High frequency Sine wave inverter -Challenges in voltage feedback

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High frequency Sine wave inverter -Challenges in voltage feedback

  1. 1. High frequency Sine wave inverter Challenges in voltage feedback www.controltrix.comcopyright 2011 controltrix corp www. controltrix.com
  2. 2. System Configuration • DSC on primary (low voltage) side • Front end Switched mode isolated converter • Boost battery voltage to 400V • Hi bridge Inverter to generate sine wave Objective • Control the output to a sine wave under various loadscopyright 2011 controltrix corp www. controltrix.com
  3. 3. Requirements of feedback circuit • High Voltage feedback • Isolation • Low cost /rugged • Linearitycopyright 2011 controltrix corp www. controltrix.com
  4. 4. RMS control mode • Measure rms voltage err. @ (50Hz/60Hz freq) • Change ONLY modulation index ‘m’ with RMSV err • Duty d = m.sin(2. .f.t) is • @ Steady state maintains sine wave • Poor /slow dynamic response approximately 100ms • Distortion under non linear loads like rectifier • Extremely low bandwidth • Low computation load req. • Low execution ratecopyright 2011 controltrix corp www. controltrix.com
  5. 5. RMS control mode feedback • Std Low grade/low cost 50Hz Potential Transformer(PT) • Std part in electric metering (widely available) • Satisfies 4 desirables  Isolation  Low cost  Ruggedness  Linearity @ 50/60 Hz output voltagecopyright 2011 controltrix corp www. controltrix.com
  6. 6. Instantaneous control • Industry term for Hi bandwidth digital control • Hi sampling frequency and code execution rate ~ 20 KHz • Great dynamics approx. • ~400 us step response • Non linear and fast changing loads handled • Large stiffness (low output impedance) • Perfect regulation even with leading /lagging PFcopyright 2011 controltrix corp www. controltrix.com
  7. 7. Feedback requirement Need • Accurate real-time fast high bandwidth voltage feedback PT limitations • PT magnetically coupled device • No output of PT for DC component in input signal • Any DC offset will not be visible to DSC/DSPcopyright 2011 controltrix corp www. controltrix.com
  8. 8. PT feedback characteristics PT input with DC offset PT output (DC offset blocked) • Low and High frequency signals undergo significant distortioncopyright 2011 controltrix corp www. controltrix.com
  9. 9. Challenge Using PT feedback would lead to… Output Voltage • Unstable or saturated intermittently (+/-400 Vdc) Result • Malfunction of Downstream equipment • Damage to Downstream equipment • Catastrophic results or shutdown !!!!copyright 2011 controltrix corp www. controltrix.com
  10. 10. Actual voltage: 200 V/div PT output output voltage and PT feedback are not correlatedcopyright 2011 controltrix corp www. controltrix.com
  11. 11. Summary • The PT works properly only @ 50/60Hz sine wave is being measured • To get accurate sine wave the PT needs to work properly in closed loop • Vicious dependency deadlock causes catastrophe !!!! • Chicken and egg problem !!!!copyright 2011 controltrix corp www. controltrix.com
  12. 12. Proposed Solution • No DC offset in output waveform • PT based feedback instantaneous control • Pure software solution • Low THD (slightly worse wave shape than ideal feedback) • Expensive, Complex ,Non linear isolation solutions not needed !!! • E.g. : Opto-couplers, voltage to frequency converters, PWM generators • Patent pendingcopyright 2011 controltrix corp www. controltrix.com
  13. 13. Experimental results Instantaneous closed loop control system Voltage output 100 V/div PT output Final output voltage waveformcopyright 2011 controltrix corp www. controltrix.com
  14. 14. Thank You consulting@controltrix.comcopyright 2011 controltrix corp www. controltrix.com

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