Determination of an Alternating current direction

[rikotech8]

Hello Guys! I have to program logic controler to control SMA on-grid invertor power producing. Here is the issue:
I have to determin in what direction current is flowing. If the current flows from the invertor toward the transfer grid I have to decrease (digitaly) the Invertor power until current reach zero. So I dont ask you for a code, it is my job. What I am asking is, Is there aproach to messure AC direction exept shunt ressistanse. I prefer some inductive sensor. Thank you in advance!

 

[FvM]

The question sounds confused. As the name suggests, AC current direction is alternating. You probably mean power direction?

You'll measure real power to determine the power flow. I'm quite sure that the on-grid inverter already knows real power, and other grid related parameters in addition. I also assume that you don't need an external controller to set an intended inverter operation point.

 

[chuckey]

I believe that you need a current sensor (transformer?) and multiply (or to phase compare) its output with a local voltage sample. This is to give a +/- output when the direction of the current changes.
Frank

 

[Dan Mills]

A currrent transformer or a Hall sensor will get the job done and are both off the shelf components.

Doing some trivial math involving the voltage and current sensors allows you to calculate both real power and reactive power figures.

Regards, Dan.

 

[rikotech8]

Sorry, My bad! I realy meant the power direction. I want to detect power feed to the grid. Let assume we have a house that produce more energy the it can consume. In this case the excessive will not be loss but fed to the grid, thus the invertor works. I want to prevent this. So my system has to produce energy only for the appliances of the household, not to fed the grid. I can regulate the invertor output power by changing the state of 4-digit (16 step) input, which control the output power with 6.25% resolution. There might be occasion that the invertor can not produce power due to sunlight absence. Then I will have power from the grid toward household. Enought background. I want to detect when the invertor start to feed the grid, but not when grid feeds the household.
Sorry about my bad English.
Thank you for answers!

 

[FvM]

Are you asking about single- or three phase grid connection? In case of single phase, you can derive a power direction signal without calculating real power, for three-phase, the real power of each phase must be calculated and summed to determine the net flow.

An energy meter chip has all required analog measurement and signal processing resources on board.

 

[rikotech8]

The system is three phase. Sout_nom = 17kVA.

 

[rikotech8]

1. When the invertor can not supply the appliances (household) the grid starts to support the invertor. In this case I have power flow from the grid toward the household.
invertor->household<-grid I dont want signal in this condition.

2. When the sun shines bright , the invertor produces more energy than the household can consume.
invertor->household->grid Now I want a signal. To reduce the invertor output power, at level that it becomes to work in "Island mode".

In other words I want the grid to act like back-up, but never drain energy from the invertor.

 

[FvM]

The system is three phase. Sout_nom = 17kVA.

If you want to design the measurement unit on your own, I suggest an energy meter chip, e.g. from Analog Devices. Or use a ready-made energy monitoring module.

 

[rikotech8]

Thank you!

 

[chuckey]

Three phase is interesting, as due to individual phase voltages both on the grid and the inverter being different, I think you have to combine the voltages in a Y configuration with a neutral and treat each phase individually.
Frank