If you are closely following the solar and wind-energy stories, then you have undoubtedly heard about "grid instabilities". As the percentage of non-central power-plant contribution goes up, risk of "grid instability" increases exponentially.
A relatively small, stand-alone grid centered in Alice Springs, Australia goes unstable at 13% "Green" energy make-up.
60 Hz, Alternating-current
In the US and in Canada, the standard "line voltage" for in-house appliances is 60 full, sinusoidal variations in line voltage every second with the peak voltage nominally between 155V and 175V with an Root Mean Square voltage of 110V to 125V.
In Europe, the frequency is 50 Hz.
The technical problem is that solar cells produce DC power...that is, no reversals in voltage. Wind power can produce alternating current but it is freakishly difficult to match it to the demands of the grid so it is usually converted to DC and then chopped up and flipped around to feed it into the grid.
The least expensive way to convert DC into AC that matches up with the grid is to reference the voltage of the grid and then to "trigger" off of it.
The grid is huge and robust and highly buffered by its loads(inductive) and line(resistance) characteristics. But, at some point the energy injected into the grid starts to jack around the voltage that is triggers when the "green" sources push energy (120 times a second) into the grid.
A complication is that the grid, as an electro-physical reality, can store energy in several modes: Capacitive and inductive. That can creates internal dynamics that can cause resonance and escalating excursions especially when simple, "step" or "impulse" (on-off) inputs are applied.
Another complication is that there might be learning-algorithms incorporated into the logic. The intent would be to make them universal in the sense of not caring if they were sent to a 50Hz grid or a 60Hz grid. Chaos and drift are not your friend when you have an energetic friend dumping power to your grid.
An obvious work-around would be to have a master-clock similar to GPS setting the timing for the (now) disjunct parts of the system. The problem is that we know that GPS can be hacked. Hacking the master-clock signal has the potential to melt wires and/or maybe short out transformers.
Not mentioned is how much power is lost doing all the conversions. Could easily be half of what is being generated.
ReplyDeleteDC to AC power conversion is quite efficient now which is part of why PV power is attractive.. Our system in California had a Solar Edge common inverter/charger that was rated at 97.5% DC to AC. It was designed to connect to the grid and provide distribution-grade power. Enphase microinverters are rated at 97.7% and they can charge the Enphase batteries too.
DeleteBoth classes of inverters provide clean sine wave (<3% THD) power, these aren't bang-on/bang-off chopper inverters (THD = 48%) anymore. The grid still needs enough balanced inductance (leading) and capacitance (lagging) reactive load along with resistive load for stability
I love this blog. Roger
DeleteLooks like I'm a bit dated & haven't kept up. Did graduate 1972.
DeleteConverting the phase of ac/dc is old technology...and you can clean up the sine wave even more by using multiple phases. It's possible to get a sine wave for power to where the ripple is only a percent or so. All old technology that's been in use for decades.
ReplyDeleteOne of the things giving the grid stability is the huge amount of rotating mass in the equipment at the large power stations. They store energy with flywheel actions, and it tends to damp out sudden impulses.
ReplyDeleteCould the rotational stability be duplicated by spinning up a large unloaded motor? I realize most motors lag in frequency because of their design, but is it possible to make one that would rotate in lockstep with the grid and thus feed back or take up fluctuations in frequency?
DeleteThat would be called a "Synchronous Motor".
Deletehttps://en.wikipedia.org/wiki/Synchronous_motor
The reason "regular" AC Inductions Motors don't spin at a synchronous speed is due to "Slip". Unless the rotating magnetic field produced by the field coils moves relative to the rotor, there won't be any current induced in the rotor, and it won't produce any torque.
https://en.wikipedia.org/wiki/AC_motor#Slip
Yes of course. I was thinking the same thing but I knew they 'cheated' with a shorting coil to get the rotation going in the desired rotation and that would be a huge loss; but then realized that was just a cheap work around and it could be made without it and even have the speed and direction controlled locally with circuitry. Have a huge magnetized armature rotating in phase would 'stiffen up' the grid. Please excuse my random thoughts on this.
DeleteAnyone with an even elementary understanding of physics will tell you that wind and solar and electric cars are pipe dreams. And…they are environmental disasters waiting to happen. Wind and solar can do small jobs. Keep some LEDs on at night, maybe run some very low wattage accessories. But the second you turn on a toaster, microwave, air conditioner, dryer… it’s all over.
ReplyDeleteIt boggles the mind, the stupidity that drives the environmentalist movement.
They are not stupid. They have a naturalist attitude which can be summed up by the notion that the planet would be better without people, except themselves. They want to destroy the capitalist economy. This is just one avenue of attack. I will grant that there are many useful idiots bleating. Roger
DeleteMox Nix, but I could swear I heard a fellow at the shop checking a 50 cycle piece of equipment destined for Canada. He was vocal because he had to find a transformer that would change the cycle inaddition to the voltage. Roger
ReplyDeleteCanada and Mexico share a distribution grid with the US, so they are 60Hz and 120VAC standard as well. You have to go a lot further south to find countries that are on the European 50Hz/220VAC standard. The Sir Adam Beck Generating Station in Niagara Falls, ONT, Canada supplies power to both the US and Canada, and it was the focal point and cause of the 1965 Great Northeast Blackout.
DeleteYour tech may have been testing something that was multi-frequency/multi-voltage or he might have been told to find the electrical equivalent of a metric crescent wrench.
I happen to know somebody who is into industrial aquaculture. His company purchased equipment from Denmark and it runs on 50Hz. The supplier said the warranty would be void if they ran it on 60Hz (frankly, most induction motors probably don't care but pumps might cavitate). In order to avoid potential warranty coverage issues, they also purchased 60Hz-to-50Hz converters from the same supplier.
DeleteI designed a few of those in the 200HP range. They're called "Static Inverters" as they have no moving parts. One shipboard installation I worked with required 400Hz power, as it was aircraft-derived. We used a "Rotary Converter" consisting of a DC motor (easier speed control than AC at the time) driving a 400Hz Alternator.
DeleteThe equipment came from Italy so the ERJ scenerio could be right on.
Delete