Abstract:
A system and method for remotely monitoring load tap changes on utility-type equipment are disclosed. In one example embodiment, a sensor measures a voltage before a tap change and compares the measured voltage to a voltage after the tap change. From this information, the direction of the tap change and the distance traveled can be determined. The system and method can also include a timer to measure an elapsed time required to complete a particular tap change and use this time information to determine the distance traveled. The direction of the tap change can be determined from a sensor that triggers the timer. The system and method thereby enable efficient and cost-effective remote monitoring of load tap changes and positions.

Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/523,770 filed Nov. 20, 2003, which is incorporated herein in its entirety by reference. 

   TECHNICAL FIELD 
   The invention generally relates to a system, software, and method for remotely monitoring load tap changes on utility-type transformer equipment. 
   BACKGROUND OF THE INVENTION 
   In service, a load tap changing transformer is supplied with an input voltage and produces an output voltage. The purpose of a tap changer is to produce an output voltage that is well-regulated or stable despite fluctuations in the input voltage and load. The load tap changer has a number of spaced-apart output terminals and performs its regulatory function by adjusting the tap position so that, for a given input voltage, the output is taken from whichever tap yields an output voltage closest to the target level. The number of taps provided depends on the environment in which the tap changer is designed to operate and which is necessary to control the output voltage. The dynamic range of the tap changer is typically rated voltage plus or minus 10%. When the input voltage is at its rated value, and the tap changer tap position is in neutral, the output voltage of the transformer is at rated voltage. Operators of large industrial electrical installations utilizing transformers with tap changers need information about tap positions on the transformers because of the associated bearing on economy of operation, maintenance, safety, and system performance. 
   Various kinds of apparatus have been developed in the past for determining the tap position of a tap changer. These prior developments have culminated in standard electromechanical tap position indicators, or meters, that are physically attached as an add-on to the tap changer mechanism. The tap changer is typically a mechanical device that changes the tap position on a transformer by physically moving the contacts from one tap to another tap. The attached tap position indicator moves with the tap changer mechanism and displays the tap position on a dial or in some other conventional manner. A conventional electromechanical meter for displaying the tap position has drawbacks, such as producing only a local meter indication, which can be read by an operator only by going to the site of the meter. Furthermore, if meter readings are converted into a signal that can be transmitted to a remote location for reading or to a centrally located computer for processing, such conversion must be performed reliably and cost effectively if it is to be a viable tool. 
   Patents relating to the monitoring or determination of the tap position of a tap changer include U.S. Pat. Nos. 4,419,619, 4,612,617 and 5,119,012. U.S. Pat. No. 6,472,850 discloses a method and apparatus for determining a voltage regulator tap position. The devices and methods shown in these patents all have various drawbacks, including relative complexity and lack of applicability in retrofit situations. 
   In view of the foregoing, there is a need in the industry for a method and a system for efficiently and cost effectively monitoring load tap changes on a transformer. In addition, providing this information from a remote location and with a high level of accuracy would substantially meet the needs of utility companies for monitoring the tap movement range and real time position of remote tap changing transformers. 
   SUMMARY OF THE INVENTION 
   The invention described herein provides convenient methods to remotely track the position of load tap changing equipment that is used for voltage regulation on power transformer equipment. 
   Embodiments of one method and system according to the invention utilize a sensor to sense current flow in the load tap changer (LTC) motor circuit and present this as a status event to a remote terminal unit (RTU) connected to a supervisory control and data acquisition (SCADA) system. When the status change is sensed, a voltage reading is initiated by the RTU. When the status reverts to its non-asserted condition, a second voltage reading is initiated by the RTU. The voltage difference between the first reading and the second reading is then determined and divided by a configurable volts per step value (0.75 volts per step for most configurations) to determine the number of steps involved in the tap change, rounded to the nearest integer. This number is then added to or subtracted from the present value of the tap position. To initiate the system, the present value of the tap position is a data input to a software program as part of the initial configuration. The tap position can be checked periodically and adjusted if necessary. 
   Embodiments of a further method and system according to the invention utilize two sensors to sense current flow in an LTC motor circuit and present the sensed current flow as a status event to an RTU connected to a SCADA system. A first sensor is initiated by an LTC raise circuit and a second sensor is initiated by an LTC lower circuit. When a status change is sensed in an asserted condition, a timer resident in a software application is started. When the status reverts to a non-asserted condition, the timer is stopped. The elapsed time as measured by the timer is then divided by the average time per tap change, a value configurable in the software application, to determine how many steps were traveled. This value is then added or subtracted as appropriate to the present value of the tap position. The direction of the tap change is determined by whichever sensor, the first or second, initiates the timer function. To initiate the system, the present value of the tap position is input as a data point into the software application as part of the initial configuration. As in the previous method, the tap position can be checked periodically and adjusted as needed. 
   The system and method of the invention substantially meet the aforementioned needs of the electric utility industry in providing a monitoring system that easily transmits load tap change data back to the transformer operator. 
   The above summary of the invention is not intended to describe each illustrated embodiment or every implementation of the invention. The figures and the detailed description that follow more particularly exemplify these embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawing, in which: 
       FIG. 1  is a block diagram of a system of remotely tracking load tap changes at a transformer according to one embodiment of the invention. 
       FIG. 2  is a flowchart of a method for determining a load tap changer position change according to the embodiment of  FIG. 1 . 
       FIG. 3  is a block diagram of a system of remotely tracking load tap changes at a transformer according to another embodiment of the invention. 
       FIG. 4  is a flowchart of a method for determining a load tap changer position change according to the embodiment of  FIG. 3 . 
   

   While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The invention is generally directed to a method and system for remotely tracking the position of load tap changing equipment that is used for voltage regulation on power transformer equipment. The invention can be more readily understood by reference to  FIGS. 1-4  and the following description. While the invention is not necessarily limited to such an application, the invention will be better appreciated using a discussion of example embodiments in such a specific context. 
   In one embodiment illustrated by  FIG. 1 , a load tap changer monitoring system  100  includes a load tap changer (LTC)  102  and a hardware current status sensor  110 . Sensor  110  is an ACST sensor, made by Cannon Technologies of Golden Valley, Minn., in one embodiment, or another similar and compatible sensor known to those skilled in the art, that is coupled to an LTC motor circuit  120  and a transformer  130 . A custom remote terminal unit (RTU) software program  140 , for example the Cannon SUBSTATION ADVISOR System or another similar and compatible software program known to those skilled in the art, resident in an RTU  150 , is also coupled to sensor  110 . The transformer secondary voltage is measured by a voltage transformer  160 , which is connected to a voltage transducer  170 . LTC  102  typically utilizes a sixteen-step raise and a sixteen-step lower scale, each consisting of ⅝% per step to provide the ability to vary power transformer  130  output voltage by plus or minus 10%, i.e. 16 steps×⅝% per step=10% output voltage variation or change in each direction. 
   The system and method described herein utilize sensor  110  to sense current flow in LTC drive motor  120  and present sensed current flow as a status event to RTU software application  140  connected to a SCADA system. When the status change is sensed, a voltage reading is initiated by RTU  150 . When the status reverts to a non-asserted condition, a second voltage reading is initiated by RTU  150 . The voltage difference between the first reading and the second reading is then determined and divided by a configurable volts per step value to determine the number of steps involved in the tap change, rounded to the nearest integer. The volts per step value is 0.75 in this one example embodiment. The determined number of steps is then added, or subtracted, to the present value of the tap position. To initiate the system, the present value of the tap position is input as data to software application  140  as part of the initial configuration. The tap position can be checked and verified periodically and adjusted if necessary to maintain accuracy. 
   In another example embodiment, the method of the invention monitors the real time voltage change produced by LTC  102  and divides this voltage change by the volts per step of LTC  102 . Access to real-time information previously difficult or impossible to obtain is now facilitated in this operation. As described above, most LTCs use sixteen steps in each direction to raise and lower the voltage of transformer  130  in causing a 10% voltage change in either direction. This results in a ⅝% change in voltage per step, which is about 0.75 volts/step on a 120 volt based system for this example. The actual amounts are dependent on system  100  changes that can take place, but these should cause minimal effect due to the “step” nature of the mechanism that produces a fairly coarse change per step. For instance, if LTC  120  moves two steps, a 0.75×2, or 1.5-volt, change is expected. Due to simultaneous system  100  changes, the measured change might be 1.2 volts, 1.6 volts, etc; however, when divided by 0.75 and rounded to the nearest integer, each produces a value of “2.” The voltage measurements should be taken in real time, however, and the process should be initiated by current status sensor  110  that monitors operation of LTC motor  120  in one embodiment. 
   Referring to the flowchart of  FIG. 2 , by way of load tap monitoring system  100  as described above with reference to  FIG. 1 , a method  200  of measuring real-time voltage for determining LTC  102  position changes includes sensing the movement of LTC motor  120  by current sensor  110  at step  202 . This sensed movement is passed to RTU  150  and software application  140  as a status change at step  204 . At step  206 , the status change keys RTU  150  to read the regulated voltage input V 1 . When LTC motor  120  stops running at step  208 , the status reverts, keying a second voltage reading V 2  at step  210 . 
   The two voltage readings are then subtracted to approximate the voltage change produced by LTC  102 . This voltage change is then divided by a configurable value, which in this example is about 0.75 volts per step. Again as described above, this is based on a standard +/−10% LTC that changes voltage at about ⅝% per step on a 120-volt base, wherein ⅝% of 120 volts=0.75 volts per step each direction for raise and lower directions of movement. This result is rounded to the nearest integer to represent the number of taps moved to complete step  212 . The number generated at step  212  is then added for voltage increases, or subtracted for voltage decreases, to the previous LTC position to obtain the new value at step  214 . This new value is used for subsequent tap changes in order to continue monitoring system  100  and repeating method  200 . Further, this new value then becomes the LTC position for the next calculation. 
   Referring to  FIGS. 3 and 4 , embodiments of a further method  400  according to a system  300  of the invention are illustrated. System  300  utilizes two sensors to sense current flow in LTC motor drive  120  and present the sensed current flow as a status event to RTU software application  340  connected to a SCADA system at RTU  350 . A first sensor  310  is initiated by an LTC raise circuit and a second sensor  312  is initiated by an LTC lower circuit. When a status change is sensed in an asserted condition at step  402 , a timer resident in software application  340  is initiated at step  404 . When the status reverts to a non-asserted condition at subsequent step  406 , the timer is stopped at step  408 . At step  410 , the elapsed time as measured by the timer is then divided by the average time per tap change, a value configurable in software application  340 , to determine how many steps were traveled. This value is then added or subtracted as appropriate to the present value of the tap position to determine the new LTC position at step  412 . The direction of the tap change is determined by which sensor, the first  310  or second  312 , initiates the timer function. To initiate the system, the present value of the tap position is input to software application  340  as part of the initial configuration. As in the previous method, the tap position can be checked periodically and adjusted as needed. 
   In one example embodiment according to the invention, a load tap changing monitoring system includes an ACST sensor, made by Cannon Technologies of Golden Valley, Minn.; a voltage transducer; and a software application. This load tap changing monitoring kit is effective in embodiments of a system as described with reference to  FIG. 1  to carry out a method as described with reference to  FIG. 2 , for example. 
   In another example embodiment according to the invention, a load tap changing monitoring system includes first and second ACST sensors, made by Cannon Technologies of Golden Valley, Minn.; a voltage transducer; and a software application. This load tap changing monitoring kit is effective in embodiments of a system as described with reference to  FIG. 3  to carry out a method as described with reference to  FIG. 4 , for example. 
   The various embodiments disclosed and described herein provide several advantages over other known systems, including determining the number of steps the tap moved in addition to determining the direction of the tap movement. The invention divides a measured voltage change by a configurable value to determine how many steps were taken as well as a direction of tap movement along the transformer. The invention can also divide an elapsed time by the average time per tap change to determine how many steps the tap traveled. Further, the invention is compatible with and retrofitable to other transformers or controls, as the invention is intended to address any tap changer and has no dependence on the control used. 
   In addition, the tap changer monitoring system of the invention is not limited to a control self-contained and resident on the particular transformer, as with other designs. Further, the invention is applicable to voltage regulator applications. The invention uses software resident in the substation RTU, or gateway, and can be applied to multiple transformers without concern or connection to a specific control unit. The invention also uses a distributed approach to retrieving the tap changing information. 
   The invention may be embodied in other specific forms without departing from the spirit of the essential attributes thereof; therefore the illustrated embodiment should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.