Cidel 2010 Velasquez Enu

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CIDEL-2010-Assessment FRAFinal.doc 1 / 6

Ttulo Assessment of the integrity of the active part of power

transformers with Frequency Response Analysis: Case Studies N de Registro (Resumen) 149

Empresa o Entidad OMICRON Electronics GmbH

Autores del Trabajo Nombre Pas e-mail

Juan L. Velsquez Contreras Austria [email protected]

Miguel A. Sanz-Bobi Spain [email protected]

Samuel Galceran Spain [email protected]

Charles Sweetser USA [email protected]

Hyder DoCarmo USA [email protected]

Palabras Clave

Frequency Response Analysis, Diagnosis, Defects, Failures, Active part, Power Transformers

Abstract. The Frequency Response Analysis is a powerful method for the detection and diagnosis of defects and failures in the active part of power transformers. The assessment of the results is based on comparisons in which the results of an actual test are compared to a reference or baseline. Three methods are commonly used to assess the measured traces: time-based, construction-based and phase-based comparison. Unfortunately, in the majority of the cases, time-based comparisons cannot be carried out because of the lack of fingerprints. In these cases, the construction-based and phase comparisons can be used. In this work the assessment of FRA results under the different comparison philosophies is illustrated by means of real cases studies.

1. Introduction to the FRA method

The Frequency Response Analysis (FRA) has been proven to be a powerful tool for the detection and diagnosis of the active part of power transformers [1-3]. In contrast to

traditional diagnostic methods, the FRA method is able to detect geometrical deformations in the windings before the occurrence of a major or catastrophic failure. When talking about FRA it is important to distinguish between Impulse Frequency Response Analysis (IFRA) and Sweep Frequency Response Analysis (SFRA). This work focuses the attention on the SFRA method. As illustrated in Fig.1, the SFRA consists in applying a frequency variable low-level sinusoidal signal "U" at one end of a winding and from this point a reference signal "U1" is measured. Simultaneously, the output or response signal at the other end of the winding "U2" is measured. Subsequently, the transfer function H(f) is computed. It can be easily demonstrated that H(f) is only dependant on the measurement resistance of the FRA instrument (Rm) and on the impedance Z(f) of the transformer.

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The most common way of representing the results is in the form of a bode diagrams as shown in Fig. 2. In the majority of the cases only the plot of the magnitude is used for interpretation purposes. Nevertheless, the plot of the phase also provides important information.

RMC

RLC Network

U1 Rref=50 U2Rm=50

50

U

CMC

Figure 1. Measurement setup

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

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-90

-80

-70

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-50

-40

-30

-20

H0 H1 H0 H2 H0 H3

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

100

150

Ma

gn

itu

de

(d

B)

Ph

as

e (

)

Figure 2. Representation of FRA results as a

Bode diagram

2. Capabilities of FRA measurements

FRA measurements provide information on the whole components of the active part of power transformers. Geometrical change in any of the components will in the majority of the cases lead to changes in the parameters RLC of the transformer what at the same time lead to changes in the FRA plot. The failure modes that can be detected by FRA are presented in Table 1.

Table 1. Failure modes detected by FRA

Abbreviation Failure modes

EFST Short-circuit between turns/strands EFTG Short-circuit to ground EFOC Open-circuit failure EFCR Contact resistance failure EFFP Floating potential CFSL Short-circuited core laminations CFAL Air gaps between core laminations* CFMG Multiple core grounding* CFUC Ungrounded core TFCC Cooking of the conductor TFCM Melting of the conductor MFCT Conductor tilting MFMCB Conductor bending MFAI Axial instability MFMBM Bulk movement MFMAC Axial collapse MFMB Buckling MFMLC Loose clamping structure* MFMLD Lead deformations

*These failure modes are more difficult to be detected

3. Assessment methodologies

FRA is a comparative measurement method. This means that results of an actual test are compared to a reference or baseline. Three methods are commonly used to assess the measured traces: 1. Time-based comparison (current FRA

results will be compared to previous results of the same unit or fingerprint)

2. Construction-based comparison (FRA of one transformer will be compared to another of the same design)

3. Phase-based comparison (FRA results of one phase will be compared to the results of the other phases of the same transformer)

The preferred method is the time-based comparison. Unfortunately the so called fingerprint is in the majority of the cases not available. Nevertheless, by phase-based comparison or by construction-based comparison, a successful assessment of the results can be achieved. Next, in section 4 different case studies are used for illustrating the application of the before mentioned assessment methods.

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4. Case studies

Case Study 1

The FRA plots of a 750 MVA generator transformer manufactured in 1978 were measured for failure investigation. Due to the lack of a fingerprint, a phase-based comparison was used for assessing the results. According to the results shown in Fig. 3, it is obvious that significant deviations similar to the pattern of typical of electrical failures (shorted turns) take place at low frequencies. When an electrical failure is suspected, it is necessary to see the behaviour of the FRA plots of the other windings. The FRA plots of the 27 kV windings were also tested. In this case, the trace "yz" deviates. Taking into account that the vector group of the transformer is YNd5, it is to expect also deviations in this trace under the presence of an electrical failure, because for this vector group the winding of the phase W is magnetically coupled with the winding between the terminals "y" and "z". From these results the presence of a short-circuit between turns (EFST) was successfully diagnosed. Other electrical tests were used for determining if the failure took place in the 420 kV or in the 27 kV windings.

N U N W N V*

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

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Figure 3. FRA plots of the 420 kV windings

x y y z z x

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

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Case Study 2 The FRA fingerprint of a 100 MVA, 550/230 kV with a 14kV tertiary winding, single-phase Autotransformer manufactured in 1980 was measured on 22-04-2008. On 26-09-2008 both the differential protection (87) and sudden pressure relay (63) tripped. On 01-10-2008 a FRA test was carried out for failure investigation. The time-based comparison of the FRA plot of the 550 kV winding is shown in Fig. 5. As can be seen, there are deviations at low frequencies, but in this case the deviations are mainly due to a clear change in the parallel capacitance of the winding what can be seen between 500 Hz and 10 kHz. This is an indication of short-circuit to ground (EFTG). This result also correlated well with the insulation resistance test. Based on these results, the owner of the transformer decided to perform an internal inspection for localizing the failure. It was found that the insulation between a lead of the tap changer and a lead holder was damaged as shown in Fig. 6, what gave place to a hot spot point, generation of gasses and to a trip of the protections. Between 100 kHz and 500 kHz it was also observed a change in the FRA plot. From this it was suspected that a component of the active part suffered a mechanical deformation. After dismounting the transformer it was found that as a result of the fault current the electromagnetic forces lead to a deformation in the lead between the bushing and the winding as shown in Fig. 7.

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H0 H1* Test2 H0 H1 Test 1*

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005 1.000e+006

dB

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Figure 5. FRA plots of the 550 kV winding

Hot spot in a lead holder

plus detereoration of

the insulation

Figure 6. Insulation failure of a lead of the tap changer

Figure 7. Lead deformation of the lead between the winding and the bushing

The time-based comparison of the FRA plots of the 230 kV winding is shown in Fig. 8. At low frequencies the plots look well, but above 10 kHz clear deviations were observed. This kind of deviations is usually related to changes in the series and parallel capacitances of the windings, what could at the same time be related to radial or axial deformations. But for sure, a deformation in windings was diagnosed. After performing a physical inspection the diagnosis was confirmed. It was found that a conductor tilting failure (MFCT) took place in the winding as shown in Fig. 9.

x0 x1 Test2* x0 x1 Test1*

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005 1.000e+006

dB

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Figure 8. FRA plots of the 230 kV winding

Figure 9. Conductor tilting failure found in the 230 kV winding

Case Study 3

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An inner short-circuit in a 33 kV current transformer caused a short-circuit current to flow across one 18 MVA, 132/33/33kV transformer of an off-shore wind generation power plant. After this event, the differential protection of the transformer tripped and fault gasses were found in the Buchholz relay. FRA measurements were performed on the windings of the primary and on the two secondary sides of the transformer. Because a fingerprint was not available, the interpretation was performed by means of phase-based comparisons. The results of the measurements performed on the 132 kV windings are shown in Fig. 10. According to these results, neither electrical problem nor mechanical deformations were found. The integrity of the HV windings was suspected.

N U N V N W

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

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The FRA results measured in the 33 kV windings are shown in Fig. 11 and 12 for the secondary windings 1 and 2 respectively. Between 160 kHz and 300 kHz some deviations were observed in the secondary 1 (Fig. 11), specially the trace w1u1 deviates from the other traces. Taking into account that the two secondary windings should be from the construction point of view very similar, if no deviations were found in the secondary winding 2, it was expected not to find deviations in the secondary winding 1.

Because of this inconstancy, the deviations found in the secondary winding 1 were questionable and reason of concern. Before taking a final decision on the condition of the windings, a winding resistance tests were performed. The results obtained are shown in Fig. 13. A clear increase of the winding resistance measured between the terminal 2w1 and 2u1 was found. This correlates well with the FRA results shown in Fig. 11, where it was observed that the trace w1u1 deviates a little bit with respect to the other traces. In conclusion, from the FRA and winding resistance results, a high resistance failure (EFCR) was diagnosed.

u1 v1 v1 w1 w1 u1

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

dB

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(secondary winding 1)

u2 v2 v2 w2 w2 u2

f/Hz1.000e+002 1.000e+003 1.000e+004 1.000e+005

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(secondary winding 2)

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Figure 13. Winding resistance test performed at

the secondary windings

Based on these results, the owner of the transformer decided to send it to the manufacturer where the windings were dismounted. A physical inspection of the winding confirmed the diagnosis done by the tests. The high contact resistance gave place to very high temperatures that leaded to a conductor cooking failure (TFCC), as shown in Fig. 14.

Figure 14. Conductor cooking failure found at the

secondary winding 1

5. Conclusions

The FRA measurements are able to detect electrical and mechanical failures in power transformers. The case study 1 showed that electrical failures such as short-circuits between turns can be detected at low frequencies and the effects of an electrical failure on one winding are reflected on the FRA plot of the magnetic coupled winding. In this case, the vector group helps in the interpretation of the results. Traditional electrical tests such as, ratio, excitation current, winding resistance and

frequency response of stray losses complement the information provided by FRA for the diagnosis of electrical failures. The case study 2 showed the application of the time-based comparison assessment. By analysis of the deviations found at different frequency ranges it was possible to distinguish among simultaneous failure modes in the transformer. Short-circuit to ground caused a change in the parallel capacitance what was observed at low frequencies. A lead deformation as well as conductor tilting failure caused deviations at high frequencies. The case study 3 illustrated the application of phase-based comparison. Some deviations found in one of the secondary windings was interpreted by using the FRA plots of the other secondary winding, what showed how to apply the construction-based comparison as well for assessment of the results. Winding resistance measurements helped to confirm the presence of a high contact resistance problem. In conclusion, the diagnosis performed by experts using FRA and complemented by other diagnostic methods was confirmed by physical inspections, what shows the potential of this method. The present challenge is to represent the actual knowledge of the experts in the form of knowledge rules in order to make the use of this method also accessible to non-expert testers. References

[1] S.A. Ryder, Diagnosing Transformer Faults Using Frequency Response Analysis, IEEE Electrical Insulation Magazine March/April, Vol. 19, No. 2, pp.16-22 (2003). [2] J.L. Velsquez, A. Hedgecock, Enhanced Transformer Diagnosis with Frequency Response Analysis: Real Case Studies, 5th CIGRE international conference GCC Power 2009, Saudi Arabia, (2009). [3] Castro J.C, Aponte G, Casos Prcticos en la Evaluacin del Estado Mecnico de los Transformadores Mediante el Anlisis de su Respuesta en Frecuencia, ALTAE Conference 2009, Medellin, Colombia (2009).

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