As a key material to perceive the discharge and vibration signals of power equipment, piezoelectric materials have been commonly used in the fields of vibration monitoring, discharge detection, flaw detection, temperature measurement, voltage sensing and so on.
Piezoelectric materials are used in a variety of piezoelectric sensor parts, the core of which is the conversion of mechanical energy and electric energy: Piezoelectric materials by mechanical vibration (piezoelectric vibration sensor), sound waves (acoustic pressure sensor), such as mechanical lattice deformation, external force caused by the change of polarization state, output of sensor signal, or through the piezoelectric materials under electric fields of measurement to reflect on the deformation of the electric field size (piezoelectric voltage sensor).
The working principle of piezoelectric voltage sensor is primarily based on the inverse piezoelectric effect, which converts the electrical signal applied to the piezoelectric material into the displacement or deformation signal, and then further detects through other ways, so as to accomplish the voltage signal measurement.
Piezoelectric voltage sensors can be divided into piezoelectric voltage sensors based on stress detection, piezoelectric voltage sensors based on glow detection and piezoelectric voltage sensors based on capacitance value detection.
1 Piezoelectric voltage sensor based on stress detection
In the early stage of piezoelectric voltage sensor research, K.Awamura et al. used pressure sensors to detect the electrically induced strain of piezoelectric materials. The peak detection voltage can reach 26kV, the measurement error is less than 2%, and the frequency measurement range is 0~ 2.5khz. Nevertheless, the detection range and accuracy of this type of sensor are easily limited by additional pressure sensors, and additional power supply is required, which increases the complexity of voltage sensors. It is hard to meet the requirements of miniaturization, passivity and strong anti-interference ability of current sensors, and it is difficult to be applied in practice.
2 Voltage sensor based on light detection
Compared with stress detection, it is more useful to calculate the deformation of piezoelectric materials by utilizing passive optical devices. K.m. Boohnert et al. combined quartz piezoelectric crystal with dual-mode fiber, and measured the deformation of piezoelectric material by detecting the phase change of coherent light in the fiber. The frequency measurement range was 50HZ-11khz, and the measurement voltage was as high as 520kV, but the equipment volume was comparatively large. Other researchers combined PZT and other high-voltage piezoelectric ceramic polycrystal and grating devices to convert the piezoelectric material deformation, which is hard to accurately measure, into the grating center wavelength change for detection, which successfully enhanced the test accuracy.
G. Fusiek et al. used multiple PZT piezoelectric ceramic sheets of 4mm thickness to form a laminated structure to amplify the displacement of the piezoelectric ceramics under the same voltage. The maximum measuring range of the sensor is 5kV, and the frequency measurement range is 50Hz~20kHz. The researchers used an external aluminum structure to transfer the displacement of the PZT to the FBG, which decreased the requirement on the width of the single PZT sheet, but the multiple transfers of the displacement may insert additional measurement errors.
3 Voltage sensor based on capacitance value detection
In addition, Xue Fen et al. superimposed two layers of PVDF piezoelectric films with opposite polarization directions and fixed ends into the upper electrode of the capacitor, and added the fixed lower electrode of the capacitor to form the piezoelectric voltage sensor. When the applied electric field changes, the structure of the PVDF film bends and the capacitance plate changes, which leads to the change of the capacitance value. The information of the applied electric field can be deduced by calculating the capacitance value in real time.
Although the resonant frequency of stretching vibration of piezoelectric polymer film (PVDF) in the thickness direction is considerably higher than that of ordinary piezoelectric ceramic, and the measurement range of almost 10MHz wide band response and 22kV/cm can be attained, the piezoelectric coefficient is much smaller than that of ordinary piezoelectric ceramic, and the deformation is mostly at the nm level, even if the use of optical devices is hard to detect it. Consequently, the difficulty of utilizing PVDF for voltage/electric field sensing research is to convert the micro-nano deformation into other measurable and easy physical quantities.