Earthing of MV and LV Earthing of MV and LV

Distribution Systems: Distribution Systems:

A multi-faceted A multi-faceted

problem.problem. Hendri Geldenhuys

Gareth Stanford

Industry Association Resource Centre (IARC) Eskom

Considerations related to:Public , Customer (end

user?)And Line’s man safety

considerations

Merged with

System “protection” and performance considerations

Artwork: A Dickson

LV Earth

LV Feeder

Service Box

Serviceconnection

Communicationline

Distributiontransformer

MV Earth

MV Line

Consider the Bonding and Earthing of the structure:

• There are no single solution that fits all applications and environmental conditions

• All solutions has risk, some less than others,

Design Philosophy

LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel

Design Philosophy

Design PhilosophyBIL wires on all shared structures (no gap no insulation of BIL wire.)BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.)No BIL wireBIL wire on all shared structures- insulate bottom 2m of down wireNo BIL wire- Double arresters on transformers on MV sideBIL wires on all – Move gap down below LVBIL wires on all – Split air gap above & below LV to prevent LV Faults

BIL wire on all shared structures (no gap no insulation of BIL wire.)

LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel LV Feeder

MV Feeder

Telephone

BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.)

LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel LV Feeder

MV Feeder

Telephone

No BIL wire

LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel LV Feeder

MV Feeder

Telephone

LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel LV Feeder

MV Feeder

Telephone

BIL wire on all shared structures- insulate bottom 2m of down wire

LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel LV Feeder

MV Feeder

Telephone

No BIL wire- Double arresters on transformers on MV side

BIL wires on all – Move gap down below LV

LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel LV Feeder

MV Feeder

Telephone

LV Feeder

MV Feeder

Telephone

Wood,

Concrete

or Steel LV Feeder

MV Feeder

Telephone

BIL wires on all – Split air gap above & below LV to prevent LV Faults

Safety Risks That occurs in MV LV

Distribution Systems

Separate MV and LV Earths

L N E

TANK

N

- 242 V+ 242 V

LV EARTH

HV TO LV NEUTRAL ARRESTOR

SURGE ARRESTOR

MV EARTH

FEEDER LINE ( 3Ph , Ph-Ph or SWER )

fuses

MV Voltage Transferred to LV Earth

L N E

TANK

N

- 242 V+ 242 V

LV EARTH

HV TO LV NEUTRAL ARRESTOR

SURGE ARRESTOR

MV EARTH

FEEDER LINE ( 3Ph , Ph-Ph or SWER )

fuses

> 5 kV

> 5 kV

MV-LV Supply System

L N E

N

L N E

MV-LV Supply System

L N E

N

L N E L N E L N E

2.5 kW10A

2.5 kW10A

20A

-100V -100V 100V 100V

LV Neutral Break

MV

LV

Neutral SA

Step potential 30 Ω 70 Ω

161 V [ =230 x (70/100) ]

I = 2.3 A

☻The ratio between the MV and LV earth electrode resistance determine the voltage on the LV neutral- earth

69 V

[ =230 x (30/100) ]

Equipment Damage

5 x Iph

Earthing and Bonding in different Environments

Pollution LowPollution High

Lighting Low No BIL down wire or bonding of hardware

Bonding of hardware (no BIL down wire)

Lighting High BIL wire and co-ordinated gap

Bonding of total structure, no gap, high insulator BIL.

Artwork: A Dickson

LV Earth

LV Feeder

Service Box

Serviceconnection

Communicationline

Distributiontransformer

MV Earth

MV Line

AC power system related risk

Design Philosophy MV Conductor drop onto LV system

MV conductor contact to BIL down wire only

LV contact to BIL down wire

BIL wires on all shared structures (no gap no insulation of BIL wire.)

Fast clearing of MV fault (1sec). Auto Reclose repeat

to lock out. High GPR on LV Protective

Earth.Exposure to all LV

installations and BIL wire locationsMed risk

Slow clearing of fault (10 sec) even a small risk of not

clearing fault.Very High GPR on BIL wire.

Med risk

Fault not clearedBIL wire stay live

High risk

BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.)

Fault not clearedFewer BIL wires stay live

High risk

No BIL wire

Fast clearing of MV fault (1sec). Auto Reclose

repeat to lock out. High GPR on LV Protective

Earth.Exposure to all LV

installationsMed risk

No risk No risk

BIL wire on all shared structures- insulate bottom 2m of down wire

Slow clearing of fault (10 sec) even a small risk of not

clearing fault.Very High GPR on BIL wire.

Fault not clearedBIL wire not accessible

Med risk

No BIL wire- Double arresters on transformers on MV side

No risk No risk

BIL wires on all – Move gap down below LV

BIL wire will remain live until it flashes to LV

protective earth. If Fault to LV occurs-clearing of the

fault is the same as column to the left.Med risk

Low risk

BIL wires on all – Split air gap above & below LV to prevent LV Faults

Low risk Low risk

Lightning risk

Design Philosophy Effective earthPublic&Consumer

lightning riskEquipment Damage

BIL wires on all shared structures (no gap no insulation of BIL wire.)

MV earth + LV earth +BIL wire earths.

Best practice(not totally safe)

Best Practice

BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.)

MV earth + LV earth +fewer BIL wires

Best practice(not totally safe)

Best Practice

No BIL wireMV earth+ LV earth

onlyHigh risk High risk

BIL wire on all shared structures- insulate bottom 2m of down wire

MV earth + LV earth only

Best practice(not totally safe)

Best Practice

No BIL wire- Double arresters on transformers on MV side

MV earth + LV earth +BIL earths

High risk Medium risk

BIL wires on all – Move gap down below LV

MV earth + LV earth + BIL earths

Best practice(not totally safe)

Best Practice

BIL wires on all – Split air gap above & below LV to prevent LV Faults

MV earth + LV earth + BIL earths

Best practice(not totally safe)

Best Practice

AC power risk

Overall Risk

Lightning Risk

Design Philosophy

MV Conductor

drop onto LV system

MV conductor contact to BIL down wire only

LV contact to BIL

down wire

MV conductor contact to BIL

down wire only

LV contact to BIL

down wire

BIL wires on all shared structures (no gap no insulation of BIL wire.)

MED MED HIGH HIGH LOW LOW

BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.)

MED MED HIGH HIGH LOW LOW

No BIL wire MED LOW LOW HIGH HIGH HIGH

BIL wire on all shared structures- insulate bottom 2m of down wire

MED LOW MED 2 MED LOW LOW

No BIL wire- Double arresters on transformers on MV side

MED LOW LOW HIGH HIGH MED

BIL wires on all – Move gap down below LV MED MED MED 3 MED LOW LOW

BIL wires on all – Split air gap above & below LV to prevent LV Faults

MED LOW LOW 1 MED LOW LOW

LV Feeder

MV Feeder

Telephone

50 mm LV gap

500 mm BIL wood gap

LV Feeder

MV Feeder

Telephone

50 mm LV gap

500 mm BIL wood gap

A (simple) Wood Pole Structure has complex safety

and performance considerations

Designs can be optimised for specific areas and

applications

No Power System is ever 100% safe

Safe and Reliable