SnowFlakeUsername2 t1_iqph2bm wrote
So if I'm reading this right, it is using the grids frequency to communicate/regulate the power sources. I read three lengthy articles on this and all have a very vague explanation of how this is done. It would be really interesting to know the frequency varies based on loads. Anyone know more about this than the OP article?
mastapsi t1_iqply3w wrote
It sounds like the prime frequency of the microgrid is set by a battery system and its inverter. Everything else uses it as a reference frequency. When the battery is discharging and needs to be charged, it will lower it's output frequency, as a signal that the other generation sources need to increase production. If the battery is charging and needs to discharge, it will raise the frequency.
Frequency only really matters for three phase prime movers. Since the micro grid doesn't have any of those, there is no problem with frequency deviating from the nominal frequency by a half a Hz on either side. Once upon a time, it mattered for clocks, but we use solid state clocks with crystal resonators now.
AnotherFuckingSheep t1_iqpmr6z wrote
This is what it sounds to me but also this sounds like the battery is the controller. Is there some natural tendency in the system to increase the frequency when supply is bigger than the load?
mastapsi t1_iqpp7g2 wrote
Yes, when you have three phase prime movers (whether that is a motor or a generator). When a generator is over-generating for its load, it will spin faster and frequency will increase. When it is under-generating it will slow down and frequency will decrease. Similar for three phase motors, if there is too much power going into the motor, it will spin faster and force the frequency up, and vice versa.
If there are no three phase prime movers, then there is no natural tenancy for this. In the case of the microgrid, it's entirely manufacturers for signaling purposes.
AnotherFuckingSheep t1_iqppi5s wrote
I see so this system is mimicking the system already used in generator and at the same time allows existing diesel generators to also tie into the micro grid
SnowFlakeUsername2 t1_iqpuf8g wrote
This makes sense. But couldn't the same thing be accomplished via signaling sent at a different frequency? I'm assuming both would require a simular control system, but with the controller needing a simple low power transmitter VS the inverters varying the power frequency.
mastapsi t1_iqpzl4y wrote
The advantage here is it basically ties in with existing droop governors, so you don't need specialized signalling at all. You just need the specialized system driver (the battery system+inverter) that sets the reference frequency.
Knackered_lot t1_iqqsen7 wrote
This is not correct. You may be confusing frequency with voltage.
mastapsi t1_iqrkx61 wrote
No, it's not voltage. In an AC system, voltage will be determined more by reactive loads and generation than by real load mismatches. If your generators are not producing the right amount of VARs to negate the VARs produced by the system, then your voltage will deviate from your intended system voltage. This is probably not a huge deal on microgrids, since most loads will be unity power factor loads.
Now you can get voltage changes from real load mismatches, but usually you'll trip generators from deviating too far from nominal frequency long before that happens. Rotating generators are very sensitive to frequency, they are designed to run at a specific speed (which correlates directly to system frequency) and deviating too far can cause serious problems, so there is protection on them to trip them offline if they deviate too far from nominal.
Knackered_lot t1_irrqji2 wrote
Oh ok. I understand the AC generator part of things, but I am less sure about how this frequency works on a renewable energy microgrid. Does the variable frequency work in a way with the AC (capacitive/inductive) reactance equation? for say, you need a lower current so in an inductive circuit, higher frequency -> higher reactance -> higher impedance -> lower current.
mastapsi t1_irrytcm wrote
I don't think it would have much impact at all. The amount of deviation from nominal frequency is minimal, half a Hz in each direction, which is less than 1%. That's why they are using it for signaling.
Most renewables are designed to run their inverters based on a reference frequency from grid mains. For devices with no regulation capability, there would be no change to how this works. But devices that have some regulation (battery storage, hydrogen, wind) would have extra modules to interpret that frequency signal just like a droop controller would and vary their output to help balance the system.
Plus if you have existing rotating generation with droop control, like microhydro or diesel, they can tie in and still function.
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