there has to be an absorbing load on the receiving bus at the ‘other end of the line’.
However, distant distribution networks on the southeast coast would more than likely also be energy self-sufficient during a large part of the day. Furthermore, because there is so little time-difference between Adelaide (138° E), Melbourne (144 ° E), Canberra (149° E), Sydney (151° E), and Brisbane (153° E) that charge/discharge time differences are perhaps an hour or so, i.e., charging and discharging being correlated so narrowly doesn’t make energy transfers over the transmission grid a solution.
The proposal for BESS-grid forming inverter/converters at terminal stations would render substations effectively, independent or close to, of the transmission grid. Existing transmission lines and generation would ‘top up’ terminal station BESS, and the southeastern grid would still be a contiguous, synchronised system. A fundamental hurdle is that the scheme proposed here cuts across regulations (distribution networks cannot be generators), and energy markets.
But we should recognise the reality that the mums and dads of Australia, with encouragement of state and Commonwealth governments, are rapidly becoming self-sufficient, energy-wise. However, the wide variety in rooftop solar PV makes their orchestration nigh on impossible, and rather than evermore complicated schemes to reward their owners, cajoling and curtailing them to prevent reverse power, a scheme to recognise the reality of AEMO’s forecasts for DER is needed.
The reason for not allowing power export is that the engineering and investment needed in distribution networks would be massive—to name a few; voltage control, protection and control, transformer upgrading, phase balancing reconductoring. Turning one-way distribution into a dynamic, two-way system is a major task but so is the construction of long transmission lines (meeting charging current limitations, voltage control, thermal limits, security—and ‘social license’). Not only that, but the engineering and control implications of distant, REZ-inverter-generated electricity is an enormous technical challenge, that publicly, at least, the engineering community remains ‘shtum’ about.
The edge-of-network solar inverters are voltage sourced, with controlled current, and therefore requiring phase-locked loop (PLL) operation. These are untuneable and are therefore subject to (confused by) phase jumps depending on angle size (for example due large loads switching in and out). They have anti-islanding properties that vary amongst them, making power support during fault induced, delayed voltage recovery impossible to predict. If over-modulated through failure in fast control loops or maximum power point tracking, they can inject low frequency harmonics and interfere with proper operation of protection relays.
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All this would need to be taken account of, as well emerging vehicle charging and V2G. As already stressed, re-engineering distribution is a major task. Re-engineering terminal stations, utilising BESS-grid forming inverters with voltage-reactive power loop control, power-frequency control and frequency-inertia control is costly but it would be laying the foundations for sensible renewable integration. Modularity (adding more paralleled H-bridges) to the same fast inner loop control for sine wave modulation and outer loop plant control, as well as more battery modules, would allow keeping pace with consumption/generation of prosumers.
The alternative, the preferred way—the continual interminable energy summits and talkfests, is for a generation and transmission system based on renewable (remote) energy zones, large BESS and wind, in particular offshore. Yet, in the mean time individual electricity consumers are encouraged to install batteries, making them increasingly independent of externally supplied power. Yet, they need to stay connected to their distribution networks, which in turn, need to stay connected to the transmission grid, for a stable source of voltage required for the PLL-synchronised inverters.
There should already have been a fundamental reassessment of the national electrical energy policies but it appears to be studiously avoided. A national engineering review should be undertaken, now!
