One of the issues that comes up over and over is folks wanting to connect their PV DC production with the grid's AC supply. DC can be inverted to simulate AC. The 2 AC (grid and inverter) supplies can than be put into parallel. While similar to 2 DC supplies in parallel there is several areas that are very different. AC voltage is not a fixed condition. It goes from a max positive value to a max negative value in a cycle. This cycle is referred to as hertz or hz (cycle of pos to neg in a second. Thus 60hz is 60 cycles per second).
So think of it like two cars driving down the freeway at 60mph. In order for them to connect in parallel they need to match speed, direction and be across from each other in order to hookup. Once they hookup one car can pickup the other car's load by the driver pushing down on the gas pedal. The car that load has been picked up from can decrease the amount of gas pedal while the 2 cars maintain speed. If it did not decrease the 2 parallel cars would exceed the 60mph and break the speed limit. While in parallel these 2 cars are acting in sync. What affects one affects the other.
PV Grid tie inverters (they have circuitry that parallels with grid) attempt to pick up all the load they can in order to maximize PV production. The grid responds by decreasing its supply. If the grid goes away, grid tie inverters disconnect as there is no supply to parallel with. Off grid inverters create an AC supply but do not have circuitry to parallel with a grid and pick up load. To try and hook grid and off grid together is going to be like hooking 2 cars together that are driving at different speeds going different directions. Disaster is the likely results.
Hybrid or grid assist inverters can parallel with grid and operate in a several of ways when it comes to picking up load. They can be zero export in that they only pick up the equivalent loading of the home. They can be grid assist in that they see that the grid is not enough to supply the load and pick up the load above what the grid will provide. Some can even export additional supply to grid based on programed times of days or state of battery charge. But because they parallel with the grid they will almost always require an interconnect agreement with the utility company. It is difficult to prevent momentary export to grid anytime you run in parallel with grid.
So think of it like two cars driving down the freeway at 60mph. In order for them to connect in parallel they need to match speed, direction and be across from each other in order to hookup. Once they hookup one car can pickup the other car's load by the driver pushing down on the gas pedal. The car that load has been picked up from can decrease the amount of gas pedal while the 2 cars maintain speed. If it did not decrease the 2 parallel cars would exceed the 60mph and break the speed limit. While in parallel these 2 cars are acting in sync. What affects one affects the other.
PV Grid tie inverters (they have circuitry that parallels with grid) attempt to pick up all the load they can in order to maximize PV production. The grid responds by decreasing its supply. If the grid goes away, grid tie inverters disconnect as there is no supply to parallel with. Off grid inverters create an AC supply but do not have circuitry to parallel with a grid and pick up load. To try and hook grid and off grid together is going to be like hooking 2 cars together that are driving at different speeds going different directions. Disaster is the likely results.
Hybrid or grid assist inverters can parallel with grid and operate in a several of ways when it comes to picking up load. They can be zero export in that they only pick up the equivalent loading of the home. They can be grid assist in that they see that the grid is not enough to supply the load and pick up the load above what the grid will provide. Some can even export additional supply to grid based on programed times of days or state of battery charge. But because they parallel with the grid they will almost always require an interconnect agreement with the utility company. It is difficult to prevent momentary export to grid anytime you run in parallel with grid.
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