Mark Davies Trevor Charlton Denis Baudin

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


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



Distance

VB

VA

LV Electrode

B

LV Electrode

A

Soil Surface Potential

HV Electrode

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

VT(A) = VA

VT(B) = VB



xRr

IV

HVs 4

sin2

1

rRHV

2

Earth Resistance of a Hemispherical Electrode

Surface Potential at distance x from the electrode

RHV HV Electrode Earth Resistance (Ω)ρ Soil Resistivity (Ωm)r HV Electrode Radius (m)I Current (A)x Distance from HV Electrode where the Surface Potential is Calculated (m)




Distance

VB

VA

LV Electrode

B

LV Electrode

A


HV Electrode A B

Transfer Voltage

VT(AB) = VA



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


Distance

VB

VA

LV Electrode

B

LV Electrode

A


HV Electrode

New Calculation Method

A B

VT

VBVA

RBRAEquivalent

Circuit

Transfer Voltage

Accounts for the effect of the surface potential on each LV Electrode and their relative resistances



Part of a Project to Improve Safety at 11kV Distribution Substations



Application of New Calculation Method

20m

Vertical Rod Electrode Electrodes (1.2m) HV Electrode

HV-LV Separation Distance

LV Neutral / Earth Conductor

2.4m 20m 60m

2.4m



0

10

20

30

40

50

60

70

80

90

0 5 10 15 20

Tran

sfer

Pot

enti

al (

% o

f H

V E

PR)

Separation Between HV and LV Electrodes (m)

Existing Approximate Method

Detailed Simulation

New Approximate Method

Comparison of Results from Different Methods

(CDEGS MALZ)



Effect of a Low Resistance LV Electrode

20m

Vertical Rod Electrode Electrodes (1.2m) HV Electrode

HV-LV Separation Distance

LV Neutral / Earth Conductor

2.4m 20m 60m

2.4m

This LV Rod Resistance reduced to One Fifth of the Others



0

5

10

15

20

25

0 5 10 15 20

Tran

sfer

Pot

enti

al (

% o

f H

V E

PR)

Separation Between HV and LV Electrodes (m)

Detailed Simulation (Equal LV Rod Resistances)

New Approximate Method (Equal LV Rod Resistances)

Detailed Simulation (Low Resistance Electrode at End of LV Network)

New Approximate Method (Low Resistance Electrode at End of LV Network)

Effect of a Low Resistance LV Electrode(MALZ)

(MALZ)



Application to Different ArrangementsDCBA 50m9m

5m

5m

HV Electrode

50m 50m

C

BA 50m9m5m

5m

HV Electrode

50m

D

50m

D

BA 50m9m

5m

5m

HV Electrode

50m

C

50m

Arrangement 1

Arrangement 2

Arrangement 3



Application to Different Arrangements

Arrangement Calculated Transfer Potential (% of EPR)

S.34 (Circular Plate) S.34 (Hemisphere) CDEGS MALT 1 3.88 3.85 5.76 2 4.05 4.03 6.05 3 4.18 4.15 6.24

Table A4. Comparison of Transfer Potential Results for Different Arrangements Using Three Different Methods (HV Resistance = 12.2Ω)

Arrangement Calculated Transfer Potential (% of EPR)

S.34 (Circular Plate) S.34 (Hemisphere) CDEGS MALT 1 5.75 5.70 5.76 2 6.03 5.98 6.05 3 6.23 6.18 6.24 Table A5. Comparison of Transfer Potential Results for Different Arrangements

(HV Resistance = 8.24Ω)



Practical Example – New Residential Development


11kV Distribution Substation

HV Electrode

LV Feeder Underground

Cables




Transfer Potential

(% of HV EPR)1




Transfer Potential

(% of HV EPR)1

2




Transfer Potential

(% of HV EPR)1

2

3


Effect of Soil Resistivity & Structure


300Ωm100Ωm

3m

300Ωm1000Ωm

1m


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



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

Trevor Charlton

Denis Baudin


New Design Methods to Achieve Greater Safety in Low Voltage Systems During a

High Voltage Earth Fault

Mark Davies Trevor Charlton Denis Baudin

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