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.

Transformer Back-to-Back or Sumpner Test

Sumner's Test or Back-to-Back Test of a Transformer

A full-load test of the large transformer is needed to determine the maximum temperature rise of a transformer. This kind of Transformer test is called a back-to-back test. The back-to-back test is also known as Sumpner's test or regenerative test of transformers.

Sumner's Test Circuit Diagram of Transformer

The circuit diagram or the connection diagram to carry out the transformer Sumpner's test also known as the transformer back-to-back test is shown below. Find herewith the following connection diagram where two transformers are connected in a parallel way to supply the AC voltage V₁ and the low voltage supply V₂; 

EXCAVATIONS SAFETY IN THE WORKPLACE

How Excavations Carry on Safely in the Workplace:   

Excavation and trenching are among the most hazardous construction operations. The Safety and Health Code should be followed strictly during Excavation and Trenching in the workplace. Trenching and excavation work present serious hazards to all workers involved. Especially in trafficked, crowded city areas and moreover in the rainy season is more hazardous and much more likely than other excavation-related accidents to result in worker fatalities. Excavation in soil movement and cave-ins should take special care. Other potential hazards include falls, falling loads, hazardous atmospheres, and incidents involving mobile equipment.

What is the Benefits of Bundle conductor for EHV (Extra High Voltage) Transmission Lines?

Generally, 230kV or higher system overhead transmission line uses single-stranded conductors; above this voltage system multi-stranded like two, three, or four strands with bundle conductor spacers is required.

As per the nature of current flowing on conductive materials, maximum electrons tend to flow through the surface of the conductor rather than the inner. Considering this property hollow conductor is more suitable to optimize the current flow for higher capacity power transmission in an extra-high-voltage (EHV) system.

Theoretically, the hollow conductor may be viable, but practical conductors are not economical for maintenance and erection. So, the concept of a bundle conductor is the best solution to instate a hollow conductor.

Advantages of Using bundle conductors in EHV overhead transmission system

Sponsored:

A bundle conductor in a transmission line is used to get the following advantages:

1.    Bundle conductor of transmission line reduces the reactance of the electric transmission line;

2.    Bundle conductor in power system also reduces corona loss, which improves the transmission efficiency as loss due to corona effect;

3.    Bundle conductor in EHV transmission system provide the higher capacitance to neutral when compared to single lines. This results in higher charging currents which helps in improving the power factor;

4.    In bundled conductor lines, as the effective GMR (geometric mean radius)  of the bundled conductor increases the conductor diameter, virtually acts like a larger diameter hollow conductor, and  the inductance per phase of the conductor decreases;

5.    The bundle conductor spacers give optimum spacing (8 to 10 times the diameter of the conductor) between sub conductors in bundled conductors that gives a minimum voltage gradient on the surface of bundle conductors;

6.    As the advantage of bundled conductors in transmission lines by reducing voltage gradient, the radio interference is also reduced or minimized;

7.    With the decrease in inductance of bundled conductors, the surge impedance of the line is also reduced, L=(u0/2π)* ln(Mutual GMD)/(Self GMD)) H/m/phase;

8.    Due to bundling the conductor uses the surge impedance reduction, so more SIL-surge impedance loading (Z=(L/C)^1/2)) of the conductor increases which increases the transmission capacity of the system.

The remarkable phenomenon of the overhead transmission system is the “Corona effect” that can be observed when the air medium present between the phases is charged up and start to ionize and acts as a conducting medium.

Primarily bundle conductor concept is employed to reduce the corona loss and radio interference of transmission lines. Especially EHV at around 400 kV or higher corona losses are very high that can reduce using bundled conductors.

To know more details about Corona Effect and Radio interference, read-

Mysterious Corona Effects in Transmission Lines

How Power Cable Interfere with Instrumentation and Communication Cable

Bundle Conductor Formation

Sponsored:

Usually following three types of bundle conductor formation used in power transmission systems:

1.    Duplex formation;

2.    Triplex formation;&

3.    Quadruplex.

Bundle Conductor in Overhead Line

Usually Used Bundle Conductor Formation

The geometric mean radius of a bundled conductor for a two-conductor (duplex), a three-conductor (triplex), and a four-conductor (quadruplex) arrangement is shown in Figs.

In conclusion, we can say bundle conductors are used for transmission purpose as it helps in obtaining better voltage regulation and efficiency by reducing the inductance and skin effect that are present in the power lines.

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.

Sponsored:

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

Electric Actuator RA-300 User Manual and Specification

Specification and User Manual for Electric Actuator R-300

The RA-3000 series synchronous motor-driven reversible actuators are generally available for 3-point (floating) or with electric positioned for 0…10 V control.

You will get feature factory-calibrated pressure switches to provide specified close-off ratings.

The RA-3000 series actuators are available in three sizes 1600 N, 1800 N, and with 3000 N nominal force and can be used with JC-fanged valves according to the maximum close-off pressure ratings specified.