High frequency Sine wave inverter -Challenges in voltage feedback

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High frequency Sine wave inverter

Challenges in voltage feedback

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


Requirements of feedback circuit• High Voltage feedback• Isolation• Low cost /rugged• Linearity


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


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


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


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


• Low and High frequency signals undergo significant distortion

PT input with DC offset PT output (DC offset blocked)

PT feedback characteristics


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 !!!!


output voltage and PT feedback are not correlated

Actual voltage: 200 V/div

PT output


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 !!!!


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


Final output voltage waveform

Voltage output 100 V/div

PT output

Experimental results Instantaneous closed loop control system


Thank [email protected]

High frequency Sine wave inverter -Challenges in voltage feedback

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