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F28M36P63C2: Parallel operation of inverters

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Replies: 25

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Part Number: F28M36P63C2

Dear all,

I have two inverters that work well in stand-alone mode and i want now to operate them in parallel. All the control part is implemented in the Simulink add-on for concerto microcontrollers. The microcontroller i am using is the F28M36P63C2.

The problem i am facing is as follows: when i start operating one of the inverters (inverter 1) with the load and then i want to synchronize the second inverter ( inverter 2), inverter 2 needs to read the voltage at the connection point without connecting (i.e. without closing the relays). Even though at the output of voltage sensor of the 2nd inverter, the waveform is very good, in the CCS graph, every 10-15s this voltage gets very noisy for some seconds and then returns to normal. Thus, it is not safe to try connect.

Maybe the problem is related to the fact that i use the ePWM ADC SOCA for triggering the ADC inputs. Note that when the relays are open, the PWM signals of the second inverter do not correspond to the connection point voltage, which i am trying to read (the pwm signals of the second inverter are not those that create the connection/load voltage i am trying to read, which is created by inverter 1). I am not sure if this exactly is the problem or how i should proceed.

Has anyone faced anything similar before? Any hint would be very helpful.

Thanks in advance.

Alex

  • Hi,

    Are both inverters switching at same PWM frequency and in synchronization all the time?
    Is it possible that the instance you are trying to read the voltage there is noise due to switching on the other inverter (in this case inverter 1)?

    Alexandros Paspatis
    i want to synchronize the second inverter ( inverter 2)

    It seems like you are trying to synchronize the PWMs here - is this correct?

    Also, is the noise only in the measurement or you actually see the noise when you probe the voltage signal?
    I wonder this is due to switching noise from inverter 1 being coupled into asynchronous measurement at inverter 2.

    Alexandros Paspatis
    Maybe the problem is related to the fact that i use the ePWM ADC SOCA for triggering the ADC inputs.

    Which PWM event are you using for generation of SOCA? Did you try sampling the signals at a different time instance?

  • In reply to Subrahmanya:

    Hi, 

    Thanks for your reply.

    Both inverters operate at the same switching and sampling frequency.

    The same exactly code is run on 2 different computers, one for each microcontroller of each inverter.

    Since the first inverter is initially operating alone (with the load), what i am trying to do is to synchronize the second inverter on the existing power system ( consisting of the inverter 1 and the load). The voltage read is very good but every 10-15s it gets seriously distorted so i have not tried to synchronize them yet.

    If the noise was coming from the switching of inverter 1, wouldn't it be permanent?

    What do you mean be synchronizing the PWMs?

    The noise does not exist neither in the probed voltage nor in the 0/3V output of the sensor. It only exists in the CCS graph where i read the ACD output value from my code. So it seems that it is initiated in the ADC conversion.

    I am using an event trigger at EPWM2A, at first event with CTR=ZERO and CTR=PERIOD. I also tried using only CRT=ZERO or CTR=PERIOD and second event start.

    I hope i answered your questions. Let me know if you need anything else.

    Alex

  • In reply to Alexandros Paspatis:

    Hi,

    1. PWM outputs can be synchronized. Even if they are on 2 different microcontrollers, you can send a synchronization input from one MCU to another.
    Please refer to "Figure 7-7. Time-Base Counter Synchronization Scheme 4" of device TRM. This allows a GPIO signal to be used an a synchronization input.
    One MCU can generate EPWMxSYNCO and can be connected to another MCU as EPWMxSYNCI.

    2. Since you mentioned that there is no noise in the measured signal - when plotted on the scope, but noise only exists in the CCS graph, i'm wondering if its a graphing issue. Is there a possibility that the graph buffer is being overwritten with some other data? any Overflow in data captured?
    Also, please cross check your graph properties and make sure that the buffer settings etc. are accurate.


    Alexandros Paspatis
    The noise does not exist neither in the probed voltage nor in the 0/3V output of the sensor. It only exists in the CCS graph where i read the ACD output value from my code. So it seems that it is initiated in the ADC conversion.


    This makes me suspect that point 2 above could be the case. It's very unlikely that the ADC is introducing such noise - it may be a graph artifact. Hence it would be worth while check above. Also, try to inspect the ADC data (not just the graph) and check if you see the same anomaly.


  • In reply to Subrahmanya:

    Many thanks for the reply.

    I dont think its point 2 for 2 reason:

    1) When i operate the inverter in stand-alone (if i just connect inverter 2 to the load) everything works perfect. There is no noise and the graph is clear with exactly the same settings.

    2) The output of the ADC is connected to some protection logic system which in case of an overvoltage for example would open the relay. When trying to read the voltage in the synchronisation case, this protection system is enabled. Thus, this noise exists in the actual output of the ADC and does not come up in the graph only.

    Using a connection between the MCs would be a strict requirement. Can't i manage to read the voltage accurately without this? Is the problem indeed due to the triggering coming from the PWM?

    Let me know if you have any more insight after my above comments.

    Thanks a lot.

    Alex

  • In reply to Alexandros Paspatis:

    Hi,

    Alexandros Paspatis
    Using a connection between the MCs would be a strict requirement. Can't i manage to read the voltage accurately without this? Is the problem indeed due to the triggering coming from the PWM?



    I can't say for sure if the problem is indeed due to the triggering of PWM. Again, even in this case I would expect that you would see the noise on probed voltage or output of the sensor. But you are saying there is no noise observed on the ADC input.

    When you say noise, what kind of noise are you observing? Is there a pattern? How many samples are out of expected range?

  • In reply to Subrahmanya:

    Hi,

    Note that during the synchronisation process, the second inverter is not connected to the load (relays are open). Thus, the probed load voltage is solely produced from the inverter 1. In inverter 1 everything is running smoothly. So, why would you except if there is an issue in the inverter 2 sampling the load voltage to be affected?

    I can see 4 full periods of the sine wave voltage in the CCS graph. Every 10-15s, about 8-12 periods are getting heavily distorted (the waveform is not sinusoidal and there are also some spikes). After that the voltage returns to normal. And again after 10-15s, the same thing happens.

    I hope that helped.

    Alex

  • In reply to Alexandros Paspatis:

    Hi,

    We also sent the voltage after the adc into an ecap module and pictured it in the oscilloscope. The same problem persists. So for sure it is not an issue of the memory or the graph.

    Alex

  • In reply to Alexandros Paspatis:

    Hi,

    Alexandros Paspatis
    We also sent the voltage after the adc into an ecap module and pictured it in the oscilloscope. The same problem persists. So for sure it is not an issue of the memory or the graph.


    Thanks for confirming this.

    Alexandros Paspatis
    The noise does not exist neither in the probed voltage nor in the 0/3V output of the sensor. It only exists in the CCS graph where i read the ACD output value from my code. So it seems that it is initiated in the ADC conversion.

    You've confirmed 2 things.
    1. These is no noise in the sensed signal going into ADC.
    2. Noise reflects on ADC results and observed via graph as well as other methods (eCAP).

    Is there any noise on the other inputs (like Vref hi/lo, power supply, IO switching?) coinciding with noise?

    I would also refer to ADC experts. 

  • In reply to Subrahmanya:

    Hi,

    Yes i have confirmed both.

    Furthermore, i tested the ADC ouput without the low pass filter that i was originally using. It seems that every 15s, big spikes occur at the sinusoidal form (the voltage was around 20Vac and the spikes were around 300V). The same spike was observed at 50Vac as well.

    Note that this spike does not exist at the output of the voltage sensor.

    Both microcontrollers are connected through 2 USBs (each one) to each computer.

    Can you understand anything more according to these? Aren't there ADC experts in TI that could take part in the discussion?

    Thanks.

    Alex

  • In reply to Alexandros Paspatis:

    Hi,

    Alexandros Paspatis
    Can you understand anything more according to these? Aren't there ADC experts in TI that could take part in the discussion?



    Yes - that is what i meant. I'll loop in ADC experts at TI.