Cable Condition Revealed

May 1, 2012
Field cable assessments at RWE Rhein-Ruhr Netzservice prove to be technically viable and economically valuable.

RWE Rhein-Ruhr Netzservice GmbH has a competence centre for cable testing and measurement technology at Bad Kreuznach with more than 20 years of cable diagnostic testing experience on low- and medium-voltage cables. The southern network region deploys five cable test vans that carry equipment for cable and sheath testing, for tan delta (tan d) diagnostics and partial-discharge measurement. The equipment for fault location is also installed in the vehicles ready for use when required. Therefore, the teams have the facilities to test new and old cable systems for operational safety, reliability and condition assessment. These cable test vans are primarily used for diagnostic testing on plastic-insulated cables (XLPE) because of their wide-scale use, but these procedures also can be used on paper-insulated mass-impregnated cables.

There is a significant difference between diagnostic procedures and test procedures. Cable and sheath testing yield a pass or fail result for operational reliability and safety. Diagnostic procedures provide an insight into the condition of cable and reveal weak points, which may lead to future failures. Therefore, there is a distinction between local and global diagnostics.

One of the key objectives of cable diagnostics is the optimization of maintenance costs. Medium-voltage cable systems and other operating equipment can be expected to experience age-related failures. Hence, it is important to take countermeasures. Driven by the German regulatory authorities, RWE must ensure the actions implemented are cost-effective and the network retains maximum availability.

Assessment Procedures

The assessment of cable systems helps to identify the most appropriate maintenance practices for the network. However, cable evaluation and steps for condition-based maintenance are not based solely on current measurement values. Other factors are a part of the assessment. The required supply security of the cable network is a key factor. Another factor is basic data on the cable: manufacturer, type, installation date, insulation material or joint type, number of defects and fault history form. All this information is stored in a database for archiving and analysis. This enables a systematic evaluation of the cable condition based on various weighted factors, allowing the best possible use of the maintenance budget.

The normal AC cable testing using several times the rated voltage prior to commissioning (e.g., testing of XLPE cables with three times rated voltage for over an hour), recommended by the German Association for Electrical, Electronic & Information Technologies [VDE]), only confirms that the cable can withstand the test voltage. It is no indication of the aging process, and on older cables, there is the risk of cable damage caused by high test voltage stress. Therefore, for evaluating older cables, alternative tests that do not stress the cable are preferable. Depending on the objective, the main options are sheath testing, tan δ measurement and partial-discharge measurement.

DC sheath testing enables the safety of the cables to be checked and problems detected. These may include damage during cable installation or water penetration, which impairs the insulation's effectiveness and poses a safety hazard for the public. In the event of sheath damage, the position can be pinpointed by sheath fault location techniques to enable repair.

With older cables, it is common practice to check for damage caused by water trees. This requires tan δ measurement, also known as dissipation factor measurement. The method determines the dielectric dissipation factor: the ratio of real to reactive power of a cable section. It is an integrated or “global” method, a determination of “average aging” for the complete cable circuit.

The tan δ of an undamaged XLPE cable is initially relatively high, but the value decreases with time as the cable out-gasses. Later, the value increases depending on the frequency and size of water trees.

The measured value of tan δ depends on the measurement voltage. New, intact XLPE cables have a low value, which is not significantly higher at twice-rated voltage than at full- or half-rated voltage. Aged cables already show a somewhat higher value at half-rated voltage and significantly higher values at full- or twice-rated voltage. Thus, reliable classification can be performed based on the measured values at various test voltages.

Recommendations and comments are added to the tan δ measurement logs by field technicians according to the limit values. These are available in the cable database for other teams and for future measurements. These recommendations include, for example, repeating the diagnostic testing on partially damaged cables at shorter intervals (after two years, for example). Repeated measurements like this reveal aging trends.

At a frequency of 0.1 Hz, tan δ is particularly informative because the values are better differentiated than at the network frequency of 50 Hz or 60 Hz. For very high reproducibility and comparability of the results, it is necessary that the measuring voltage is symmetric and not be influenced by the connected load and length of cable. RWE Rhein-Ruhr Netzservice depends on the generator from BAUR that provides a “true sinusoidal voltage,” which ensures the measured results are reproducible.

A further advantage of very low frequency (VLF) measurement (0.1 Hz) is that the voltage sources are smaller than comparable devices for operating frequency. The VLF voltage generator also allows conventional cable testing, tan δ measurement and partial-discharge measurement to be performed.

Partial-Discharge Measurement

Partial-discharge measurement increases the reliability of assessment for XLPE-insulated cables and is also used for insulation diagnostics for paper-insulated cables. XLPE cables and other insulated cables are also subjected to partial-discharge measurement following indicative results of a problem during tan δ measurement.

Partial-discharge measurement locates and identifies the places with a greater occurrence of electrical trees that can develop from water trees. It is often revealed that only one phase or only a part of the cable has weak points, so targeted repair measures can be cost-effectively undertaken.

For partial-discharge measurement following tan δ measurement, the use of a VLF true sinusoidal voltage source is advantageous, because measurements at 0.1 Hz alternating current lead to faster, directed growth of water trees compared to other voltage waveforms or frequencies. These then develop into electrical trees at which partial discharge occurs, which often sparks within a few minutes. A high rate of detection is achieved with short measurement times.

With “healthy” XLPE cables, partial-discharge measurement typically yields no results as partial discharges occur primarily at joints and terminations. These are then an indication of defects or installation errors.

Diagnostic Procedure Effectiveness

In addition to other applications, tan δ measurement can be used to check the integrity of a reconstruction or repair. For example, XLPE cables with water trees in the insulation where electrical trees have not yet formed often can be repaired by silicone treatment that displaces the water trees. It is not good practice to determine the dissipation factor immediately after a repair as the new silicone can give false (high) results for tan d. But after a few years, the silicone is fully cured and is chemically and electrically stable. A repeated measurement then reveals whether the cable can be classified as noncritical.

Installation Diagnostics

Diagnostic procedures prove useful for aging cables and also for newly installed cables. RWE Rhein-Ruhr Netzservice supplements AC cable testing and DC sheath testing with diagnostic procedures because installation errors can only be detected by partial-discharge measurement. This enables warranty claims to be made in a timely manner and defects that could lead in-service failures to be corrected before operational usage begins.

For example, in an urban area in 2010, two 2-km (1.24-mile) sections of paper-insulated mass-impregnated cable laid parallel in steel pipes were replaced by XLPE-insulated cables. Both cables were subjected to cable and sheath testing followed by partial-discharge measurements to check the quality of the installed joints, which were spaced at intervals of about 300 m (984 ft).

The first XLPE-insulted cable tested was without problems, but the second cable laid by the same installation team two weeks later showed conspicuous partial discharge at many joints. Joint problems were suspected. But, as the cold-shrunk joints have a higher contact pressure after a few days, the measurement was repeated eight days later. Comparison of the values indicated that the joints were indeed incorrectly installed, a situation that would not have been found by cable and sheath testing.

The cold-shrunk joints on the second cable were installed without preheating when the outdoor temperature was -5°C (23°F). Before installation, the joints should have been heated for a long period or installed in a heated environment (such as a tent). Therefore, the service provider had to remake the joints by heating to about 60°C (140°F) for two hours. Subsequent partial-discharge measurement showed the cable to be free of faults. In this example, the additional diagnostic measurement avoided the consequential damages likely to occur after the end of the warranty period. The costs of correcting the problems were borne by the service provider.

Diagnostics Value

Diagnostic measurements have proved to be technically and economically valuable for the maintenance of existing and new underground cable networks. Depending on the application, tan δ measurement and/or partial-discharge measurement are used. Overall cost savings are achieved, despite investments in the test equipment and the time required for measurements and their associated switching operations. Diagnostics provide information on older cables that enable targeted, cost-effective maintenance of the network based on the probability of cable failure. Therefore, the resources available for network maintenance can be used in a manner that maintains or even improves the quality and reliability of the network within budgets.

For new XLPE-insulated cables, partial-discharge measurement proves an effective instrument for detecting and correcting installation errors before an in-commission circuit failure. Diagnostic procedures have enabled the south region of RWE Rhein-Ruhr Netzservice for the past 15 years to reduce the number of cable faults attributable to water trees in XLPE-insulated cables.

Moreover, recent random partial-discharge measurements on newly installed cables have raised quality awareness on this procedure and with the service providers, resulting in a reduction in the number of faulty installations.

Andreas Borlinghaus ([email protected]) has been involved with maintaining the medium-voltage facilities at RWE Rhein-Ruhr Netzservice GmbH since 1980. Since 1986, he has managed the testing and measurement team for low and medium voltage for electrical special service in the south region. The facility in Bad Kreuznach also has assumed the role of a competence centre for cable testing and measurement technology within RWE Rhein-Ruhr Netzservice GmbH. Borlinghaus is head of the competence centre for cable testing and measurement technology, and he and his team have used VLF cable diagnostics successfully for 10 years.

XLPE Cable Classification Based on RWE Rhein-Ruhr Netzservice Experience

Tan δ for measurements at 0.1 Hz Classification of XLPE cables tan δ2Uo < 1.2 × 10-3 and tan δ2Uo - tan δ1Uo < 0.6 × 10-3 Still safe for operation tan δ2Uo ≥ 1.2 × 10-3 and tan δ2Uo < 2.2 × 10-3 and tan δ2Uo - tan δ1Uo ≥ 0.6 × 10-3 Partially damaged tan δ2Uo ≥ 2.2 × 10-3 and tan δ2Uo - tan δ1Uo > 1.0 × 10-3 No longer safe for operation Note: Uo = rated voltage

Examples of Diagnostic Testing

Diagnostic measurement was necessary in a system following a transient ground fault. Cable testing of the circuit confirmed the system would have been certified as “good.” However, partial-discharge measurement revealed the cause of the transient ground fault: a defective joint that was replaced, eliminating the risk of future failures.

A further example of the value of diagnostic measurement is more apparent when, on a cable almost 2 km (1.24 miles) long between two distribution substations, water trees were expected because of repeated disruptions. However, replacement of the complete cable had to be avoided for cost reasons. Measurement of the tan δ revealed only one phase was severely damaged. Partial discharge was found on sections of the cable when corresponding measurements were made, so cable replacement was limited to two 300-m (984-ft) sections.

To check the success of these actions after the cable was replaced, dissipation factor measurements were performed in addition to cable testing. This confirmed the condition of the cable system as being healthy. Identifying the location of the fault reduced by one-third the cost of laying replacement cable, resulting in a cost saving of 130,000 euros (US$186,000).

The costs of diagnostic measurement are negligible as the time taken to complete a dissipation factor and partial-discharge measurement is about an hour. Thus, the cost of diagnostic measurement is covered when the replacement of a few meters of cable is avoided.

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