Tuesday, July 31, 2012

Medium Voltage Cables test

A . Visual and mechanical inspection :
1- Inspect exposed sections of cables for physical damage and evidences of  
    overheating and corona.
2- Inspect terminations and splices for evidences of overheating and corona.
3- Inspect all bolted electrical connections for high resistance using one of the following methods:
i  - Use of low – resistance ohmmeter.
ii - Verify tightness of accessible bolted electrical connections by calibrated torque-wrench method in accordance with manufacturer's published data.
iii- Perform thermo graphic survey .
4- Inspect comparison – applied connections for correct cable match and identification.
5- Inspect for shield grounding , cable support , and termination.
6- Verify that visible cable bends meet or exceed ICEA and/or manufacturer's minimum allowable bending radius.
7- Inspect fireproofing in common cable areas , if specified.
8- If cables are terminated through window –type current transformers, make an inspection to verify that neutral and ground conductors are correctly placed and  that shields are correctly terminated for operation of protective devices.
     B .Electrical Tests:
1- Perform a shield – continuity test on each power cable by ohmmeter method.

 2- Perform an insulation – resistance test utilizing a  megohmmter with a voltage output of at least 2500 volts.Individually test each conductor with all other conductors and shields grounded . Test duration shall be one minute.

3- Perform resistance measurements through all bolted connections with a low resistance ohmmeter, if applicable .

4- Perform a dc high – potential test on all cables . 
adhere  to all precautions and limits as specified in the applicable NEMA/ICEA standard  for specific cable. Perform tests in accordance with ANSI/IEEE standard 400.test procedure shall  be as follows , and the results for each cable test shall be recorded as specified herein .test voltages shall not exceed 60 percent of cable manufacturer's factory test value or Maximum test voltage in the table.

Note:

1- Insure that the input voltage to the test regulated
2- Current – sensing circuits in test equipment shall  measure only the leakage current associated with the cable under test and shall not include internal leakage of equipment .
3- Record wet – and dry-bulb temperatures or relative humidity and temperature.
4- Test each section of cable individually.
5- Individually test each conductor with all other conductors grounded. ground all shields
6- Terminations shall be adequately corona – suppressed by guard  ring , field reduction sphere , or other suitable methods as necessary.
7- Insure that the maximum test voltage does not exceed the limits  for terminators specified in NSI/IEEE Standards 48 or manufacturer's  specifications.
8- Apply a dc high – potential test in at least five equal increments  until maximum test voltage is reached . no increment shall exceed the the voltage  rating of the cable .Record dc leakage current at each step after a constant  stabilization time consistent with system charging current.
9- Raise the conductor to the specified maximum test voltage and hold for five minutes, Record readings of leakage current at 30 seconds and one minute intervals thereafter.
10- Reduce the conductor test potential to zero and measure residual voltage at discrete intervals
11- Apply grounds for a time period adequate to drain all insulation stored charge.
 
C .Test Values:

1- Compare bolted connection resistances to values of similar connections.
2- Bolt – torque levels shall be in accordance with manufacturer specification.
3- Microhm or millivolt drop values shall not exceed the high levels of the
    normal ranges as indicated in the manufacturer's published data .if
    manufacturer's data is not available , investigate any values which deviate
    from similar connections by more than 25 percent of the lowest value.
4- Shielding must exhibit continuity . investigate resistance values in excess of
    ten ohms per 1000 feet of cable.
5- Graphic plots may be made of leakage current versus step voltage at each
    increment and leakage current versus time at final test voltages.
6- The step voltage slope should be reasonably linear.
7- Capacitive and absorption current should decrease continually until steady
    state leakage is approached.
8- Compare test results to previously obtained results.

 
INSULATION-RESISTANCE TEST VOLTAGES FOR ELECTRICAL APPARATUS

 
Maximum Maintenance Test Voltages (KV,dc)

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