Patent Publication Number: US-2017363483-A1

Title: Surface acoustic wave (saw) based temperature sensing for electrical conductor

Description:
TECHNICAL FIELD 
     The present disclosure relates to systems for monitoring temperature of an electrical conductor, and in particular, to systems for monitoring temperature of an electrical conductor enclosed in at least a (semi)conductive layer, for example, an electrical conductor of an electrical power cable in a power distribution system. 
     BACKGROUND 
     Medium and high voltage power distribution systems play an important role in modern society. Safety and security are always considerable factors for the “health” of a power distribution system. Accordingly, there should be a technology that enables monitoring of the “health” of the power distribution system. 
     In a power distribution system such as a medium or high voltage power distribution system, the temperature of conductors of electrical cables may increase as currents carried by the cables increase. Accordingly, the “health” of such system can be assessed by monitoring the temperature of the on-line electrical conductor, for example, at the cable splices or the junctions, which may be the weak points, in such a system. Usually, normal currents flowing through the cable splices or the junctions may create a temperature of up to, for example, about 90° C. If the temperatures of the cable splices or the junctions were to increase beyond that, it could be an indication that something may be wrong in this power distribution system. On the other hand, it is also useful to know if the existing power distribution system is at maximum current carrying capacity, to know if additional power can be reliably distributed with the existing system, or, to know if additional infrastructure expenditures are needed. 
     SUMMARY 
     On-line power cables, as well as the cable splices and the junctions, for example, in medium or high voltage power distribution systems are typically insulated and protected by a number of insulative and (semi)conductive layers and/or are commonly buried underground or are positioned high overhead. There is a desire to directly monitor or measure the temperature of the on-line electrical conductor, for example, directly at the cable splices or the junctions. 
     Briefly, in one aspect, the present disclosure describes systems and methods for directly sensing, measuring, or monitoring the temperature of an electrical conductor of a power cable. Some embodiments described herein provide a surface acoustic wave (SAW) temperature sensor that is in thermal contact with the electrical conductor. The SAW temperature sensor includes an antenna to receive a wireless signal. The received signal can be converted into a SAW signal that can vary with the temperature of the electrical conductor. The temperature of the electrical conductor can be sensed, measured, or monitored by measuring the SAW signal. 
     In one aspect, a temperature-sensing apparatus for sensing a temperature of an electrical conductor enclosed in at least a (semi)conductive layer, is provided. The apparatus includes a surface acoustic wave (SAW) temperature sensor including a substrate having a major surface, a transducer disposed on the major surface of the substrate, and one or more antennas electrically connected to the transducer. The one or more antennas are configured to receive or send an electromagnetic signal, and the transducer is configured to conduct conversion between the electromagnetic signal and a SAW signal that propagates on the major surface of the substrate. At least a portion of the substrate of the SAW temperature sensor is disposed in thermal contact with the electrical conductor, and the SAW signal varies with the temperature of the electrical conductor. 
     In another aspect, an electrical cable assembly includes an electrical conductor, a (semi)conductive layer enclosing the electrical conductor, and a temperature-sensing apparatus. The temperature-sensing apparatus includes a surface acoustic wave (SAW) temperature sensor including a substrate having a major surface, a transducer disposed on the major surface of the substrate, and one or more antennas electrically connected to the transducer. The one or more antennas are configured to receive or send an electromagnetic signal, and the transducer is configured to conduct conversion between the electromagnetic signal and a SAW signal that propagates on the major surface of the substrate. At least a portion of the substrate of the SAW temperature sensor is disposed in thermal contact with the electrical conductor, and the SAW signal varies with the temperature of the electrical conductor. The SAW temperature sensor is disposed between the electrical conductor and the (semi)conductive layer, and is enclosed by the (semi)conductive layer. The (semi)conductive layer is configured to provide electromagnetic shielding for the power carried by the electrical conductor, while allowing the electromagnetic signal of the one or more sensor antennas to pass therethrough. 
     In yet another aspect, a method of sensing a temperature of an electrical conductor enclosed in at least a (semi)conductive layer, is provided. The method includes providing a surface acoustic wave (SAW) temperature sensor. The SAW temperature sensor includes a substrate having a major surface, a transducer disposed on the major surface of the substrate, and one or more antennas electrically connected to the transducer. The one or more antennas are configured to receive or send an electromagnetic signal, and the transducer is configured to conduct conversion between the electromagnetic signal and a SAW signal that propagates on the major surface of the substrate. The method further includes disposing at least a portion of the substrate to be in thermal contact with the electrical conductor such that the SAW signal varies with the temperature of the electrical conductor, providing a transceiver unit configured to be in electromagnetic communication with the one or more antennas of the SAW temperature sensor, detecting, via the electromagnetic communication between the transceiver unit and the one or more antennas, the SAW signal that varies with the temperature of the electrical conductor, and determining the temperature of the electrical transmission line based on the detected SAW signal. 
     Various unexpected results and advantages are obtained in exemplary embodiments of the disclosure. One such advantage of exemplary embodiments of the present disclosure is that some passive SAW temperature sensors used herein are hermetically sealed to provide accurate temperature measurement with no external physical stress or change in the mechanics of the device even in harsh temperature environments. In addition, the embodiments described herein allow the passive SAW temperature sensors to be in efficient electromagnetic communication with an outside, remote transceiver unit. 
     LISTING OF EXEMPLARY EMBODIMENTS 
     Exemplary embodiments are listed below as aspects. It is to be understood that any of embodiments 1 to 14 and 15 to 17 can be combined. 
     Embodiment 1 is a temperature-sensing apparatus for sensing a temperature of an electrical conductor enclosed in at least a (semi)conductive layer, the apparatus comprising: 
     a surface acoustic wave (SAW) temperature sensor including a substrate having a major surface, a transducer disposed on the major surface of the substrate, and one or more sensor antennas electrically connected to the transducer, the one or more sensor antennas being configured to receive or send an electromagnetic signal, and the transducer being configured to conduct conversion between the electromagnetic signal and a SAW signal that propagates on the major surface of the substrate, 
     wherein at least a portion of the substrate is disposed in thermal contact with the electrical conductor, and the SAW signal varies with the temperature of the electrical conductor. 
     Embodiment 2 is the apparatus of embodiment 1, wherein the transducer includes an interdigital transducer (IDT). 
     Embodiment 3 is the apparatus of embodiment 1 or 2, wherein the SAW temperature sensor further includes one or more reflectors disposed on the major surface of the substrate, the one or more reflectors each being disposed to reflect at least a portion of the SAW signal back to the transducer. 
     Embodiment 4 is the apparatus of any one of embodiments 1-3, wherein the SAW temperature sensor further comprises a metallic housing to accommodate the substrate with the transducer, and the sensor antennas are disposed outside the metallic housing. 
     Embodiment 5 is the apparatus of any one of embodiments 1-4, wherein the SAW temperature sensor is disposed between the electrical conductor and the (semi)conductive layer, and is enclosed by the (semi)conductive layer. 
     Embodiment 6 is the apparatus of any one of embodiments 1-5, wherein the substrate includes one or more piezoelectric materials. 
     Embodiment 7 is the apparatus of any one of embodiments 1-6, further comprising a transceiver unit in electromagnetic communication with the one or more sensor antennas, and the transceiver unit being configured to send out a signal representing the SAW signal and the temperature of the electrical conductor. 
     Embodiment 8 is the apparatus of embodiment 6, wherein the transceiver unit is disposed outside of the (semi)conductive layer. 
     Embodiment 9 is the apparatus of any one of embodiments 1-8, wherein the electromagnetic signal has a frequency in a VHF/UHF range. 
     Embodiment 10 is the apparatus of any one of embodiments1-9, wherein the electrical conductor carries an electrical power having a frequency of 60 Hz. 
     Embodiment 11 is an electrical cable assembly comprising: 
     an electrical conductor; 
     a (semi)conductive layer enclosing the electrical conductor; and 
     the temperature-sensing apparatus of any one of embodiments 1-10, 
     wherein the SAW temperature sensor is disposed between the electrical conductor and the (semi)conductive layer, and is enclosed by the (semi)conductive layer, and 
     wherein the (semi)conductive layer is configured to provide electromagnetic shielding for the power carried by the electrical conductor, while allowing the electromagnetic signal of the one or more sensor antennas to pass therethrough. 
     Embodiment 12 is the electrical cable assembly of embodiment 11, wherein the (semi)conductive layer includes strips of electrically conductive tapes that extend along a longitudinal axis of the electrical conductor. 
     Embodiment 13 is the electrical cable assembly of embodiment 11 or 12, wherein the (semi)conductive layer includes one or more electrically conductive tapes that are configured to have gaps serving as windows to allow the electromagnetic signal of the one or more antennas to pass therethrough. 
     Embodiment 14 is the electrical cable assembly of embodiment 13, wherein the (semi)conductive layer includes an insulating layer that allows for wrapping the one or more electrically conductive tapes around the electrical conductor. 
     Embodiment 15 is a method of sensing a temperature of an electrical conductor enclosed in at least a (semi)conductive layer, the method comprising: 
     providing a surface acoustic wave (SAW) temperature sensor, the SAW temperature sensor including a substrate having a major surface, a transducer disposed on the major surface of the substrate, and one or more antennas electrically connected to the transducer, the one or more antennas being configured to receive or send an electromagnetic signal, and the transducer being configured to conduct conversion between the electromagnetic signal and a SAW signal that propagates on the major surface of the substrate; 
     disposing at least a portion of the substrate to be in thermal contact with the electrical conductor, the SAW signal being variable with the temperature of the electrical conductor; 
     providing a transceiver unit configured to be in electromagnetic communication with the one or more antennas of the SAW temperature sensor; 
     detecting, via the electromagnetic communication between the transceiver unit and the one or more antennas, the SAW signal that is variable with the temperature of the electrical conductor; and 
     determining the temperature of the electrical transmission line based on the detected SAW signal. 
     Embodiment 16 is the method of embodiment 15, further comprising providing a (semi)conductive layer to enclose the SAW temperature sensor and the electrical conductor, and the SAW temperature sensor being disposed between the (semi)conductive layer and the electrical conductor. 
     Embodiment 17 is the method of embodiment 15 or 16, wherein the (semi)conductive layer is configured to provide electromagnetic shielding for the power carried by the electrical conductor, while allowing the electromagnetic signal of the one or more antennas to pass therethrough. 
     As used in this specification: 
     “(semi)conductive” indicates that the layer may be semi-conductive or conductive, depending on the particular construction. 
     “thermal contact” between two articles means that the articles can exchange energy with each other in the form of heat. 
     “direct contact” between two articles means physical contact. 
     Various aspects and advantages of exemplary embodiments of the disclosure have been summarized. The above Summary is not intended to describe each illustrated embodiment or every implementation of the present certain exemplary embodiments of the present disclosure. The Drawings and the Detailed Description that follow more particularly exemplify certain preferred embodiments using the principles disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying figures, in which: 
         FIG. 1  is a schematic block diagram of a SAW temperature sensor, according to one embodiment. 
         FIG. 2  is a schematic block diagram of a system for monitoring temperature of an electrical conductor, according to one embodiment. 
         FIG. 3A  is a perspective side view of a SAW temperature sensor, according to one embodiment. 
         FIG. 3B  is a perspective side view of a SAW temperature sensor, according to another embodiment. 
         FIG. 4  is a partial cut-away schematic view of application of a system for monitoring temperature of an electrical conductor in a cable splice assembly, according to one embodiment. 
         FIG. 5  is a sectional view of a portion of the electrical conductor in a cable splice assembly having a passive SAW temperature sensor, according to one embodiment. 
         FIG. 6  is a partial cross-section side view of a SAW temperature sensor, according to one embodiment. 
       In the drawings, like reference numerals indicate like elements. While the above-identified drawing, which may not be drawn to scale, sets forth various embodiments of the present disclosure, other embodiments are also contemplated, as noted in the Detailed Description. In all cases, this disclosure describes the presently disclosed disclosure by way of representation of exemplary embodiments and not by express limitations. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides embodiments of systems and methods for monitoring a temperature of an electrical conductor of, for example, medium or high voltage (e.g., &gt;1 kV or &gt;10 kV) power cables. It may be particularly useful to perform such monitoring by means of a “passive” apparatus, by which is meant an apparatus that does not require an internal power source (e.g., battery) and that does not need to be physically connected to an external power source. In this disclosure, one type of passive apparatus that can find use in such applications relies on a temperature sensitive surface acoustic wave (SAW) device or a SAW temperature sensor. 
       FIG. 1  illustrates a schematic block diagram of a SAW temperature sensor  20 , according to one embodiment. The SAW temperature sensor  20  includes a transducer  20 T disposed on a major surface of a substrate  20 S. The substrate  20 S can be, for example, a piezoelectric substrate including one or more piezoelectric materials. The SAW temperature sensor  20  further includes an antenna  20 A configured to receive and send electromagnetic signals. In some embodiments, the electromagnetic signals can be in the very high or ultra-high frequency (VHF/UHF) band (e.g., from 30 MHz to 3 GHz). The antenna  20 A is electrically connected to the transducer  20 T. The transducer  20 T is configured to receive the electromagnetic signal from the antenna  20 A and convert the received electromagnetic signal into a SAW signal by, for example, a converse piezoelectric effect. The SAW signal can propagate on the major surface of the substrate  20 S as acoustic waves. In the embodiment of  FIG. 1 , the SAW temperature sensor  20  further includes one or more reflectors  20 R. At least a portion of the acoustic waves can be reflected by the reflectors  20 R back to the transducer  20 T where the reflected SAW signal can be re-converted into electromagnetic signals to be sent out by the antenna  20 A. 
     It is to be understood that the reflectors  20 R can be optional. The SAW temperature sensor  20  can include any suitable elements for guiding, modulating, or converting the acoustic waves. In some embodiments, the SAW temperature sensor  20  may not include the reflectors  20 R, and instead can include a second transducer to receive a SAW signal as acoustic waves from the transducer  20 T, without first reflecting from a reflector, and re-convert the received SAW signal into electromagnetic signal to be sent out by a second antenna electrically connected to the second transducer. 
     In some embodiments, some components of the SAW temperature sensor  20  including the substrate  20 S with the transducer  20 T and the reflector  20 R disposed thereon can be hermetically sealed inside a package. The package can be, for example, a hermetically sealed ceramic or metal package. The antenna  20 A can be disposed outside the package and electrically connected to the transducer  20 T via, for example, pins of the package and a transmission wire such as, for example, a coaxial cable. 
     The temperature of the substrate  20 S can affect properties (e.g., velocity, amplitude, phase, frequency, etc.) of the acoustic waves propagating thereon. When the temperature of the substrate  20 S of the SAW temperature sensor  20  changes, the acoustic waves propagating on the major surface of the substrate  20 S can be modulated by the temperature change. Accordingly, the properties of the electromagnetic signal re-converted from the SAW signal can be modulated. In some embodiments disclosed herein, the SAW signal can be used to sense, measure or monitor the temperature of the substrate  20 S. When the SAW temperature sensor  20  is placed in thermal communication or contact with a portion of a power cable, a change in temperature of that portion of the power cable can cause the temperature of the temperature sensitive SAW device to change commensurately. This temperature change can modulate the SAW signal and the correspondingly re-converted electromagnetic signal, which can be detected and used to infer the temperature of that portion of the power cable. 
       FIG. 2  is a schematic diagram of a system  100  for monitoring a temperature of an electrical conductor  31  according to one embodiment. The system  100  includes the passive SAW temperature sensor  20  of  FIG. 1 , a transceiver unit  40 , and a control unit  50 . The passive SAW temperature sensor  20  is disposed to have at least a portion of the substrate  20 S to be in thermal contact with the outer surface of the electrical conductor  31  such that the acoustic waves propagating on the substrate  20 S can be variable with the temperature of the electrical conductor  31 . 
     In some embodiments, the passive SAW temperature sensor  20  can receive an electromagnetic signal from the transceiver unit  40  and send out a feedback electromagnetic signal that varies with the temperature of the electrical conductor  31 . The control unit  50  can communicate with the transceiver unit  40  to determine a value of the temperature of the electrical conductor  31  based on the feedback electromagnetic signal. In some embodiments, the system  100  may further include an optional central monitoring unit (not shown in  FIG. 2 ). The optional central monitoring unit can communicate with the control unit  50  wirelessly (e.g., through mobile network) or through wires to receive the determined value of the temperature of the electrical conductor  31  and make decisions accordingly. 
     In some embodiments, during operation, if there is a need to monitor the temperature of the electrical conductor  31 , the control unit  50  may send out an instruction signal S 1  to the transceiver unit  40 . Once the transceiver unit  40  receives the instruction signal S  1 , it then sends out an electromagnetic signal S 2  to the passive SAW temperature sensor  20 . The passive SAW temperature sensor  20  can receive the electromagnetic signal S 2  and convert it into a SAW signal. The SAW signal can vary with the temperature of the electrical conductor  31 , for example, being modulated by the temperature change of the electrical conductor  31 . The SAW signal then can be re-converted into a feedback electromagnetic signal S 3 . The transceiver unit  40  can detect the feedback electromagnetic signal S 3  from the passive SAW temperature sensor  20  and then send out a signal S 4  to the control unit  50 . The feedback electromagnetic signal S 3  and the signal S 4  contain the information representing the SAW signal of the passive SAW temperature sensor  20 , which can be variable with the temperature of the electrical conductor  31 . The control unit  50  can determine a value of the temperature of the electrical conductor  31  based on the ascertained signal S 4 . 
     In some embodiments, the absolute temperature of the electrical conductor  31  can be determined by the control unit  50  based on the measured feedback electromagnetic signal S 3 . In some embodiments, a temperature change of the electrical conductor  31  can be determined by the control unit  50  based on the measured feedback electromagnetic signal S 3  and the absolute temperature of the electrical conductor  31  can be determined accordingly. 
     In some embodiments, the system  100  may further include an optional energy harvesting unit  60 . The energy harvesting unit  60  can be adapted to harvest electrical power from the electrical conductor  31  when an AC current flows through the electrical conductor  31  and to supply the harvested electrical power to the transceiver unit  40  and/or the control unit  50 . 
     In some embodiments, the passive SAW temperature sensor  20  can measure the temperature of the electrical conductor  31  in a temperature range of, for example, from −55° C. to 150° C. with a temperature accuracy of, for example, +/−2° C. or better. 
       FIGS. 3A-B  illustrate two examples  21  and  22  for the passive SAW temperature sensor  20  of  FIGS. 1 and 2 , according to some embodiments. The passive SAW temperature sensor  21  of  FIG. 3A  includes a piezoelectric substrate  21 S, an interdigital transducer (IDT)  21 T disposed on a major surface  211  of the substrate  21 S, and an antenna  21 A electrically connected, via a wire  212 , to the IDT  21 T. 
     The antenna  21 A is configured to receive a wireless signal such as, for example, an electromagnetic signal in the VHF/UHF band from the transceiver unit  40  of  FIG. 2 . The IDT  21 T is configured to convert the electromagnetic signal received by the antenna  21 A into a SAW signal S 21 . The SAW signal S 21  propagates on the major surface  211  of the substrate  21 S as acoustic waves. The passive SAW temperature sensor  21  further includes one or more reflectors  21 R disposed on the major surface  211  of the substrate  21 S. The reflectors  21 R each are configured to reflect at least a portion of the SAW signal S 21  back to the IDT  21 T. The reflected SAW signal S 22  can be received by the IDT  21 T and re-converted into a feedback electromagnetic signal to be sent out by the antenna  21 A. 
     In some embodiments, the piezoelectric substrate  21 S can include one or more piezoelectric materials. The piezoelectric material can be any suitable natural or synthetic materials that exhibit piezoelectricity including, for example, barium titanate, lead zirconate titanate, potassium niobate, lithium niobate, lithium tantanate, sodium tungstate, sodium potassium niobate, bismuth ferrite, sodium niobate, bismuth titanate, sodium bismuth titanate, polymers such as polyvinylidene fluoride, etc. 
     During operation, at least a portion of the piezoelectric substrate  21 S is in thermal contact with the electrical conductor  31  of  FIG. 2 . When the temperature of the electrical conductor  31  is changed, the acoustic waves can be modulated by the temperature change. The temperature of the electrical conductor  31  can be determined based on the feedback electromagnetic signal. In the embodiment of  FIG. 3B , the passive SAW temperature sensor  22  includes a series of reflectors  22 R disposed on two sides of the IDT  21 T and two antennas  22 A electrically connected to terminals of the IDT  21 T where the IDT  21 T is disposed in a central portion of the piezoelectric substrate  21 S. In the embodiment of  FIG. 3A , the IDT  21 T is disposed adjacent to an edge of the piezoelectric substrate  21 S. It is to be understood that one or more IDTs and one or more reflectors can be arranged in various ways as long as the passive SAW temperature sensor can work properly. 
     In the embodiment of  FIGS. 3A-B , the IDT  21 T includes electrodes that are arranged in an interdigitated comb configuration including an arrangement of electrically conductive lines or “fingers”. The electrodes can be disposed on or embedded into the major surface  211  of the piezoelectric substrate  21 S. The electrodes can be made of any appropriate electrically conductive materials such as, for example, metals, metal alloys, metal-filled polymers, etc. The fingers can be disposed parallel to each other with a space therebetween. When an input electrical signal is received by the IDT  21 T from an antenna (e.g.,  21 A or  22 A), the input electrical signal can create alternating polarity between the fingers of the IDT  21 T. The alternating polarity can create alternating regions of tensile and compressive strain on the major surface  211  of the substrate  21 S between the fingers of the electrode by a piezoelectric effect of the piezoelectric substrate, and can produce a mechanical wave thereon known as a surface acoustic wave (SAW). The wavelength of the mechanical or acoustic wave can be the space between the fingers of the electrodes. The frequency f 0  of the acoustic wave can be represent as the following equation: 
     
       
         
           
             
               f 
               0 
             
             = 
             
               
                 v 
                 p 
               
               p 
             
           
         
       
     
     where V p  is the phase velocity of the acoustic wave and p is the space between the fingers. 
     The generated mechanical or acoustic wave can propagate away from the IDT  21 T. In some embodiments, one or more mechanical absorber can be added between the IDT  21 T and the edges of the piezoelectric substrate  21 S to prevent interference patterns or control insertion losses. The acoustic wave travels across the surface of the substrate and can be reflected by one or more reflectors back to the IDT  21 T and re-converted into electromagnetic feedback signals by a piezoelectric effect. In some embodiments, the acoustic wave can travel to other IDT, converting the acoustic wave back into a feedback signal by the piezoelectric effect. Any changes that were made to the mechanical or acoustic wave can be reflected in the feedback signal. In the present disclosure, the SAW signal varies with the temperature of the electrical conductor which can be determined based on the feedback signal. 
       FIG. 4  illustrates application of the system  100  of  FIG. 2  including the passive SAW temperature sensor  20 , the transceiver unit  40 , and the control unit  50  for monitoring or measuring temperature of the electrical conductor  31 , for example enclosed in a cable splice assembly  30 , according to one embodiment. 
     In the cable splice assembly  30 , two sections of an electrical cable  10  are spliced. Each section of the electrical cable  10  includes the electrical conductor  31 , an insulation layer  33 , and a (semi)conductive layer  35 . The insulation layer  33  and the (semi)conductive layer  35  enclose the electrical conductor  31 . A connector  12  concentrically surrounds the spliced electrical conductors  31 . A first (semi)conductive (or electrode) layer  13 , in this case a metallic layer, concentrically surrounds the spliced electrical conductors  31  and the connector  12 , forming a shielding Faraday cage around the connector  12  and the electrical conductors  31 . In some embodiments, “(semi)conductive” indicates that the layer may be semi-conductive or conductive, depending on the particular construction. An insulating layer  11  (containing geometric stress control elements  16 ) surrounds the first (semi)conductive layer  13 . The foregoing construction is placed inside a second (semi)conductive layer  14 , in this case a metallic housing, which functions as a shield and ground layer. A resin can be poured into the metallic housing  14  through one of the ports  18  to fill in the area around insulating layer  11 . A shrinkable sleeve layer  15  serves as an outermost layer. 
     In this embodiment, portions of the electrical conductors  31  are covered by the connector  12  and then are enclosed by the first (semi)conductive layer  13 , the insulating layer  11 , the second (semi)conductive layer  14 , and the shrinkable sleeve layer  15 . In this embodiment, the shrinkable sleeve layer  15  includes two overlapping sections  151  and  152  to leave a passage  153  between the overlapping portions. The passage  153  is from the outside of the shrinkable sleeve layer  15  through the port  18  on the second (semi)conductive layer  14  to the inside of the second (semi)conductive layer  14 . 
     As shown in  FIG. 4 , the passive SAW temperature sensor  20  is positioned adjacent to one of the electrical conductors  31  and inside the first (semi)conductive layer  13 . Preferably, a portion of the electrical conductor  31  is exposed between the insulation layer  33  of the electrical cable  10  and the connector  12 , and the passive SAW temperature sensor  20  may be positioned at an outer surface of the exposed portion of the electrical conductor  31 . More detailed description about the position of the passive SAW temperature sensor  20  will be given hereinafter with reference to  FIG. 5 . 
     The transceiver unit  40  is positioned outside the first (semi)conductive layer  13  and inside the second (semi)conductive layer  14 , i.e. between the first (semi)conductive layer  13  and the second (semi)conductive layer  14 . The transceiver unit  40  can include an antenna that can be any type of antenna including, for example, an inductive coil, a printed antenna, etc. The transceiver unit  40  can include two or more antennas that can be positioned around the insulating layer  11  of  FIG. 4 . In some embodiments, the antenna of the transceiver unit  40  and the antenna  21 A of the passive SAW temperature sensor  20  can be located in a same cross section, so as to improve the electromagnetic communication therebetween. More detailed description about embodiments of the transceiver unit  40  and its positioning will be provided hereinafter with reference to  FIG. 5 . 
     In some embodiments, pairings of the passive SAW temperature sensor  20  and the transceiver unit  40  can be located at various locations of the electrical cable  10 . The passive SAW temperature sensor  20  can be disposed adjacent to the electrical conductor  31  and enclosed by the (semi)conductive layer  35  and the insulation layer  33  of the electrical cable  10 . The transceiver unit  40  can be located outside the (semi)conductive layer  35  and configured to be in electromagnetic communication with the antenna  20 A of the passive SAW temperature sensor  20 . A series of such pairings can be distributed along the electrical cable  10  to provide a temperature distribution of the electrical conductor  31 . 
     Referring again to  FIG. 4 , the control unit  50  is configured to communicate with the transceiver unit  40  through a wire  51 . The wire  51  can be accommodated within the passage  153  so that the wire  51  can extend from the transceiver unit  40 , through the port  18 , to the control unit  50 . The optional energy harvesting unit  60  including a power inductive coil  61  can be located outside the assembly  30  and around the cable  10 , or located between the second (semi)conductive layer  14  and the shrinkable sleeve layer  15 . The energy harvesting unit  60  can be used to supply power to the transceiver unit  40  and/or the control unit  50  through a wire  52 . Throughout this specification, although the wire  51  and the wire  52  are each referred to as a “wire,” it should be understood that either or both of wire  51  and wire  52  may include multiple wires as needed for the system to function. 
     In some embodiments, the inductive coil  61  of the optional energy harvesting unit  60  can include, for example, an iron-core current transformer, an air-core current transformer, or a Rogowski coil. The inductive coil  61  can be positioned outside the first (semi)conductive layer  13 , or outside the second (semi)conductive layer if one is used. Preferably, the energy harvesting unit  60  may be used mainly to provide the harvested electrical power to the transceiver unit  40 , so the energy harvesting unit  60  can be positioned outside the layer in which the transceiver unit  40  is located. Thus, the energy harvesting unit  60  may be electrically connected with the transceiver unit  40  via one or more wires. In some embodiments, the energy harvesting unit  60  may further include an optional rectifier circuit to adapt the harvested electrical power right for the transceiver unit  40  and/or the control unit  50 . 
       FIG. 5  illustrates a closer perspective view illustrating an exemplary location of the passive SAW temperature sensor  20  of  FIG. 4  that is placed on the electrical conductor  31  adjacent to the connector  12 .  FIG. 6  is a cross-sectional view of the passive SAW temperature sensor  20 , according to one embodiment. In the embodiment of  FIG. 5 , the shrinkable sleeve layer  15  is continuous and a hole has been cut in the shrinkable sleeve layer  15  to accommodate the port  18  and allow the egress of the wire  51 . 
     As an example, the passive SAW temperature sensor  20  of  FIG. 6  includes the antenna  20 A and the substrate  20 S with the transducer  20 T, the reflector  20 S and other components disposed thereon. The substrate  20 S and the components disposed thereon are hermetically sealed inside a package  20 P. The package  20 P can be, for example, a hermetically sealed ceramic or metal package. In some embodiments, the package  20 P can provide a housing with a cavity to receive the substrate  20 S where the substrate  20 S can be mounted on a wall of the housing. The housing can be made of electrically conductive material such as, for example, copper. The antenna  20 A and the transducer  20 T (not shown) on the substrate  20 S are electrically connected via a transmission line  220  which can be, for example, a coaxial cable. 
     A fixture  210  is provided to install the antenna  20 A and the package  20 P. In the embodiment of  FIG. 6 , the fixture  210  includes a main body  2101  and a channel  2102 . The channel  2102  is adapted to accommodate the electrical conductor  31  to have the electrical conductor  31  pass through the channel  2102 . The main body  2101  has a chamber  2103  to accommodate the package  20 P and the chamber  2103  can communicate with the channel  2102  in a way that at least a portion of the substrate  20 S inside the package  20 P can be in thermal contact with the outer surface of the electrical conductor  31  in operation. The antenna  20 A can be adapted to various configurations/geometries to promote the electromagnetic communication with the transceiver unit  40  that is disposed outside of the first (semi)conductive layer  13  as shown in  FIG. 5 . The fixture  210  further includes a cover  2104  to enclose the main body  2101 . It is to be understood that two or more antennas  20 A, and/or two or more packages  20 P can be accommodated in the fixture  210  where the antennas and the IDTs inside the packages can be electrically connected in various ways. 
     Referring again to  FIGS. 6 and 7 , at least a portion of the substrate  20 S of the passive SAW temperature sensor  20  is disposed in thermal contact with the electrical conductor  31 . In some embodiments, the package  20 P that seals the substrate  20 S can adhere to the surface of the electrical conductor  31  by, for example, a thermal-conductive paste. In some embodiments, the package  20 P can be in direct contact with the surface of the electrical conductor  31 . It is to be understood that the package  20 P can be any suitable shapes as long as a suitable thermal contact surface can be provided to effectively exchange heat between the substrate  20 S and the electrical conductor  31 . 
     In some embodiments such as the embodiment shown in  FIGS. 4 and 6 , the passive SAW temperature sensor  20  including the antenna  20 A is located inside an electromagnetic shielding layer such as the first (semi)conductive (or electrode) layer  13  or the (semi)conductive layer  35 , and the transceiver unit  40  is located outside of the electromagnetic shielding layer. The electromagnetic shielding layer surrounds the electrical conductor  31  and/or the connector  12 , providing an effective shield of the electrical power carried by the electrical conductor  31 . For example, the first (semi)conductive (or electrode) layer  13  can shield angular discharges on the connector  12  caused by crimping. In some embodiments, the power carried by the electrical conductor  31  has a frequency of, for example, 60 Hz. The present disclosure recognizes that an electromagnetic shielding layer such as the first (semi)conductive (or electrode) layer  13  or the (semi)conductive layer  35 , if improperly designed, may affect the electromagnetic communication between the antenna  20 A of the passive SAW temperature sensor  20  and the transceiver unit  40 . 
     Some embodiments in the present disclosure to be described below provide one or more (semi)conductive layers such as the first (semi)conductive (or electrode) layer  13  or the (semi)conductive layer  35 . The (semi)conductive layer surrounds and encloses the electrical conductor  31  and the passive SAW temperature sensor  20 . The transceiver unit  40  is disposed outside the (semi)conductive layer. The (semi)conductive layer is configured to provide electromagnetic shielding of the power carried by the electrical conductor  31 , without significantly affecting the electromagnetic communication between the antenna  20 A of the passive SAW temperature sensor  20  and the transceiver unit  40 . 
     In some embodiments, the (semi)conductive layer can include one or more electrically conductive tapes that surround the electrical conductor  31 . The tapes can be, for example, finely woven mesh tapes including electrically conductive meshes. Example tapes are commercially available from 3M Company (Saint Paul, Minn., USA) under the trade designations Scotch 24 Electrical Shielding Tape, which are conducting metal taps being woven of tinned copper wire and capable of operating at a temperature of 130° C. In some embodiments, multiple tapes are arranged to have a gap or space therebetween. In other embodiments, a single tape can be used that includes gaps or spaces between electrically conductive meshes thereof. The gaps or spaces can serve as windows to allow electromagnetic communication between the antenna  20 A of the passive SAW temperature sensor  20  and the transceiver unit  40 . The gaps or spaces can have a dimension of, for example, from 0.05 mm to 25 mm, or from 0.1 mm to 10 mm. Without the spaces or gaps, the (semi)conductive layer may block the electromagnetic signal from the antenna  20 A or the transceiver unit  40  to be transmitted therethrough. 
     In some embodiments, the (semi)conductive layer can further include an insulating base layer that allows for wrapping the one or more electrically conductive tapes around the electrical conductor  31  to form an electrically conductive surface. The electrically conductive surface with the gaps or spaces can form a frequency selective surface, which can be relatively transparent to electromagnetic signals of a specific range of frequencies (e.g., in a VHF/UHV range) while relatively shielding to the electrical power carried by the electrical conductor  31 . 
     In some embodiments, the (semi)conductive layer can include strips of electrically conductive tapes that extend along a longitudinal axis of the electrical conductor and wrap around the outside of the electrical conductor. The electrically conductive tapes would not form a cylindrical current loop and possible eddy currents can be suppressed. The suppression of the eddy currents can help an electromagnetic signal in the VHF/UHV range to transmit therethrough. 
     Some embodiments described herein provide temperature-sensing apparatus that include a passive SAW temperature sensor. The passive SAW temperature sensor can be hermetically sealed system which can be exposed to harsh temperature environments and measure the temperature of an electrical conductor with no external physical stress or change in the mechanics of the sensor. Some passive SAW temperature sensors described herein can undergo many cycles of measurement without inducing failure mechanisms such as, for example, mechanical stress. 
     Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment,” whether or not including the term “exemplary” preceding the term “embodiment,” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the certain exemplary embodiments of the present disclosure. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the certain exemplary embodiments of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. 
     While the specification has described in detail certain exemplary embodiments, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. Accordingly, it should be understood that this disclosure is not to be unduly limited to the illustrative embodiments set forth hereinabove. In particular, as used herein, the recitation of numerical ranges by endpoints is intended to include all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). In addition, all numbers used herein are assumed to be modified by the term “about.” Furthermore, various exemplary embodiments have been described. These and other embodiments are within the scope of the following claims.