Mine underground distribution protection -- part 2 [Industrial Electrical Power Systems]

The final element in the protection system is this relay, which ensures the integrity of the neutral grounding resistor. If this latter device should open or short-circuit, the NGRM will operate to either alarm on trip. The problems experienced with solid grounding:

• High fault currents -- only limited by inherent impedance of power system

• Solid grounding means high ground fault currents

• This damages equipment extensively

• Leading to long outage times -- lost production, lost revenue

• Heavy currents in ground bonding gives rise to high touch potentials -- dangerous to human life

• Large fault currents are more hazardous in igniting gases (explosion hazard).

++++ Ground fault lockout

++++ NGR monitor

++++ Problems -- Pilot wire monitor

These can be overcome by introducing an ground barrier between the phases so that all faults become ground faults then controlling the ground fault current levels by the neutral grounding resistor.

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M Pilot wire monitor Pilot R 2.5A 1.25 A 1.25 A

++++ Solutions Copper/nylon screens Multi-strand cores Polychloroprene rubber sheath Pilot line core.

++++ Screened trailing cable Air Creapage paths

++++ Investigation

Grave Concern

Over failure of 11 kV cable couplers

1. Government mining engineer

1.1 Possible ignition of methane

1.2 Giving rise to a disaster of horrendous magnitude

1.3 Considering banning this type of equipment

2. Mine authorities

2.1 Same concerns as above

2.2 Alternatives increase production downtime:

- Initial installation takes longer and is more difficult

- Lose facility to quickly extend network

- Lose facility to quickly locate faults

3. Manufacturers

3.1 Same concerns as 1 and 2

3.2 Business reputation threatened

+--+ Concerns over failures

• Phase segregation eliminates phase-to-phase faults

• Resistance grounding means low ground fault currents

• Fault damage minimal -- reduces fire hazard

• Lower outage times -- less lost production, lost revenue

• Touch potentials kept within safe limits -- protects human life

• Low ground fault currents reduce possibility of igniting gases (explosion hazard)

• No magnetic or thermal stresses imposed on major plan during fault

• Transient overvoltages limited - prevents stressing of insulation, MCB restrikes.

Air ionizes to cause phase-to-phase flashover. Hence, phase segregation is achieved by insulation barriers, which are made of silicon rubber.

Phase Barriers

• Molded out of silicon rubber

• Material has suitable dielectric strength and insulating properties; no carbon content so it has minimal tracking capabilities

• Flexible - allows push-on fit over insulator ports and gives a cleaning action

• Is compressed as coupler halves bolt together

• Acts as a 'heat sink' for any arc that occurs, thereby assisting quenching

• Can be easily retro-fitted to existing couplers in service.

Proving tests Fault energy formula =

2 IRt ×× Where

= fault current

= resistance of fault arc

= time in seconds fault is on.

I R t

++++ Solution-rubber phase barrier

Khutala fault throwing tests

Fault current = 4000A

Clearance time = 350 ms

Assume arc resistance of 1 ?

Fault energy = 4000 1 0.35 ×××

= 5.6 MJ

If clearance time reduced to 100 m/s

Fault energy = 4000 1 0.1 ×××

= 1.6 MJ

70% reduction

If steps could be taken to also reduce level of fault current then major strides would be made.

Fault location:

In mining industries, identification of a fault location is a critical requirement. The following sketches/ pictures show how couplers are useful in identifying the fault location.

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Flags to Identify; Fault; Location

• Flags indicate path to ground fault

• Flags automatically reset on restoration of normal load current

• Just plugs in between new or existing couplers

• Obviates uncoupling and re-coupling cable couplers in wet polluted environment when fault-finding.

Major problem

++++ Great difficulty in locating ground faults CINDI; Half couplers

++++ Fast fault location

EWA Audible alarm

'Check CINDIs' Interrogate digital read-out to determine level of E/F current Highly sensitive early warning alarm relay + core balance CT

++++ Early warning alarm system


• Universal Voltage independent

Any voltage up to 3.3 kV (500 V, 1000 V, 3300 V)

• High sensitivity 250 mA ground fault, 40 ms

Coordinates with upstream relays

Operates when closing onto a fault

• Self-reset--Automatically resets of restoration of normal load current

• Self-powered--No separate power supply required

• Stable flag--Magnetic disk pulse operated

Can be mounted in any position

Retains status through shock, vibration and power failures, etc.

• Installation--Quick and easy

Just plugs in between two half couplers

Ideal for retro-fitting

No special tools required

No breakdown of prepared joints

• Tamperproof--Electronics potted in epoxy-resin

Intrinsically safe circuitry

• Flameproof, tested

GME certificate obtained


'CINDI' Cable Ground Fault Indicator System Offers Major advantages

• Fast fault location

- Less production downtime

- Less lost revenue

• Fast payback period

- Savings integrate as time goes by

• Increased safety and equipment reliability

- Obviates the need to uncouple and recouple cable couplers in dirty environment when fault finding.

+--+ Advantages

CINDI-Major Advantages

Other factors


• Potential to save mining industry millions of dollars in lost revenue due to production down time

• Cost-effective Same price as a half coupler

• Fast payback period-- Cost of one extended outage would more than pay for equipping mine with these units

• Return on investment--Excellent.-- Benefits integrate as time goes by New developments

• Field experience so far has been excellent and requests have been received to develop: 11 kV (1 A) model This is the network that grows. Prototypes now in the field 3.3 kV (80 mA) model For transformer and continuous miner

To coordinate with 100 mA E/L relay

Prototypes also in field.


Other factors


• Phase segregation a must

• System grounding - restricts neutrals currents to a low value with a resistor

• Install fast, highly sensitive, ground-leakage protection

• Pilot wire monitor, an essential and equal partner

• Fit an ground fault lockout system

• Install phase barriers and cable ground fault indicators to reduce production downtime and improve safety levels.

+--+ Summary of recommendations

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Wednesday, January 16, 2013 12:24