[Radio Wrangler]

SMPS have a nasty characteristic that they try to look like constant power loads. If the supply voltage is low, they compensate by taking more current by changing their switching periods.

So current goes UP as voltage goes DOWN. Um, that's negative resistance!

The avionics industry is worried about this. As more and more systems, instriments, radios radar use 'smart' switching power regulators, a lower and lower negative resistance gets imposed as the load on a battery and generator. The stability of the battery voltage control of the generator comes into doubt.

I suspect that something similar is looming for the electricity distribution network. The thermostatic load issue is undestood, but the SMPS business is on a cycle by cycle basis.

The distribution system has not only many loads being brought back on line, but also many different generators with different regulation schemes scattered all over the place. Think of the numbers of small solar inverters. They are arranged to shut off feeding the grid if the grid voltage goes too low, but the business of the grid coming up will bring them on-line in various ways.

It's all rather scary once you start thinking about it. The new requirement for surge suppression in new installations starts to look sensible.

David