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

Expectations from any proposed resolution 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 4/20/2017 ERCOT Limited

The Challenge – Scenario 1 Operations & Market Systems XFMR Impedance XFMR Tap info. NMMS Vrated = Windingrating Vnominal = Bus kV Vneutral = Windingrating @100 MVA & Vnominal Transformer Test Data (converted to appr. SI units using rated kV & 100MVA as base) Planning System XFMR Impedance XFMR Tap info. @100 MVA & Vrated 4/20/2017 ERCOT Limited

The Challenge – Scenario 2 Operations & Market Systems XFMR Impedance XFMR Tap info. NMMS Vrated = Windingrating Vnominal = Bus kV Vneutral = Windingrating @100 MVA & Vnominal Normalized Transformer Test Data (converted to appr. SI units using nominal kV & 100MVA as base) Planning System XFMR Impedance XFMR Tap info. @100 MVA & Vrated 4/20/2017 ERCOT Limited

Allow direct correspondence with field telemetery. 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. 4/20/2017 ERCOT Public

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, 4/20/2017 ERCOT Public

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. 4/20/2017 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. 4/20/2017 ERCOT Limited

A Nominal Equivalent – What works and what doesn’t Operations & Market Systems XFMR Impedance XFMR Tap info. NMMS Vrated = Bus kV Vnominal = Bus kV Vneutral = Windingrating @100 MVA & Vnominal A Nominal Equivalent for an Off-Nominal Tranformer Planning System XFMR Impedance XFMR Tap info. @100 MVA & Vrated 4/20/2017 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. Current Updates needed 4/20/2017 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) 4/20/2017 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. 4/20/2017 ERCOT Limited

Modeling Off Nominal Transformers in NMMS Questions? 4/20/2017 ERCOT Public