Earthing systems in mines - Phillip Wall
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Transcript of Earthing systems in mines - Phillip Wall
Phillip Wall - MIEAust [email protected]
Earthing Systems in Mining Operations – managing transfer voltage hazards
25th Electrical Engineering Safety Seminar - Nov 2015
Segregate and contain? OR
Bond all systems?
Earthing Systems in Mines
A high Earth Potential Rise (EPR) needs to be adequately managed
Should we:
Slide 1
Earthing Systems in Mines Managing transfer
voltage hazards
Fault Current
Impedance to Ground
Buried Earth Grid
Voltage = Earth Potential Rise
(EPR)
Voltage = Earth Potential
Rise (EPR)
V Touch Voltage
Transfer Hazard V
Step Voltage V
Mandatory commonly bonded systems:
Power Systems AS/NZS 3007 AS 2067 Single combined earthing systems Unless:
• Underground mining • Voltage limits cannot be achieved
Mandatory commonly bonded systems:
Lightning Protection Systems
AS/NZS 1768 & AS/NZS 3007 Single combined earthing systems Unless:
• Underground mining
• Reduce: impedance of the earthing system, earth fault current or fault clearing time • Surface insulating layers, grading rings or bonding concrete reinforcement • Separation of HV and LV earthing systems • Combined HV and LV earthing systems • Isolation • Protective barriers or signs • Remove non compliant infrastructure (eg telco pit)
Risk Management Treatment Methods
Mitigation methods for Lightning effects
- Minimizing the lightning collection area
- Supplementary bonding underground in accordance to the principles of AS/NZS1768
- Insulation between separate earthing systems
- Reducing earth connection resistances
Lightning Protection Systems in Mines
• Surface installations, protection - AS/NZS 1768
• For underground mines, bonding principles in underground - AS/NZS 1768
Transfer of lightning to other parts of the mine?
• Minimised through separated earthing systems.
Boreholes:
• Voltages can be controlled with PVC insulation. (Dielectric Isolation)
• Stopping the casing short (say 10m above roof mesh) is a similar outcome to PVC for current.
ACARP PROJECT FINDINGS Project C22003 published 1/6/2015
‘Investigation of the Potential Lightning Impacts on Underground Coal Mines’
Source: ‘Investigation of the Potential Lightning Impacts on Underground Coal Mines’ by Prof. David Cliff
ACARP PROJECT FINDINGS Boreholes (cont):
• Attenuation ↑ lower soil resistivity
deeper the mine
• Impulsive volts ~ tens of kV on roof mesh
• Gaps in roof mesh - reduced currents
Project C22003 published 1/6/2015
Direct strike to surface transformers feeding U/G parts
ACARP PROJECT FINDINGS Project C22003 published 1/6/2015
• Large Voltages (~MV) transferred by cable screens to U/G parts
• Potential difference in U/G
soil → cable screen armour was found to be very high.
• Voltages relatively independent of mine depth, frequency and length of power cable
• Smaller than direct strike BUT still significant
• The potentials in U/G Parts↑significantly
for increasing cable lengths for higher freq.
ACARP PROJECT FINDINGS Project C22003 published 1/6/2015
Indirect strike to a cable due to a horizontal lightning channel
Why would we separate earthing systems?
• Majority of cases safer to commonly bond.
– Improves overall system impedance
– Lower EPRs and touch hazards
– Simplest and easiest configuration to maintain
– Less damage to equipment
Separate or Common Earthing Systems?
Why would we separate?
A note on separations – two scenarios
1. Separation between power systems EPR from a power system fault:
Main Substation
mine infrastructure (or Mine Surface Earth)
What is the source of electrical energy?
A note on separations
2. Separation for lightning transfer
surface
underground
Separated Power Earthing Systems
When EPR cannot be managed through Common Bonding. Sparse networks
Separation comes with a few difficulties:
• Confusion from multiple earthing systems
• Hazards within yard due to different potentials on earths
• Lightning Flashover – correct insulation levels
• Earth switches/switchboard earths maintenance
• Single point bonding / cable damage
Common Bonded vs Separated
Q: Do the scales tilt at any point?
A: Yes, however the optimum arrangement is not always apparent;
Lower touch voltages with more exposure
vs
Higher touch voltages with less exposure
Each system differs and requires detailed assessment.
Independent
Earthing System
Impedance Earthed - IT systems:
The earthable point of the power system is either isolated from earth or earthed through an impedance
Provide for the safe management of voltages during earth faults
Impedance Earthed systems
According to AS/NZS 3007:2013
“There is potential benefit for electrical supplies entering underground being impedance earthed systems”
Readily controls touch and transfer potential
Typical Earthing systems … and how they go together:
Network Earth
Mine Surface Earth (MSE)
Mine Underground Earth (MUE)
Lightning Earth
Network Earth … is the Earthing system associated with the incoming supply.
Irrespective of earth connection to the upstream Network
substation, the earth fault belongs to network.
Mine Earth
Q: What is a Lightning Earth?
A: An earth termination intended to discharge lightning currents into the general mass of the earth.
Lightning Earth and transfer effects
What happens when the structure is used as a downconductor?
Can there be Lightning and Power System separation?
Lightning Earth and transfer effects
Lightning Earth and Mine Earths
Unless suitably protected, all surface structures can be assumed to be incorporated into a lightning earth.
Power Systems Separation
Neutral Earthing Resistor
Power Systems Separation
What is this earth?
Where to earth?
Power Systems Separation
Insulated and isolated earth
Power Systems Separation
Two earthing systems within the one yard
Power Systems Separation
Insulated and Isolated
Earth Bar
Power Systems Separation
Power Systems Separation
NER connected to Network Earth
How to connect these?
Power Systems Separation
Screens bared back and insulated Single point
bonded at Mine Earth
Power Systems Separation
Controlled Area
Single point bonded at Mine Earth
Power Systems Separation
Single point bonded at
Network Earth
Feeder Maintenance
Breaker 1 open Earth Switch 1 closed
Breaker 2 open Earth Switch 2 closed
Lightning transfer effects and U/G mines
According to AS/NZS 3007:
No likelihood of transfer:
AS/NZS 1768.
If likely transfer effects:
No direct connection
Lightning earth Mine Undergroud Earth (MUE)
Where do we provide the separation?
Mine Earths – Surface and Underground?
Screens bared back
Mine Underground Earth
• Transfer mechanisms
- Transition from Surface to UG
- Through different districts in UG areas
• Areas of interest:
- sealed areas
- working faces
- return airways
U/G Bonding Practices
Borehole Mitigations
• Depends on the construction type of borehole and what services use the borehole
U/G Bonding Practices
- High Voltage feeders
- Gas Drainage / Dewatering / Submersible pumps
- Ballast / Concrete / Stone dust drop holes
- Steel lined air shafts
- Piezometers and Extensometers
- Tube bundle (caternaries) and communications
Before sealing up longwalls
• Breaks in mesh in gate roads
• Removal or breaking of pipe lengths and cables
• Removing or treatment of mesh at seals
• Careful attention to pipe penetrations through seals
U/G Bonding Practices
Mines are being asked to implement controls to minimise lightning effects. Effective earthing systems such as separated systems are shown to reduce energy transfer.
Summary