South Australia’s accidental experiment is taking it up a notch. A report commissioned by the state government from the market operator sets out the dire consequences if uncontrollable rooftop PV installations continue to grow at current rates. These are keeping pace with AEMO’s highest forecasts as shown in the graph below.
Solar supporters are keen to dub the collective capacity of rooftop solar panels as the state’s largest power station. AEMO has been pointing out for some time that having the largest power station in a region be incapable of responding to instruction by the market operator at times of need, and in fact, acting in a way that is all but invisible to them, is a risk to overall system security.
AEMO forecasts that net demand (underlying demand less rooftop solar output) in SA could hit aero or below within 3-4 years as shown below.
AEMO believes this is unprecedented globally at a large system scale (although there’s a chance WA could get there first…). The problem it creates is that if net demand is zero, then none of the large power stations around the state need to run. But, if they’re all switched off then where does SA get its essential system services from, like voltage and frequency control? Rooftop panels don’t provide these. Worse, if something does spike voltage, say, many of the solar panels are likely to trip off simultaneously. Given the different inverters in use, each of which can have different settings, one might think that diversity would take care of this. In practice it seems not, but the aggregate response is not sufficiently predictable for AEMO to work with it. It leaves SA vulnerable to the impact of a large cloud that takes hundreds of megawatts of solar out rapidly. This state of affairs also compromises emergency management such as bushfire response and system restart.
Interconnectors can help to some extent, allowing AEMO to source services from other regions. But this doesn’t solve the problem fully and still leaves SA vulnerable to “islanding” if the interconnectors have to be shut down – as can happen when system security is threatened. Nonetheless, AEMO emphasises that they see the proposed NSW-SA interconnector as being critical to future security.
This is only one of the 31 measures AEMO recommends. Other key recommendations are around the need to move quickly to start implementing the capacity for AEMO to remotely control rooftop PV systems via their inverters, specifically with a view to turning them off in certain extreme scenarios. Some analysts believe that curtailing solar at times of negative prices (which indicate excess supply, and so could align with the times that AEMO needs to intervene to manage security) can actually improve its value. Others, however, caution that it is harder than it seems to make this work, especially if we move beyond control of new systems to retrofitting existing installations. In any case, the technical challenges may even be outweighed by the political challenges of fronting up to solar owners who have had years of being told how much good they are doing the grid and the planet.