Latest Trends in Cable Condition Monitoring and Aging Assessment
Electronic Characterization and Diagnostic (ECAD®) System
The ECAD system is a fully automated electrical characterization and diagnostic system that is used in the nuclear industry for monitoring the conditions of the instrumentation and control cables installed in nuclear power plants. The ECAD system, is a PC-driven data acquisition system. It measures various standard electrical characteristics as well as providing the TDR signature. The ECAD system measures DC and AC resistance, impedance, capacitance, DF, inductance, quality factor, phase angle, and TDR signature of the cables. This computer-based storage/retrieval system provides capability for accurate, lumped data comparison with previous historical information on the same device. The TDR signature permits easy and quick identification of faults in both magnitude and location of the cable. The ECAD system can be used for condition monitoring, troubleshooting, and trending of the measured data. The ECAD system maintains the data in a retrievable format. Reports can be generated quickly with built-in analysis packages to perform the necessary checks on each type of circuit.
Traditional electrical tests, such as insulation resistance and hi-pot testing, are not sensitive enough to detect the level of age-related deterioration in cables. Present electrical tests do not detect aging-induced cracks in the insulation that penetrate to the conductor if the cable is dry. Therefore, mea- surement of the mechanical properties of the cable polymers is the best way to track the vulnerability of cables to age-induced cracking, which could lead to a cable failure in a moist or wet environment. The cable indentor aging monitor developed by the Franklin Research Center and the Electric Power Research Institute (EPRI) is used to perform the in situ, nondestruc- tive test for assessing age-induced degradation. The cable indentor consists of an anvil that is pushed against the surface of the cable jacket or insulation and, depending on the depth of penetration of the indentor for a given force, the hardness of the cable insulation is determined. It is expected that the depth of penetration for a given force will decrease as the cable materials age, thereby indicating the age of the cable insulation.
Oscillating Wave (OSW) Testing
This method was selected by a CIGRE task force as an acceptable compro- mise using the following criteria:
1. Ability to detect defects in the insulation that will be detrimental to the cable system under service conditions, without creating new defects or causing any aging
2. Degree of conformity between the results of tests and the results of 50 or 60 Hz tests
3. Complexity of the testing method
4. Commercial availability and costs of the testing equipment
The purpose of the OSW testing method is to detect defects that may cause failures during service life without creating new defects that may threaten the life of the cable system. Although OSW testing does not have a wide reputation with respect to cable testing, it is already used for testing in met- al-clad substations and is being recommended for gas-insulated cable test- ing. The following description is based on the information given in IEEE std 400-2001, “IEEE guide for field testing and evaluation of the insulation of shielded power cables systems.”
General description of test method The test circuit consists of a DC voltage supply that charges a capacitance and a cable capacitance. After the test voltage has been reached, the capacitance is discharged over an air core coil with a low inductance. This causes an oscillating voltage in the kilohertz range. The choice of and depends on the value of to obtain a frequency between 1 and 10 kHz.
• OSW method is based on an intrinsic AC mechanism
• Principal disadvantages of DC (field distribution, space charge) do not occur
• Method is easy to apply
• Method is relatively inexpensive
• For both HV and MV cable systems, f * OSW/DC is low (0.2 to 0.8), indicating the superiority of OSW over DC voltage testing
• Effectiveness of the OSW test method in detecting defects is better than with DC but worse when compared with AC (60 Hz)
• In particular for medium-voltage cable systems, the factor f * OSW/60 Hz voltage is approaching 1, indicating the mutual equivalence
• For HV cable systems, f * OSW/60 Hz is significantly higher (1.2–1.9), which means that OSW is less effective than 60 Hz
Note: f * OSW/60 Hz is the ratio of breakdown values for a dielectric containing a standard defect when using, respectively, OSW voltage and 60 Hz voltage.
The cable is charged with a DC voltage and discharged through a sphere gap into an inductance of appropriate value so as to obtain the desired frequency.
The voltage applied to the cable is expressed as:
Most of the tests carried out so far are of an experimental nature. Artificial defects like knife cuts, wrong positions of joints, and voids in the insulation were created and subjected to different testing procedures of which one method was the OSW testing. These test procedures were intended to obtain breakdown as a criterion for comparison. The general testing procedure is as follows:
1. Start to charge the cable with a DC voltage of about one or two times the operating voltage
2. Increase with steps of 20–30 kV
3. Produce 50 shots at each voltage level
4. Time interval between shots to be 2–3 min
5. Proceed until breakdown occurs
Broadband Impedance Spectroscopy Prognostic/Diagnostic Technique The broadband impedance spectroscopy (BIS) technique was developed by the Boeing Company under the sponsorship of the U.S. Federal Aviation Administration (FAA) to monitor the condition of installed aircraft wiring.
Under the sponsorship of the U.S. Nuclear Regulatory Commission (NRC), the BIS technique was evaluated for application to electric cables used in nuclear power plants. Cable samples, which are representative of a commonly used type of low voltage instrumentation and control cable in nuclear power plants, were prepared and received accelerated aging to simulate various types of degradation expected in actual plant service conditions, including the following:
• Global thermal degradation
• Global thermal degradation plus localized hot spots
• Thermal degradation with cracking
• Abrasion damage
The cable samples were then tested in the laboratory using the BIS method, and the data were analyzed to draw conclusions on the effectiveness of the method. Several test configurations were evaluated, including
• Constant temperature and humidity along the cable with no load attached
• Constant temperature and humidity along the cable with a load attached
• Varying environments along the length of the cable with no load attached
The results of the NRC sponsored research demonstrated that the BIS method can be used on nuclear power plant cables, or on cables used in industrial plants. This method may represent a breakthrough in the prognostics and diagnostics of installed cable systems. The technique provides a nondestruc- tive means of monitoring cable systems in their installed configuration. Age- related degradation can be detected in an incipient stage prior to failure.
The following are the specific conclusions from this research:
• The BIS method was clearly able to detect the presence of thermal deg- radation associated with the cables used in this study. Specifically, the impedance phase spectra of the cables tested were observed to increase as the amount of thermal degradation on the cable increased. This increase can be used as an indicator of global thermal degradation.
• The BIS method was able to detect the presence of localized thermal degradation, or hot spots on the cables. Specifically, a shift in the zero crossings of the impedance phase spectra was observed when a hot spot was present on the cable.
• An approach was developed for locating hot spots within a cable using models of the cable electrical properties. The models were able to predict the hot spot locations within ±10%.
• The BIS method was not sensitive enough to distinguish between the different severities and sizes of hot spots simulated at low
frequencies. However, high-frequency data were able to distinguish between the severity levels. Additional research is warranted to establish the sensitivity limits for this technique.
• The BIS method was able to detect and locate the presence of abrasion- related damage on a cable. The models and approach used are similar to that for detecting and locating thermal hot spots.
• The BIS method was demonstrated to be effective for detecting and locating degradation on cables with an attached load. This is impor- tant since it is desirable to have a technique that can test cables in their installed configuration, without having to disconnect them from attached equipment.
• The BIS method was able to detect simulated cracking damage on cables. However, the simulation method used in this study was determined not to accurately represent the cracking phenomena. Additional research is warranted to more accurately evaluate the BIS method on actual cable cracking.
• The BIS method was able to detect the presence of localized thermal degradation, or hot spots on the cables even with a varying environment along the external surface of the cables.
While the BIS method shows great promise as a prognostic and diagnostic technique for installed cable systems, additional research is continuing before this method can be applied in the field to power cables.