On-load tap changers (OLTCs), besides transformer oil, are the only moving part in a typical power transformer. This makes them especially vulnerable. The statistical analysis of transformer problems shows that the faults located on OLTCs represent a significant number of total transformer faults, and that, on average, 1 in 20 tap changer faults lead to the transformer main tank failure. Issues occur for various reasons, such as poor design, contact wear, mechanism failure, etc. Since the OLTC mechanical system is very complex, a significant number of faults can be caused by incorrect maintenance or bad assembly. Also, a study has shown that 12% of OLTCs require maintenance before the manufacturer’s suggested period. This raises the need for improved testing and maintenance.
There are several diagnostic methods which are used for OLTC condition assessment. The most commonly used method is dissolved gas analysis (DGA). This method provides a very good indication of problems in their early stages. However, the major disadvantage of the DGA method is that it cannot locate which part of the tap changer is causing the critical DGA results. OLTC needs to be disassembled and all contacts must be visually inspected. This can be a long and difficult process. If the same oil is used for both the power transformer and the OLTC, it becomes difficult to locate the source of the problem. To overcome this, an electrical non-intrusive test method, DVtest (also called DRM – Dynamic Resistance Measurement), complementary to DGA, can be applied. This test is very useful as it can pinpoint the location of the fault. Numerous OLTC issues, such as mechanism misalignment, worn-out selector contacts, diverter switch defects, broken springs, and open circuits during tap changer transition can be detected with this test.
This paper will explain the basics of DVtest method and show two case studies where bad selector contacts have been detected using this method.
DVTEST (DRM) BASICS
DVtest is an off-line, non-destructive test that is used for on-load tap changer (OLTC) condition assessment. The test is performed by injecting a DC current through the transformer winding and the OLTC. Once the desired test current is injected, voltage from the DC source is fixed. This way, all changes in resistance are reflected in the test current. As current is inversely proportional to the total resistance of the circuit (winding resistance + OLTC contacts resistance + test cables resistance), the test current will drop whenever the resistance is increased, and vice versa.
The injected current, also called DVtest current, is recorded a few seconds before, during, and a few seconds after a tap changing process. The outcome of the test is a graph of DVtest current in time. The graph reveals what happens during the tap changing process, helping to determine any issues that may occur. Figure 1 shows the typical DVtest graph of one transition of a resistor-type OLTC. The red vertical cursor represents the beginning of the transition, i.e. the moment when the first transition resistor is introduced in the circuit. The blue vertical cursor is the end of the transition; it is a moment when the test current stops flowing through the second transition resistor and starts flowing through the main current carrying contact.
There are two major parameters that can be observed from the DVtest graph – ripple and transition time. The ripple represents maximum drop of current during a transition and is expressed in percents of the starting test current. The transition time is the time between the beginning and the end of the transition (the time between red and blue cursor). It represents the time during which transition resistors are in the circuit. It is expressed in ms, since the tap changing process is very fast (it usually lasts less than 100 ms for resistor-type OLTCs). For that reason, DVtest must be recorded with a high sampling rate, in the range of few kHz. The larger the sampling rate, the better the resolution of the DVtest graph, the easier the analysis of the graph is, and the more precise transition time is obtained.
Together with DVtest, it is also possible to record the OLTC motor current and vibration and plot them on the same graph. These two parameters are useful for detecting issues related to the OLTC mechanism. Figure 2 shows the example of DVtest graph with motor current and vibration signals.
DETECTING BAD SELECTOR CONTACTS USING DVTEST
As the DVtest current is recorded even before the transition process starts, it is possible to evaluate the condition of diverter switch and tap selector contacts in stationary positions. The part of DVtest graph before the beginning of the transition (left from the red cursor in the Figure 1) is actually the test current flowing through transformer winding and OLTC contacts, both diverter and selector, while tap changer is in stationary position. When all contacts are clear and firmly tight, this current should be perfectly flat. Unstable current before the transition may indicate contact problem, either due to a weak connection between moving and stationary contacts, or due to insulation layers that may be formed on selector contacts. The weak connection between moving and stationary contacts is usually caused by broken springs that push moving contact towards stationary contacts, or by loosened bolts which hold stationary contacts in firm position. The formation of insulation layers, such as silver sulfide deposits on stationary selector contacts made of silver, is a common issue, especially on those stationary contacts which tend to be unused for a long period.
The test object was a 50 MVA, 72.5 kV / 11.3 kV three-phase transformer with MR type M on-load tap changer. This was a routine field testing, and DVtest was performed as one of the standard tests.
By analyzing DVtest graphs, it was easily observed that current is unstable before each transition, in each phase (Figure 3, Figure 4, and Figure 5). In this part, the current flows through diverter contact, selector contact, and change over selector.
As bad contacts might be the consequence of insulation layers (dust, dirt, or some other insulating material created by chemical reactions) over contacts, it is recommended to operate on-load tap changer several times up and down through all tap positions. This way, contacts can be cleaned. To get better cleaning, a DVtest current should be injected during these multiple OLTC operations. Arcing that occurs during tap change process will burn insulation layers. Therefore, it is good to inject as high current as possible.
After the recommended “cleaning” was performed, DVtest was recorded again. This time, current was significantly more stable on the so-called static part of the graph, before the transition resistors are introduced. This is clearly visible on Figure 6, Figure 7, and Figure 8.
It is important to emphasize that winding resistances were measured in all 25 taps on the high voltage side where OLTC is placed. The resistance values of different phases were similar and within the acceptable deviation range. The resistance deviation between phases was from 0.2% to 0.39% which is acceptable according to the IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors, IEEE std C57.152TM. This level of contacts degradation wasn’t detected using winding resistance test. The issue would probably become worse over time, and at certain moment it could be detected using winding resistance test. But the question is would transformer fail before the issue is detected. For that reason, it is important to perform DVtest.
The test object was a 40 MVA, 132 kV / 11.5 kV three-phase transformer with MR type M on-load tap changer. All standard electrical tests, such as winding resistance, turns ratio, excitation current, leakage reactance etc., showed good results. But DGA results showed increased “low temperature” gases, particularly methane and ethane. The results are given in the TABLE I.
The transformer owner decided to perform DVtest to check if it will reveal any issues related to the OLTC. The graph of third phase showed unstable current before the beginning of almost every transition. This is shown in Figure 9. Unstable DVtest current is marked with black circles.
This indicates a bad, unstable contact in electrical circuit of phase C. The assumption is that selector contacts are either worn out or contain layers of insulation (dust, dirt, or some other insulating material created by chemical reactions). Selector contacts are placed inside the transformer main tank oil compartment, which means they share the same oil with the transformer main tank. Therefore, it is likely possible that increased gases are the consequence of bad selector contacts. Another possibility is that unstable DVtest current is caused by bad diverter switch contacts. But since the diverter switch contacts are placed in a separate oil compartment, this option is less probable.
The graphs of phases A and B are shown in Figure 10 and Figure 11, respectively. They don’t indicate bad contacts. This is clearly visible when two phases are directly compared, as presented in Figure 12, where graphs of phase A (red) and phase C (blue) are overlaid.
DGA tests were repeated two more times, in two and three months. Methane and ethane levels were still above caution threshold, but did not increase gradually, so the final decision was to leave the transformer online, without any further action.
The recommendation for next time is to operate OLTC several times through all taps up and down, with high test current injected, to clean the contacts, before recording DVtest.
November 2, 2023