Lesser known facts about TTR testing that are affecting your results
Authors: Dinesh Chhajer, Daniel Carreno and Ken Petroff
Transformer Turns Ratio (TTR) testing is one of the most common ways of assessing the condition of a transformer’s windings and core. Throughout the life of a transformer, TTR results are compared against the nameplate ratings to reveal insulation deterioration, shorted turns, core heating or other abnormalities. TTR testing is simple, so is often taken for granted without fully understanding the basis of the test. As a result, when measurements are not within expected limits, it is hard to determine the cause and resolve the problem.
This article focusses on some of the lesser known aspects of TTR testing, such as the effect of applied test voltage, step-up versus step-down excitation; differences between nameplate ratio, voltage ratio and turns ratio; sources of error; per phase testing vs true three-phase testing; and more.
Transformers transfer power between circuits, usually at different levels of voltage and current, by electromagnetic induction. This function depends on the relationship between the number of turns of a specific pair of windings in the transformer. As this relationship is so important, TTR testing is typically performed many times throughout the life of a transformer – during manufacture, at acceptance and then during routine maintenance and as an aid to fault finding.
Transformer turns ratio (TTR) is simply the ratio of the number of turns in a pair of windings and can be written as follows:
Where Np is the number of turns in the primary winding and Ns is the number of turns in the secondary winding. Typically, transformer users will not know Np and Ns, so will work with the transformer nameplate ratio (TNR), which can be calculated as:
Where VLLp is the primary line-to-line voltage and VLLs is the secondary line-to-line voltage, both taken from the transformer nameplate.
Modern TTR instruments will work by applying a voltage on one winding of the transformer (VP), measuring the resulting voltage on another winding (VS) and then calculating the ratio of these two voltages. This is the transformer voltage ratio (TVR), but it should be noted that for three-phase transformers, a correction factor, which depends on the vector configuration of the windings, has to be applied.
As TTR measurements are made under no load conditions, impedance will have a negligible effect on the results. The measured value of TVR will, therefore, be approximately equal to TTR, the turns ratio. For this reason, it is standard industry practice to validate TTR with an instrument that in reality measures TVR.
Understanding the results
A TTR test instrument presents three quantities for each measurement: TVR, excitation current and phase deviation. The measured TVR can be compared with the expected TVR calculated from the nameplate data and, if necessary, the winding configuration correction factor. According to IEEE Std C57.152, 2013, the measured and calculated values of TVR should match within ± 0.5%.
Figure 1: Turns ratio error at various test voltages
The excitation current measurement can be used to detect problems in the magnetic core structure, winding defects, like shorted turns, and tap changer problems. This measurement can also be performed using a power factor test set, as it is normally made at rated frequency and voltages up to 10 kV. The results are voltage dependent and, as measurements evaluation relies heavily on pattern recognition, the results obtained during TTR testing – even at considerably lower voltages – can be a useful diagnostic tool.
Phase deviation depends primarily on the quality of the material used in the construction of the transformer core. Building a transformer core with high permeability, low loss material and with no inter-lamination defects helps to minimize eddy currents and, therefore, phase deviation. Significant phase deviation therefore indicates an inefficient core.
As documented in IEEE Std C57.152, 2013, there are special cases involving a transformer with a load tap changer on the LV side and a small number of overall turns. With these, the variation per tap may be outside the normal ± 0.5% tolerance. In such cases, measurements at the extreme ends of the tap changer should be within the ± 0.5% tolerance band and for all taps, the three phases should all have the same voltage ratios.
Correlation with other tests
When TTR test results suggest that there may be a problem, it is useful to know how those results relate to other tests that can be performed in the field.
The excitation current of a transformer is the current flowing in an energized winding, with all other windings open circuited. Measuring the excitation current can help to identify major problems in the core structure, issues with tap changers, turn-to-turn faults and grounded windings.
Winding resistance tests can provide information about insulation issues, such as turn-to-turn shorts and problems in tap changers that, in extreme cases, will affect TTR measurements. Finally, an inductive interwinding test, which is one of several types of measurement possible when performing sweep frequency response analysis (SFRA) testing, can be used to obtain a good approximation of the voltage ratio of a transformer.
Sources of error
An assumption made in TTR testing is that under no-load conditions the voltage ratio of a transformer is equal to the turns ratio. Another assumption is that all the flux produced by one winding links with the second winding. However, in reality there is always flux leakage, which means that the voltage in the secondary winding will always be lower than that given by a simple calculation based on turns ratio. These factors, along with eddy current and hysteresis losses, excitation losses and the effects of applied excitation voltage and core permeability, contribute to errors in the measured turns ratio. Other external factors that can influence TTR measurements include the type of transformer (two-winding, three-winding, autotransformer with tertiary etc.), the transformer configuration (Dy, Yd, Yy, Dd etc.), the connections made between the transformer and the test instrument (HV winding excitation or LV winding excitation), single phase or three phase excitation, the loading of delta windings (when present), the magnitude of excitation voltage, and the value of the turns ratio itself. These factors are examined in the following sections of the article.
A TTR test is generally performed by energizing the HV winding of a transformer and measuring the voltage on the LV winding. This is the step-down test method. The voltage used to energize the winding can, however, affect the results. When the test voltage is applied, magnetic flux directly proportional to the volts/turn is induced in the transformer core. Most but not all of this flux links with the secondary winding, and the flux that does so is known as the mutual flux. The flux that does not link with the secondary is the leakage flux.
The mutual flux depends on winding inductances, core design, construction and permeability of the core. Since the flux in the core is a function of volts/turn, a higher excitation voltage may be required to obtain higher mutual flux linkage and overcome errors due to leakage flux, excitation losses and core losses. Also, since the permeability of the core increases with increasing excitation voltage, it is beneficial to use higher test voltages. Figure 1 shows TTR results at various test voltages for a Dyn1, 138 kV to 4.365 kV transformer energized from the HV side.
In practice, for any transformer there is an excitation voltage beyond which voltage dependence reduces. TTR results are consistent at any higher voltage.
Three-phase transformers are produced with a wide range of winding configurations and, in general, it is more difficult to test accurately if the LV winding is delta configured. This is because TTR testing assumes that the secondary is open circuit and has no load connected. With a delta connected LV winding and measurements performed on a per-phase basis, this assumption does not hold, as the winding under test is loaded by its connection with the other two windings in the delta loop. The current circulating in the delta loop leads to internal losses and impacts the accuracy of the TTR measurement.
In these cases, it is recommended either to energize the HV winding line-to-line or to use three-phase excitation. Even better is to excite the LV winding and measure the voltage induced in the HV winding (step-up mode). Figure 2 shows the effect these measures have on TTR results and it is worth noting that even when a test voltage of just 8 V was used with step-up excitation, the results were more accurate than when a test voltage of 80 V was used in step-down mode.
Figure 2: TTR error for a YNd transformer with various excitation methods
Figure 3: Comparison of errors in step-up and step-down modes for an autotransformer with tertiary winding
With three-winding transformers and autotransformers with tertiary windings, it is difficult to get a good ratio measurement from HV to tertiary. The tertiary winding is usually closest to the core with the HV winding being the outermost winding. With this arrangement, when the TTR test is performed from the HV side, the coupling coefficient between HV and tertiary winding is lower than in a typical two-winding transformer. The situation is worse when the turns ratio is high – experience shows that any ratio greater than 20:1 gives problems for HV to tertiary ratio measurements when the step-down test mode is used. Further, the tertiary winding is usually delta connected, which creates additional difficulties, as discussed earlier.
To address these challenges, it is recommended that TTR measurements are performed in step-up mode from the tertiary side. It is, however, important to keep the LV excitation voltage low to guard against producing dangerously high voltages on the HV side.
Usually, the step-up excitation test voltage is chosen based on the maximum voltage the test instrument can measure safely and accurately in the HV winding.
Figure 3 shows the HV-to-tertiary turns ratio measurements for a 288.7 kV/95.2 kV/26.4 kV autotransformer. Tests were performed from the HV and tertiary sides to allow comparison. As can be seen, with HV winding excitation, the first group of taps gave results that were outside the IEEE ± 0.5% limit. With tertiary winding excitation, however, all of the taps were within tolerance.
HV winding vs LV winding excitation
As has already been discussed, the transformation ratio measured in a TTR test is influenced by the mutual flux that links the HV and LV windings. This in turn depends on the geometry of the windings, the number of turns, and the permeability of the core. For a given transformer, the first two of these factors are fixed, but the permeability of the core is not constant.
For the type of steel used in transformer cores, permeability increases rapidly with increase in magnetic field strength H. Applying a higher excitation voltage increases H, which increases the permeability of the core and results in more effective coupling between the windings. This improves the accuracy of TTR measurements, as is shown in Figure 4, where the excitation voltage is applied to the HV winding.
Figure 4: TTR errors with single-phase HV winding excitation using various test voltages
Even greater benefits can be obtained, however, by using the step-up mode of testing, where the transformer is energized from the LV side. Since flux is a function of volts/ turn, the same excitation voltage will produce more flux if it is applied to the LV side. In addition, as the LV winding is usually closer to the core, there is better coupling between the windings from LV to HV. Better coupling and more flux mean that more accurate TTR results are obtained. Examples of results from step-up TTR testing are shown in Figure 5 and it is worth noting that, as the excitation voltage is increased, the ratio error moves in a positive direction, which is the opposite of what happens in step-down testing.
Figure 5: TTR errors with single-phase LV winding excitation using various test voltages
A further benefit of step-up mode testing is that it provides better accuracy when high levels of interference are present. With step-up testing, modern measurement and signal processing techniques allow reliable results to be obtained even in the most difficult conditions in the field.
Single-phase vs three-phase excitation
Three-phase power transformers are often tested on a per-phase basis with a single-phase source, using relays to switch the power from one phase to another as necessary. The limitations of per-phase methods have already been discussed and to compensate for these, it is recommended that a higher excitation voltage is used, along with step-up test mode. Energising two phases by testing phase-to-
phase is also desirable as, with two windings energized, the coupling between the windings is improved and the dependency on excitation voltage is reduced.
Even better results are obtained by using a three-phase source and testing the three phases simultaneously. Flux distribution will be more uniform, leading to higher coupling between windings, so the results are less sensitive to excitation voltage. Excitation losses during the test are shared by all the three sources, giving much more accurate results than those obtained with single- or two-phase excitation. Additional benefits are that simultaneous measurement of all three phases minimizes testing time and, because it reduces the need for swapping test leads and climbing up and down ladders, it increases safety. In addition, switching relays are no longer needed in the test instrument, which improves both reliability and longevity.
Figure 6 shows single-phase and three-phase test results in step-up mode for four test voltages and it will be seen that the three-phase test gives smaller errors in every case.
Figure 6: Single- and three-phase excitation at various voltages
When the three phases are measured simultaneously, better comparisons can be made between the results for each phase. Other advantages include the ability to test the ratio of phase-shifting transformers, increased accuracy of phase deviation measurements and the potential for using vector recognition techniques on transformers with limited nameplate information. Performing three-phase TTR measurements in step-up mode is even better, as it combines the benefits of both techniques, as is convincingly shown in Figures 7 and 8.
Figure 7: Error in three-phase step-down TTR measurements on a transformer with OLTC
Figure 8: Error in three-phase step-up TTR measurements on a transformer with OLTC
Figure 7 shows the three-phase step-down excitation results for the HV-side OLTC of a Dyn1 138 kV/4.365 kV three-phase transformer. It can be seen that the errors are large and inconsistent, with some of the taps exceeding the ± 0.5% IEEE tolerance limit.
Figure 8 shows results for the same transformer and OLTC using three-phase step-up excitation. This time the errors are much smaller and more consistent between phases, and all taps are well within the IEEE tolerance limit.
Summary and conclusions
TTR testing is an important aid to assessing the condition of a transformer’s windings, core and insulation. The test is simple, but many factors, such as core permeability, leakage flux, excitation losses and winding configuration, can affect the accuracy of the results. The test techniques used also influence accuracy, potentially leading to significant differences between measured and nameplate ratios. Many of these factors are outside the control of the person performing the test, but there are steps that can be taken to enhance the accuracy and repeatability of the results. Best practices can be employed to select the most appropriate winding to be energized (step-up or step-down mode), to choose an excitation voltage that minimizes voltage dependence, and to decide whether to energize multiple windings (line-to-line or three-phase excitation) so that errors due to excitation losses and the configuration of the windings are minimized. Field tests show that simultaneous three-phase excitation and step-up mode testing greatly improve accuracy, but where three-phase excitation is not possible, line-to-line testing is an acceptable alternative. Step-up testing gives better coupling, generates more flux and reduces voltage dependence when compared with step-down mode.
With these best practices, TTR testing becomes safer, more efficient and allows for transformers that are sensitive to applied excitation voltage, auto transformers with tertiary windings and transformers with delta LV windings to be tested accurately to obtain ratio errors that can be compared with IEEE limits to provide a reliable assessment of winding and insulation condition.