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Lifetime of HCPP Film Capacitor for Marx Modulator

Lifetime of HCPP Film Capacitor for Marx Modulator. Tao Tang, Mark Kemp, Craig Burkhart Power Conversion Department. Outline. Background of ILC Marx Modulator High Crystalline Poly-Propylene (HCPP) self healing film capacitor Capacitor life time model (DC) Voltage related lifetime change

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Lifetime of HCPP Film Capacitor for Marx Modulator

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  1. Lifetime of HCPP Film Capacitor for Marx Modulator Tao Tang, Mark Kemp, Craig Burkhart Power Conversion Department

  2. Outline • Background of ILC Marx Modulator • High Crystalline Poly-Propylene (HCPP) self healing film capacitor • Capacitor life time model (DC) • Voltage related lifetime change • Thermal related lifetime change • AC capacitor lifetime scaling • Image of failed capacitor films • Future work

  3. Marx Modulator • ILC Marx Modulator • Pulse Voltage: 120 kV • Voltage Regulation: ±0.5% • Pulse Current: 140 A • Pulse Length [flat-top]: 1.6 ms • Repetition Rate: 5 Hz • Total # of RF Stations: 576 • Current statues • Installed in ESB: summer 2009 • Operating with klystron: Sept. 2009 • Life time testing now : modulator ran 43 days last year (April 2010- April 2011) • Capacitance degradation • RF power start to droop after a few months running • Degradation depends on depth of discharge • Residual voltage on capacitors (as high as 1kV) • 105hr theoretical life time103 hr

  4. HCPP Film Capacitor • High Crystalline Poly-Propylene (HCPP) film capacitor • High energy density • Used in industry applications • DC: storage capacitor, filter capacitor • Deep discharge: defibrillator • Marx application • Compact system  High energy density • In new operation region • Discharge 20%-40% • Like DC operation(?) Capacitor electrode Film(4.8µm) metallization 2-4Å Solder point

  5. Capacitor Life Time Model (DC) • Lifetime scaling relationship model widely used in literature for film capacitors • Suggested by manufacture • Definite end-of-life >5% capacitance change • Other fail mechanism after >5% change • Formula: • DC voltage & Temperature • Design and testing for P2 capacitors • Optimize capacitor design • Testing at elevated voltage stress (standard method used in industry) 20% droop • Conclusion: 264V/um for >10^5 hr life time • P1 capacitor(40% droop): 194V/um

  6. Voltage Related Life Time Change • Voltage related shorten lifetime • Residual voltage induced uneven voltage distribution • Improvement • Replace end-of-life capacitors • Add balancing elements: balancing resistors • Results: capacitance decrease at same rate

  7. Thermal Related Life Time Change • Average power • Very small • P1 Capacitor thermal design is very conservative • Change repetition rate did not help • Instantaneous heating • Manufacture defect on edge connection of large capacitors is sensitive to instantaneous power • Testing of small capacitors  same degradation rate

  8. Capacitor Life Time Model(AC) • AC capacitor fail model • Anodic oxidation of Al metallization(Corona related) • AC voltage (ie. Depth of discharge) related • DC field can not initiate or sustain the corrosion process • Relation of life time and depth of discharge • ESB operation data (I^2 t also changed accordingly) • Controlled environment (same I^2 t) at B015

  9. Image of the Film • Scanned image of films • Only on one polarity of the film (cathode or anode) • Almost perfect round • Optical microscope image • Can not see pin hole • SEM • Film charging • Material analysis: can not find Al in metalized area

  10. Future Work • Solution for current issue • Decrease depth of discharge to 20% • Shorter pulse width (short term) • Double capacitance (long term) • Understanding the failure mechanism • Find the relation of AC voltage to life time (increase with AC voltage or sharp change after certain threshold) • Find the threshold field strength for this failing mechanism

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