Transformer Inrush Current

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Transformer Inrush Current Reduction

• Doug Taylor
• January 29, 2009

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Overview

• What is Inrush Current?
• Electromagnetic Phenomenon
• Magnetic Properties
• Causes of Inrush
• Thesis Work
• Problem Statement
• Experimental Measurements
• Pre-fluxing
• Future Work

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Inrush Current

• Occurs when energizing transformers
• 5-6 times normal operating current
• Stresses insulation
• Thermal Stress
• Physical Stress
• Causes Protection/Power Quality Issues

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Source Wikipedia
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Magnetic Hysteresis
Source http//hyperphysics.phy-astr.gsu.edu/Hbase
/Solids/hyst.html
Magnetic Permeability
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Normal Transformer Operation
Source ABB App. Guide
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Inrush Current Causes

• Based on two main factors
• Residual Flux
• Energizing (or Switching) Instant
• Ideally switch when prospective flux equals
  residual flux
• Deviations from this instant result in inrush
  current

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Inrush Incorrect Switching Time
Source ABB App. Guide
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No Inrush Proper Switching Time
Source ABB App. Guide
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Ideal Single Phase Switching Demagnetized Case
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Ideal Single Phase Switching Demagnetized Case 2
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Ideal Single Phase Switching Magnetized Case
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Worst Case Single Phase Switching
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Ideal Three Phase, Three Pole Switching
Fr87 of Fmax
Fr0 of Fmax
Fr-87 of Fmax
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Problem Statement

• Set Residual Flux in Single-Phase Transformer
• Pre-fluxing
• Set as high as possible
• Controlled Energization
• Precise switching

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Experimental Setup

• 55 kVA Transfomer
• Primary/Secondary Winding 230 V
• Tertiary Winding 25 V
• LabVIEW
• Computer-based Measurement Software
• Measure Flux
• Precision Switch
• Allows Controlled Energization

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LabVIEW
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Experimental Hysteresis Measurements

• Nominal Winding Voltage - 25 V
• Sequence of plots
• 8.5 V Applied
• 17 V Applied
• 19.5 V Applied
• 25 V Applied

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Hysteresis Loop Family
Nominal Operation
Source Wikipedia
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Tertiary Winding at 8.5 Vrms
Voltage
Flux
Magnetizing Current
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Tertiary Winding at 17 Vrms
Voltage
Flux
Magnetizing Current
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Tertiary Winding at 19.5 Vrms
Voltage
Flux
Magnetizing Current
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Tertiary Winding at Rated 25 Vrms
Voltage
Br_max 0.95 T
Flux
Magnetizing Current
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Inrush Current Demagnetized Case

• Case 1 Unloaded Steady-State Operation
• Case 2 Proper Switching No Inrush
• Case 3 Worst Case Switching Maximum Inrush

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Unloaded Steady-State Operation (230 V Winding)
Voltage
Magnetizing Current
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Demagnetized Proper Switching Time (90 deg)
Voltage
Magnetizing Current
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Demagnetized Proper Switching Time (90 deg)
Voltage
Magnetizing Current
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Demagnetized Improper Switching Time (0 deg)
Voltage
Magnetizing Current
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Demagnetized Improper Switching Time (0 deg)
Voltage
Magnetizing Current
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Demagnetized Improper Switching Time (0 deg)
Voltage
Magnetizing Current
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Prefluxing

• Send a pulse of energy to the transformer
• Circuit Used
• Series capacitor and diode
• Fairly Simple
• Effective

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Series Capacitor Diode
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Pre-Flux 9 mWb (75 of Fr_max)
Voltage
Flux
Magnetizing Current
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Pre-fluxed Proper Switching Time (124 deg)
Voltage
Magnetizing Current
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Pre-fluxed Proper Switching Time (124 deg)
Voltage
Magnetizing Current
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Future Work

• Sensitivity Testing
• Non-Ideal Pre-Fluxing
• Non-Ideal Switching
• Keep Inrush To 1.5-2 p.u.
• Increase Device Practicality

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Ideal Three Phase, Three Pole Switching
Fr87 of Fmax
210 Deg
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Non-Ideal Pre-Fluxing
Non-Ideal Pre-Fluxing/ Switching
Fr50 of Fmax
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Non-Ideal Switching Demagnetized (114, 24
error)
Voltage
Magnetizing Current
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Pre-fluxed Proper Switching Time (124 deg)
Voltage
Magnetizing Current
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Future Work (Cont)

• Device Sizing
• Dependent upon Transformer Size

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Questions?