275 / 2018-09-25 11:13:53

Integrated Detection of Temperature and Partial Discharge on Cables Based on FBG

Cables; temperature; PD; FBG; integrated detection

Cable is an important part of power system, and both temperature and partial discharge (PD) are main indicators to evaluate the condition of cables. The integrated detection system of temperature and PD on cables based on fiber Bragg grating (FBG) realizes the goal of distributed measurement of temperature and PD on cables via the same system, which simplifies the whole detection system.
The temperature and PD integrated FBG detection system for cables consists of a tunable laser, an optical coupler, many FBGs, a photo detector, a data acquisition card and a computer. These FBGs with different Bragg wavelengths are connected in series and distributed in different positions on cables, forming a quasi-distributed detection system.
The reflection spectrum of every FBG is got by the tunable laser scanning FBGs. Because the reflection spectrum of the FBG belongs to Gaussian distribution and its peak corresponds to the Bragg wavelength of the FBG, we calculate the Bragg wavelength by the peak. Then, temperature can be obtained according to the calibrated correspondence between the Bragg wavelength of the FBG and temperature. However, the ultrasonic vibration generated by PDs shifts the reflection spectrum of the FBG and makes the Bragg wavelength change randomly. Thus, the temperature on cables can’t be measured accurately. For this reason, we use the tunable laser to scan FBGs slowly many times to eliminate the influence from PDs. Of course, the time interval of each scan is designed specially to deal with the period characteristic of PDs. The exact temperature of every measuring point can be obtained utilizing the mean reflection spectrum of every FBG.
The reflection spectrum of the FBG has approximate linear regions. Before PD being detected, the wavelength corresponding to the linear region where slope is maximal is calculated by the computer, and set as the output wavelength of the tunable laser. After a PD occurs, the Bragg wavelength of the FBG is shifted by the ultrasonic wave generated by the PD. As the output wavelength of the tunable laser is unchanged, the Bragg wavelength shift causes a fluctuation in the intensity of the light reflected off the FBG. With the help of the photo detector, the fluctuation in optical intensity is converted into an electric signal displayed on the computer. The Bragg wavelengths of all FBGs are set as the output wavelengths of the tunable laser successively to detect the PD of every measuring point, achieving the distributed detection of PD on cables. But the change of temperature will shift the Bragg wavelength, and make the output wavelengths of the tunable laser deviate from the maximal slope of the reflection spectrum, resulting in a decrease in sensitivity. For ensuring the output wavelengths always correspond to the maximal slope of the reflection spectrum at different temperatures, the output wavelengths of the tunable laser are recalculated just before the PD measurement.
An experiment was conducted to verify the feasibility and sensitivity of the integrated detection system of temperature and PD on cables based on FBG. FBGs were glued on the oversheath of the cable by epoxy resin adhesive, and different sizes of current was injected into the cable for getting different temperatures; a spine fault was made on the cable to produce PDs, and a partial discharge detector was added into the experiment system as a reference of the FBG detection. The experiment results show the system can detect the change of temperature and PD on cables. The system is able to provide an accurate and comprehensive dependence for evaluating the operation status of cables.