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

2. 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

3. 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

4. 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

5. 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

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

9. 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

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 High Step (RMA1)
Voltage or phase 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

11. A Nominal Equivalent – What works and what doesn’t
Voltage or phase High Step (RMA1)
Current
Updates Needed
Voltage or phase Low Step (RMI1)
4/20/2017
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.
4/20/2017
ERCOT Limited

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