As electricity is transmitted and distributed from generation plants to the end-users, it passes through numerous sections of low voltage (below 1000 V) and high voltage (defined in Singapore as more than 1000 V) cables.
In addition to these transmission cables, pilot cables (which consist of 5 pairs or 10 pairs of color-coded wires) are used to connect the secondary outputs of current transformers (CT) deployed at both ends of the transmission stations (also known as sub-stations).
To safeguard sensitive equipment and avoid the escalation of faults that may distress the entire transmission network, these pilot cables assist to detect any unbalances between the corresponding pairs of CT, which indicate fault conditions. Whereas only one pair of wire is required to complete the connections, operation, and redundancy planning call for a minimum of three functional pairs of wire at any one time.
It’s unbearable to have a single cable that spans the vast distance between sub-stations. The result comes numerous joints are formed to link different sections of cables together. Such joints are the weakest links in the system because they are imperiled to failures affected by environmental factors that include thermal stress and moisture absorption. The failures will highlight when current readings taken at the secondary of CT deviate from the proportional primary current readings.
During such failure is recounted, the utility’s maintenance team is entitled to repair the fault. They have to travel to the sub-station at one end to separate the affected CT and pilot cable at that sub-station. Then they need to travel to the sub-station at the other end to check the insulation resistance of the pilot cable. In this condition, the acceptable insulation resistance is 10 Mohms minimum with 500 V DC applied.
While they are at it, they will check the pair of wires that have been used, as well as all the remaining 4 or 9 pairs of wires in the same bunch of pilot cable, and ensure at least three pairs are functional. However, some wires may have permanent damage, there are prospects that different color wires are mixed in order to attain the minimum 3 functional pairs. As a result, the team must validate the exact inter-connect pairs and they term this work as “phasing”. This is done by examining the continuity of the respective wire pairs. When a joint is verified faulty, it will be cut out and a short jumper cable will be added and two new cable joints are formed.
Afterward, insulation resistance as well as “phasing” must be checked again to ensure no abnormality.
In addition to reported faults, the same tests can be assigned by a new extension of cables, or diversion of cables due to civil engineering works (ie. new housing project development). Time is a perilous consideration as the maintenance team is always hard-pressed to complete the troubleshooting and repairs in the shortest possible time. They find the Fluke 1587 Insulation Multimeter a very multipurpose tool as it can perform insulation resistance tests as well as voltage measurements and continuity checks.
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