Int J Thermophys (2011) 32:2343–2350DOI 10.1007/s10765-011-1080-1

Temperature Measurements on Hot Spots of PowerSubstations Utilizing Surface Acoustic Wave Sensors

M. A. M. Cavaco · M. E. Benedet · L. R. Neto

Received: 6 March 2010 / Accepted: 22 August 2011 / Published online: 21 September 2011© Springer Science+Business Media, LLC 2011

Abstract In several applications in the field of metrology, the direct connection ofthe sensor element with the respective signal-processing unit of the measurement sys-tem is not trivial. It can be mentioned, as an example, the measurement of hot pointsin electric power substations because of the high electrical potential. To solve thatproblem, two alternatives were studied, one using active surface acoustic wave (SAW)sensors and other using passive SAW tags. For the passive sensor, a SAW radio-fre-quency identification (RFID) temperature detector was used. That technology is widelyapplied for typical transport identification (grain transportation, road traffic control),but its application in the field of metrology is innovative. The variation in temperaturemakes an alteration in the characteristics of the piezoelectric material of the SAWmatrix, changing mostly the resonance frequency. Using SAW–RFID, the problem ofmeasuring temperature basically is directed to the identification of the frequency ofresonance of the SAW. The use of active SAW sensors has been demonstrated to bemuch more satisfactory for the solution of such a problem because of the limitationin the range of the passive sensors.

Keywords Active sensors · Hot spots · Passive sensors · Radio-frequencyidentification (RFID) · Surface acoustic wave (SAW)

1 Introduction

With the advancement in sensor technology measurements, applications that werepreviously unresolved begin to be implemented. In this direction are the measurementapplications that do not allow physical connection between the transducer and the

M. A. M. Cavaco (B) · M. E. Benedet · L. R. NetoFederal University of Santa Catarina, Florianópolis, Brazile-mail: cavaco@labmetro.ufsc.br

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signal-processing unit. The temperature measurement in electrical connectors used indistribution substations of electric power is one of them.

The use of radio waves to communicate between the transducer and signal-process-ing unit has been shown to be a good alternative for such situations. However, to usethese channels of communication, some considerations should be highlighted, amongthem we can mention: electromagnetic interference, signal-to-noise ratio, carrier fre-quency, and transmission range [1–3].

One of the promising techniques to be employed in the area of wireless measure-ments is the use of sensors that work according to a variation from the “radio-frequencyidentification” (RFID) technique. This technology is known in the literature as a sur-face acoustic wave (SAW), which is used to measure the temperature in a passive way(with no power). The operating principle is based on the theory of radio detectionand ranging (RADAR) and changes in the behavior of piezoelectric materials withtemperature [4–8].

Although the most common use is in identification applications, the fact that thepiezoelectric material forming the SAW label is affected by temperature and strainmakes this technology an important tool for measurements that aim to decouple thetransducer from the signal-processing unit.

The measurement of temperature in hot spots is crucial for evaluation of operatingconditions and to determine the best time for maintenance by the power company.Currently, this measurement is performed with FLIR cameras (infrared) that, besideshaving a high measurement uncertainty and high cost, are used once a year, and inother words do not monitor the substation on a daily basis.

With that comes the opportunity to implement the SAW–RFID technology formonitoring the temperature on hot spots. Using this technology coupled with a mobilephone via general packet radio service (GPRS), one can monitor temperature at thesehot points remotely and continuously.

Parallel to the development of passive sensors, adjustments were implemented andtests were carried out with active sensors. The main difference of active sensors incomparison with the passive ones is that the former are provided with a power supply.Constructive modifications were carried out in order to use these sensors in the harshenvironments of an electrical power substation. Tests have shown the feasibility andlimitations of both methods.

2 Temperature Measurements with Passive Sensors

Because of the characteristics of the environment where the measurement of temper-ature occurs, electromagnetic interference, high voltage, and difficulty of access playimportant roles. Such problems could be circumvented with measurement systemsacting actively or passively with wireless technology. The system we use is knownin the literature as SAW sensors. The operating principle is based on the theory ofRADAR and the change in behavior of piezoelectric materials with temperature.

The temperature measurement is based on the change of electromagnetic signals.Initially, the RFID reader (transceiver) sends a pulse of an electromagnetic signal tothe RFID tag SAW (SAW sensor), and it converts the electromagnetic pulse in a wave

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(vibration) that travels inside the piezoelectric material to find the spotlight on theSAW sensor. Then, the wave returns to the spotlight in the form of a number of pulsescorresponding to the number of reflectors present in the SAW sensor. With a changein temperature, the piezoelectric material undergoes a change in size (expansion/con-traction) and the speed of propagation of the acoustic wave. These two effects occursimultaneously and show a linear relationship with the variation of temperature. Thechange of the speed of propagation and the size of the sensor causes a change to theresonance frequency of the tag. Thus, a measure of the temperature is performed bydetection of the resonance frequency of the SAW sensor subjected to a change intemperature.

SAW sensors have numerous applications and constructive aspects. To develop thiswork, SAW sensors were used with the following specifications:

• Material: Quartz• Operating frequency: 433 MHz (ISM)• Frequency range: 1.7 MHz (minimum)• Input impedance: 50 �

• Maximum signal loss: 2.5 dB• Device: delay line or resonator

The developed electronic circuit used just one PIC16F877A microcontroller thatcommunicates with two CC1100 devices, one dedicated to the transmission and theother to the signal reception, in order to meet the demand of 5 µs required by thesensor during measurement.

A fundamental part in the development of the passive system is the architecture ofthe interrogator device. Basically, an interrogator comprises a transmitter, a receiver,and a management unit. The major difficulties encountered during the developmentof the unit were related to the extremely short time of unloading of the SAW sensorand the low amplitude of the reflected signal.

The issue of time was resolved with the implementation of a deterministic systemthat ensures the sampling of a point on the unloading ramp of the SAW sensor, whichcarries the information on the resonant frequency of the sensor at that temperature.This procedure is performed three times for each frequency during the scan, takingthe mean value from the obtained temperature data.

The intensity of the reflected signal from the SAW sensor has been improved whendifferent types of antennas are tried. SAW antennas were designed and simulated ona computer, and then selected from the best performance. The frequency used for thedesign of the antenna was 433 MHz, which is the resonant frequency of the sensor,corresponding to a wavelength of 692.8 mm.

The requirements for antenna design were:

• small size, not much above the dimensions of the sensor;• good directivity, i.e., capture and transmission greater in the direction desired; and• low cost.

The following antennas were tested: stick-type antennas, helical antennas, and dif-ferent configurations of planar antennas (Hilbert, K4, loop KR, and rectangular loop).Among the tested antennas, those that performed better were those of the planar type.

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However, the result is still very limited in terms of measuring distance (average of0.8 m). One can improve the signals with amplification of the signal sent by the inter-rogator.

With the measurement system completed, several tests were carried out for improve-ments and equipment validation. The tests consisted of measuring the temperature invarious parts of the measuring range and with different configurations of the methodof scanning the resonance frequency. These tests could show the relationship betweentime and quality scanning results for the various scan settings.

To obtain satisfactory results with regard to the temperature in a relatively shorttime,a configuration has been established in which the sensor takes about 2 min and50 s to determine the temperature. This time is in line with expectations since the tem-peratures in the hot spots of the substation vary slowly because of inertia of the temper-ature at these points. The configuration chosen first scans frequencies correspondingto increments of 5 ◦C, in order to get the range of interest within the measurement.Within this range, frequencies are scanned at increments of 1 ◦C. For each value oftemperature, readings were performed for 20 levels of the signal power. The maximumsignal power corresponds to the resonant frequency and, therefore, the temperature ofthe measured point.

The results obtained in the tests were compared with a calibrated thermometerwith a resolution of 0.01 ◦C. The metrological analysis of the system proved that themeasurement uncertainty of the passive system was less than 1 ◦C, which was muchbetter than the uncertainty required for the detection of hot spots in electric powersubstations. The range of such systems was limited to 0.6 m to 1 m (distance from thetag to the receiver), which implies a strong limitation for field use.

3 Temperature Measurement with Active Sensors

Parallel to the development of the passive sensor and because of the low range (lessthan 1 m) of the RF signal transmitted by the SAW sensor, a study of an alternativesolution to the problem of temperature measurement in hot spots was started. In thisscenario, we began the adaption of commercial active industrial sensors to the harshenvironment of the power substations.

The main difference between the passive sensors and active ones is that the latterare provided with a source of voltage (battery). This battery, which lasts up to fiveyears in active sensors, gives a significant gain in the range of the radio-frequencysignal as the signal prior to being sent to the receiver can be amplified.

To develop the active system, the following specifications were imposed:

• Measuring range from 0 ◦C to 120 ◦C;• Uncertainty of ±2 ◦C;• Resolution of 1 ◦C;• Wireless communication;• Long battery life;• Range of at least 40 m;• RS232 serial communication; and• Communication protocol TCP/IP.

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Table 1 Average temperature recorded by the sensors

Test 1: Day1 (average of1 h test)

Test 2: Day2 (average of1 h test)

Test 3: Day3 (average of1 h test)

Average ambient temperature(◦C

)25.5 30.4 29.9

Sensor 1 average temperature(◦C

)27 33 34

Sensor 2 average temperature(◦C

)28 33 33

With the active system, tests were performed using different types of antennas wherethe best result was again obtained with planar antennas. The tests consisted of mea-suring the signal strength received by the receiver.

After performing several tests showing great improvement in range using activesensors and with different antennas configurations, the actual tests measuring real hotspots in electric power substations were started.

Software was developed to monitor the measured temperature values sent by theactive TAGs. Through this software, it was possible to follow the received data to keeptrack of the temperature and signal strength. In addition, the software stores data in afile.

The software displays the parameters read for each sensor. In laboratory tests, seventags identified as: 101, 102, 103, 104, 201, 1002, and 1003 were used. These numbersare also used in the communication protocol with each tag identifying them since theyall operate on the same frequency.

Field tests were conducted with active sensors in order to determine the influenceof the electromagnetic field present in electric power substations. It was noted thatthe presence of an electromagnetic field did not result in deterioration of the measure-ment of temperature inside the substation plant. One can verify this with the data inTable 1. It shows three tests performed on different days under the same conditions(except that in the third test, a planar antenna was used, the other two used a stick-typeantenna for the receiving signal). Moreover, it was observed that physical obstacles arehighly detrimental to the transmission of the signal sent by the active sensors (TAGs).Furthermore, the use of a planar antenna resulted in a large gain in the range of themeasurement (typically 10 m to 15 m).

For the installation of the sensors on the transmission line, where the electromag-netic field is critical and can damage the electronics of the system, some form ofshielding was implemented. The biggest difficulty at this stage was to develop a deviceto prevent the formation of electrical arcs between the transmission line and the elec-tronic components of the tags, allowing at the same time, the transmission of the signalto the receiver.

Figure 1 shows one of the adjustments made in the laboratory for use in fieldtests. The holes machined in the capsules for circuit protection were designedin order to permit the transmission of the radio-frequency signal without impair-ing the Faraday cage effect, which provided an electrical shield protection to thesensor.

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Fig. 1 Protection of the TAGsdesigned for instrumentation inthe field

Fig. 2 Active system installed in a hot-spot substation

After laboratory tests were carried out with the protection system, field tests intransmission lines were initiated. Figure 2 shows the device instrumented at a criticalpoint of a power substation.

Nine configurations of the antenna in the substation were analyzed, and the qualityof signal reception was assessed. Three sensors were placed on each phase of thepower line. One sensor, identified as 102, was allocated on the C-phase, 101 on theB-phase, and finally 1002 on the A-phase. At the beginning of the tests, it was notedthat the sensor 1002 stopped working because of the lack of protection. This prob-lem did not happen with the other two sensors, because of the protection against theelectromagnetic field generated by the 230 kV power line.

In Fig. 3, one can observe the nine tests performed with the active sensors. In thefirst three tests, the antenna was close to the phase B, being gradually approximatedto the sensor 101, where one can notice that the closer the distance between the trans-ducer 101 and the antenna, the better the signal reception of this sensor, and on the

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Fig. 3 Average reception of signals from sensors in different tests (RSSI received signal strength indicator)

Fig. 4 Tests of the active system in the field

other hand, the intensity of reception of the signal received from sensor 102 graduallydecreased.

In tests of numbers four to six (Fig. 3), the reverse situation is observed, once theantenna was deployed near the A-phase power line and is being increasingly approx-imated to the sensor 102.

In tests seven and eight, the antenna was placed about 20 m from sensor 102. Testseven used the antenna perpendicular to the sensors, and it was not possible to capturethe signal coming from sensor 102. Test eight used the antenna positioned at a 45◦angle leading to better signal reception.

Test nine was conducted with the antenna about 30 m from sensor 102 as shown inFig. 4, being only possible to capture the sensor 101. After verifying the effectivenessof the shielding, other tests were performed. In Fig. 4, one can observe a test rangewhere the antenna is located about 30 m away from the sensors that are installed in hot

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spots. The data obtained during the measurement showed that at this point the signalis still high and the antenna could be located even further.

4 Results

The passive system for measuring the temperature in hot spots has been successfullydeveloped although it has a strong limitation in the distance between the hot spot andthe receiver. The tests show the efficiency of the method of temperature measurementby SAW–RFID technology. This efficiency was confirmed by checking the valuesobtained using the developed system and a calibrated thermometer.

Uncertainties obtained with the passive measurement system can be considerednegligible for the suggested application, where temperature variations are consideredcritical if they are an order of magnitude greater. The uncertainties obtained in thetests were less than 1 ◦C.

As for the active transducers, the covers (protections) have been successfully testedin the field. The distance range obtained with the radio-frequency signal sent by thetags was sufficient for hot-spot monitoring in power lines.

The protection devices are designed to permit the transmission of the radio-fre-quency signal without losing the characteristic of shielding required for the tags in aharsh environment.

The tags were instrumented and protected with an antenna positioned 30 m fromthem. The strength of the signal received during the tests indicates that this proximityis more than enough for a proper measurement of hot spots. Tests have shown thatpassive tags were not adequate in field implementation because of their limitation inrange. Typical ranges for such tags (passive) are close to 1 m, which is very short foractual field implementation (10 m to 30 m).

Acknowledgment This work was supported by CNPq “National Counsel of Technological andScientific.”

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