The DC voltage tests conducted on cable are insulation resistance measurement and DC hi-pot test.
The DC hi-pot test can be performed as leakage current versus voltage test, leakage current versus time test, or go, no-go overpotential test. It is always appropriate to conduct the insulation resistance measurement test first, and if data obtained looks good, then proceed with the DC overpotential test. After DC overpotential test is completed, then perform the insulation resistance again to assure that the cable has not been damaged during the DC overpotential test.
DC Overpotential Test Connections and Procedures
The test connections for this test are similar to test connections shown in Figure 2.7a, and for three-conductor cable are similar to those shown in Figure 2.7b and c. The test procedures are the following:
• Cable to be tested must be de-energized, opened at both ends, and grounded to discharge any electrostatic charge on the cable. Switches, potential transformers, lightning arresters, jumpers from potheads to feeders, fuses, cutouts, and any switchgear should be disconnected.
If impossible to disconnect any or some of connected equipment, the test voltage should not exceed the value that could overstress these devices connected to the cable.
• DC test voltage should be applied from phase to ground on each conductor with other conductors, shields, and metallic sheath connected to ground or other conductors guarded with shield and metallic sheath grounded.
• Ensure that the hi-pot set main “on–off” switch is in off position and the high-voltage on switch is in the off position with voltage control switch turned to zero position before beginning the tests.
• Connect the hi-pot test set safety ground stud to a good electrical ground and make sure the connections are tight. Never operate the DC hi-pot test set without this ground connection. Also connect the shield ground strap of the shielded cable under test to the test set ground stud.
• Connect the return line from other conductors not under test to the earth ground terminal or to the guard terminal of the test set as desired. The hi-pot grounding switch should be switched into the appropriate position. Normally, 100 V insulation is required on the return line. Connect the shield and sheath to ground and also to the ground terminal of test set. The guard terminal is provided to bypass the current due to corona and surface leakage around the microammeter so that corona and surface leakage currents are not included in the test readings.
• Connect one end of the output or line cable to the desired phase of the cable under test, making sure that the connections are tight and without any sharp edges. Where corona currents may be expected owing to the application of high voltages, it is recommended that the connections be taped, covered over with clear plastic bags, or use a corona ring or corona shield. The other end of the output or line cable is connected to the output or line stud of the test set.
• Cable used for connecting the hi-pot test set to the cable under test, that is, the line or output cable, should be short and direct and supported along its length so that it is not touching the ground or grounding materials or surfaces. If extension cables are to be used with the output or line cable to reach the cable under test, shielded cable should preferably be used for this purpose. The shields of the extension cable and hi-pot cable should be connected with a shield jumper, which should be run away from the splice to prevent leakage. In case of the extension cable being nonshielded, care should be taken to keep the nonshielded wire away from the grounding surfaces as explained previously.
When shielded cable is being tested, it is • recommended that the shield be trimmed back about 1 in. for every 10 kV. The shield on the test set end of the cable is connected to ground as explained previously. The shield on the other end of cable can be taped and left hanging without any connections made to it.
• Test set now should be plugged into a 115 V, 60 Hz outlet. It is important that the AC supply voltage have good line regulation, because the DC output voltage of the test set depends upon the AC line input voltage. The test voltage kilovolt range should be selected before beginning the test. The power now can be turned on and the test begun either as step-voltage or as a go, no-go test.
• After the test is completed, turn the high-voltage switch of the test set to off position. Allow the cable just tested to discharge either through the internal test set discharge circuit or external ground applied to the cable by means of hot stick at 2 kV or below. Do not touch the cable until it is fully discharged.
• Connect a ground to the cable that was tested and leave it connected for at least four times the length of the test time or until the cable is connected into the system.
Reference:Electrical Power Equipment Maintenance and Testing