# Optimization of Cable Fault Recognition Using Fisher’s Ratio Feature Selection

**Info:** 8218 words (33 pages) Dissertation

**Published:** 17th Dec 2019

**Tagged:**
EngineeringElectronics

# Table of Content

Page ABSTRACT………………..................................... i TABLE OF CONTENT....................................... ii CHAPTER 1. INTRODUCTION...................................... 1 1.1 Research Background............................... 1 1.2 Problem Statement................................. 3 1.3 Research Objective................................ 4 1.4 Research Scope................................... 4 1.5 Research Contribution.............................. 4 2. LITERATURE REVIEW.................................. 5 2.1 Prior Work...................................... 13 2.2 Summary....................................... 16 3. PROPOSED METHODOLOGY............................. 17 4. CONCLUSION........................................ 21 REFERENCES.......................................... 22# Chapter 1: Introduction

**1.1 Research Background**Power cables are very important and sensitive devices in the power system, and they play an important role in the safety of the power load and reliable transmission of electricity. Nowadays, the majority of power cables are insulated with polymeric material. Cross-linked polyethene (XLPE), as the main polymeric insulation, is widely used as electrical insulation material for high voltage distribution power cables. The power cables may be exposed to high currents and voltages and they are critical parts of the transmission infrastructure. Therefore, they are expected to have high resistance against possible failures [1]. Damage of insulation can lead to equipment failure and other disorders. The insulation degradation is inevitable during the operation and the failure rate of XLPE cable increases with service time [2]. The cables are permanently exposed to thermal ageing during operation and it may change in dielectric parameters of cables and irreversible damage of cable insulation. High reliability of the electrical insulation system is of key importance to any high voltage apparatus. The effect of many parameters or mechanisms needs to be carefully assessed when designing electrical insulation systems. The effects of humidity, temperature, pollution/contaminants, mechanical and electrical stress, and external pressure need to be assessed in order to assure high reliability of the insulation system. So, the research on the application of detection and diagnosis technology for insulation defects in HV XLPE cable line is particularly urgent. Studies have shown that defects of power cable insulation at an early stage can be detected by partial discharge sensitively. Partial discharge is also a major cause of insulation degradation, so it is often seen as one of the main parameters for the electrical equipment insulation status. Partial discharges are localized electrical discharges which partly bridges the insulation between the electrodes. The detection and continuous monitoring of PD data can provide useful information regarding the insulation condition. As PD occurs before the complete breakdown, PD monitoring can alarm for necessary emergent actions in order to remove the system component before the occurrence of catastrophic failure [2]. Further, it has been observed that polymeric insulation materials, like XLPE, may have a complete breakdown within a few days after the inception of partial discharge. Therefore, many researchers still aim to relate partial discharge to the lifetime of insulation materials. However, defining such a quantitative relationship is difficult to confirm. Partial discharge measurements are performed with the help of Phase Resolved Partial Discharge Analyzer (PRPDA). This technique is used to analyze the PDs with respect to the phase angle of applied voltage [5]. The PD pattern recorded with the help of PRPDA can be used to recognize the insulation defects which are the root cause of partial discharges. The aim of many researches regarding the PD measurements is to automate the PD pattern recognition process which allows predicted behavior of partial discharge activity [4]. It is believed that each type of PD mechanism has a unique set of statistical parameters like skewness, kurtosis [3-5] and others. The variations in PD pattern with respect to phase angle can be reflected by a change in these statistical quantities. Different techniques of automated learning such as neural networks, fuzzy logic and clustering are used to compare the PD patterns which allows the prediction of insulation degradation. Furthermore, the ultrawideband characteristics of the individual discharge pulses observe by means of nonconventional PD detection methods allows the detection of various type of ageing mechanisms taking place inside the insulation [6]. Thus, it’s quite possible to devise an intelligent and automated insulation diagnostic system based on the quantification of the partial discharge signals. Feature extraction selection is one of the most important tasks for dimensionality reduction in pattern recognition problems. There are two steps in feature extraction: first, the information relevant to the classification is extracted from raw data to a parameter vector X with m dimensions; second, feature vector Y with n dimensions (n < m) is extracted from the parameter vector X. While the parameter vector X has high dimensionality and requires a large amount of computation, feature extraction should have the capability to map original features into a smaller number of features for reducing the dimensionality of data presented to the classifier and yet improve the classification efficiency. Principal component analysis (PCA) and Fisher linear discriminant analysis (FLDA) are among the most common techniques of linear dimensionality reduction for feature extraction in pattern recognition problems. Both methods search optimal directions for the projection of input data onto a lower dimensional space. The task of linear discriminant feature extraction algorithms is to reduce the dimensionality of pattern observation space by finding a suitable linear subspace in which the class separability is optimally maintained. Therefore, in this work, Fisher’s Ratio Feature Selection is proposed.

**1.2 Problem Statement**Feature extraction is one of the important steps in any pattern recognition task. The efficient feature extraction technique extracts the feature which is able to discriminate one pattern from another accurately. If the feature vector is of high dimension, it is reduced to lower dimensional subspace using the techniques like Principal Component Analysis (PCA) and Linear Discriminant Analysis (LDA). These techniques are also known as dimensionality reduction techniques. Dimensionality reduction of the features is done so that the computational cost and system complexity for subsequent processing can be decreased. Linear Discriminant Analysis [7][8][9] finds a linear transform by maximizing the Fisher’s ratio and is found to be good in discriminating patterns. In a big dimension of PD data, not all of the input data extracted has the same significance level for PD pattern recognition. By using input PD data with high significance level and ignoring the low significance data, classification accuracy under noise contamination could be improved. One of the major challenges of on-site partial discharge PD measurements is the (PD) recovery of PD signals from a noisy environment. The different sources of noise include thermal or resistor noise added by the measuring circuit and high-frequency sinusoidal signals that electromagnetically couple from radio broadcasts and or carrier wave communications. For a sensitive PD measurement, these disturbing signals have to be rejected. In the case of development and routine tests, the PD measurements are carried out in the manufacturer’s shielded laboratories, with filtered mains, to reach the demanded measurement sensitivity. However, the problems faced in PD measurements performed in unshielded laboratories as well as on-site conditions, is the strong coupling of external noise, particularly from broadcasting stations.

**1.3 Research Objectives**The key objectives of this work are:

- To propose feature extractions using Fisher’s Ratio Feature Selection from PD data and artificial intelligence classifiers to identify PD types from cable joint defects.
- To compare the performance of the proposed Fisher’s Ratio Feature Selection based input data against normal input features under noisy condition using different classifiers.

**1.4 Research Scope**In this work, the research seeks to do an analysis on the feature extractions impact towards classifying the faults or defects in the cable. In this case, the proposed feature extraction method is the Fisher’s Ratio Feature Selection. As stated earlier, feature extraction is a vital procedure in order to increase the accuracy in classifying partial discharge (PD) pattern. Feature extraction will be able to reduce the dimension of a feature vector by extracting out the data which have a lower significance level. By keeping data with high significance level and inputting them into artificial intelligence classifier would have a better and more accurate result. After performing feature extraction, the data will input to artificial intelligence classifier in order to check whether the classifier performs a good pattern recognition of the cable faults. In this research, an investigation on which classifier gives the highest accuracy in recognizing partial discharge (PD) pattern is performed. Last but not least, classification under noisy environment will also be performed in order to imitate the situation in real life as close as possible.

**1.5 Research Contribution**The main contribution of this research are:

- A feature extraction named Fisher’s Ratio Feature Selection which could possibly increase the accuracy of PD classification with or without noise contamination has been proposed.
- Imitation of real time event on site by experimenting using actual cable joint samples.
- Investigation on different classifier in artificial intelligence which results in the highest accuracy in PD classification is performed.

# Chapter 2: Literature Review

Partial discharges (PD) are one of the main causes of deterioration of insulating material in power transformer windings. Many researches already had been done to find PD mechanism, the PD detection techniques, the relationship between PD and the damage they cause to insulating materials and insulating system, the location of PD sources and the problems related to avoid external interference [10]. The reliability of power transformers mainly depends on the condition of insulation. Therefore any indication of terrible failure must be detected at early stage. It is widely accepted that the presence of PD may start and lead to failure of power transformer. Even though there is no direct relationship between occurrences of PD in power transformer failure, it is widely accepted that the presence of PD may start and lead to failure of power transformer. If the PD pulse allowed to growing over time, it can cause the insulation to deteriorate which may lead to complete breakdown of the power transformer insulation [11]. PD is an electrical discharge or sparks that bridge small fraction of insulating between two conducting electrodes. PD can occur when electric field strength goes beyond the breakdown strength of insulation and can lead to flashover [12]. PD is most common phenomena which occur in high voltage equipment especially power transformer and power cables. Several causes of occurrence of PDs are ageing in the insulation and electrical overstressing on equipment or presence of defects (voids, cracks) presented during manufacturing [13]. Power transformer reliability can be seriously affected by partial discharge (PD) [14].**Fisher linear discriminant analysis (FLDA) is a mathematical transformation method from multidimensional to one-dimensional space which can be used in dimensionality reduction and pattern classification. The basic idea of FLDA is to project an n-class m-dimensional data to one direction (one straight line, called projection line) and to possibly separate classes (maximizing between-class distance and minimizing within- class distance). Let us consider a two-class problem {w1,w2 } as well as the linear discriminant function. Assume that x is d-dimensional original samples, m1 and m2 are the means of the two class samples, S1 and S2 are the within-class scatter matrices of the two samples, then Sw is the total within-class scatter matrix Sw = S1 + S2 and Sb is the between-class scatter matrix of the class samples. Then a projection or transformation is carried out to the original samples y=wTxwhere w is the weight vector or projection direction, µ1 and µ2 are the transformed mean of two samples, σ1 and σ2 are the transformed within-class scatter matrices, σw is the transformed total within-class scatter matrix, σb is the transformed between-class scatter matrix. The objective of FLDA is now to seek the w*which is the most easily classification vector or the best projection direction from d-dimension to 1-dimension and make the projected ratio of between-class distance and within-class distance maximum. Thus the definition of Fisher linear discriminant function is: JFw=μ1-μ22σ1+σ2=wTSbwwTSww (1) So, the best weight vector or the best projection direction is a vector w*which makes JFwmaximum. Then w1 or could be classified only by a threshold θ’s achievement after the w*’s determination. y=w*x-θ>0, x ϵ ω1<0, x ϵ ω2=0, indefinite (2) The definition of the threshold θ has several different methods, a commonly used formula is: θ=N1μ1+N2μ2N1+N2 (3) In formula (3), N1 and N2 are the numbers of the two samples.**

__FISHER LINEAR DISCRIMINANT ANALYSIS__**Different types of ANN have been successfully employed for PD classification problems. The most used ANN for such classification purposes are the Back Propagation Network (BPN) [15, 17, 18, 20-24], the Kohonen Self- Organizing map [21], Nearest Neighbor Classifier (NNC) [22] and the Learning Vector Quantization (LVQ) network [21,22]. These networks differ in their capabilities, since each of them has its own structure and training procedure, for example, the network may be trained in either a supervised fashion or in an unsupervised fashion. The main advantage of the BPN is its ability to create clustering shapes that are highly nonlinear and capable of forming arbitrary decision boundaries between the classes. On the contrary, LVQ is piecewise linear, since it is based on the nearest neighbor rule. Further, Modular Neural Network (MNN) based on task decomposition is also used for classification of PD [25]. More recently, Counter Propagation NN [26] and Adaptive Resonant Network [27] have been used for identification of PD source. The progress in computer measurement techniques has made it possible to calculate various statistical parameters from all discharge pulses being detected during the measurement period. The computer measurement technique showed the possibility of more accurate diagnosis. The important parameters that characterize PD are discharge magnitude,**

__ANN (ARTIFICIAL NEURAL NETWORK)__*Q*, number of pulse count,

*n*of PD pulse and phase angle of the applied voltage,

*φ.*Since, each defect has its own characteristic discharge parameters, different discharge sources will result in different patterns. Thus, PD pattern recognition may be effectively employed to discriminate between different types of PD activities that occur in the insulating systems of electrical apparatus. Further improvements in characterizing discharges have been also shown by applying statistical operators that could provide an even more effective means for the discrimination of information. The PD patterns are composed of above three parameters are

*φ*-q,

*φ*-n, q-n and

*φ*-q-n patterns. However, the complexity of these patterns makes it difficult to find effective parameters for the diagnosis. A

*φ*-q pattern indicates the mean PD magnitude of the pulses in each phase window. To obtain a

*φ*-q-n pattern, the matrix like windows is set for phase angle and PD magnitude. The number of pulse count, n, which is defined as the number of PD in each cell per unit time and is normalized by the maximum value among all cells. In both

*φ*-q and

*φ*-q-n patterns, one can see the difference between the patterns before and after the discharge initiation. In addition, it seems that one can discriminate more accurately with

*φ*-q-n patterns than with

*φ*-q patterns, because

*φ*-q-n patterns provide a larger amount of information. However, the quantitative expression of these patterns is difficult because of the complexity of the patterns. It is expected that the ANN is able to recognize the differences between these patterns. Following sections review the architecture of different ANN used for PD classification problems.

- Back Propagation Network:

- Kohonen Self-Organizing Map

- Nearest Neighbor Classifier

**For example, as shown in Figure 3, the vectors A and B denote the two pulse shapes of the PD occurring within two cavities of different size respectively. When a new pulse shape enters the network, the distance between the input pulse shape and all the stored pulse shapes is calculated and sorted to determine the prototype with the least distance. The NNC structure is very simple because there is no iterative training involved. However, it is computationally very demanding, since all training PD pulse shapes must be stored and the distance computations and sorting for each pulse to all prototype may take a long time on the computer, especially for complex problems with many dimensions.**

- Learning Vector Quantization Network

- Modular Neural Network based on Task Decomposition

- Counter propagation Network

- Adaptive Resonant Network

**2.1 Prior Work**

**Two researchers named Pradeep Kumar Shetty and T.S.Ramu address the problem of recognition and retrieval of relatively weak industrial signal such as Partial Discharges (PD) buried in excessive noise. The major bottleneck being the recognition and suppression of stochastic pulsive interference (PI) which has similar frequency characteristics as PD pulse. In their paper, they provide techniques to de-noise, detect, estimate and classify the PD signal in a statistical perspective. A multi-resolution analysis based technique is incorporated to discard the huge amount of redundant data in acquired signal. A scale dependent MMSE based estimator is implemented in undecimated wavelet transform (UDWT) domain to enhance the noisy signal. characterize the PD and PI pulses using a statistical model as the first moment of multi variate Gaussian distribution and its parameters are estimated using maximum likelihood (ML) and maximum aposteriroi probability (MAP) based techniques. A statistical test known as generalized log likelihood ratio test (GLRT) was incorporated to ensure the existence of the pulse. The decision as to whether a pulse is a noise or the desired signal has been made based on a weighted-nearest neighbor methodology.**

__AN UNDECIMATED WAVELET TRANSFORM BASED ENHANCEMENT, STATISTICAL FEATURE EXTRACTION AND DETECTION-CLASSIFICATION OF PD SIGNALS__**Partial discharge (PD) classification in power cable accessories and high voltage equipment in general is essential in evaluating the severity of the damage in the insulation. In this article, the PD classification was realised as a two-fold process. Firstly, measurements taken from a high-frequency current transformer (HFCT) sensor were represented as features by means of a transformation to the classifier and secondly, the probabilistic neural network (PNN) classifier itself was capable of effectively recognising features coming from different types of discharges. The feature that was used as a fingerprint for PD characterization was extracted from the moments of the probability density function (PDF) of the wavelet coefficients at various scales, obtained through the wavelet packets transformation. The PNN classifier was used to classify the PDs and assess the suitability of this feature vector in PD classification. Four types of artificial PDs were created in a high voltage laboratory, namely corona discharge in air, floating discharge in oil, internal discharge in oil and surface discharge in air, at different applied voltages, and were used to train the PNN algorithm. The results obtained here (97.49, 91.9, 100 and 99.8% for the corona, the floating, the internal and the surface discharges, respectively) are very encouraging for the use of PNN in PD classification with this particular feature vector. This article suggests a feature extraction and classification algorithm for PD classification, which when combined together reduced the dimensionality of the feature space to a manageable dimension, and achieved very high levels of classification.**

__FEATURE EXTRACTION OF PARTIAL DISCHARGE SIGNALS USING THE WAVELET PACKET TRANSFORM AND CLASSIFICATION WITH A PROBABILISTIC NEURAL NETWORK__**In this research, a new hybrid feature extraction method combining adaptive optimal radially Gaussian kernel (AORGK) time-frequency representation with two dimensional non-negative matrix factorization (2DNMF) is proposed for partial discharge (PD) classification. Firstly, AORGK is applied to obtain the time-frequency matrices of PD ultra-high-frequency (UHF) signals. Then 2DNMF is employed to compress the AORGK amplitude (AORGKA) matrices to extract various feature vectors with different (d1, d2) combinations, i.e. (5, 5), (5, 10), (10, 5) and (10, 10). Finally, the extracted features are classified by fuzzy k nearest neighbor (FkNN) classifier and back propagation neural network (BPNN). 600 samples sampled from four typical artificial defect models in Laboratory are adopting for testing of the proposed feature extraction algorithm. It is shown that the success rate by FkNN and BPNN are all higher than 80%, and FkNN has superior classification accuracies than BPNN under four circumstances of (d1, d2) combinations. In addition, FkNN achieves the highest classification accuracy 93.73% with (10, 5) combination. The results demonstrate that it is feasible to apply the proposed algorithm to PD signal classification.**

__A NEW HYBRID FEATURE EXTRACTION METHOD FOR PARTIAL DISCHARGE SIGNALS CLASSIFICATION__**In order to analyze the type of XLPE cable partial discharge, and grasp characteristics of partial discharge caused by different defects. In this research, they use a 10kV 30-meter-long XLPE cable and design different types of defects. Then they collect the PD data of different defect model by PD data acquisition system. To characterize Partial discharge signals generated by different defect model by use of discharge capacity qs, discharge times n , the average discharge amount qn and phase distribution function include the maximum discharge amount distribution HQmax(φ) , the average discharge amount distribution HQmean(φ) , the discharge times phase distribution HN(φ) as well as φ-Q-N distribution map and so on. The results show that the partial discharge signals produced by different defect model have different statistical characteristics.**

__PATTERN RECOGNITION OF PARTIAL DISCHARGE IN XLPE CABLE BASED ON PHASE DISTRIBUTION__**The presence of metallic particles can adversely affect the reliability of Gas-Insulated Substation (GIS) by initiating partial discharges (PDs). Therefore, the investigation of PD characteristics and particle size and position on the spacer surface are the significant steps toward the reliability improvement of the GIS equipments. This paper presents the use of Back-Propagation Artificial Neural Network (BP-ANN) technique supplemented with Principal Component Analysis (PCA) as the PD pattern recognition tools for the estimation of the particle size (length) and position on the spacer surface in a simulated GIS. PD features acquisition was performed by collecting their fingerprints from the measurements carried out using IEC 60270 method. The role of PCA is to reduce the dimension of the collected PD fingerprint data. The obtained results show that PCA can significantly improve the BP-ANN performance in terms of execution time. Without PCA, 88% and 92% accuracies can be achieved when BP-ANN was implemented with 1 and 2 hidden layers, respectively. With the integration of PCA, execution times were greatly reduced while retaining fairly high accuracy, i.e. 88% and 88%. Thus the proposed method is a contribution in the development of the tool for improving the reliability of GIS.**

__PARTIAL DISCHARGE ANALYSIS USING PCA AND ANN FOR THE ESTIMATION OF SIZE AND POSITION OF METALLIC PARTICLE ADHERING TO SPACER IN GAS-INSULATED SYSTEM__# PARTIAL DISCHARGE PATTERN RECOGNITION VIA SPARSE REPRESENTATION AND ANN

In this study, seventeen samples were created for classifying internal, surface, and corona partial discharges (PDs) in a high voltage lab. Next, PDs were measured experimentally to provide a dictionary comprising the types. Due to the huge size of the recorded dataset, a new and straightforward preprocessing method based on signal norms was used to extract the appropriate features of various samples. The new sparse representation classifier (SRC) was computed using ℓ 1 and stable ℓ 1 -norm minimization by means of Primal-Dual Interior Point (PDIP) and Basis Pursuit De-noise (BPDN) algorithms, respectively. The pattern recognition was also performed with an artificial neural network (ANN) and compared with the sparse method. It is shown that both methods have comparable performance if training process, tuning options, and other tasks for finding the best result from ANN are not taken into account. Even with this assumption, it is shown that SRC still performs better than ANN in some cases. In addition, the SRC technique presented in this paper converges to a fixed result, while the results after training the ANN vary with every run due to random initial weights.- Summary

# Chapter 3: Proposed Methodology

In this research, analysis on the PD classification pattern based on the 5 defects which usually occur on site in actual XLPE cable joints. In order to obtain high level significance input while discarding the low level significance input, feature extraction was proposed which is the Fisher’s Ratio Feature Selection. It is vital to perform feature extraction on the input data because it helps to increase the accuracy when those inputs were used as training for different classifier in artificial intelligence. After extracting out the useful input features, those input features will be fed to various artificial intelligence classifier to identify multiple PD patterns. Different classifier will bring different result and the best result will be chosen as the best work in classifying PD pattern in this research. Normally, input feature without noise contamination will be only fed to the classifier for training but in this research, input feature with noise contamination will be used in order to bring this up a level so that this experiment will be similar to the event which is happening on site. All the simulation in this research will be done using a software which is MATLAB. Since MATLAB contains Artificial Neural Network toolbox which provides algorithms, pre-trained models, and apps to create, train, visualize, and simulate both shallow and deep neural networks. It is possible to perform classification, regression, clustering, dimensionality reduction, time-series forecasting, and dynamic system modeling and control using MATLAB.**Gantt Chart**

**Figure 7: Gantt Chart outlining FYP work. The above Gantt Chart is the outline of the task during the whole period of completing this research. The project started in May 2017 and it will last until January 2018. The Gantt Chart covers progress from the very beginning which is preparation on project title until final submission of final year project dissertation. This outline will serve a guide as the research progresses so that I will be able to complete all the required tasks and objectives on time and complete my final year project.**

**Flow Chart**Figure 8: Flowchart of general flow of the program

**The flowchart above represents the overall program’s framework. Generally, the framework will be as Figure 8 but the in each step, the works need to be very detail and precise in order to generate an efficient program. Firstly, the obtained PD data need to be imported into MATLAB in order to be able to work with it, then using MATLAB coding, a feature extraction which is Fisher’s Ratio Feature Selection will be performed. The next step would be checking the optimum value of high significance data input in order to feed into artificial intelligence classifiers. Multiple artificial intelligence classifiers will be tested in order to get the highest accuracy in classifying different PD patterns. Data with noise contamination will be used in order to test the capability of feature extraction and classifiers working under noise contamination.**

# Chapter 4: Conclusion

**In conclusion, there are lots of works and research have been completed on the recognizing different PD pattern using different techniques. Artificial Neural Network has been a very popular thing in the field of partial discharge as there are many works used ANN in recognizing PD patterns. As for Fisher’s Ratio Feature Selection, there are still no works which use Fisher’s Ratio to do feature extraction, thus it would be wise and interesting to propose this method in this project. Furthermore, Fisher’s Ratio Feature Selection is popular in performing classification and reducing large dimensionality of data which is what I aim for and therefore this will be a good investigation to perform.**

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