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Hot!VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system

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RDD@EAPL
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2020/11/23 22:35:41 (permalink)
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VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system

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    RDD@EAPL
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 22:38:56 (permalink)
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    [[[[<font]Thanks in advance!
    #2
    ric
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 22:38:57 (permalink)
    +1 (1)
    Did you mean to post a question?
     

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    RDD@EAPL
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 22:40:24 (permalink)
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    I am not able to post the Problem Statement
     
    #4
    ric
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 22:50:17 (permalink)
    +1 (1)
    Are you trying to copy text from a Word document or a web page?
    Try just typing it in.

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    PStechPaul
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 23:06:18 (permalink)
    +1 (1)
    You will need to connect the neutral to digital ground to get a reference for each of the phase voltages (L-N). If you want to read the phase-to-phase voltages, you will need to take simultaneous samples of the phase voltages and use the difference for calculation. And you will need to perform a true-RMS calculation based on these samples over one or more integral multiples of the period of the waveform.

     
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    RDD@EAPL
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 23:08:37 (permalink)
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    I have opened a New Thread where the Problem Statement is mentioned in the form of .txt Attachment.
    Please Refer the thread: https://www.microchip.com/forums/m1158678.aspx
     
    Regards,
    RDD
    #7
    PStechPaul
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 23:13:32 (permalink)
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    Please DO NOT open new threads. Keep the discussion here. Your text:
    VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system

    Hello Dear Members,

    I am not able to Post the Problem Statement here. Don't know the reason!

    Please go through the Attachment to refer the Problem Statement.

    Problem Statement:
    I am developing software code for simple VOLTMETER for 3Phase-4Wire system.
    Existing Hardware is as per attachment. I am stepping down the Analog AC voltage through 1MOhms resistor. Then as microcontroller ADC can't read -ve voltage so I am shifting the AC voltage on positive side by Potential Divider circuit. As VCC = 5V DC and I am using equal value resistors of 6.81KOhms for Potential Divider, then AC voltage will be shifted by 5/2 = 2.5V DC. Now DC shifted voltage is given to 10-Bit ADC as Analog Input. ADC reads this Input by taking few samples and calculates Line to Neutral voltage by Root-Mean-Square formula accurately.
    Another formula is used to calculate Line to Line voltage as,
         VRY(L-L)      =     Square root of (  (VR(L-N))2  + (VY(L-N))2  + ( (VR(L-N)) * (VY(L-N)) )   )

    Till here everything is working perfectly. It's displaying R-Y-B (Line to Neutral) and R-Y-B (Line to Line) voltages accurately. Even if different voltages are present in R-Y-B lines also it is calculating both Line to Neutral and Line to Line voltages perfectly.
    If only one phase voltage (say R-Ph) is connected between R-terminal & Neutral-terminal then also it is calculating both Line to Neutral and Line to Line voltages perfectly.
    The issue is, in Single phase as only one phase voltage (say VRN = 240V) is available and if it is connected to all three Terminals (R-terminal, Y-terminal & B-terminal) [actual Y-Phase & B-Phase voltages are absent in single phase, so it is as good as Y & B terminals are shorted to R-terminal ] then as single phase voltage is given to all three Terminals, Line to Neutral voltages are perfectly calculated (R-N, Y-N & B-N voltages are equal since they are shorted together)
    Now in above condition as single phase voltage (L-N = 240V) is given to all three Terminals,  Line to Line voltages are Zero as there is no 1200 seperation between the phases so the phase difference is 00. But as per above formula for Line to Line voltage, there is no technique to identify Phase Angle so VOLTMETER is calculating the Line to Line voltages as 415V only. So all three Line to Line voltages (VRY, VYB  & VBR ) are calculated as 415V instead of ZERO volt.
    I agree that above situation is not a Standard Recommended one as per connection, since for 3-PH 4W System all three phases shall be 1200 apart in Balanced Load. For Single Phase, only  R-terminal & Neutral-terminal shall be connected. But in case if we give single phase voltage to all three Terminals then showing 415V is also not correct.
    Kindly suggest a technique in software to detect in case of Single phase connected to all three Terminals (R-terminal, Y-terminal & B-terminal), How to calculate & show  Line to Line voltages as ZERO.
    Thanks in advance!

    Regards,
    RDD


     
    #8
    ric
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 23:17:46 (permalink)
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    PStechPaul
    You will need to connect the neutral to digital ground to get a reference for each of the phase voltages (L-N).
    ...

    But that will cause big problems if the low voltage supply is generated form the HV supply via a diode bridge.
    It might be better to put DC blocking capacitors on each phase.

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    #9
    PStechPaul
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 23:27:51 (permalink)
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    We need a more complete schematic as well as program code. DC blocking capacitors will introduce phase shift, which could be problematical. The low voltage 5 VDC power supply might be best generated using a resistor (perhaps with a capacitor) and zener diode, from L-N. If there is no neutral, it could be created by using resistors from each phase to ground. It is possible that the circuit as shown will work on an open delta (no neutral), but it might not be very reliable.

     
    #10
    al_bin
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 23:43:17 (permalink)
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    No schematic or code needed as OP claims they work perfect.
    For each ADC signal sample,
    also calculate the RMS value from the difference of two phases samples.
    Use as result or phase fault indicator (when zero)
     
    Albert
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    PStechPaul
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/23 23:55:33 (permalink)
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    Your formula is basically the same as V(L-L) = V(L-N) * sqrt(3).

     
    #12
    al_bin
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/24 01:03:27 (permalink)
    +1 (1)
    In the correct 3-phase system, yes. In other cases, no.
    #13
    oliverb
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/24 05:01:51 (permalink)
    +1 (1)
    OK I think I get this, processing the phase voltages individually returns 3 RMS voltages, but no information about phase, so there's no obvious way to verify phase sequence.
     
    To resolve phase you could use zero-cross detection, that is converting the sine wave to a square and timing the transitions.
     
    Alternatively two or three phases could be measured in quick succession. Simultaneously would be good but would require an external track/hold circuit to capture all 3 voltages together. If the program loops through the inputs quickly enough then it should be near enough to simultaneous for the phase error to be less than 5 degrees. Given the instantaneous voltages you can calculate differences between them (as stated by PStechpaul already) and then convert the differences to RMS.
     
     
     
     
    #14
    Mysil
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/24 05:52:12 (permalink)
    +2 (2)
    Hi,
    You may sample all the voltages R_N, Y_N and B_N as fast as possible.
    Also record the time when sign of each voltage change sign, above or below the bias value Vdd/2.
    This will give information about phase sequence, and also show if 2 or 3 phase voltages are the same,
    or if one or two of the inputs are unconnected, and measure just the bias value all the time.
     
    Calculate the differential voltages ((R_N) - (Y_N)),  ((Y_N) - (B_N)) and ((B_N) - (R_N)) ,
    these are the instantaneous line to line voltages. These differential voltages must be calculated as signed values.
    Calculate the square of each line to line voltage, this will give values that may be treated as unsigned integer for average accumulation, and square root extraction.
    Accumulate all the squared measurements during 1 or more exact period cycles, 
    to calculate true RMS value of each line to line voltage.
     
    Measuring differential voltages in the same sequence, during both rising and falling parts of AC curve,
    then small differences caused by measurements not beeing exactly the same time,
    will even out during the whole AC cycle.
     
    There have been a long thread in this forum, a couple of years ago,
    discussing many of these questions, and with schematics and program examples.
    https://www.microchip.com/forums/FindPost/959814
     
        Mysil
     
    Edited to distinguish line to line voltage from phase voltage.
    post edited by Mysil - 2020/11/27 02:21:04
    #15
    radikn
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/26 02:27:32 (permalink)
    +2 (2)
    I did power measurements (voltage and current) for tongs for slaughtering pigs in slaughterhouses. I'm sampling a 50Hz sine wave. I applied a sine approximation to the samples and multiplied the calculated amplitude by the square root of 2. The results were sufficient. I enclose a scheme for inspiration. I was inspired here https://www.ti.com/tool/TIDA-00555
     

    #16
    PStechPaul
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/26 21:44:26 (permalink)
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    That is an unnecessarily complex and expensive means to accomplish the OP's requirements (which would require three circuits as shown). And I question how you can multiply the calculated amplitude by 1.414 to get the correct (RMS) value. If you measure peak of a sine wave, you can divide by square root of 2 to get RMS. But it is really not difficult to compute true RMS from a series of samples. Even a modest 500 samples per second will give quite accurate results.

     
    #17
    davea
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/26 23:42:50 (permalink)
    0 (2)
    But in case if we give single phase voltage to all three Terminals then showing 415V is also not correct.

     
    if you have ADC_value[3] in a ring buffer, most likely you do
    you scan the array for MAX value and compare there location in buffer
    are if to close to each other discard 1 or 2 of there calculations (less then 90 deg)
     
     
    #18
    vexorg
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    Re: VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system 2020/11/30 05:45:11 (permalink)
    0 (2)
    That original circuit is one dangerous way to measure it. Pretty sure the OP will be dead or zapped soon as you dont mess about with 3 phase unless you know what you're doing.
     
    As a power engineer, it's not even worth the time to explain how to do it, the other posts are entertaining enough..
    #19
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