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					     Network Modeling Group

Modeling Off Nominal
Transformers in NMMS
   By Nitika Mago, Model Administration
Agenda



 •   Off-Nominal Transformer – Modeling challenges faced with prevailing
     software

 •   Expectations from any proposed resolution

 •   A Nominal Equivalent for two winding & three winding tranformers

 •   Nominal Equivalent – Report Card

 •   Questions




         4/12/2012                                                            2
                                                              ERCOT Limited
The Challenge – Scenario 1




                                                    XFMR Impedance

                                                    XFMR Tap info.

    Transformer
       Test Data
     (converted to
    appr. SI units       Vrated = Windingrating
    using rated kV
    & 100MVA as          Vnominal = Bus kV
         base)
                         Vneutral = Windingrating



                                                    XFMR Impedance

                                                    XFMR Tap info.




             4/12/2012                                                           3
                                                                 ERCOT Limited
The Challenge – Scenario 2




                                                    XFMR Impedance

                                                    XFMR Tap info.

     Normalized
    Transformer
      Test Data
    (converted to
    appr. SI units       Vrated = Windingrating
    using nominal
    kV & 100MVA
                         Vnominal = Bus kV
       as base)          Vneutral = Windingrating



                                                    XFMR Impedance

                                                    XFMR Tap info.




             4/12/2012                                                           4
                                                                 ERCOT Limited
Any resolution must…..



  – Provide a clean & simple method for appropriately representing an off-
    nominal transformer.

  – Standardize the submission & consumption of off-nominal transformer
    information from the model.

  – Allow direct correspondence with field telemetery.

  – Minimize software changes necessary to implement the resolution.

  – Maintain system reliability & operations.




          4/12/2012                                                                5
                                                                    ERCOT Public
A Nominal Equivalent for an Off-Nominal 2 Winding Transformer

                                      ztotal
                           zscaling            z
                                                        NH:NL
                      IH                                             Is


                Vin                                VH           VL        VH = Vnominal-high
                                                                          VL = Vnominal-low




  – A combination of a nominal transformer with its impedance scaled to
    compensate for the off-nominal-turns-ratio.

  – The scaled impedance of this resulting transformer (ztotal) can be derived
    using the off-nominal transformer’s impedance (z) as,
                    Vnominal
          If, k =            Then , z total = k 2 z
                     Vrated


         4/12/2012                                                                                     6
                                                                                        ERCOT Public
A Nominal Equivalent for an Off-Nominal 2 Winding Transformer

                                        ztotal
                             zscaling            z
                                                          NH:NL
                        IH                                             Is


                  Vin                                VH           VL        VH = Vnominal-high
                                                                            VL = Vnominal-low




  – Any associated TapChangers must be modeled using prevalent modeling
    practices, taking care to ensure
     •   Step Voltage Increment is provided in percentage per unit of the nominal voltage.

     •   Neutral Step represents the step at which rated voltage is produced

     •   Neutral Voltage reflects the winding’s rated voltage.




          4/12/2012                                                                                      7
                                                                                          ERCOT Public
A Nominal Equivalent for an Off-Nominal 3 Winding Transformer



  – Three winding transformers are modeled in the NMMS as a set of 3 two-
    winding transformers connected at a common 1kV node.

  – Model consuming applications such as the Topology Processor are designed
    to read this T-equivalent and convert it to true three-winding Transformer
    model.

  – When any of the windings in such a transformer is off-nominally rated, its
    nominal equivalent follow all principal’s defined for a two-winding
    transformer.




         4/12/2012                                                                8
                                                                  ERCOT Limited
A Nominal Equivalent – What works and what doesn’t




                                                   XFMR Impedance

                                                   XFMR Tap info.

    A Nominal
    Equivalent
    for an Off-
     Nominal            Vrated = Bus kV
    Tranformer
                        Vnominal = Bus kV
                        Vneutral = Windingrating



                                                   XFMR Impedance

                                                   XFMR Tap info.




            4/12/2012                                                           9
                                                                ERCOT Limited
A Nominal Equivalent – What works and what doesn’t



  – Topology Processor is currently tuned to extract the following TapChanger
    information by relying on the TransformerWinding.ratedkV attribute in CIM.
     •   Winding Turns Ratio (WINDV1)
     •   Voltage or phase angle @ High Step (RMA1)
     •   Voltage or phase angle @ Low Step (RMI1)


  – With nominal equivalent for an off nominal transformer the computations for
    these is affected negatively.

  – Winding Turns Ratio (WINDV1)
     •   Current




     •   Updates needed




          4/12/2012                                                                10
                                                                   ERCOT Limited
A Nominal Equivalent – What works and what doesn’t



  – Voltage or phase angle @ High Step (RMA1)
     •   Current




     •   Updates Needed




  – Voltage or phase angle @ Low Step (RMI1)
     •   Current




     •   Updates Needed




          4/12/2012                                                  11
                                                     ERCOT Limited
A Nominal Equivalent – What works and what doesn’t



  – Resolution options (in order of preference, ease of implementation)
     •    Update the computations in Topology Processor      to extract all TapChanger related
         information from the TapChanger instance in CIM.

     •   Use MOD to submit corrective PMCRs.

     •   Post Process the RAW output from existing Topology Processor to correct the associated
         fields with information from NMMS.




          4/12/2012                                                                               12
                                                                                ERCOT Limited
Modeling Off Nominal Transformers in NMMS




                        Questions?




        4/12/2012                                          13
                                            ERCOT Public

				
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