ARGUMENTS FOR THE USE OF NEW TECHNOLOGY VLF (ULTRA LOW FERCUENCIA) TESTING OF HIGH VOLTAGE ARGUMENTS FOR THE USE OF NEW TECHNOLOGY VLF (ULTRA LOW FERCUENCIA) TESTING OF HIGH VOLTAGE
ARTICLE MADE BY HIGH VOLTAGE INC.
This document answers many questions about Hipots AC VLF y sus aplicaciones. No pretende argumentar las pruebas de cables AC VLF frente a DC. Esto se ha realizado durante muchos años, existiendo un acuerdo mundial casi unánime acerca de que las pruebas DC no solo dañaban el dieléctrico del cable sino que también es un sistema ineficaz para determinar la calidad del aislamiento del cable. Prueba suficiente es el hecho de que las normas europeas solicitan las pruebas VLF de cables desde hace años, y en USA, IEEE, EPRI, CEA y otras denuncian las pruebas DC desde hace tiempo, por lo que IEEE ha presentado una nueva norma para VLF que pronto será implementada.
On the other hand, Inc. also manufactures HV Hipots DC, like other manufacturers VLF. Not VLF manufacturers who are forcing the use of VLF, but that local and global utilitys are seeking an effective method for testing AC cables.
What is VLF?
VLF stands commonly used to refer to Very Low Frequency. VLF is generally considered to be 0.1 Hz or less.
too mysterious Many people consider testing when Hipot VLF VLF and VLF tests are very simple. It is simply a AC Hipot, working Very Low Frequency. Have been used for decades for testing AC Hipots various equipment. Now we can use for testing cables and rotating machinery.
VLF Where used?
VLF test is used in any application requiring AC tests on high-capacitance loads. The major application is to test the cable dielectric, followed by testing of large rotating machines (IEEE 433-1974), and occasionally for testing large insulators, switches and enclosures. The VLF Hipot is also a powerful tool for burning in cable failures.
Even if a utility does not take VLF cable testing extended, one of the best reasons to use VLF is to check the joints. Many failures are due to poor execution. The VLF Hipot tester is a large joints, as well as an excellent tool for testing wire insulation.
What tensions are available in Hipots VLF?
The VLF Hipots are available in voltage ranges from 27 kV to 240 kV. VLF HV Hipots Inc. manufactures from 40 kV to 240 kV, the highest rank in the industry and it is the only VLF company offering units for testing cables.
0.1 Hz
Why?
A 3,000-meter cable has approximately 1 mfd capacity. Capacitive reactance at 60 Hz is 2,560 Ohms; to 0.1 Hz is 1.6 Mohms. A test at 60 Hz to 53 kV AC require 20 A for a test of 1,060 kVA and a team weight of 4,500 kg the same cable at 0.1 Hz, requires only 33 mA, or 1.8 kVA and a team of 45 kg
0.1 Hz Is AC?
Yes, because the waveform is sinusoidal and the characteristics of the team to try not to vary, because the capacity does not vary with frequency.
How is the test?
The test is very simple. With the cable to test isolated, connect the output of the VLF high voltage protection for the driver and connected to the mainland. Like any Hipot, apply the test voltage for the required time.
What is the test voltage and duration?
agencies IEEE / EPRI / CEA and other world bodies engineering and standardization recommended test levels for dielectric cables of 2 to 3 times the voltage for 15 minutes. For a 15 kV cable, which usually has a line-ground voltage of 7.2 kV to 8 kV, the test is performed at 22 kV. A 25 kV system is tested at 40 kV and 69 kV system is tested at 120 kV.
VLF Among the different units with different waveforms, what is the best?
All units Inc. VLF HV output give a perfect sine wave. The original German design, which still has not given a sine wave output. These teams are waveforms with trapezoidal or square waveform transitions cosine between poles. They often try to prove it than the sine wave: an argument hard. There are many options for this argument: Do not make sine wave VLF. How can you argue with a sine wave?
addition, for a VLF unit can be used for diagnostic tests, both of Tangent Delta and Partial Discharge, should produce a sine wave. Working with a sine wave drive is essential.
VLF "- is a destructive test?
depends on your meaning. VLF Hipot test is not destructive to the cable and the DC test, where the real damage to insulation occurs during the test, causing future failures. The VLF test does not cause degradation of the insulation. Is AC voltage, which is for the cable that is designed for the service. In fact, the cable is tested at the factory with AC voltage to higher levels in field trials.
When people say that the VLF tests are destructive, are right only if the cable is faulty, the board or the joint under test should fail, which is exactly what should happen. How any engineer can doubt the effectiveness and efficiency of AC voltage testing of cables designed to carry AC voltage?
But my cables may fail during the test
This is precisely the intention of the VLF test. It is not a diagnostic test. Is an AC voltage test. No need to take readings of leakage current. (DC leakage currents say little about the quality of the cable). A cable or blood test holds or fails. If a cable is defective and can not hold 2 or 3 times the normal stress will not last long. Use testers VLF to avoid failure in service. Who
VLF supports?
Almost all engineering bodies in the world. EPRI, IEEE, CEA (Canadian Electrical Association), other engineering organizations in other countries, almost all manufacturers of cables, and many utilities around the world have accepted the effectiveness of the VLF tester. The German VLF testing standards (DIN-VDE 0276-620 and 0276-1001) there for 6 years, IEEE has written the standard VLF Cable Test (IEEE 400.2) will soon be implemented, and IEEE 433 for VLF testing of rotating machines already existed 20 years ago.
Why
VLF has not been used more widely?
VLF is not new. However, the reason for not being more widely used until 3 or 4 years, because only in the last 5-10 years has been determined to test the dielectric DC cable damage and is an ineffective means of determining the quality isolation. In addition, the original European VLF units today have been and are large, heavy, expensive and some do not produce the desired sinusoidal output.
was not until HV Inc. developed the first truly portable line, cheap and sinusoidal, producing units that were feasible and economic VLF field testing. Hundreds of VLF HV units Inc. have been delivered in the last 3 years since its introduction, with thousands of stakeholders.
What alternatives are there to VLF cable tester?
Not many. The DC test can not be used for a long time. Damaged wire insulation, and says little about its quality. DC tests analyzing problems, many utilities have reduced the traditional test for 4-5 times the normal voltage to well below. Many have reduced voltage test cables of 15 kV to 15 kV and 15 kV DC is a little less than the peak AC voltage cables in service. This test is useless, except perhaps for exposure to short circuits.
Some have tried to develop methods for detecting partial discharge testing of cable insulation in the field, to determine the quality of insulation without risk of failure during testing. None of the methods used to date have proven to be effective. DP testing must be finely substation equipment, but there are severe limitations for testing cables. Experience has shown DP test that is unreliable, with unrepeatable results due to many variables related to the cable tester, temperature, humidity, number and type of fixtures, fittings of various cables, and interpretation of the operator. The results have many interpretations. In addition, equipment and / or service is very expensive. Many utilities have spent hundreds of thousands of dollars hiring DP testing services, only to replace cables and joints in good condition while leaving faulty cables to fail soon.
tests are conducted Tan Delta (similar to power factor), where VLF unit is used to power the cable loss measured readings. This method is good, while there are fewer joints, gaskets, etc.., Along the length of cable and there is one type of cable under test, which does not normally occur, if not most of the extensions of the cable is formed by cables mixed with different loss factors. HV Inc. offers equipment VLF Tan Delta.
The safest way to check cables and accessories is just make a simple test with an AC Hipot, as we do with insulators, switches, disconnectors, etc. The cable under test should fail if you have a severe defect, but this is the intention of the test. If a cable can not withstand 2 or 3 times the normal voltage for 10 or 15 minutes, is likely to fail soon. It is better to fail when appropriate, to wait for one to fail, often at the worst possible time. Summary
A company faced with a choice of how to test the cables underground. Many do nothing and the cables fail, having to make emergency repairs, unhappy customers and lost revenue: it is a good practice. Many VLF testers have taken to find out bad cables and accessories with great results. Hundreds more will the coming years, especially once they release the new IEEE standard. Many cable failures are due to poor workmanship. The VLF has been proven to be a great tester cables. At least, each utility will test the main driver and each new installation or repair.
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