Network Modeling Group

Modeling Off Nominal Transformers

in NMMS

By Nitika Mago, Model Administration

2ERCOT Limited

04/19/23

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

3ERCOT Limited

04/19/23

The Challenge – Scenario 1

Transformer Test Data

(converted to appr. SI units using rated kV & 100MVA as

base)

XFMR Impedance

XFMR Tap info.

Vrated = Windingrating

Vnominal = Bus kV

Vneutral = Windingrating

@100 MVA & V nominal

@100 MVA & Vrated

4ERCOT Limited

04/19/23

The Challenge – Scenario 2

Normalized Transformer Test Data

(converted to appr. SI units using nominal kV & 100MVA

as base)

Vrated = Windingrating

Vnominal = Bus kV

Vneutral = Windingrating

XFMR Impedance

XFMR Tap info.

@100 MVA & V nominal

@100 MVA & Vrated

5ERCOT Public

04/19/23

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.

6ERCOT Public

04/19/23

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,

Vin VH

Is IH

VL

NH:NL

VH = Vnominal-high

VL = Vnominal-low

ztotal

z zscaling

z k =z ,Then V

V =k If, 2

totalrated

nominal

7ERCOT Public

04/19/23

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.

Vin VH

Is IH

VL

NH:NL

VH = Vnominal-high

VL = Vnominal-low

ztotal

z zscaling

8ERCOT Limited

04/19/23

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.

9ERCOT Limited

04/19/23

A Nominal Equivalent – What works and what doesn’t

A Nominal Equivalent for

an Off-Nominal

TranformerVrated = Bus kV

Vnominal = Bus kV

Vneutral = Windingrating

XFMR Impedance

XFMR Tap info.

@100 MVA & V nominal

@100 MVA & Vrated

10ERCOT Limited

04/19/23

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

11ERCOT Limited

04/19/23

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

12ERCOT Limited

04/19/23

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.

13ERCOT Public

04/19/23

Modeling Off Nominal Transformers in NMMS

Questions?