Smart Grid Sensing Solutions Leveraging SAW Technology

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1 Smart Grid Sensing Solutions Leveraging SAW Technology Summary Reliability is a big concern when it comes to managing power distribution systems. Switchgear, which serve as important points of control within a power distribution system are amenable to failure if not closely monitored and controlled. Increased loads can greatly stress switchgear. The resulting increases in temperatures of critical switchgear components can cause significant degradation of metal contacts and insulation. In turn, this increases the probability of internal short-circuits, which, if unchecked can lead to system failure. The problem is especially relevant in emerging economies like India and China where the burgeoning need for power greatly stresses legacy and outdated electric power infrastructure. One solution is to monitor the temperature of switchgear boxes to elicit early warning signs of imminent failure. Conventional methods of monitoring switchgear temperature are expensive and not entirely effective. Surface Acoustic Wave (SAW) technology can provide a passively powered (battery-less), wireless temperature measurement solution that is ideally suited for switchgear. This note explores how a SAW based solution can provide a reliable, safe and cost effective means of monitoring switchgear temperature. Impact of Increased Loads on Switchgear Switchgear establish critical points of control within an electric power distribution system. A typical switch consists of a combination of electrical disconnects, fuses and/or circuit breakers that is used to isolate electrical equipment 1. A part of the electrical energy flowing through the switchgear is converted into heat due to the inherent resistance of the bus bars but also due to resistive contacts and loose connections. High temperatures, sustained over long durations of time, cause degradation in metal contacts, insulators and other critical switchgear components. This creates a vicious cycle of thermally induced degradation leading to internal shortcircuits and further increases in temperature. The weakest points or the points most likely to fail within switchgear include 2 : Cable connections Bus bar connections Isolator or circuit breaker connections If unchecked, the failure of these components can lead to overall system failure. Therefore, it is important to ensure that the temperature of critical switchgear components remain within established limits and that if these limits are exceeded, corrective action be taken. Infrared Thermography of Switchgears Infrared(IR) thermal imaging is one of the best methods presently available to monitor the temperature of switchgear. Hot Spots within the switchgear are readily identified by taking thermal images of cable connections, insulators and bus bar connections. Some IR cameras offer the ability to fuse a visual image with an infrared image making it easier to isolate fault areas and take corrective action. The non-contact testing afforded by IR cameras address a variety of safety concerns with respect to operating in a high voltage/high current environment. Page 1

2 However, IR cameras also have distinct disadvantages associated with them: Spot Assessments: IR cameras capture the temperatures of switchgear components at a moment in time and do not allow for the continuous monitoring of temperature. They do not provide for the detection of adverse events (or the prelude to adverse events) between image captures. Cost: IR cameras have very significant costs associated with them. It is not uncommon to pay in excess of $30,000 for an industrial grade IR camera. It also takes an experienced and trained operator to correctly set up and use an IR camera. The costs associated with hiring or retaining and training a thermographer can be very significant. Reference Images in Winter and Spring: In order to determine if a system component is unusually hot, the thermographer typically compares the obtained image with a reference image taken when the system is operating under normal conditions. It is recommended that reference images be created during winter or early spring, when the consumption of electricity is relatively low. There is a large demand for electricity during summer. It can therefore take a significant amount of calendar time to set up a thermal imaging library for a thermographic maintenance program. Dust: IR cameras can provide misleading readings if sufficient amounts of dust accumulate over switchgear components. The true temperature of joints can literally be hidden under layers of dust. The use of sensors is an alternative to measuring temperature with IR cameras. However, these sensors have to be wireless and batteryless. Wired temperature sensors pose safety concerns in the high voltage/current environment of switchgear; they also increase the probability of arcing. Battery powered sensors pose the unique challenge of requiring the replacement of batteries at regular intervals. While a limited number of switchgear installations are manageable, battery maintenance in a large installed base of switchgear is untenable. The typical number of temperature sensors required per switchgear box makes the cumulative environmental impact of battery powered sensors very significant. Surface Acoustic Wave (SAW) temperature sensors, which address the above mentioned disadvantages, provide a passive (no batteries), wireless solution that is ideally suited for switchgear temperature monitoring. Figure 1: Thermal Image of Faulty Insulators within a Switchgear Page 2

3 Wireless Surface Acoustic Wave (SAW) based Temperature Sensing Traditional methods of measuring temperature have relied on the temperature dependence of resistance (thermisters or Resistance Temperature Detectors - RTDs), the temperature dependence of fluid expansion (thermometers) and the emission of infrared radiation from heated objects (IR thermometers). SAW based temperature sensors on the other hand take advantage of the piezoelectric effect. SAW based temperature sensing, which is described in detail below, involves electrically inducing a surface acoustic wave into a piezoelectric material and then reconverting the energy of the wave (influenced by the temperature to which the sensing element is exposed) back into an electrical signal for temperature measurement. One significant advantage of SAW devices is their low power consumption, which makes them very amenable to wireless interrogation. A wireless SAW based temperature sensing solution consists of a wireless interrogator (RF Transceiver) electromagnetically linked to a SAW sensing element as shown in Figure 2. Reflector IDT A typical interrogation cycle includes the following steps: Figure 2: Wireless SAW Temperature Sensing System The wireless interrogator generates a Radio Frequency (RF) signal which is transmitted by the interrogator antenna. This signal is received by the sensor antenna and is used to induce a surface acoustic wave in the piezoelectric sensing element via an Interdigital Transducer (IDT). The IDT consists of two interlocking comb-shaped metallic patterns applied to a piezoelectric substrate for the specific purpose of converting microvoltages to surface acoustic waves and converting surface acoustic waves back into microvoltages. The surface acoustic wave is reflected back to the IDT by structures called reflectors, thereby creating a resonator. The resonant frequency of the surface acoustic wave resonator is influenced by the temperature to which the sensing element is exposed. It is this phenomenon that is exploited to obtain a temperature measurement. The IDT converts the natural oscillation of the surface acoustic wave resonator into an RF signal, which in- turn, is transmitted back to the interrogator via the same antenna set. A change in the frequency of the received RF signal is indicative of a change in the measured temperature. Page 3

4 SAW Based Switchgear Temperature Sensing A SAW based temperature measurement solution for switchgear includes SAW Temperature sensors mounted in different locations within the switchgear box, as shown in Figure 4, and a wireless interrogator capable of interrogating multiple SAW temperature sensors in rapid sequence. The interrogator antenna is mounted within the box, while the interrogator itself is mounted outside. Each sensor is connected to a sensor antenna that, in turn, is electromagnetically linked to the interrogator antenna. A SAW based temperature measurement system addresses many of the disadvantages associated with IR Thermography: Continuous Monitoring: A SAW based temperature measurement solution allows for the continuous monitoring of temperature and thereby provides for the ability to continually monitor the switchgear for adverse events or the prelude to an adverse event. Cost: The cost of a SAW based temperature monitoring system is a fraction of what an IR Thermographic program costs. No Seasonal Effects: The sensors can be calibrated at any temperature within their stated operating range and so seasonal effects do not affect a SAW based temperature monitoring solution. Further, a SAW based system moves away from a reliance on reference images (and the calendar time disadvantages) associated with IR Thermographic solutions. Dust: SAW based temperature measurement solutions are significantly more immune to the effects of dust accumulation than are IR Thermographic solutions. Wireless Temperature Sensors Wireless Interrogator Figure 3: Sensors mounted within Switchgear Box Page 4

5 TempTrackr SAW Sensor System SenGenuity has developed a SAW based temperature sensing solution that successfully addresses the unique needs of switchgear temperature monitoring. The TempTrackr SAW Sensor System is capable of wirelessly monitoring six to twelve passive SAW temperature sensors. Temperature data can be accessed either via a PC based application or via the CAN network protocol for feedback to control systems. The key solution differentiator of the TempTrackr system is that it measures temperature at the source of expected failure: the bus bar connection, the cable connection and the circuit breaker connection. Unlike other solutions that measure the ambient temperature of the air within the switchgear box, the TempTrackr system measures the temperature at the most vulnerable points within the switch. The core value of the system lies in its ability to provide early warning signs of expected failure by continuously monitoring the temperature of critical switchgear components. Figure 4: The TempTrackr SAW Sensor System Contact Information Please do not hesitate to contact our Application Engineering group at support@sengenuity.com for more information about the TempTrackr SAW Sensor System. References , June 2009 Robinson M., Infrared Inspection Windows: Where Do I Start?, Global Maintenance Technologies, March 2009 Application Note, Substations and Switchgear, March 2009 DISCLAIMER Vectron International reserves the right to make changes to the product(s) and or information contained herein without notice. No liability is assumed as a result of their use or application. No rights under any patent accoumpany the sale of any such product(s) or information Vectron International All Rights Reserved - Rev. B SenGenuity 267 Lowell Road, Hudson NH 03051, USA Tel: Fax: Page 5