Transformer Inrush Current Reduction Doug Taylor January 29

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: http: //hyperphysics. phy-astr. gsu. edu/Hbase/Solids/hyst. html 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 Φr=87% of Φmax Φr=0% of Φmax Φr=-87% of Φmax

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

Experimental Setup • 55 k. VA Transfomer – Primary/Secondary Winding – 230 V – Tertiary Winding – 25 V • Lab. VIEW – Computer-based Measurement Software – Measure Flux • Precision Switch – Allows Controlled Energization

Lab. VIEW

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 Nominal Operation Source: Wikipedia

Tertiary Winding at 8. 5 Vrms Voltage Flux Magnetizing Current

Tertiary Winding at 17 Vrms Voltage Flux Magnetizing Current

Tertiary Winding at 19. 5 Vrms Voltage Flux Magnetizing Current

Tertiary Winding at Rated 25 Vrms Voltage Br_max = 0. 95 T Flux Magnetizing Current

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 m. Wb (75% of Φr_max) Voltage Flux 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 1. 5 -2 p. u. – Increase Device Practicality

Ideal Three Phase, Three Pole Switching Φr=87% of Φmax 210 Deg

Non-Ideal Pre-Fluxing Φr=50% of Φmax Non-Ideal Pre-Fluxing/ Switching

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?