Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> A. OLTC <s> On load tap changers (OLTC) are widely used for voltage regulation in electricity networks. Noninvasive vibration methods for condition monitoring of internal electrical contacts have been recently proposed in the literature. The vibration signals emitted during the tap changes are usually recorded and post-processed using spectral analysis and some pattern classifier technique. To reduce the complexity of the classifier, a new technique based on genetic algorithm is proposed in this paper. A description of the data acquisition system and the corresponding collected experimental data are presented. The proposed technique is detailed and preliminary experimental results are discussed <s> BIB001 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> A. OLTC <s> On-load tap changers (OLTCs) are electromechanical devices used for voltage regulation on electrical networks. A noninvasive monitoring technique was developed using Prony's method for condition assessment of on-load tap-changers using vibration analysis signals. The transient vibration signals emitted during a tap change are recorded and processed using Prony's spectral analysis and standard pre-processing techniques. Traditional condition monitoring techniques exhibit a certain degree of complexity, which may be a serious drawback when designing portable low-cost maintenance equipment. In order to reduce this complexity, we introduce a new index based on Prony's analysis. The proposed technique is presented and its diagnosis capabilities are evaluated using experimental results collected from field installed equipments during their maintenance inspection <s> BIB002 </s> Vibro-Acoustic Methods in the Condition Assessment of Power Transformers: A Survey <s> A. OLTC <s> Mechanical failure is the major failure type of on-load tap changers (OLTC) which makes the mechanical characteristic analysis of OLTC to be significant for fault diagnosis. Being different from traditional time and frequency domain analysis, vibration signals are analyzed from a new perspective in this paper based on the chaotic dynamic characteristic analysis. The key parameters of the analysis, namely the embedding dimension and delay time are obtained via chaotic methodology. Then the vibration signal is reconstructed successfully in multidimensional phase space and the maximum Lyapunov exponent is calculated to further verify the chaotic feature of vibration signal. Finally, the chaotic dynamic characteristics of OLTC vibration are illustrated with the Poincare section. Analyses of recorded data from field experiments shows that the apparent dynamic characteristic distinction is detected between vibrations of normal and fault condition which lays foundation for the effective OLTC fault diagnosis. <s> BIB003
In BIB001 and BIB002 , OLTC vibration fingerprint was modelled as a second-order spring-damper system (4): where, x(t), m, c and k are the linear displacement, mass, damping factor and spring constant, respectively. System exhibits an underdamped behaviour and a sum of three underdamped sine waves was used to approximate the vibration signals (5): where a(t), A i , α i , t 0i are the vibration signal estimate, amplitude, damping coefficient and time delay (phase shift), respectively. The authors reported that using less than 3 sine waves is not precise enough and for more than 3, the algorithm becomes too slow without significant improvements in precision. Following signal preprocessing, estimated parameters were amplitude, damping coefficient and the phase shift with minimisation of mean square error as the criterion (objective function). Genetic algorithm was used for optimisation of the criterion in BIB001 while Prony's analysis was used in BIB002 . Both of these methods have a low computational cost. However, it is noted in BIB002 that some contacts operate only a few times during their life cycle and in such cases Prony's method may not be able to establish good diagnostics. Chaos theory was used for OLTC diagnostics in both BIB003 and  for distinguishing contacts with weak spring condition from those in good condition. For practical application, external influences such as temperature and load variations will affect the results, and they should also be taken into account. 