Parasitic Capacitance Troubleshooting

Parasitic Capacitance Troubleshooting 

Introduction: This document mainly explains why there is an inverter error of “leakage current protection “of the inverter and the solutions. 

1. Fault description 
Leakage current, also known as matrix residual current, is caused by parasitic capacitance between the photovoltaic system and the ground. When the photovoltaic system doesn’t have a transformer and parasitic capacitance, the photovoltaic system, and power grid form a loop, the loop impedance is relatively small. This means the common mode voltage will form large common mode current on the parasitic capacitance between the photovoltaic system and the ground, namely leakage current. 

If leakage current occurs in the PV system, including both DC and AC components, that are connected to the grid, it will cause grid current distortion, electromagnetic interference, and other problems. These will affect the operation of equipment in the grid. Leakage current may also make the inverter shell live posing a threat to personal safety 

2. Cause analysis

2.1 System problems
1. Poor ground insulation of PV modules and DC cables 
2. Poor insulation of AC cable to ground 
3. The large parasitic capacitance of PV module to the ground leads to large leakage 
4. The actual leakage current measured by the field system is large  

2.2 external environmental problems 

1. In rainy weather or after heavy rain when the air is humid it will increase the parasitic capacitance of PV modules and shows leakage current protection 
2. Poor grounding of inverter 
3. The installation position of inverter is relatively humid 

2.3 inverter problems 
1. The leakage current sensor of the inverter may fail, or the leakage current detection circuit fails 

2. The leakage current protection threshold of the inverter is lower than the leakage value on site 

3. Solutions 

3.1 Tool preparation on site 
• Multimeter 
• MC4 connector wrench 
• Megohmmeter 
• Oscilloscope 
• Current probe amplifier 
• Current probe 
• LCR meter 

3.2 Solutions to system end problems 

1. Remove the DC line of the inverter and measure the DC voltage of PV series to ground with the DC gear of the multimeter. Connect the red lead to the PV positive or negative pole, and the black lead to the ground. Observe whether the DC voltage is reduced to less than 20V: 

2. During on-site inspection, the insulation resistance impedance of PV + / PV - cable on module side to ground should be greater than 150k Ω by using megger 

3. Remove the AC cable connected to the inverter and measure the insulation value of the AC cable to the ground with a megohmmeter 

4. Remove the DC line of the inverter and test the parasitic capacitance of the components to the ground by the LCR meter. Generally, the parasitic capacitance of a string to the ground is 5 ~ 10nF. If it exceeds this range, the parasitic capacitance is large. According to the calculation formula of leakage current, I = 2 π FCU, (U is the AC component, mainly related to the inverter control algorithm, F is the frequency of the grid, C is the parasitic capacitance value), the leakage current increases with the increase of parasitic capacitance 

5. The current probe of oscilloscope is used to detect the leakage current value of the inverter. the leakage current value of three-phase unit is measured by clamping the three-phase live wire on the AC side with the current probe; the leakage current value of single-phase unit is measured by clamping the live line and zero line of the AC side with the current probe. 

3.3 Solutions to external environmental problems 
1. Check whether the component frame is grounded and whether the grounding is good 

2. Check whether the support is well grounded 

3. Check whether the PV module terminal is in good contact and whether it is possible to be soaked in water. 

3.4 Solutions to inverter problems 

1. For leakage current sensor failure or leakage current detection circuit failure, replace the main board or control board

 2. When on site the continuous residual leakage current exceeds the continuous residual leakage current threshold set by the inverter and the inverter shows leakage 04, update the software and modify the inverter insulation detection and protection threshold value

 a. Process of setting inverter leakage current threshold:
 i. Advanced settings 
ii. Password 0010 
iii. Special functions
 iv. Zero crossing comparison
 v. ILeak-Limit value (set according to local regulation) 

3. Try to change the filter level of inverter to 02 to verify whether the effect is effective 
a. Process of setting inverter filter level: 
i. Advanced setting 
ii. Password 0010 
iii. Special function setting 
iv. Power grid filtering level setting 
v. Power grid filtering level number = 02 

4. Summary 

4.1. System installation suggestions 

1) PV modules are well grounded 
2) The inverter is well grounded 
3) DC series connection needs reliable wiring to avoid electric leakage caused by water immersion 

4.2. Suggestions on inverter operation and maintenance

1) If the machine reports a fault, you can check the inverter fault time. If the time is in the morning or after rain, the fault is caused by high air humidity, which is a normal phenomenon 
2) Check whether the inverter is well grounded

Thomas is the author of this solution article.

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