Abstract:
A method and system is provided for electronically simulating, indicating and controlling the temperature of liquid cooled multi-phase electric power transformers. The temperatures of transformer windings are electronically computed from the actual top oil temperature and the computed incremental additional temperatures resulting from the transformer winding currents to determine the hottest spot temperature. The signals representative of the winding current for each phase of the multi-phase transformer are time processed to simulate the rate of rise of winding temperature resulting from those currents. A square-law function is generated to make a winding bias parameter for each winding. The signals representative of the actual and incremental additional temperature are added for each transformer phase and the largest signal, representative of the hottest winding is used to indicate and control the transformer temperature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for simulating, indicating and controlling the temperature of electric power transformers of the type used by public utilities. These types of transformers typically have their transformer windings immersed in a liquid coolant. During operation of the power transformer it is important to know the temperature of the hottest transformer winding, the maximum temperature each winding has reached, when cooling fans and pumps should be turned on and off and when power to the transformer should be turned off. 
     2. Discussion of Prior Art 
     This invention represents a variation of that described in U.S. Pat. No. 4,745,571 which issued from application Ser. No. 772,133 filed Aug. 30, 1985 and Ser. No. 829,214 filed Feb. 14, 1986 of Joseph F. Foster. It is known from prior systems to simulate the winding temperatures by measuring the top oil temperature of the transformer and biasing the thermometer reading for each winding, by an amount proportional to the corresponding transformer load, or winding current, so as to simulate each winding temperature. Such a system employs thermometers with sensing elements measuring the top oil temperature, current transformers to provide signals proportional to transformer loads, heater coils to provide the bias and auto transformers to adjust the current supplied to the heater coils. 
     It is an object of this invention, rather than have biasing heaters, to modulate the actual oil temperature in a transformer, to provide a method and apparatus to electronically determine the winding temperatures based on the oil temperature in the transformer and the increased temperatures that result from currents flowing through the transformer windings. 
     It is a further object to provide an improved adjustable transformer winding temperature simulator, indicator and controller which may be adjusted or programmed to provide variable temperature increments to the oil temperature in accordance with the multi-phase transformer manufacturers specification for temperature change at various transformer loads or winding currents. A different temperature increment is provided for each relay output to accommodate accurate control, and a complete separate simulator circuit for each transformer winding. 
     SUMMARY OF THE INVENTION 
     The invention aims to provide a new solution to the above-mentioned problems, that is, a multi-phase electronic temperature simulator, monitor and controller. Specific winding current temperature rise relationships can be programmed for any class of power transformer. Transformer top oil temperature is continuously measured. A sample of each transformer winding current is also continuously measured. Each winding current signal is then processed with a specific time constant such that its load current heating effect is time-scaled to approximate actual winding temperature rise due to its load current. Winding temperature rise is proportional to winding power dissipation and power dissipation is proportional to the &#34;square&#34; of winding current. Due to this square-law relationship, the load current signal is then applied to a function generator that creates a square law relationship between its input and output voltages. The function generator output is the winding temperature rise for the phase, and this winding temperature rise is then added to the top oil temperature analogy to accurately simulate and indicate the temperature of this winding. The signal representative of the highest winding temperature of the various phases is automatically selected by a high value selector. This hottest winding temperature analogy is then used to operate switches that operate transformer cooling devices, shut offs, and alarms. The hottest simulated temperature reached by each winding, and the hottest liquid (top-oil) temperature measured, are all stored in a non-volatile electronic memory, and these stored values can be displayed. 
     The multi-phase Electronic Temperature Controller allows separate programming of important parameters for Winding Temperature Simulations. The input current range, winding bias value, and time constant for each phase, as well as output relay set points which can be chosen independently so that an electric power transformer can be simulated more precisely than previously possible, and the simulation process retains its accuracy as conditions change. 
     An important feature over prior devices is the use of a complete, separate simulator circuit for each phase of a multi-phase transformer, so that each winding temperature is continuously available to the maximum memory circuitry and the remote winding temperature indicators through constant current sources provided. 
    
    
     DESCRIPTION OF THE FIGURES 
     FIG. 1 is a representation of one form of a prior art winding temperature simulation system. 
     FIG. 2 is a schematic block diagram of a multi-phase electronic temperature simulator, indicator and controller of this invention. 
     FIG. 3 is a schematic block diagram of one winding temperature simulator, including the Resistance Temperature Detector Interface. 
     FIG. 4 represents the function generator square-law curve, Vin v.s. Vout, showing break points and line segments. 
     FIG. 5 is an application example of using the square-law curve of FIG. 4. 
     FIG. 6 is a front view of the multi-phase electronic temperature controller. 
     FIG. 7A is a Current Transformer current example. 
     FIG. 7B is the simulated winding temperature example signal. 
     FIG. 8 is a front view of the indicator panel. 
    
    
     DESCRIPTION OF THE INVENTION 
     Because direct measurement of power transformer winding temperatures is noteconomically feasible, means to simulate these temperatures have been devised in the past. 
     One such prior system is shown in FIG. 1 and uses a heater coil in the transformer oil to increase the temperature of the oil near the heat sensor, by an amount approximate that which would occur from the current in the transformer winding. 
     In FIG. 1 the power transformer 10 has oil 11, a thermometer indicator 12 whereby an observer may view the reading of the power transformer winding temperature. The heater coil and sensing element 14 is provided to generate the input signal to the thermometer indicator 12 via capillary element 20. The sensing element portion of the heater coil and sensing element 14 responds to the temperature of the transformer oil 11 near the sensor to provide the reading information to the thermometer 12. Alarm leads 22 control alarm apparatus, not shown. The heater coil 14 is controlled by the current transformer 16 which is normally available from the transformer manufacturer who additionally is aware of the various temperature differences which may be expected to result from different load currents i.e. winding currents. A current balancing auto-transformer 18 responds to the current transformer 16 signal which is proportional to transformer load current and provides the heater coil 14 with the current to heat the oil near the sensor by an amount corresponding to that expected for a given load current. In this way it is seen that prior systems used a sensor to detect the top oil temperature, which was locallymodified or biased by a heating coil responsive to transformer winding current. Examples of prior art systems using a heater coil to heat the oilor liquid in accordance with winding current are shown or described in U.S.Pat. Nos. 2,834,920; 3,144,770; 3,148,349; and 4,258,570. Many electric power transformers contain three winding temperature simulators, one for each winding phase. 
     Turning now to the present invention, FIG. 2 is a schematic block diagram of the temperature simulators, indicator and controller. In essence, liquid oil temperature and the incremental temperature due to transformer winding current induced heating effect are added to generate simulated winding temperature values, which are used to determine the hottest winding temperature and control corrective apparatus such as fans, alarms,trips and winding temperature memory devices. 
     In a preferred embodiment of the invention, as shown in FIG. 2, a D.C. voltage from 0 to 3.33 volts is generated by the oil temperature circuitry24 responsive to a Resistance Temperature Detector (RTD) 26 and is applied to conductor 28 which then has a signal representative of liquid temperature. The indicator 30, when the &#34;Temp&#34; button 32 is depressed, displays the liquid temperature. 
     FIG. 3 includes a schematic block diagram of the RTD interface and amplifier functions. Liquid temperature signal 28 is also connected to theliquid temperature memory 34 where the hottest value will be stored. This stored value can also be displayed on temperature indicator 30. 
     Continuing with FIGS. 2 and 3, the current transformers 36, 38, and 40, forthe three phases, are connected to a complete winding temperature simulatorfor each phase. FIG. 3 is the schematic block diagram for one simulator. Each simulator first converts the input current from the appropriate current transformer C.T. to a D.C. voltage that represents present windingload current. This is accomplished by the Current Transformer Interface Circuit 42. The amplitude of this load signal is then programmed by winding bias adjustment 44 to provide the desired load current-to-winding-bias relationship at the required load current data point. Next, the load current signal 45 is time processed by its own time-constant (T.C.) circuit 46, such that the rate of rise and rate of decay of this load signal accurately approximates the time-constant response characteristic of load current heating of actual transformer windings. This time processed load signal on conductor 48 is then applied to its function generator circuit 50. This generator makes use of the square-law relationship between winding current and temperature rise to create the winding bias or ΔT, differential temperature, signal on conductor 52. The differential temperature or temperature differential represents the change in fluid temperature to be expected from the currentthrough the transformer winding. In the case of a 3 phase transformer therewill be three differential temperatures to be simulated, based on the current through each winding. The winding bias signal 52 is then added to the liquid temperature signal 28 in the adder circuit 54. The output 56 ofthe adder circuit 54 represents the present winding temperature for this phase. 
     A simulator for each phase creates the winding temperature signal for each phase. The highest winding temperature signal 57, indicating the hottest phase, is automatically selected by the high value selector 58. The high value selector 58 is in effect a comparator which permits the passage of the largest phase signal to be applied to conductor 57. This highest valueis displayed via Display Switching and Driver 29 on the display meter 30 and used to operate relays for cooling devices, alarms, and trip circuits.Any of the relays could also be operated from the liquid temperature signal28. The highest oil temperature and the highest winding temperature for each phase are stored in the maximum temperature memory 34, and can be displayed on the display meter 30. The memory devices retain the value of the highest winding and liquid temperatures measured during the time of interest. Between these times they can be manually reset. These memory devices are non-volatile memories. 
     Each of the output relays 60, 61, located on an Input-Output Termination module 62, responds to the winding or liquid temperature voltages on conductors 28 or 57. In each case a comparator 70, well known in the electronics art, compares the selected temperature signal with a preset temperature limit so that a relay driver will activate a relay 60 or 61 upon the event of the selected temperature value exceeding the preset limit. Each comparator circuit 70 has a preset differential, that is a hysteresis, such that when a relay is activated (the set point is exceeded) the relay will remain activated until the temperature value drops below the set point by an amount equal to the hysteresis. Hysteresisfor a fan circuit is typically set to 15° C. Exceeding the preset limit for a comparator will cause its relay to be activated or deactivated, as determined for each relay by jumper selection in its relaydriver circuit. 
     FIG. 3 includes a block diagram of the resistance temperature detector (RTD) circuit 24. Current source 25 and amplifier circuits 27 create a DC voltage on conductor 28, that is the analogy for liquid temperature measured by resistance temperature detector 26. In the preferred embodiment, 0 to 3.33 VDC on conductor 28 indicates and represents liquid temperatures of 0° C. to 120° C. 
     The interface circuits 42 and viewing resistor R, for C.T. current inputs from the constant current transformers (CT) 36 generate DC voltages that represent the winding load currents. The scale factor, for each interface,is set to 1.0 VDC per AC ampere of input CT current at conductors 43 and this scale factor is then divided and individually adjusted for each winding by setting the winding bias adjustment 44 for each winding input. In this way, an input current value can be adjusted to become a winding bias signal of 0 VDC to a portion of each interface output. The winding bias output 45 is then connected to time constant circuits 46 such that each winding load signal is time-scaled to accurately represent load current heating effects. The present devices are programmed for an eight minute time constant; that is, a total response time of forty minutes. Theoutput of each time constant circuit on conductor 48 amplifies its winding bias signal input by a factor of 100 such that the D.C. output fed to the function generator 50 can have any scale factor of from zero to 1.47 VDC per AC ampere of C.T. current, adjustable for each winding. 
     A characteristic of power transformers and their winding temperatures is that different transformers have different temperature characteristics. For each power transformer there will be a characteristic relationship between the incremental temperature variations ΔT and the winding currents. 
     FIG. 4 illustrates the function generator characteristic curve. The horizontal axis, Vin from the time-constant circuit conductor 48, is the bias-scaled, time-processed winding current signal. The vertical axis, Vout on conductor 52, is the function generator output voltage. The generator circuit creates a `square-law` relationship between Vin and Vout. Vout then represents the appropriate winding bias signal 52 which isthen added with the top oil signal 28 in the adder circuit 54 such that theadder output signal 56 represents simulated winding temperature. The simulated winding temperature signal for all phases are continuously evaluated by the high-value selector circuit 58 which selects the highest simulated winding temperature signal for display and control purposes. This selected highest winding temperature represents the hottest spot winding temperature which is used to control the output relays. These relays can be used to control cooling devices, alarms, trip circuits, annunciators etc. Any number of relays can be controlled in this manner and any relay can be activated by comparators that act upon either liquid temperature or the hottest winding temperature. 
     The square-law curve shown in FIG. 4 is easily applied to transformer winding temperature simulation as demonstrated in FIG. 5. The operating line (D) is constructed for an example transformer that exhibits a 160° C. winding temperature rise at 160% rated load. This operatingline, once constructed, will determine the winding temperature rise for other load levels. Because the time constant circuits hold back the winding bias calculations for five time constants, a time-constant bypass switch is provided to temporarily eliminate the time constant. As long as the T/C bypass switch is depressed, any input current that creates a winding bias signal will appear immediately on a winding temperature conductor. 
     The temperature signals are scaled such that 27.78 mVDC represents 1° C., for both winding and oil temperatures. 
     The following example describes operation of a simulator as shown in FIG. 3A. Winding bias has been set for VBias=1 V./A. of C.T. current. See FIGS.7A and 7B. 
     I. Initial &#34;steady-state&#34; conditions: at time T=0. 
     1. Oil Temperature=45° C. 
     2. C.T. (1) current=1.9 A.:V Current=1.9 VDC 
     3. VBias=1.9/100=19 mVDC on conductor 45 
     4. T/C Signal=1.9 VDC=1 V/A 
     5. ΔT Signal=280 mVDC=See FIG. 4. 
     6. Oil Temperature Signal=1.25 VDC 
     7. Winding Temperature Signal=1.53 VDC 
     Indicating a present winding temperature of 55° C. 
     II. Rising Temperature Condition: Time T&gt;0 
     1. At T=5 min. 
     a. C.T. current, FIG. 7A abruptly rises from 1.9 A to 3.6 A and remains at 3.6 A for 65 minutes: VCurrent becomes 1.9 VDC. 
     b. VBias immediately changes to 36 mVDC where it remains for 65 minutes. 
     c. T/C signal 48 begins charging exponentially from 1.9 VDC toward a final value of 3.6 VDC. 
     d. ΔT signal begins rising exponentially from 280 mVDC toward a finalvalue of 970 mVDC. 
     2. At 5≦T≦45 min.: 
     a. ΔT signal is rising with a time constant of 8 minutes, toward 970 mVDC. 
     b. Winding temperature output is rising exponentially toward a final value of a final value of 80° C.--See FIG. 7B. 
     III. At 45 min.≦T≦70 minutes=all signals are stabilized, simulated winding temperature remains constant at 80° C. 
     IV. Falling temperature condition: At 70 minutes≦T≦110 minutes 
     a. Load current, FIG. 7A, abruptly drops from 3.6 A to the original value of 1.9 A and remains at 1.9 A for all remaining time. 
     b. VBias immediately drops back to 19 mVDC where it remains for all time. 
     c. T/C signal begins decaying exponentially, time constant=8 minutes, from 3.6 V back to the original 1.9 VDC value. 
     d. ΔT signal begins dropping to its original value of 280 mVDC. 
     e. Simulated winding temperature drops back to 55° C. 
     V Final steady state=Time&gt;110 minutes 
     All conditions have returned to the same values as those in Section I. 
     An example of designing function generators may be found in the publication &#34;Electronic Circuits: Discrete and Integrated&#34; by Donald L. Schilling and Charles Belove published by McGraw-Hill Book Company, Library of Congress Catalog Card No. 68-19493, pages 42-49, 1968. 
     FIGS. 6 and 8 show the physical configuration of the multi-phase electronictemperature monitor. The main component assemblies are electronic housing 72, input/output module 74, and remote front panel 76. All &#34;Field&#34; connections for the multi-phase device are made to and from the input/output module terminal block 78. FIG. 6 illustrates the modular construction of the multi-phase controller. Functional assemblies are mounted on the mounting plate. Within these assemblies, individual electronic modules are connected to perform required tasks. Modules are easily added, removed, repaired, replaced and/or examined. Similarly, configuration changes can be accomplished on a module-by-module or a function-by-function basis. 
     It should be unerstood that the initial load current signal may be obtainedfrom the current transformer 36 normally provided by the manufacturers of the power transformer. 
     In addition, the initial liquid temperature signal may be obtained from anyResistance Temperature Detector 26 such as described in the Transducer Interfacing Handbook edited by Daniel H. Sheingold pp. 2-5, 10 and 11, as well as other thermal sensors such as thermocouples, thermistors, optical sensors, etc. The Transducer Interfacing Handbook was published by Analog Devices, Inc., Norwood, Mass. in 1980 and 1981 (Library of Congress Catalog Card No. 80-65520. 
     It should be understood that the computations and signal representations may be in any one of several forms or parameters. The parameter utilized in the preferred embodiment is D.C. voltage and the description here is based on that parameter. 
     In addition to the details provided herein, the circuitry shown in the boxes of FIGS. 2, 3, etc. is of the type shown and described in the related U.S. Pat. No. 4,745,571 which issued from application Ser. No. 772,133 filed Aug. 30, 1985 for a Modular Electronic Temperature Controller and Ser. No. 829,214 filed Feb. 14, 1986 for a Tri-Phase Electronic Temperature Controller.