High voltage cable testing: methodology, norms, terms

Any conductor of the highest quality, designed for increased voltage, may have technological damage during installation work. To avoid emergency situations during commissioning, when an increased load will be applied, it is necessary to make sure that the cable line is intact. During operation, the inevitable processes of destruction of the material from which the conductor is made occur, so it loses its insulating characteristics. To ensure safe operation, it is necessary to periodically test the cable with increased voltage. Next, we will tell you exactly how the test work is carried out.

Typical cable damage

According to statistical data, the most frequent damages that cause the failure of electrical cables are:

• Damage to the integrity of the containment as a result of improper technological work.
• Destruction of insulation due to aging of the material from which the cable is made, due to violation of the test technology.
• The appearance of cracks and ruptures in the protective screen, which disrupt the insulating function.

Varieties of tests

In accordance with the accepted standards and rules for testing electrical equipment, it is necessary to make sure that the declared characteristics of the cable comply with the requirements. If any inconsistencies are revealed, it is strictly forbidden to carry out delivery, and even more so to operate such lines.

Test types:

• Insulation violation is checked by determining the value of its resistance using a device called a megohmmeter, by applying a voltage of 2.5 kV. If the insulation resistance turns out to be higher than 500 kOhm, then it is considered that this is sufficient for cable lines up to 1000 V. If the voltage is more than 1000 V, there is no standardization, but according to PTEEP (clause 6.1. And table 37) and PUE (clause 37 and table 1.8.34), the value should not be lower than 10 megohms. More details about how to use a megohmmeter, you can find out from our article.
• The presence of damage can be determined by performing a high voltage test. In this method, observe leakage currents, namely, their phase asymmetry and character. This method is more effective because it allows you to identify insulation damage that was not detected with a megohmmeter. The increased load produces a breakdown in problem areas. To carry out such a test, voltage is applied to one of the cores of the cable, and the remaining cores and the sheath are grounded.

The figure above shows: a - electrical circuit for checking insulation; b - shows a high-voltage installation for testing. In the diagram:

• 1 is a generator (source) of increased load;
• 2 - conductor checked for integrity.

Different types of insulation take a certain amount of time to establish a breakdown. So, for example, testing a cable line for an overvoltage of 2000-35000 V requires 5 or 10 minutes of time to apply a constant load for each core. If the tests are intended for a cable line rated for 110,000-500,000 V, voltage is applied to the cable for 15 minutes. During the test, the unbalance of the current distributed across the phases shall not exceed 50%.

If a cable is operated in parallel with another, it must be phased. This is achieved by applying an operating voltage to one end of the cable and measuring the voltage at the other end.

• High-voltage line with oil-filled insulation, which is usually used in the mains where it is transmitted load 110-500 kV, the oil or other liquid filling it is tested for compliance with the declared characteristics.
• The high voltage cable communication line is tested for corrosion protection:
  1. When the cable has a metal sheath, and the products are used for laying in the ground, its specific resistance does not exceed 20 Ohm / m.
  2. When the conductor has a metal sheath, and the products are used for laying in the ground, its specific resistance is less than 20 Ohm / m.
  3. When the shell is armored and it must be checked for damage, as well as the destruction of the protective covers.
  4. When the cable is intended in the high-pressure zone of steel pipelines, and the soil has a different degree of aggressiveness. The high-voltage cable communication line is subjected to measurements of the values ​​of potentials and currents wandering in the shell.
• The high voltage line of the cable communication is checked for the integrity of the conductive cores, as well as phasing by means of an ohmmeter. For this, one core is determined and relative to it they continue to carry out, in turn, measurements of the resistances of the closed circuits of all cores. A known undamaged conductor can be used as a reference conductor.

where: 1 - ohmmeter device; 2 - tested item.

• A high-voltage line designed for operation at an increased voltage of 20,000 V and more, it is necessary to set the resistance value of each individual core of the cable under test.
• Checking the distribution of current through the veins. The value of unevenness along the veins should not exceed more than 10%.
• The high voltage cable communication line (from 110,000 V to 500,000 V), with oil-filled insulation, is subjected to the determination of the content of insoluble gases. For such lines, their value should not exceed 0.1%.
• The cable line, where there is an overvoltage of 20 kV and above, is subjected to determination of the electric capacitance value. As a rule, in such cases, two methods are used: using a voltammeter, using a determination method using a bridge circuit.

1 - load source; 2 - tested item.

• A high voltage line (110,000 V to 500,000 V) with oil-filled insulation should be checked for not only insoluble gases but also soluble gases. For this, a chromatographic method for the determination of such substances is used.
• Also, resistance tests of grounding devices, couplings of end and cable terminations, metal structures that make up cable wells, as well as make-up points are performed.
• High-voltage cable communication lines (110,000 V), the shells of which are made of plastic, are tested for 1 min by applying an increased rectified voltage.

What else is important to know?

After carrying out the test work, the result is entered into the protocol, such as on the sample:

As for the timing of the tests, they are as follows:

Well, it is also important to say that devices such as IVK-5, AID-70 and AII-70 are most often used for work!

Finally, we recommend watching a useful video on the topic:

So we examined how the cable is tested with increased voltage. Now you know why you need to make checks and what methods exist today!

We also recommend reading:

• How to find a short circuit in the network
• Phase sequence in a three-phase network
• Grounding resistance measurement techniques