Electrical Diagnosis Techniques for Power Transformers: A Comprehensive Review of Methods, Instrumentation, and Research Challenges
Abstract
:1. Introduction
Importance of Power Transformer Diagnosis
- Elimination of high economic losses associated with partial or total repair [14];
- Increase in the uptime of equipment;
- Improvement in the quality of equipment performance and avoid untimely degradation [15];
- Reduced outages in the production and distribution of electricity;
- Prevention of conditions that put operators in risk [9];
- Increase in the life expectancy of the assets.
2. Review Methodology
(“All Metadata”: Power transformer) AND (“All Metadata”:diagnos*) OR (“All Metadata”: condition* monitoring) NOT (“All Metadata “: words or phrase to exclude)
Data Segregation and Account for Bias
(“Document Title”: Power transformer) AND (“Document Title”: diagnos*) OR (“Document Title”: condition* monitoring) NOT (“Document Title”: words or phrase to exclude): 2014–2024
3. Power Transformer Diagnosis Techniques
3.1. Frequency Response Analysis
3.2. Partial Discharge
3.3. Dielectric Dissipation Factor/Tan Delta
- Transportation: A key challenge highlighted is the large size and weight of measurement devices, which limit transportation flexibility. End users prefer lightweight and compact designs to reduce transportation, handling, and storage costs.
- Testing or measurement: Another challenge in dielectric loss measurement is selecting the right measuring device, as post-processing availability and reliable, repeatable results are crucial for electrical engineers and researchers to take accurate actions.
- Processing the results: After measurement, analyzing and post-processing the results is essential, with accuracy being a primary concern.
- Standards and others: Testing procedures and experimental results should adhere to international standards to assess the test object’s condition. Strict adherence to these standards poses a challenge to manufacturers and researchers.
3.4. DC Insulation Resistance
3.5. Polarization Index
3.6. Transformer Turn Ratio
3.7. Dielectric Response Analysis
3.7.1. Recovery and Return Voltage Measurement
3.7.2. Polarization Depolarization Currents (PDCs)
3.7.3. Frequency Domain Spectroscopy (FDS)
- Un-Grounded Specimen Test (UST) Mode: This mode assesses the condition of the insulation between the LV and HV windings by measuring the current flowing through the HV winding to the LV winding, thus completing the circuit path. In this configuration, only the current passing directly from HV to LV contributes to the results (see Figure 12b) [101,102].
3.8. Breakdown Voltage Test
3.9. Power Factor and Capacitance
4. Advances in AI/ML for Power Transformer Diagnosis
5. Instrumentation
6. Discussion
7. Conclusions
Author Contributions
Funding
Conflicts of Interest
Abbreviations
FRA | Frequency Response Analysis |
SFRA | Sweep Frequency Response Analysis |
IFRA | Impulse Frequency Response Analysis |
AI | Artificial Intelligence |
PD | Partial Discharge |
UHF | Ultra-High Frequency |
IR | Insulation Resistance |
DAR | Dielectric Absorption Ratio |
PI | Polarization Index |
TTR | Transformer Turn Ratio |
DRA | Dielectric Response Analysis |
RVM | Recovery Voltage Measurement |
PDC | Polarization Depolarization Currents |
FDS | Frequency Domain Spectroscopy |
BDV | Breakdown Voltage |
PF | Power Factor |
DDF | Dielectric Dissipation Factor |
IEC | International Electrotechnical Commission |
IEEE | Institute of Electrical and Electronics Engineers |
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Winding Rated Voltage | Insulation Resistance Test Voltage |
---|---|
<1000 | 500 |
1000–2500 | 500–1000 |
2501–5000 | 1000–2500 |
5000–12,000 | 2500–5000 |
>12,000 | 5000–10,000 |
Thermal Class Rating | Minimum PI Value |
---|---|
Class 105 (A) | 1.5 |
Class 130 (B) and above | 2.0 |
Test Mode | 1 min | 10 min | PI |
---|---|---|---|
Primary–GND | |||
Secondary–GND | |||
Tertiary–GND | |||
Primary–Secondary | |||
Primary–Tertial | |||
Secondary–Tertiary |
Technique | Method | Advantages | Disadvantages | Domain |
---|---|---|---|---|
Recovery voltage measurement | Application of DC voltage with increasing charging and discharging time while maintaining . | Simple and non-destructive. | Time consuming. Affected by environmental conditions (e.g., temperature). | Time domain |
Return voltage measurement | Application of single charging DC voltage in the kV range. | Highly resistant to interference from external electric fields in the surrounding environment. Fast diagnosis. | Experienced experts are required, as RVM spectra can be challenging to differentiate from the combined effects of oil and paper insulation. | Time domain |
Polarization depolarization current (PDC) | Application of DC voltage and measurement of polarization and depolarization current using an electrometer over a period (typically 20,000 s). | Other diagnostic parameters such as polarization index (PI), dissipation factor (%), dielectric adsorption ratio (DAR), paper moisture (%pm), etc., can be determined by analyzing the PDC data. | PDC data affected by de-trapped current. PDC measurement is affected by low-frequency field noise and temperature variation. Time-consuming, and thus, temperature variation can affect measurement. | Time domain |
Frequency domain spectroscopy (FDS) | A sinusoidal voltage with frequencies ranging from 100 μHz to 1 kHz is applied to a test specimen, and the resulting current through the test object is measured | The dielectric frequency response method enables the direct evaluation of heterogenous (paper–oil) insulation by distinguishing between the effects of oil and paper insulation. | Interpreting and comparing FDS output curves is a complex task that demands specialized expertise in the field. | Frequency domain |
Method/Standard | Limit Value for Equipment with Voltage in the Range: 69–230 kV |
---|---|
ASTM D877 | |
ASTM D1816 (0.04) | |
ASTM D1816 (0.08) |
Data Acquisition | Data Analysis/ Diagnosis Methods | Research Gaps/Challenges | Refs. | |
---|---|---|---|---|
Detection Methods/Tests | Measurand | |||
Frequency Response Analysis (FRA) | Impedance, admittance or transfer function vs. frequency | Comparison of measured FRA signature with its healthy state, sister transformer, or other phases FRA signature of the transformer, statistical methods, ML, AI | Uncertainties interpretating FRA signature and correlation signature to specific faults Online IFRA at high voltages | [11,24,28,29,35] |
Partial Discharge | Apparent Charge (pC) | Expert analysis of PRPD and TRPD spectrums, advanced signal processing techniques, such as discrete wavelet transform (DWT), AI (neutral networks, decision trees, etc.) | Onsite noise during online PD Measurements Localization of PD sources Examination of the correlation between PD and breakdown under DC voltage | [16,42,45,48,50,134] |
Tan Delta | Dissipation factor/currents capacitance | Analysis of dielectric responses curves (frequency vs. tan), comparison of measures values with standards | Selection of right measuring instrument Testing with variable frequency and waveform Limitation on the use of DC excitation source | [56,58,62,63] |
DC Insulation Resistance | Resistance (ohm) | Guidelines from standards Statistical methods | Impact of Environmental Variables on Test Accuracy Effects of Aging and Wear on Insulation Resistance | [18,56,71,112] |
Polarization Index | Current | Guidelines from standards Statistical methods | hybrid approaches that combine PI with other diagnostic techniques Influence of ambient and test conditions of PI | [56,66,73,74] |
Transformer Turn Ratio (TTR) | Current | Guidelines from standards Comparison of results with nameplate data | Automating TTR analysis through AI/ML Integrated testing systems Standardized hybrid framework combining TTR with FRA | [18,34,57,75] |
Recovery/Return Voltage Measurements (RVMs) | Voltage | Comparison of the return voltage curve with the curve from the maxwell model Using the time constants and and the r-factor as diagnosis tools | Integrating RVM into multi-modal diagnostic platforms Comparative studies of RVM with other diagnostic methods Effectiveness of RVM under varying load and temperature conditions Quantification of the effects of aging and moisture of RVM | [57,84,86] |
Polarization Depolarization Current (PDC) | Current | Modeling the monotonically decreasing dielectric response using various insulation models (e.g., Debye model) | Methods for compensating for temperature effects on the dielectric response function Assessment of the effects of de-trapped charges (de-trapping current) on PDC measurement Development of advanced filtering techniques to mitigate the impact of field noise on PDC measurements Incorporation of advanced forecasting methods to reduce PDC measurement time | [88,94,97,98] |
Frequency Domain Spectroscopy (FDS) | Time and frequency domain analysis; fitting models | Development of effective temperature compensation techniques Use of AI for data interpretation Research into simpler, cost-effective setups or portable solutions | [98,101,102,104,107] | |
Breakdown Voltage Tests | Voltage (kV) | The use of cumulative Gaussian probabilities Guidelines from Standards | Integration of BDV with other diagnostic techniques for fault detection The effects of particle type, size, and distribution on BDV Synergistic effects on combined impact of moisture and particulate contamination on BDV | [112,113,118,119,120] |
Power Factor and Capacitance | Capacitance PF | Monitoring the trend of measures Comparison of measurement results with name plate data healthy state measurement The use of absolute limits imposed by standards | Incorporation of AI models with online measurements Discrimination or elimination of the effect of stray capacities on the test results | [125,126,128] |
Diagnosis Technique | Implementation of AI/ML | Results | Refs |
---|---|---|---|
Frequency response analysis (FRA) | This paper applied clustering analysis and cross-correlation methods to interpret frequency response analysis (FRA) data for diagnosing power transformer winding faults. It uses statistical clustering techniques to group different types of faults (short circuits, axial displacement, and radial deformation) and cross-correlation to measure the similarity between healthy and faulty transformer states. | The proposed approach successfully classified and diagnosed various winding faults with high accuracy. The clustering method provided a systematic way to distinguish different fault types, reducing reliance on expert judgment. | [137] |
This study combines logistic regression, discrete wavelet transform (DWT), and artificial neural networks (ANNs) to enhance fault detection in transformer windings. DWT is used for feature extraction, logistic regression selects the most effective wavelet bases, and ANN classifies different fault types. | The proposed model achieved a 97% accuracy rate in detecting transformer faults and reduced misclassification to 2.9%. | [138] | |
The paper implements unsupervised machine learning using the k-means clustering method to classify power transformer states into groups based on failure probability. Supervised machine learning techniques, including artificial neural networks (ANNs) and adaptive neuro-fuzzy inference systems (ANFISs), are used to detect fault severity in power transformers of different lifetimes. | The k-means clustering method effectively groups transformers based on their health state, while ANN and ANFIS provide accurate fault severity detection. | [139] | |
The paper proposes a data augmentation technique using a Conditional Wasserstein Generative Adversarial Network with Gradient Penalty (Conditional-WGAN-GP) to generate synthetic frequency response analysis (FRA) data. This augmented dataset is used to train fault diagnosis models, including support vector machines (SVMs), to detect winding deformation faults in transformers. | The proposed data augmentation technique significantly improves the accuracy of fault diagnosis models, with an average improvement of 5% compared to baseline models. | [140] | |
Partial discharge (PD) | The study proposes using an artificial neural network (ANN) combined with Cepstrum analysis to classify multiple partial discharge (PD) sources under noisy conditions. The ANN is trained on features extracted from PD signals using various methods, including discrete wavelet transform (DWT) and discrete Fourier transform (DFT), and then tested with noisy signals. | The Cepstrum-ANN method demonstrated the highest classification accuracy compared to other feature extraction techniques when classifying PDs under noisy conditions. | [141] |
The authors applied multiple machine learning techniques including support vector machine (SVM), k-nearest neighbors (kNN), naïve Bayes, random forest, and probabilistic neural network (PNN) to classify different types of PDs in paper–oil insulation. Feature extraction is performed using the power spectral density (PSD) of acoustic emission (AE) signals. | The SVM classifier with a polynomial kernel achieved 100% accuracy, while kNN also performed well. Random forest and naïve Bayes classifiers achieved over 97% accuracy, demonstrating the effectiveness of ML for PD identification. | [142] | |
This study combines k-nearest neighbors (kNN) for imputing missing data and support vector machine (SVM) for classifying and localizing PD sources in power transformers. The dataset used is based on dissolved gas analysis (DGA), where kNN helps handle missing values before SVM processes the classification. | The combination of kNN and SVM significantly improved accuracy in PD classification and localization compared to traditional approaches. The study showed that handling missing values properly led to higher precision in PD diagnosis. | [143] | |
The paper compares several machine learning and deep learning models, including support vector regression (SVR), back-propagation neural networks (BPNNs), convolutional neural networks (CNNs), and extreme gradient boosting (XGBoost), for the three-dimensional localization of PDs inside power transformer tanks. The models use electric field sensor data to predict PD locations. | CNN outperformed other methods in terms of localization accuracy, followed by XGBoost and SVR. The study found that using a single electric field sensor with ML techniques can provide high-precision PD localization, reducing the need for multiple sensors. | [41] | |
Tan delta | The authors proposed ANN model trained with a hybrid meta-heuristic algorithm called the particle swarm-based crow search algorithm (PS-CSA) to predict the aging of transformer insulation oil. The model uses input parameters including tan delta. The PS-CSA algorithm optimizes the weights of the ANN to minimize the error between predicted and actual aging outcomes. | The PS-CSA-ANN model outperforms LM-ANN, PSO-ANN, and CSA-ANN, showing significant improvements in error metrics like RMSE, MAE, and SMAPE. It achieves 49.6% lower RMSE than LM-ANN, 15.3% lower than FF-ANN, and 26.9% lower than CSA-ANN at a 25% learning rate. | [144] |
IR and PI | The paper employs computational optimization techniques, specifically the hill-climbing algorithm combined with the 1/5th success rule, to refine the analysis and classification of power transformer insulation resistance. | The optimized method achieved an 88.9% accuracy rate in classifying insulation resistance conditions of power transformers. | [73] |
TTR | No applicable papers (to the author’s knowledge). | Not applicable. | --- |
RVM | No applicable papers (to the author’s knowledge). | Not applicable. | --- |
PDC | The paper uses a neural network (NN) model to forecast polarization current (PDC) and reduce measurement time. The model predicts long-duration PDC data using only a short-duration sample, minimizing the need for extended testing while maintaining accuracy. | The NN model effectively forecasts polarization current, reducing measurement time from several hours to just 10 min while maintaining diagnostic accuracy. | [88] |
A residual LSTM model is used to forecast polarization current, reducing the need for lengthy PDC measurements. The model incorporates spatial shortcut connections to improve learning efficiency and is compared against LSTM, Attention-LSTM, GRU, and CNN. Monte Carlo dropout is used for uncertainty estimation. | The residual LSTM outperforms other models, achieving the lowest error metrics and the least uncertainty in forecasts. | [145] | |
FDS | The paper proposes a GA-SVM model for diagnosing moisture in transformer insulation using frequency domain spectroscopy (FDS). It introduces a novel method to predict FDS curves with limited data, improving classification accuracy. | The GA-SVM model achieves 99.15% classification accuracy, outperforming standard SVM and particle swarm optimization SVM (PSO-SVM). | [108] |
The paper uses machine learning (ML) models to classify moisture levels in transformer paper insulation based on frequency domain spectroscopy (FDS) data. It evaluates the support vector machine (SVM), artificial neural network (ANN), and k-nearest neighbors (KNN) algorithms for their ability to classify insulation moisture levels. | SVM outperformed ANN and KNN, achieving the highest classification accuracy and fastest training speed. | [146] | |
BDV | The paper develops an artificial neural network (ANN) model to predict the breakdown voltage (BDV) of transformer oil, considering the effects of barriers and contamination. The ANN model is trained on 784 experimental samples with various parameters such as electrode configurations, barrier properties, and temperature. | The ANN model achieves a prediction accuracy of 98.4% for training and 97.34% for testing, demonstrating its high reliability in estimating BDV. | [147] |
The study integrates genetic algorithm-optimized artificial neural networks (GA-BP-ANNs) and partial least squares regression (PLS-R) to predict the breakdown voltage of transformer oils using ATR-FTIR spectroscopy data. The GA optimizes feature selection, improving the ANN’s predictive performance. | The GA-BP-ANN model outperforms PLS-R, achieving a higher correlation coefficient (R2 = 0.9891) and lower RMSEP (0.2874), making it a highly accurate tool for oil condition monitoring. | [148] | |
PF and capacitance | No applicable papers (to the author’s knowledge). | Not applicable. | --- |
Diagnosis Method | Instruments Required | Online | Offline | Standard(s) |
---|---|---|---|---|
Sweep Frequency Response Analysis (SFRA) | ISA SFRA 5000 test instrument wide bandwidth current and voltage sensors | X | ✓ | IEC 60076-18 IEEE C57.149-2012 |
Impulse Frequency Response Analysis (IFRA) | ✓ | ✓ | IEC 60076-18 (not dedicated) | |
Partial Discharge | MPD600 system High-Frequency Current Transformers (HFCT) | ✓ | ✓ | IEC 60270 |
Tan Delta | Doble M4100 Omicron’s DIRANA Current and voltage instrument transformers | ✓ | ✓ | IEC 60851-5:2008 |
DC Insulation Resistance Polarization Index | Megger MIT 1525 DC voltage generator Current and voltage ITs | X | ✓ | IEC 60076-3:2013 |
Transformer Turn Ratio (TTR) | AC voltage generator Voltage instrument transformer | X | ✓ | IEEE C57.12.90-2021 IEC 60076-1:2013 |
Recovery/Return Voltage Measurements (RVMs) | RVM 5462b Recovery Voltage Meter Electrometer | X | ✓ | Not applicable |
Polarization Depolarization Current (PDC) | Electrometer (e.g., Keithley 6517B Electrometer) | X | ✓ | Statistical Analysis) (not dedicated) |
Frequency Domain Spectroscopy (FDS) | IDAX-300 insulation diagnostic analyzer | X | ✓ | IEC 60076-18 IEEE 62-1995 |
Breakdown Voltage Tests | Standard test electrodes made of disc or spherical electrodes | ✓ | ✓ | IEC 60156:2025 ASTM D1816 ASTM D877 |
Power Factor and Capacitance | M4000 insulation analyzer | ✓ | ✓ | IEC 60076-21-2011 |
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Mwinisin, P.; Mingotti, A.; Peretto, L.; Tinarelli, R.; Tefferi, M. Electrical Diagnosis Techniques for Power Transformers: A Comprehensive Review of Methods, Instrumentation, and Research Challenges. Sensors 2025, 25, 1968. https://doi.org/10.3390/s25071968
Mwinisin P, Mingotti A, Peretto L, Tinarelli R, Tefferi M. Electrical Diagnosis Techniques for Power Transformers: A Comprehensive Review of Methods, Instrumentation, and Research Challenges. Sensors. 2025; 25(7):1968. https://doi.org/10.3390/s25071968
Chicago/Turabian StyleMwinisin, Peter, Alessandro Mingotti, Lorenzo Peretto, Roberto Tinarelli, and Mattewos Tefferi. 2025. "Electrical Diagnosis Techniques for Power Transformers: A Comprehensive Review of Methods, Instrumentation, and Research Challenges" Sensors 25, no. 7: 1968. https://doi.org/10.3390/s25071968
APA StyleMwinisin, P., Mingotti, A., Peretto, L., Tinarelli, R., & Tefferi, M. (2025). Electrical Diagnosis Techniques for Power Transformers: A Comprehensive Review of Methods, Instrumentation, and Research Challenges. Sensors, 25(7), 1968. https://doi.org/10.3390/s25071968