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Determining Winding Hot Spot Gridsense

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                                                                                                          APPLICATION NOTE


                              
 



Determining Winding Hot Spot

INTRODUCTION

ANSI and IEC loading guides both describe the relationship between the hottest point in a
transformer’s winding and the rate at which the transformer is aging. Because this is the
primary means of calculating the life of a transformer, it is vitally important to determine the
temperature of the hottest insulation point as accurately as possible.


HISTORICAL METHOD

The common winding temperature indicator does not measure winding hot spot or any other
winding temperature. Instead, it measures the top oil temperature and adds to it a temperature
gradient which is proportional to the square of the winding load.

This additional heat is added with a heater coil wrapped around the top oil temperature probe,
which adds heat with current set up by a current transformer placed on the transformer
bushing. This is defined as the winding hot spot gradient (or copper rise) and is shown in plot
below.




                                  Oil Depth
                                                       Oil Temperature


                                                                        Winding Temperature

                                                                A        D E          A - Top Oil (Measured)
                                                                                      B – Bottom Oil (Measured)
                                                                                      C – Average Winding (Measured)
                                                                           A-E=
                                                                    C                 D – Ideal Hot Spot Temperature
                                                                           “Copper
                                                                                      E – True Hot Spot Temperature
                                                                           Rise”
                                                                                          (Reported for each winding
                                                                                           on Transformer test report)
                                               B                65         80
                                                                                °C Rise




GridSense, Inc.                                                         CHK GridSense PTY Ltd.
2568 Industrial Blvd., Ste. 110    Tel: 916-372-4945                    Unit 3 20-36 Nancarrow Ave.       Tel: +61 2 8878-7700
West Sacramento, CA 95691          Fax: 916-372-4948                    Meadowbank, NSW 2114, Australia   Fax: +61 2 8878-7788

                                                                                                                  gridsense.com
                                                             
     
     
     


                           
 
    This traditional technique introduces a number of potential sources of error. Using this
    century-old approach, the best attainable margin of error for the winding hot spot is ± 5°
    Celsius. Potential sources of error arising from the traditional technique include:

           Winding CTs and heater coils are chosen based on calculated winding gradient, not
            following heat run testing
           Error resulting from selecting stock heater rather than the most appropriate heater
           Trip and alarm set points are selected on gauge dials as opposed to electronic set
            points
           Repeatability of alarm and trip points using bi-metallic elements degrades with time.


    ELECTRONICALLY CALCULATED HOT SPOT

    Using a microprocessor to calculate the winding hot spot temperature removes a number of
    potential error sources (except that of the original calculation of winding gradient). With this
    method, a common 100-ohm platinum resistance (RTD) is used to measure the top oil
    temperature, either in a dry-well or magnetically mounted sensor with a tip measurement error
    of less than 1°C.

    The current is measured by means of current transformer or rogowski coil with an error margin
    of <1%. These error margins represent a tremendous improvement over heater-type winding
    temperature gauges. The hot spot temperature is then calculated as follows:

    Hot Spot Temperature =Top Oil Temp + (Hot Spot Gradient above Top Oil at PU
    Load)*Loadfactor2*m

    The constant m is the winding exponent dependent
    on the type of oil cooling. For traditional heater
    element winding temperature gauges, the heat
    applied to a top oil temperature probe will be
    proportional to the square of the load. This equates
    to m=1.0, which is the most conservative result
    providing the highest hot spot temperature.

    Electronic winding temperature monitors such as the
    Gridsense TransformerIQ allow for setting this m
    value.


    DIRECT HOT SPOT MEASUREMENT WITH OPTICAL PROBES

    In the early 1990’s fiber optic methods were introduced that allowed for direct measurement of
    the winding hot spot. Within the last ten years, flexibility and durability of these probes has



    Determining Winding Hot Spot                                                                       2

     
     
     
     


                           
 
    improved dramatically, allowing for transformer manufacturers to be able to confidently install
    them.

    The components of a fiber optic system include:

        1. The probe to be placed in the winding
        2. A tank wall feed-thru which must be designed
           to prohibit water ingress
        3. An external length of probe typically from the
           transformer cover to control cabinet
        4. Electronics to convert the optical signal to a
           digital or analog output
        5. Processor to communicate to SCADA via
           serial or analog means and to allow for fan
           controls and alarm/trip relaying.
    Fiber optic equipment will surely replace other
    equipment as it becomes less expensive, more
    reliable and more accurate than any other method.




    Determining Winding Hot Spot                                                                      3

     

								
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