3 phase voltage measurement using SCC

[freya]

Good day. I would like to seek advice about monitoring 3-phase voltage at the supply point of 230V.  The main aim of the project is to monitor power quality disturbances like transients, voltage swells etc. The experiment equipment consists of 3 single phase transformers, NI SCC AI01, SC2345 Shielded carrier with BLACK BOX CA-100 enclosure, and a computer.

The transformer is used to step down the voltage and to provide isolation. To measure the phase voltage, is it correct to connect the  transformers  in star-star,with primary wye grounded? Also, is it possible to measure line-line voltage using BNC input? If yes, how should I connect the wires? This is because the BNC input is shielded to ground through the computer ground point. That is why I have doubts about how to measuring the line-line voltage using BNC input.

Apart from the maximum input voltage, what are the other differences between the SCXI and SCC signal conditioning module? Is the experiment setup  the same for both? Since transformer is used for isolation as safety measures. From what I have heard, if I connect the DAQ board directly to the supply point of 230V through a voltage step-down probe, the circuit might trip due to grounding problem, ie. the PC and DAQ device is grounded, excessive current might flow through the supply N and G. In order to fix this problem, transformer is used to provide the isolation. Is that correct?

Thank you very much for the time and  help. 

[Neil_S.]

freya,

Thanks for posting to the NI forums.  Let me see if I can address some of your questions. 

1. To measure the phase voltage, is it correct to connect the  transformers in star-star,with primary wye grounded?

I am not a familiar with transformer design, I may defer this question to someone with more experience in this area.  However, I will note that the SCC-AI01 is isolated but this isolation is lost when you connect your signal source ground to the shield of the BNC connector.  Since this BNC connector shield is connected to the chassis of the SC-2345 it is also connected to the DAQ card ground.  To make this phase measurement and retain isolation it may be preferable to send the ground through a separate BNC connector (through the center pin).

2. Is it possible to measure line-line voltage using BNC input?  If yes, how should I connect the wires?

Since the AI01 is isolated to make a line-line voltage connect one signal to the pulse terminal on the AI01 and the other signal to minus terminal on the AI01.  You will not connect the ground reference to the AI01.  The voltage measured on the AI01 channel will then be the difference between the two signals.

3. Apart from the maximum input voltage, what are the other differences between the SCXI and SCC signal conditioning module?

SCXI and SCC are two different signal conditioning platforms.  The SCXI is our higher end product that satisfies applications with larger channel counts and that need more rugged hardware.  Specific differences between SCC and SCXI will depend on which modules are being compared.  There can be differences in maximum sample rates, signal bandwidth, and voltage ranges.  There can also be differences in the level of isolation.  If you have more specific questions about SCXI modules let me know.

4. Is the experiment setup  the same for both?

The setup should be similar.  However, since the connections are different, obviously the pin numbers and the like will not match up.

5.  From what I have heard, if I connect the DAQ board directly to the supply point of 230V through a voltage step-down probe, the circuit might trip due to grounding problem, ie. the PC and DAQ device is grounded, excessive current might flow through the supply N and G. In order to fix this problem, transformer is used to provide the isolation. Is that correct?

If you connect the ground of your external circuit to the ground of the DAQ board or to the chassis there can be ground loops if the ground reference of your external circuitry varies from the computer ground.  However, since the SCC-AI01 is isolated from the DAQ board you can connect the ground of your external signal to the - terminal of the SCC-AI01 without any trouble, given the level of the ground does not vary any more than 60 V from computer ground.

Let me know if I need to clarify any of the answers I provided.  Good luck with your application.

Regards,

Neil S.
Applications Engineer
National Instruments

[freya]

To: Neil S

Thanks for replying to all my questions. Now i get a better idea about the setup. : )
However may i ask for your help on some further questions below.

1. Please correct me if my understanding is wrong. The input of the SCC-AI series has 2 number of channels, differential, which are the positive terminal and negative terminal of the AI01. Since there are 2 BNC connectors per panelette, these 2 BNCs are actually for the positive and negative terminals. Negative terminal is at the lower part of the panelette. Do i need to manually configure the negative terminal, ie put an - sign to each data collected? Or is the negative terminal already a differential input in the circuit to deduct from positive terminal? Because i am quite confuse with the connections. If i connect 2 signals to 2 BNC in the same panelette, i get 2 signals on my computer instead of 1 signal. Does that mean differential process must be manually done through data manipulation?

2. The grounding of the BNC shield at the enclosure end is for noise reduction,right? Will the data measurement  be affected by noise if i do not ground the shield of the BNC connector to retain isolation? because i am trying to reduce noise and prevent ground loop at the same time, but not sure how. Will transformer solve the problem by providing the electrical isolation?

Thanks again for your help. =p

[Hal_L]

Hi,

The SCC AI-01 has 2 differential inputs. For differential connection already has the + and - terminals configured for that operation. All you need to do is to make sure that you have "differential" selected under your input configuration in LabVIEW or Measurement and Automation Explorer (MAX).

It looks like you are doing everything properly if you are connecting 2 signals when you connect 2 signals. Here is the link to a tutorial about connecting signals and reducing noise that you may find helpful: Field Wiring and Noise Considerations for Analog Signals.

Regards,
Hal L.