problem sensing off-grid inverter output voltage

[Xtal011]

Hello everyone. I am designing an off-grid inverter and recently run into a problem whil trying to sense the output voltage with a differential amplifier.
The problem is that as soon as I try to sense the voltage at the output the retrieved waveform is totaly distorted. Is there any way to sense the output voltage witout distorting the waveform??
Attached to this post are the images with the schematics and the waveforms simulated in OrCAD capture.

To further clarify the voltage sensed (yelow waveform) will later be fed into an RMS to DC converter.

[dtmouton]

The one thing I picked up is that the ratio of the values of the resistors in your voltage divider is very large. In this case the capacitance of the resistors come into play. Google the phrase "frequency compensated voltage divider".

[Xtal011]

thanks for the idea, later I will simulate it and comment on the result.

Adding to this answer, i simulated it and it did not work

[richard.cs]

Looking at your schematics V_OUT_N is bouncing up and down at 400 V pk-pk which is not at all suppressed by your unbalanced filter (VO_L- and V_OUT_N are the same net). That's a bad idea from an EMC perspective, but it also means that the voltage measurement circuit needs to have very good common mode rejection to measure it the differential voltage in the presence of that signal. Does the measurement circuit rely on a floating supply to work? Capacitance across the isolation barrier can upset that sort of thing.

I think I would start by fixing the filter, it needs to change to avoid producing awful noise, but that will probably also help your measurement.

Side note, it also makes a difference which switching strategy you use. One approach puts the PWM on all the transistors, and the other just PWMs one side whilst driving the other at a leisurely 50/60 Hz.

[Circlotron]

Low tech would be an effective solution here. A very small mains stepdown transformer would do the trick very nicely. No need for tight resistor matching. No common mode problems. Ground referenced secondary voltage. What's not to like, besides being unfashionable maybe?

[NiHaoMike]

Standard power transformers are not very linear. That could be worked around by using the transformer at less voltage than it's designed for, such as a 240V transformer on 120V or a 380V transformer on 240V.