Pruebas A Cables De Media Tension

WIRE AND CABLE ENGINEERING GUIDE (FOR INFORMATIONAL PURPOSES ONLY)

DC Hi Pot Testing Excerpt from PRYSMIAN’S

700 Industrial Drive Lexington, South Carolina 29072 Phone: 1.800.845.8507 Fax: 1.803.951.1142 www.na.prysmian.com/energy

DC Hi-Pot Testing Revision 1 March 09,2010 Page 1 of 6

DC HIGH POTENTIAL TESTING
DC Hi-Pot testing can be applied as a withstand test (a Type I –Destructive Test) or a leakage current measurement technique (a Type II – Non-Destructive test). The Hi-Pot withstand test is a Pass/Fail test that has been applied to many types of cable and accessories. The Hi-Pot leakage current technique is a diagnostic test which involves the measurement of leakage current when a high potential (above nominal) is applied to the conductor while the metallic shield ofthe cable is grounded. The behavioral characteristics of the leakage current are evaluated to determine the condition of the cable, specifically the insulation. As part of any cable testing program, many factors must be considered to properly characterize the test results obtained. Many of those factors are controllable as part of material testing, qualification testing, and production testing.Unfortunately, field testing does not allow control over various factors, two of which are temperature and humidity. Keeping track of the factors that influence cable testing results and accounting for them can be the difference between passing or failing results. It is important to recognize that published documentation provides details showing DC hi-pot testing mostly finds conductive type grossworkmanship errors in extruded † dielectric cable systems . Consequently, the practical use of the DC hi-pot testing is recommended only for paper insulated cable systems and for performing a safety check before switching an extruded cable system into service (to prove that the system is not grounded). Additional details on DC high potential testing can be found by referencing IEEE 400.1 IEEE Guidefor Field-Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5kV and Above with High Direct Current Voltage.

GENERAL INFORMATION:

• DC Hi-Pot Testing is commonly applied at much higher voltage levels than megger testing. The elevated voltage levels place more electrical stress on the dielectric† (megger testing is typically used at voltages up to 5kV). • DC Hi-Pot Testingis typically reserved for shielded, medium-voltage cables †. • Several types of currents are flowing at the beginning of this test. o Capacitive current – Current required to charge the capacitance of the insulation; after all, cable is a long capacitor. This current quickly declines to zero †. o Absorption or Polarization current – Current absorbed by the insulation during molecular changes in theinsulation. This current may go to zero quickly or possibly linger for an extended period of time†. o Leakage current – This current is the primary current to be concerned with. This current quickly reaches and maintains a constant value †. o Total Current – is comprised of the Capacitive Current, the Absorption Current and the Leakage Current †. • In order to obtain leakage current readings thatare most representative of the cable system, and to insure against flashovers to other structures, all bus ties and/or aerial jumpers from the terminals should be removed before performing such tests†. • It is sometimes necessary to “smooth” any sharp edges or points of the hardware on the terminal top fittings using a soft, puttylike insulating compound. This helps reduce stress concentrationand corona discharge off such sharp projections. Alternatively, or in addition, a clean, dry plastic bucket or trashcan (containing NO metallic fittings) is placed over the top of the terminals to effectively lengthen the leakage path to a surrounding grounded structure. Note that the far-end terminals must be treated in the same manner.

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