Abstract:
Temperature-indicting system including a mechanical temperature-indicating system including a bourdon tube for directly driving an indicating pointer, and an electrical temperature-indicating system including an electronic sensor, a transmitter and one or more electrical switches. The transmitter is modular and easily replaceable. Output of the pointer and the transmitter is synchronized relative to accuracy so that comparison of switch points of the switches that correlate temperature determined by the mechanical system and temperature determined by the electronic system enables detection of a failure or inaccuracy in one or both systems.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to instruments used to monitor and control power transformers that are subject to harsh environments and furthermore, to power transformer monitoring and control instruments having improved reliability in that they feature a dual and redundant monitoring and control system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Traditionally, control of power transformers entails use of a mechanical thermometer with a high torque mechanism that can operate up to six switches to control cooling apparatus of the power transformer, such as circulating oil pumps and fans, with the switches being sellable by the user. The switches set in the highest part of their possible range are typically used for high temperature alarms and emergency shut down. 
         [0003]    The two major types of power transformers are oil temperature controls and winding temperature controls. A winding control sensor is normally mounted in a thermowell that includes a heater having a current that is proportional to the main transformer load current. With the advent of computers, it has become popular to generate an electronic signal from both the oil and the winding control sensor for input to a computer for remote monitoring and to collect the temperature data history. 
         [0004]    Thermometers with switches as output to control equipment have been widely used. For example, U.S. Pat. No. 3,114,903 (Murphy) includes a switch inside a case of a pressure gauge or thermometer for use as an alarm or safety shutdown. 
         [0005]    Attempts have been made to replace the mechanical controls with electronic controls that offer advantages such as calculating and compensating functions. In this regard, U.S. Pat. No. 4,745,571 (Foster) describes a conventional mechanical system for control and monitoring of power transformers using separate winding and oil mechanical systems and improves on this conventional system by incorporating fully electronic controls. While some of these electronic controls are in use, the resistance of users to depend on electronics is strong in view of the harsh environments that electronics are subject to including for example, temperature extremes, both high and low humidity levels, high electromagnetic fields, and lightning. It is more common to combine the electronics as an output feature into the traditional filled system remote mechanical controls. 
         [0006]    One common method for combining an output for computer monitoring is described by Messko in a technical brochure IN2100/00/01 entitled “Universal Retrofit Kit”. In the Messko control technique, a thin film pressure transducer is soldered into the liquid-filled thermal system within the case housing, and a correlation is made between pressure and temperature and converted to a 4 mA to 20 mA current output corresponding to 0° C. to 160° C. The current is typically connected to a precision resistor and the voltage across the resistor is read by an Analog to digital converter for use in a data acquisition computer, this 4 to 20 milliamp current loop being a common technique used in industrial automation and control. 
         [0007]    Another method of integrating the output signal with a mechanical control is to use a precision potentiometer connected to the shaft of the mechanical control pointer in combination with a circuit to convert to a current loop signal. Both of these methods have the limitation of being dependent on the integrity of the filled systems, liquid filled in both cases, and having accuracy, repeatability and environmental errors no better than the filled system that they are dependent upon. Another disadvantage of these methods is that the entire control system would need to be replaced in the case of a failure requiring the transformer to be shut down. 
         [0008]    The prior art systems inherently include errors due to ambient temperature changes because the liquid in the capillary and head usually expands with higher exposure temperature and contracts with lower exposure temperatures. A compensator is normally included in the instrument housing to offset this error at one point, usually at the mid point of the span, but no attempt is made to compensate for the change of the capillary temperature. The compensation techniques may include their own accuracy offset and need to be tested, sorted and matched to the thermal system and only at one temperature point. In addition, the compensation is not integrated in the electronic output thereby causing wide discrepancies between the mechanical and electronic systems, which in turn, cause an ambiguity regarding what the actual temperature is and thus whether to rely on the temperature measurement provided by the mechanical system indicator or by the electronic output. 
       SUMMARY OF THE INVENTION 
       [0009]    An indicating temperature switch in accordance with the invention includes an electronic temperature sensor integrally mounted in the top of a filled system bulb having substantially the same thermal path to the process being measured, and connection wires protected within capillary armor and entering a thermometer case with a connecting capillary. The capillary is connected to a low volume bourdon tube that directly drives an indicator pointer and multiple switches. The sensor wires are connected to a modular transmitter that can easily be replaced by removing a front window of the case detaching and reattaching the sensor and loop control wires and closing the front window. 
         [0010]    A low volume enhanced gas thermal system with a low expansion and low elastic temperature coefficient is used to avoid the need for a mechanical system compensator. Such a low ambient thermal system is described, for example, in the inventors&#39; earlier U.S. Pat. No. 4,646,406, incorporated by reference herein. The pointer and switches are directly operated to provide 180° angular rotation to improve accuracy in temperature and switch settings and to maintain a low switch differential. 
         [0011]    The rotating bourdon tube drives a plurality of switches, e.g., six or more, with a shifting micro switch arrangement, such as that described in U.S. Pat. No. 6,492,608 incorporated by reference herein, to avoid the additive effect of friction. 
         [0012]    The combination of the duality and redundancy of the mechanical and electronic systems with the low ambient effect thermal system and the common thermal path of the mechanical and electrical sensor renders the present invention advantageous in comparison to conventional temperature indicating systems. 
     
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0013]    The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings wherein like reference numerals identify like elements. 
           [0014]      FIG. 1  shows a prior art filled system with a pressure transducer attached to a capillary. 
           [0015]      FIG. 2  shows a prior art filled system with a potentiometer attached to a pointer shaft. 
           [0016]      FIGS. 3A and 3B  are exploded views of a dual temperature indicating system of a preferred embodiment of the invention. 
           [0017]      FIG. 4  shows a thermal system and cross-sectional view of the sensor. 
           [0018]      FIG. 5  is a frontal drawing of the controller and transmitter access. 
           [0019]      FIG. 6  is a graphical chart showing data for several sensors. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring to the accompanying drawings wherein like reference numerals refer to the same or similar elements,  FIG. 1  shows a prior art liquid-filled thermal system including a spiral bourdon tube  1 , and a sensing bulb  2  attached to the bourdon tube  1  by a connecting capillary  3 . The connecting capillary  3  can be only a few meters in length up to 20 meters long. The connecting capillary  3  and bourdon tube  1  are attached with a connector  4  that also includes a fill tube  5  and another capillary  6  that is attached to a pressure transducer  7 , e.g., soldered thereto. The pressure transducer  7  senses the internal pressure of the system at the case near the bourdon tube  1 . The spiral bourdon tube  1  rotates in a clockwise manner with increasing pressure and drives a pointer and several switches, not shown. 
         [0021]      FIG. 2  illustrates another arrangement of the prior art liquid-filled system including similar elements as shown in the system of  FIG. 1 , namely, a sensing bulb  8 , connecting capillary  10 , connector  11  and a spiral bourdon tube  12 . In this embodiment, the spiral bourdon tube  12  is attached to a shaft  13  suspended between bearing plates  15  and  16  and drives an indicating pointer  18  over a scale  17 . The shaft  13  is also attached to a rotation sensor  14 , which may be, for example, a precision potentiometer. The rotation sensor  14  is part of a circuit  19  that converts the angle of rotation sensed by rotation sensor  14  to, for example, a 4 to 20 milliamp output. The advantage of this embodiment over the embodiment shown in  FIG. 1  is that adjustment of the bourdon position adjusts the potentiometer as well as the pointer  18 , and adjustment of the pointer  18  alone does not generate the 4 to 20 mA output signal. The adjustment is generally called an offset adjustment and is used to make one point of the range more accurate. 
         [0022]    Another element connecting the bourdon tube  12  to the shaft  13  is a bimetallic compensator  20  that also has a rotation effect to counteract, or compensate for the effect of ambient temperature error as a result of the volume expansion of the liquid in the bourdon tube  12  and capillary  10 . Bimetallic compensator  20  also compensates for the effect of temperature on properties of the bourdon tube  12 , e.g., the length expansion and strength, spring rate and other changes of the bourdon tube  12 . 
         [0023]    The prior art thermal system of  FIG. 1  may also include a compensating bimetallic compensator. Other systems not described here use counteracting or counter bellows to drive a mechanism effecting rotary motion, and in these constructions the counter bellows is expected to compensate for ambient temperature changes. The same counter balanced spiral bourdons could also be used; however, these systems restrict the output and/or increase the internal pressure causing less accuracy and resolution and higher ambient error at the extremes of the temperature indicating range. 
         [0024]    All of the prior art systems described above have only a single point temperature compensation and have the transmitter output directly responsive to the mechanical system, whereby a failure of the mechanical system also causes the electrical system to fail, and a failure of the electrical transmitter requires the replacement of the entire system. A slow leak, for example, would cause the mechanical system and the electronic output to both read low and some time may pass before the low readings are discovered. 
         [0025]    Referring now to  FIGS. 3A and 3B  which illustrates a preferred embodiment of a system in accordance with the invention,  FIG. 3A  shows the arrangement of the components of the system including a mechanical temperature-responsive system having a low ambient temperature error and being filled with a thermometric filling material, and an electronic temperature system including a temperature sensor and a replaceable electronic transmitter including a temperature sensor in thermal contact with the temperature sensing chamber of the mechanical temperature responsive system. The mechanical and electronic systems provide substantially the same temperature reading over a wide ambient temperature range. 
         [0026]    More specifically, the system includes an uncompensated, edge welded, spiral bourdon tube  25  mounted between an upper mounting plate  26  and a lower mounting plate  27  with a shaft  28  rigidly attached to a hub  29  of the bourdon tube  25 . Bourdon tube  25  may be a low volume, edge-welded bourdon tube, and additionally or alternatively, may have either a spiral or a helical form. 
         [0027]    To reduce friction, the shaft  28  is pivoted through an upper bearing  30  and a lower bearing  31 . A bourdon tube mounting bracket  32  is brazed to the bourdon tube  25  and attached to the lower mounting plate  27  through an adjustable gear mechanism, not shown in  FIG. 3A , that attaches to a zero offset adjustment shaft  33 . The upper and lower mounting plates  26 ,  27  are rigidly attached to one another by an attachment mechanism, such as a pair of spaced apart posts  34  and  35 . 
         [0028]    An adjustable switch assembly  36  rides on the upper mounting plate  26  and has a switch or set point indicator  37  that coincides with the divisions in a scale  38 . Switch assembly  36  may be a cam drive switch assembly. Although only one switch is shown in the switch assembly  36 , typically up to six switches are used for control and alarm functions. A 4 to 20 milliamp transmitter  39  has a center hole that fits over the zero offset adjustment shaft  33  and is mounted to an auxiliary mounting plate  40  secured to the lower mounting plate  27 . A cover plate  41  is attached to standoffs  42  and  43 , for example, with screws  44  and  45 . 
         [0029]    The entire assembly is inserted into a protective case  46  that includes a detachable 14-pin electrical connector  47 . The thermal system, i.e., the system including the temperature sensing chamber or temperature bulb, is attached to the spiral bourdon tube  25  by a capillary and case through a fitting. Once assembled, the 4 to 20 milliamp transducer can be easily accessed by the removal of the two screws  44  and  45  and the cover plate  41 . 
         [0030]    An access hole in the cover plate  41  allows offset adjustment of the mechanical system without removing the cover plate  41 . 
         [0031]      FIG. 3B  shows the inside view of the exploded system. Gears  48  and  49  are attached to the zero offset adjustment shaft  33  and to the bourdon tube mounting bracket  32  to enable adjustment of the mechanical indicator once the cover plate  41  is removed, as described above. 
         [0032]      FIG. 3B  also shows the bourdon tube assembly partially inserted into the case  46 , and the case  46  is cut away for viewing the internal components. On the bourdon tube shaft  28 , a switch arm assembly  50  is attached to operate the adjustable switch assembly  36 . The offset adjustment gears can be seen attached to the back side of the lower mounting plate  27  and connected to the zero offset adjustment shaft  33  providing easy access to adjust the position of the pointer  66  (see  FIG. 5 ). 
         [0033]    The 4 to 20 milliamp transmitter  38  is easily attached and removed with two mounting screws  52  and  53  that are threaded into an auxiliary mounting plate  40 . The zero offset adjustment shaft  33  extends through the auxiliary mounting plate  40  and through a center hole  54  of the transmitter  39 . During installation and assembly of the transformer or other equipment the control system is used for, it may be desirable not to have the transmitter  39  connected because high frequency waves from arc welding and other processes during the construction of the transformer may damage the transmitter  39 . 
         [0034]    An electrical connector  47  is shown attached to the lower part of the case  46 . Wires to the switches and to the 4 to 20 milliamp loop connections of the transmitter  39  are soldered to the internal side of pins  55 . A harness, not shown, attaches to the connector  47  and connects the switches to, for example, circulating pumps and fans to cool the transformer and also to a high temperature alarm and to a computer interface to convert the 4 to 20 milliamp signal to a voltage that can be converted to a digital signal for computer monitoring and data logging by a computer or data logger, or other processing unit. 
         [0035]    The assembly is described in this manner to demonstrate the independence of the mechanical and electronic systems so that the commonality of accuracy over a wide environmental range provides a new benefit of enhanced reliability for critical control applications. 
         [0036]    A discrepancy between the temperature readings provided by the mechanical and electronic temperature-responsive systems in this embodiment of the present invention is an indication of a failure of either the mechanical or electronic system that would not otherwise be detectable in currently employed systems. 
         [0037]    The control systems described herein as the preferred embodiment applies to the control of large power transformers where the life of the transformer can be greatly increased and the operation of the transformer is not compromised by the need to power down for service. Application of the principles are not, however, limited by this example and the invention may be applied to many critical control situations. 
         [0038]      FIG. 4  shows the sensor that is coupled to the bourdon tube  25  of the system shown in  FIGS. 3A and 3B . The sensor comprises a rigid wall tube  56  defining a cavity having a fixed volume. The tube  56  can have a length of, for example, 100 mm and an outside diameter of, for example, 10 mm, and may be made from for example, stainless steel. A bottom plug  57  is welded or otherwise attached to one end of the tube  56  and a bulb connector  58  is welded or otherwise attached to the top of the tube  56 . This is commonly called a temperature sensing bulb or simply a bulb. The bulb, in a gas filled system, may be filled with activated charcoal to adsorb nitrogen with decreasing temperature and desorb (outgas) nitrogen with increasing pressure thereby causing pressure changes with temperature that exceed the changes due to the natural gas laws where absolute pressure and temperature are directly proportional. 
         [0039]    Bulb connector  58  includes a nose section that is welded or otherwise attached to a capillary  59  made from, for example, stainless steel. The capillary  59  can be anywhere from a few meters up to 20 meters. For short capillary systems, the volume of the capillary  59  is not significant and filling the bulb with a liquid, instead of activated carbon and gas, will work as the effect of the liquid expansion in the capillary due to temperature change is small when the capillary is short. 
         [0040]    The capillary  59  is attached to a case connector  60  and a filling capillary  64  is also connected to the case connector  60 . The inside of the case connector  60  is attached to a connecting capillary  62  leading to the spiral bourdon tube  25 . All connections are made to provide air-tight seals and with minimum volume. The capillary  59  has a very small bore of about 0.13 mm diameter, and the coil may be constructed of, for example, edge-welded, 43% Nickel alloy to minimize internal volume and a change of elastic properties with temperature as with the inventor&#39;s earlier invention. The low volume and stable properties of the bourdon tube  25  are most important to having a low ambient temperature error and thus the system does not require a temperature compensator. 
         [0041]    A PT100 resistance detection device (RTD) sensor  63  may be located in a hole in the bulb connector  58 . Sensor  63  is in close thermal contact and substantially follows the same temperature of the entire bulb. 
         [0042]    In addition, a spare sensor, not shown, can be embedded into the other side of the bulb connector  58  for use at a future date. 
         [0043]    A thermocouple or thermistor can be used as the electronic sensor  63 . The RTD sensor  63  includes three wire leads  61 ,  61 ′ and  61 ″ that follow the capillary  59  and are within a protective casing or flexible armor  69  and feed through the case connector  60  to the case assembly where the wires are attached to the aforementioned transmitter  39  by, for example, a screw terminal attachment. The third wire, commonly configured, is used to compensate for the line length and the resistance of the wires. 
         [0044]    By removing the case window and then removing screws  44  and  45 , access to the transmitter  39  and transmitter connections becomes possible. 
         [0045]    Referring to  FIG. 5 , a front view of the control system is shown with the cover plate  41  removed. A pointer  66  is fixed to the shaft  28  and is rotatably driven by the spiral bourdon tube  25 . A pointed end of the pointer  66  travels over the divisions in the scale  38 . The scale  38  itself is normally printed on metal, laser etched or printed by a sublimation process. The laser etched method and the sublimation printing may be controlled by a computer enabling the nonlinearity of the scale to exactly match the output. Alternately, the scale  38  may be made as a mask having translucent divisions and numbers and is back-lighted with an electro-luminescent panel powered by alternating current attached to two tabs  67  and  68 . 120 VAC 60 hertz or 220 VAC 50 Hertz can power the EL panel directly for readability at night or in the dark. 
         [0046]    The zero offset adjustment shaft  33  has a slot or as shown in this embodiment, an Allen key recess, and is located inside the center hole of the transmitter  39 . To replace the transmitter  39 , a user must detach the wire connections from the sensor  63 , i.e., wires  61 ,  61 ′ and  61 ″, and disconnect the loop 4 to 20 mA wires  65  and  66  and the two transmitter mounting screws  52  and  53 , and then the transmitter  39  can easily be lifted off the zero offset adjustment shaft  33 . A replacement transmitter can then be reattached in the reverse order. 
         [0047]    For more critical applications as already explained, a second sensor may be attached to the bulb connector  58  and three spare wires are provided (not shown) so that in this manner, a sensor failure can also be easily repaired. 
         [0048]    Transmitters are available that can be individually calibrated to improve accuracy, and this calibration is based on at least 5 points within the temperature range. A preferred transmitter is of a type that can be calibrated by a computer and the calibration data saved. One such transmitter is manufactured by PR Electronics in Denmark, model PR5335. The original calibration data may be saved according to the serial number and downloaded to a new transmitter for retrofit purposes. 
         [0049]    Referring now to  FIG. 6 , a plot of three slightly nonlinear curves for three sensors are shown, the differences are actually exaggerated for the sake of illustration. The plot is of the calibration data and shows a slight deviation from a straight line of a PT100 resistance detection device. Data for any serial number can be downloaded to a replacement transmitter. Replacement transmitters can also be supplied with the control system so that time is not wasted when failure of a transmitter is discovered. Generally, environmental conditions, such as lightning strikes that cause transients and what is called a ring wave, can damage transmitters, and salt air and moisture are also causes for electronic failure. 
         [0050]    The independent dual, mechanical and electrical systems herein described have another advantage in that they can detect inaccuracies in either system. A current sensor attached to the switch wiring provides another input to the computer along with the transmitter analog signal. 
         [0051]    When the temperature rises, the pointer  66  passes over the switch indicator  37 , see  FIG. 5 , at 44° C. and the switch of the switch arm assembly  36  is activated by the switch arm positioned below the scale  38  as described above. A history of the transmitter signal when this switch is actuated is an indication of the status of the accuracy of both the mechanical and electronic system. If, for example, the transmitter  39  and switch indicator  37  indicate that over a period of time, this switch actuates at progressively higher temperatures, then this trend is an indication of a small leak in the filled system. Alternately, if the switch indicator  37  precedes the expected temperature, then a shift in the electronic signal indicates a failure of the electronic probe or transmitter  39 . 
         [0052]    Accordingly, a processor, such as a computer or data logger, is provided to compare a plurality of points over time at which the switch indicator, or an associated switch current sensor, determines that the switch of the switch arm assembly  36  is switched between on and off positions to the output signal of the transmitter  39 . The processor also determines when the switch point deviates from preset limits indicative of failure or inaccuracy in either the mechanical temperature indicating system or the electronic temperature sensor  63 . 
         [0053]    A control system with a replaceable transmitter is an advantage as the cost of replacement is very small compared to powering down the equipment. In the invention, this is accomplished by the independence of the design of both the mechanical and electrical systems as described. Transmitters capable of generating output signals, such as RS232 or RS485 digital communication or even wireless signals, are also possible within the scope of the invention. 
         [0054]    The foregoing structure thereby achieves and provides a dual and redundant temperature control for power transformers incorporating the reliability of a mechanical switch controller with a separate electronic sensor and transmitter. It also provides dual mechanical and electronic systems that may be matched in output to provide equal accuracy over the entire temperature measuring range and over a wide variation of ambient temperature conditions. Further, the structure enables easy replacement of an electronic transmitter without the need to power down the transformer with which the transmitter is associated. The invention also provides a method of comparing the mechanical and electronic temperature reading and automatically self checks to determine a fault in either of the independent systems. 
         [0055]    While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.