Traditional Earth Fault protection schemes revolve around the concept of determining residual current which appears on the modern distribution network when a fault connection to earth is made. This fault could be anything from a conductor on the ground to a tree brushing up against powerlines, but fundamentally the result is that these faults are risks for utilities and the general public and should be eliminated.
The Ground Fault Neutraliser (GFN) is an emergent technology which measures the shift of voltages across three phases in response to this fault, and injects additional voltage to attempt to compensate for the loss of energy through the earth fault. By using this strategy, GFNs are able to reduce earth fault current levels at a fault site to close to zero. However, this strategy does not allow for a faulty conductor to be isolated or disconnected by traditional protection schemes, as the rapid extinction of earth fault current results in a terrifying scenario: a live conductor lying on the ground with a negligible earth fault signature for protection devices.
Whilst this form of protection is exceptionally effective in underground networks, it brings new challenges to utilities with overhead networks particularly when the GFN is employed to reduce bushfire risk. A common bushfire mitigation strategy is to employ a Rapid Earth Fault Current Limiting (REFCL) technology to reduce the incidence of earth faults on overhead lines, however with no earth fault signature remaining and a feeder long voltage imbalance, utilities wishing to reduce their fire risk have no option other than to disconnect the entire faulted feeder for a manual patrol to find the fault site. For most utilities, the cost of losing supply to so many customers is a burden too great to handle.
Fortunately, through collaboration with utilities around the world NOJA Power has implemented a suite of functionality in the OSM® series reclosers to address the challenges of working together with a GFN.
As most modern utilities with overhead lines have multiple reclosers installed per feeder, there must be a signature which indicates the presence of a local earth fault even under the operation of a GFN. The secret to this capability arises in the clever use of situational electrical indicators at fault sites to develop a comprehensive understanding of the location of the fault. Firstly, the greatest indicator of a GFN operation is a movement in system Neutral voltage. As all REFCL compensations are based on neutral voltage injections, the NOJA Power Recloser can be tuned to listen in for this function. Once the recloser is aware of the GFN injection, intelligent upgrades to the OSM® series sensors and an algorithm within the controller monitors the rate of change of earth fault current at a resolution never before seen in pole top reclosers: less than 100mA.
We can already hear the experienced utility engineer choking on their coffee whilst reading this, but this level of sensitivity is what is required to achieve reliable fault location whilst utilising a GFN. 100mA is far below the noise floor of standing neutral current on a network, but by examining the rate of change of earth fault current during operation of the GFN with respect to steady state network conditions, a NOJA Power OSM® Recloser has the capability to report if the fault location is downstream from the device. The OSM® Recloser can then relay this information back to the SCADA station, so that instead of de-energising an entire feeder, utilities can make intelligent decisions to sectionalise the fault location and safely restore power to their unaffected customers.
“It is clear this new functionality implemented by our R&D team in consultation with our Australian Electricity Utility customers will allow them to implement GFN technology as a further enhancement to their bush fire mitigation programs without affecting reliability of supply” said Neil O’Sullivan NOJA Power’s Managing Director.
NOJA Power’s new RC10 capabilities are available for use across the world. Through careful field trial and analysis of network operating signatures of Ground Fault Neutralisers and Rapid Earth Fault Current Limiting Technology, NOJA Power’s RC10 controller demonstrates again why it is clearly the most advanced recloser controller available on the market today. By intelligent use of reliable sensor data, utilities can adapt their network infrastructure to gain the best value from their asset investments, mitigating bushfire risk whilst providing reliable business performance.