Diagnosis of the Short Circuit Duty of Power Transformers

Presentation 11.1

Diagnosis of the short circuit duty of power transformers Alexander Kraetge, OMICRON Abstract: A survey about various aspects of the short circuit performance of power transformers is given. Starting with the effects of electromagnetic forces originated by short circuit currents in transformer windings, manufacturers' efforts to assume the windings withstand capability are shown. Hereafter the short circuit testing of new transformers is briefly described. Finally, methods to assess the short circuit duty of aged transformers are represented.

voltage (lv) windings are acting in opposite directions. The radial forces, depending on the axial component of the magnetic stray flux, distends the hv windings outwards and presses the lv windings inwards to the legs as shown in figure 1. Hence, the outer windings are affected by tensile stress which has to be sustained by the copper of the winding conductors and the friction of windings, known as the Capstan-effect.

Introduction Transformers are usually very reliable parts of transmission and distribution networks, but in the case of a fault, the effects are extreme for the equipment and the stability of the system. High transient currents occurring during inrush or short circuits lead to significant mechanical forces within and between the windings of power transformers. To avoid damage caused by these forces, the windings of new transformers are mechanically supported and prepressed. The dimensioning values of the compression are usually the forces of the highest possible current peak under short-circuit conditions. Unfortunately, the clamping pressure does not remain constant during the lifetime of the transformer. The main reason for its decrease is the degradation and thus shrinking of the main insulation. This is also valid for the other organic structures inside the tank, such as paper, pressboard and support carriers. Additional factors are shock, damage during transport, repeated thermal cycling, operational vibrations or mechanical forces during inrush and short circuit. Another reason could be the effect of a drying procedure applied too intensively. All of these influences can lead to a reduced clamping pressure involving winding slackness and thus to a reduction of short circuit withstand capability.

1. Dynamic short circuit forces The electrodynamic forces in transformer windings are related to the square of the winding currents and consequently rise steeply with the rating. The overall forces could be calculated by integrating the vector product of current density and leakage field intensity over the winding's volume. Based on this idea, the transformer manufacturers developed calculation programs. Usually the short circuit forces within and between the windings, as well as the forces between the windings and the support structures are divided, dependant upon their main directionality, into radial and axial forces. In both cases the forces on the high voltage (hv) and low

Figure 1: Radial forces between windings [1]

While the strain can usually be handled, the inner windings are exposed to compression which is more difficult to contain. A typical defect caused by this kind of force is buckling (see figure 2).

Figure 2: forced buckling (a) vs. free buckling (b) [1]

Buckled windings do not inevitably lead to an outage of the affected transformer but its short circuit duty can be dramatically reduced as a result of reduced isolation distances (fig. 3), torn insulation paper (fig. 4) and general electromagnetic imbalance, which inceases the stresses during any subsequent fault.

© OMICRON electronics GmbH 2006 – Diagnostic Measurements on Power Transformers

Presentation 11.2

be divided into three main components. The currents cause oscillating vibrations of the windings at double the line-frequency. During the occurrence of the positive or negative current wave the layers or discs of the windings are pressed against each other, relieving the strain on the clamping structures while the windings unbend during zero crossing. At this point the clamping structures have to carry the upward pressure. This principle is shown in figure 6.

Figure 3: reduced isolation distance due to buckling [1]

Figure 6: principle of winding vibrations [1]

Figure 4: buckling leading to ripped insulation paper [2]

Another failure mode initiated by radial electromechanical forces is limited to the area around the exit leads of the inner winding. Due to high pressure during a short circuit, a twisting movement of the windings can occur, leading to excessive bending of the exit leads with a danger that the insulation nay tear off and thus reduce the isolation quality. This effect is described in the literature as spiralling. Figure 5 shows a winding damaged by the spiralling effect.

These compression forces can cause bending of conductors between the spacers or the tilting of complete discs raising the risk of interturn faults and an increase of clamping pressure. Figure 7 illustrates the tilting effect.

Figure 7: tilting of winding disks due to axial forces [1]

Layer windings are vulnerable to axial displacements of adjacent conductors whereby the insulation can be damaged. Such a case is illustrated in figure 8. A particularly problem is forces between concentric circular windings which are not of equal length or are axially displaced which tend to shift them further apart due to the electromagnetic imbalance. These forces can cause severe damage on winding ends and clamping structures. Figure 9 shows a broken press ring cracked by the effects of axial forces. Figure 5: transformer winding damaged by spiralling [3]

The radial component of the magnetic stray flux causes the occurrence of axially directed forces. Even though these are significantly smaller than the radial forces they are more difficult to contain. The axial forces can


Presentation 11.3

Figure 8: axial shift in a layer winding [1]

Figure 9: broken press ring due to axial forces [3]

Another kind of axial force appears on tapped windings and tends to enlarge the gap on the exit leads. It is to be assumed that all kinds of forces appear simultaneously, leading to multi-stress loads inside and between the windings and between the windings and the support and clamping structures.

2. Verification of the short circuit duty According to the definitions of IEC "Transformers together with all equipment and accessories shall be designed and constructed to withstand without damage the thermal and dynamic effects of external short circuits under the conditions specified…". The most important fundamentals of short circuit withstand capability are correct customer specification, good design and accurate manufacturing. Usually the ability to withstand the dynamic effects of short circuits is proven by calculations. An important quality control tool is the internal or external design review. During manufacturing, special attention must be paid to the winding's clamping. Usually the windings becoming prestressed twice – once after the winding process, during or after oil impregnation (see fig. 10) and secondly during the final assembly after removal of the active part from the vacuum drying oven.

Figure 10: pressing of a winding in a transformer factory

The short circuit testing of power transformers is expensive and is not very common, particularly for transformers of category III (above 100 MVA). For this case it is a special test, the conditions of which have to be agreed upon between purchaser and manufacturer according to IEC 60076-5 or IEEE Std. C 57.12.90 – 1999. Just a few testing laboratories worldwide are able to perform short circuit tests on large power transformers. One laboratory reported a high rate of failures [4]. Besides visual inspections and a repetition of the electrical routine tests; the only method required by the standards to consider the transformer to have passed the short circuit test, is the short circuit reactance measurement. Although it is commonly accepted as a reliable diagnostic tool, cases have occurred in which deformations were not conclusively identified by reactance measurement. Furthermore, deformations and displacements in supporting structures and clamping systems can not be detected by the reactance measurement. Those problems can be revealed during visual inspections in the testing laboratory but could stay unnoticed on-site remaining a potential risk factor as a consequence.

3. Diagnosis of the short circuit duty In principal, three ways to assess short circuit duty are available. Various methods are available to integrate sensors (force and displacement transducers) in to the transformer [5], [6], [7] to monitor the remaining clamping pressure directly. These methods have not become common because the durability of the sensors is much lower than the operating life span of the observed transformers. In addition a retrofit with such devices is complex and expensive. Other kinds of diagnostic measurement are provided by different electrical methods. In addition to the above mentioned reactance measurement, there are also; the low voltage impulse method (LVI); the measurement of the frequency response of stray losses (FRSL); and the frequency response analysis (FRA). A common factor in all of these methods is their use of comparison


Presentation 11.4

techniques. Since these topics are covered in detail elsewhere they will not be elaborated upon in this paper. The third set of diagnostic measurement techniques for the winding clamping pressure and thus the residual short circuit duty of power transformers are those utilizing vibro-analytic methods. An assessment of the vibrations induced by the core and the windings should provide information about the state of the clamping. To determine the vibration data different techniques are used. Data acquisition via piezoresistive or piezoelectric accelerometers mounted on the tank wall holds some uncertainties as the behavior of the tank wall as an oscillator with a large number of vibration nodes, irregularities in and pollution of the paint of the vessel, or an extensive dependency on the temperature may all affect the results.

short circuit or inrush current ⇓ mechanical stress in the windings ⇓ winding movements ⇓ flow processes through oil displacement and excitation ⇓ measurable oil pressure oscillation Figure 11: Principle of TOP measurement

Figure 12 illustrates the operation of TOP measurement. The heart of the TOP measurement is the differential pressure transducer. For offline use it is mounted on the breather valve of the Buchholz relay. In case of online-monitoring the transducer has to be integrated on top of the transformer tank. The pressure signals are transmitted potential-free via optical fibres. Figure 13 shows a photograph of a pressure transducer installed on the Buchholz relay of a 63 MVA transformer.

Another method to acquire vibration data is a measurement directly in the oil. This requires access to the oil circuit which is usually limited. Nevertheless, it is possible to measure oscillations in the oil related to the state of clamping; a proven method is introduced in the following section.

4. Transient oil pressure measurement The transient oil pressure (TOP) measurement provides a diagnostic system which makes it possible to assess the clamping pressure of power transformers offline or online. The TOP measurement is based on the recording of dynamic pressure oscillations of the oil which are initiated by the movements of the windings and clamping parts inside the transformer, in the event of short circuit or inrush currents. For a given current excitation, the amplitude of the oil-pressure oscillation increases with decreasing axial clamping pressure. Figure 11 shows the principle of the TOP measurement. If the clamping pressure is reduced, the windings can move more, so that their elongation under the force of the transient currents rises. As a consequence the amount of displaced oil increases thus initiating a higher level of shock wave causing oil pressure changes.

Figure 12: Operation of TOP measurement

The TOP measurement proceeds by switching on the unloaded power transformer from the HV side while the oil pressure and the phase currents, responsible for the excitation, are recorded synchronously. Figure 13: Differential pressure transducer on Bucholz relay

The inrush current amplitudes are determined by the point of the voltage wave at which the switching happened and the magnitude and polarity of the residual magnetism which may be left in the core after previously switching off. As a consequence of the


Presentation 11.5

fluctuation of inrush currents, it is advisable to repeat this switching several times. The German experience shows meaningful results with five switching operations of older transformers and ten switch-on operations for new transformers. The total testing time is usually less than five hours. The results will be evaluated in different ways [8]. The measured current peaks are put into relation to the highest possible current peak under short circuit conditions calculated in equation (1): îSC = √2 (κ IFL)/ VZ x 100% (1) îSC κ IFL VZ

= = = =

highest possible short circuit current peak asymmetry factor full load current percentage impedance voltage

Here the pressure increase factor D is introduced to achieve a normalization of the TOP measurement: D = Pm (ISC / Im)² D Pm Im

(2)

= pressure increase factor = measured pressure amplitude = measured current amplitude

Based on the pressure increase factor the short circuit duty of the transformer can be assessed. Another way of evaluating the results is to search for trends between the different switching operations within a measuring cycle on a transformer. Significantly increasing values of the current-related oil pressure or changes of the frequency characteristic are an indicator of a critical loss of clamping pressure inside the device under test. This is also valid with respect to the comparison of results previously obtained for the same transformer (finger prints). In addition, it is possible to compare several transformers of the same construction and aging level in the absence of older fingerprints. Thus trend analysis is an important part of the TOP measurement method. The TOP measurement can also be installed as an online monitoring system [9]. This method seems to be especially useful for transformers which are frequently switched, e.g. in peak-demand power stations, short circuit laboratories or for industrial furnace transformers.

5. Results of on-site measurements Figure 14 shows a typical result of a TOP measurement on a 400 MVA power transformer in Berlin. The upper three channels show the inrush phase currents, the fourth channel shows the oil pressure signal. The recording time was 20 seconds including two seconds pre-trigger. The inrush currents are fading away slowly while the pressure signal, after a steep rise, follows a damped oscillation with a slow natural frequency.

Figure 14: Typical TOP measurement result on a 400-kV power transformer (400 MVA)

A comparative measurement of two transformers (110kV/31.5MVA) of the same construction is shown in figure 15. Both were built in 1977 and operate in the same substation but one of them was refurbished in 2003, including re-clamping of the windings. The waveshape of the pressure signals shows good alignment but the average of the pressure increase factor was reduced from 7.05 mbar (untreated transformer) to 1.78 mbar (re-clamped transformer). The four-fold increase in the value of the untreated transformer proves the increase of transient oil pressure with decreasing clamping force. The Figure shows respectively the highest oil pressure curves of each measurement with the same scaling.

Figure 15: Comparative results red: untreated transformer – blue: re-clamped transformer

6. Measurements in high power labs The TOP measurement is useful to monitor short circuit tests in high power laboratories. The following example describes a comparison between TOP and reactance measurement during a short circuit test. The tested object was a 42-year old distribution transformer (6kV/630kVA), short circuit time was 300ms. During the test sequence every winding had to carry the highest current once, after three tests the supply voltage was increased. Starting at 3 kV the voltage was increased step by step up to the rated voltage. After every short circuit a reactance measurement was performed as claimed in the testing standards. Figure 8 shows the three pressure curves at 5.3 kV and the first curve at 6 kV. It can be seen, that all four graphs are very similar


Presentation 11.6

and the increase of the voltage provides a moderate increase of oil pressure in the green curve.

7. Conclusions A survey of various aspects regarding the short circuit duty of power transformers is given. The main effects of electromagnetic forces acting in different directions during short circuit events are described; examples of failed windings are illustrated. Measures taken by manufacturers of transformers to assess the short circuit withstand capability are listed. Possibilities for the diagnosis of the short circuit duty are suggested with special attention to the TOP measurement.

Figure 8: Three oil pressure curves at 5.3kV – green curve at 6kV, p_max = 3 mbar

Figure 9 presents the same results as figure 8 and additionally the second short circuit at 6kV. The difference between the first and the second 6kV-curve is clearly visible. The rise of oil pressure is a clear indication of a decreasing short circuit duty. At that point the reactance measurement still gave no indication of dangerous changes inside the transformer.

8. References [1] G. Bertagnolli, transformers

Short-circuit

duty

of

power

[2] ABB Transformatori, Golinelli Editore, Legnano 1996, Source: Weidmann ACTI [3] J. Foldi et.al., Recent achievements in performing short-circuit withstand tests on large power transformers in Canada, Cigre Session, Paris, 2000 [4] R.P.P.Smeets et. al., Significant failure rate observed at short-circuit testing of large power transformers, IEEE PES T&D Conference, Dallas 2003 [5] PJ de Klerk, JP Reynders, Winding slackness monitoring as a diagnostic for insulation ageing in oil-paper insulated power transformers, ISH, London, 1999

Figure 9: Contrasting red curve indicates loss of short circuit duty, p_max = 4.5 mbar

The third short circuit at 6kV produced a failure due to an inter-winding fault with a severance of high voltage winding U. The oil pressure increased to 84 mbar (fig. 10).

[6] P. Kienast, Messung der axialen Wicklungseinspannkräfte mit Dehnungsmessstreifen an Leistungstransformatoren, Elektrizitätswirtschaft, Heft 10 , Jg. 87 (1988), S. 519 – 521 [7] A. Marinescu et. al., HV Power transformer direct monitoring of windings axial clamping forces, CMD, Changwon, Korea, 2006 [8] A. Kraetge et.al., Diagnostic of the short circuit duty of power transformers, ISH, Beijing, 2005 [9] A. Kraetge et al., Online monitoring of windings clamping pressure at a high power testing transformer, CMD, Changwon, Korea, 2006

Figure 10: Oil pressure curve during inter-windingfault compared to the preceding tests


Diagnosis of the Short Circuit Duty of Power Transformers

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