I. Background In power grid, circuit breakers in switchgears are often the last line of defense when big systems must be protected from faults [1], sudden switchgear failures could cause long outages, huge economic losses and even present threats to the public safety. Based on field experience, the major causes of switchgear failure are loose or corroded metal connections, degraded cable insulation and external agents (e.g. dust, water) [2]. Due to ohmic loss at these weak points, the causes of switchgear failure are always accompanied with an increased thermal signature over time. With continuous thermal monitoring inside the switchgear, adequate data can be collected to make timely failure predication and prevention, especially for equipment deployed in harsh environment. II. Objective This paper presents the design of a passive radio frequency identification (RFID) sense tag, which measures temperature at critical spots of the switchgear and wirelessly (EPC C1G2 standard) transmits the data to the base station for real-time analysis. Compared with infrared imaging [2], surface acoustic wave (SAW) sensing system or fiber bragg grating (FBG) sensing system [1][3], no cables for power and communication are necessary, which avoids potential side effects like arcing in the grid after the addition of the sensor. The use of standard CMOS process results in a cost effective solution and the proposed passive wireless sensor can be easily retrofitted to existing switchgears with simple bolted connections. III. Passive Tag Design and Measurement Results Fig. 1 shows the proposed passive tag with temperature sensing capability. The power management unit in the chip harvests the incoming RF wave (860 ∼ 960 MHz) and sustains all the other on-chip building blocks (sensor, clock, memory, baseband, modem). The energy harvesting efficiency is in the range of 15%∼25% based on the operating mode of the tag. With 10 uW system power, the effective reading distance of this tag is 4.5 m ∼ 6 m. The on-chip temperature sensor adopts PNP bipolar as the sensing device, which has a temperature sensitivity of ∼2 mV/oC [4]. By using a triple-slope analog-to-digital converter (A/D), temperature-sensitive voltages are digitized and transmitted back to the reader after modulation by the modulator. Because there's no battery or other energy sources on the device, the power consumption of the tag, especially the sensor should be in the order of sub-mA to maintain the tag sensitivity. In this work, a passive integrator instead of an active one is used in the A/D, where its caused nonlinearity error is compensated by adding an additional nonlinear current into the temperature signal. The overall power consumption of the sensor is 0.75 uW and achieves 10 bit sensing resolution (0.12oC/LSB) within 10 ms conversion time, corresponding to a resolution FoM of 1.08x102 pJ√K2, which is among the most energy-efficient embedded sensor designs. Fig.2(a) shows the micro-photo of the designed passive RFID sense tag. Fig.2(b) shows its ceramic package, which can be readily installed on the spot locations with bolted connection. By designing the antenna with an additional ground layer, this tag is able to work in switchgear with full metal enclosure [5]. The measured tag sensitivity is -12 dBm. By measuring and calibrating multiple samples, the overall sensing precision of the tag is +/-1.5oC, which is enough for switchgear thermal monitoring, as shown in Fig.3(a). Thanks to the designed on-chip supply protection circuit, the sensor performance does not degrade much with the reading power or reading distance (received power ∝ or ∝ 1/distance2), as shown in Fig.3(a).IV. Conclusion The combination of passive RFID tags with sensors enables a lot of new applications and it help to bring embedded intelligence to the legacy power grid. The designed passive sense tag is of low-cost and with robust sensing performance after optimizing the tag at the system level and the use of low-power circuit design techniques. By re-designing the tag package, it can also be used in other applications like cold supply chain or highly flammable goods monitoring. Acknowledgement This work is in collaboration with Land Semiconductor Ltd., Hangzhou China and thanks Mr. Qibin Zhu and Mr. Shengzhou Lin for helping the measurement. Reference [1] G.-M. Ma et al., “A Wireless and Passive Online Temperature Monitoring System for GIS Based on Surface-Acoustic-Wave Sensor,” IEEE Trans. on Power Delivery, vol.31, no.3, pp. 1270 - 1280, June 2016. [2] Top Five Switchgear Failure Causes, Netaworld. [Online]. Available: http://www.netaworld.org/sites /default/files/public/neta-journals/NWsu10-NoOutage-Genutis.pdf, accessed Oct. 2017. [3] Fundamentals of Fiber Bragg Grating (FBG) Optical Sensing, NI White Papers. [Online]. Available: http://www.ni.com/white-paper/11821/en/, accessed Oct. 2017. [4] B. Wang, M.-K. Law and A. Bermak, “A Precision CMOS Voltage Reference Exploiting Silicon Bandgap Narrowing Effect,” IEEE Trans. on Electron Devices, vol. 62, no.7, pp.2128-2135, July 2015. [5] Chong Ryol Park and Ki Hwan Eom, “RFID Label Tag Design for Metallic Surface Environments,” Sensors, vol. 11, no. 1, pp. 938 - 948, Jan. 2011.


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