# Building a CIM Network Model

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```					     Network Modeling Group

Modeling Off Nominal
Transformers in NMMS
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

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

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

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

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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