High Voltage test for Power Cable

High Voltage Test Guide for Power Cables

A complete high voltage test guide for power cable is essential for both of cable users and contractors to get clear idea about cable pre-commissioning and commissioning. 

Underground power cables should be tested before on-site commissioning or re-commissioning is known as SAT or Site-Acceptance-Test. Although cables are tested by the manufacturer in their factory is known as FAT or Factory-Acceptance-Test before delivery. 

FAT ensured the quality and specifications of the cables that followed as per international standard (like IEC, IEEE, BS) and customer’s requirements; on the other hand SAT is limited in that it gives no information about the quality of the circuit. 

The summary of this test results is simple just PASS or FAIL information; but this test gives the user a very degree of confidence that when they switch on to energize the line will operate satisfactorily.
Cable testing at site gives the satisfaction that during laying and carrying there happened no damage. So, cable testing including joints, terminations and all others accessories in site before switch on the power is mandatory for line construction companies and operators. 

Cable should be tested as per national or international specified specifications where applied voltage level and duration mentioned.
e.g- for 33kV medium voltage underground cable Hipot test voltage 3 times of rated voltage for 15 minutes duration. Means rated voltage per phase 33kV/1.732=19kV, so applied voltage = (19x3)kV = 57kV DC for 15 minutes.
Similarly for 132kV cable, test voltage is (132/1.732)x3=228kV for 15 minutes. Of course time duration varies cable manufacture to manufacture specification.

If at the end of the test time the circuit has withstood the applied voltage, then it gives the satisfactory result PASS; otherwise FAIL.

Getting Ready to Start High Voltage Test:

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Consider the following points before start the Hi-Pot or High Voltage test performed.

All equipment must be disconnected from the cable circuit, i.e., taps, motors, circuit breakers, surge arrestors, etc. This will prevent damage or interruption to such equipment from excessive leakage current.
Establish adequate clearance between the circuit test ends and any grounded object, and to other equipment not under test, also keep all personnel in safe distance.
Ensure grounding of all circuit        conductors  except under test with all cable shields including nearby equipments.
Ensure specified maximum test voltage, recommendations time duration for termination and others accessories.

Procedure to apply voltage:

Voltage may apply Continuous method or step method as per specification.
Continuous Method: Apply test voltage at an approximate rise rate of 1 kV per second or 75% of the rated current output of the equipment, whichever is less. Some equipment will take longer to reach the maximum test voltage because of the amount of charging current.
Step Method: Apply test voltage slowly in 5 To 7 Increments of equal value to the maximum specified. Allow sufficient time at each stop for the leakage current to stabilize.

Caution: Don’t forget, DC charges on cable can build up to potentially dangerous levels if grounds are removed too quickly. Maintain solid grounds after the test on the cable for at least 4 times the duration of the test. 



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