Why Power Cable Test After Installation
HV/EHV power cable testing at field after installation is the common requirements of cable user. While MV-medium voltage, HV-high voltage and EHV-extra high voltage power cables are carefully tested by the manufacturer before consignment with AC-alternating or DC-direct voltage. Some defects may not be detected or, more likely, damage may occur during shipment handling, carrying to storage and site, or during installation. To make over-sure additional testing carry out after installation and prior to being placed in service, including joints and terminations, may be conducted. Moreover, many users find that, gradually cable systems degrade and service failures which become troublesome for them. Due to that they desire to reduce or eliminate those failures performing periodic tests after some time in service. Determining the economic replacement of downgraded cable, users need special diagnostic test.
Power cable test in field after installation may be broadly divided into following two categories:
A. Field tests are intended to detect defects in the insulation of a cable system in order to improve the service reliability after the defective part is removed and appropriate repairs performed. These tests are usually achieved by application of relatively elevated voltages across the insulation for prescribed duration.
B. Field tests are intended to provide indications that the insulation system has deteriorated. Some of these tests will show the overall condition of a cable system and others will indicate the locations of discrete defects which may cause the sites of future service failures. Both varieties of such tests may be categorized as “pass/fail” or “go/no-go” and are usually performed by means of moderately elevated voltages applied for relatively short duration, or by means of low voltages.
Applied Voltage Type for Cable Test:
Both of Direct Current Voltage and Alternating Current Voltage has been accepted for many years as the standard method.
a. DC test: DC testing method is performing for long time as high voltage tests on cable insulation systems, especially for field test. But recent research has shown that dc testing is the causes to certain types of defects and that it can make worse the deteriorated condition of some aged cables insulated with extruded dielectrics and affected with water trees.
b. AC test: Alternating voltage tests at alternating voltages are highly acceptable since the insulation is stressed in a similar way to normal operation and the test is similar to that used in the factory on new reels of cable. But high voltage AC tests equipment normally heavy, bulky and expensive test transformers which may not be readily transportable to a field site.
During high voltage test carry out of power cable in site after installation the grounding system should be sufficiently, due to long periods of time following such tests can experience dangerous charge build-ups and cause an accident. To avoid any mistake recommended grounding procedure and appropriate working rules should be followed.