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Network Modeling Group
Modeling Off Nominal
Transformers in NMMS
By Nitika Mago, Model Administration
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
5/23/2017
2
ERCOT Limited
The Challenge – Scenario 1
XFMR Impedance
XFMR Tap info.
Transformer
Test Data
(converted to
appr. SI units
using rated kV
& 100MVA as
base)
Vrated = Windingrating
Vnominal = Bus kV
Vneutral = Windingrating
XFMR Impedance
XFMR Tap info.
5/23/2017
3
ERCOT Limited
The Challenge – Scenario 2
XFMR Impedance
XFMR Tap info.
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.
5/23/2017
4
ERCOT Limited
Any resolution must…..
– Provide a clean & simple method for appropriately representing an offnominal 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.
5/23/2017
5
ERCOT Public
A Nominal Equivalent for an Off-Nominal 2 Winding Transformer
ztotal
zscaling
z
NH:NL
Is
IH
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,
If, k =
Vnominal
Then , z total = k 2 z
Vrated
5/23/2017
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ERCOT Public
A Nominal Equivalent for an Off-Nominal 2 Winding Transformer
ztotal
zscaling
z
NH:NL
Is
IH
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.
5/23/2017
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 twowinding 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.
5/23/2017
8
ERCOT Limited
A Nominal Equivalent – What works and what doesn’t
XFMR Impedance
XFMR Tap info.
A Nominal
Equivalent
for an OffNominal
Tranformer
Vrated = Bus kV
Vnominal = Bus kV
Vneutral = Windingrating
XFMR Impedance
XFMR Tap info.
5/23/2017
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
•
Updates needed
5/23/2017
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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)
•
Current
•
Updates Needed
5/23/2017
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
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.
to extract all TapChanger related
5/23/2017
12
ERCOT Limited
Modeling Off Nominal Transformers in NMMS
Questions?
5/23/2017
13
ERCOT Public