Transformer Inrush Current Reduction Doug Taylor January 29, 2009

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

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

Source: Wikipedia

Magnetic Hysteresis Source: Magnetic Permeability

Normal Transformer Operation Source: ABB App. Guide

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

Inrush – Incorrect Switching Time Source: ABB App. Guide

No Inrush – Proper Switching Time Source: ABB App. Guide

Ideal Single Phase Switching – Demagnetized Case

Ideal Single Phase Switching – Demagnetized Case 2

Ideal Single Phase Switching – Magnetized Case

Worst Case Single Phase Switching

Ideal Three Phase, Three Pole Switching Φ r =87% of Φ max Φ r =-87% of Φ max Φ r =0% of Φ max

Problem Statement Set Residual Flux in Single-Phase Transformer – ‘Pre-fluxing’ – Set as high as possible Controlled Energization – Precise switching

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

LabVIEW

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

Hysteresis Loop Family Source: Wikipedia Nominal Operation

Tertiary Winding at 8.5 Vrms Flux Magnetizing Current Voltage

Tertiary Winding at 17 Vrms Flux Magnetizing Current Voltage

Tertiary Winding at 19.5 Vrms Flux Magnetizing Current Voltage

Tertiary Winding at Rated 25 Vrms Flux Magnetizing Current B r_max = 0.95 T Voltage

Inrush Current – Demagnetized Case Case 1: Unloaded Steady-State Operation Case 2: Proper Switching – No Inrush Case 3: Worst Case Switching – Maximum Inrush

Unloaded Steady-State Operation (230 V Winding) Voltage Magnetizing Current

Demagnetized – Proper Switching Time (~90 deg) Voltage Magnetizing Current

Demagnetized – Proper Switching Time (~90 deg) Voltage Magnetizing Current

Demagnetized – Improper Switching Time (0 deg) Voltage Magnetizing Current

Demagnetized – Improper Switching Time (0 deg) Voltage Magnetizing Current

Demagnetized – Improper Switching Time (0 deg) Voltage Magnetizing Current

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

Series Capacitor & Diode

Pre-Flux 9 mWb (75% of Φ r_max ) Flux Voltage Magnetizing Current

Pre-fluxed – Proper Switching Time (124 deg) Voltage Magnetizing Current

Pre-fluxed – Proper Switching Time (124 deg) Voltage Magnetizing Current

Future Work Sensitivity Testing – Non-Ideal Pre-Fluxing – Non-Ideal Switching – Keep Inrush To p.u. – Increase Device Practicality

Ideal Three Phase, Three Pole Switching Φ r =87% of Φ max 210 Deg

Non-Ideal Pre-Fluxing Non-Ideal Pre-Fluxing/ Switching Φ r =50% of Φ max

Non-Ideal Switching– Demagnetized (114°, 24° error) Voltage Magnetizing Current

Pre-fluxed – Proper Switching Time (124 deg) Voltage Magnetizing Current

Future Work (Con’t) Device Sizing – Dependent upon Transformer Size

Questions?