Tektronix 3 Series MDO - RoadTest Application

(c) Why did you apply for this particular roadtest?

I applied for this roadtest as I have an interest in exploring the capabilities of the Tektronix 3 Series MDO to carry out condition monitoring on high voltage rotating electrical plant. There are three tests that involve the use of an oscilloscope.

• • Partial discharge analysis of insulation systems

• • Data collection from rotor flux probes
  • Recording of rotor surge oscillograph (RSO) traces

My primary area of interest for the instrument is that of monitoring partial discharge. The systems that I utilise to monitor for this are relatively basic and can produce false positives due to their sensitivity to electrical noise. They do not have the ability to capture singular partial discharge pulses due to the very fast nature of them.

I aim to carry out tests and monitoring of existing apparatus utilising the Tektronix 3 Series MDO to capture partial discharge pulses and examine their shape to determine if they fit the required criteria, allowing me to develop a greater understanding of partial discharge within the insulation systems of high voltage apparatus.

I am keen to investigate other areas of functionality of the oscilloscope to see how it can be used to analyse rotor flux probe and oscillograph traces, that the current oscilloscope can only capture and provide visual analysis.

The ultimate aim is to show how the oscilloscope can be used to work alongside specialised test apparatus to enhance the data analysis and reporting, or indeed provide a more cost effective option for collecting and analysing the condition of high voltage rotating electrical plant.

This review will require both workshop and more field based applications, giving the opportunity to look at the aspects of transporting the oscilloscope to different work areas and operating in different environments and ambient conditions.

(d) What is your testing procedure or project plan (Be as specific as you can)?

The Tektronix 3 Series MDO will be first used to capture the partial discharge test signal from the Iris TGA-B. I will then analyse this signal and will use the waveform generator aspect of the oscilloscope to play the signal back into the TGA-B with modified amplitude and pulse rates and observe the effects that this will have on the findings reported in the TGA-B software. This will be a workshop based task.

The second element of this will then involve collecting live data from the partial discharge couplers on either a generator or a transformer. The data obtained on the oscilloscope will be compared to the data obtained on the TGA-B. Below is the typical physical setup for data collection on a generator. To the right is the system setup showing where the oscilloscope will be used in place the TGA-B. This will be field based work, requiring the use of the oscilloscope outside of a laboratory / workshop environment.

The final element of this will be to record partial discharge pulses on a test cable and a high voltage supply using a Rogowski Coil / Capacitive Coupler. This data will be compared to the data collected by the UltraTEV Locator instead of the TGA-B.

The test cable will be powered by a high voltage test set with the ability to manually control the voltage applied up to 40kV AC. I have specialist training from the Faraday Centre and am authorised by my company to use such test sets. I have specialist training from HVPD and Iris on monitoring apparatus for partial discharge.

Different defects will be added to the cable under test to develop an understanding of the pulse shape in comparison to the defect applied to the cable. A 3 core 3.3kV cable specimen and capacitive coupler will be obtained from redundant plant to carry out these tests. A further single core 132kV cable will be available for further testing with a Rogowski coil only. The Rogowski coil is part of the UtraTEV monitoring package. This series of tests will be conducted in a workshop / test area environment.

Partial discharge can be detected by monitoring of either voltage or current pulses via sensors. A genuine partial discharge pulse will have a specific shape, generally determined by the initial fast rise time and short length, that I hope to capture with the oscilloscope. Oscillation after the initial spike and overall pulse length is determined by the type of partial discharge as depicted below. (Picture courtesy of F. Viola and P. Romano).

Whilst partial discharge covers the monitoring of high voltage stator windings, the flux probe and RSO tests cover the low voltage rotor winding.

Data collection from flux probes is carried out in a similar manner to partial discharge and the test process will be very similar. Signals can be captured from the sensors installed within the generator and displayed on the oscilloscope screen. This work is field based testing next to the generator.

The signal comprises of opposite polarity halves to one cycle as seen on the left below. Analysis is carried out by filtering the signal to remove noise, and then inverting the second half of the signal and overlapping it over the other half to determine differences between the peaks as seen on the right.

To do this with the current oscilloscope requires the use of the reference waveform that prevents any further analysis, other than visually comparison of the signals. I will investigate the use of the 3 Series MDO maths functionality, to improve on this analysis.

The aim of the testing will be to see how much analysis can be preformed using an oscilloscope against more specialised test apparatus specifically for monitoring flux probes, such as the RFA-II.

Where as partial discharge and rotor flux is captured on a machine in operation, RSO tests are carried out with the machine isolated. A rotor reflectometer is utilised to inject the test signal and an oscilloscope used to capture and analyse it.

The first element of this test is to see how the 3 Series MDO can be utilised to check the calibration of the rotor reflectometer in place of the Keysight 53220A53220A counter currently utilised.

Calibration is carried out using a delay line to represent the rotor and by directly connecting the reflectometer to the scope. The pulses emitted by the reflectometer can be verified using the measurement functions of the oscilloscope and recorded as evidence of the calibration checks. This will be a workshop based task.

For the actual tests, the delay line is replaced with a generator rotor. The particular rotor utilised is from a generator in preservation and was used during my Road Test of the Megger MIT420/2MIT420/2 insulation tester, so these test will be conducted out in the field.

The RSO tests works on the principles of time domain reflectometry and works by sending a pulse through the rotor winding and capturing the reflected pulse, the connections are then reversed and a pulse sent in the opposite direction and the reflection again captured.

I wish to use the Tektronix 3 Series MDO to capture these reflected pulses and use the mathematical functions to carry out analysis.

The current oscilloscope cannot analyse the signal traces as the reference trace must be used to display both signals. So again I am stuck with visual assessment of the signal.

The first screenshot above shows the two reflected pulses, but the oscilloscope used can only record one pulse at a time. So one pulse is saved as a reference waveform, but this scope does not have the ability to carry out maths using a reference waveform. The second screenshot shows the initial pulse alongside the two reflected waveforms.

I will investigate the mathematics functionality of the 3 Series to see if it can capture and subtract the two waveforms and produce an analysis waveform, that should be a straight line for a rotor in good condition.

My proposal summary to test the RTM3K-COM4 would be;

1) Unboxing and bench top review

2) Carrying out performance tests to verify operation of the instrument

3) Utilise the analogue and digital functions of the oscilloscope to monitor operation of generator synchronisers

4) Utilise the arbitrary waveform generator function to create test signals for air gap search coil analyser used on generators

5) Utilise the digital inputs to verify the wiping action of contacts on electro-mechanical protection relays

6) Utilise the high bandwidth of the oscilloscope and spectrum analysis functions to investigate faults on peak detection relays

7) Utilise the protocol analysis functions to monitor RS485 communication with a power monitor

I will plan to carry out a separate blog on each of the above items before writing the road test review as an overview of the tests carried out.

I have successfully carried out the road test review of a Megger MIT420/2MIT420/2 insulation tester using this methodology last year.

MIT420/2 Road Test

My proposal in depth;

1) Unboxing and bench top review

This will be an overview of the instrument and the items contained within the package, a look at its build quality and basic operation

2) Carrying out performance tests to verify operation of the instrument

To carry out performance tests on the instrument, I will have access to a high speed digital delay line capable of creating delay pulses from 10ns to 1000s, AC injection test sets to produce 3 phase voltages from 5V to 480V, DC through to 1000Hz AC and a 1GHz RF generator for testing bandwidth.

3) Utilise the analogue and digital functions of the oscilloscope to monitor operation of generator synchronisers

I have access to both analogue and digital synchronisers used on generators. Two analogue channels from the RTM3K-COM4 would be used to monitor the 110V synchronising voltages and five of the digital inputs would be utilised to monitor the frequency / voltage raise and lower signals and the breaker closing signals. As synchronising can take up to 1 minute to complete, I would look to utilise the history and memory functions to record the activity and then analyse the synchronism of the 110 voltages when the breaker close command is released. The images below show two old analogue synchronisers in the first photo and the test apparatus in the second.

The oscilloscope can also be used for testing the quality of the closing signal from the synchronising by setting up to measure the contact bounce of the contacts as depicted in the schematic below.

4) Utilise the arbitrary waveform generator function to create test signals for air gap search coil analyser used on generators

Air gap search coils are used on generators to determine the presence of shorted insulation turns in rotor windings whilst the machines are in operation. To capture the data, an IRIS RFA-II analyser is used. I aim to create a test signal using the arbitrary waveform generator function of the RTM3K-COM4 to inject into the RFA-II and check the calibration of the unit by analysing this signal. The first photo shows a similar bench test setup of the RFA-II I am proposing to create with the RTM3K-COM4. The second photo shows the output of the RFA-II after capturing a signals

If a machine is in operation during the road test period, I will use the RTM3K-COM4 to capture this signal and test the mathematical functions of the unit to see if it can carry out the analysis of the waveform directly. The photos below are of an AGSC signal capture and the current manual analysis of the signal using a reference waveform saved within an oscilloscope.

5) Utilise the digital inputs to verify the wiping action of contacts on electro-mechanical protection relays

Electromechanical protection relays have a contact wiping motion during operation that is designed to keep the contacts clean. At the moment, this wiping operation is verified visually by manually operating the relay coil and visually inspecting the operation of the contacts as seen in the pictures below. I plan to use the RTM3k-COM4 to enhance this test by utilising the digital inputs to monitor the operation contacts whilst the relay is operated using a depth micrometer.

6) Utilise the high bandwidth of the oscilloscope and spectrum analysis functions to investigate faults on peak detection relays

I currently have a specific fault on a voltage controlled overcorrect relay that tests out of specification using a 3 phase injection test set. The relay is a peak detection device, so I plan to use the RTM3K-COM4 to monitor the output of the test set to see if harmonic peak outputs are responsible for causing the incorrect tripping times. I will also investigate the use of the spectrum and power analysis functions of the RTM3K-COM4 to monitor the output of the test sets to verify the quality and performance of their outputs. The pictures below are of the MCVG relay and the associated trip times captured, showing the multiple out of tolerance values.

7) Utilise the protocol analysis functions to monitor RS485 communication with a power monitor

I have a Gossen Metrawatt A2000 power monitor which has a standard industrial RS485 interface, that I will use to test out the protocol analysis functions offered with the oscilloscope.

This will be with the view to enabling the power monitor to communicate with a computer. I also have Micom protection relays and Vaisala temperature monitors, also with RS485 interfaces, to expand this option should time permit.