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#580 – Electrical Archeology

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  1. Just a comment on Dave’s talk about the grid operators turning off solar: Not sure if it’s Australia wide. But Solar remote disconnection is definitely a thing in South Australia. https://www.abc.net.au/news/2020-08-27/authorities-power-to-switch-off-south-australia-solar-panels/12602684

    After our state wide blackout a commission into SA’s power grid imposed new guidelines for solar installations. One guideline is that South Australia Power Networks, (SAPN) should have the ability to enact “PV shedding” to maintain grid stability.

    The way it works in practice: In the agreement with SAPN to connect your system upto the grid you must have a “smart meter”, if you don’t have one, it’s installed before your solar can be hooked up. This talks back to SAPN via either 4G or a local peer-to-peer ISM network (via other smart meters).
    These smart meters have 2 separate “channels” that provide independent measuring, but also the ability to disconnect. (i.e. if you don’t pay your bills).
    The solar installation must run back through the second channel. If your system is not installed this way then SAPN have the authority to boot you off their grid.

    So essentially if SAPN deem it necessary to enact “PV shedding” in much the same way they’re able to perform “rolling blackouts”, they turn off this second channel in your smart meter remotely, and to your solar inverter it’s as if you flipped your solarmain breaker, and the inverter is disconnected from the grid. The inverter doesn’t need a wifi connection or any extra smarts for this to work.

    This system is only in place on new installations or if you upgrade an existing installation.

  2. Utility Operational Technology engineer here… Chris is right; there’s not one person who knows it all at the electric utilities (even the operators, though they often have a good understanding of the power system dynamics). There’s even a pretty significant difference between transmission and distribution operations.

    In most cases, smart meters on residential and small commercial customers do have a full-load (200 Amp) disconnect built into them so the power company can turn the power on and off remotely. This is often used for move-in (new customer) and move-out processes. This can also be used for pre-pay services, which not a lot of utilities offer, but are an option available to utility regulators who have options to reduce late payment charges. Of course, the utility can also use the smart meter disconnect to shut you off for non-payment, but many public utility commissions have their own rules about how the utility goes about doing that.

    As for putting disconnect only on solar panels, I’m not aware of.a utility that does that, at least on a typical residential rooftop solar system. (Larger solar systems, perhaps 1 MW or greater, might have “transfer trip” load-breaking switch in front of them that the utility can operate for safety or emergency purposes, but that’s not generally done for rooftop systems.) The confusion may also be due to the fact that most inverters used with solar panels are so-called “grid following” inverters, meaning they stop functioning when the grid voltage goes away. (Essentially they function as current sources that track the voltage on the line.) So if a utility remotely disconnects your meter they also remotely disconnect your grid-following solar inverters. Inverters can also shut themselves off when line voltage and/or frequency go outside a certain window and those set points, to a large part, are set by the utility. The direction we’re headed seems to be that utilities will be able to communicate with inverters to adjust those windows (i.e. set points) in real time. Many utilities, at least in the US, follow the IEEE 1547 interconnection standard that covers most of these issues related to how a grid-connected inverter is supposed to operate.

    As Dave points out, with rooftop solar penetration so high, the distribution system is getting more complex and harder to operate. One thing I’d like to mention is that when the grid starts to have excess capacity (Watts) *frequency* goes up, not voltage. With lots of incentives to put solar in (at least in some places), there have been places like Germany where they have had to figure out how to adjust the inverters to back off on power production as frequency increases instead of all cutting out at a certain frequency. (See the “50.2 Hz problem”- https://solarindustrymag.com/north-american-solar-seeks-to-learn-from-germanys-grid-integration-trials.)

    A comment about “channels”… In the electrical utility metering world, a “channel” is merely one measurement, usually energy, but not necessarily so. A common “four channel” meter records, kWh delivered (from utility to customer), kWh received (customer to utility) kVARh delivered, kVARh received, but that’s all through one electrical path through the meter. There’s no “channelling” in the meter such that you can shut off power flow in one direction but not the other.

  3. 1) Is the Northen Territory still running the solar power cable to Indonesia, or has that system been cancelled?

    2) Some systems with batteries can be triggered charge the batteries from the mains when power is cheap, and also sell back to the grid when power is expensive. You split the profits with the provider.