Off-Frequency Current Injection Testing

As part of our earthing design services, Tesla can inspect and measure existing site earth grid characteristics by undertaking an earth site safety review.

A recent engagement involved on-site earthing testing for the owner of a geothermal power station. The site is a heavily industrialised environment that includes multiple earthing systems likely to be inadvertently bonded together through steam and reinjection pipelines. These pipelines can result in fortuitous earthing between different earthing systems. This can result in hazardous touch voltages along the pipeline due to an increased impedance establishing a voltage gradient around the site during an earth fault.

This site work involves:

  1. Digging up and inspecting sections of the installed earth grid to identify any deterioration or lack of bonding,
  2. Establishing injection testing traverse locations, this includes:
    a) A remote current injection point for the testing, placed at a distance far enough away to be considered at remote earth.
    b) A voltage traverse for measuring the earthing system impedance and maximum Earth Grid Voltage Rise (EGVR) of the site. This traverse is ideally placed orthogonal to the current injection traverse to minimise inductive coupling between test leads.
  3. Perform injection tests using specialised test equipment to:
    a) Measure the site’s earthing system impedance and maximum EGVR,
    b) Identify locations of Earth Potential Rise (EPR) contours
    c) Identify sources of transferred potentials onto adjacent metallic infrastructure (eg steam pipelines),
    d) Carry out touch and step voltage measurements around the site,
  4. Undertake structure bonding/continuity tests to confirm connections to primary plant.

Following the on-site testing, detailed calculations were carried out to assess the worst-case earth return current at the site. This allowed the measured test results to be scaled up and representative of voltage gradients present during a real power system earth fault.

The client received a detailed test report documenting the measured test results (in comparison to tolerable voltage limits) earthing calculations, identified hazards and a recommended programme of works.

For further information contact rob.orange@tesla.co.nz or tyler.patterson@tesla.co.nz

Testing in the field

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