TRAX Multifunction transformer and substation test system

With DC resistance measurements, a central objective is to try to saturate the transformer core, as this reduces the effective inductance of the winding and allows the test current to stabilise more quickly. Saturation typically occurs when the DC test current is over 1 % of the rated current for the winding. It is usually advantageous to use a somewhat higher test current than this to minimise the effect of noise on the measurements. If the test current is too low, you will often find that successive measurements give inconsistent results. Nevertheless, you must avoid using test currents of more than 15 % of the rated current, as they are likely to lead to erroneous results because of the resultant heating of the winding. In most cases, the optimum test current is between 1 % and 15 % of the rated current.

When performing a TTR or WR test with the TRAX, the form settings give you the option of testing by tap or testing by winding. These options have different advantages depending on the TRAX accessories you have and what test you are performing. You should perform TTR tests by tap or by winding, depending on the availability of a switchbox.TTR by winding (no switchbox):We recommend performing the test by winding if you have a TRAX without a three-phase accessory and are performing TTR testing on multiple taps. This testing approach allows you to connect leads one time to the phase under test and then proceed through all the taps. After completing TTR testing on all taps in the first phase, ensure the TRAX output is de-energised before you switch the leads to the next phase. Continue testing, beginning at the tap position on which you finished testing on the previous phase winding. There is no need to move the tap changer back to the position on which you began Phase 1 tests; instead, test the second phase in the reverse tap order. Make sure to have “Reversed order for next connection” checked. That way, if you begin testing on the highest tap for the first phase, the software will assume that you will commence testing on the lowest tap for the second phase, then reverse once again on the third. Using this method, you only need to connect to the transformer three times to perform all TTR tests. Conversely, if you tested ‘by tap,’ you would have to change test connections after every measurement, thus changing leads frequently. The number of lead changes, as an example, would amount to three times per tap multiplied by the number of the transformer’s tap positions.  TTR by tap (with a switchbox):We recommend performing the TTR test by tap if you have the automated three-phase switchbox (TSX 303) or the manual three-phase switchbox (TSX 300). With the automated switchbox, the TRAX will measure one phase, automatically switch to the next phase, and then the next. When all three phases have been tested, the TRAX will prompt you to switch the tap position to proceed testing. The manual switchbox allows you to conveniently change the dial from one phase to another in between tests instead of changing lead connections on the transformer. With the switchbox, you only need to connect to the transformer once, and you only need to cycle through the tap changer positions once.  Winding resistance (by winding):When performing a WR test on a transformer with an on-load tap changer (OLTC), we recommend testing by winding whether or not the TRAX is equipped with a switchbox. This test progression allows the WR application to test the ‘make before break’ functionality of the load tap changer. The TRAX will detect interruptions in current by keeping the current continuously running through the winding while changing taps. Interruptions in the current can indicate a faulty tap changer. Additionally, by testing one winding and cycling through the taps, the core saturates on the first measurement, and successive measurements on the same phase are performed much more quickly.  

The TRAX ecosystem offers several ways to add additional tests or retest. Suppose you want to repeat a test that has already been recorded and overwrite the results. In that case, you click on the line you want to test, make the appropriate connections, and select the play button. This action will overwrite the previous data. The software will advance to the following line, so make sure you select the next appropriate test if not retesting sequentially. If you have already left an application and, while viewing the report, discover that you need to repeat one of the tests, you have two options. To overwrite previous results, you must go to the report section and click on the “Edit” button in the upper right corner. Once clicked, a small taskbar will appear at the top right of each test sequence on the report. From here, you can choose to re-measure by clicking the “Play” button. The software will open the appropriate app with the previously measured data populated when you click “Play”. Click on the line you want to retest and test as usual. There may be instances where you want to perform the same test but keep previous results. If this is the case, select the instrument selection button and then the appropriate app; this will create a new test sequence on the same report.  

Since the TRAX has multiple inputs and outputs at various settings, it may perform the same test using different connections. Check that you are using the appropriate voltage or current setting for the measurement; e.g., when performing the winding resistance test, there are three options, 1 A, 16 A, and 100 A. Each setting uses a different current output connection on the TRAX. You can utilise optional switchboxes if performing TTR or WR tests. If using the manual switchbox (TSX 300) for TTR and WR, make sure that the “Manual switch box” option is checked (underneath the vector diagram) in the TTR and WR app. If using the automated switchbox (TSX 303), the TRAX will automatically detect that the TSX 303 is connected and adjust the connection diagram.

Make sure to take the reading only after the voltage has stabilised. You may need to toughen your ‘stable’ criterion setting to 99.95 % for more than 5 seconds. Waiting for more stable readings should reduce the deviation. Sometimes the variation in a measurement that has a very low absolute value, such as 1 mΩ, can result in a substantial divergence between phases. Still, the actual value may be just a few µΩ greater than expected, and there is no genuine issue with the transformer winding.If the asset under test is a large power transformer, consult reference measurements with the manufacturer or compare your results against factory test results. Construction constraints of these large transformers might result in larger, yet acceptable, WR differences between phases. A good example of this is a transformer with an LTC in one of the short segments of the tank, which will result in considerably different lengths for the conductors used between the tap changer and each tap winding.

The algorithm finds the timing for the OLTC based on the expected curve. A faulty tap changer can result in a curve that is different from the expected value. Examine the graph and look at the curve transition, where you should find the timing and/or problem with the tap changer.

The TRAX offers complete manual control of inputs and outputs - a unique tool for immediate troubleshooting. You can reproduce or modify routine test sequences using the manual control features to vary voltage, current and/or frequency. A notable caveat is that the discharge function is not implemented in manual mode. Therefore, while you can run a contact resistance test in manual mode, you should not attempt a winding resistance test.The TRAX is a portable metrology laboratory ideal for advanced users, research institutions, and root cause analysis specialists. Manual control gives you access to manage and operate:

• 10 generators (AC and DC; voltage and current)
• 6 measurement channels (AC and DC; voltage and current)
• Electrical formulae calculator
• Real-time oscilloscope