Power Cable Test Methods Flashcards

A NETA technician is preparing to perform an acceptance test on a newly installed 15 kV, 100% insulation level, shielded power cable. According to NETA standards, which Very Low Frequency (VLF) withstand test parameters are correct?

Answer: 19 kV rms for 30 minutes

According to NETA ATS-2021, Table 100.6.3, the acceptance test voltage for a 15 kV rated shielded power cable is 19 kV rms (sinusoidal waveform) for a minimum duration of 30 minutes. The other options list incorrect voltages, durations, or test types (DC vs. VLF AC).

A technician performs a Tan Delta (or Power Factor) test on a service-aged, medium-voltage XLPE cable. The results show a significant increase in the Tan Delta value as the test voltage is stepped up. What is this phenomenon called and what does it most likely indicate?

Answer: Tip-Up; it indicates widespread degradation such as water treeing.

The increase of the Tan Delta (dissipation factor) value with increasing test voltage is known as "tip-up." This indicates that the dielectric losses are voltage-dependent, a characteristic sign of insulation degradation like widespread water treeing or moisture ingress. Healthy insulation would show a relatively flat, stable Tan Delta value across all voltage steps.

A client requests a DC high-potential (Hipot) test on a 20-year-old, 15 kV XLPE insulated feeder cable during a plant shutdown. Which of the following is the MOST critical reason for a NETA technician to advise against this test and recommend an alternative like VLF?

Answer: DC testing can induce a space charge in the aged insulation, leading to failure after the cable is returned to service.

DC Hipot testing on service-aged extruded dielectric cables (like XLPE) is strongly discouraged by industry standards (including IEEE and NETA). The primary reason is that the DC voltage can create a trapped electrical charge, known as a 'space charge,' within the insulation. When the cable is re-energized with AC voltage, this trapped charge can create a localized high-stress point, potentially causing the cable to fail days or weeks later.

During a maintenance insulation resistance test on a medium-voltage cable, a technician records a reading of 800 megohms at 1 minute and a reading of 2000 megohms at 10 minutes. What is the Polarization Index (PI), and how should it be classified according to NETA standards?

Answer: PI = 2.5; Good/Acceptable

The Polarization Index (PI) is calculated as the ratio of the 10-minute insulation resistance reading to the 1-minute reading. In this case, PI = 2000 MΩ / 800 MΩ = 2.5. According to NETA standards, a PI value of 2.0 or greater is generally considered good or acceptable for this class of insulation.

An underground medium-voltage cable has failed a VLF withstand test during commissioning. Which of the following test methods is specifically designed to pre-locate the physical position of the fault along the cable's length?

Answer: Time-Domain Reflectometry (TDR)

Time-Domain Reflectometry (TDR) is the most effective method for pre-locating cable faults. It operates like radar, sending a low-voltage pulse down the cable and analyzing the reflections that occur at points of impedance change, such as opens, shorts, or splices. The time it takes for the reflection to return allows for an accurate calculation of the distance to the fault.

An offline Partial Discharge (PD) test is being performed on a newly installed cable system. This test is most effective at detecting which of the following types of defects?

Answer: Small voids or contaminants introduced into a splice during assembly.

Partial discharge testing is a highly sensitive diagnostic used to detect small electrical discharges occurring in localized defects within an insulation system, such as voids, impurities, or sharp points. It is particularly effective at identifying workmanship issues in accessories like splices and terminations, which are common sources of failure.