VERY LOW FREQUENCY TEST FOR POWER CABLES
Very Low Frequency (VLF) High Voltage Tests for cables rated below 22 kV
High voltage equipment such as switchgears, transformers and cables need to be tested prior to be put into service. This is to determine that the whole system is built according to specifications and most importantly, that it is safe to be energised. High voltage tests are thus carried out in accordance with engineering standards and practices. In this section, we will discuss briefly high voltage tests for cables rated 22 kV and below.
DC High Voltage Tests on Cables rated 22 kV or less
Typically, cables have characteristics of a capacitor with low resistance. We know that electrically, capacitors present a "short circuit" to alternating current, i.e. AC passes through a capacitor like it is non-existent. If we were to test a cable using AC, we would require a test set with a very high capacity. The high capacity is required to charge and discharge the capacitor every cycle during the test. This would not be practical on site.
To overcome this, DC test was devised. We know that capacitors present an "open circuit" to direct current. So, if we were to apply DC to a cable, it would charge up the capacitor (cable) uni-directionally. The capacitor will be fully charged over a short time, typically 1 minute or so depending on the length of the cable. After which, the charges remain in the conductor and if the voltage is maintained, then we will be stressing the insulation of cable. In other words, we are doing a DC High Voltage Test on the cable.
Traditionally, in Singapore, we carry out High Voltage Tests for cables rated 22 kV and below using DC voltages. Typically, for a new 22 kV cable, the cable is tested at 50 kV DC for a duration of 15 minutes.
We also know that high voltage tests are destructive, more so for XLPE insulation if DC voltages were to be applied to it. Polyethylene is a good electrical insulation material (superior to PVC) but with poor mechanical properties due to its laminar chemical structure. To overcome this, scientists produce a chemical link "linking" the layers and thus strengthening and enhancing its mechanical property. This compound is called Cross-linked Polyethylene (XLPE).
High DC voltages tend to destroy the cross-links in XLPE. Over time, mechanical and electrical properties will deteriorate and fail. For this reason, we always carry out insulation resistance test before and after the DC high voltage test as a gauge of whether the insulation has deteriorated after being subjected to DC high voltage.
AC High Voltage Tests on Cables rated 22 kV or less
So, the natural thing to do is to test XLPE cables using AC. And how do we overcome the need to use a large capacity test set?
The reactance of cables (ignoring the small resistance of the cables) is inversely proportional to 2 pi f C. Current = Voltage divided by reactance, in this case, Current = Voltage x 2 pi f C.
If the frequency is 50 Hz, then the charging current will be = 100 pi C x V. And this has to charge/discharge the capacitor 50 times per second or every 20 ms.
If we reduce the frequency to 0.1 Hz, then the charging current will be = 0.2 pi C x V. This is 1/500 times the above. On top of that, the charging/discharging takes place 1 time every 10 seconds.
Eureka! Now we can test XLPE cables using AC voltages.
VLF Test Sets
We carry out VLF hi-pot tests for cables using sine wave test set. Tests are conducted complying with IEEE 400.2. IEEE 400.2 dictates that a cable is deemed to have complied with the requirements of IEEE 400.2 if it has withstood the prescribed voltage for the duration of the tests. Insulation resistance is carried out before subjecting the cable to high voltage. This is to ensure the cable is not shorted to earth before commencing the test. IEEE 400.2 does not prescribe insulation resistance test after the high voltage test.
This is a pass/fail test. If the test voltage is maintained throughout the duration of the test, the cable is considered to have passed the test.
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