Mine underground distribution protection -- part 1 [Industrial Electrical Power Systems]

General

A typical colliery underground network. The protection required for the medium-voltage (11 kV) network from the surface substation to the mobile transformer will require considerations about the trailing cable. However, this could be the standard protection that is followed in a normal industrial substation, taking care of short-circuit and overcurrent conditions.

Underground substation; Mobile flameproof transformer; Surface 11 kV substation; Flexible trailing cable 11 kV cable couplers (flameproof).

++++ Typical colliery UG network

However, it’s important to pay particular attention to the protection of the low-voltage 'front-end electrics' - especially at the coalface where most activity takes place, increasing the possibility of electrical faults occurring.

In coal mining, the normal protection found in the flameproof gate-end boxes comprises:

• Ground-leakage protection

• Pilot wire monitors (ground continuity monitors)

• Ground fault lockout

• NGRM (neutral grounding resistor monitors). These features will now be discussed in detail point by point.

Ground-leakage protection

Ground-leakage protection is primarily employed to protect life. It must therefore detect and isolate faulty equipment as soon as possible to protect the rest of the system and to minimize fault damage.

Consequently, it needs to be as sensitive and as fast as possible. However, ultra-sensitivity and high speeds can lead to nuisance tripping so a compromise is necessary. Generally, one only needs to consider protecting against indirect contact. This is considered justified, as only qualified persons should have access to live terminals, equipment and interlocks being designed accordingly.

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Ground-Leakage Protection

• Primarily employed to protect human life

• Isolates faulty equipment asap: to (a) protect rest of the system; and (b) minimize fault damage

• Needs to be sensitive and fast as possible

• Ultra-sensitivity and high speeds may lead to nuisance tripping - hence compromise necessary

• Protects against indirect contact only but considered justified as only qualified persons should have access to live terminals. Equipment/ interlocks designed accordingly.

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Ground-leakage protection

Sensitivities

The factors that influence relay sensitivities are:

• Stray capacitance

• Unsymmetrical mounting of core balance CTs

• Motor starting in-rush currents

• Transients:

- Switching surges/point-on-wave switching

- Lightning

- Voltage dips

- Harmonics (especially 3rd and 9th, etc.).

Unbalances can be caused by one or more combinations of the above. Relay sensitivities of 250 mA were found to be immune from the above whereas levels of 100 mA were susceptible so they had to be time delayed by 100 ms to ride through the transient disturbances.

Using 250 mA instantaneous sensitivity, typical relay coordination for ground-leakage protection is shown.

2.5A; 1A; 500mA; 375mA; 250mA; M

++++ Typical ground-leakage current sensitivities

Ground-leakage; Sensitivities

• Factors influencing relay sensitivities:

- Stray capacitance

- Unsymmetrical mounting of core balance CTs

- Motor starting in-rush currents

- Transients (a) Switching surges/point-on-wave switching (b) Lightning (c) Voltage dips

- Harmonics (especially 3rd, 9th, etc.): Unbalances could be caused by one or more combinations of the above.

- Relay coordination: Grading is achieved on a current/time basis ---- the diagram.

+--+ Ground-leakage sensitivities

Clearance times

The faster speeds are desirable as they are much less than the 'T' phase resting period of the heart.

Ground Fault Clearance Times

• USA equipment: Ground leakage (250 mA instantaneous) = 30 ms Industrial type contactor = 30 ms Total 60 ms

• UK/USA equipment (NCB): Ground-leakage (80 or 100 mA delayed) = 100 ms

*British contactor = 50 ms Total 150 ms

*Industrial type contactors are not used because these are outside the operating limits laid down in the British/European/USA standard specification.

+--+ Clearance times

Pilot wire monitor

This is a very important and sophisticated relay as it carries out the following functions:

• • Prevents on-load uncoupling of cable couplers
• • Ensures continuity and measures ground bond resistance
• • Detects pilot-to- ground short-circuit
• • Permits remote start/stop of contactor using pilot
• • Unit has to be fail safe.

EL PWM Motor R S T Pilot Ground, Gate end box Stop, Start RM

++++ Pilot wire monitor ( ground continuity monitor)

It’s designed to meet very fine tolerances as shown. As it’s continuously monitoring the resistance of the ground bond to keep equipment within safe touch - potential limits, it can be regarded as important, if not more so, than the ground-leakage relay.

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Control relay--Must not--Pick up with 30ohm run resistor in circuit at 120%V Pick up with 23? in circuit all conditions Pick up with 3? in circuit all conditions--Hold in at 20%V or less--Pick up 75%V and hold in at 60%V Pick up with 21? in circuit at 120%V Pick up with 10? in circuit at 100%V Pick up with 30? run resistor in circuit at 120%V --Pick up with 3? in circuit at 75%V Pick up with 19.5? in circuit at 100%V Pick up with 10-12? in circuit at 100%V Hold in at 20%V or less Hold in at 50%V Must not NCB P130 and IEC3101 Requirement parameters Operate parameters Setting parameters

Note: The contactor must be capable of following the relay under all of the above conditions.

++++ Pilot wire monitor operating characteristics

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Ground fault lockout

As an additional safety measure, an ground fault lockout feature is installed. After the contactor has been tripped, a DC signal is injected onto the power conductor via the resistor bank to monitor the insulation. Closing is prevented if this drops below the pre-set value. As an example, this would ensure safety on start-up should a rock fall have occurred during the off-shift period.