Standard for Shielded Power Cable Diagnostic Testing Methodologies

EXECUTIVE SUMMARY

The advancement and evolution of the development and use of Partial Discharge as the technology to monitor and alarm the insulation integrity of both electrical power apparatus, accessories and the entire power systems’ Assets continues to gain credibility, accuracy and reliability. The ability to monitor an Owner’s Assets of this significance; continuously, without outage interruption has enormous value – is self-evident.

The most cost-effective applications continue to be power transformers, Iso-Phase Bus and all rotating equipment – and most applicable accessories. Regarding shielded power cable, though, the most practical, pragmatic approach for this application is the use of off-line, Tan Delta Diagnostics, Withstand or the very popular combination – Monitored Withstand Field Tests, which achieve significant, objective and reliable results that have proven to deliver real value to the Owners of significant power cable Assets.

 

For purposes of efficiency; we will address the application of shielded, solid-dielectric insulated power cable through 38KV; underground residential, network, bus-tie, station get-away, overhead, direct bury and associated applications. Additionally, we will only address Tan-Delta (Dissipation Factor and related Power Factor) and Partial Discharge technologies. There are other methods outlined in IEEE Std 400-2012; Std 400.2-2013; Std 400.3-2006 which we will use as the guide basis of this paper. Diagnostic Testing can be defined as Non-Destructive, meaning it does not induce undetected defects in a defect-free cable system and does not intentionally cause a failure during the test. A destructive test is defined as a test which (a) requires that a defect produce a fault during the test, and/or (b) produce substantial cable system degradation which goes undetected during the test.

Diagnostic Testing of high voltage electrical power equipment and conductor can be divided into two categories. On-Line (or energized) and commonly referred to as Level 1 (non-invasive; on-line) Diagnostics and Off-Line (de-energized); both methodologies require experienced, trained personnel in Safe electrical practices within a high voltage environment.

Partial discharge diagnostic testing has proven to be effective as a permanent, on-line monitoring technology in switchgear, bus and transformer applications; utilization of RFI bandwidth allows for an overall “umbrella” or pre-locating application as a system. Additionally, the use of portable, Level 1 Diagnostic PD detection for large, power transformers, lightening arrestors and rotating equipment is developing into a reliable methodology and technology, as directed by the pre-locate of the RFI signatures. Both permanent and portable Level 1 applications are beginning to establish objective, known recorded values over many years of use and many test subjects; with the resulting reliable, useful operational diagnostic information as the goal; unfortunately, there remains contention regarding detailed, objective diagnostic PD levels generated by the installed Cable System and most notably the Accessories. Off-Line PD will not measure any water-tree defects, only Splices and Terminations (Accessories).

Cable application of On-Line PD measures and records noise along the length of the conductor, allowing identification of “where in the cable length” the appreciable discharges are located. Tan-Delta (Dissipation Factor) measures and records the tangent angle of the entire cable length (test specimen).

Tan Delta Testing for Shielded Power Cable – What Is Tan δ, Or Tan Delta?

Tan Delta, also called Loss Angle or Dissipation Factor testing, is a diagnostic method of testing cables to determine the quality of the cable insulation. This is done to try to predict the remaining life expectancy and, in order to prioritize cable replacement and/or injection. It is also useful for determining what other tests may be worthwhile.

How Does It Work?

If the insulation of a cable is free from defects, like water trees, electrical trees, moisture and air pockets, etc., 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. If there are impurities in the insulation, like those mentioned above, 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. It will be 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. Below is a representation of a cable. The tangent of the angle δ is measured. This will indicate the level of resistance in the insulation. By measuring IR/IC (opposite over adjacent – the tangent), we can determine the quality of the cable insulation. In a perfect cable, the angle would be nearly zero. An increasing angle indicates an increase in the resistive current through the insulation, meaning contamination. The greater the angle, the worse the cable, as a system.

Doble Power Factor: CABLES AND POTHEADS

The overall power-factor test for cable insulation is confined largely to lengths of cable that come within the current limits of the Doble test set being used; however, regardless of the length of cable, Hot-Collar tests may be applied to all types of potheads.

Tabulations of power-factor data for cables are provided in the Cables and Accessories Section of the Doble Power Factor Test-Data Reference Book; valuable additional information is included in the Doble Reference Book on Cables and Accessories and in Doble Conference Minutes (refer to Indexes of Minutes). Some typical power-factor data for common cable-insulation systems is as follows:

Paper 0.5 to 1.0% @ 20°C
Cross-Linked Polyethylene 0.05 to 0.10% @ 20°C
Ethylene/Propylene Rubber 0.50 to 1.0% @ 20°C
Rubber (and Kerite) 3.0 to 5.0% @20°C
Varnished Cambric 4.0 to 8.0% @ 20

Note: Modern cable has a relatively flat power-factor versus temperature relation. The power factor of old cables made around 1920-1930 may increase more rapidly with increase in temperature.

TAN DELTA MEASUREMENT GRAPHIC DISPLAY

VLF WITHSTAND ONLY

 

TAN DELTA BRIDGE in Series w/ VLF

Cables which do not have a metallic sheath or grounded shield but are installed in metallic conduit may have power factors somewhat higher than the values listed above and in the Doble Power-Factor Test-Data Reference Book.

Cables which do not have a metallic sheath or grounded shield and are installed in fiber or other forms of nonmetallic conduit may have normal power factors considerably higher than the values listed above because of the poor dielectric circuit to ground. Grounding adjacent cables will improve the dielectric circuit when testing cables of this type. Ungrounded-specimen tests between unshielded cables in the same conduit may be used to confine the test primarily to the cable insulation, excluding the duct material.

Tests performed with either the 2.5- or 10-kV Doble test sets will usually detect the presence of moisture in the cable insulation; however, multi-voltage tests (at least up to the line-to-neutral voltage) should be performed. “Tip-up” in power factor with increase in test potential is indicative of losses due to ionization. In general, cable that has a power factor noticeably above that of similar-type cables or an appreciable “tip-up” in power factor should be investigated for moisture and/or corona deterioration.

Results of the Hot-Collar tests (loss/current) are graded largely by comparison of data taken on similar type potheads. Abnormally high dielectric loss and current usually indicate the presence of moisture. Increases in losses with increased test potential indicate the presence of voids. Below-normal test currents indicate the absence of compound or oil.

Partial Discharge Testing for Shielded Power Cable

Wikipedia states:

Partial Discharge (PD) is a localized dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high voltage stress. While a corona discharge is usually revealed by a relatively steady glow or brush discharge in air, partial discharges within an insulation system may or may not exhibit visible discharges, and discharge events tend to be more sporadic in nature than corona discharges.

IEEE states:

A partial discharge is an electrical discharge (formation of a streamer or arc) that does not bridge the entire space between two electrodes. The discharge may occur in a gas-filled void within the extruded cable insulation, at the interface between a shield protrusion and the insulation, at a shield skip, at the boundaries of a contaminant, or at the tip of a well-developed water tree when a cable is subjected to moderately high voltage. Partial discharges can also occur in a cable termination, in a joint, in air, or within a cable.

Partial discharge tests. There are several methods for detecting and measuring PD. Some methods involve de-energizing, disconnecting, and powering the cable from a special voltage source, while other methods allow the cable to remain energized at normal line voltage. Both methods will detect and measure partial discharge in Pico Coulombs (PC). The authors of the IEEE study on water trees state that electrical trees will likely progress to failure quickly, so PD testing would be more valuable if performed in conjunction with dissipation factor/power factor (DF/PF) testing. We will address On-Line Partial Discharge Diagnostic Testing only.

PD SENSORS

SET-UP

DATA CAPTURED

Of course, all Field Testing can be influenced by elevation, temperature and humidity.

TREND EVALUATION

Proper Asset Management should always employ trending over time of the Test activity.

The following IEEE Paper (significant NEETRAC influence) explains concisely.

Interpretation of dielectric loss data on service aged polyethylene based power cable systems using VLF test methods

The more targeted approach requires an assessment of the health of cable systems. It is increasingly common for the assessment of aged cable systems to be made through the application of diagnostic measurements. A recent study has shown that Very Low Frequency (VLF) Tan δ is the most commonly deployed cable system diagnostic. The practical use of this technique has been supported by the international standards IEEE Std. 400-2001 and IEEE Std. 400.2-2004. A key part of these standards is the guidance provided to a user that is detailed in the “Figures of Merit”.

These enable users to identify cable systems that are more likely to fail in service near term. To aid these decisions a series of criteria have been developed. The benefit of the criteria described here is that the process for their determination is rational, reproducible, and transparent. The outcomes are supported by a probabilistic assessment of service performance.

Published in: IEEE Transactions on Dielectrics and Electrical Insulation (Volume: 20, Issue: 5, Oct. 2013) Page(s): 1699 – 1711 Date of Publication: 21 October 2013
Print ISSN: 1070-9878 ; INSPEC Accession Number: 13849660
DOI: 10.1109/TDEI.2013.6633700
Publisher: IEEE; Sponsored by: IEEE Dielectrics and Electrical Insulation Society

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References and Bibliography

IEEE Std 400-2012; Std 400.2-2013; Std 400.3-2006
IEEE Power Engineering Society; NEETRAC
EC&M Oct 2003
NETA World 2006
TechIMP
Doble Engineering
High Voltage Inc.
Emerson