Introduction
This aspect of the review is at the core of the reason I applied of this RoadTest. I have an unquenchable thirst for knowledge around partial discharge in high voltage insulation systems, that I had previously experimented with using the MDO34 and attempted to rebuild partial discharge patterns with its built in waveform generator. I had struggled to obtain satisfactory results with this, so I hoped that the increased capabilities of the AFG31052 would overcome the previous constraints.
Playback of original pulses in basic mode
I still have the original partial discharge data capture from my tests with the MDO34, so my initial task was merely to play this back via the AFG31052 and compare the results to the playback from the MDO34.
This gives me my first issue, trying to load a file into the AFG31052.
The original data is captured on the MDO34 and has been saved as both a CSV file and in the ISF native file format to Tektronix. However, I found that the AFG31052 would not load either of these formats using the Recall function in the arbitrary waveform function.
The screenshot above shows the various files loaded on the USB drive. The screenshot below from the AFG31052 shows that only the tfw and tfwx files are detected.
To overcome this initially, I had to use the ArbExpress software to convert the file in a tfwx format that the AFG31052 would understand. The other method was to use the ArbBuilder program on the AFG31052 that could also perform the same conversion as seen below.
When attempting to load this file as an arbitrary waveform, an error message is displayed stating 'Invalid waveform length' at the top of the screen and not waveform is loaded. The waveform has over 500k points and appears to be too large for the AFG31052 whilst in basic mode. The ArbBuilder program has a resample function, seen in the screenshot above along the bottom options.
Using this function, it is possible to change the number of points of the waveform. I found I had to take the waveform down below 128k points in-order to get the waveform to load. The result of course is that a lot of detail will be lost in doing this, but it does then load into the AFG31052 and can be displayed on the oscilloscope. This is what the waveform looks like on the screen of the AFG31052.
Captured on the oscilloscope and zoomed in to looked at an individual pulse, shows a basic square pulse over 300 ns long which is incorrect.
Playback of original PD pulses in advanced mode
The AFG31052 has two modes of operation, basic and advanced. Up until now I have been using the basic mode, that gives access to all the build in waveforms and allows a single file to be loaded into the arbitrary waveform function and played back through either of the output channels. The advanced option gives extended memory for the waveforms and the potential to create a waveform from multiple files. Both modes are accessed from the main screen.
Switching to the advanced mode allows the larger file sizes to be loaded and played back. On the left of the screenshot below, in the list, are the two waveforms I created. Conveniently, the number of points is also shown next to the waveform. The waveform is selected by dragon it over to whichever channel in the list I want to assign to it.
Two other changes were made. In the repeat column, the default number '1', is changed to 'infinite' so that the waveform is constantly played. The second change is to set the sample rate to match the frequency output required. This value defaults to 500 MS/s, I changed this to 25 MS/s for the 500 kpts waveform to output at 50 Hertz.
I had envisaged that the play back of this file in this manner, would improve the waveform capture on the oscilloscope, but this was not the case. The results of a couple of screen captures can be seen below.
| {gallery} Original PD Pulse Waveform Playback |
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Positive PD Pulse from Advanced Mode Playback |
Negative PD Pulse from Advanced Mode Playback |
As before, this is not the desired result, so I looked for further improvements.
Creating PD pulses in advanced mode
Instead of loading a captured PD pulse into the ArbBuilder, I used the ArbBuilder to create my own pulses. The new file option in ArbBuilder initially shows an option window from which the main parameters of the new waveform can be defined. The only option I changed is the 'Total Points'. Defaulted to 1.000 kpts, I lowered this to 250 points, as the PD pulses are very short in nature.
Initially, I created a positive pulse by drawing it manually on the touch screen using the point tool. Using this tool allows points to be defined along the waveform and then the software automatically joins up the dots for you, dependent upon the interweave setting. Another option is to draw the curve in manually as though you were using a pencil.
| {gallery} ArbBuilder Waveform Creation |
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New waveform created with point tool |
Interpolate options available for point tool |
Editable points table available once waveform is created |
Empty points table when trying to edit after reloading a waveform |
Waveform invert function to create an opposite polarity signal |
When using the point tool, a table of the points is also available to assist with further editing of the waveform. I did notice that after I had saved the waveform and then reloaded to carry out some more editing, the points table would be empty. It seems that this mode is a bit like a bitmap drawing package, that once saved, the previously edited part are merged into the file.
Partial discharge pulses appear as positive and negative pulses. The ArbBuilder package has a handy 'invert' function that was used to create an identical negative pulse once the positive pulse had been created. Once the pulse files have been created and saved, it is time to create the waveform sequence.
Naturally, the partial discharge pulses are periodic and split up by periods of no activity. Initially, I created these periods of inactivity by just creating empty slots within the sequence. When trying to run this though, the AFG31052 protests.
So I came up with the idea of creating a flat waveform to fill in these empty slots. This is just a 250 point file with a flat line across at zero volts as shown below.
With this file put in place of the 'empty' rows, the sequence can be played and displayed on the oscilloscope. To get the waveform to playback in a continuous manner, I set the 'Go To' box on the last row to return back to the first row. The waveform then continuously plays until it is manually stopped.
This gets me a straight line of pulses across the scope screen, but no 50 Hz reference waveform. So my next step is how to create this. Originally I planned to use the 'External Add' function available for Channel 1 as seen in the bottom right of the screen shot below. This takes a waveform from one of the connectors on the rear of the AFG31052 and combines it with th ewavform out of Channel 1. Unfortunately, this function only appears to be available when the AFG31052 is operated in basic mode and cannot be used in advanced mode.
The actual connection for the 'Add In' function is on the rear of the AFG31052, as seen in the picture below.
I did come up with a scheme to use another waveform generator to supply the 50 Hz reference signal into Channel 2 of the oscilloscope and overlay them. To synchronise the reference waveform to the PD Pulse train, I used the second channel from the waveform generator to generate a 5 volt pulse and connect this to the trigger input of the AFG31052. To get the PD Pulse train to be triggered, the first line in the sequence has its 'Wait Event' function activated and set to 'External - Positive'
| {gallery} PD Pulse Synchronisation Scheme |
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UTG962 set to provide reference and synchronise pulses |
Wait Event trigger set for line 1 |
Reference waveform and PD Pulse Synchronised on Oscilloscope |
Zoomed in on PD Pulse Train |
To an extent this does work. However, the partial discharge apparatus requires a combined reference waveform and PD pulse train to take the readings and this setup has them on separate channels. Somehow I need to combine these two signals together. There are external amplifiers available that can do this, as could a second AFG3000 series function generator. This is adding further apparatus to a bench top that is already crowded, so is not a good option.
Merging the PD Pulses with the Reference Waveform
I decided to capture the PD Pulse train on its own with the MDO34 as a CSV file to load back into the AFG31052 whilst it is in basic mode. This would then allow me to utilise the second channel of the AFG31052 to play this as an arbitrary waveform into the 'External Add' input of Channel 1, that would be set to provide the 50 Hz reference waveform. The danger is that I will loose definition of the partial discharge pulse train, but it is my only option at this moment in time. It also makes it slightly more time consuming by adding in the waveform capture and then conversion from CSV to tfwx format using the ArbBuilder application. The connection configuration is shown below.
I am pleased that this methodology seemed to work without loosing the PD Pulse Definition. The screenshot below of the oscilloscope, shows the result of the combined signals.
Below I have partially zoomed in so that the three clusters of individual pulses can be clearly seen.
Zooming in even further and the five individual PD pulses can now be seen and appear to have good definition.
The further benefit of this method of creating the pulses, is that I can easily adjust the overall amplitude and phase position of the partial discharge pulses in relation to the reference waveform. Alongside the ability to draw the individual partial discharge pulse and build them up into a sequence, this has become a very powerful test method for investigating partial discharge and testing the performance of monitoring apparatus.
| {gallery} PD Pulse Phase Shift |
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A 20 Degree phase shift places the PD Pulses at the zero crossing of the reference waveform |
A 30 Degree phase shift places the PD Pulses mid-slope of the reference waveform |
A -25 Degree phase shift places the PD Pulses at the peaks of the reference waveform |
Part of my scope was also to compare the InstaView screen to the oscilloscope screen for the pulse patterns. This was very short lived, as when using the 'Add In' facility, InstaView only showed the waveform set on Channel 1 and not the combined output that the oscilloscope was seeing as seen on the two InstaView screen captures below. The first screenshot shows both of the channels with InstView on for Channel 1 and Channel 2 showing the added waveform.
Expanding Channel 1 out with InstaView on, shows only the 50 Hz reference waveform, with no sign of any of the pulses being added into Channel 1.
Unfortunately, due to the current situation in the UK, I have been unable to go into work and test the PD Pulse patterns on the monitoring apparatus. I feel that I have achieved proof of concept with the work undertaken so far. The resulting PD patterns will be based upon the nature of the pulse sequences created and with AFG31052, this is now a very flexible and relatively easy task.
Conclusions
The AFG31052 has allowed me to progress the methods used to created partial discharge pulse patterns from collecting them with an oscilloscope, modifying them in Excel and playing them back through the built in waveform generator.
Waveforms are relatively easy to create with the ArbBuilder application allowing a waveform sequence to be created in advanced mode. With this method there are endless variations to creating partial discharge patterns for analysis.
Not having the ability to use the external Add In function for Channel 1 in advanced mode, means that extra steps must be taken to merge the created partial discharge pattern with the reference waveform to allow for the analysis system to work. An easier approach would be to be able to merge the two waveforms whilst operating in advanced mode, but this would require extra hardware.
The extra steps required did not appear to affect the resolution of the partial discharge pulse pattern. The Add In function works extremely well in this scenario and allows for amplitude and phase shifting of the PD pulses to be carried out using the channel controls and provide more variability to the patterns, without having to go back and redraw the pulses or create new test sequences.
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