Testing And Commissioning :

Progress in cable-manufacturing technology has been accompanied by the development of testing standards. In the early days there were only a few cable manufacturers, and they developed their own testing requirements to ensure reliability of their product. As the industry grew, national standardization organizations stepped in to unify testing procedures, and international standards were subsequently established. The International Electro technical Committee (IEC) is nowadays the main body specifying cable testing procedures, which have been adopted by most countries around the world. Specifications for testing of HV cables with extruded insulation are contained in two separate standards, namely (a) HV, 30 to 150 kV in IEC60840, and (b) extra high voltage (EHV), 150 to 500 kV in IEC62067. These two standards, in their most recent editions, follow almost the same pattern regarding test requirements for design, production quality, and insulation integrity after installation. Even the clause numbering has been coordinated. In terms of test classification they are divided into five main categories:

Prequalification (PQ) tests:

Tests made before supplying, on a general commercial basis, a type of cable system in order to demonstrate satisfactory long term performance of the complete cable system.

Type tests:

Tests made before supplying, on a general commercial basis, a type of cable system in order to demonstrate satisfactory performance characteristics to meet the intended application.

Sample tests:

Tests made by the manufacturer on samples of complete cable, or components taken from a complete cable or accessory, at a specified frequency, so as to verify that the finished product meets the specified requirements.

 Routine tests:

Tests made by the manufacturer on each manufactured component (length of cable or accessory) to check that the component meets the specified requirements.

Tests after installation:

Tests made to demonstrate the integrity of the cable system as installed.

Types of Cable Testing:

• IR Test (conductor).
• Conductor Resistance Test.
• Phase identification (conductor).
• Cross Bonding Test.
• Sequence Impedance Measurements.
• Serving Test.
• Sheath Resistance Test.
• SVL Test.
• High Voltage Test (conductor)

IR Test:

The Insulation resistance (IR) test measures the dc resistance of the insulation of the cable installation using an Insulation Resistance Tester. It involves measuring both the ph-ph & ph-earth insulation resistances. Because of cable capacitance, the IRT shall be applied until a stable reading is obtained. IR tests shall be performed after laying and prior to jointing each section of cable. Screen to Earth test will record an IR after one (1) minute. This test shall be performed first to assist in the reconnection process of the screen to earth connection. Phase to Screen and Phase to Phase testing requires the measurement of IR at 1kV after one (1) minute duration.

Conductor Resistance Test:

This is carried out to determine the effectiveness of the conductor joints and terminations by measuring the dc loop resistance. Results are compared with the manufacturer’s conductor resistance usually expressed in ohms/km in the cable specification.

Phase Identification:

Phase identification should be checked by the use either of the phasing resistors or a continuity check of each individual core wherever possible after all jointing work has been completed. Alternatively, the current injection phasing method may be used. Phase identification checks must be carried out by jointing staff to ensure cables have been connected correctly in line with the system phasing diagrams. Phase identification relates to both conductors and sheaths.

Cross Bonding Test:

Cross bonding tests on the cable sheaths shall be performed to verify the integrity of the cross bonding system. This test ensures circulating sheath currents generated by induced voltages at full load will not adversely affect the cable rating. This test involves injection of a 3 phase current into the cores of the cable, and measuring of voltages and currents induced into the sheaths at each cross bonding point along the complete cross bonding section. An injection current of greater than 50A should be used. Sheath currents and voltages are to be measured at each cross bonding point, earth point and termination. The cross bonding connections shall be rearranged to prove incorrect connection, and checked again after correct restoration. The procedure shall be repeated for each complete cross bonding section of the cable run. Current and voltage measurements are to be scaled up to the rated load current. Voltage values at isolated (not solidly earthed) cross bonding points shall be less than the rated load current scaled voltages of the installation design.

Sequence Impedance Measurements:

These tests are carried out for protection settings, earth potential rise and fault analysis. The tests are circuit dependent. The cable measurements shall include DC resistance, positive, negative and zero sequence impedances, and shall be expressed at a reference temperature of 20deg C. The measured values shall be compared with the calculated theoretical values or those provided by the cable manufacturer.

Serving Tests:

A high voltage DC test between the metallic sheath to earth is performed to test the integrity of the outer sheath. An IR test is performed to and after the high voltage test to assess the insulation integrity of the cable. The test voltage level and test period for serving tests on sub transmission cables are:

• 33kV & 66kV – 10kV/1min.
• 132kV – 15kV/1min.

Sheath or Screen Resistance Test:

The dc resistance of the metallic sheath and connections is measured and compared with the manufacturer’s sheath resistance usually expressed in ohms/km in the cable specification.

Sheath Voltage Limiters (SVL):

SVL’s are connected to the cable sheaths to limit the transient voltage rises to avoid puncturing the cable servings under fault conditions. The units are tested to ensure their compliance with their original characteristics. The test shall be carried out in accordance with the manufacturer’s recommendations.

HV Tests:

The requirement is to complete the prescribed tests as specified. All cables must be fully discharged for a time duration equal to the test time upon completion of each test by means of discharge function on test equipment or an independent earth. HV tests shall be carried out after laying & bedding has been completed.