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  1. Network Modeling Group Modeling Off Nominal Transformers in NMMSBy Nitika Mago, Model Administration

  2. 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 ERCOT Limited

  3. The Challenge – Scenario 1 Planning System Operations & Market Systems NMMS XFMR Impedance XFMR Impedance Transformer Test Data (converted to appr. SI units using rated kV & 100MVA as base) @100 MVA & Vnominal XFMR Tap info. XFMR Tap info. Vrated = Windingrating Vnominal = Bus kV Vneutral = Windingrating @100 MVA & Vrated ERCOT Limited

  4. The Challenge – Scenario 2 Operations & Market Systems Planning System NMMS XFMR Impedance XFMR Impedance Normalized Transformer Test Data (converted to appr. SI units using nominal kV & 100MVA as base) @100 MVA & Vnominal XFMR Tap info. XFMR Tap info. Vrated = Windingrating Vnominal = Bus kV Vneutral = Windingrating @100 MVA & Vrated ERCOT Limited

  5. 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. ERCOT Public

  6. A Nominal Equivalent for an Off-Nominal 2 Winding Transformer • 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, ERCOT Public

  7. A Nominal Equivalent for an Off-Nominal 2 Winding Transformer • 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. ERCOT Public

  8. 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. ERCOT Limited

  9. A Nominal Equivalent – What works and what doesn’t Operations & Market Systems Planning System NMMS XFMR Impedance XFMR Impedance A Nominal Equivalent for an Off-Nominal Tranformer @100 MVA & Vnominal XFMR Tap info. XFMR Tap info. Vrated = Bus kV Vnominal = Bus kV Vneutral = Windingrating @100 MVA & Vrated ERCOT Limited

  10. 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 ERCOT Limited

  11. 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 ERCOT Limited

  12. 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. ERCOT Limited

  13. Modeling Off Nominal Transformers in NMMS Questions? ERCOT Public