Surface Acoustic Wave Technology based Temperature Monitoring of High Voltage and High Current Switchgear Boxes Summary Safety is a big concern when it comes to managing power distribution systems. High voltage and high current switchgear boxes serve an important role in establishing points of control within the power distribution system. The high voltages and current flowing through switchgear boxes greatly increase the probability of electric arcing and arc blasts. An arc blast is characterized by intense amounts of heat, pressure, shrapnel, and molten copper. Great strides have been made in building arc resistant switchgears, nevertheless accidents have happened and the state of technology is far from completely eliminating them. One solution to this problem 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 switchgears. This application note explores how a SAW based solution can provide a reliable, safe and cost effective means of monitoring switchgear temperature. Electric Arc Explosions in Switchgear Boxes Switchgear boxes are designed to protect both the power distribution system and personnel operating them from adverse events and accidents. The high levels of voltage and current passing through switchgears combined with defective or aging components make electric arcs and arc explosions likely. The by-products of arc explosions include intense amounts of heat, pressure, shrapnel and molten copper. Figure 1 shows the after effects of such an explosion 1. While great strides have been made in improving the safety of switchgears, the large installed base of legacy systems make accidents as likely today as they were in the past. The problem is further compounded in emerging economies like China and India, where the burgeoning demand for power combined with aging equipment result in overloaded systems that are prone to failure. The weakest points or the points most likely to fail within switchgears include 2 : Cable connections Bus bar connections Isolator or circuit breaker connections There are a variety of circumstances that result in internal arcs in electrical switchgear. Often times this failure occurs when the breaker fails during routine switching or when clearing a through fault. Other causes of internal failure include partial discharge activity that weakens insulation over time 3. Monitoring the temperature of critical points within the switchgears can provide early warnings of imminent adverse events like arc explosions. An additional layer of safety is established by taking preventive actions when the temperature of critical switchgear components breach threshold limits. Page 1 Figure 1: After effects of an Arc Explosion
Infrared Thermography of Switchgears Infrared(IR) thermal imaging is one of the best methods presently available to monitor the temperature of switchgears. 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 and properly implemented can offer significant advantages. However, IR cameras also have distinct disadvantages associated with them: 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. Further, the costs associated with hiring or retaining and training a thermographer can be very significant. Open Doors: In order for IR cameras to work effectively, they need to have direct line of sight with target areas within the switchgear. This often requires the doors of the switchgear cabinet to be opened when taking IR images. Arc explosions have been known to occur in these instances and casualties have occurred. One solution to this problem is to create infrared inspection windows to allow the IR camera to take images without the need to open cabinet doors. However, the cost and logistics associated with retrofitting legacy systems with these windows can be 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. Time of Day: The time of day is an important factor in collecting thermal images. Readings in the early morning may avoid the effects of solar reflections and wind for switchgear boxes mounted in the open. These effects also apply to switchgears housed within buildings, especially in tropical climates. 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. 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 battery-less. Wired temperature sensors pose safety concerns in the high voltage/ current environment of switchgears; they also increase the probability of arcing. Further, 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 switchgears is untenable. Further, the typical number of temperature sensors required per box makes the cumulative environmental impact of batteries very significant. Figure 2: Thermal Image of Faulty Insulators within a Switchgear Surface Acoustic Wave (SAW) temperature sensors, which address the above mentioned disadvantages, provide a passively powered (no batteries), wirelessly interrogated solution that is ideally suited for switchgear temperature monitoring. Page 2
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 3. Figure 4 shows an actual wireless interrogator and a SAW temperature sensor. Reflector IDT Figure 3: Wireless SAW Temperature Sensing System Figure 4: Wireless Interrogator and SAW Temperature Sensor Page 3
A typical interrogation cycle includes the following steps: 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. SAW Based Switchgear Temperature Sensing A SAW based temperature measurement solution for switchgears would include SAW Temperature sensors mounted in different locations within the cabinet as shown in Figure 5 and a wireless interrogator capable of interrogating multiple SAW temperature sensors in parallel. The interrogator antenna is mounted within the cabinet, while the interrogator itself is mounted outside. Each sensor is connected to a sensor antenna that, in turn, is electromagnetically coupled to the interrogator antenna. A SAW based temperature measurement system addresses many of the disadvantages associated with IR Thermography: Cost: The cost of a SAW based temperature monitoring system is a fraction of what an IR Thermographic program costs. Open Doors: While direct line of sight needs to be maintained between the sensor antennae and the interrogator antenna, the location of the interrogator antenna makes it possible to take temperature measurements without ever opening cabinet doors. 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. Time of Day: Temperature measurements can be taken any time of the day since an absolute measurement of temperature is made with the SAW based system as opposed to a relative comparison as in the case of 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. Dust: SAW based temperature measurement solutions are significantly more immune to the effects of dust accumulation than are IR Thermographic solutions. Page 4
Wireless Temperature Sensors Wireless Interrogator Figure 5: Sensors mounted within Switchgear Cabinet References 1. 2. 3. 4. 5. http://www.magnaelectric.com/images/image001.jpg, March 2009 Robinson M., Infrared Inspection Windows: Where Do I Start?, Global Maintenance Technologies, 2007 http://www.magnaelectric.com/content/view/28/42/, March 2009 Application Note, Substations and Switchgear, http://support.fluke.com/, 2009 http://www.goinfrared.com/images/gallery/ir_0406.jpg, March 2009 2009 Vectron International All Rights Reserved SenGenuity 267 Lowell Road, Hudson NH 03051, USA Tel: 1.603.578.3025 Fax: 1.603.578.4060 www.sengenuity.com Page 5