NERC sounds alarm on solar tripping in ‘sobering’ summer reliability report

NERC sounds alarm on solar tripping in ‘sobering’ summer reliability report
utilitydive.com

This is entirely the electricity grids own doing. They told everyone who wanted solar that they must use inverters that are super safe and self disconnect if anything unexpected happens. The regulations were written super tight, with frequency limits, harmonic limits, reactive power limits, rate of change of frequency limits, residual current limits, DC injection limits and vector shift detection requirements, presumably because rulemakers were hoping that having a massive thick rulebook would make rooftop solar too expensive. Unfortunately for them, all the complexity is pushed to software, and when that's written once it can be deployed lots almost free.

Fast forward 10 years, and this stuff is widely deployed, meeting all the very strict criteria. Except the grid only stays within those strict limits during the good times. When the grid is stressed, for example low frequency due to insufficient generation, it will go outside the frequency limit, and all rooftop solar generation statewide will disconnect all at once. And now the grid has lost gigawatts of solar production, making a minor incident far far worse. The end result is a cascading failure and within a matter of seconds half the state has a blackout.

Smart rulemakers back in the 2010's would have dropped islanding protection, and vector shift protection (they actively destabilise the network). They would also remove underfrequency protection (again - destabilises the network) and slacken substantially undervoltage protection.

The only protections important to leave in are rate of change of frequency (upwards only), overfrequency, and overvoltage.

What's ironic is there is a more sensible requirement they could have required to make rooftop solar cost more and add substantially to grid stability. They could have required solar systems have a load line equivalent to coal. That pretty much means if the grid frequency decreases, the solar systems must output more power. But solar can't output more power than the sun provides. So in turn, to meet this requirement all solar systems would have to be constantly throwing away say 10% of capacity, so that it could be delivered if and only if the frequency suddenly dropped.

The rulemakers were expecting solar to be a failure and there to never be anything more than a handful of geeks with it installed. The problems only become major when 1+% of statewide generation is solar. 1% is (very approximately) the amount of generation an electricity grid can suddenly lose without failure.