An Electrical Insulator Defects Detection Method Combined Human Receptive Field Model

Nondestructive inspection of electrical insulators subjected to the high electrical stress and environmental damage is fundamental for reliable operation of a transmission lines. The breakage and defect of the insulator have great influence on the safe of transmission lines, and insulator defect detection with difference types is a complex work. This paper proposed an insulator defect detection method inspired by human receptive field model, which meets the requirements for detecting defect insulator in a simple background. In thismethod, the defect detection combined human receptive fieldmodel of human visual system is constructed and applied on the different insulators, so as to achieve accurate detection of the insulator defected parts. Experimental results show that the method can accurately and robustly detect the defect (such as cracks and damage) of electrical insulator in case of noise affect.


Introduction
High-voltage transmission lines and transmission towers in China are usually in a harsh environment where their components are often eroded by rain or damaged by unpredictable foreign objects.When an electrical insulator is in operation at the high-voltage transmission line systems, those devices are subjected to a strong electrical stress and also damaged by the severely environmental conditions [1].It is well known that the presence of voids and inclusions are introduced in the manufacturing process, or generation and propagation of defects, such as holes and cracks inside and outside of the electrical insulators, when those devices are in operation under high-voltage stress, a partial discharge begins as localized dielectric breakdown.Furthermore, partial discharge can also occur along the boundary between different insulating materials, ultimately leading to electrical breakdown and, eventually, explosion with stop of energy transportation [2][3][4].Therefore, the defect of electrical insulator is an important reason causing electrical network accident, so it is a crucial work to detect the defect of insulator periodically and minimize interruption of energy accidents.The traditional way of monitoring and troubleshooting is to conduct manual inspection of the videos and images of the transmission lines photographed by the helicopter.But it is too time-consuming and labor-intensive to get efficient defect detection, usually easy to make wrong judgements because of the large amount of data.At present work, D. Zuo et al. used SVM classification to detect missing insulators [5].Qaddoumi et al. introduced artificial neural networkbased near-field microwave to accurately test damage of outdoor insulators [6].Armando et al. detected the cracks and operation defects in the insulators and breaker by a portable low cost computed radiography systems [7].Chen et al. extract single insulator disk in infrared image of insulator string [8].Huang et al. proposed some back propagation neural network method for equipment fault diagnosis [9,10].However, the above methods are operated in the special imaging situation to achieve the insulator defect detection, it has great limitation to the natural captured insulator images in damage detection.With the developing of highspeed digital imaging and processing technique, intelligent identification technology began to be used in high-voltage safe operation and long-distance power transmission lines inspection [11][12][13][14][15][16]. Figure 1 is the illustration of electrical safe operation and transmission inspection, the electrical insulator (glass/ceramic material) image is captured by UAV (unmanned aerial vehicle), which is suitable for several kinds electrical safe inspection.In view of the above details, this paper proposed an electrical insulator defect detection method inspired by human brain receptive field (RF) model, aiming to achieve accurate detection including defect, crack even dirty of different damaged insulators, so as to improve safety and reliability of high-voltage transmission line.Experimental results show that this method can quickly and accurately detect whether the insulator is broken or damaged.
The paper is organized as follows.In Section 2, we detailed the biology basis of human brain RF and the proposed method combined RF model.Section 3 discussed important parameter selection and experimental performance evaluation.Finally, we give a brief summary and draw a conclusion in Section 4.

Biology Basis of the RF Model.
A large number of biological experiments show that the primary visual pathway of the human brain can obtain the main information of the object and play a key role in the overall perception of the object [17,18].In Figure 2(a), visual information reaches the eye through the cornea and pupil, passes through the photoelectric conversion of photoreceptor cells in the retina, then passes into the lateral geniculate nucleus (LGN), and projects to V1 area, finally creating visual perception of the brain [19,20].
LGN is an important step to V1 area of HVS, Figure 2(b) represents two types of LGN RF model, that is, center-on ("+") and center-off ("-") cells.In these cells, the central region is excitatory (marked by red in center-on and blue in center-off) and the surrounding is inhibitory (color opposite to excitatory area), respectively.From Figure 2(b), it can be seen that the center of the on-type is surrounded by off-type and the center off-type around is surrounded by on-type.Both of these cells are arranged and their total number is substantially equal and detect contrast changes.Some researchers have proved that the on-type and off-type channels are located between the LGN and the visual cortex is fully parallel separation [21,22].The process of LGN receptive field is defined as a 2D Gaussian function in the following: In this equation,  and  represent spatial variables respectively,  represents the type of cell (that is sensitivity of excitory and inhibition area of RF), which is "+" or "-",  is a mapping function of , when  is "+", a center-on RF cell is defined as () = , else () is defined as the negative of (), that is () = −, so a center-off RF cell is  −  (, ) = − +  (, ). is the standard deviation of the outer Gaussian function, which represents the size of excitatory and inhibitory area of RF, usually is set by 0.5, which is in accordance with electron-physiological findings of LGN cells in mammals.

Construction of the Detection Method with RF Model.
For the above details, the proposed electrical insulator defect detection method is inspired by human brain RF model, which is followed in [21].Figure 3 shows the relationship of above described LGN cell and the main information of an image (object).In Figure 3, center-on LGN RF cell is marked by green and center-off LGN RF cell is marked by red, V1 simple cell RF is marked by blue circle.The center of RF is positioned on an edge (marked by points) which gives rise to model LGN cell responses rendered in the left two columns of Figure 3.The eight small points represent the RF centers of eight subunits, four of center-on (in green), and four of center-off (in red) type.For the considered example, it results in the inclusion of eight subunits in the model.The number of subunits depends on the number of circles we considered and the specific input pattern presented at the time of configuration.In this insulator defect detection method, each subunit included is represented in parametric form by a tuple (, , , ), where the parameters  represent the polarity of subunit,  is the scale of the involved model LGN cells, the radius , and the polar angle  of the RF center of the subunit relative to the RF center of the model cell [21,23], respectively.
We denote by  = {(  ,   ,   ,   ) |  = 1 ⋅ ⋅ ⋅ } 4tuples of the configured subunits of model.According to the right three subparts in Figure 3, we explain the general configuration process of the proposed detection method in more detail below.A cell group computes the sum of the weighted responses of the model LGN cells it receives input from.These models LGN cells have the same polarity (on or off), RF size and neighboring RFs.In this way, a subunit detects contrast changes, similar to a model LGN cell.A subunit can be thought of as a dendrite branch of a simple cell which receives input from a pool of adjacent LGN cells.The concentric receptive field of a single LGN cell can be characterized as (1).For an LGN cell that located at coordinate (, ), the response can be obtained by convolving the intensity distribution function of the image with the cell receptive field, that is represented by the following: Considering the different  may lead to    (, ) being negative value, but the self-firing rate is positive in biology, the nonnegative half-wave operation [22,24] is adopted to obtain single LGN cell response as follows: All the feature characteristic LGN cells can be combined to a group, which is defined as follows similar to [25].
In ( 4),  represents the quality of cell group included in each cell group.For each specified (  ,   ,   ,   ),  and  represent the single cell relative to the center of cell group.For the LGN cell group located at (, ), the response is described as follows: The parameters satisfied the following conditions:   Define  ,  as the simple cell response of the direction   , which is determined by all cell group response in the defined region.In Hubel-Wiesel's work, only the response of all LGN cell group included in given region can excite simple cell.
In order to show the biological nature, all of the cell group responses are multiplied to get geometry average, so as to represent the simple cell response, which is mathematical explained in the following: In the above equation, it is satisfied as   =  −  /2 2 and   = (1/3)max ∈{1,2,⋅⋅⋅ ,||} {  }, the direction function is defined as  = {(2/) | 0 ≤  < },  = 0, 1, ⋅ ⋅ ⋅ , , all of these simple cell responses are combined to get the overall response of V1 area in the following:

Key Parameter Selection.
The appropriate selection of parameter  is a key step in the proposed detection method, because the structure of the electrical insulator is relatively simple, in order to show the detection efficiency by different parameters on insulator defect detection.In order to verify the parameter selection difference, we choose the image as an example from RUG (university of Groningen) image dataset [20].Because the texture of the natural image (such as basket) is more complex and more detailed than a simple insulator image, as shown in Figure 1.The configured detection model cell response to a natural image is illustrated in Figure 4.When n=4, there are a large number of missed detections of the model and it is difficult to recognize the basic appearance of the object from the output information.The basic appearance of the object can already be basically resolved when n=8, but the included edge information is still incomplete.The output of the model contains relatively complete edges when n=12, which can clearly identify the appearance of the object.The last one in Figure 4(g) is n=16, the model will get more complete local contour information.As the above analysis, the parameter is selected as n=16 in our experiment.When the orientation selection is n=16, some simple cell with different orientations are shown in Figures 4(h)-4(l).This illustration proved that a single simple cell just sensitive to the specific orientation, different orientation cell groups can better describe various redundant detailed features.

Experimental Results.
In this section, the evaluation performance of the proposed model in the electrical insulator defect detection task is shown.Although several types of insulators are used in China, this paper only showed two main types of insulators: one type is the white ceramic insulator and the other is the dark red insulator.
For the reason of lacking public electrical insulator dataset, all of the electrical insulators used in our experiment are from our team's captured dataset.The dataset includes   In addition, except for the subjective experimental result, objective evaluation result such as BRISQUE metric [26] is also provided here.Objective experimental evaluation value reflects that the insulator defect detection is more acceptable, it is easy to achieve further fault judgement.

Robust Detection.
Experiments in Section 3.2 are electrical insulator captured in a good natural condition, the image is clear and clean.However, the electrical insulator has been exposed in outdoor electricity transmission line for a long time in normal situation, so the captured insulator image including noise.In order to verify the robust of our method, the experiment is added in this section.

Conclusion
The application of image processing and machine learning method is a popular development trend in future electricity power transmission line inspections.Inspired by human brain visual pathway LGN and V1 simple cell RF characteristic, this paper proposed an electrical insulator defect detection method combined computational model in area V1 of visual cortex, different feature orientation selectivity is achieved by combining operation of a collection of LGN cells with center-surround RF.Demonstration of different experimental results shown that the proposed method can achieve accurate electrical insulator defect detection, even complete robust detection of noisy insulators.Because the background of the insulator image acquired by aerial photography is complicated and there is more than one insulator type, defects are difficult to detect, so that our further work will focus on optimizing this method, identifying and locating the insulator defect by segmentation, and extending it to more wide application.The aim is greatly improving the efficiency of electricity power transmission line and easy to find defect or fault in the system, so as to provide completely and timely guarantee for electricity grid dispatching and electrical equipment maintenance.

Figure 4 :
Figure 4: Selection of the appropriate parameter n and orientation comparison.

Figure 5 :
Figure 5: Some electrical insulator images in our dataset.

Figure 4 (
a) is the original image, Figures 4(b)-4(d) are the basic contour of original image in various parameters condition, and Figures 4(e)-4(g) are the binary detection result by different parameters, which takes 8, 12, and 16, respectively.

(Figure 8 :
Figure 8: Robust detection test of original circle and different noisy insulator.

Figure 6
shows defect detection result of electrical insulator with simulated crack.Figures6(a1) and 6(b1) are real original ceramic electrical insulators.Figures 6(a2) and 6(b2) give the edge detection of original ceramic electrical insulators without crack.In Figures6(a3) and 6(b3), there is a simulated crack located on the surface of real ceramic electrical insulator respectively.Figures6(a4) and 6(b4) clearly show the crack and achieve accurate defect detection.(B)Real Defect Detection.In Figure7, four real damaged ceramic electrical insulators are used in our experiment.Figures 7(a1)-7(d1) are real original damaged insulator images from our dataset, we can see that there are obviously damaged on the surface of electrical insulators.Figures 7(a2)-7(d2) is the defect detection result of these damaged electrical insulators.In order to further provide the efficiency of the proposed method, the results of Figures 7(a2)-7(d2) are reversed operation to show accurate detection result, which is shown in Figures 7(a3)-7(d3) and marked in red rectangle.

Figures 8 (
a1)-8(c1) represent the original clean circle image, the noisy circle image additive with standard deviation is 0.01, and the noisy circle image additive with standard deviation is 0.05.The damaged part of the insulator is marked by red rectangle.The type of noise is Gaussian noise.In Figures 8(d1)-8(f1), we can see the detection result of fore-referred circle images.Figures 8(a2) and 8(a3) are noisy version of the same electrical ceramic insulator (the standard deviation is 0.01 and 0.05, respectively).Experimental results shown in Figures 8(b2) and 8(b3) are the detection results; the damaged part of the noisy defect electrical insulator is accurately detected and it is very clear.For the purpose of further show the robust detection result, Figures 8(c2) and 8(c3) are operated reversely in order to show more clear damaged part of the electrical insulator.