VOLTMETER - Line to Line Voltage calculation in 3Phase-4Wire system
Hello Dear Members,
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!
Hello Dear Members,
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!
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