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Frankfurt (Germany), 6-9 June 2011 Mark Davies Trevor Charlton Denis Baudin Mark Davies UK Session 2 Paper 0376 New Design Methods to Achieve Greater Safety in Low Voltage Systems During a High Voltage Earth Fault Slide 2 Frankfurt (Germany), 6-9 June 2011 The transfer potential from HV to LV earth electrodes is an important safety consideration, e.g. in UK TNC-S (PME) systems. UK practice has been to assume that LV transfer potential is simply the worst case soil surface potential, i.e. where it is closest to the HV electrode. This is an overly cautious approach. A new calculation approach is described and illustrated via case studies. The work is part of a wider R&D project funded by UK Electricity Distribution Companies via the Energy Networks Association (ENA). Background Mark Davies UK Session 2 Paper 0376 Slide 3 Frankfurt (Germany), 6-9 June 2011 Existing Calculation Method (UK, ENA ER S.34) A B The Transfer Potential from the HV Electrode to each LV Rod Electrode is assumed to be equal to the surface potential at the LV Electrode location Transfer Voltage V T(A) = V A V T(B) = V B Mark Davies UK Session 2 Paper 0376 Slide 4 Frankfurt (Germany), 6-9 June 2011 Earth Resistance of a Hemispherical Electrode Surface Potential at distance x from the electrode R HV HV Electrode Earth Resistance () Soil Resistivity (m) r HV Electrode Radius (m) I Current (A) x Distance from HV Electrode where the Surface Potential is Calculated (m) Existing Calculation Method (UK, ENA ER S.34) Mark Davies UK Session 2 Paper 0376 Slide 5 Frankfurt (Germany), 6-9 June 2011 A B Transfer Voltage V T(AB) = V A Mark Davies UK Session 2 Paper 0376 Existing Calculation Method (UK, ENA ER S.34) The Transfer Potential from the HV Electrode to a group of LV Rod Electrodes is assumed to be equal to the surface potential at the LV Electrode closest to the HV Electrode Slide 6 Frankfurt (Germany), 6-9 June 2011 New Calculation Method A B VTVT VBVB VAVA RBRB RARA Equivalent Circuit Transfer Voltage Accounts for the effect of the surface potential on each LV Electrode and their relative resistances Mark Davies UK Session 2 Paper 0376 Slide 7 Frankfurt (Germany), 6-9 June 2011 Part of a Project to Improve Safety at 11kV Distribution Substations Mark Davies UK Session 2 Paper 0376 Slide 8 Frankfurt (Germany), 6-9 June 2011 Application of New Calculation Method Mark Davies UK Session 2 Paper 0376 Slide 9 Frankfurt (Germany), 6-9 June 2011 Comparison of Results from Different Methods (CDEGS MALZ) Mark Davies UK Session 2 Paper 0376 Slide 10 Frankfurt (Germany), 6-9 June 2011 Effect of a Low Resistance LV Electrode This LV Rod Resistance reduced to One Fifth of the Others Mark Davies UK Session 2 Paper 0376 Slide 11 Frankfurt (Germany), 6-9 June 2011 Effect of a Low Resistance LV Electrode (MALZ) Mark Davies UK Session 2 Paper 0376 Slide 12 Frankfurt (Germany), 6-9 June 2011 Application to Different Arrangements Mark Davies UK Session 2 Paper 0376 Slide 13 Frankfurt (Germany), 6-9 June 2011 Application to Different Arrangements Mark Davies UK Session 2 Paper 0376 Slide 14 Frankfurt (Germany), 6-9 June 2011 Practical Example New Residential Development Mark Davies UK Session 2 Paper 0376 11kV Distribution Substation HV Electrode LV Feeder Underground Cables Slide 15 Frankfurt (Germany), 6-9 June 2011 Practical Example New Residential Development Mark Davies UK Session 2 Paper 0376 LV Electrodes ExistingMethod New MethodDetailedSimulation 1984625 Transfer Potential (% of HV EPR) 1 Slide 16 Frankfurt (Germany), 6-9 June 2011 Practical Example New Residential Development Mark Davies UK Session 2 Paper 0376 LV Electrodes ExistingMethod New MethodDetailedSimulation 1984625 2982618 Transfer Potential (% of HV EPR) 1 2 Slide 17 Frankfurt (Germany), 6-9 June 2011 Practical Example New Residential Development Mark Davies UK Session 2 Paper 0376 LV Electrodes ExistingMethod New MethodDetailedSimulation 1984625 2982618 3981916 Transfer Potential (% of HV EPR) 1 2 3 Slide 18 Frankfurt (Germany), 6-9 June 2011 Effect of Soil Resistivity & Structure Mark Davies UK Session 2 Paper 0376 300m 100m 3m 300m 1000m 1m Slide 19 Frankfurt (Germany), 6-9 June 2011 A new calculation method is described which: More accurately approximates transfer potential between HV and LV earthing systems in distributed LV networks (e.g. PME in the UK). Accounts for the beneficial reduction in LV transfer potential provided by larger LV electrodes which are located further away from the HV substation (as previously demonstrated by measurement). Is in good agreement with results from detailed simulation software (uniform soil). The work is currently being used to develop new guidance / calculation tools for UK standards. LV earthing design practice should be reviewed. The work suggests that LV electrode should be installed in parts of the network furthest from the HV electrode, e.g. at the end of LV feeder cables. Work is progressing to evaluate the new calculation method with different arrangements, soil resistivity, etc. Conclusions Mark Davies UK Session 2 Paper 0376 Slide 20 Frankfurt (Germany), 6-9 June 2011 During the design of MV distribution substations, LV electrodes should be biased towards the areas of the network furthest away from the HV electrode. In existing networks where there is an unacceptably high transfer voltage on the LV earthing system. Additional LV electrode would be installed at strategic locations in areas remote from the HV electrode. The approach could be extended to EHV Substations and the transfer potentials which may exist onto other adjacent HV electrodes or telecommunication circuits. Application Mark Davies UK Session 2 Paper 0376 Slide 21 Frankfurt (Germany), 6-9 June 2011 Mark Davies Trevor Charlton Denis Baudin Mark Davies UK Session 2 Paper 0376 New Design Methods to Achieve Greater Safety in Low Voltage Systems During a High Voltage Earth Fault