Inverter insulation detection detection function and software implementation

Hazards of leakage current

When the current exceeds a certain value, it will cause certain harm to the human body.

The total leakage current (Total current) of a photovoltaic system can be regarded as composed of two parts: the common mode leakage current (Leakage current) connected to the AC terminal and the residual current (Residual current) connected to the PV terminal.

Relevant Industry Standards

According to NB32004-2013 standard 7.10.1 square matrix insulation resistance test

Detecting the insulation resistance of the square array is a mandatory standard and requirement for the inverter. When the insulation resistance of the photovoltaic array is detected to be less than the specified value, the inverter must display a fault. The non-isolated inverter must be shut down and cannot beConnect to the grid

7.10.1.1 Inverter connected to ungrounded photovoltaic array

The inverter connected to the ungrounded photovoltaic array measures the insulation resistance between the input end of the photovoltaic array and the ground before the system starts. If the impedance is less than Umaxpv/30mA (Umaxpv is the maximum output voltage of the photovoltaic array):

a) For the inverter with electrical isolation, the fault should be indicated, but other operations can still be performed during the fault, and it is allowed to stop the alarm when the insulation resistance meets the above requirements.

b) For non-isolated inverters or inverters with isolation but their leakage current does not meet the requirements, the fault should be indicated and its access to the grid should be restricted. At this time, it is allowed to continue to monitor the insulation resistance of the square array.And when the insulation resistance meets the above requirements, it is allowed to stop the alarm and also allow access to the power grid.

Hardware detection method and circuit design

As shown below,

1. Before the inverter system works, first check the PV+ and PV- voltages Vpv1 and Vpv2.
2. Disconnect the K_ISO relay and detect the UIso voltage V1.
3. Close the K_ISO relay to detect the UIso voltage V2.
4. According to Kirchhoff's law, the equations as shown in the figure are simultaneously solved, and the solvable PV+, PV--to-ground impedance: Rx, Ry

Software implementation

Use the ADC sampling function of DSP to sample the voltage signal, then calculate the impedance value according to the equation group, and finally control the state related to the inverter.