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Guidance understanding power direction, please ?

Externet

Well-Known Member
Hello.
By some reason the effect escapes my simple understanding or analogy.

The electric utility feeds many houses in my neighborhood block.
My solar generation inverter feeds the same grid and neighborhood too.

Am missing the concept when my generation feeds the grid + neighbors, and when the utility feeds the neighborhood + me.
In terms of voltage diferentials and current demands, I can somewhat visualize the flow, but in terms of Watts, it appears harder for me to understand the flow direction. Any clarification will be appreciated.
In order to deliver/inject any power into the grid, my generation voltage has to be higher than the grid; am I right ? How higher ? Half a Volt ? Much more ? What is the effect ?

Diver300

Well-Known Member
There will be a small voltage drop between source and load, the size of which depends mainly on the resistance of the wires, so how far you are from the source makes a big difference.

The voltage at the sockets at my house dropped from 242 V to 236 V when I turned on a 7 kW electric shower. That indicated an impedance of around 0.2 Ohms.

I would assume that if I were to put 7 kW back into the grid, with a good power factor, the voltage would go up by about 6 V.

rjenkinsgb

Well-Known Member
In order to deliver/inject any power into the grid, my generation voltage has to be higher than the grid; am I right ?
Not necessarily.

The solar inverter will control both its output voltage and phase, relative to the grid connection.

It will initially synchronise with both voltage difference and phase difference at zero to allow it to go online. That's essential to avoid high current in either direction.

Then, to feed power into the grid it will advance it's output phasing slightly, as much as needed to get the required output current.

It may also increase voltage slightly to further control current, but I suspect a very small amount if it changes it at all.

Diver300

Well-Known Member
rjenkinsgb makes a good point that in an ac system, inductance has an effect and to get current to flow though an inductive load, the phase angle has to be advanced.

However, for a solar system generating power which will probably feed loads that are also fed by the same distribution transformer, the inductance of the distribution system is probably not that important.

Inductance is really important with conventional rotating electrical machines. On those, when running at no load, they will generate the correct voltage and can be synchronised with very little current in the field winding. In order to produce current into a load, the EMF generated has to overcome the considerable inductance of the generator's windings, so the excitation has to be increased, and the phase angle will be in advance of the load. The voltage regulator is needed to control the terminal voltage and / or the power factor, depending on whether there are other generators on the system.

Automotive alternators also have large excitation at large loads to overcome their winding's inductance. Load-dump surges are the result of the excitation being so large and only reducing slowly if the load is removed and there is no battery to absorb the excess current.

On a solar inverter system, there is more direct control of the voltage, and very little inductance, so they are not affected in the same way as rotating generators.

fourtytwo

Active Member
Can I just correct something about a GTI, it is a current source, not a voltage source, so it does not control it's output voltage but rather it's current. As for phase, those I am familier with strictly phase lock to the grid, usually at zero cross then synthesize there own sine wave current waveform for injection IN PHASE with the grid (they also frequency lock to ensure the synthesised waveform is the correct frequency). An LCL filter is usually placed at the output to both filter HF noise and buffer the extremely low grid impedance from the inverter and allow a voltage difference to exist. I hope this explanation clarifies rather than confuses (I apologize if it is the latter).

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