Transformer Inrush Current
Reduction
Overview
•
What is Inrush Current?
–
Electromagnetic Phenomenon
–
Magnetic Properties
–
Causes of Inrush
•
Thesis Work
–
Problem Statement
–
Experimental Measurements
–
Pre-fluxing
Inrush Current
•
Occurs when energizing
transformers
•
5-6 times normal
operating current
•
Stresses insulation
–
Thermal Stress
–
Physical Stress
•
Causes Protection/Power
Faraday's Law
Magnetic Flux
Density
Magnetic Field
Intensity
Vp Np
d
t
d
B
Acore
H
Np IP
MPL
Magnetic Hysteresis
B
0
r
H
Source: http://hyperphysics.phy-astr.gsu.edu/Hbase/Solids/hyst.html
Normal Transformer Operation
Inrush Current Causes
•
Based on two main factors:
–
Residual Flux
–
Energizing (or Switching) Instant
•
Ideally switch when prospective flux equals residual
flux
Inrush – Incorrect Switching Time
No Inrush – Proper Switching Time
Ideal Single Phase Switching – Demagnetized Case
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1
System Voltage
Winding Voltage - Demagnetized Case
Time (s) Vo lt ag e (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1
Prospective Flux
Transformer Flux - Demagnetized Case
Time (s) F lu x (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Residual Flux Prospective Flux
Transformer Flux - Demagnetized Case
Time (s) F lu x (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Winding Voltage System Voltage
Winding Voltage - Demagnetized Case
Time (s) Vo lt ag e (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Transient Flux Prospective Flux
Transformer Flux - Demagnetized Case
Ideal Single Phase Switching – Demagnetized Case 2
0.01 0 0.01 0.02 0.03 0.04
1.5 1 0.5 0.5 1 Transient Flux Prospective Flux
Transformer Flux - Demagnetized Case
Time (s) F lu x (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Winding Voltage System Voltage
Winding Voltage - Demagnetized Case
Ideal Single Phase Switching – Magnetized Case
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Transient Flux Prospective Flux
Transformer Flux - Magnetized Case
Time (s) F lu x (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Winding Voltage System Voltage
Winding Voltage - Magnetized Case
Time (s) Vo lt ag e (p u)
0.01 0 0.01 0.02 0.03 0.04
2 1 1
Transient Flux Prospective Flux
Transformer Flux - Magnetized Case
Time (s) F lu x (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Winding Voltage System Voltage
Winding Voltage - Magnetized Case
Worst Case Single Phase Switching
0.01
0 0.01 0.02 0.03 0.04 1 1 2 3 Transient Flux Prospective Flux
Transformer Flux - Magnetized Case
Time (s) F lu x (p u) 0.01
0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Winding Voltage System Voltage
Winding Voltage - Magnetized Case
0.01 0.005 0 0.005 0.01 0.015
1 0.5 0.5 1
Winding Voltage - Phase A System Voltage - Phase A Winding Voltage - Phase B System Voltage - Phase B Winding Voltage - Phase C System Voltage - Phase C
Winding Voltage Time (s) Vo lt ag e (p u)
0.01 0.005 0 0.005 0.01 0.015
1 0.5 0.5 1
Transient Flux - A Phase Prospective Flux - A Phase Transient Flux - B Phase Prospective Flux - B Phase Transient Flux - C Phase Prospective Flux - C Phase
Transformer Flux Time (s) F lu x (p u)
Ideal Three Phase, Three Pole Switching
Φ
r=87% of Φ
maxProblem Statement
•
Set Residual Flux in Single-Phase Transformer
–
‘Pre-fluxing’
–
Set as high as possible
•
Controlled Energization
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
Experimental Hysteresis
Measurements
•
Nominal Winding Voltage - 25 V
•
Sequence of plots
Hysteresis Loop Family
Tertiary Winding at 8.5 Vrms
Flux
Tertiary Winding at 17 Vrms
Flux
Tertiary Winding at 19.5 Vrms
Flux
Tertiary Winding at Rated 25 Vrms
Flux
Magnetizing Current
B
r_max= 0.95 T
Inrush Current – Demagnetized
Case
•
Case 1: Unloaded Steady-State Operation
•
Case 2: Proper Switching – No Inrush
•
Case 3: Worst Case Switching – Maximum
Unloaded Steady-State Operation (230 V Winding)
Voltage
Demagnetized – Proper Switching Time (~90 deg)
Voltage
Demagnetized – Proper Switching Time (~90 deg)
Voltage
Demagnetized – Improper Switching Time (0 deg)
Voltage
Demagnetized – Improper Switching Time (0 deg)
Voltage
Demagnetized – Improper Switching Time (0 deg)
Voltage
Prefluxing
•
Send a pulse of energy to the transformer
•
Circuit Used
–
Series capacitor and diode
Pre-Flux 9 mWb (75% of Φ
r_max
)
Flux
Voltage
Pre-fluxed – Proper Switching Time (124 deg)
Voltage
Pre-fluxed – Proper Switching Time (124 deg)
Voltage
Future Work
•
Sensitivity Testing
–
Non-Ideal Pre-Fluxing
–
Non-Ideal Switching
0.01 0.005 0 0.005 0.01 0.015
1 0.5 0.5 1
Winding Voltage - Phase A System Voltage - Phase A Winding Voltage - Phase B System Voltage - Phase B Winding Voltage - Phase C System Voltage - Phase C
Winding Voltage Time (s) Vo lt ag e (p u)
0.01 0.005 0 0.005 0.01 0.015
1 0.5 0.5 1
Transient Flux - A Phase Prospective Flux - A Phase Transient Flux - B Phase Prospective Flux - B Phase Transient Flux - C Phase Prospective Flux - C Phase
Transformer Flux Time (s) F lu x (p u)
Ideal Three Phase, Three Pole Switching
Φ
r=87% of Φ
maxNon-Ideal Pre-Fluxing
0.01 0 0.01 0.02 0.03 0.04
2 1 1
Transient Flux Prospective Flux
Flux - 3 Phase Case (190 deg)
Time (s) F lu x (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Winding Voltage System Voltage
Winding Voltage - 3 Phase Case (190 Deg)
Time (s) Vo lt ag e (p u)
0.01 0 0.01 0.02 0.03 0.04
2 1 1
Transient Flux Prospective Flux
Flux - 3 Phase Case (210 deg)
Time (s) F lu x (p u)
0.01 0 0.01 0.02 0.03 0.04
1 0.5 0.5 1 Winding Voltage System Voltage
Winding Voltage - 3 Phase Case (210 Deg)
Time (s) Vo lt ag e (p u)
Non-Ideal Pre-Fluxing/
Switching
Non-Ideal Switching– Demagnetized (114°, 24° error)
Voltage
Pre-fluxed – Proper Switching Time (124 deg)
Voltage
Future Work (Con’t)
•
Device Sizing
