Timely detection of the incipient fault of a power transformer is of utmost importance to prevent potential malfunctioning. Monitoring the condition of the transformer insulation oil is the essential step as it contains most of the transformer’s health diagnostic information. Thus, the temperature of the oil is considered the most crucial parameter that has to be monitored continuously in real-time. Distributed optical fiber sensors for temperature monitoring has various advantages over traditional methods. In this study, an optical fiber-based distributed temperature measurement of insulation oil of a fully energized 100 kVA commercial distribution transformer is demonstrated. Two types of techniques were deployed to monitor the temperature of the transformer inside and outside of the tank using a distributed Optical Frequency Domain Reflectometry (OFDR) and a quasi-distributed Fiber Bragg Grating (FBG) sensor array respectively. The insulation oil temperature was monitored with different load conditions in a distributed fashion and compared with the conventional method of single point thermocouple and infrared thermal imaging. The test results show very good agreement between the conventional methods and proposed distributed fiber temperature sensors.

An active magnetic bearing (AMB) rotor system has such advantages as no friction and active vibration control. However, harmonic current caused by mass imbalance and sensor runout of magnetically suspended rotor system will lead to harmonic vibration. The harmonic current frequency is changed with the variation of rotation speed. The harmonic current suppression at variable rotation speed is a challenging research topic. In order to suppress harmonic current in AMB whose frequencies are integer multiples of the rotation frequency of rotor and to maintain system stability, multiple phase-shift notch filters are proposed in this paper. The closed-loop stability is ensured by adjusting the phase at variable rotation speed. Experiments are carried out on vibration measurement table and magnetically suspended flywheel prototype to show the effectiveness of the proposed method.

This paper investigates a hybrid fractional repetitive control (HFRC) scheme for magnetically suspended rotor systems to suppress harmonic current caused by mass imbalance and sensor runout. With parallel structure, HFRC consists of a frequency-adaptive dual-mode repetitive controller (DMRC) and a phase-shift notch filter. By introducing fractional delay filters, the proposed DMRC provides frequency adaptability to eliminate odd and even harmonics independently at fixed sampling rate. Its delay time is halved compared with the conventional repetitive control scheme, and the control gains can be weighted to ameliorate system dynamic response according to harmonics distribution. Moreover, as the domination of control current, fundamental frequency current is additionally rejected by the notch filter; thus, achieving smaller overshoot and faster suppression performance. An improved phase compensator is designed in HFRC to expedite current convergence rate while stabilizing the overall system. Experimental results on a magnetically suspended flywheel demonstrate the advantages of the proposed method.

This article investigates using optical fibers with enhanced backscattering profiles to improve distributed fiber sensor performance and reduce instrumentation costs. Using a femtosecond (fs) laser direct writing technique, the Rayleigh backscattering profile of a standard telecom fiber was enhanced by more than 40 dB to improve the signal-to-noise ratio (SNR) for optical frequency domain reflectometry (OFDR). The enhanced backscattering signals enable effectively distributed strain measurements using a low-cost tunable laser (TL). Median filtering is applied to denoise cross correlation results to further improve measurement outcomes. Results presented in this article show that a TL with a 1-nm tuning range, which is far less than the tuning range used in commercial OFDR interrogators, can perform effectively distributed strain measurements using sensing fibers with enhanced backscattering profiles. The sensing fiber with over 40-dB backscattering enhancement achieved 4.8-cm spatial resolution in strain measurements with a root mean square accuracy of less than 2.70 $\mu \varepsilon $ when 10–50 $\mu \varepsilon $ were exerted to the sensing fiber. Results presented in this article reveal both the potential and limitations of sensing fibers with enhanced backscattering for OFDR-based distributed fiber sensors.

This paper demonstrates high-frequency dynamic strain measurements using Fiber Bragg Grating (FBG) sensors embedded in metal parts. Using an ultrasonic additive manufacturing (UAM) process, FBGs inscribed in polyimide coated optical fibers were embedded in aluminum parts. An electromagnetic shaker was used to exert dynamic events on the embedded FBG sensors with frequencies from 1–10 kHz. The high-speed interrogation of FBG sensors was accomplished using a tunable vertical-cavity surface-emitting laser (VCSEL) and a high-speed interrogation system sampling at 120 kHz. The strain response measured by the FBG sensors was compared with real-time measurements using a laser velocimeter. Finite Element Analysis was performed to simulate responses to both static strain and high-frequency dynamic strain. Results show that strains as small as 2.5 μ can be resolved at frequencies up to 10 kHz.

Magnetically suspended rotor has the characteristics of no friction and active vibration control. The mass unbalance and sensor runout of the rotor induce harmonic current, which will cause harmonic vibration. The ratio of the sampling rate to the fundamental frequency of harmonic current is often not an integer. The existing repetitive control (RC) for the suppression of harmonic current in magnetic bearing is applicable for the integer case. Harmonic current suppression performance is degraded drastically if the ratio is fractional. In this paper, combined finite-impulse response (FIR) and fractional-order RC is proposed. FIR filter and fractional-order RC are combined to perform harmonic current suppression at arbitrary frequency. Phase compensator is designed to enhance the stability of magnetically suspended rotor system. Experimental results are given to verify the effectiveness and superiority of the proposed method.