NDE | Cable ageing

Testing old cables

3 September 2012



Different types of electrical tests can reveal defects in cables or in their insulation, without needing to physically remove or detach the cable. One vendor is attempting to integrate many of these tests into a single device. By H. M. Hashemian


Cables are the link between a nuclear power plant’s control and safety systems through the signals they send to plant operators, control equipment, and safety systems. Accident situations expose cables to heat, humidity, thermal and mechanical shock, and radiation, which can cause them to lose their desired characteristics earlier than expected and threaten the safety of the plant.

As the 2011 Fukushima tragedy made clear, it’s critical for the plant maintenance personnel of operating nuclear power plants to know whether monitoring signals are reliable as well as the cause and exact location of any signal anomalies. They also need to know whether a signal problem originates from the cable, the connectors, the end device (a sensor, a motor, a valve, etc.) or a problem in the plant process.

The condition and reliability of cable insulation, conductors, or connectors must therefore be determined through periodic cable testing and cable condition monitoring to identify, locate, and repair problems in important cables. As more nuclear plants outlive their original life spans, awareness of the importance of cable ageing management has grown.

In some countries (for example, Hungary and its Paks nuclear power plant), ageing management strategies have taken the form of replacing some of its critical cables. However, replacing cables, except in short sections, is expensive, radiation-intensive, and typically impractical. As a result, utilities operating nuclear power plants are not adopting wholesale replacement of cables as a strategy. Rather, they are searching for ageing management techniques that can identify cable problems and areas where maintenance or replacement is needed.

Tests

Over the years, a wide range of cable testing methods have been developed. For example, inductance, capacitance, and resistance (LCR) are facets of impedance and they quantify the severity of cable conductor, insulation, connector, or end device faults. However, they do not provide the distance to a fault.

Insulation Resistance (IR) quantifies the quality of cable insulation by energizing the cable conductor and measuring the leakage of current through degraded insulation.

Time Domain Reflectometry (TDR) is an electrical test technique that measures the distance to and severity of a fault in a cable conductor, connectors, or insulation using techniques that involve the injection of a test signal through the cable and measurement of the reflected wave. The TDR test is typically performed using a pulse generator and a recorder or oscilloscope. The TDR technique involves sending an electrical signal with a fast rise time through the cable and measuring its reflection to identify the location of any impedance change in the cable and the end device (load). Reflected voltage waves occur when the transmitted signal encounters an impedance mismatch or discontinuity in the transmission line (Figure 1). The resulting wave is captured in the time domain and is a ratio of the incident signal and the reflected signal expressed in terms of Reflection Coefficient (RHO). The speed that the incident and reflected signals travel in the transmission line is known as the velocity of propagation (VP), which is a percentage of the speed of light in a vacuum and is determined by the relative permittivity or dielectric constant of the media’s insulating material. Distance to any impedance change can be determined by multiplying the VP of the transmission line by half the time it takes for the incident wave to travel to the impedance change and get reflected back to the generator. TDR test results are usually presented in terms of a plot of the reflection coefficient (RHO) versus distance and is referred to as the TDR trace or TDR signature (Figure 2).

The author’s organization has helped to develop an additional technique that is closely related to TDR: reverse time domain reflectometry (RTDR). RTDR is used primarily to test the quality of the shielding around the conductor of a coaxial or triaxial electrical cable (due to their physical construction) by simulating the coupling of electrical noise signals into a signal transmitted on an instrument cable. These type of cable systems are often used to transmit low-level signals where the effects of unwanted external noise must be minimised. The electrical noise interference typically couples at poor connections or terminations in the cable circuit that tend to degrade through the ageing process. However, such interference may also result from damage to the cables or inherent properties of any inline devices. RTDR detects the location of degraded connectors or cable shields by using time delays to determine where the electromagnetic interference (EMI) couples into the cable system.

Much like the TDR method, RTDR is performed by sending a fast-rise-time, rising-edge electrical pulse with a slowly decaying falling edge onto the shield of a coaxial or triaxial cable. Simultaneously, the centre conductor is monitored in the time domain for any capacitive coupling of the pulse. As the pulse travels down the shield, its high-frequency components will couple to the centre conductor if there is a loss of the 360° shielding where the shield and centre wire appear to be parallel conductors. If a return signal is received, the time delay determines the distance to the point of coupling.

Similar to the TDR test, location can be determined by multiplying the VP of the transmission line by half the time it takes for the incident wave to travel to the coupling point and back to the generator. Standard TDR signatures are typically used in conjunction with RTDR to determine the location of cable connections. The RTDR test is particularly important in I&C systems that have low signal levels (<100 mV) that are easily affected by electrical noise intrusion such as source-range nuclear instrumentation systems. Figure 3 shows an RTDR trace detecting an EMI coupling point.

A relatively new twist on TDR is called frequency domain reflectometry (FDR). FDR outperforms TDR in detecting problems with the cable insulation material such as a discontinuity, large cracks, or physical/mechanical damage on a cable. The size of the wave that the TDR method sends down a tested cable is limited by the bandwidth of the pulse and sampling circuitry. Because it sends only very broad DC pulses, the TDR method can locate only DC open- or short-circuit conditions. In contrast, the FDR technique uses a selected set of much smaller or narrower bandwidth frequencies, which enables it to locate radio frequency (3 kHz-300 GHz) faults in cables. There are three types of FDR, based on the sine wave property they measure—namely, frequency, magnitude, and phase—to calculate distance.

Typically, the TDR and FDR methods are complementary, and both produce similar signatures or traces. The most significant differences are in the method used to inject the test signal in the cable and in the method used to process the resulting data.

Discoveries such as the “time-frequency domain reflectometry apparatus and method” [1] show promise in improving the diagnostic capabilities of TDR and FDR-type methods.

The following techniques can be integrated with the above tests to distinguish between cable faults and end device faults.

  • Loop current step response (LCSR) is an in-situ test of the response time of an RTD or thermocouple. The test is performed by heating the sensing element by applying a small electric current to the sensor extension leads and measuring the time for the sensor to respond to the step change in temperature. LCSR has also shown to be an effective method for separating cable problems from sensor problems.
  • Noise analysis tests are performed using passive data acquisition techniques from the instrument cabinets in the control room during normal operation of a sensor as installed in the plant process. This technique has been used to measure the dynamic characteristics of end devices such as pressure transmitters and neutron detectors and to distinguish between sensor problems and cable or connector problems.
  • Current to voltage (I-V) curves are produced by applying increasing, incremental voltages to the end device under test while measuring the device’s current. Analysis of the IV test data can often reveal age-related problems in end devices such as neutron detectors.

Despite the variety and relative effectiveness of these methods for testing cable ageing, no individual testing method is adequate in itself. As the U.S. Nuclear Regulatory Commission’s Draft Regulatory Guide 1.218 (2012) observes, “no single, nonintrusive, currently available condition monitoring method can be used alone to predict the survivability of electric cables under accident conditions.” Traditional techniques like elongation-at-break, compressive modulus, and density tests, reveal only the condition of the specific tested location. Similarly, many conventional cable test methods can only determine if an electrical problem exists. Others, such as TDR, FDR and RTDR, can identify the fault location within the length of cable, but may not differentiate whether the problems are in the connection or the end device; additional tests are normally required (for example, LCSR, noise analysis, I-V).

Integrated cable testing system

Through two research projects sponsored by the U.S. Department of Energy, the author’s organisation has developed, tested, validated and sold four portable integrated cable testing system (ICTS) prototypes to Duke Power Company. Utilising an 80-channel external multiplexer accessory that harmonizes both electrical and mechanical test methods for low-voltage cables

(<1000 V), this ICTS prototype successfully integrates 8 of 10 planned electrical cable test techniques: voltage, LCR, IR, TDR, RTDR, waveform capture, I-V, and DC resistance. (The final two techniques, FDR and DTDR, are under development and will be integrated by the time of the system’s delivery in 2013.)

To unite these various modules performing these disparate tests, existing testing modules manufactured by National Instruments (NI) were evaluated for each test, new custom modules were created wherever NI hardware was unavailable, and proprietary software was written to control the test sequence and analyse the data.

The ICTS software-based characterization module characterizes cable condition and the end device through the following sequence of test measurements: voltage waveform and noise voltage from digital multimeter–voltage (DMM-V); impedance direct current resistance (RDC), alternating current resistance (RAC), and inductance/capacitance (L/C) at 100 Hz, 1kHz, and 10 kHz from LCR meter; IR from power supply and DMM-V; TDR and dynamic TDR from TDR pulser and digitizer.

During data acquisition five separate software modules are called on depending on typical testing: calibration, characterization, waveform, I-V, or FDR. Each software module communicates with software in the test equipment over a network connection. As each test is performed, the data is sent back to the software module in the control computer. After the test finishes, the data is passed to the main program, WinCharm, for analysis.

 


Author Info:

H.M. Hashemian, founder and president, AMS Corporation, AMS Technology Center, 9119 Cross Park Drive, Knoxville, TN 37923 USA

This article was originally published in the August 2012 issue of Nuclear Engineering International

 


References

[1] "Application of joint timeâ€"frequency domain reflectometry for electric power cable diagnostics," by J. Wang, P.E.C. Stone, Y.-J. Shin, and R.A. Dougal, IET Signal Processing, doi: 10.1049/iet-spr.2009.0137, ISSN 1751-9675

[2] "Time-frequency domain reflectometry apparatus and method," United States Patent 7337079, Inventors: Park, Jin-bae, Shin, Yong-june, Yook, Jong-gwan, Powers, Edward J., Song, Eun-seok, Kim, Joo-won, Choe, Tok-son, Sung, Seung-hoon, Application Number: 10/519414, Publication Date: 02/26/2008, Filing Date: 07/07/2003 http://www.freepatentsonline.com/7337079.html


Figure 1 Figure 1
Figure 2: A TDR signature trace, based on a posited cabling installation Figure 2: A TDR signature trace, based on a posited cabling installation
Figure 3: Sample RTDR trace Figure 3: Sample RTDR trace


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