HI

[Power Monitoring and control unit]

HI

I am doing a project to monitor current, voltage, power and power factor of an ac power system. I am using NI-USB 6008 unit for this purpose.

To get the voltage I use a potential transformer and stepdown the voltage and send to USB-6008.

To get the current I use a current transformer and send the voltage signal to the DAQ. However the signal coming out from the current transformer is not big enough to be detected from the DAQ. Therefore I am using an amplifier to increse the voltage. I am sending that increased voltage signal to the USB-6008.

To find "voltage magnitude and angle" and "current magnitude and angle" I am using EMP_Phasor vi. When I run the program the current angle that I am getting is always fluctuating (even the polarity changes) and is always out of phase with voltage.

To find power and power factor I am using EMP_power.vi and I am sending the same current and voltage signal to calculate power and power factor. Thse two values always change as well. The reason for that has to be because the current signal angle changes always.

Can you please provide me a solution to keep the current angle to a steady value.

I have attached the labview program that I have developed .

Thank you.

[cosmin]

Hi,

I think you posted something similar in another thread. I a bit confused, you are saying that the secondary of CT's are connected directly to DAQ AI channel?

You are aware that the impedance in secondary circuit of CT directly influence the accuracy of the measurement and consequently angle error of the CT. A large impedance could even damage the CT. I used high accuracy low resistance resistor in secondary circuit and measure the voltage drop across resistor. 

cosmin