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Original question:

CCS: SFRA

CCS: SFRA DC vs AC.

Michael Koval27
Intellectual 990 points

Other Parts Discussed in Thread: SFRA

Tool/software: Code Composer Studio

Hello, Manish. We work with SFRA project _IQ  , F28035. Only SFRA. We got good stable plant, test it , all OK. DC  Voltage is controlled very stable. Thank you much for your advices. But , when we launch  AC on this board  - it gives very good sine if open loop, But  when we insert 3P3Z coefficients which we got for DC ( the board, all inductives, and capacitors are the same - all the board is the same-project HV_SOLAR_DC_AC) , so when we try to control AC through these coefficients we can not get even normal sine form. We control Voltage.  Question :  are coefficients which are good for DC are good for AC? (Concerning the same topolgy)  And how it is possible to make a sine which is getting through 24 kHZ PWM with filter (3P3Z - in real 2P2Z) which has the bandwidth only 1.5 kHZ?  Again - HV_SOLAR_DC_AC - excelent sine when open loop. Not sine at all - when 2P2Z ( Voltage control). PWM rate is 24 kHZ,  control rate the same 24 kHz.  Voltage control. DC vs AC.

  • 0
    TI__Mastermind 43730 points

    Michael, 

    1. Yes a plant stable under DC can be unstable under AC if you have only measured DC at one point and verified that it is stable only at one DC point. Remember under AC it will go through a variety of DC points and it needs to be stable across for the AC to be stable.

    2. Are you using cascaded loops for current or voltage? without using a cascaded loop for voltage and current it will be very tough to tune the system.

    3. Also given your switching frequency is so low, 24kHz overlaying a voltage loop will even be more challenging. You will need to use PR controllers for the voltage and not a simple 3p3z. please see 

    4. you may be able to get away with just the voltage loop, if the performance requirement is low, but you will have to use PR controllers at 24kHz there is not enough room there to get enough gain at grid and harmonic frequencies to be a good voltage source.