Checking the PV System for Ground Faults


Qualified person

If the red LED is glowing and the event number 3501, 3601 or 3701 is being displayed in the Results menu on the inverter user interface, there may be a ground fault present. The electrical insulation from the PV system to ground is defective or insufficient.

DANGER

Danger to life due to electric shock when touching live system components in case of a ground fault

If a ground fault occurs, parts of the system may still be live. Touching live parts and cables results in death or lethal injuries due to electric shock.

  1. Disconnect the product from voltage sources and make sure it cannot be reconnected before working on the device.
  2. Touch the cables of the PV array on the insulation only.
  3. Do not touch any parts of the substructure or frame of the PV array.
  4. Do not connect PV strings with ground faults to the inverter.

WARNING

Danger to life due to electric shock from destruction of the measuring device due to overvoltage

Overvoltage can damage a measuring device and result in voltage being present in the enclosure of the measuring device. Touching the live enclosure of the measuring device results in death or lethal injuries due to electric shock.

  1. Only use measuring devices with a DC input voltage range of 1000 V or higher.

Procedure:

In order to check the PV system for ground faults, perform the following actions in the prescribed order. The exact procedure is described in the following sections.

  • Check the PV system for ground faults by measuring the voltage.

  • If the voltage measurement was not successful, check the PV system via insulation resistance measurement for ground faults.

Test by Measuring the Voltage

Proceed as follows to check each string in the PV system for ground faults.

Procedure:

  1. DANGER

    Danger to life due to high voltages

    1. Disconnect the inverter from all voltage sources Disconnecting the Inverter from Voltage Sources).
  2. Measure the voltages:
    • Measure the voltage between the positive terminal and the ground potential (PE).

    • Measure the voltage between the negative terminal and the ground potential (PE).

    • Measure the voltage between the positive and negative terminals.
      If the following results are present at the same time, there is a ground fault in the PV system:

      All measured voltages are stable.

      The sum of the two voltages to ground potential is approximately equal to the voltage between the positive and negative terminals.

  3. If a ground fault is present, determine the location of the ground fault via the ratio of the two measured voltages and eliminate the ground fault.
  4. If a definite ground fault cannot be measured and the message is still displayed, measure the insulation resistance.
  5. Reconnect the strings without ground faults to the inverter and recommission the inverter (see inverter installation inverter).
  6. Location of the ground fault

    The example shows a ground fault between the second and third PV module.

    5451103500

Test by Measuring the Insulation Resistance

If the voltage measurement does not provide sufficient evidence of a ground fault, the insulation resistance measurement can provide more exact results.

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Schematic diagram of the measurement

Calculating the insulation resistance

The expected total resistance of the PV system or of an individual string can be calculated using the following formula:

The exact insulation resistance of a PV module can be obtained from the module manufacturer or the datasheet.

For the resistance of a PV module an average value can be assumed: for thin-film PV modules approximately 40 MOhm and for polycrystalline and monocrystalline PV modules approximately 50 MOhm per PV module (for further information on calculating the insulation resistance see the Technical Information "Insulation Resistance (Riso) of Non-Galvanically Isolated PV Systems" at www.SMA-Solar.com).

Required devices:

  • Suitable device for safe disconnection and short-circuiting

  • Measuring device for insulation resistance

Device required for safe disconnection and short-circuiting of the PV array

The insulation resistance can only be measured with a suitable device for safe disconnection and short-circuiting of the PV array. If no suitable device is available, the insulation measurement must not be carried out.

Procedure:

  1. Calculate the expected insulation resistance per string.
  2. DANGER

    Danger to life due to high voltages

    1. Disconnect the inverter from all voltage sources Disconnecting the Inverter from Voltage Sources).
  3. Install the short circuit device.
  4. Connect the measuring device for insulation resistance.
  5. Short-circuit the first string.
  6. Set the test voltage. The test voltage should be as close as possible to the maximum system voltage of the PV modules but must not exceed it (see datasheet of the PV modules).
  7. Measure the insulation resistance.
  8. Eliminate the short circuit.
  9. Measure the remaining strings in the same manner.
  10. If the insulation resistance of a string deviates considerably from the theoretically calculated value, there is a ground fault present in that string.
  11. Reconnect to the inverter only those strings from which the ground fault has been eliminated.
  12. Reconnect all other strings to the inverter.
  13. Recommission the inverter.
  14. If the inverter still displays an insulation error, contact the Service Contact). The PV modules might not be suitable for the inverter in the present quantity.