1. PV array insulation test
For an ungrounded photovoltaic array, the connected inverter should have the ability to measure the insulation resistance between the DC input and the ground, and a fault must be indicated when the insulation resistance exceeds the limit requirement. Insulation resistance limit R The test steps are described below:
①Ungrounded photovoltaic array, connect a resistor with a size of 90% of the limit value between the DC input terminal and the ground, observe the response of the inverter, and indicate the fault for the inverter with isolation;
② For inverters without isolation and inverters with isolation but the contact current and ignition leakage current do not meet the requirements, the fault should be indicated and the access to the power grid should be restricted;
③ For functional grounded photovoltaic arrays, there are two cases. When the designed grounding resistance is greater than R, the insulation resistance value measured by the inverter should not be less than 80% of the designed grounding resistance value, otherwise the inverter should respond. The response method is the same as that of the ungrounded photovoltaic array. When the designed grounding resistance is less than R, the test principle is the same as the residual current mutation detection principle.
2. PV array residual current detection
①There is a danger of electric shock between the input terminal of the ungrounded photovoltaic array and the ground. When the inverter is not isolated, or the inverter has isolation measures but cannot guarantee to limit the contact current within a suitable range, when the user touches the When live parts of the array are connected to ground, the connection to the grid and ground will provide a return path for touch currents, creating a shock hazard.
② Regardless of whether the photovoltaic array is grounded or not, the occurrence of ground faults will cause conductor parts or structures that should not carry current to carry current, thereby causing a fire hazard.
In order to eliminate the above two dangers, the inverter can be equipped with residual current detection (RCD) and residual current monitoring (RCM) to provide protection. Whether RCD additional protection is required depends on whether the touch current and ignition leakage current meet the specified limits. If the contact current is greater than 30mA and the ignition leakage current is greater than 300mA, residual current detection protection is required. If the measured contact current and ignition leakage current are both less than the limit requirements, it means that the electrical isolation and insulation resistance of the inverter are good, and there is no potential electric shock and Fire hazard, no RCD and RCM required.
(1) Test principle
① First, test the contact current and ignition leakage current.
②The touch current uses a test network that simulates the impedance of the human body.
③ Fire leakage current Use a multimeter to directly measure the current between the PV input terminal and the ground.
④ If the measured contact current and ignition leakage current are greater than the limit requirements, the inverter needs to add residual current detection and monitoring devices.
To further measure the protection function of the device, continuous residual current detection and sudden change residual current detection are required.
(2) Touch current measurement steps
① Build a test network according to the circuit shown in Figure 1, which can simulate the impedance of the human body.
②Connect the A and B terminals of the test network to the PV input anode and ground respectively.
③Close the switch on the AC/DC side of the inverter, and the inverter will run in parallel with the grid.
④Use a multimeter to measure the voltage U2 of C1 terminal in the test network, and divide the peak voltage by 500Ω to calculate the weighted contact current.
(3) Fire leakage current test steps
①Connect to the main circuit, the DC input terminal is not grounded, and the AC output terminal has one pole grounded
②Connect the ammeter between the positive pole of the DC input and the ground.
③Close the AC/DC switch of the inverter, connect to the grid, observe and record the ignition leakage current of the DC input positive pole to ground.
④ The same method is used to measure the fire leakage current between the negative pole of the DC input and the ground.
(4) Continuous residual current test steps
①Remove the inverter insulation resistance detection function.
②Connect an adjustable resistor and an ammeter between the positive electrode of the DC input and the ground, and the initial value of the adjustable resistor should be as large as possible, so that the initial residual current is less than 300mA.
③ Start the inverter to run at the rated power point.
④ Gradually reduce the resistance value and record the current value when the residual current protection device operates.
⑤Use the same method to test the residual current detection function between the negative electrode of the DC input and the ground.
(5) Test steps of sudden change residual current
①Remove the inverter insulation resistance detection function.
②Add an oscilloscope to the test circuit to observe residual current and grid-connected current signals.
③Remove the continuous residual current detection function, start the inverter to run the rated power point, and adjust the adjustable resistance value to make the residual current 30mA.
④ Disconnect the adjustable test resistance first, then close the adjustable test resistance switch, the resistance is connected to the circuit that produces a sudden change, and the oscilloscope records the tripping time.
⑤ Use the same method to test the 60mA, 150mA mutation leakage current detection function.