Background

Power Control Engineers were engaged by a NSW Coal Mine to conduct site wide earth testing and lightning protection audits on their 66kV and 11kV private networks. The motivation for the project was to validate the existing earth systems at multiple significant locations and ensure prospective touch and step voltages were maintained at a safe magnitude, as per AS/NZS 3000:2018 and AS 2067:2016. Additionally, existing lightning protection systems were evaluated by PCE as per AS/NZS 1768:2007 through visual inspection and risk assessment calculation.

66kV Switchyard, 11kV Switchroom & Power Factor Correction Yard

Comprehensive earth tests were conducted at the two major switching sites of the mine: the 66/11kV switchyard and 11kV switchroom, with associated power factor correction yard. At these sites, an off-frequency injection test was carried out to validate the earth system by applying a reduced simulated earth fault at a remote location and investigating the earth systems response. Under these test fault conditions, resultant touch and step potentials were measured to ensure compliance to the above standards. Current distribution measurements were likewise taken for alternate return paths under test.

In addition, continuity tests were undertaken to validate all equipotential bonding at the site and maintain the safety and performance of the earthing systems.

Kiosk Substations

The Three-Point Fall of Potential earth grid test method was implemented at three kiosk substations at the mine, due to the isolated nature of the assets and the simplicity of the earthing system at these points. This test was conducted to verify the existing data on the kiosk earth grids by inducing a current between the grid and a remote point and investigating the voltage profile in the soil surrounding the system under test.

Difficulties

The major difficulty experienced in this project was the extremely dry soil at the mine site. With the local area under a severe drought it proved difficult to create a low resistance at the remote injection point for the off-frequency injection test. As a result of this, test source voltages were becoming unconventionally high to create the required current magnitudes for the investigation. To address this, PCE implemented a larger, more complex electrode system to ensure the loop resistance was reduced.