Objectives
Replication of the neutral current diversion issues experienced in DNO LV networks using simulation and physical testing.
Measurement of electrical quantities at the secondary substation and load on a test rig with the view to determine neutral current diversion detection criteria.
Verification of commercial detection solutions (if available).
Outcomes
An LV cable network model was created in Matlab/Simulink to replicate neutral current diversion, which occurs during breakages in the neutral conductors. This was based on data provided by SP Energy Networks, where neutral current diversion was experienced. The level of neutral current diversion and the associated phase voltages variations align well with the LV network measurements collected from SP Energy Networks and provide a degree of confidence in the model; the caveat is that there are assumptions made with some unknown parameters (pipework impedance, connectivity, to name a few).
Through the initial testing plan and incorporated additional testing, the behaviour of the test rig has been captured remotely via a Beckhoff measurement acquisition system and the results presented and discussed in this report. When carrying out the physical testing the time taken to make safe and reconfigure between tests was a limiting factor in the number of scenarios in each test category.
The commercial solution identified, monitoring the supply side of the feeder, did not trip on any of the scenarios during the live testing. This is due to the location on the network that it was monitoring, the supply side of ECB1 and this confirmed the difficulty of detecting open neutral events from the source transformer. The voltage imbalance upstream of the breakage is not sufficient to cause this device to react.
It has been demonstrated on the test rig that the inclusion of equipotential bonding on the TN-C-S system can provide a low impedance route back to the transformer to bypass the breakage in the neutral conductor. This has the impact of keeping the phase voltages generally within statutory limits along the feeder but at the expense of introducing dangerous currents on the bonded pipework. It has also been shown that the responses when no equipotential bonding (plastic pipe) is being used in PME TN-C-S system with a broken neutral event is very similar to that of the TN-S system.
Next steps
Potential follow up core projects or complementary non-core or academic activities could include:
Development of techniques or methods for the detection of diverted neutral currents.
Development of techniques or methods for finding the location of a broken neutral.
Informing future network design and fault finding policies for network operators.
Investigations into impacts different magnitudes of ground impedance have on diverted neutral currents.
Impact of different levels of harmonics on the system during an open neutral event.
Get in touch
PNDC project lead: Eddie Corr .
For further information on this case study or to discuss collaborative opportunities, please get in touch.
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