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Category: High voltage cable testing

Cables are very important electrical apparatus for transmission of electrical energy by underground means. They are also very important means for transmitting voltage signals at High Voltage Test on Cables. For power engineers, large power transmission cables are of importance, and hence testing of power cables only is considered here. Of the different electrical and other tests prescribed, the following are important to ensure that cables withstand the most severe conditions that are likely to arise in service.

Here only the electrical tests are described, i. For High Voltage Test on Cables and withstand tests, samples have to be carefully prepared and terminated; otherwise, excessive leakage or end flashovers may occur during testing.

The normal length of the cable sample used varies from about 50 cm to 10 m. The terminations are usually made by shielding the end conductor with stress shields or terminations to relieve the ends from excessive high electrical stresses. A few terminations are shown in Fig. During power factor tests, the cable ends are provided with shields so that the surface leakage current is avoided from the measuring circuits.

The dielectric power factor test is done using the high voltage Schering bridge see Section 9. The maximum value of the power factor and the difference in power factor between the rated voltage and 1. Sometimes, difficulty is felt in supplying the charging voltamperes of the cable from the available source.

high voltage cable testing

In such cases, a choke is used or a suitably rated transformer winding is used in series with the cable to form a resonant circuit. This improves the power factor and raises the test voltage between the cable core and the sheath to the required value, when a source of High Voltage Test on Cables and high capacity is used. The Schering bridge has to be given protection against overvoltages, in case breakdown occurs in the cables.

Cables are tested for withstand voltages using the power frequency a. As a routine test, the cable is tested applying an a. No damage to the cable insulation should occur.

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Type tests are done on cable samples using both high voltage d. The d. For impulse tests, impulse voltage of the prescribed magnitude as per specifications is applied, and the cable has to withstand five applications without any damage. Usually, after the impulse test, the power frequency dielectric power factor test is done to ensure that no failure occurred during the impulse test.

Partial discharge measurements and the discharge locations are important for cables, since the life of the insulation at a given voltage stress depends on the internal discharges. Also, the weakness of the insulation or faults can be detected with the help of these tests; the portion of the cable if weak may be removed, if necessary. The general arrangement for partial discharge tests is the same as described in Sec. The equivalent circuit of the cable for discharges is shown in Fig.

If the detector is connected through a coupling capacitor to one end of the cable as in Fig. Thus, the detected response is the combination of the above two transient pulses.

But, if the connections are made as in Fig. Now two transients will arrive at both the ends of the cable, and the superposition of the two pulses is detected. This can be obtained by adding the responses of the two transients. The superpositions of the two responses may give rise to a serious error in the measurement of the discharge magnitude. The magnitude of the possible error may be determined mainly by the shape of the response of the discharge detector.

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The voltage dip caused by a discharge at a fault or a void is propagated as a travelling wave along the cable. This wave is detected as a voltage pulse across the terminals of the cable ends. By measuring the time duration between the pulses, the distance at which the discharge is taking place from the cable end can be determined.

The shapes of the voltage pulses depend on the nature of the discharges. Typical waveshapes are given in Fig.Very low frequency VLF withstand testing is the application of an AC sinusoidal waveform, generally at 0. During the test cables are subjected to a test voltage significantly higher than what they experience during normal operating conditions.

The higher test voltage allows for weak points or pre-damaged areas within the cable to breakdown during the test, rather than while they are in service. From the above relationship, when comparing VLF 0. This has the added benefit that the size of the test instrument can be significantly reduced to allow for a very portable high voltage tester. Due to the sinusoidal waveform, VLF can also be used for tan delta diagnostics and partial discharge diagnostics.

What are the causes of cable failure during a VLF withstand test? When a cable is subjected to a considerably higher test voltage than what it typically sees in service, any defects in the cable will see higher stress levels that may grow within the insulation.

Electrical trees are channels of carbonization that arise from partial discharge activity within the insulation. Once an electrical tree grows big enough and bridges the electrodes of the cable system, a breakdown of the cable insulation is created. Water trees are tree-like structures that form from the electrochemical interaction of the electric field and water ingress within the cable. Their growth is extremely slow, but they act as stress enhancements, which can help to initiate an electrical tree.

Below is a photograph of a water tree growing into an electrical tree. What are the recommended test voltage levels and testing times? Below is an overview of the recommended voltage levels that should be applied during installation, acceptance, and maintenance testing of medium voltage distribution cables depending on the cable system rating phase to phase voltage. VLF testing times should last between 15 and 60 minutes, depending on the age of the circuit and what type of test is conducted.

For example, a minimum test time of 30 minutes is recommended for aged cable circuits. Extending the time to 60 minutes should be considered for particularly important circuits, such as feeder circuits. The times recommended for VLF withstand testing stem from studies conducted on tree growth rate on partial discharge defects in XLPE cable systems.

A typical 15 kV medium voltage cable in USA has an insulation thickness of 0.All tests made following cable installation and during the warranty period must be performed in accordance with the applicable specifications. Testing should be performed by qualified personnel taking all appropriate safety precautions. The responsible safety officer should be consulted regarding the equipment and the appropriate personnel protection requirements.

Humidity, condensation and actual precipitation on the surface of a cable termination can increase the leakage current by several orders of magnitude.

High voltage cable testing

Humidity also increases the corona currentwhich is included in the total leakage current. Wind prevents the accumulation of space charges at all bare energized terminals. This results in an increase of corona. It is desirable to reduce or eliminate corona current at the bare metal extremities of cable or terminations. This may be accomplished by covering these areas with plastic envelopes, plastic or glass containers, plastic wrap, or suitable electrical putty.

Direct current test equipment is commercially available with a wide range of voltages. Test equipment should be supplied from a stable, constant voltage power source.

high voltage cable testing

Do not use the same source that is supplying arc welders or other equipment that may cause line voltage fluctuations! The use of a portable, motor driven alternator to provide power to the test set is recommended. The DC test voltage may be applied either continuously or in predetermined steps up to the maximum value in accordance with the applicable specifications.

Some equipment will take longer to reach the maximum test voltage because of the amount of charging current. The test voltage is applied slowly in 5 to 7 increments of equal value up to the maximum specified.

Allow sufficient time at each step for the leakage current to stabilize.

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Maintain the test voltage at the prescribed value for the time specified in the applicable specifications. On exceptionally long cable lengths, it may be necessary the increase the grounding time. It is also advantageous to maintain these grounds longer and while reconnecting circuit components.

Acceptance testing is performed to detect any defects in cable insulation and terminations which may have resulted from poor workmanship or mechanical damage. This proof test confirms the integrity of the insulation and accessories before the cable is placed into service. After installation and before the cable is placed in regular service, the test voltages specified in Table c.

Record the leakage current at one minute intervals for the duration of the test.JavaScript seems to be disabled in your browser. You must have JavaScript enabled in your browser to utilize the functionality of this website. Inthe insulated conductor industry determined that dc withstand testing of the plastic XLPE insulation systems either in the cable factory as a routine production test or after installation as the higher voltage proof test was detrimental to the life of the insulation and therefore discontinued recommending dc testing.

Medium voltage EPR insulating systems are not subject to the same aging characteristics and, therefore, can be dc tested as required in accordance with the tables below.

When an insulated cable arrives on the job site, the recipient should be able to confidently assume it will attain the designed service life.

The time-honored methods of proof testing in the field involve high potential direct current dc. The advantage of the dc test is obvious. Since the dc potential does not produce harmful discharge as readily as the ac, it can be applied at higher levels without risk or injuring good insulation. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance. Field tests should be utilized to assure freedom of electrical weakness in the circuit caused by such things as mechanical damage, unexpected environmental factors, etc.

Field tests should not be used to seek out minute internal discontinuities in the dielectric or faulty shielding systems, all of which should have been eliminated at the factory, nor should the dc potential be excessive such that it would initiate punctures in otherwise good insulation. For low voltage power and control cables it is general practice to use a megger for checking the reliability of the circuit.

This consists essentially of measuring the insulation resistance of the circuit to determine whether or not it is high enough for satisfactory operation. For higher voltage. Even at the lower voltages, high voltage dc tests are finding increasing favor.

The use of high voltage dc has many advantages over other types of testing procedure. It is general practice, and obviously empirical, to relate the field test voltage upon installation by using a percentage of the factory applied dc voltage. This means that prior to being connected to other equipment, solid extruded dielectric insulated shielded cables rated 5kV and up may be given a field acceptance test of about volts per mil.

The actual test values recommended for the field acceptance test are presented in the Table below. If other equipment is connected it may limit the test voltage, and considerably lower levels more compatible with the complete system would be in order. Rated Voltage Phase to Phase. Note: If the leakage current quickly stabilizes, the duration may be reduced to 10 minutes.

The dc leakage can be affected by external factors such as heat, humidity, windage, and water level if unshielded and in ducts or conduits, and by internal heating if the cable under test had recently been heavily loaded.

These factors make comparisons of periodic data obtained under different test conditions very difficult. If other equipment is connected into the cable circuit this makes it even more difficult. In the event hot poured compound filled splices and terminations are involved, testing should not be performed until they have cooled to room temperature. The relays in high voltage dc test equipment are usually set to operate between 5 and 25 milliamperes leakage.

From the standpoint of safety as well as data interpretation, only qualified personnel should run these high voltage tests. After the voltage has been applied and the test level reached, the leakage current may be recorded at one minute intervals.

As long as the leakage current decreases or stays steady after it has leveled off, the cable is considered satisfactory. If the leakage current starts to increase, excluding momentary spurts due to supply-circuit disturbances, trouble may be developing and the test may be extended to see if the rising trend continues. The end point is, of course, the ultimate breakdown.

High voltage cable testing

This is manifested by an abrupt increase in the magnitude of the leakage current and a decrease in the test voltage. At the conclusion of each test, the discharge and grounding of the circuit likewise requires the attention of a qualified test engineer to prevent damage to the insulation and injury to personnel.

It may be justifiable in the case of important circuits to make periodic tests during the life of the installation to determine whether or not there had been significant deterioration due to severe and perhaps unforeseen operational or environmental conditions. Furthermore, a dc test failure is seldom burned-out, and visual analysis may disclose the cause and permit corrective action.

Further, there is the danger of mechanically injuring the dielectric during the seal removal and end preparation.

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In the case of power plants, it is customary to schedule desired maintenance proof tests to coincide with planned major shutdowns. It is not necessary or justifiable to check every circuit each year.This article provides an overview of some commonly used maintenance and diagnostic techniques that are commercially available for performing tests in the field on medium- and high-voltage power cables.

Photo: TestGuy. Field testing of medium- and high-voltage cables may performed for various reasons, such as acceptance after installation, charting the gradual deterioration of insulation over time, verification of splices and joints, and for special repairs. These sources may be used to perform insulation-withstand tests, baseline diagnostic tests such as partial discharge analysis, and power factor or dissipation factor.

Due to the various cable testing methods available, a test method selection should only be made after an evaluation of each test method and a thorough review of the installed cable system by a certified testing agency and the cable owner. When testing cables, personnel safety is most important. All cable and equipment tests should only be performed by qualified persons on isolated and de-energized systems, except where otherwise specifically required and authorized.

There are cases in which switches may be connected to a cable end and serve to isolate the cable from the rest of the system. Exercise extreme caution after de-energizing power cables as they are capable of holding large capacitive charges, use the correct PPE and electrical safety tools to properly discharge cables before and after testing. Field diagnostic tests can be performed on cable systems during various stages of their operating life. As defined by the IEEE standardcable tests are defined as:.


Which test method to use depends greatly on the age and type of cable system installed. Many of the methods described in this article can be performed as an acceptance or maintenance testdepending on conditions such as the test voltage applied or duration of test. The objective of any diagnostic test is to identify problems that may exist with a cable - in a non-destructive way - so that preventative measures can be taken to avoid potential failure of that cable when in service.

Diagnostic assessments may apply to cable systems comprising of the cable itself and the associated accessories such as splices and terminations. The dielectric withstand test is a basic electrical stress testconducted to ensure an insulation system will provide adequate service life. For the withstand test, the insulation under test must withstand a specified applied voltage that is higher than the service voltage across the insulation for a specified period without breakdown of the insulation.

The magnitude of the withstand voltage is usually far greater than that of the operating voltage and the amount of time applied is dependent on service age and other factors.

high voltage cable testing

Dielectric withstand testing is a relatively easy test to perform. If no evidence of distress or insulation failure is observed by the end of the test, the specimen is considered to have passed.

If the applied voltage results in sudden breakdown of the insulation material, however, a strong leakage current will flow and the insulation is determined to be unsuitable for service as it might pose a shock hazard. When conducting a DC hi-pot testvoltage is gradually raised to the specified value with a steady rate of rise that produces a consistent leakage current until the final test voltage is reached.

Anywhere from a minute to 90 seconds is generally considered sufficient for reaching the final test voltage. The final test voltage is then held for minutes and if leakage current is not high enough to trip the test set, the insulation is found to be acceptable.

This type of test is usually performed after cable installation and repair. The test voltage values for DC hi-pot tests are based upon final factory test voltage, which is determined by the type and thickness of insulation, the size of conductors, the construction of cable, and applicable industry standards. It's important to know that DC hi-pot testing does not provide a thorough analysis of cable conditionbut instead provides sufficient information as to whether the cable meets a specific high-voltage breakdown strength requirement.

In the past, DC dielectric withstand testing has been the most widely used test for acceptance and maintenance of cables. However, recent studies of cable failures indicate that the DC over potential test may be causing more damage to some cable insulation, such as cross-link polyethylene XLPEthan the benefit obtained from testing.

When maintenance testing existing in-service cables using a DC hi-pot, many factors should be considered in order to properly select the correct dielectric withstand test voltage. Note: If the cable cannot be disconnected from all the connected equipment, the test voltage should be reduced to the voltage level of the lowest rated equipment connected. Cables and accessories may also be withstand tested using power frequency voltage, although this is normally not done because it requires heavy, bulky, and expensive test equipment that may not be readily available in the field.

The AC test equipment used should have adequate volt-ampere VA capacity to supply the required cable charging current requirements of the cable under test. AC hi-pot tests can only be conducted as go-no-go test and therefore may cause extensive damageshould the cable under test fail.

If AC hi-pot acceptance and maintenance tests are to be conducted on cables, then it should be recognized that this test is not very practical. While it may not be very practical in the field, the AC hi-pot test has the distinct advantage of stressing cable insulation comparable to normal operating voltage.As with insulators, it is vitally important to perform appropriate high-voltage testing on cables to ensure they meet required safety standards, and also to monitor any deterioration over time so you can take appropriate action when required.

In a voltage test, a voltage is applied at any frequency between 25 and Hz. The voltage must be approximately sinusoidal in shape and is gradually increased to the full value.

Once the full value has been reached, the voltage is maintained for no less than 15 minutes. This voltage is maintained for the 15 minutes between conductors and also between the conductors and the sheath.

Each country has its own specifications for the test voltages, depending on the voltage designation, which can be found in the local specification. The high-voltage tests need to be conducted both before and after a bending test.

Bending tests are extremely important in the testing process—the cable needs to be bent around a cylinder that has a specific diameter for a complete turn.

Then, the cable is unwound and rewound in the opposite direction. The winding and unwinding need to be executed three times. As mentioned above, the voltages are specified according to local regulations. By way of example, if the designation is 11 kV, there are usually 4 other voltages provided in the specification:. High-voltage dielectric power factor tests are only conducted on 33 kV cables. The tests are conducted at room temperature at a single phase alternating current 50 Hz.

When testing, the critical element is the power factor. In no cases should this exceed 0. There are various other more complex calculations that must also be satisfied. Sample tests at manufacture site are conducted only on cables that will eventually be installed vertically.

Bending tests as explained aboveand also dripping and drainage tests are usually required. This is almost identical to the high-voltage acceptance tests detailed above, but the voltages are reduced. Pressurized cables often require a broad range of tests to ensure their safety and robustness. Type approval tests are very important in this regard. The type approval tests are conducted on both the minimum AND maximum conductor sizes for every single design and voltage rating.

The good thing is that, if the cable passes the type approval test, no additional tests are necessary unless the design changes.

For this test, you need to create a test loop that includes the cable and each accessory, which are then subjected to load cycles. This test is conducted only on kV cables, very similar to the loading cycle test, with the voltage at 1.

Unique to this test is the testing duration—6 hours minimum is the standard requirement. The test needs to be performed for this long to ensure that the cable is stable thermally. For impulse tests on cables, a test voltage at a ratio of 6 times the working voltage is applied for 10 positive and 10 negative pulses.Tables in In the absence of consensus standards dealing with insulation-resistance tests, the Standards Review Council suggests the above representative values.

Test results are dependent on the temperature of the insulating material and the humidity of the surrounding environment at the time of the test. Insulation-resistance test data may be used to establish a trending pattern. Deviations from the baseline information permit evaluation of the insulation.

high voltage cable testing

Is there any latest techniques to identify the faults in EHV cables in the shortest possible time. During High votage. What is the permissible leakage current value for a 11kv cable. If there any formula for calculating leakage current, cable rated voltage Any one please explain the Hi pot Pressure Test testing procedure for 3 core 70 sq.

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Need some advise on doing some end to end phasing test on 3km kv cable, the cable impedance is 34ohms, one end of line is overhead and it then goes underground and going overhead again, overhead cable one side is about 3km then undergrond of 3kms and overhead again for 30kmscan we use a generator to inject current into this cable.

Or The capacitive inrush will defeat the generator…. Our client has required us to perform tests to prove that metallic screen of 11kV cables are capable of sustaining the specified earth fault current.

Can you please elaborate this. Who will perform these test? How can these tests can be performed. It is my understanding that the metallic screen on 11kV cables is not there to carry fault current.

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The screens are to control the electrical stress produced under load. Whilst it is true that the screen must be connected to earth in order to function correctly, it is not designed to carry any significant load. Its only other purpose is to act as an electrical protection device guarding against punctures of the cable. This would produce a localised short circuit between conductor and the earthed screen, which should immediately operate the protection device.

Hi Syed, Stephen. Most 11KV screened cable is designed to carry fault current. Great article as always. Dear I tested a Mv cable 5kv first i applied insulation test on 5kv result was Gega ohm. Search for:.

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