Abstract:
A load tap changer (LTC) having a plurality of windings is coupled to one of the primary and secondary of a power transformer in order to regulate the output voltage of the transformer. The LTC includes a plurality of taps physically and electrically connected to and along the windings and a contacting element is selectively moved along the taps to increase or decrease the output voltage of the transformer. The power transformer and the LTC windings are placed in a main tank and the taps are placed in an LTC tank. The temperature in the main tank and the temperature in the LTC tank are monitored by means of first and second temperature probes whose outputs are used to sense the temperature differential (T DIFF ) between the main tank and the LTC tank and to determine if the LTC tank temperature exceeds the main tank temperature for a period of time exceeding a specified time period. Also included is circuitry for sensing the rate of change of T DIFF  and determining if it exceeds a predetermined value.

Description:
This invention claims priority from provisional application Ser. No. 60/716,996 titled Load Tap Changer Condition Monitoring Method filed Sep. 14, 2005 and provisional application Ser. No. 60/717,000 for Load Tap Changer Position Monitoring Method filed Sep. 14, 2005. 

   BACKGROUND OF THE INVENTION 
   This invention relates to apparatus and method for sensing certain components of a load tap changers (LTC) under various operating conditions. 
   Load Tap Changers (LTCs) are used in electric power systems to regulate the voltage distributed from substations and along the power lines. An LTC, as used and defined herein and in the appended claims, may be connected in the primary circuit of a power transformer, XFR, as shown in  FIG. 1 , or in the secondary circuit as shown in  FIG. 2 .  FIG. 1 , is a highly simplified version of a prior art system illustrating use of one type of LTC connected in the primary circuit of a power transformer (XFR). In  FIG. 1 , there is shown the primary (P 1 ) of a power transformer (XFR) to which is coupled the windings  100   a  and taps  100   b  of a load tap changer (LTC),  100 . Note that in the discussion to follow and in the appended claims, windings  100   a , whether connected in the primary or the secondary of the power transformer, may also be referred to as the LTC windings. The LTC may be used to change the effective turns ratio (N 1 :N 2 ) of the primary and secondary of the power transformer XFR and thereby its output voltage (Vout). The LTC  100  of  FIG. 1  is shown to include several taps (T 0 -T M ) which are contacted with a movable contacting element, or contact, C 1 . The number of taps may vary from a few to many. The movable contact C 1  is shown mounted on a tap changer mechanism  105  which is caused to move along the taps T 0 -T M  by a rotatable shaft  103  driven by a motor M 1 . The shaft  103  can move in a clockwise direction or in a counterclockwise direction and causes contact C 1  to advance from tap to tap. For purpose of illustration, in  FIG. 1 , the contact C 1  is shown to be movable in either a down to up direction (from T 0  to T M ) or in an up to down direction (from T M  to T 0 ). In actual systems, the taps may be physically arranged in a circular pattern and the contacting element would then move along a rotary or other suitable path, rather than linearly up and down. 
   In  FIG. 1 , the windings  100   a , extending between nodes  14  and  16 , are connectable in series with the primary windings (P 1 ) of the power transformer XFR. One end  11  of P 1  is connected to an input power terminal  17  while the other end  13  of P 1  is connected to the top end  14  of the windings  100   a . Taps T 0  through T M  are disposed along the LTC windings, with the lowest tap, T 0 , corresponding to node  16  and the highest tap, T M , corresponding to node  14 . For ease of illustration, contact C 1 , shown mounted on a movable arm depending from mechanism  105 , is electrically connected to input power terminal  19  and provides a very low impedance connection between terminal  19  and whichever tap it is contacting. The input power Vin is applied between terminals  17  and  19  and is redistributed via the secondary of the power transformer, XFR, onto output power lines  21 ,  23 . When C 1  is connected to tap T 0  the primary winding P 1  is connected in series with all the windings  100   a  of the LTC and the effective turns ratio of the primary (e.g., N 1 ) to the secondary (e.g., N 2 ) has been increased. For this condition, the output voltage (Vout) produced at the output of the secondary (SEC 1 ) is decreased. When C 1  is connected to tap T M  the effective turns ratio of the primary to the secondary is decreased and the output voltage (Vout) produced at the output of the secondary (SEC 1 ) is increased. 
   In the operation of the system (see  FIGS. 1 and 2 ) the voltage Vout, across the secondary of the power transformer is supplied, via a transformer PT 10 , to a tap change controller  101  which senses the voltage and produces signals identified as K 1  (lower) and K 2  (raise). Signals K 1  and K 2  are applied to the motor M 1  and determine whether the motor is driven in a clockwise or counterclockwise direction causing shaft  103  to turn so as to raise or lower tap changer mechanism  105  causing C 1  to move along the taps of the LTC windings  100   a . If Vout is below some desired level, the controller  101  produces signals (K 1 , K 2 ) which function to tend to raise Vout to the desired value. Likewise, if Vout is above some desired level, controller  101  produces signals (K 1 , K 2 ) which function to tend to lower Vout to the desired value. 
   As noted, motor M 1  causes the rotation of drive shaft  103  on which is mounted tap changer mechanism  105  which controls the movement of contacting element C 1  along the taps  100   b  of LTC windings  100   a . Mechanism  105  may include gears, cams and switches (not shown) which cause the contact C 1  to make contact with the taps in a predetermined sequence. 
   In the configuration of  FIG. 2 , windings  100   a  are connectable in series with the windings of the secondary of the power transformer. As in  FIG. 1 , which one(s) of the windings  100   a  get connected in circuit with the secondary windings is a function of which tap is contacted by contact C 1 . For the condition of contact C 1  connected to tap T 0 , the turns ratio of the primary to secondary is decreased (Vout is increased). For the condition of contact C 1  connected to tap T M , the turns ratio of the primary to secondary is increased (Vout is decreased). In  FIG. 2 , as in  FIG. 1 , the voltage across the secondary is coupled via a transformer PT 10  to a tap change controller  101  which drives a motor M 1  which drives a shaft  103   a  which causes a mechanism  105   a  to raise or lower the contact C 1  to produce a desired Vout. Thus in  FIGS. 1 and 2  there is a feedback loop including controller  101  which functions to try to maintain the output voltage at a desired value. 
   It should be noted, as detailed below, that the power transformer is normally located in a main, oil filled, tank and the LTC taps are located a separate, oil filled, tank, referred to herein as the LTC tank. Generally the temperature of the main tank is significantly higher than the temperature of the LTC tank. However, problems exist in that, for some operating conditions, the temperature of the LTC tank may increase and be greater than the temperature of the main tank. For example, some of the taps may be, or become inoperative. When this occurs the temperature of the LTC tank may rise considerably and exceed the temperature of the main tank. The increase in temperature, especially if it persists for a long time, may result in a highly dangerous situation. Also, due to some malfunctions, the temperature of the LTC tank may rise at a faster rate than a specified amount. 
   It is an object of this invention to monitor the temperature of the main tank and of the LTC tank and to identify problem conditions to prevent sensed increases in temperature from resulting in a dangerous condition. 
   SUMMARY OF THE INVENTION 
   Systems and methods embodying the invention include: (a) means for sensing and monitoring the LTC tank temperature versus the main tank temperature to determine if, and when, the temperature of the LTC tank exceeds that of the main tank; and (b) means for determining the rate of rise of the LTC tank temperature (or a differential temperature rise) to monitor any change occurring at a relatively rapid rate. 
   The temperatures of the main tank and of the LTC tank are continuously monitored to determine if, and when, the temperature of the LTC tank exceeds that of the main tank and if the condition persists for more than a predetermined period of time. This measurement is generally intended to sense the occurrence of a relatively slowly developing problem. In accordance with the invention, the rate of rise of the LTC tank temperature is also monitored to determine whether any rapidly evolving problems (e.g., due to arcing) are present. 
   Sensing and monitoring slowly and rapidly evolving problematic conditions results in an improved and efficient system for generating alarms and taking necessary steps to prevent significant damage and/or a dangerous condition from becoming overwhelming. 
   In one embodiment, the arithmetic difference of the temperature between the main tank (T TANK ) and the LTC tank (T LTC ) is calculated to determine whether the temperature in the LTC tank is more, or less, than the temperature in the main tank. This is monitored to determine if, and when, the temperature of the LTC tank exceeds the temperature in the main tank. If the LTC tank temperature (T LTC ) exceeds the main tank temperature (T K ) by a preset amount for longer than a preset period of time, alarm conditions are produced indicating that a problem may be present. In addition, for each tap position the corresponding temperature of the LTC tank is monitored to determine whether there are any heating problems associated with that tap position. This information is important to determine whether a tap position is defective and whether corrective action should be taken (e.g., the contacting element may be moved to another tap and the defective tap by-passed at this time and in the future). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawing like reference characters denote like components; and 
       FIGS. 1 and 2  are highly simplified semi block, semi schematic, diagrams of prior art circuits including a power transformer with a load tap changer (LTC); 
       FIG. 3  is a simplified block diagram of a main tank housing a power transformer side by side with an LTC tank housing the LTC taps with their temperature probes and also showing a control cabinet; 
       FIG. 4  is a simplified semi block, semi schematic, diagram of circuitry used to practice the invention; 
       FIG. 5  is a block diagram of circuitry for processing temperature information in accordance with the invention; 
       FIG. 6  is a diagram of waveforms illustrating the detection of slowly evolving heating conditions; and 
       FIG. 7  is a diagram of a waveform illustrating the detection of a rapidly evolving heating condition. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Note that certain aspects of this invention are also described in my co-pending application titled APPARATUS AND METHOD FOR MONITORING TAP POSITIONS OF LOAD TAP CHANGER bearing Ser. No. 11/520,821 and filed on the same day as this application and the teachings of which are incorporated herein by reference. 
   As shown in  FIG. 3 , the main power transformer, XFR, the LTC windings  100   a  and the potential sensing transformer PT 10  may be housed in a main tank  401 . The LTC taps  100   b  (taps T 0 -T M  connected to windings  100   a ) may be housed in a different, adjacent, LTC tank  403 . The tap change controller  101  and the motor M 1 , as well as some of the system electronics, may be located in an adjacent control cabinet  405 . The tanks  401  and  403  may be filled with a fluid (e.g., oil) for distributing the heat generated by their respective components and preventing any hot spots. A main tank temperature probe, TP 1 , (also called the top oil temperature probe) may be used to measure the temperature of the main tank  401 . The LTC temperature probe, TP 2 , may be used to measure the temperature of the LTC tank. In general, the main transformer tank  401  and the LTC tank  403  are separate tanks and do not share the same fluid. However they are thermally connected. The volume of oil in the main tank is generally much greater than that in the LTC tank. As shown in  FIG. 4 , the outputs of probes TP 1  and TP 2  are fed via analog to digital converters to a microcontroller for processing the temperature information and for comparing measured temperature signals versus specified values. 
   An aspect of the heating problem may be better understood by noting that the main tank  401  contains the transformer primary and secondary windings and, usually, the LTC windings  100   a  and potential transformer PT 10 . With loading, these windings generate heat due to I 2 R losses in the windings and eddy currents in the steel core. The heating in the main tank influences the temperature in the LTC tank. But, the temperature of the main tank should generally be higher than the temperature of the LTC tank since there is no significant source of heat in the LTC tank, when the LTC is operating correctly. However, heating within the LTC tank may be caused by a number of factors. For example, heating can be caused by arcing due to dielectric breakdown or, if equipped with vacuum interrupters, a breach in the interrupter. Another source of heating may occur in the LTC tank due to carbonization of the switching contacts. This phenomenon is also known as “coking”. For example, the oil in the LTC tank  403 , which is present between a contact and a tap position, may begin to polymerize due to conduction between the contact and the tap. As this polymerization takes place the resistance of the contacts increases. At first it may be virtually undetectable. However, the polymer film may begin to burn and, as it carbonizes, there is a further increase in the contact resistance. This gives rise to a vicious cycle that eventually causes the contacts to get so hot that the oil in the LTC tank may become hotter than that of the main tank. Abnormal heating may cause the evolution of combustible gases, which create high pressure within the LTC tank leading to catastrophic failure. Coking and polymerization effects tend to develop slowly. Problems such as arcing evolve quickly with little warning. The malfunctions discussed above may result in damage, which may be irreversible, to the LTC and to the power transmission system. It is therefore important to have reliable information regarding both types of problem conditions and to be able to process the information accurately. 
   In accordance with one aspect of the invention, the arithmetic difference of the temperature between the main tank and the LTC tank is calculated to determine whether the temperature in the LTC tank  403  is more, or less, than the temperature in the main tank  401 . This is monitored to determine if, and when, the temperature of the LTC tank exceeds the temperature in the main tank. If the LTC tank temperature exceeds the main tank temperature for longer than a preset period of time a problem may be present and an alarm signal is produced. 
   In accordance with another aspect of the invention, the LTC tank temperature is monitored for each tap position to determine potential problems associated with a tap generating excessive heat. This information is important to identify defective or “bad” tap positions. A tap position is defective (“bad”) when that tap is being contacted by the contacting element and the LTC tank temperature is greater than the main tank temperature (or some specified value of temperature) for an extended period of time (e.g., a period of several hours). Each defective or “bad” tap position is identified and recorded and the system (e.g., microcontroller  150  in  FIG. 4 ) is programmed to cause the contacting element to move off the bad tap and, if needed, to by-pass the “bad” tap in the future. The by-passing of a bad tap requires careful system programming to ensure that the feedback loop including tap change controller  101  accepts the value of Vout produced by contacting the next tap (up or down) to a bad tap. 
   As already noted,  FIG. 4 , shows that the temperature in the main tank is constantly monitored via the top oil temperature probe TP 1  whose output is fed via an A/D converter  201  to the microcontroller  150 . Likewise the temperature in the load tap changer (LTC) tank is constantly monitored via LTC temperature probe TP 2  whose output is fed via an A/D converter  203  to the microcontroller. 
   Applicant recognized that the main tank temperature is generally higher than the LTC tank temperature since under normal operating conditions there are substantial heat sources in the main tank and very few in the LTC tank. Therefore, in order to sense a possible problem, the system is designed to sense the LTC tank temperature (T LTC ) minus the main tank temperature (T K ). So long as T LTC  is less than T K , there is no problem. However when T LTC  is higher than T K , by some predetermined amount and this temperature differential exceeds a predetermined value for longer than a predetermined amount of time, it is indicative of the existence of a problem. Consequently, the system is designed to alert the user or operator that there is a problem or malfunction which needs to be addressed. 
   In particular, reference is made to  FIG. 5  which shows TP 1  measuring the main tank (Top Oil) temperature applied to A/D converter  201  and TP 2 , measuring the LTC tank temperature, is fed to A/D converter  203 . The digital word representing the temperature of the tanks is fed into Main Tank Register  501  and LTC Tank Register  503  respectively. These registers are then fed into a subtractor  511  which computes T LTC −T TANK =T DIFF . The value of T DIFF  may vary as shown in  FIG. 6  and may be characterized as generally representing relatively slowly changing temperature conditions. Note that T LTC  is compared to T TANK . So long as T LTC  is less than T TANK , there is no need for concern and hence no output. It is only when the temperature differential (T DIFF ) between T LTC  and the main tank temperature (T K ) exceeds a predetermined set point (an amount shown as delta T 1  at time t 2  in  FIG. 6 ) and identified as Tsp, that a timer is set and begins to count the length of time that T DIFF  exceeds the set point temperature, Tsp. Signals corresponding to T DIFF  and Tsp (shown as Ref 1 ) are applied to comparator  531  which functions to detect when T DIFF  exceeds Tsp, the output of comparator  531  is fed to a timer  533  preset for a given time period and a latch  537 . A clock  506  is applied to timer  533  and, if the comparator output persists for the preset time period, a signal is applied to latch  537  causing an alarm to be generated (e.g., alarm  1  at time t 3  in  FIG. 6 ). 
   In accordance with the invention the rate of change in T DIFF  is also calculated and used to provide an indication of rapid changes. The rate of change is accomplished by means of registers  507  and  509  and a subtractor  511 . The registers  507  and  509  are clocked by clock  504  and function to compare a present value of temperature (at a time t 1 ) with a previous value of temperature (obtained or clocked at time t 0 ). Subtracting the two values of temperature and dividing by the time differential provides the value of “Delta T DIFF ” as shown in  FIGS. 5 and 7 . Delta T DIFF  and a ref 2  are applied to a comparator  513 . Ref 2  represents a specified value of permissible at which the temperature can change. If exceeded, it is indicative that the temperature is rising (changing) too quickly and that there may be a malfunction. Accordingly, when this happens, comparator  513  outputs a signal denoted as alarm 2  which is fed into an OR gate  535 . The other input to OR gate  535  is the alarm signal responsive to T DIFF . Thus, the system is designed to provide an alarm indication when there is a slow changing temperature problem condition and when there is rapid changing temperature problem condition. 
   Note that the circuit of  FIG. 5  is presented for purpose of illustration and that the microcontroller may be programmed and/or designed to provide the functions described above. 
   The steps to perform temperature sensing in accordance with the invention include:
         1—measure the main tank temperature (T K );   2—measure the LTC tank temperature (T LTC );   3—calculate T Diff =[(T LTC )−(T K )]; (normally T K  is greater than T LTC );   4—determine when T Diff  becomes positive; i.e., when (T LTC )&gt;(T K );   5—as an option, introduce an offset such that T LTC  must exceed T K  by some set temperature level (e.g., Tsp) to define an alarm condition. Tsp may range from zero to ten or more degrees.   6—specify the length of time (T LTC ) must exceed (T K ) for an alarm condition to be defined;   7—sense how long (T LTC ) exceeds (T K  ) for establishing an alarm condition and compare to specified period.   8—Concurrently, the rate at which T DIFF  changes as a function of time may be calculated by selecting a time increment (Delta t) and comparing the value of T DIFF  per time increment. For example:
           (i) A=[T DIFF  =T LTC −T K ] at time t=t 0 ;   (ii) B=[T DIFF  =T LTC −T K ] at time t=t 1 ; and   [A−B]/delta t, where delta t is equal to t 0 −t 1 , gives a rate of rise for the delta t selected   
           9. Specify the amount of permissible/specified change and compare to the calculated/measured value.   10. The rate of rise has been calculated for T DIFF , but a similar calculation could be done for T LTC .   11. Alarm signals are generated if the rate of rise of T DIFF  is greater than the maximum rate specified and/or if the LTC tank temperature exceeds the main tank temperature by a specified level for a specified period of time.
 
As discussed above, the temperature differential (T DIFF ) is equal to the temperature of the LTC tank (T LTC ) minus the temperature of the main tank (T K ). As shown in the figures and as discussed, circuitry or programming is provided to sense the rate of change of T DIFF  by including means for sensing T DIFF  at different points over a predetermined time interval (e.g., T DIFF  at a first time (t 1 ) and T DIFF  at a second time (t 2 )) where the time interval t 2 −t 1  is a pre-selected time interval. The time interval could be per minute, per hour or any other selected time. The actual rate of change is the determined by calculating T DIFF  at time t 2  minus T DIFF  at time t 1  divided by the time interval t 2 −t 1 . The obtained rate of change can then be compared to a maximum specified or desirable rate of change and circuits are provided to produce an alarm if the rate is exceeded.