1. In a typical single-phase medium voltage cable, what material is commonly used for the main insulation layer?
XLPE (Cross-linked Polyethylene) or EPR (Ethylene Propylene Rubber).
2. What is the purpose of the inner semiconductor layer in a medium voltage cable?
To smooth out the electric field at the conductor interface and prevent electrical stress concentrations.
3. Term: PILC
Definition: Paper Insulated Lead Covered cable, a type of laminated dielectric insulation.
4. What defines a 'local problem' in a medium voltage cable system?
A defect constrained to a specific segment or accessory, such as a termination or splice.
5. What defines a 'global problem' in a medium voltage cable system?
An aging or degradation issue that is spread along the entire length of the cable insulation.
6. Which diagnostic test is specifically used to detect incipient installation or manufacturing problems through discharge activity?
Partial Discharge (PD) testing.
7. Which test is used to identify global aging issues, such as water trees, in XLPE cables?
Tan Delta testing.
8. What is the result of moisture ingress and electrical stress in polymeric insulation over long periods?
The generation and growth of water trees.
9. At what point does a water tree become an immediate threat to cable integrity?
When it converts into an electrical tree, which involves progressive carbonization of the insulation.
10. How long does the water tree growth process typically take before reaching a critical size?
At least five years.
11. What is the primary aging phenomenon associated with paper-insulated cables?
Cellulose degradation caused by thermal and chemical processes.
12. What are the two main goals of preventive maintenance like VLF testing?
To increase system reliability and reduce outage minutes.
13. What does 'VLF' stand for in the context of cable testing?
Very Low Frequency.
14. What is the standard frequency used for a VLF over-voltage test?
$0.1\text{ Hz}$.
15. VLF testing is essentially a 'pass/fail' test also known as an AC _____ test.
Hipot (or withstand).
16. According to IEEE 400.2, what is the typical voltage range for a VLF test?
Between $2U_0$ and $3U_0$, where $U_0$ is the rated rms phase-to-ground voltage.
17. What is the standard duration for a VLF withstand test according to industry guidelines?
Between $30$ and $60$ minutes.
18. What are the main primary wave shapes used in VLF testing technology?
Sinusoidal (Sine) (CR).
19. What is a major advantage of the VLF Sine wave shape over the Cosinusoidal-Rectangular wave shape?
It allows for the performance of Tan Delta testing.
20. Why is the BAUR is the best in the VLF cable testing?
Baur has been working in the cable testing and diagnositic for more then 30 years.
21. If the VLF Sine frequency is reduced from $0.1\text{ Hz}$ to $0.01\text{ Hz}$, how must the test time be adjusted?
The test time must be increased tenfold to maintain the same electrical tree growth rate.
22. Which VLF wave shape allows for the measurement of leakage current during the 'plateau' of the wave?
Cosinusoidal-Rectangular.
23. What standard specifically covers VLF testing of shielded power cable systems?
IEEE 400.2.
24. Why is VLF testing considered safer for the cable than service failures?
It occurs at a reduced energy level, which limits collateral damage compared to a high-energy service fault.
25. Tan Delta testing is a global diagnostic method used to determine the _____ of cable insulation.
Remaining life expectancy.
26. The Tan Delta value represents the ratio between which two current components in a cable?
Resistive current and capacitive current.
27. In a perfect insulation system, what is the theoretical phase angle between voltage and current?
$90$ degrees.
28. As cable insulation ages and degrades, does the Tan Delta value typically increase or decrease?
Increase (due to rising resistive current).
29. What specific waveform is required to perform a Tan Delta measurement?
Sinusoidal wave shape.
30. What are the three specific voltage steps used during a standard Tan Delta test?
$0.5U_0$, $1.0U_0$, and $1.5U_0$.
31. In Tan Delta testing, what does the 'Mean Tan Delta' parameter represent?
The average of the individual Tan Delta values measured at a specific voltage step.
32. How is 'Delta Tan Delta' (also known as Tip-up) calculated?
The Mean Tan Delta at $1.5U_0$ minus the Mean Tan Delta at $0.5U_0$.
33. What does the 'Tan Delta Stability' parameter measure?
The standard deviation of Tan Delta measurements at a constant voltage step.
34. According to IEEE 400.2, what are the three condition assessment categories for Tan Delta results?
No Action Required, Further Study Advised, and Action Required.
35. What is the recommended action if a Tan Delta test result is 'No Action Required'?
Return the cable to service and retest at a later date, typically in four years.
36. Why should a decision to replace a cable never be based on a single Tan Delta measurement?
Trending over time and comparison with historical data are necessary to accurately assess degradation.
37. How do adverse weather conditions, such as rain, typically affect Tan Delta measurements?
They tend to increase the measured Tan Delta values due to surface leakage and moisture.
38. What is the typical maximum cable length limit for a standard Tan Delta test?
Approximately $3\text{ km}$.
39. Why is it difficult to interpret Tan Delta results for 'mixed' cable circuits (e.g., XLPE joined to PILC)?
The different insulation types have different base loss characteristics, which masks specific defects.
40. What is the first connection made when setting up a VLF test unit for safety?
The safety ground.
41. In the Megger VLF Sine 45 simulation, what must the operator press to actually initiate high voltage?
The high voltage 'ON' button (green button).
42. Which standard covers the use of Damped AC (DAC) wave shapes for partial discharge testing?
IEEE 400.4.
43. What is a 'monitored withstand' test?
A VLF withstand test performed while simultaneously monitoring Tan Delta or Partial Discharge as diagnostic indicators.
44. How does EPR insulation differ mechanically from XLPE?
EPR is more flexible, making it easier to handle and use for test leads.
45. Between XLPE and EPR, which is generally considered the superior insulation for high voltage (above $110\text{ kV}$) due to lower losses?
XLPE.
46. What is the typical insulation resistance value for a healthy solid dielectric medium voltage cable?
In excess of $100\text{ M}\Omega$, often reaching into Giga-ohms or Tera-ohms.
47. What is the 'after-laying' or 'decommissioning' test voltage commonly used in the IEC world for new cables?
$3U_0$ for $60$ minutes.
48. In a Tan Delta graph, what does a 'flat' line as voltage increases indicate for XLPE insulation?
The insulation is in good condition with little to no voltage-dependent losses.
49. If a Tan Delta test shows high values at the start that decrease as the test continues, what might be occurring?
Moisture in the cable may be evaporating due to the test current.
50. What is the purpose of a 'cable replacement program' based on Tan Delta results?
To prioritize the replacement of the most aged cables rather than just the oldest ones.
51. The trigonometric function used to correlate resistive current to capacitive current is called _____.
Tan Delta.
52. Which VLF waveform provides a change of polarity that most closely matches power frequency ($60\text{ Hz}$)?
Cosinusoidal-Rectangular.
53. Why is DC testing no longer recommended for aged XLPE cables?
It can cause space charge accumulation that leads to premature failure after the cable is returned to service.
54. What component of a cable serves as the return path for fault current and provides a neutral connection?
The metallic shield or concentric neutral.
55. What is the primary drawback of using the VLF Sine wave at $0.1\text{ Hz}$ compared to Cosinusoidal-Rectangular?
The units are often unable to test very long cables due to lower capacitance limits.
56. Concept: Electrical Tree
Definition: A permanent, carbonized discharge path within the insulation that eventually leads to a complete breakdown.
57. True or False: VLF testing alone provides comprehensive information about the remaining life of the cable insulation.
False (it is a pass/fail test; Tan Delta provides condition assessment).
58. What parameter is measured by Tan Delta to indicate the presence of moisture or contamination in a cable joint?
An increase in resistive current (loss factor).
59. How many individual measurements are typically taken at each voltage step during a Tan Delta test?
$8$ to $10$ measurements.
60. In a Tan Delta test, if the standard deviation is high at a specific voltage, what does it suggest about the measurement?
The Tan Delta values are unstable or dispersed, possibly indicating active discharge or varying conditions.
61. Which standard is used for testing paper-insulated cables (PILC) using VLF?
IEEE 400.1.
62. What is the 'bathtub curve' in the context of cable life expectancy?
A model showing high failure rates at the beginning (teething) and end (aging) of a cable's life.
63. Why is the safety ground connected to the metallic shield during a VLF test?
To provide a safe return path for the test current and ensure the metallic components remain at ground potential.
64. What feature on a VLF unit prevents accidental activation of high voltage by unauthorized personnel?
The high voltage interlock key.
65. In Tan Delta diagnostics, what does a 'negative tip-up' (lower value at higher voltage) often indicate?
The presence of liquid water inside a joint or accessory.
66. Which VLF technology typically results in lighter and more portable test units?
Sinusoidal (Sine).
67. What is the effect of excessive torque on a cable holder regarding the insulation?
It can cause mechanical damage and deformation, leading to a local weak spot.
68. In a 3-core belted cable, how many shields are typically present?
One common shield surrounding all three conductors.
69. What does a significant 'Action Required' assessment in Tan Delta testing imply for the operator?
The cable is at high risk of failure and should be repaired or replaced immediately.
70. If performing a monitored withstand test, what does a rising Tan Delta trend during the withstand period suggest?
The cable is actively degrading or approaching failure under the test stress.
71. Term: $U_0$
Definition: The rated phase-to-ground RMS voltage of the cable system.
72. Why does a water tree not immediately cause a cable failure?
It is a non-conductive chemical degradation; failure only occurs once it triggers an electrical tree.
73. What is the purpose of the 'outer semiconductor' layer in a cable?
To maintain a uniform electric field between the insulation and the metallic shield.
74. In the IEC standard, what is the 'Hail Mary' test for a highly troubled, aged cable?
Applying $1.7U_0$ for $5$ minutes to ensure it won't fault immediately upon re-energization.
75. Which insulation type is more sensitive to bending radii: solid core or stranded core?
Solid core conductors are more sensitive to bending.
