This paper discusses DVtest or Dynamic Resistance Measurement, a new diagnostic method used in detecting on load tap changer (OLTC) operational problems, as applied to reactor-type tap changers. Several cases of tap changer problems are presented, which were detected using the DVtest method – a high speed recording of a DC test current as the OLTC goes from tap to tap. The focus of the article is on the reactor-type tap changers. These reactor OLTC constructions apply Preventive Autotransformers (PA) for current control instead of resistors. Specific features in the current traces due to this particularity are explained. Finally, the article presents a discussion of the principles, DVtest features and defect cases for tap changers in a series transformer/booster arrangement.
Keywords: tap changer, DRM, DVtest, reactor, transformer, booster
On Load Tap Changer (OLTC) testing and diagnostics used to be based on simple resistance measurement at each tap position to establish if the tap changer contacts are in good condition. Possibility to see the problem with its operation, if any, was based only on flicker in the test voltage or induced voltage when performing some AC tests, such as turns ratio while operating the changer. With modern electronics, recording of a test current at high frequency allows us to see the performance of an OLTC at high speed, providing the important information of its mechanical motion, and contact bouncing, opening, coking, etc. The DVtest methodology is only 10 years old, but it has shown a great potential in detecting problems, otherwise invisible to repair technicians.
The reactor-type tap changers are predominant in the USA networks, while European tap changers use resistors for circulating current limitation during the tap transition.
2. The DVtest or Dynamic Resistance Measurement (DRM)
Winding resistance measurement is a well-known method in the transformer maintenance procedure.
The measurement is performed with a low DC current through the winding to make sure losses are as expected on a new transformer, and to check if the values have not changed due to defects when measuring units already in service. This measurement can also be called static resistance measurement.
The dynamic measurement is the same test where, in addition, the current during the tap change operation from one position to the other is recorded. This can be done through the whole range of positions. A current drop (the graph ripple) when transition resistors or reactors are connected can be observed and analyzed. The operation time for each transition of the OLTC is measured.
The test should be performed applying a DC current value that exceeds the knee point of the magnetization current to keep the core saturated during the complete measurement, in order to reduce the inductance as much as possible. If appropriate, windings not measured should be short-circuited in order to reduce the inductance even further. An example of graphs showing difference when the inductance is minimized, with a much larger trace inherent resolution, is provided further in the paper as compared with the one where it is left unchanged.
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