— The most serious condition of a non-ceramic insulator is when the core is damaged, increasing t he possibility of a catastrophic failure resulting in the conducto r being released. This condition is unacceptable, as it may lead to safety issues both for the public and workers. Furthermore , core damage may occur without any detectable sign on the surface of the insulator. This paper presents a method aiming at the detection of core damages by testing the insulator based on mechanical excitation and measurement of the result ing vibration in the ultrasonic range. The testing arra ngement and results on new and damaged insulators are presented in this paper. Keywords— composite, insulator, diagnostics, vibrati on, core fracture I. INTRODUCTION Composite insulators are widely used in transmissio n lines because of their excellent properties, such a s high toughness, low weight and good insulating propertie s. However, as several insulators in service have rupt ured without warning, some operators have become wary of using composite insulators. This is particularly true in places where live work is carried out because the extra load of the workers and the equipment can be the trigger that causes th e insulator to rupture, endangering both the workers and the pu blic. Unexpected rupture of insulators is the result of degradation of the insulator core, which in many ca ses shows no external signs of degradation. During operation, the insulator is constantly subjected to tensile, torsi onal, and tension forces, and vibration. Insulators are extre mely resilient and robust, but if the core develops some minor damage, e.g., due to a factory defect, deliberate d amage, or improper handling, it can become more severe over t ime, leading to the rupture of the insulator. Partial di scharges inside the insulator can also significantly weaken the core by burning the resin in the fiberglass core. Water ent ering the insulating sheath can also have serious consequence s, as it is responsible for two different processes that can de grade the insulating core. One of these problems is the incre ase in leakage current [1],[2], which increases the temper ature of the insulator and exposes it to increased aging. Th e other problem is that water entering the insulator can fo rm acids, which damage both the sheath and the insulator core [3]. The main problem for system operators is that, alth ough the potential faults are known, most diagnostic tec hnologies are optical measurements (visual inspection, UV, IR cameras, hydrophobicity testing) [4],[5] which focus on iden tifying external damage of the insulator housing. Therefore , there is currently no diagnostic method that can effectively test the insulating core in situ, but at the same time is ec onomically feasible. This paper presents a method based on wave propagat ion analysis, which can be used to identify core damage , even in cases where the insulator exterior is intact. The i dea is that in a homogeneous medium, a traveling wave propagates a t a constant speed until it reaches the medium boundary , where it is reflected. The medium is usually not lossless , so the waves are continuously attenuated, but more importa ntly, any defect in the medium causes inhomogeneity. Such inhomogeneous parts are called discontinuity points at which the propagating wave is reflected. In the TDR method, known in cable diagnostics, a lo w- voltage pulse is induced in the cable, and it is po ssible to identify various faults based on wave reflections. In its original form, the method is not applicable to comp osite insulators, but vibrations caused by mechanical exc itation can be detected with a piezoelectric sensor. The sensor converts the mechanical wave energy into an electrical charg e so that inhomogeneities inside the insulator, and hence, de fects can theoretically be detected using an oscilloscope in a similar way to TDR. As of the knowledge of the authors, the method has not yet been used to diagnose composite insulators, but other fields of science have had a similar approach to di agnosing different structural units.[6]–[8] The measurement procedure and initial results are presented, in the following sections. II. MEASUREMENTS To test the method, waveforms detected on intact an d damaged insulators were compared. Since detecting d efects in the core of insulators is costly even under labo ratory conditions, the damaged sample was not collected fr om the field; instead, an intact insulator was artificiall y damaged. The aim of the study was to investigate the applica bility of the method for the detection of damaged insulato rs. Therefore, we tested the method under laboratory co nditions. However, we did not exclude its future application in the field, so we tried to design the measurement setup to be similar to field conditions. This means that the in sulator under test is in a hanging position, and one of its end f ittings is in a fixed position to which the piezoelectric sensor is attached to. The sensor is connected to an oscilloscope via a si gnal amplifier. In service, the other end fitting would support the weight of the conductor, but here we have examined three different cases. In the first case, the end fitting hangs freely; in the second case, a small weight (~7 kg) was atta ched to the end fitting using a rope to reduce the sway of the insulator. In the third case, a 250 kg weight was attached direct ly to the end fitting, and the insulator was tightened withou t the weight lifting off the floor. The first two cases are more straightforward and qu icker to use in the laboratory because the wave will be r eflected entirely at the free end fitting, which is easy to handle. The third case represents field conditions, where the i nsulator is stressed by a heavy weight, and complicated wave re flection is expected at the weighted end. The sketch of the measurement setup is shown in Fig. 1. Fig. 1. Sketch of the measurement setup. To ensure the repeatability of the results and to c ompare waveforms effectively, the process of mechanical wa ve generation had to be standardized, as the shape of the wave is strongly influenced by the location, angle and magn itude of the impact. We solved this by using a spring-loaded device to provide the excitation during the measurements. The device had two different fixed positions at the end fittin g of the insulator. This was necessary because, although the preliminary assumption was that longitudinal waves would make it easier to identify the fault, we could not be sure, and it might be easier to generate transverse waves in the field. Therefore, one fixed position design was responsibl e for generating longitudinal waves, and the other one fo r generating transverse waves. III. RESULTS In signal shape studies, conclusions can be drawn f rom multiple properties of the signal. Observable prope rties include the amplitudes, phases, repetition, attenua tion and frequency components of the signal. There are many methods for evaluating results, but in many cases, they req uire some prior knowledge of the signal. Therefore, the resul ts presented here are the results of simple observatio ns gained from a limited number of measurements. In most case s, several differences between signals of intact and d amaged insulators could be observed, but many of these did not match on repeated measurements, or the differences were n ot significant enough to be conclusive. The most signi ficant differences observed are highlighted below. The res ults will be presented based on the load fixed to the free en d fitting. In all the following cases, the generated waves las ted for less than 20 ms. From each of the detected waveform s, 500,000 data points were taken, and using those dat a points, the signal was reconstructed in MATLAB for further investigation. This allowed us to compare the wavef orms more easily, calculate the cumulative energy of the waves (which is calculated as the square of the deviation of each data point from 0 and summed), and analyze the wave forms in the frequency domain. A. There is no load on the free end fitting 1) The excited wave is longitudinal to the insulator c ore In this case, the amplitude of the signal associate d with the damaged insulator decreased quicker than that o f the intact insulator. In this case, the cumulative ener gy was lower in the case of the damaged insulator. The signals f or both the intact (top) and damaged (bottom) insulators can be seen in Fig. 2. Fig. 2. Signals detected during the measurements. In the frequency domain there is a significant diff erence between the signals of the intact and the damaged i nsulators, as the frequency component around 60 kHz is missing in the case of the damaged insulator. Wave signals in the frequency domain are shown in Fig. 3. for both intact (top) a nd damaged (bottom) insulators. Fig. 3. Signals detected during the measurements. 2) The excited wave is transverse to the insulator cor e In the case of transversal waves, the waves detecte d by the sensor could be easily separated in the time do main during the test of the intact insulator. Fig. 4. shows an example of the waveform. In the case of the damaged insulator such separation could not be made on the signal. The cum ulative energy of the wave was higher in the case of the da maged insulator. Fig. 4. Signals detected during the measurements (i ntact insulator). In this case, the spectral patterns were very simil ar for intact and damaged insulators, with the only signif icant difference being the reversal in the proportion of components around 7 kHz and 18 kHz. Fig. 5. Spectrum of the signal (intact insulator). B. 7 kg load on the free end fitting 1) The excited wave is longitudinal to the insulator c ore The amplitude of the detected signal associated wit h the damaged insulator decreased faster. In this case, t he cumulative energy was higher in the case of the dam aged insulator. In the frequency domain, as in the unloaded case, w e found that the frequency component around 60 kHz do es not appear in the case of a damaged insulator. 2) The excited wave is transverse to the insulator cor e In this case, no significant difference was found b etween the time-domain signals of damaged and intact insul ators. The cumulative energy of the signal associated with the damaged insulator was higher. The only dominant frequency in the spectrum of the intact insulator was around 18 kHz. In the case of damaged insulators, there were significant components at a number of frequencies. C. 250 kg load on the free end fitting 1) The excited wave is longitudinal to the insulator c ore In the case of a strained insulator, the time-domai n signals are very similar for both intact and damaged insula tors. The reason for this is that there is much more signific ant reflection at the weighted end fitting of the insulator and, t herefore, more waves reach the sensor, which overlap. However , isolating several larger waves detected by the sens or is possible. Further observations could therefore be m ade by splitting the signal into smaller parts. In our cas e, however, this was prevented by the fact that the amplitude o f the signal was too large for the oscilloscope, even at the low est gain, and so it was cut off. This will require the insert ion of a signal attenuator or a change in the excitation in the fut ure. This can be observed in Fig. 6. Due to the overdrive, it is not possible to determine the cumulative energy in this case. Fig. 6. – Spectrum of the signal (intact insulator) . Spectrum analysis is also much more difficult in th is case. The frequency components in the intact and damaged spectra are different but not as significantly as before. T herefore, in Fig. 7, the spectra of the intact insulator (top) a nd the damaged insulator (bottom) are only shown up to 30 kHz, so the difference can be seen. However, repeated measu rements have not yet been able to establish a definite diff erence. Fig. 7. – Spectra of the signal (intact insulator). 2) The excited wave is transverse to the insulator cor e In this case, the same can be established in both t ime- domain and frequency-domain analysis as in the prev ious case. IV. CONCLUSIONS The wave propagation based method has not been used before for the diagnosis of composite insulators, a nd it is therefore essential to establish a solid basis for this research. For this reason, we have set up several scenarios f or testing, among which we have found several possibilities to distinguish between damaged and intact insulators u sing this method. However, further studies are needed to dete rmine the reliability of the method and the extent to which t he results can be applied uniformly to different insulator typ es. Future work will also aim to find the most suitable time a nd or frequency range for the analysis. Consequently, thi s paper only presents the initial stages of the research an d the possibility of diagnosing non-visible damage of ins ulators.