Jettysoned Earth Potential Rise – A Manapouri Shock Investigation

Manapouri Power Station is unique in many aspects; our largest hydro station, our first underground power station, and is a critical part of the New Zealand electricity system. However, foremost is its integration in the remote Fiordland National Park, which is both an incredible and challenging environment.

Manapouri has a unique earthing system, consisting of several discrete but connected components. These include the switchyard and below-ground earthing systems, and the local earthing systems of all the 11 kV installations around the wider site such as the above-ground workshops, visitor centre and jetty, and the staff hostel.

During installation of a new distribution board at the jetty a defective earthing system was identified at the jetty transformer. When inspected it appeared that the designed segregated HV/LV system had been poorly implemented and there was a small standing potential difference between the two earths. This was remedied via combining the HV and LV earths.

However, as an unintended consequence, this standing voltage now propagated through to the LV earthing system and all metallic items bonded to it, including the metallic jetty itself. Meridian staff began noticing small electric shocks when stepping on and off the ferry, and uncomfortable shocks when working on the underside of the jetty.

An investigation was launched, which involved returning to site to carry out detailed testing. We discovered that there is a permanent earth current flowing into the Manapouri earthing system, leading to a permanent earth grid voltage rise (EGVR) impressed onto the Manapouri earthing system. This EGVR is transferred through the varying earth connections and appears as a permanent touch voltage on the metal jetty.

To understand where this EGVR originates, an electromagnetic transient model of the site earthing system and the connected transmission system was prepared, using the CDEGS Right-of-Way module. This showed an imbalance in the magnetic coupling between phases and earth wires of the connected transmission lines which leads to an induced earth current returning to the Manapouri transformer neutrals. This arrangement is not unique to Manapouri – but the way in which the effect presented certainly was.

In this paper we detail the history of the earthing system designs and testing performed at Manapouri, the investigation work leading up to this find, and our proposed mitigation methods.