POWER TRANSFORMER FAILURE AND CHALLENGES IN THE TRANSFORMER DESIGN

Matthias Bucher, PhD

Computational and Applied Electromagnetics GroupInstitute for Energy TechnologyUniversity of Applied Sciences, Rapperswil, Switzerland

Infolytica Engage User Conference, October 27, 2015

Agenda

Research and development at CAEM group

Power and drive transformers

Challenges in the transformer design

Application of Infolyticas MagNet and ElecNet Equivalent HF-circuit FEM-modeling Extraction of inductances and capacitances Transient time-domain solution Comparison with measurements Visualization

Conclusions and Outlook

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Research and Development at CAEM Group

Electromagnetic field simulations at low and high frequency

Virtual prototyping and product development

Electromagnetic analysis and design optimization

Power and drive transformers, MV and HV switchgear, electric motors

Measurement and testing at our high-voltage lab 800kV lighting pulse tests 150kV AC, +/- 270kV DC Partial discharges

3

Power Transformers

4

Source: Swissgrid

Source: ABBSource: ABB

Source: ABB

Drive Transformers

5

Source: ABB

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FrequencyConverter Motor

Challenges in the Transformer Design

Transient overvoltages due to

Lightning strikes

Switching operations in circuit breakers

Ferro-resonances

can cause a non-linear voltage distribution over the transformer windings, high field strengths between turns, and eventually lead to destruction of the winding insulation.

6

Sou

rce:

EE

P

HV LV

Challenges in the Transformer Design

Solution: computation of the transient voltage distribution over the transformer winding to find hot spots and to improve the dielectric design of the transformer.

Needed: equivalent electric HF-circuit of the transformer to compute turn voltages

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Equivalent HF-circuit

Equivalent electric circuit of 4 turns

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FEM Modeling of Transformer

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rotation axisPrimary (HV)Secondary (LV)Core

3D 2D axis symmetric

FEM Modeling of Transformer

Modeling in MagNet

10

meshing

HVLV

core

HV turns modeled as individual coils

LV foils modeled as block (2x) LV-coils are short-circuited

M5 core steelEP04 epoxyAluminium

FEM Modeling of Transformer

Modeling in ElecNet

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Core is grounded HV turns modeled as individual

electrodes LV foils modeled as block (one

single electrode)

M5 core steelEP04 epoxyAluminium

Extraction of Winding Inductances

Extraction of turn inductances and magnetic couplings in MagNet1. Initialization of current in coil#1 to 1, all other turns to 02. Time-harmonic 2D simulation3. Calculation of self and mutual inductances of the transformer turns based

on flux linkage: =

4. Flux linkage column vector = first column of inductance matrix

5. Repeat for the next coil

Procedure is automatically repeated for all coils,

which results in a symmetric inductance matrix

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Extraction of Winding Capacitances

Extraction of turn capacitances and electric couplings in ElecNet1. Initialization of current in electrode#1 to 1, all other turns to 02. Time-harmonic 2D simulation3. Calculation of self and mutual capacitance of the transformer turns based

on charges: =

4. Charge column vector = first column of capacitance matrix

5. Repeat for the next electrode

Procedure is automatically repeated for all electrodes,

which results in a symmetric capacitance matrix

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Matrices

C-matrix (left) and L-matrix (left) in logarithmic scale:

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Transient Time-Domain Solution

Equivalent electric

circuit of 4 turns:

Ordinary differential equation:

EMTP-like

trapezoidal

Integration:

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L: inductance matrixC: capacitance matrixR: resistance matrix (winding losses)G: conductance matrix (losses in dielectric)T: incidence matrixx(t): branch currents and node voltagesu(t): source voltage

Transient Time-Domain Solution

Input surge voltages:

Very fast transient voltage waveform (VFT surge):

Standard lightning impulse (BIL surge):

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Comparison with Measurements

Measurement of turn voltages of a transformer in the lab

Surge generator connected to first turn

Comparison of simulation

results with measurements:

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Visualization of Results (Video)

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Voltage distribution Electric field

Turn

vol

tage

[p.u

.]

Turn number

Conclusions and Outlook

Accurate tool for lightning strike and switching surge analysis of transformer windings

Accurate computation of winding inductances and capacitances using Infolyticas MagNet and ElecNet

Visualization of the results based on transient simulations in ElecNet

Future improvements: Inclusion of frequency-dependency of winding inductances and resistances Optimization of read-out routine in MagNet and ElecNet to speed up the

simulations

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Thank you for your attention!

Power transformer failure and challenges in the transformer design AgendaResearch and Development at CAEM Group Power TransformersDrive TransformersChallenges in the Transformer DesignChallenges in the Transformer DesignEquivalent HF-circuitFEM Modeling of TransformerFEM Modeling of TransformerFEM Modeling of TransformerExtraction of Winding InductancesExtraction of Winding CapacitancesMatricesTransient Time-Domain SolutionTransient Time-Domain SolutionComparison with MeasurementsVisualization of Results (Video)Conclusions and OutlookFoliennummer 20