We have implemented and installed a grid-connected PV inverter in two different locations. In the first location, it works well, but in the second location, the line voltage to the cubicle body increases and the inverter surge arrester burns. The earth resistance in both cases is below 1 ohm, which indicates that the earthing system is good. I know that high-frequency harmonics are generated by power electronic devices and can cause leakage currents and neutral-earth voltage, but I cannot find the difference between the two conditions. I would appreciate any suggestions you may have.
in the attached pic :
Yellow: line to cubicle body ( which is earthed)
Blue: line-to-line voltage
Green: inverter current
Hello Pooya Ghani ,
In order for the RG readers to have a chance at providing you with relevant suggestions, you need to give us more information:
1) Your 'scope has four channels: CH1 (yellow) 200 V/div, CH2 (light blue) 200 V/div, CH3 (dark blue) 50 A/div, and CH4 (green) 50 A/div. You only identified 3 of the 4 channels. Can you show us 'scope pictures with only two traces at a time?
2) Your timebase is set for 10 mS/div, and the period of your waveforms in 20 mS, so your grid uses 50 Hz. Is this correct? I see a 49.998 Hz dispaly item.
3) A high frequency noise is superimposed on top of the 50 Hz signal. Do you know the frequency of this high frequency noise? Does this noise go away when the inverter is turned off?
4) What is the manufacturer and model number of the inverter?
5) Do you use separate earth/ground rods or planes for the DC part of your system, and for the AC part of your system? Or do you use the same earth/ground for both the DC and AC portions of your system?
6) You say the two different locations are similar. Does that mean the photovoltaic portion, number of panels and their series and parallel connections are identical? What about the environmental conditions? Any heat and/or humidity differences?
7) What alerted you to make the measurements of ground leakages. Was it the lights or display on the inverter? Did someone receive an electric shock? Was there no useable outpt from the inverter?
8) The cubicle you talk about. Is it the case of the inverter?
9) Did you measure any ground leakage from the DC side of your system? If so, then do these leakages go away when the inverter is turned off?
10) Is the surge arrestor on the AC output side of the inverter (hot and neutral lines), and are you saying it burns out?
Regards,
Tom Cuff
If the installation of the PV inverters is strictly the same, then the problem may come from the common mode impedance of the power source.
To go further, you would need at least a diagram of the cabling topology.
Hello,
I suggest you put the same oscilloscope signals for the inverter which is functioning well
- High-frequency harmonics: Power electronic devices like inverters generate high-frequency harmonics when they switch. These harmonics can cause leakage currents and neutral-earth voltage, which can lead to an increase in the line-to-enclosure body voltage.
- Poor grounding: If the grounding system is not properly designed or implemented, it can also lead to an increase in the line-to-enclosure body voltage. This is because the grounding system provides a path for the leakage currents to flow, which can raise the voltage on the enclosure.
- Transients: Transients are short-duration voltage spikes that can occur on the power grid. These transients can also cause an increase in the line-to-enclosure body voltage in an inverter.
To prevent the line-to-enclosure body voltage from increasing when switching is initiated, it is important to:
- Use filters to reduce the high-frequency harmonics generated by the inverter.
- Ensure that the grounding system is properly designed and implemented.
- Use surge protectors to protect the inverter from transients.
In the case of the inverter mentioned in the ResearchGate post, it is likely that the increase in the line-to-enclosure body voltage is due to a combination of high-frequency harmonics and poor grounding. The fact that the surge arrester burned out suggests that there were significant transients on the power grid. By addressing these issues, the increase in the line-to-enclosure body voltage can be prevented.
Here are some additional tips to help prevent line-to-enclosure body voltage from increasing in grid-connected inverters:
- Use an inverter with a low total harmonic distortion (THD). THD is a measure of the amount of harmonics in a signal. A lower THD means that there are fewer harmonics, which can help to reduce the leakage currents and neutral-earth voltage.
- Use an inverter with a good surge protection rating. This will protect the inverter from transients that could cause an increase in the line-to-enclosure body voltage.
- Regularly inspect and maintain the grounding system. This will help to ensure that it is properly connected and that there are no loose connections.
Please check the surroundings. If the earth around inverter is having many installations with leakage currents the Ground potential itself increases.
Check system Ground/neutral voltage wrt Earth at both locations. This may give hint.