Diagnostic information in the context of electrical cable refers to the information gathered about the cable system. Usually, tests are performed to check the condition of the cable system. The results of those tests are referred to as diagnostics information. These tests are performed using various methods, and these methods generate different information. For instance, the Tan Delta method will produce Tan Delta diagnostics information.
Power grid systems utilize overhead and underground high voltage cables. These cables have to be tested regularly in order to check their condition. This is done to avoid any failures and complications in the future.
The reasons for performing diagnostics are multiple. Cable testing can be done for approval after the installation of the system. Diagnostics are also performed to check insulation deterioration, verification of splices and joints, and before repairs.
Types of Cable Tests
Diagnostics can be performed on cable systems at various points in their lifespan. Usually, there are three types of tests:
- Installation Diagnostic Test: These tests are conducted after the cable system is installed. This is a routine test done in order to check the condition of the cables during installation. These tests are performed to check whether there has been any damage to the cable during the handling or transportation.
- Acceptance Diagnostic Test: This test is done after the installation but before the voltage is passed through the system. The acceptance test is required for the cable system to be approved. It is to check whether there was any damage to the cables during the installation process. This test also checks for damages to accessories.
- Maintenance Diagnostic Test: Maintenance tests are done from time to time to ensure that the cable system is working properly and will not fail in the near future. Repairs and maintenance are based on the results of these tests.
Methods
There are various methods of testing cables. The efficiency of these methods is decided by two factors – whether the method damages the cables in any way or form and the number of problems and issues it can uncover. Below is a list of various diagnostic methods and some details about them.
1. VLF (Very Low Frequency) Dielectric Withstand Voltage
VLF testing is usually done for maintenance or for approval after installation. Thus, it can be classified as a diagnostic test. This method of testing involves the use of an AC hi-pot at a frequency range of 0.01 Hz to 1 Hz. The upside of this method is that, unlike DC voltage testing, there is little to no damage to the insulation. Thus, this method of testing does not lead to any permanent failure of the cable system. For most VLF tests, the frequency of 0.1 Hz is used. Although, there are exceptions based on the kind of cable system that is being tested. In technical terms, the VLF test is conducted as a go/no-go test. The procedure is quite simple – if the cable is able to withstand the voltage applied for a set period of time, it is considered to have passed the test. The voltage and the duration of the test become critically important here. Any miscalculation in the voltage or the duration period can compromise the whole test and may even damage the cable system. For example, if the voltage applied is very low or very high, there is a chance that the cable system may fail.
2. DAC (Damped Alternating Current) Voltage
The DAC voltage testing is suitable for a wide range of cables – medium voltage, high voltage, and even extra-high voltage. It is an alternative method of AC voltage testing. This testing method involves the production of damped alternating current voltages by charging the cables to a certain level of voltage that is decided beforehand. The capacitance of these cables is then discharged through a suitable inductance. This method is quite advanced as it utilizes the combination of voltage withstand, partial discharge, and dissipation factor. It is not a go or no-go test.
3. Dielectric Withstand Test
The Dielectric Withstand Test is the most basic test of them all. It is a simple electrical stress-based test done to make sure that the cable system will not fail as soon as it comes into service. Thus, it is usually performed during or before the installation. The insulation to be tested withstands an applied voltage that is higher than the service voltage across the insulation. The test lasts for a set duration of time. If the insulation does not break down for that duration, it is considered to have passed the test.