Frequency Domain Spectroscopy Results Research Articles

A High-Precision Diagnosis Method for Damp Status of OIP Bushing

An accurate assessment of the damp status of oil-impregnated paper (OIP) bushings is crucial for the power industry to make informed decisions on the maintenance and replacement schedule of bushings. This article proposes a hybrid of the convolutional neural network (CNN) and the hidden Markov model (HMM) for estimating the damp status (i.e., moisture level and moisture source) of bushings especially when nonuniform moisture distribution exhibits in the bushing. First, simulation models of moisture diffusion and frequency-domain spectroscopy (FDS) of the OIP bushing were constructed using the finite element modeling (FEM) approach. Then, CNN was employed to extract informative features from FDS results of the OIP bushing, which is sensitive to both concentrations and sources of moisture. Finally, HMMs were further utilized as a strong stability tool to recognize the damp status of OIP bushings. The proposed method was implemented to identify the bushing damp status using both simulation data and real-life measurements. Identification results demonstrate that the proposed method has high accuracy in determining the moisture level and moisture source of the OIP bushing insulation.

Condition prediction for oil-immersed cellulose insulation in field transformer using fitting fingerprint database

The results of frequency domain spectroscopy (FDS) will be affected by moisture which often leads to an unreliable condition prediction of oil-immersed cellulose insulation in transformers. In view of this, a solution is proposed in this study by adopting a fitting fingerprint database (FFD) technique, which can be utilized for condition prediction of aging and moisture content. In the current work, the relevant fingerprints that could characterize the insulation aging condition and moisture content are extracted from the FDS curves and the DC conductivity of the transformer oil. The FFD is then constructed by fitting fingerprints. The accuracy of the reported FFD technique is verified from tests on oil-immersed pressboard prepared in the lab and a transformer from the field. In this respect, the present contribution highlights FFD might be used as a powerful tool for condition prediction of oil-immersed cellulose insulation in transformers.

Artificial neural network based method for temperature correction in FDS measurement of transformer insulation

Frequency domain spectroscopy (FDS) measurement has become an important method for the assessment of the condition of the insulation of oil transformers. In recent years, numerous researchers have found that temperature variation affect FDS results. The master curve technique is commonly used to correct the effect of temperature on FDS results. In this paper, an FDS experiment is carried out on a sample transformer. Then, for this transformer, insulation model parameters are determined by using a genetic algorithm based on the FDS results. Then, by using the insulation model parameters, tanδ curves are simulated and compared to real results. Finally, an FDS experiment is conducted on two other transformers at 22 °C, 30 °C, 40 °C, 50 °C, 60 °C, and 70 °C (in order to give sufficient information for a training neural network) and insulation model parameters are calculated via the genetic algorithm. In one of the transformers, the effect of temperature on the FDS curves is corrected by using the master curve technique and the FDS curves are transmitted over the reference curve. It is also shown that the transformer insulation is an Arrhenius-type dielectric. The error of this method is calculated by using a mean square error technique. In the two other transformers, the insulation model parameters related to 22 °C are considered as the target parameters. The insulation model parameters of the other temperatures are fed into an artificial neural network as input, to train it to transfer insulation model parameters related to other temperatures to the reference parameters. Finally, the errors of both methods are compared, and it is shown that this latter method for correcting the temperature effects in the FDS method is the more effective.

A Circuital Model-Based Analysis of Moisture Content in Oil-Impregnated-Paper Insulation Using Frequency Domain Spectroscopy

The explicitly numerical or well-defined correlations between the dielectrics responses (DRs) and dielectric essences of oil-impregnated paper (OIP) insulation have not been fully understood yet. As a result, it is rather difficult to quantitatively diagnose the critical insulation condition, like determining moisture content in insulation paper (MCP) of power equipment only using electrical-based techniques such as frequency domain spectroscopy (FDS). To obtain MCP value, from a new perspective of parametric study on the circuital DR equivalence of OIP insulation-extended Debye model (EDM), the present contribution introduces a novel approach by exploring the pattern when EDM parameters vary with MCP and temperatures T. Further, mathematical correlations are developed between sensitive R-C values and MCP-T values. On the above analysis, small-scale physical models of real-life transformer bushings were prepared as test samples. Their OIP condenser bodies were artificially absorbed different controlled quantities of moisture and conditioned at different temperatures to record corresponding FDS results. Then a hybrid genetic algorithm combined with the Levenberg-Marquardt algorithm was proposed to estimate the EDM parameters by fitting the measured FDS data. Sensitive parameters therein were identified using subspectrum decomposition and formulated with MCP-T values so that a reliable moisture content estimation can be achieved once the testing temperature and FDS recordings of an insulation body are known.

Partial discharge behaviour due to irregular‐shaped copper particles in transformer oil with a different moisture content of pressboard barrier under uniform field

This study presents the investigations on Partial Discharge (PD) behaviour of irregular-shaped copper particle in oil with pressboard barrier placed in two positions under uniform field with different moisture contents. Frequency Domain Spectroscopy (FDS) was used to estimate percentage moisture in pressboard. Recovery Voltage Measurement was used to confirm obtained FDS results. PD characteristics like magnitude, number, discharge power, rise time and duration time of PD pulse were analysed. Electric field due to particle in oil with moisture content pressboard barrier was studied by simulations. Phase Resolved PD (PRPD) pattern for different moisture pressboard observed to have similar patterns with variation in PD inception voltage (PDIV), magnitude and number of discharges. It was observed that PD characteristics as a function of voltage, increases nearly quadratic rate with respect to percentage moisture in pressboard. Simulation results complimented experimentally measured PDIV values for particle in oil with moisture in pressboard. Filtering and conditioning of oil is mandatory practice in a new transformer before going to any high voltage (HV) applications. PRPD pattern and PD characteristics data obtained in this study has great practical significance to assess the condition of transformer insulation systems before proceeding for HV application to avoid any potential failure.

Insulation condition diagnosis of oil-immersed paper insulation based on non-linear frequency-domain dielectric response

In this work, samples of oil-immersed paper with different moisture levels and aging condition were studied. Frequency domain spectroscopy (FDS) of the samples was examined at 40 °C under different excitation voltages. A non-linear phenomenon in frequency domain dielectric response with different moisture contents and aging condition was observed. The non-linear phenomenon appears to be related to the hindrance effect of cellulose on the movement of ions and the Poole-Frenkel effect on the concentration of ions. With aging, the nonlinear loss peaks at different frequencies caused by the effect of nonlinear conductance and interface polarization appear in the FDS results. The non-linear phenomenon was modeled and verified experimentally. The characteristic parameters and quantitative method to estimate moisture content and the degree of aging degree of oil-immersed paper can be obtained by the proposed method.

A Parameterization Approach for the Dielectric Response Model of Oil Paper Insulation Using FDS Measurements

To facilitate better interpretation of dielectric response measurements—thereby directing numerical evidence for condition assessments of oil-paper-insulated equipment in high-voltage alternating current (HVAC) transmission systems—a novel approach is presented to estimate the parameters in the extended Debye model (EDM) using wideband frequency domain spectroscopy (FDS). A syncretic algorithm that integrates a genetic algorithm (GA) and the Levenberg-Marquardt (L-M) algorithm is introduced in the present study to parameterize EDM using the FDS measurements of a real-life 126 kV oil-impregnated paper (OIP) bushing under different controlled temperatures. As for the uncertainty of the EDM structure due to variable branch quantity, Akaike’s information criterion (AIC) is employed to determine the model orders. For verification, comparative analysis of FDS reconstruction and results of FDS transformation to polarization–depolarization current (PDC)/return voltage measurement (RVM) are presented. The comparison demonstrates good agreement between the measured and reconstructed spectroscopies of complex capacitance and tan δ over the full tested frequency band (10−4 Hz–103 Hz) with goodness of fit over 0.99. Deviations between the tested and modelled PDC/RVM from FDS are then discussed. Compared with the previous studies to parameterize the model using time domain dielectric responses, the proposed method solves the problematic matching between EDM and FDS especially in a wide frequency band, and therefore assures a basis for quantitative insulation condition assessment of OIP-insulated apparatus in energy systems.

Frequency domain spectroscopy measurements of oil-paper insulation for energized transformers

Frequency Domain Spectroscopy (FDS) is proposed as a suitable diagnostic method in this paper for implementation on energized oil-paper insulation systems. In present FDS tests, the measurement method is only designed for offline applications. To overcome the deficiencies with the offline testing, this work investigates the feasibility of performing FDS measurements on energized transformers with the view to developing an online diagnostic tool using neutral connections as injection and measurement paths. In the proposed technique, a low-pass filter system is designed to remove the high frequency noise as well as the power frequency signals present in the measured neutral currents. The technique has been successfully applied in a laboratory environment on transformers with oil-paper insulation systems with different operational ages and is feasible for single-phase as well as 3-phase transformers with the accessible neutral points. Compared with the conventional FDS results, a nonlinear dielectric phenomenon is observed from the energized insulation systems as higher dissipation factor Tan5 values primarily due to the increased losses under power frequency voltage excitation. The degree of the nonlinearity is found to correlate well with the degradation status of insulation systems under test.

Investigations of dielectric monitoring on an energised transformer oil–paper insulation system

Frequency domain spectroscopy (FDS) is being used to assess the insulation condition of oil–paper power transformers. However, it has to date only been implemented on de-energised transformers, which requires the transformers to be shut down for an extended period and may cause significant costs. To solve this issue, a newly improved monitoring method based on the FDS principle is proposed to implement the dielectric measurement on energised transformers. Moreover, a chirp waveform excitation and its novel processing method are introduced. Compared with the conventional FDS results, dielectric results from the energised insulation system have higher tanδ values because of the increased losses. To further understand the insulation ageing process, the effects of the geometric capacitance are removed from the measured imaginary admittance of the insulation system to enhance the ageing signature. The resulting imaginary admittance is then shown to correlate well with the central time constant in return voltage measurements results. The proposed methods address the issues on techniques used on energised transformers and provide a clue for on-line FDS diagnostic application.

Effect of temperature on frequency dependent dielectric parameters of oil-paper insulation under non-sinusoidal excitation

Frequency domain spectroscopy is a potential tool for non-invasive condition assessment of oil-paper insulation in power equipment. On the other hand, it is well known that the results of frequency domain spectroscopy are affected predominantly by temperature. Moreover, the voltage used for frequency domain spectroscopy is sinusoidal is nature. But the actual insulation system at site is stressed by voltage that may deviate significantly from sinusoid during operation. Considering these two aforesaid facts, in this paper, the response of oil-paper insulation has been studied at different temperatures using both sinusoidal as well as non-sinusoidal excitation. An experimental sample has been prepared in the laboratory that emulates the composite insulation of oil immersed type power transformer at site. The results of frequency response using sinusoidal voltage waveform at different temperature show good agreement with the conventional results that validates the scheme of the experimental setup. Some more information is obtained from the response of non-sinusoidal excitation that could help to get a better interpretation of the actual physical condition of oil-paper composite insulation.

Condition assessment of stator insulation during drying, wetting and electrical ageing

Stator insulation failure due to moisture ingress is one of the common reasons for generator failures. Non-destructive tests such as DC ramp, insulation resistance (IR) and frequency domain spectroscopy (FDS) are usually conducted to assess the conditions of such insulations. This paper presents a laboratory investigation of generator spare windings under drying, wetting and cyclic electrical ageing conditions. First Dc ramp, IR and FDS tests were conducted on asphalt-mica and epoxy-mica insulated spare windings to obtain initial finger print values. Then the drying effects was investigated by DC ramp and FDS measurements conducted on asphalt-mica insulated winding samples by drying them up to three days at 80°C. The wetting effect was studied by wetting the polyester-mica insulated winding samples in a water bath up to one week at 25°C. Further a correlation has been established between the moisture content and FDS results by drying and wetting of two asphalt-mica insulated samples at 70°C and the room temperature respectively. The ageing effects was obtained by electrically stressing two asphaltmica and two epoxy-mica insulated winding samples at 300% of the rated voltage up to 9 weeks in a cyclic manner. For all cases, conductivity (σ) and parameters representing the response function were estimated by the Davidson-Cole and inverse power dependence functions. The estimated parameters under DC ramp, IR and FDS tests were in similar range for new samples. During ageing and drying, a clear variation was noted only on the σ values and FDS results. It was found that FDS results and estimated parameters provide useful information about the condition of generator insulation.

Effects of the Insulation Quality on the Frequency Response of Power Transformers

This paper presents results of frequency domain spectroscopy (FDS) measurements on oil-impregnated pressboard insulation, their analyses and use of the data for modeling high frequency response (FRA) of transformers. The dielectric responses were measured in a broad frequency range, i.e. from 0.1 mHz to 1 MHz, on model samples containing different amount of moisture. The responses were parameterized with terms representing dc conductivity, low frequency dispersion and Cole-Cole polarization mechanisms and they were thereafter used to model the FRA response of a three-phase transformer.

Phonons in V 2O 3 above and below the Mott transition: a comparison of time- and frequency-domain spectroscopy results

We report an optical study of the Mott transition in V 2O 3 carried out by time- and frequency-domain spectroscopies. We have observed time-resolved spectra through the Mott transition with oscillations in reflectivity associated with the fully symmetric phonon modes. Raman spectrum also shows an abrupt change of both the phonon and electronic excitations through the phase transition. We notice that although the phonon spectra obtained by the time- and frequency-domain spectroscopies are essentially similar, there are distinct differences in the phonon lineshape.