How does a grid-tie inverter control the load without overcurrent?

[Garyabc]

Hi,
Haven't been able to find an answer to this question with a search. How is the output current controlled in a grid tie inverter? It seems as though it should overload without some control of the load on the output. Let me make up a fictitious example to explain what I mean.
Suppose I have a 1KW grid-tie inverter supplied by 2KWs of solar panels and I have 5KW of loads running on the inverter output.. What happens? Does the inverter overload? Shutdown? Does the inverter limit the current to its maximum rating? .Is it necessary to control the input to the inverter to below the maximum rating of the inverter?
Not clear to me how this works.
Thanks.
Gary

[ButchDeal]

Hi,
Haven't been able to find an answer to this question with a search. How is the output current controlled in a grid tie inverter? It seems as though it should overload without some control of the load on the output. Let me make up a fictitious example to explain what I mean.
Suppose I have a 1KW grid-tie inverter supplied by 2KWs of solar panels and I have 5KW of loads running on the inverter output.. What happens? Does the inverter overload? Shutdown? Does the inverter limit the current to its maximum rating? .Is it necessary to control the input to the inverter to below the maximum rating of the inverter?
Not clear to me how this works.
Thanks.
Gary

your 1kw inverter with 2kw of modules will invert as much as it can and as much DC power is provided which in your example could be none to 1kw.
The load is not related at all. If there were zero load a grid tie inverter will do exactly the same inverting as if there is 5kw or 100kw of load, it doesn't care about load.
The load will be feed by either the grid or the inverter. If the inverter is generating 1kw then 1kw of the load will come from the inverter and the rest from the grid. There is nothing that controls this other than ohms laws.
the load being like a bucket with two hoses feeding it (inverter and grid)

The inverter output goes to your breaker panel which connects to the load AND to the grid.

The inverter has a controller for the PV modules which is an MPPT.

[kaleem]

because grid tie inverter has the ability to synchronize to interface with a utility line by using local oscillators.

[inetdog]

because grid tie inverter has the ability to synchronize to interface with a utility line by using local oscillators.

Wrong. The Grid Tie Inverter (GTI) does not have a local oscillator. It uses the grid frequency as its synchronizing source and delivers a controlled amount of current into the grid wiring at each point in the grid voltage waveform.
With no grid present there is no output.

Going back to the OP's question: An off grid inverter will supply the programmed voltage and whatever current it being consumed by the connected loads. If the load is greater than the inverter capacity the inverter will shut down.

A GTI, on the other hand, is connected in parallel with the utility grid and your local loads and will deliver all of the power available from its DC input. If that amount is less than your local load power some additional power will flow in from the grid. If that amount is more than your local load power at any time the excess power will flow out backwards through your meter to the grid.

[jflorey2]

Wrong. The Grid Tie Inverter (GTI) does not have a local oscillator. It uses the grid frequency as its synchronizing source and delivers a controlled amount of current into the grid wiring at each point in the grid voltage waveform.

It actually does. It uses the local oscillator to reconstruct the current waveform, and it uses the local oscillator to compare to the measured powerline frequency. That way it can disconnect if frequency strays too far from nominal (50 or 60Hz.) But you are right in that it will not provide power without the grid present.

[jflorey2]

Suppose I have a 1KW grid-tie inverter supplied by 2KWs of solar panels and I have 5KW of loads running on the inverter output.. What happens? Does the inverter overload? Shutdown? Does the inverter limit the current to its maximum rating? .Is it necessary to control the input to the inverter to below the maximum rating of the inverter?

The 1kW inverter cannot use all the power from the 2kW array. So it allows the array voltage to rise until the current drops off. It keeps doing this until the power delivered by the array drops to 1kW.

The inverter then places that power on the grid. If 5kW of load is also connected to the grid, then 1kW of that load is supplied by the inverter, with 4kW coming from the grid. If you keep increasing the load, eventually you will draw too much power and blow a breaker. That's the same whether you have solar or not.

[Sunking]

Looks like everyone has missed the answer. A GTI is a current source, not a voltage source like a battery or generator. The grid is an extremely low impedance compared to your home at most times. The current produced goes where ever the lowest impedance is. If that is your home due to you demanding power, you use the power. If the GTI is generating more power than you demand, goes out on the grid to your neighbors.

If the grid disconnects, the impedance the inverter sees increases significantly causing an over shoot of voltage and the Inverter shuts down until it can see a stable line voltage and low impedance from the grid.

That is the quick and dirt explanation. What most people get tripped up by is how a current source operates. They can only operate in a vaery narrow range of low impedance.