Thermal Analysis and Ampacity Calculation for Underground Cable Raceway

Thermal analysis and ampacity calculation for underground cable raceways are crucial for ensuring the safe and efficient operation of electrical systems. Here's a brief overview:

Thermal Analysis Underground Cable Raceway

The thermal analysis involves calculating the temperature rise in cables due to electrical current flow and the heat dissipation to the surrounding environment. Key methods include:

Why Need Power Cable Test After Installation

Power Cable Test After Installation

HV/EHV power cable testing in the field after installation is a common requirement for cable users. Meanwhile, MV-normal voltage, HV-high voltage, and EHV-extra high voltage power cables are carefully tested by the manufacturer before consignment with AC-alternating or DC-direct voltage. 

7 Step: How Find Cable Fault Location

How to Find Underground Electrical Energy Transmission Lines Cable Fault?

Finding the faulty point of an underground cable doesn't have to be like finding a lost item on the street. There are many faults locating methods depending on who finds the fault, and how he wants to find it -detection technologies are now much easier to find the cable fault, also important is the technique applied that makes the task much easier and less time-consuming. However, we can say that there is no single method or combination of methods that is universal. Skilled technique and proper tool selection is the major key point to finding the cable fault more easily and within a minimum time.

Types of Cable Faults

High Voltage DC Test is Killer for XLPE Insulated Power Cable

Fig- Formation of Electric Tree on XLPE Cable Insulation

High Voltage DC Test is Killer for XLPE Insulated Power Cable

The high-voltage DC (Direct Current) test carry out actually confirm the cable is capable to take the pressure. AC (Alternating Current) test instate of DC test can do if anyone interested. The problem is AC high voltage arrangement at the site is not physible to do. 

In operating conditions cables will operate the AC system, so AC high voltage test is no problem for it, rather AC is better than DC. Repeating the DC high voltage test may internal injure the cable that may cause cable failure in the future.

Why High Voltage DC Test is Danger for XLPE Insulated Power Cable?

The high voltage DC test is alternately referred to as the Hi-Po test. The high voltage DC (Direct Current)  testing method is performed for a long time on cable insulation systems, especially for field tests which are broadly known as SAT (Site Acceptance Test) for users and contractors. 

But recent research has shown that DC testing is the cause of certain types of defects and that it can make worse the deteriorated condition of some aged cables insulated with extruded dielectrics and affected with electrical-tree and water-tree. That’s why our article question “Is High Voltage DC Test Killer for XLPE Insulated Cable”.

LVDC or Low voltage DC testing is covering voltages up to 5 kV and HVDC high voltage dc testing covers voltage levels above 5 kV.

Type of DC Hi-Po Test:

DC Withstand Test: DC withstand test is a Hi-Po test where the voltage at a prescribed level is applied for a prescribed duration. The cable system will be considered acceptable if no breakdown occurs.

Leakage Current Tests: 

The leakage current test is a Hi-Po test with the function of time where the total apparent leakage output current is recorded to a prescribed voltage level. Not the absolute value, but the variations of leakage current with time is recorded which presents the diagnostic information of the cable system.

Voltage Test: 

The voltage test is also known as Leakage Current Tip-up Test where voltage is increased in small steps while the steady-state leakage current is recorded until the maximum test voltage is reached.  The relationship graph between current and voltage denotes whether the insulation system is defective or not.

Weak Points of DC Hi-Po test for Power Cable:

• This test is blind to certain types of defects, such as clean voids and cuts. 
• May not replicate the stress distribution existing with power frequency ac voltage. The stress distribution is sensitive to temperature and temperature distribution.
• May cause undesirable space charge accumulation, especially at accessory cable insulation interfaces.
• May adversely affect the future performance of water-tree-affected extruded dielectric cables.

Though the DC Hi-Po test is simple and easy recently experts are thinking DC high voltage test keeps some signs that bring bad effects for cable at age. Consequently, we can say the above-mentioned disadvantages of the DC Hi-Po test turn themselves into a killer for XLPE insulated power cable.

Type Test on Complete Underground Power Cables

Here in this article, we would like to present a sample type test report on complete underground power cable:

Bending test in accordance with IEC 60840, clause 12.3.3

A cable sample of approximately 40 m was bent six times around a test cylinder with a maximum diameter of (25 x (d+D) +5%) at ambient temperature.

The results are stated in annex A. For a picture of the bending test, a reference is made to Annex G.

 Result: The test was completed successfully.

Partial discharge test at ambient temperature in accordance with IEC 60840, clause 12.3.4.

After the bending test and short duration test, the cable samples were examined for partial discharges in accordance with IEC 60885-3 at ambient temperature. The sensitivity of the measuring circuit was checked with a calibrator, and the noise level was < 2 PC. The voltage was first raised to and held to 133 kV (1, 75 Uo) for 10 s and then lowered to 114 kV (1, 5 Uo). At this voltage, the partial discharges were measured. For the results, reference is made to Annex A.

Result: No visual internal partial discharges were established.

Measurement of the dielectric loss angle in accordance with IEC 60840, clause 12.3.5

After the partial discharge measurement as mentioned under 2.1, the dielectric losses of the test installation were measured at Uo and a conductor temperature of at least 95 °C.  The results are stated in annex A. 

 Result: The test results met the requirements.

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Heating cycle voltage test in accordance with IEC 60840, clause 12.3.6

The test installation was subjected to a power frequency test of 152 kV (2 Uo), 60 Hz for at least 480 h. During this test, the test set-up was subjected to at least 20 heating cycles. Each heating cycle consisted of 8 h heating and 16 h of natural cooling. During the last 2 h of each heating period, the conductor reached a temperature of at least  95 C (5 C above the maximum rated temperature). For the data of the test, a reference is made to Annex A.
Result: The test installation passed the test successfully. 

Partial discharge test in the hot condition in accordance with IEC 60840, clause 12.3.4

After the heating cycle voltage test, the test the set-up was examined for partial discharges in accordance with IEC 60885-3 at a conductor temperature of at least 95 °C The test was carried out as described in 2.1. The results are stated in annex A.
 Result: The test results met the requirements.

Short duration power frequency withstand voltage test in accordance with Tranco specification

After the above-mentioned tests the test installation was subjected to a power frequency test of 275 kV (3,6 Uo), 60 Hz for at least 1 minute. The test was carried out while the test installation was at ambient temperature.
Result: No breakdown occurred.

Impulse withstand voltage test in accordance with IEC 60840,  clause 12.3.7

After the above-mentioned tests, the test set-up was tested with an impulse voltage while the conductor was heated to a temperature of at least 95 C (5 C above the maximum rated temperature). The test was carried out in accordance with IEC 60230 and IEC 60060. The sample was tested with ten positive and ten negative voltage impulses of 650 kV. The oscillograms of these tests are stated in annex B.
Result: No breakdown occurred. 

Power frequency voltage test for 15 minutes in accordance with  IEC 60840, clause 12.3.7

Upon completion of the impulse withstand voltage test, a voltage test was executed with an alternating voltage of 50 Hz. The test installation was tested with a voltage of at least 190 kV  (2,5 Uo) for 15 minutes in accordance with the specification. The test was carried out while the test installation was at ambient temperature.
  Result: No breakdown occurred.

Examination of the cable and the accessories after the tests in accordance with IEC 60840, clause 12.3.8

After completion of the electrical tests as mentioned above the cable and the accessories were dismantled and inspected for electrical degradation of the insulation. For pictures of the dismantling, a reference is made to Annex I.

 Result: No signs of electrical degradation were detected. The construction of the accessories complied with the construction drawing

Tests on semi-conducting layers

Electrical resistivity before and after aging in accordance with IEC 60840, clause 12.3.9

The resistivity of the extruded semi-conducting layers was measured and calculated according to annex B of the IEC 60840. The aging treatments were carried out in accordance with IEC 60811-1-2, clause 8 at a temperature of 100 C for 7 x 24 h. The measurement was carried out at a temperature of 90 C. The test results are stated in annex F.

Result: The test results met the requirements as stated in the specification

Why Choice STL Member Type Test For Underground Power Cable

 

Each product needs verification before going to use in commercial operation, underground power cable also not out of that; not only underground cables but also all major equipment for the power transmission network. User needs to confirm carrying out the proper taste that the underground cable is safely ready to use, but the user no has test facility also can not depend on manufacturer own test. So, they need some third party test that can be reliable. The question is "who is the third party tester?" May be STL members organisation.

What is STL?

STL is shortened of Short-circuit Testing Liaison, means STL is an organization who provides voluntary collaboration internationally to meet HV/MV underground power cable testing services.

The objective of STL to harmonize and uniform the testing presentation, testing result and test data under a standard guideline.

Most of the cable users worldwide, their first choice for type test from STL members to find the reliable and good quality cable from a large number of the cable manufacturer.

What Type Test Certificates Are Required?

Basically following six type test certificates are required-

1.    Complete Type Test Certificate

2.    Dielectric Performance Test certificate

3.    Temperature Rise Performance Test Certificate

4.    Short-circuit Test Certificate

5.    Switching Performance Test Certificate

6.    Internal Arc Performance Test Certificate

Who Is the STL Member Organization?

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World recognized testing organizations like Intertek (ASTA)-UK, CESI-Italy, CPRI-India, ESEF ASEFA-France, JSTC-Japan, KEMA-The Netherlands, KERI-South Korea, PEHLA-Germany, SATS-Norway, STLNA-USA, VEIKI-Hungary and the ZKU-Czech Republic are the member of STL; well-known testing services organizations and member STL are shown in the above figure. If you interested to know more about STL may visit the website www.stl-liaison.org.

TEST OF THE OUTER PROTECTION OF THE BURRIED JOINT IN ACCORDANCE WITH IEC 60840

Fig-Typical Medium Voltage Cable Joint

Here in this article we will find a case study of type test report for direct buried type cable joint. Hope these text will help us to understand what kind of test actually required for underground power cable direct buried joints. In another article “TYPE TESTS ON COMPLETE UNDERGROUND POWER CABLE” we discussed detail a case study to make sense on type test of an XLPE cupper core underground power cable.

Dry heating cycle test in accordance with IEC 60840

On the joint three heating cycles without voltage were performed. Each heating cycle consisted of 8 h heating and 16 h of natural cooling. During the last 2 h of each heating period the conductor reached a temperature of at least  95 C (5 C above the maximum rated temperature).

Result: The test was performed in a correct way.

Water immersion and heat cycling in accordance with IEC 60840

After the three dry heating cycles, mentioned under 3.1 the joint was immersed in water, at a depth of at least 1 meter at the highest point of the outer protection. Then the joint was subjected to 20 heating cycles. In each heating cycle the water around the joint was heated to a temperature between 70 °C and 75 °C (between 15 °C and 20 °C below the maximum rated temperature of the cable conductor) and kept on this temperature for at least five hours. Hereafter the water was cooled down to a temperature less than the ambient temperature +10 °C.

Result: The water immersion and the heat cycling did not give reason for remarks.

DC voltage test on insulation joint in accordance with IEC 60840

After the water immersion and heat cycling, a voltage of 20 kV DC was applied for 1 minute between the metallic screen and the earthed exterior of the joint outer protection. Hereafter a voltage of 20 kV, DC was applied for 1 minute between the metallic screens on both sides of the joint. The test was carried out while the test installation was at ambient temperature.

Result: No breakdown occurred.

Impulse withstand voltage test on insulation between the screens and water in accordance with IEC 60840

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After the above mentioned tests the test set-up was tested with an impulse voltage of 37.5 kV. The test voltage was applied between the metallic screen and the earthed exterior of the joint outer protection. The test was carried out while the test installation was at ambient temperature.  The test was carried out in accordance with IEC 60230. The joint was tested with ten positive and ten negative voltage impulses.

Result: No breakdown occurred.

Impulse withstand voltage test on sheath sectionalizing insulation in accordance with IEC 60840

After the above mentioned tests the joint was taken out the water and the sheath sectionalizing insulation was tested with an impulse voltage of 75 kV. The test voltage was applied between the metallic screens on both sides of the joint. The test was carried out while the test installation was at ambient temperature.  The test was carried out in accordance with IEC 60230. The joint was tested with ten positive and ten negative voltage impulses.

Result: No breakdown occurred

Examination of the joint in accordance with IEC 60840

The joint was dismantled and inspected for water penetration and cracks.

Result: No signs of cracks or water penetration were detected. The construction of the joint complied with the construction drawing.

Hope this article on TEST OF THE OUTER PROTECTION OF THE BURRIED JOINT IN ACCORDANCE WITH IEC 60840 is helpful for you and your friends. If you think it’s need some add, put your comments below- comment space.

QUALITY ASSURANCE INSPECTION AND TESTING FOR POWER TRANSMISSION LINE CABLE PROJECT

The Inspection and Testing Procedure to Ensure Quality

The underground power cable transmission line, whole Plant supplied and works constructed under the contract should be subject to inspection and testing by the Engineer, by the Employer or, where appropriate by an approved inspection authority should they so require, during manufacture the equipment, erection the plant and after completion the installation works.

The inspection and testing should include but not be limited by the requirements of this article of the specification and of the detailed inspection. Prior to inspection and testing the plant and equipment should undergo pre-service cleaning and protection as specified in the section of the specification.

Following Inspection and Test should be carryout in the underground power cable transmission line project:

·     FAT or Factory Acceptance Test, before shipment the plant or equipment this test must be completed;

·  PLI or Post Landing Inspection, after arrived the plant or equipment this inspection should carry out;

·  SAT or Site Acceptance Test, during installation or erection the plant or equipment this test should carry out to satisfy the employer;

· Pre-commission, before final commissioning the plant satisfactory pre-commissioning should carry out;

·       Commission, final commissioning test must be carry out before energize the plant.

What Standard Should Follow to Ensure Quality

Where no test is specified then the various items of plant, materials and equipment should be tested in accordance with the relevant British, American or National Standards acceptable to the Engineer. Where no appropriate standard is available, tests should be carried out in accordance with the manufacturer’s standard practice which must meet with the prior approval of the Engineer. 

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Notification for Inspection and Testing

Fourteen days notice of the readiness of plant for test or inspection and every facility should be provided by the service provider and sub-service provider to enable the Engineer to carry out the inspection and witness the test.

Equipment should be packed, prepared for shipment, or dismantled for the purpose of packing for shipment only when it has been inspected and approved (or inspection has been waived) and written instructions have been received from the Engineer.

Within 30 days of acceptance of the agreement the service provider should submit a quality assurance program and a work quality program for the Engineer’s approval.

Responsibility for Quality Assurance

The Engineer should have the right to supervise and witness tests of all materials to be used and all workmanship employed in connection with manufacture.

Each part of the work should be commenced after prior approval from the Engineer. This should not relieve the service provider from any liability or obligation under the agreement and he should be responsible for the acts, defaults and neglects of any sub-supplier, his agents; employees or workmen as if they were the acts, defaults, or neglects of the service provider.

Quality Assurance Plan

For control of the quality of manufacture, material control and documentation, the service provider and each main sub-service provider should submit a copy of his manufacturing program and three unparsed copies of all guidelines and sub-guidelines from the Engineer. 

These documents will be examined and classified according to the extent of expediting, quality audit or inspection required. The approved manufacturing program should be integrated with the Engineer’s Quality Department work program. 

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Manufacturing Capacity

The Service provider should furnish evidence that he and his sub-service providers have supplied other equipment of a similar type and size that should have been in commercial operation successfully for a minimum of four years.

Engineer require reviewing and approving any manufacturing capacity; the Service provider should secure permission for such reviews to be made by the Engineer.

To ensure the inspection and testing for power transmission underground power cable project, each and every test and inspection standard and safety measure must comply with international and local or national standard.

Have a quality job in your project, comments and share more idea to others.

Cable Type Test -IEC 60840 International Standard

Why Cable Type Test is Required:

Cable type test is essential for both cable manufacturer and cable usurer; manufacturer being satisfied type testing that followed all necessary standards and quality plan and will ensure customer satisfaction; usurer also being convinced that the product they are purchasing is manufactured as per required standard. 

Ultimately cable type test makes manufacturer and customer confident and satisfied that the cable will work satisfactorily after switch on to energize the line for a long life time without trouble.

Please note that, in this post cable type test means type test only for cable; don’t confused with type test for cable systems, cables and cable accessories; also for references this post is written on the basis international standard IEC 60840 “power cables with extruded insulation and their accessories for rated voltages above 30 kV (U_m=36kV) up to 150 kV (U_m=170 kV)- test methods and requirements” and part of this contains copyright rules and ownership goes to as per original source documents. If you interested for more details, please visit main site.

Summary of cable type test as per IEC standard:

Cable type test is typically carry out by specified laboratory. To understand the cable type test first of all need to know to complete the type test actually which tests should be carry out. The following summary list will help to understand the scope of cable type test requirements before carry out the test.

1.      Range of type approval;
2. .    Electrical type tests;
3.      Test voltage values;
4.      Bending test;
5.      Partial discharge test at ambient temperature;
6.      Tan §measurement;
7.      Heating cycle voltage test;
8.      Lightning impulse voltage test followed by power    frequency voltage test;
9.      Examination;
10.      Resistivity of semi-conducting screens;
11.      Non-electrical type tests on cable components and on completed cable.

Range of type approval: 

Rated voltage range, cross-sectional area range, constructional procedure, variation of calculated electrical stress at conductor screen and insulation screen range condition should be met.

Electrical type tests:

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To pass the electrical type tests on a completed cable first of all need to choose a piece of cable at least 10 meter length. Some series of test as per IEC standard requirements need to performed on that piece of cable, such as- bending test; tan lamda measurement test; heating cycle voltage test; lightning impulse voltage test; partial discharge test at ambient temperature; examination of the cable on completion of the these tests.

Test voltage values:

As per IEC standard 60840 test voltage values are shown in the table, but important considering point is that ensure the insulation thickness is correct as per IEC standard 60811-1-1 of the piece of cable which used for test. If insulation thickness is not same as requirement, no problem, just need to adjust the test voltage value as per insulation thickness.

Table- Test Voltage Values

Bending test:

As per IEC standard the bending test procedure is as below-

The cable sample shall be bent around a test cylinder (for example, the hub of a drum) at ambient temperature for at least one complete turn and unwound, without axial rotation. The sample shall then be rotated through 180° and the process repeated.

This cycle of operations shall be carried out three times in total.

The diameter of the test cylinder shall not be greater than:

– -for cables with plain aluminum sheaths:

– -36 (d + D) + 5 % for single-core cables;

– -25 (d + D) + 5 % for three-core cables;

-for cables with lead, lead-alloy, corrugated metallic sheaths or with longitudinally applied metal foils (overlapped or welded) bonded to the over sheath:

– -25 (d + D) + 5 % for single-core cables;

– -20 (d + D) + 5 % for three-core cables;

– -for other cables:

–- 20 (d + D) + 5 % for single-core cables;

– -15 (d + D) + 5 % for three-core cables.

Where

d is the nominal diameter of the conductor, in millimeters;

D is the nominal overall diameter of the cable, in millimeters.

NOTE:  A negative tolerance is not specified, but testing at diameters below the specified values should only be done by agreement with the manufacturer.

Partial discharge test at ambient temperature:

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The partial discharge test performed as per requirements of IEC standard 60885-3, in the table test voltage values the test voltage range is mentioned in column 5 as ‘partial discharge test’; the test voltage shall be raised gradually to and held at 1.75 U0 for 10 second and then slowly reduced to 1.5 U0.

Tan lamda measurement:

The procedure of tan lamda measurement is mentioned in IEC 60840 is quoted in below- “The sample shall be heated by a suitable method and the temperature of the conductor determined either by measuring its resistance or by thermocouples on the surface of the screen/sheath, or by thermocouples on the conductor of another sample of the same cable heated by the same means. The sample shall be heated until the conductor reaches a temperature which shall be 5°C to 10 °C above the maximum conductor temperature in normal operation. The tan shall then be measured at a power frequency voltage of U0 at the temperature specified above”.

Heating cycle voltage test:

Heating cycle test voltage is mentioned in table- test voltages values are 2 times. Cable assembly as bending test/ U-bend, cable heated 5 to 10 degree above the conductor maximum temperature. Cable heated for 8 hours and stay for 2 hours then natural cooling for 16 hours. This process carried out 20 times.

Lightning impulse voltage test:

The impulse voltage shall be applied according to the procedure given in IEC 60230 on cable assembly heated 5 to 10 degree above the conductor maximum temperature. This test followed by a power frequency voltage test at 2.5 times of test voltage U0 for 15 minutes as shown column 4 in test voltage values table.

Examination:

Physical check and examination is carried out during test, if there any degradation visible in cable.

Resistivity of semi-conducting screens:

This measurement of resistivity of semi-conducting test is carried out on separate piece of cable.

Non-electrical type tests:

The non-electrical type test may perform by checking and testing as follow:

-Cable construction;

- Mechanical properties of insulation;

- Mechanical properties of over sheaths;

- Compatibility of materials;

- Mass test on PVC over sheaths;

- Pressure test at high temperature on over sheaths;

- PVC over sheaths low temperature test;

- Heat shock test for PVC over sheaths;

- Ozone resistance test for EPR and HEPR insulations;

- Hot set test for EPR, HEPR and XLPE insulations;

- Density of HDPE insulation;

- Carbon black content of black PE over sheaths;

- Shrinkage test for PE, HDPE and XLPE insulations;

- Shrinkage test for PE over sheaths;

- Hardness of HEPR insulation;

- Elastic modulus of HEPR insulation;

- Fire conditions test;

- Water penetration test, etc.

After completed the type test of sample for a series of cable (considering the cross sectional area of cable, voltage rating of cable, construction procedure of cable etc.) it is considered that the other cables within the same series are valid and certified that no need separate test for individual lot.