In-situ cable condition monitoring8 March 2017
Elongation at Break (EAB), measuring the tensile properties of insulation, is the standard technique for determining the remaining useful life of a cable. It is laboratory-based and destructive method, so it cannot be applied to installed cables. Frequency Domain Reflectometry (FDR) can identify, locate, and quantify cable insulation degradation in nuclear power plants. By H.M. Hashemian*
As nuclear power plants pursue licence extensions to operate for 60, 80 or more years, managing ageing of key components that cannot easily or economically be replaced becomes critical to safety, reliability, and availability. This includes electrical cables with polymer insulation material that can become brittle, crack, or degrade over time from exposure to the harsh environment of a nuclear power plant, which may include heat, radiation, humidity, vibration and mechanical shocks.
The I&C system cables generally constitute nearly 90 percent of important cables that can affect the safety and availability of nuclear power plants. Wholesale replacement of cables is expensive and impractical and must be avoided or minimised. Methods that can identify degradation or ageing in cable insulation material are necessary to isolate those cables or sections of cables that must be repaired, rejuvenated or replaced.
Traditionally, cable condition monitoring and ageing management has been performed by one or more of the following approaches: visual inspections through walk downs; monitoring of electrical characteristics such as insulation resistance (IR) and inductance, capacitance, and resistance (LCR) measurements; in-situ testing of insulation hardness using an indenter device; and EAB testing of sacrificial cables samples periodically removed from cable baskets that are installed in harsh environments.
Each of these methods has shortcomings. Visual inspection by walk-down only provides a qualitative assessment and is limited to those cables that can be seen and therefore excludes cables that are in conduits, buried, covered by other cables, or hidden from plain view. Electrical measurements such as IR and LCR are straightforward, but the results do not provide distance to fault information. Monitoring of indenter modules (IMs) is useful only on hot spots if they are visible and accessible. EAB measurement of sacrificial cables is effective, but only a few nuclear plants have such cable depots.
These shortcomings motivated the validation of the Frequency Domain Reflectometry (FDR) technique for in-situ cable condition monitoring in nuclear power plants. With the FDR technique fully developed and correlated with EAB, a conservative approach for cable ageing management would be to perform FDR to locate a problem and assess its severity using FDR-versus-EAB correlation for the particular cable, to measure the IM at the location that FDR has identified using an indenter to confirm the FDR results.
FDR technique and correlation with EAB
In an FDR test an electrical signal is injected through the cable conductor and its reflection is measured. The reflected signal is recorded as a plot of impedance changes as a function of distance. It can register problems such as cracks, hardening and gouges in the cable insulation.
In a research project just completed with funding from the US Department of Energy, simultaneous FDR and EAB measurements were made on representative nuclear power plant cables as they were aged in laboratory ovens. The results were correlated for cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) – two of the most commonly used cable insulation types in nuclear I&C systems (Figure 1). Part of the FDR data for the XLPE cable involved in this research is shown in the diagram. This cable was aged in an oven for one year at 130°C and periodically removed for the FDR and EAB tests. The localised ageing or “hot spot” enlarged in Figure 2 shows progressive changes in the FDR signal peak relative to the cable’s baseline characteristic impedance.
Additional research is planned to produce FDR versus EAB correlations for the remaining polymers from different cable manufacturers and vintages (see Figure 3 showing those most commonly used). When this is done, the nuclear industry will be able to use FDR to perform routine cable condition monitoring, ageing management or remaining useful life estimation as recommended by the US Nuclear Regulatory Commission (NRC) in Regulatory Guide 1.218.
FDR was used at the Oyster Creek nuclear plant in September 2016 to determine whether a group of cables that was installed during plant construction (about 40 years ago) should be replaced. These cables are routed from control room cabinets to the turbine area and pass through the plant’s condenser bay, where they are exposed to elevated temperatures that can degrade their insulation.
FDR measurement revealed that the cables are in good working condition, have maintained their qualification and most are far from reaching their end of qualified life. This helped the plant avoid unnecessary replacement of these cables, saving tens of thousands of dollars for the utility as the cost of replacing nuclear-qualified cable could exceed $2000 per foot. The diagram below shows an FDR trace (converted from frequency domain to time domain) for one of the Oyster Creek cables. The test signal was applied in the control room at the cable termination point to assess the condition of the portion of the cable that ran along a hot region. Note that there is a decrease in reflection coefficient indicating insulation ageing that manifests itself in a reduction in impedance.
Cable ageing and condition monitoring has been the subject of much research, regulation and standards produced over the last two decades. The Electric Power Research Institute (EPRI) has published nearly a dozen reports on this subject, the International Electrotechnical Commission (IEC) has published two series of standards in the last decade, and the International Atomic Energy Agency (IAEA) completed a co-ordinated research project in 2015. The US Nuclear Regulatory Commission has produced a number of documents and has sponsored research at the National Institute of Standards and Technology, Sandia National Laboratory and elsewhere to uncover any safety issues with ageing of nuclear power plant cables.
Cable condition monitoring and ageing management is important because a crack in cable insulation can allow moisture to enter the cable, especially during a loss of coolant accident, and prevent the plant personnel from operating a pump, closing a valve or reading process variables such as temperature or pressure. The consequences can be dangerous and costly. FDR can help avoid such consequences by providing early warning to isolate faulty cables and providing clues as to whether or not a cable should be repaired, replaced or rejuvenated.
FDR is also useful in aviation safety and accident prevention in a variety of other industries such as chemical and petrochemical plants and manufacturing facilities. The public is aware of aviation catastrophes – such as the crash of TWA flight 800 off the east coast of the USA – rooted in cable problems. This and other types of cable-related industrial accidents can be prevented by predictive maintenance using FDR.
*H.M. Hashemian is president and CEO at Analysis and Measurement Services Corporation