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Managing Underground Assets

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The Power industry has been developing into a challenging environment due to ongoing restructuring and deregulation. The structural change has directed the electrical utilities to reduce the operating costs and optimize usage and maintenance of electrical assets without sacrificing the quality and reliability of the power delivered to the consumers.

Underground distribution systems are valuable assets of electrical utilities, which supply power to end customers at low voltages. Many of the system components, particularly Underground Cables fail over a period of time for a variety of reasons like deterioration of dielectric or due to external impact.

Underground cables in a power supply system are like nerves in a human body. The importance of the same can also be well understood if we compare its function to that of the human body. Underground cables carry and circulate current in the entire distribution system. However, any flaw or defect in the cable obstructs the flow of current to its destination and thus cuts off the power supply to the consumers.

Any malfunction in the cable leading to obstruction of power supply can be called as a FAULT in a cable.
Effective fault location is dependent on a methodical approach, an understanding of the characteristics of the fault to be located and a good working knowledge of the attributes and limitations of the equipment available. Moreover, information of the distribution network helps to:


  • Cut down the time and labor required for cable fault location.
  • Expenses incurred due to indiscriminate digging and subsequent damage to the municipal properties.

It is important to analyze the nature of cable fault, then select appropriate instrument for locating the fault and correct technique of locating the fault by using that instrument.

“A cable fault is any defect that affects its performance.”

Most common types of fault are either -

  • Contact faults (short circuit).. Breaks (open circuit)

However they vary so much that they take on a multitude of forms.

A power distribution cable of any size, working voltage, insulation etc. can develop following faults.

The Contact Fault : A short circuit fault between two core or between two cores and metallic sheath.

The Break : This type of fault is called as open circuit fault. One or all cores of a cable break at a point having infinite resistance between two ends.

Low Insulation Fault : This type of fault occurs mainly due to external impact, which lowers the required insulation level of the cable.

Flashing Fault : The cable withstands certain voltage level and beyond this level it fails. It flashes at a spot that acts like a spark gap.

Intermittent Faults : These types of faults are highly unstable in nature. These occur mostly due to water or moisture ingress into the cable.

High Resistance faults : These types of faults exhibit a high resistance (instead of infinite resistance) between cores or core to sheath. They often need to be treated by short term burning before being located.

Low Resistance Faults : These type of faults exhibit low resistance (instead of exact short circuit) between cores or core to sheath. They often need to be located by Low Voltage fault locators.

The variety of faults calls for different techniques and instruments for cable fault location.

SYSTEMATIC CABLE FAULT LOCATION

Cable fault location activity starts with basic tests like Insulation resistance (I R) tests. These test results normally indicate the nature of fault and serve as basic guide for the choice of cable fault location equipment. Generally fault location activity can be classified into two parts. PRELOCATION and PINPOINTING.

Prelocation :
There are many situations where in cables of long lengths are laid underground. In case, a fault develops in such long cable, it becomes strenuous, tedious and time consuming affair to walk down the total length of the cable in order to locate the fault. TIME DOMAIN REFLECTOMETERS (TDR) or a CABLE RADAR with a facility of short term arc reflection are can be used to locate the distance to the fault from one end of the cable. If distance to the fault is known, it reduces down time and time taken for locating faults in long cables. However, these instruments are not manufactured locally and are expensive. Investment in these instruments can be justified only with the failure rate of cable and extent of cable network.

PINPOINTING :
This is the most important part of cable fault location. As the name suggests one can actually pinpoint the fault with aid of Surge Testers or THUMPERS as more popularly called. The Thumper or a SURGE TESTER thumps energy into the faulty cable at regular intervals. Wherever the insulation of the cable goes low (low insulation fault), the energy is released from the cable into the underground strata making strong vibrations underground. These vibrations propagate through the underground strata up to the surface, which are picked by a sensitive receiver (DUOPHONE) having seismic microphone. These strong vibrations make loud sound, which can be heard into the headphone and its intensity can be seen on a meter. The point of loud sound or highest vibration is the fault point.

It is imperative to locate route of the cable before pinpointing the fault. A search along the wrong track would lead nowhere. Seismic receivers like DUOPHONE have facility of magnetic indication, which keeps the operator on track of the route simultaneously while pinpointing the fault.

To pinpoint a short or low resistance fault one needs a high power audio generator and receiver to locate it, where in high voltage thumping does not work.

Every cable has to be subjected to HIGH PRESSURE or HIGH POT TEST after being laid underground and before commissioning to ensure its healthiness before charging the cable. Megger tests are NOT recommended for testing the insulation of the cable as we do not have Meggers operating on a voltage higher than 5 kV.

Regulatory authorities for electrical utilities are now making HI-POT tests mandatory.
IS- 1255 explicitly mentions the test voltages for different class of HV cables.

There are many factors that influence the choice of equipment. Extent of cable network, cost of outages, fault incidence and budget, generally should be considered before the selection of equipment. Equipment offered by indigenous manufacturers is a better choice, as post sales service support is better warranted as against imported equipment.

Generally more attention is given to breakdown maintenance; however focus should be on preventive maintenance. There are new technologies and instruments available, that would determine the age of the cable and probable occurrence of the fault before it actually happens. Root cause of cable faults should be studied: few factors that lead to cable faults are faults on cable joints, terminations, mechanical damage and ageing of cable.

Some tests like Partial discharge tests, Very Low Frequency (VLF) testing and Tan-Delta tests are non destructive tests and provide valuable information on the dielectric of the cable, its status and also the life expectancy of the cable.

Cables are valuable and expensive assets laid underground, mere fault location and repairs after fault location are activities under break down maintenance. Condition monitoring and preventive maintenance of cables, its management or underground asset management should be given more importance.

ELECTROCON SYSTEMS offers a wide range of indigenous Cable Fault location, Cable Testing instruments and comprehensive systems suited best to the client’s exact requirements.

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