internationreach

 

Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers

Cable Testing

An important component in overall power system reliability is the power cable. Whether it is a main feeder, a branch feeder, or subfeeder, its insulation testing should be regarded as a preventive maintenance function to avoid danger to personnel, equipment damage, and unexpected failure and downtime.cabletesting

Many times, cables are damaged during installation. Damage may not show up until load is applied or the cables approach maximum current carrying capacity. Low insulation resistance values frequently indicate low dielectric strength and potential equipment failure. Although megohmmeter testing of high voltage cables is a common practice, sometimes it is not considered as a satisfactory method of determining whether cable insulation has been damaged. The megohmmeter readings can be influenced by temperature variations so the value of the readings is to indicate an upward or downward trend.

Declining megohmmeter readings are indicative of only a general condition which could be due to dirt or moisture influences. Low readings which return to normal at subsequent tests are not indicative of cable deterioration. Only continued decrease in megohmmeter readings is significant and indicative of reduced insulation factors. Megohmmeter power cable testing is relatively reliable if cables are kept clean and dry. Only in that conditions an annual high voltage megohmmeter test should show up any degradation of insulation.

To provide more assurance in testing results a d-c high potential test of all power cables is recommended as addition to this test method. During this testing, cables should be tested in groups or pairs since the dielectric strength of the insulation is measured between adjacent conductors and not to ground. insulation failure to ground is secondary to the capability test of the conductor insulation. Before testing both ends of the cable need to be disconnected. After that, the test equipment should be connected and the test voltage applied as specified in testing procedure for a period of ten minutes. Generally leakage current should start at a relatively high value and drop off rapidly, becoming constant at a low value. The fact that this current becomes constant and levels off is more important than the magnitude of the leakage. If, after the ten minute test, the leakage current has not dropped, or more significantly, after dropping it starts to rise again, it is a strong indication of trouble on the circuit and the test should be extended. The test should be continued as long as the current continues to rise, or until a failure occurs, to permit locating the weak spot. Also, this test method can be influenced by the temperatures of the cable being tested.

To avoid the temperature influence on testing results of these traditional test methods, a modern technique for checking for faulted cables is to use an acoustic type fault detector.

New or old cable that fails repeatedly at other than repaired locations should be replaced. Cable with dry, brittle outer covering, or cracked or obviously deteriorated inner insulation should be replaced and the expense of testing eliminated.

Also, power cable testing should be always performed in accordance with CEC and suppliers or IEEE specifications.

 

Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers