transformer thermal modeling...transformer model a (structural part) to be conservative, the model...

© 2016 Electric Power Research Institute, Inc. All rights reserved.

Randy Horton, Ph.D., P.E.Senior Technical Executive

Transformer Thermal Modeling

NERC GMD TF MeetingFebruary 22, 2017

2© 2016 Electric Power Research Institute, Inc. All rights reserved.

Background

EPRI recently completed a study to assess the effects of MHD-EMP (E3) on the U.S. Transformer Fleet.The tools and assessment procedure used in the study can

be leveraged in GMD assessments.– Time-domain transformer model– Thermal model parameters– Assessment criteria (temperature limits)


Time-domain Thermal Model

Electrical

Response

Thermal

ResponseGIC

Transformer Thermal Model

Σ

Top Oil Temperature

TotalHotspot

Temperature

HotspotTemperature

Rise

H(z)

Transformer Thermal Model(Impulse Response)

X(z) Y(z) HotspotTemperature

RiseGIC



Hotspot Temperature Rise of a Power Transformer Injected with dc Current

Step Response

(time domain)

Unit Step Response

(time domain)

ImpulseResponse

(time domain)

Derivation of time-domain thermal model



ImpulseResponse

(time domain)

DifferenceEquation

ImpulseResponse(Z domain)

ImpulseResponse(S domain)

asymptotic behavior

EffectiveGIC (current and

previous time step)

hotspot riseprevious time step

hotspot risecurrent time step

time step

thermal time constant

Total Hotspot Temperature


Transformer Thermal Models

A total of 5 models were used in the study.

Model Model Type

Transformer Type

Thermal Time Constants(Seconds) Transformer

DetailsHeating Cooling

A Structural Part

Auto w/ tertiary 600 650 1-phase 230 kV:115 kV 240 MVA

B Structural Part

Conv. 780 780 1-phase 500 kV:16.5 kV 400 MVA

C Structural Part

Three-winding Conv.

800 500 3-phase 410 kV:120 kV:21 kV 400 MVA

D Winding Conv. 150 150 1-phase 345 kV:24.5 kV750 MVA

E Winding GSU 150 150 1-phase 415.8 kV:20.5 kV1299 MVA


Transformer Model A (Structural Part) Single Phase 230kV:115kV 240 MVA Autotransformer Factory test and FEM simulation results were evaluated to determine the

basis of the model.FEM Model (Strong Source)

Largest Simulated TemperatureFactory Test (Weak Source)

~130°C~100°C


Transformer Model A (Structural Part)

To be conservative, the model for Transformer A was based on the FEM simulation results.

Hotspot temperature rise corresponding to various dc current levels

Heating: τ1 = 600 seconds

Cooling: τ2 = 650 seconds

Model Validation


Transformer Model B (Structural Part) Single-phase 500kV:16.5kV 400 MVA conventional transformer1

1L. Marti, et al, “Simulation of Transformer Hotspot Heating due to Geomagnetically Induced Currents,” IEEE Transactions on Power Delivery, Vol. 28, No. 1, January 2013.

Model Validation

Hotspot temperature rise corresponding to dc current level of 5 Amps/phase




Transformer Model C (Structural Part)

3-Phase 410 kV:120kV:21 kV GY-GY-D 400 MVA conventional transformer1

Model was based on the highest recorded hotspot temperature observed during the field test.

1J. E. M. Lahtinen, "GIC Occurrences and GIC Test for 400 kV System Transformer," IEEE Transactions on Power Delivery, vol. 17, no. 2, 2002.

Measured Temperature dc Current

dc Current

dc C

urre

nt (A

mps

in n

eutra

l)

Hot

spot

Tem

pera

ture

(°C

) Model Basis


Transformer Model C (Structural Part)



Hotspot temperature rise corresponding to various dc current levels

Model Validation


Transformer Model D (Winding)

Single-phase 750 MVA 345kV:24.5kV transformer (winding)1

1L. Marti, et al, “Simulation of Transformer Hotspot Heating due to Geomagnetically Induced Currents,” IEEE Transactions on Power Delivery, Vol. 28, No. 1, January 2013.

Hotspot temperature rise corresponding to dc current of 60 Amps/phase

Model Validation




Transformer Model E (Winding)

Single-phase 1299 MVA 418.8kV:20.5kV GSU (Winding)1

1R. Girgis, "Calculation Techniques and Results of Effects of GIC Currents as Applied to Two Large Power Transformers," IEEE Transactions on Power Delivery, vol. 7, no. 2, 1992.

Hotspot temperature rise corresponding to dc current of 60 Amps/phase

Model Validation




Example Results

Benchmark Waveform (GICpeak = 75 Amps/phase)


Large Scale Assessments

Things to consider when performing large-scale, “planning-level” assessments– Initial temperature of structural parts and windings– Magnitude and duration of GIC are important time-domain models– Transformer condition: temperature limits in IEEE C57.163 assume

transformers are in new condition– Exceeding temperature limits ≠ transformer failure– Bubble formation is not instantaneous and does not guarantee failure

(temperature limits are very conservative)

Temperature limits used in the EPRI study were based on Condition-Based GIC Susceptibility Category:– PTX Condition Code (based on trends of dissolved gases) – Transformer age (proxy for moisture content)– Effect of transformer design is accurately accounted for in the thermal

models


Assessment Criteria

For comparison, IEEE C57.163 limits are 200°C for structural parts and 180°C cellulose insulation (windings).

Condition-based GIC Susceptibility Categories

Conservative Temperature Limits


Example Results

Benchmark Waveform (GICpeak = 75 Amps/phase)


Together…Shaping the Future of Electricity

transformer thermal modeling...transformer model a (structural part) to be conservative, the model...

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