The high-frequency noises in the grid seem to be caused by its inductive and capacitive parasitic parameters, or the grid-connected power electronics systems.

of course, I know the grid is causing this event... The question is what can be causing this abnormal PQ in the lower part of the waveform. It is an strange sudden drop and the a small oscillation.

Francisco G. Montoya This small oscillation could be caused by:

1) Resonance caused by the parasitic capacitances of the measuring instruments (e.g. voltage sensor) and the inductances of the power grid;

2) Resonance caused by the parasitic capacitances and inductances inside the power grid;

3) High-frequency noise caused by the random events inside the power grid or grid-connected systems.

For the first two reasons, the small oscillation usually presents repeatable characteristics. But the third reason will cause non-repeatable or random oscillations, and it is often difficult to find which specific event results in this oscillation in the case of unknowing the detailed structure/circuit diagram of the power grid.

Hope this reply can be beneficial to you.

In your house circuit, there are non-linear loads, likely have transistors rectifier circuit。So，the waveform have 3, 5, 7, 9,..... harmonics.

The distortions are most likely caused by non-linear loads which can produce unwanted harmonics in the system. The best solution of compensating this type of issue is to use active power filters.

In your house probably you have none linear loads which causes instability mostly these are simple small gadgets with simple electronic circuits hard to suspect but it should be a simple fix and if the neighbours don’t have the same issue then it’s not on the grid system.

Long lines work as a good inductivity. So higher frequencies can be seen in the curves, although the nominal line impedance is very low. This sudden drop and the ringing may be caused by a Power Factor Correction circuit (PFC), that runs under low load. These circuits may be hidden in any pump or motor or electronic device. If these curves occur, try an AC - capacitor with some Microfarads (and for line voltage!). At least, the ringing should be lower, what would be good for the whole grid. The capacitor has lower impedances at higher frequencies and can compensate the inductivity of the line.

Dear Francisco,

The voltage form contains clear 7th harmonic about 6 - 7 %. This is probably caused by diode rectifiers within variety of power supplies and adapters in your power grid. However, the discontinuity during the grid event seems to be a sudden frequency or phasor angle change of the main harmonic. It may be some switching event in the supply grid as for example reactive power capacitors or maybe of a solar inverter power fluctuation causing voltage phasor change in your power grid. This can be seen if you compare the distances of positive and negative amplitudes of the waveform during the event. The distances differ. What time you experience the events?

BR

Jüri

yes, Jüri Joller you are right. There is a significant 7th harmonic content in my grid. See attached picture.

Regarding the sudden change in frequency or angle (and later oscillation) I cannot figure out what is causing this event. As far as I know, there is no big PV plant or similar near my village. I live in a small village in the southeast of Spain.

These events are repeated every 20-30 seconds during all day.

@Francisco: Thanks for the interesting informations. I have one question: Is there any inverter driven apparatus like a pump, a climatisation or refrigerator? There should be no backward effects to the line, but that depends on the quality of the line filters in the inverter electronics. Modern domestic apparatus often have very hidden inverters with a power consumption between 50 and 200 Watts.

@Francisco: The question of Mr. Prodinger seems to be close to the reality. Maybe you have some forgotten floor heating or some other consumer switching on and off regularly? I think you already have switched off all your loads. Other possibility is that the same power transformer feeds some industry having current peaks, as from powerful point welding or melting apparatus. If not, it may be there is some farming equipment as milk cooler, grain dryer, pump, boiler, distiller, etc.

The first and second positive and second negative half-waves are identical. The chipping of the top is due to the operation of the network on a rectifier with a capacitive filter. It doesn't happen otherwise. The first negative half-wave raises questions. There is some kind of discharge of energy into the network. Can you demonstrate 6-8 periods? It is not clear what causes this form of stress. Is this process periodic or is it a one-time switching surge when switching on or off a consumer.

The problem could be the measuring device which is not able to capture the correct waveform. The best way to determine the quality problem is to take several measurements at different times and with different measuring devices. Whenever the sine wave is not uniform as it should be, then the judgement on quality is that the transmission is compromising on not using components the balance and filter power, hence sending power with harmonics.

Mystery solved. The problem was caused by the meter itself. A new update on a processing thread was responsible for the malfunctioning. Thanks anyway for your valuable help and ideas.