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High frequency Sine wave inverter -Challenges in voltage feedback
1. High frequency Sine wave inverter
Challenges in voltage feedback
www.controltrix.com
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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 loads
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3. Requirements of feedback circuit
⢠High Voltage feedback
⢠Isolation
⢠Low cost /rugged
⢠Linearity
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4. RMS control mode
⢠Measure rms voltage err. @ (50Hz/60Hz freq)
⢠Change ONLY modulation index âmâ 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 rate
copyright 2011 controltrix corp www. controltrix.com
5. RMS control mode feedback
⢠Std Low grade/low cost 50Hz Potential Transformer(PT)
⢠Std part in electric metering (widely available)
⢠Satisfies 4 desirables
ď Isolation
ď Low cost
ď Ruggedness
ď Linearity @ 50/60 Hz output voltage
copyright 2011 controltrix corp www. controltrix.com
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 PF
copyright 2011 controltrix corp www. controltrix.com
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/DSP
copyright 2011 controltrix corp www. controltrix.com
8. PT feedback characteristics
PT input with DC offset PT output (DC offset blocked)
⢠Low and High frequency signals undergo significant distortion
copyright 2011 controltrix corp www. controltrix.com
9. Challenge
Using PT feedback would lead toâŚ
Output Voltage
⢠Unstable or saturated intermittently (+/-400 Vdc)
Result
⢠Malfunction of Downstream equipment
⢠Damage to Downstream equipment
⢠Catastrophic results or shutdown !!!!
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10. Actual voltage: 200 V/div
PT output
output voltage and PT feedback are not correlated
copyright 2011 controltrix corp www. controltrix.com
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. 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 pending
copyright 2011 controltrix corp www. controltrix.com
13. Experimental results
Instantaneous closed loop control system
Voltage output
100 V/div
PT output
Final output voltage waveform
copyright 2011 controltrix corp www. controltrix.com
14. Thank You
consulting@controltrix.com
copyright 2011 controltrix corp www. controltrix.com