As with insulators, it is vitally important to perform appropriate high-voltage testing on cables to ensure they meet required safety standards, and also to monitor any deterioration over time so you can take appropriate action when required.
Tests on cables are roughly divided into the following groups:
High-Voltage Acceptance Tests at Works
1. Voltage tests
In a voltage test, a voltage is applied at any frequency between 25 and 100 Hz. The voltage must be approximately sinusoidal in shape and is gradually increased to the full value. Once the full value has been reached, the voltage is maintained for no less than 15 minutes.
This voltage is maintained for the 15 minutes between conductors and also between the conductors and the sheath. Each country has its own specifications for the test voltages, depending on the voltage designation, which can be found in the local specification.
The high-voltage tests need to be conducted both before and after a bending test. Bending tests are extremely important in the testing process—the cable needs to be bent around a cylinder that has a specific diameter for a complete turn.
Then, the cable is unwound and rewound in the opposite direction. The winding and unwinding need to be executed three times.
Next, you need to know the testing voltages.
As mentioned above, the voltages are specified according to local regulations. By way of example, if the designation is 11 kV, there are usually 4 other voltages provided in the specification:
• For belted cables, between the conductors, the test voltage of the cable as manufactured should be 24 kV minimum
• For belted cables, between the conductor and sheath after the bending test, should be 36 kV minimum
• For single-core, S.L., and screened cables, between the conductor and sheath, the cable as manufactured should be tested at 15 kV minimum
• For single-core, S.L., and screened cables, between the conductor and sheath after the bending test, should be 22 kV
2. Dielectric power factor
High-voltage dielectric power factor tests are only conducted on 33 kV cables. The tests are conducted at room temperature at a single phase alternating current 50 Hz. It’s important to test at voltages of 9.5 kV, 19 kV, 28.5 kV, 38.0 kV.
When testing, the critical element is the power factor. In no cases should this exceed 0.01. There are various other more complex calculations that must also be satisfied.
High-voltage sample tests at works
Sample tests at manufacture site are conducted only on cables that will eventually be installed vertically. Bending tests (as explained above), and also dripping and drainage tests are usually required.
High-voltage tests on installed cables
This is almost identical to the high-voltage acceptance tests detailed above, but the voltages are reduced. This is usually by around 20%.
Tests on pressurized cables
Pressurized cables often require a broad range of tests to ensure their safety and robustness. Type approval tests are very important in this regard. The type approval tests are conducted on both the minimum AND maximum conductor sizes for every single design and voltage rating. The good thing is that, if the cable passes the type approval test, no additional tests are necessary unless the design changes.
The various tests available are as follows:
1. Loading cycle test
For this test, you need to create a test loop that includes the cable and each accessory, which are then subjected to load cycles. 20 cycles are completed in total and the temperature needs to be more than 5 degrees more than the design value and one and a half times the working voltage. It’s also important to make sure that during testing, the cable is under a minimum internal pressure.
2. Thermal stability test
This test is conducted only on 132 kV cables, very similar to the loading cycle test, with the voltage at 1.5 times the working voltage and the current loading adjusted to a MAXIMUM of 5 degrees more than the design value. Unique to this test is the testing duration—6 hours minimum is the standard requirement. The test needs to be performed for this long to ensure that the cable is stable thermally.
3. Impulse test
For impulse tests on cables, a test voltage at a ratio of 6 times the working voltage is applied for 10 positive and 10 negative pulses.
4. Dielectric thermal resistance test
This is a very simple test where you measure the thermal resistance of a cable.
5. Binding test
For this test, the cable undergoes three binding cycles around a diameter that is 20 times the diameter of the pressure retaining sheath. All production lengths of the cable need to be tested in this way.
There are several other less common tests: cold power factor/voltage tests and mechanical reinforcement tests.
High-voltage cable testing
All cables need to undergo a standard testing procedure to make sure that the cables are fit for service. Fortunately, most of the tests are relatively simple and can easily be performed by experienced engineers such as ours.