Results of a two-year field demonstration, sponsored by the Electric Power Research Institute, indicate that magnetostrictive sensors might provide a possible alternative to linear variable differential transformers (LVDTs) for monitoring turbine valves in power generation applications.
In this field demonstration, the magnetostrictive sensors performed reliably and with results identical to the LVDTs — even continuing to produce results when the LVDT failed. This technology has several potential advantages since magnetostrictive sensors are able to operate without field calibration, are capable of being mounted in a variety of configurations and eliminate some of the failure modes associated with current installations of LVDTs.
LVDT failures
LVDTs are electrical transformers used for measuring linear displacement, and are commonly deployed for position feedback in servomechanisms. Failures of LVDT in steam turbines are a leading cause of plant trips. More than 200 LVDT failures have been reported to the Institute of Nuclear Power Operations
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LVDT sensors also require periodic recalibration, because they are prone to drift. In addition, because of the nature of the way that LVDTs work, the calibration uses a best-fit curve and has varying degrees of nonlinearity, requiring periodic recalibrations for critical applications.
An industry effort is under way to consider alternative technologies to LVDTs. A viable replacement for LVDTs would need to meet, at a minimum, the following conditions:
- Perform more reliably and/or eliminate the failure mode associated with LVDTs;
- Provide the same or better output than LVDTs, in terms of responsiveness and sampling rates;
- Withstand a power plant environment; and
- Be capable of being applied within current instrumentation configurations.
Magnetostrictive sensors
One possible alternative to LVDTs is a magnetostrictive sensor. Magnetostriction is the property of ferromagnetic materials to change shape in the presence of a magnetic field. The most familiar example of magnetostriction is the hum from a transformer. The electromagnetic fields created by AC changes the shape of the ferrous components inside a transformer relative to the AC frequency.
Because the deformation occurs at the AC frequency, the resulting vibration occurs at a harmonic to the AC frequency (normally 50 or 60 Hz), and if the vibration is within sonic range, a hum is created.
Magnetostrictive sensors use the magnetostrictive property of a ferromagnetic measuring element to determine the position of a magnet relative to a fixed detector. Figure 1 shows how magnetostriction is used in a position sensor. A movable position magnet generates a direct-axis magnetic field in the waveguide. When a current pulse passes through the waveguide, a magnetic field is created radially around the waveguide. The interaction between the magnetic field of the waveguide and position magnet generates a strain pulse. This strain pulse travels at a constant sonic speed from its point of generation, the measurement point, to the end of the waveguide, where it is detected by the sensor electronics.
The position of the magnet is determined with high precision and speed by accurately measuring the time elapsed between the application of the interrogation pulse and the arrival of the resulting strain pulse.
Magnetostrictive sensors are used in hydraulic cylinders and in several process and manufacturing environments, such as plastic injection molding machines and sawing equipment in saw mills. In power generation, the sensors are used to provide position feedback for valve position and blade pitch.
Advantages
Magnetostrictive technology might provide several possible advantages to LVDT technology in power generation applications. First, because magnetostrictive sensors are resistant to shock, vibration and other harsh environments, they could meet utility reliability and durability requirements.
