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?
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