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Underground Mine Earth Fault Grading

This underground mine was supplied by an 11 kV distribution system which included sections of overhead lines. The overhead lines were subject to frequent faults due to lightning and bird strikes.

Underground operations relied on six cable feeders, each set to trip on an earth fault current of 2 Amps. Unexplained outages of the underground feeders, sometimes all six at one time, were causing lost production and putting safety at risk.

AEE were engaged to investigate and report on the protection operations. The outcomes of the report are summarised below.

During earth faults anywhere within the 11 kV system, the normally balanced cable charging current in a healthy feeder, experiences two significant changes:

  1. The magnitude of the capacitive current in the two healthy phases increases, &
  2. The magnitude of the capacitive current in the faulted phase reduces to near to zero.

The net result is that the vector sum of the capacitive currents in the three phases no longer equals zero, but increases to a level equal to three times the steady state charging current. Furthermore, this capacitive current flows to earth and is, therefore, sensed by the earth fault protection.

Because of the extent of the underground workings, the underground feeder cables drew high levels of steady state charging current. Consequently, during remote 11 kV earth faults, usually on the overhead network, large unbalanced capacitive charging current would flow in the underground feeders causing them to trip.

The trip times of the feeders’ sensitive earth fault relays exacerbated the problem. They had been set with trip times of between 200 ms and 500 ms. The underground feeders were, therefore, tripping well before the time delayed earth fault protection elements in the 11 kV overhead system could operate.

To eliminate the nuisance tripping, AEE recommended:

  • Increasing the trip time delay in the underground feeders’ sensitive earth fault relays to from 0.2 seconds to 6 seconds,
  • Augmenting the underground feeders’ earth fault protection schemes by utilising earth fault elements with trip time delays of 0.4 seconds with pick up currents set above the capacitive earth fault currents that flow during remote 11 kV system earth faults,
  • Re-grading the entirety of the 11 kV system to ensure coordination between:
    • Grid incomers,
    • Local diesel generators,
    • 11 kV overhead distribution &
    • 11 kV underground distribution.