Puzzle in the DER196

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I had read the DER196 and learn much from it .But I had a puzzle in the article like the following picture:
And Is this cuase the self-excitation in the begin?

It is due to the measurement method and noise at 10Hz. Normally at very low frequency, such as 1Hz to 10Hz, the measured values are jumping around, so you can see those abrupt change at very low frequency. As the frequency scan is going on, the measurement is getting more and more steady. So please focus on the frequency response over 20 or 30Hz. There was no self-excitation happened. Please also take a look of the start-up drain waveforms and step load dynamic response in this report.