Acceptance and Maintenance Testing Medium-Voltage Electrical Power Cables

Thomas D. Sandri Fall 2020 Features, Features

As time progresses and a cable system ages, the system’s bulk dielectric strength degrades. During this aging process, artifacts such as water trees, delamination, voids, and shield corrosion raise the local stress placed on the cable during normal operation. The exact way in which the strength of a device degrades depends upon many factors including voltage, thermal stresses, maintenance practices, system age, cable system technology and materials, and environment. 

As reliability demands grow, testing methods have been developed that can provide a better indication of the integrity of cables, splices, and terminations. To use these methods effectively, the operator must understand the mechanisms of aging and failure in cable systems.

Cable Testing Options

Over the years, several methods and/or philosophies regarding testing underground electrical power cable in the field have evolved. The Insulated Conductor Committee of the IEEE Power & Energy Society has divided these methods or philosophies into two fundamental categories: Type 1 Field Tests and Type 2 Field Tests.

Type 1 tests are intended to detect defects in a cable system’s insulation in order to improve service reliability after the defective part is removed and appropriate repairs are performed.  These tests are usually achieved by applying moderately increased voltages across the insulation for a prescribed duration of time. Such tests are categorized as pass/fail. Type 1 tests include:

  • Direct Current Withstand: IEEE 400.1, Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage
  • VLF Withstand: IEEE 400.2, Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)
  • High Potential Power Frequency: This is typically considered a factory test, not a field test.

Type 2 tests are intended to provide indications that the insulation system has deteriorated; hence, they are termed diagnostic tests. Type 2 tests include:

  • Dissipation Factor (Tan Delta) Testing: IEEE 400.2, Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)
  • Partial Discharge: IEEE 400.3, Guide for Partial Discharge Testing of Shielded Power Cable Systems in a Field Environment
  • Damped Alternating Current: IEEE 400.4, Guide for Field Testing of Shielded Power Cable Systems Rated 5 kV and Above with Damped Alternating Current (DAC) Voltage

High Potential Testing with Direct Current (DC)

For years, high-voltage DC testing has been the traditionally accepted method for judging the serviceability of medium-voltage cables. Direct current tests have worked well for conducting high potential and condition assessment tests on paper-insulated lead-covered (PILC) cable.  When plastic-insulated cables were first introduced in the 1960s, DC continued to be the preferred method.

As time moved on, plastic-insulated cables became more abundant and began showing effects of service age. Direct current continued to be the dominant test, but concerns began to grow over this test’s effectiveness. In the early 1990s, reports began to surface indicating that DC high potential testing could be to blame for latent damage experienced by extruded medium-voltage cable insulation (cross-linked polyethylene and ethylene propylene rubber). After receiving these reports, the Electrical Power Research Institute (EPRI) funded a study (EPRI Report TR-101245) related to cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) cables that yielded three conclusions regarding XLPE cable:

  1. DC high potential testing of field-aged cable reduces its life.
  2. DC high potential testing of field-aged cable generally increases water tree growth.
  3. DC high potential testing before energizing new medium-voltage cable does not cause any reduction in cable life.

Current versions of most industry recommendations no longer include DC high potential testing of extruded cables (XLPE and EPR) as a maintenance test. Of those that still do, all have reduced the recommended test duration from 15 minutes to only 5 minutes. None endorses DC high potential testing as a factory test for extruded cables, but most documents continue to include DC high potential testing as an acceptance test on newly installed extruded cable. These industry recommendations and guides also no longer endorse DC high potential testing as a maintenance test for extruded cables that have been in service for more than five years.

High Potential Testing Performed at Power Frequency

As a field test, power frequency testing suffers a serious disadvantage: At increased voltage levels, the test sets require heavy, bulky, and expensive transformers. Field-portable AC high potential test sets of the power frequency variety are used worldwide for field testing vacuum bottles, switchgear, reclosers, circuit breakers, etc. These units typically offer ratings of 3 to 7 kVA and are portable and cost effective for these applications.

The reason large transformers are required for cable testing has to do with the capacitance of the load being tested. Capacitive reactance (Xc) changes as a function of frequency as seen in the following formula:

Therefore, if we test a 15 kV-rated cable of approximately 10,000 feet, the capacitance would be around 1uF. Based on the formula, the capacitive reactance at 60 Hz would be 2,654 ohms:

To apply the IEEE-recommended 22 kV test voltage would require a power supply rated for 8.3 amps or 183 kVA:

The physical size and weight of a transformer capable of this rating is obviously not practical as a portable field unit.

High Potential Testing Performed at Very-Low Frequency (VLF)

The main advantage of testing at very-low frequency is that it significantly reduces the size, weight, and cost of the power supply required and thus offers greater attraction for field testing medium-voltage cables. If the test frequency were dropped to 0.1 Hz, the capacitive reactance, as calculated earlier, becomes 1.6 megohms. The same 22 kV would now only draw 14 mA or 0.303 kVA. Therefore, the size, weight, and portability of the power supply become very convenient for field use. VLF power supplies can be constructed as either a cosine-pulse (rectangular) waveform or sinusoidal waveform output.

Cosine-Pulse Waveform VLF

The cosine-pulse waveform version is constructed using a DC test set that acts as the high-voltage source. A DC-to-AC converter then changes the DC voltage to the VLF AC test signal. The converter consists of a high-voltage inductor (choke) and a rotating rectifier that changes the polarity of the cable system being tested every 5 seconds. This generates a 0.1-Hz bipolar wave. A resonance circuit, consisting of a high-voltage inductor and a capacitor in parallel with the cable capacitance, assures sinusoidal polarity changes in the power frequency range. The use of a resonance circuit to change cable voltage polarity preserves the energy stored in the cable system. Only leakage losses have to be supplied to the cable system during the negative half of the cycle.

The intent of the VLF cosine-pulse waveform test is to generate a 0.1-Hz bipolar pulse wave that changes polarity sinusoidally. Sinusoidal transitions in the power frequency range will then initiate a partial discharge at an insulation defect, which the 0.1-Hz pulse wave develops into a breakdown channel. During the test period, typically within minutes, a defect can be detected and forced to break through. After the defect breaks through and faults, it can then be located with standard, readily available cable fault-locating equipment. Identified faults can then be repaired during the scheduled outage. When a cable system passes the VLF test, it can be returned to service.

Sinusoidal Waveform VLF

The VLF test set generates sinusoidally changing waves that are less than 1 Hz (0.1 Hz, 0.05 Hz, or 0.02 Hz). When the local field strength at a cable defect exceeds the dielectric strength of the insulation, partial discharge starts. The local field strength is a function of applied test voltage, defect geometry, and space charge. After partial discharge is initiated, the partial discharge channels develop into breakthrough channels within the applied test period. When a defect is forced to break through, it can then be located with standard, fault-locating equipment during the scheduled outage. 

Dissipation Factor (Tan δ) Testing

Dissipation factor testing, also called tan δ or loss angle testing, is a diagnostic method of testing cables to determine the quality of the cable insulation. This is done to try to predict remaining life expectancy and to prioritize cable replacement and/or injection. Injection technology, also known as cable insulation rejuvenation, is an option to cable replacement. Cable injection technology involves the injection of a diffusive, water-reactive material into the conductor core of a buried power cable insulated with solid dielectric materials. Once inside the cable, the fluid diffuses into the cable’s insulation and chemically combines with the water content contained within. This process retards the growth of water trees, a cause of cable failure in aged solid dielectric cable.

Tan δ is also useful for determining which other tests, such as VLF withstand or partial discharge, might be worthwhile.

If the insulation of a cable is free from defects like water trees, electrical trees, moisture, and air pockets, the cable approaches the properties of a perfect capacitor. It is very similar to a parallel plate capacitor with the conductor and the neutral being the two plates separated by the insulation material. In a perfect capacitor, the voltage and current are phase-shifted 90 degrees, and the current through the insulation is capacitive (Figure 1).

Figure 1: Ideal Model for Insulation

If there are impurities in the insulation, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted 90 degrees but will be shifted something less than 90 degrees. The extent to which the phase shift is less than 90 degrees is indicative of the level of insulation contamination, hence quality/reliability. This loss angle is measured and analyzed (Figure 2).

Figure 2: Typical Insulation Model

The tangent of the angle delta is measured. This indicates the level of resistance in the insulation. By measuring IRp/ICp, we can determine the quality of the cable insulation. In perfect cable insulation, the angle would be nearly zero. An increasing angle generally indicates an increase in the resistive current through the insulation, meaning contamination. Keep in mind, however, that different insulation materials have higher or lower dielectric losses; therefore, the angle or tan δ value may be higher for some insulating materials due to their associated dielectric losses.

A voltage supply is required to energize the cable under test to the desired test voltage(s).  Although power frequency can be used, and is used in factory testing, VLF is typically chosen as the power supply for field application. As mentioned earlier, to test a cable with 60 Hz power requires a very-high power supply. When testing in the field, it is not practical — or possible in many locations — to test several thousand feet of cable with a 60 Hz supply. A typical VLF frequency of 0.1 Hz takes less than 0.2 percent of the power to test the same cable compared to 60 Hz and therefore provides a significant size, weight, and cost advantage in field testing. Secondly, the magnitude of the tan delta numbers increases as the frequency decreases, making measurement easier. As shown in the following equation, the lower the frequency (f), the higher the tan δ number.

Partial Discharge Testing

The obvious advantage of online PD testing is that it does not require disconnection or an outage. The main disadvantage when testing cables under operation is that the test is only performed at the operating voltage level and cannot be adjusted. In comparison to offline testing, where voltages can be adjusted to simulate transients or other over-voltage conditions, online methods detect a lower percentage of defects in the cable’s insulation system.

Partial Discharge Inception and Extinction Voltage

For partial discharge to occur, sufficient voltage must be applied to the system to meet the minimum voltage required to start partial discharge activity. This is known as the partial discharge inception voltage (PDIV). Once the PDIV has been reached, voltage may be lowered, and PD will remain present at the lower voltages until it finally extinguishes at what is referred to as the partial discharge extinction voltage (PDEV). The PDEV is therefore less than the PDIV. If the PDEV voltage level is lower than the system operating voltage (phase-to-ground), this implies that an over-voltage surge on the insulating system could initiate PD. The PD activity might then continue even when the system voltage returns to normal. Partial discharge activity that can continue at operating voltage is therefore more likely to result in insulation failure than PD that extinguishes above normal operating voltage. 

Power Frequency and Alternative Test Voltage Sources

As stated, for partial discharge to occur, sufficient voltage must be applied to the system to meet the minimum voltage required to start partial discharge activity. When testing cables, the online testing approach uses the system voltage of a constant fixed magnitude. In an offline approach, a temporary voltage source will be required. 

Voltage sources that are used for commercially available field partial discharge measurement systems will fall into the general categories of power frequency and alternative voltage sources such as very-low frequency (VLF).

VLF Test Voltage

Depending on the type of defect, sinusoidal VLF voltage sources (usually 0.1 Hz) for extruded dielectric systems may require a higher test voltage to generate the same partial discharge level compared with tests performed with power frequency voltages. For example, the conductivity of the surface of a cavity that has been exposed to PD increases, which allows any charges deposited on the surface by PD to leak away; this lowers the electric field in the cavity. As more charge can leak away between polarity reversals at VLF than at power frequency, the PDIV at sinusoidal VLF will be larger than that at power frequency. If there has been no previous PD activity to increase the conductivity of the cavity surface, the PDIV at sinusoidal VLF and power frequency will be similar (IEEE 400.3). 

A VLF cosine-pulse waveform generates a 0.1 Hz bipolar pulse wave that changes polarity sinusoidally. Since the sinusoidal transitions are in the power frequency range, the PDIV measurement will be comparable to power frequency. The VLF cosine-pulse voltage works according to the principle of 50/60 Hz slope technology. This is particularly important for PD diagnosis, since reliably evaluating the measured results requires direct comparability with the power frequency. Partial discharge characteristics change in the case of large frequency differences, making reliable evaluation to power frequency impossible. The 50/60 Hz slope technology ensures comparability for both voltage wave shapes.

Figure 3 shows a typical example of how PD measurement is carried out during the slope of the applied voltage. The steepness of the VLF cosine-pulse slopes in comparison to the 0.1 Hz sine wave can be clearly seen. It is precisely this rise in voltage that is so important for the PD inception voltage. Therefore, the 0.1 Hz sine wave test voltage cannot be directly compared to the 50/60 Hz power frequency, and critical partial discharge defects are therefore not always reliably detected.

Figure 3: VLF Sine Wave vs. VLF Cosine-Pulse

Damped AC Voltage (DAC)

Another approach to reduce the size and weight of the test voltage supply from that of a conventional power frequency supply is the damped AC voltage (DAC) technique. For the purpose of partial discharge analysis, the cable under test is charged to the pre-selected peak value by a direct-current high-voltage source within a couple of seconds and then shorted with an electronic switch via a resonance coil. This creates a sinusoidal oscillating AC voltage with low damping (Figure 4). The frequency is fixed in a range from 50 Hz to several 100 Hz, depending on the capacitance of the test object. Since the frequency of the test voltage is close to nominal service conditions, all measured PD activities can be effectively evaluated and compared to that of power frequency.

Figure 4: Damped AC Voltage

Due to the decaying amplitude of the test voltage, the partial discharge extinction voltage can easily be determined.

Conclusion

Numerous insulation tests can assist in assessing the quality and condition of a cable’s insulation. Pass/fail tests provide the means to identify gross defects, while diagnostic tests provide us with an understanding of the severity of degradation or the extent of contamination in the insulation.

Technology and philosophies toward testing have advanced and pushed toward predictive maintenance solutions. A key element in predictive maintenance is monitoring the trend of diagnostic test results. Not all tests are appropriate to all circumstances nor can any single test give you the complete answer. Each type of test serves as a window into the condition of the cable, and you build a more complete picture by putting together a number of different tests.

Thomas D. Sandri is the Training Development Manager at Shermco Industries. He has been active in the field of electrical power and telecommunications for over 30 years. During his career, he has developed numerous training aids and training guides and has conducted domestic and international seminars. Tom supports a wide range of electrical and telecommunication maintenance application disciplines. He has been directly involved in supporting test and measurement equipment for over 20 years and is considered an industry expert in application disciplines, including battery and dc systems testing and maintenance, medium- and high-voltage cables, ground testing, and partial discharge analysis. Tom holds a BSEE from Thomas Edison University in Trenton, New Jersey.