Creating A Virtual Power Line for Communications Channel Testing

Author(s):
Bruno Sabbattini - ABB

Industry:
Telecommunications

Products:
LabVIEW,

The Challenge:
Developing a tool for measuring medium voltage power lines used as communication channels, thereby reducing the need for field tests, and leading to lower cost and improved development speed.

The Solution:
Virtually emulating the power line using National Instruments LabVIEW and data acquisition.

Introduction
Because users can use power lines as a medium for communications, characteristics such as impulse response, group delay, impedance, and noise are critical values to consider when testing. The fact that measurement points often are located in small remote substations or even on pole-mounted switches creates a need for portable equipment. However, we aimed to create a lab-based field tester to reduce the need of taking tests in the field.

System Setup
We used a digital signal processor (DSP) board and a power amplifier as signal sources at the injection point. At the receiving location we chose a PCMCIA DAQCard-AI-16E-4 with LabVIEW for acquiring and post-processing the signals.

With this combination, users can extract all needed characteristics for a portable measurement tool.
We acquire processing signals in the lab, which are then sent out via an analog out channel in the DAQ board AT-MIO-16E-2. This provides for processing telegrams in the lab as if they were passing through a power line. Therefore, users can implement tests with new communication equipment in the lab before taking it out into the field, resulting in a major reduction in cost. In contrast to field tests, highly sophisticated development tools make the use of virtual field tests in the lab possible.

Measuring Impulse Response
Measuring the impulse response was a critical factor for us. To do this, we transmitted a digital pseudo noise sequence of maximum length through one end of the line using LabVIEW running on a DSP board. The signal shows only -V and +V more or less randomly, but has the following properties:

• The RMS value is V
• The crest factor (peak absolute value to RMS value) is 1
• The period is equal to the maximum length
• The auto correlation function is almost 0, with exception of a high triangle at t=0 repeated every period

The auto correlation is similar to a periodic sequence of Dirac pulses. By injecting periodic Dirac pulses into the channel, the channel responds accordingly with its impulse response. By assuming the correlation is linear, we can inject the pseudo noise sequence into the channel at any point, acquire the data at the receiving station, and calculate the cross correlation.This action results in the impulse response, except that the originating signal was not a Dirac pulse, but a high triangular signal.

Measuring Noise
We chose two different strategies for measuring noise. First we needed to know how the spectral density of noise varies over time. Recorded noise sequences were mixed afterwards with communication signals to emulate real world conditions in the lab.

After measuring these parameters in the field, we had all the elements necessary to build a virtual power line in the lab. First, we acquired the signal of a transmitted telegram using a DAQ board. Then, we took the impulse response measured earlier in the field and processed it with a filter that helps to compensate for the non-ideal originating signal. The result is then filtered by the impulse response of the power line.

These operations were not done in real time, as the frequencies of interest are up to 100 kHz and convolutions of a certain length are rather time-consuming. In fact, it is more important that the virtually received signals can be mixed with the recorded noise sequence of the power line and output via an analog output channel of a DAQ board to the receiving communication equipment.

Results
We were able to use the measurement tool we built with LabVIEW and DAQ products in the field to identify characteristics of power line channels. The virtual power line we built, in combination with the measurement tool, substantially reduces the need for field tests and increases the speed and quality of power line modems development.

For more information, contact:

Bruno Sabbattini

ABB

Telephone: 41 56 406 7087

E-mail bruno.sabbattini@chcrc.abb.ch

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