TESTING HIGH VOLTAGE CABLES WITH DAMPED AC (DAC)
BAKGRUNN
A power cable is considered to be a capacitive component. To charge cables with a constant AC voltage, such as AC resonant test, you need a great deal of energy. For the same reason, this is often a unsuitable method for testing cables in the field and is mainly used in factory testing.
There are mainly two different methods for testing cables in the field
Very Low Frequency (VLF) and Damped AC (DAC)
With VLF technology, the test frequency is lowered to 0.1-0.01 Hz. With DAC technology, the cable is charged to the desired test voltage with a unipolar DC voltage. When the voltage is reached, the inductance in the test equipment is short-circuited and a resonant network is formed consisting of the inductance in the test equipment and the capacitance of the cable itself. This results in a oscillating damping AC voltage with the same voltage characteristic as in alternating voltage. This is a major advantage in PD diagnosis.
Damped AC is a proven and recognized technology for high voltage testing with diagnosis according to IEEE, IEC, CENELEC and Cigrè. With a low noise llevel and a test frequency between 20-300 Hz, same conditions as normal 50 Hz is achieved. This also exposes small weaknesses that are real during normal operation.
High voltage cable diagnosis with Damped AC
DAC PD diagnose
PD diagnosis with DAC will reveal small weaknesses in cable, joints and end terminations that would otherwise pass an pressure test.
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PD measurement according to; IEC 60270, IEC 60885-3, IEEE 400.3
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All types of cables from 6.3 to 132 kV
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With unilateral measurement, 10-20 km of cable can be tested.
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For extra-long cables, two-sided measurement can be used.
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PD mapping (Localization <1 meter)
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PDIV (Start voltage)
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PDEV (Power Off)
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PRPD (PD pattern)
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PD level (Amplitude)
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PD measurements and results comparable to normal 50 Hz
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Reveals typical installation defects in joints and terminations such as;
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Poorly prepared pex surface such as knife cuts and impurities
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Poor semiconductor removal
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Incomplete shrinkage
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Misalignment of field distributors
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Air pockets
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Is cable insulation stressed by DC during a dac test?
NO
According to the basic elements of electric fields theory, a DAC test uses damped alternating voltages, and as such it stresses insulation with AC voltages.
damages dac testing the cable insulation?
NO
Healty cable insulation will not damage during a DAC test. A DAC test is always combined with PD monitoring and the test can be terminated before cable damage. If there are strong inhomogeneous defects in the insulation, this is detected by the sensitive PD measurement.
A positive DAC test result not only confirms that cable has passed the stress test, but that cable / joints / end terminations are PD free.
Can dac testing be used for
commissioning of new high voltage cables?
YES
Like AC resonant testing (20-300Hz) or 24 hour SOAK testing, DAC testing is one of the alternative methods of testing newly installed cables up to 350kV.
can dac testing be done on newly repaired or aged cable systems?
YES
Using monitored DAC testing for maintenance purposes is a good way to check the installation quality of the repaired cable by combining DAC stress testing with standardized PD detection.
On aged cable, the combination of PD detection and TanDelta diagnosis is a very powerful tool for condition assessment of PEX and oil-impregnated insulation.
Can DAC testing perform sensitive
standardized
PD detection in field?
YES
Unlike other mobile test systems with high noise levels. Due to low electromagnetic interference, the DAC systems are characterized by a very low PD background level, below 5pC.
PD measurement with DAC meets the requirements of IEC 60270 for standardized PD detection.
Is dac testing standardized?
YES
Damped AC is in the range of 20-300 Hz and is a sinusoidal AC voltage and is mentioned and recommended for field testing:
● IEC 60840: Power cables with extruded insulation and the accessories for rated voltages above 30kV up to 150kV Test methods and requirements;
● IEC 62067: Power cables with extruded insulation and the accessories for rated voltages above 150kV; Furthermore standards which are applicable for on-site testing with damped AC are:
● IEC 60060-3: High Voltage test techniques Part 3: Definitions and requirements for on-site testing;
● IEEE 400: Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems Rated 5 kV and Above;
● HD 632 S2 (CENELEC): Power cables with extruded insulation and their accessories for rated voltages above 36kV (Um=42kV) up to 150kV (Um= 170kV)
● IEEE 400.4: Guide for Field-Testing of Shielded Power Cable Systems Rated 5 kV and above with DAC