Post on 15-Jun-2015
copyright 2011 controltrix corp www. controltrix.com
High frequency Sine wave inverter
Challenges in voltage feedback
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copyright 2011 controltrix corp www. controltrix.com
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
copyright 2011 controltrix corp www. controltrix.com
Requirements of feedback circuit• High Voltage feedback• Isolation• Low cost /rugged• Linearity
copyright 2011 controltrix corp www. controltrix.com
RMS control mode• Measure rms voltage err. @ (50Hz/60Hz freq) • Change ONLY modulation index ‘m’ with RMSV err• Duty d = m.sin(2.p.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
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 voltage
copyright 2011 controltrix corp www. controltrix.com
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
Feedback requirementNeed• 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
• Low and High frequency signals undergo significant distortion
PT input with DC offset PT output (DC offset blocked)
PT feedback characteristics
copyright 2011 controltrix corp www. controltrix.com
ChallengeUsing 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
output voltage and PT feedback are not correlated
Actual voltage: 200 V/div
PT output
copyright 2011 controltrix corp www. controltrix.com
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
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
Final output voltage waveform
Voltage output 100 V/div
PT output
Experimental results Instantaneous closed loop control system
copyright 2011 controltrix corp www. controltrix.com
Thank Youconsulting@controltrix.com