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A lot different: backfeeding the grid is not about stabilizing the grid but instead stabilizing the local MICROgrid of our homes, essentially a solar inverter t

by kkfx 4y ago

A lot different: backfeeding the grid is not about stabilizing the grid but instead stabilizing the local MICROgrid of our homes, essentially a solar inverter take time to ramp up production when a load appear, take time to step down production when load drop. The grid compensate, with strict limitation, for instance in France you can't push more than 3kW if mono-phase, in Italy 6kW, even if you are allowed to have a monophase 10+kW for instance to being able to recharge your EV at 7kW, the needed common minimum to keep cells in balanced state (3kW slow charge is simply too low for most vehicles, they recharge but the battery might suffer). A simple practical example: my dishwasher start heating water is +2kW in an instance, my solar inverter take 2-3" to ramp up to +2kW. The home microgrid need or the national grid or a battery inverter to avoid collapsing due to a too big frequency drop. The dishwasher have finished heating water, again a sudden -2kW, again I need to divert extra energy to the grid or a battery to avoid a frequency peak big enough to make the inverter disconnect. AC grids are like a tense rope with various "needles" attached to it, some push the rope at a certain frequency, let's say 50 or 60Hz, some are dragged by that frequency. Those who push are generator, like a solar inverter, a NPP etc, those dragged are loads. Those who drag need to keep the frequency so when a new load arrive they need to push stronger at the same rate, when a load detach itself they need to push less at the same rate. It's NOT easy. We have made public grid of a certain size, not too little, not too big, to average the load, we have observed that at a certain size demand is "constant enough" to made easy keep the frequency. Below or above is hard, the resulting grid is unstable. Now, since served people became bigger and grid infra was not evolved with them we start to make "grids interconnections" to help compensate, that's worked a bit, but also sometimes that have made catastrophic cascading effects. We are at a point we can't compensate anymore. Witch means grid instability on grid complex enough that restoring might take time, non in seconds or minutes but in hours or days. Some hope that grid-tied EVs so batteries and powerful, fast inverters, can help absorbing excess energy when grid loads drop, pushing energy to the grid when grid load ramp up. I'm not in the field but given the very small example of my home system (solar + lithium storage) I really doubt such idea can work in practice. Also being an IT guy I fear about it's potential crapload of vulnerabilities. The sole solution I see is re-do the public grid, at State level, for current and foreseeable future needs, with enough NPP, gas PP and distribution networks to have again a stable national grid, no smartness needed. Since I doubt that's happen that's why I invest in a system with lithium storage witch here (France) is definitively NOT convenient economically speaking, preparing to more and more frequent blackouts, longer and longer...

Johnny555 4y ago
A simple practical example: my dishwasher start heating water is +2kW in an instance, my solar inverter take 2-3" to ramp up to +2kW. The home microgrid need or the national grid or a battery inverter to avoid collapsing due to a too big frequency drop. The dishwasher have finished heating water, again a sudden -2kW, again I need to divert extra energy to the grid or a battery to avoid a frequency peak big enough to make the inverter disconnect. That doesn't really answer my question. If I have a home solar installation that can send up to 10KW into the grid, how is that any different than an EV battery that can send the same amount of power into the grid? I could see the problem if people were sending 100's of KW into the grid with EV batteries, but that's not the case, they're sending a similar amount of power that a home would use. 3kW slow charge is simply too low for most vehicles, they recharge but the battery might suffer I charge my EV at 1KW (9 amps at 120V), it's enough to recharge my 30 mile commute overnight. I'm not aware of any cars (in the USA) that have a minimum charge rate.
- kkfx 4y ago
  > If I have a home solar installation that can send up to 10KW into the grid, how is that any different than an EV battery that can send the same amount of power into the grid? Your system have a purpose: give you energy NOT stabilizing the grid. Your system (like mine) actually disturb the grid and that's why countries have regulations on them. The actual grid have issues ALSO due to the augmented presence of p.v. If you want to stabilize the grid with cars you do the very opposite, your system need not to give you energy using the grid as a kind of "energy buffer" but to be itself an energy buffer. Let's say we have such systems: at a certain point in time locally our domestic power plants see a sudden frequency drop, around 1Hz. As fast as they can they start pushing energy to the grid. All connected vehicles/plants. As a result the frequency peak in 2-3". Again as fast as they can all connected systems try to get energy from the grid to help the frequency going down. Again a network effect. The frequency drop. They reverse. In a minute you have generated enough oscillation that no one is able to keep the frequency, local grid segments got disconnected, all generators in the segment shut down. Few big enough power plants can coordinate themselves to monitor and decide, too many small and spread can't. Even with frequency shifting witch is the fastest and simplest way possible we can't. Trying proving the above: if you have a monitoring system fast enough, like an MQTT frequency sensors or a multimeter with frequency measure and graphing ability try to see a minute at night, with pv disconnected, no battery etc and a minute when it work. On grid ONLY you'll see a far more stable frequency. On p.v./battery you'll see a schizophrenic variation. Try to interpolate at a grid scale the resulting mess. If we just inject to the grid our inverters simply try to push energy watching frequency, they push less as the frequency goes up but nothing more. It's up to the big power plant regulating. Being one-direction only they generate disturbances but still something manageable at least at a certain scale. A bi-direction system I can't really say if it's sustainable nor I know if a far bigger presence of p.v. can be allowed... Edit: sorry, forgot a part, yes you can recharge and EV even at 100W but the battery do not balance cells, as a result it last less time. Witch means for instance 8 years instead of 10 of usable life. 7kW is generally the "needed mean" to balance for most EVs, normally you have such issue mentioned in the manual.

by kkfx 4y ago

Johnny555 4y ago

A simple practical example: my dishwasher start heating water is +2kW in an instance, my solar inverter take 2-3" to ramp up to +2kW. The home microgrid need or the national grid or a battery inverter to avoid collapsing due to a too big frequency drop. The dishwasher have finished heating water, again a sudden -2kW, again I need to divert extra energy to the grid or a battery to avoid a frequency peak big enough to make the inverter disconnect. That doesn't really answer my question. If I have a home solar installation that can send up to 10KW into the grid, how is that any different than an EV battery that can send the same amount of power into the grid? I could see the problem if people were sending 100's of KW into the grid with EV batteries, but that's not the case, they're sending a similar amount of power that a home would use. 3kW slow charge is simply too low for most vehicles, they recharge but the battery might suffer I charge my EV at 1KW (9 amps at 120V), it's enough to recharge my 30 mile commute overnight. I'm not aware of any cars (in the USA) that have a minimum charge rate.

kkfx 4y ago
> If I have a home solar installation that can send up to 10KW into the grid, how is that any different than an EV battery that can send the same amount of power into the grid? Your system have a purpose: give you energy NOT stabilizing the grid. Your system (like mine) actually disturb the grid and that's why countries have regulations on them. The actual grid have issues ALSO due to the augmented presence of p.v. If you want to stabilize the grid with cars you do the very opposite, your system need not to give you energy using the grid as a kind of "energy buffer" but to be itself an energy buffer. Let's say we have such systems: at a certain point in time locally our domestic power plants see a sudden frequency drop, around 1Hz. As fast as they can they start pushing energy to the grid. All connected vehicles/plants. As a result the frequency peak in 2-3". Again as fast as they can all connected systems try to get energy from the grid to help the frequency going down. Again a network effect. The frequency drop. They reverse. In a minute you have generated enough oscillation that no one is able to keep the frequency, local grid segments got disconnected, all generators in the segment shut down. Few big enough power plants can coordinate themselves to monitor and decide, too many small and spread can't. Even with frequency shifting witch is the fastest and simplest way possible we can't. Trying proving the above: if you have a monitoring system fast enough, like an MQTT frequency sensors or a multimeter with frequency measure and graphing ability try to see a minute at night, with pv disconnected, no battery etc and a minute when it work. On grid ONLY you'll see a far more stable frequency. On p.v./battery you'll see a schizophrenic variation. Try to interpolate at a grid scale the resulting mess. If we just inject to the grid our inverters simply try to push energy watching frequency, they push less as the frequency goes up but nothing more. It's up to the big power plant regulating. Being one-direction only they generate disturbances but still something manageable at least at a certain scale. A bi-direction system I can't really say if it's sustainable nor I know if a far bigger presence of p.v. can be allowed... Edit: sorry, forgot a part, yes you can recharge and EV even at 100W but the battery do not balance cells, as a result it last less time. Witch means for instance 8 years instead of 10 of usable life. 7kW is generally the "needed mean" to balance for most EVs, normally you have such issue mentioned in the manual.