Research on Electric Field Detection of Degraded Insulators Based on a Sensitive Detection Method under Complex Operating Conditions

Abstract

Insulators are the basis of the stable operation of transmission lines, and it is particularly important to regularly detect the insulation performance of insulators. After several years of research, contactless space electric field detection technology for degraded insulators, based on an unmanned aerial vehicles (UAV) platform, has developed rapidly. However, the existing technology is only limited to the typical vertical/horizontal state research category and cannot apply to complex operating conditions such as attitude disturbance and path tilt. This paper builds a simulation model of insulators at a voltage level of 220 kV by COMSOL finite element software and analyzes the space electric field distribution characteristics of degraded insulators and the influence of different surface conditions, attitude disturbance, path tilt, etc., on the space electric field distribution of insulators. To reduce the error of UAV detection under complex operating conditions of insulators, a method for detecting the electric field of degraded insulators based on sensitive insulators is proposed and verified by experiments. According to the research results, surface conditions of insulators have little influence on their space electric field, while the attitude disturbance and path tilt of UAVs have a great influence on the electric field of insulators. Sensitive insulators at a level of voltage below 220 kV are generally located at the low-voltage end. The principles of sensitive insulators are combined with the space electric field measurement method to measure the space electric field distribution, thus effectively judging whether there are degraded insulators in the string. The method specified in the paper greatly improves the accuracy and detection efficiency of the contactless electric field detection of insulators, which is of great significance for the promotion of the electric field detection method of insulators in practical applications.

1. Introduction

The insulator is the most important link in transmission line equipment [1,2,3,4]; the stability of the insulator’s insulation performance seriously affects the safe operation of the power grid, and the regular deterioration detection of insulator strings is particularly critical [5,6].

At present, the most common detection methods of degraded insulators are infrared detection methods, ultraviolet detection methods, and electric field detection methods [7]. In the infrared detection method, zero-value insulators basically generate no heat, so it is impossible to judge a significant difference between zero-value insulators and normal insulators. Only the heating of low-value insulators can be used for judgment [8,9]. In the ultraviolet detection method, the corona discharge of insulators can be detected by ultraviolet methods. It is impossible to judge the situation without corona discharge [10,11]. In addition, infrared and ultraviolet detection methods easily suffer from adverse effects of environmental factors [12,13,14], such as temperature and the humidity of the air, which make it more difficult for insulator strings in transmission lines to obtain accurate detection results, thus causing misjudgment and becoming a hazard for transmission line safety.

The electric field detection method is used to detect degraded insulators by judging the space electric field waveform of insulator strings. Under typical conditions, the space electric field curve of insulators is a curve similar to a U shape. Whether the insulator strings are degraded can be detected by observing its space electric field waveform [15,16]. In the case of de-graded insulators among insulator strings, the electric field distribution of insulators will change, resulting in distortion [17,18,19]. The literature [20] has conducted research on the electric field strength of insulator strings with different positions and numbers of degraded insulators by the finite element method. According to the results, the degradation of insulators at a high-voltage end has the greatest influence on the electric field of insulators. Another work [21] established an electric field simulation model of 110 kV porcelain suspension insulators and conducted a space electric field experiment of insulators to prove that the live detection of degraded insulators can be completed through the measurement of the space electric field. The literature [22] has also explored the space electric field distribution characteristics of suspension insulator strings and indicated that degraded insulators can be located by analyzing the change of the axial electric field of insulator strings.

According to the electric field characteristics of insulators, electric field detection methods of insulators under typical conditions are of the contact type and contactless type [23]. Most electric field detection devices for contact insulators are climbing tower robots, but they are expensive and time-consuming, with a threat to personal safety [24,25]. The contactless detection method of insulators has very high application prospects. Researchers [26] have built an inspection platform of UAV for both the airborne end and the ground end of the UAV and detected a conduction defect of the composite insulator mandrel. Others have [27] developed a space electric field detection device and a zero-value insulator detection algorithm, and the proposed method can effectively identify zero-value insulators. At the actual site, due to the influence of regional climate characteristics, extreme terrain tower erection, insulator string weight, etc., there may be differences in the electric field characteristics of insulators, which also poses a challenge to the detection method of space electric field of insulators. However, most research on the contactless electric field detection methods of insulators is still limited to the typical vertical/horizontal state, which cannot be applied to complex operating conditions such as the change of the insulator surface state, attitude disturbance, and path tilt, which makes it difficult for the extensive popularization and application of current electric field detection methods.

To improve contactless electric field detection methods of degraded insulators to obtain more accurate measurement results, this measurement method conforms to field operating conditions. According to the space electric field distribution characteristics of porcelain insulator strings, this paper establishes a 3D simulation model of insulators by COMSOL finite element software and explores the influence of the electric field characteristics of degraded insulators and complex operating conditions on the space electric field of insulators, and an optimization strategy of the detection method under complex working conditions is proposed. Based on the equivalent circuit method, the theoretical circuit structure of insulators was analyzed, and the electric field judgment method of sensitive insulators applicable to complex operating conditions was obtained from the essential circuit structure of insulators. The accuracy of this method was verified by a simulation test of degraded insulators.

2. Electric Field Characteristics of Insulators under Complex Operating Conditions

The section first discusses the electric field characteristics of degraded insulator strings and then explores the electric field distribution characteristics of insulators under complex operating conditions in three parts (surface state, attitude disturbance, and space electric field simulation of insulators under path tilt) according to the erection mode of insulators under actual working conditions.

2.1. Establishment of Insulator Model

The XWP2-70 double-umbrella porcelain insulator string was modeled by SolidWorks software, and specific parameters are shown in the following

Table 1. Structure parameters of insulator.

Model		Parameter		Outline/Structure
	Height/mm	Umbrella Diameter/mm	Creepage Distance/mm
XWP2-70	146	255	400

and

Table 2. The relative dielectric constant of each dielectric material.

Dielectric material	Air	Porcelain	Cementing agent
Relative dielectric constant	1	5.71	14

.

During modeling, the influence of tower cross arm, transmission line, and other factors are considered; models are then combined into an assembly and imported into COMSOL finite element software for simulating the calculation of the space electric field of insulators. In the simulating calculation, the electric field solution of the insulator model surface and its surrounding air needs to satisfy the following electric field equation:

$$\nabla \times E=0$$

(1)

$$\nabla \cdot \epsilon E=\rho$$

(2)

where E is the electric field strength of the medium area (V/m), ε is the dielectric constant of the medium, and ρ is the electric charge volume density (C/m³). After the influence of the space charge on the simulation is ignored in the electric field simulation, the potential function φ obtained satisfies the following requirements:

$${\nabla }^2\phi =0$$

(3)

During the simulating calculation, type 1 boundary conditions adopted for the test model are:

$$\phi |{}_{{\mathrm{\Gamma }}_0}=0$$

(4)

$$\phi |{}_{{\mathrm{\Gamma }}_k}=U_0$$

(5)

where Γ₀ represents the boundary and ground of the air domain; in this paper, the infinite element domain is used as the infinite boundary of the air domain for the problem of the open field domain. Γ_k represents the high-potential simulation model, and U₀ represents the potential of the conductor, which is set as the peak phase voltage of 220 kV, that is, 179.63 kV. Equations (3)–(5) constitute the boundary conditions of the electric field simulation model.

2.2. Insulator Degradation Electric Field Curve

To compare changes of the surrounding electric field when insulators in different positions are degraded, by changing the relative dielectric constant and conductivity of the materials, the high-voltage end, the medium-voltage end, and the low-voltage end of the insulators are set to zero values, successively and respectively, and the degradation curve of the insulators obtained by simulation calculation is shown in the Figure 1 below.

According to the insulator degradation curve, when the insulator string is degraded at the high-voltage end, medium-end voltage, and low-voltage end, the value of the electric field in the surrounding space is significantly reduced. The field strength of the high-voltage end degradation point is 67.8 kV/m, with a decrease of 16.7% compared with normal; the field strength of the medium-voltage end degradation point is 29.7 kV/m, with a decrease of 9.8%; the field strength of the low-voltage end degradation point is 28.2 kV/m, with a decrease of 15.2%. It is obvious the space electric field distortion is the most obvious at the high-voltage and low-voltage ends of the insulator string. The traditional electric field distribution detection method is to collect electric field information of insulator strings through a fixed detection path and compare the electric field curves of insulator strings with good performance and with degradation to make judgments.

2.3. Influence of Insulator Surface State on Space Electric Field

To prevent pollution flashover accidents of transmission lines, room temperature vulcanized (RTV) coating is usually sprayed on the surface of transmission line insulators in heavily polluted areas to improve the external insulation level. The simulation model is shown in the following Figure 2 and Figure 3.

According to the voltage nephogram of insulators, whether the insulator is coated basically does not affect its voltage distribution. The possible reason is that the voltage distribution of insulators is mainly affected by capacitance parameters, and the RTV material on the insulator surface causes no change of insulator capacitance.

The RTV coating on the insulator surface will appear to be aging with the increase of the operation life, which is manifested as the change of the dielectric constant of coating; it is necessary to consider the influence of this situation on an electric field. The simulation comparison results of the space electric field curves of aged RTV insulators, newly coated RTV insulators, and uncoated insulators are as follow Figure 4.

According to the electric field curve of RTV insulators, the probability of whether the insulator string is coated with RTV and the aging of RTV makes the maximum space electric field strength of the insulator strings decreases to a certain extent (0–4%), but the influence is very small. Therefore, coating RTV materials on the surface of insulators will not affect the accuracy of the electric field detection of degraded insulators, and contactless electric field detection based on degraded insulators can be applied to different surface states of insulators.

2.4. Space Electric Field Distribution of Insulators under Attitude Disturbance

In the actual line, when UAV and other contactless equipment are used for insulator patrol inspection, the path may actually joggle under the influence of crosswind and other factors. As for the traditional electric field detection method, it is difficult to make all insulators at the same detection distance, so it is necessary to study the influence of the attitude disturbance of detection devices on detection results.

As shown in Figure 5, assuming the detection distance is 300 mm, the red line in the figure simulates the actual operation curve of UAV patrol inspection, where Δx represents the offset of UAV and other detection devices due to actual site reasons. The simulation results of the space electric field of insulator strings are as follows Figure 6.

According to the simulation curve, the detection distance is 300 mm. In the case of path joggle at the high-voltage end, the space electric field of the insulator string changes the most. When the path offset distance is 50 mm, the corresponding insulator space electric field change rate is about 8.6%; when the path offset distance is 30 mm, the corresponding insulator space electric field change rate is about 5.9%; the change rates of the space electric field of an insulator at a path offset of 50 mm and 30 mm in the middle of an insulator string are about 5% and 3.3%, respectively; the change of measurement results caused by path goggle at the low-voltage end is the smallest, and the change rates of the space electric field of insulators are about 0.9% and 0.54%, respectively, when the path offset distances are 50 mm and 30 mm. In conclusion, path joggle affects the distance between the detection device and the insulator string to be detected, thus further affecting the reliability of detection results. Therefore, in the process of detecting degraded insulators by contactless equipment, the traditional detection method shall be improved, and the offset distance of the UAV detection path will be controlled within 50 mm to prevent misjudgment.

2.5. Space Electric Field Distribution of Insulators under Path Tilt

As shown in Figure 7, in the practical application scenario of the insulator strings of overhead transmission lines, due to comprehensive stress, installation mode, and other factors, insulator strings may be subject to “sagging” similar to the conductor, while the traditional detection path of UAV is mostly straight, so it is necessary to analyze the influence of insulator string “sagging” on the detection results.

In the Figure 8, it is assumed that the detection distance is 300 mm, the red line represents the actual detection path of the detection device, and the blue line represents the “sagging” of the insulator string due to stress and the actual electric field detection path to be measured due to the natural sagging of the insulators. This situation is bound to have a certain influence on the electric field measurement results. This paper sets “sag angle” δ to 165° and proposes a method of adding path interest points at the bottom of the arc to reduce the influence. The simulation results are as follows Figure 9 and Figure 10.

It can be known through a simulation study that under the detection distance of 300 mm, when the “sag angle” δ is set to 165 and the detection path is straight without considering sag, the electric field strength of the first to tenth insulator strings is higher than of the detection path considering sag, and natural sag in the middle of insulator strings has significant influences on the electric field distribution of the insulator string, with a maximum deviation of about 4.8%. Therefore, in the process of electric field detection with contactless devices, it is necessary to improve detection means, thus avoiding the natural drop in the middle of insulator strings affecting electric field detection results. Combined with the simulation results, it can be seen that the maximum error of the measured electric field distribution is reduced to 0.4% after optimizing the path. Therefore, the error caused by sag can be effectively reduced by increasing path interest points.

3. Electric Field Detection Method of Degraded Insulators Based on Sensitive Insulators

The space electric field detection method of insulators can be applicable to different surface states; however, due to the attitude disturbance, path inclination, and other complex working conditions, if the traditional contactless electric field detection method is used for electric field detection, it will affect detection results. Therefore, it is necessary to improve the traditional detection method.

3.1. Criterion for Sensitive Insulators

Assuming the insulator string is dry and free from pollution, with good insulation performance, infinite resistance, and no leakage conductivity considered, the insulator string is equivalent to the pure capacitor chain, and the equivalent circuit diagram of insulators is as follows Figure 11.

The capacitance of insulators in the figure is C_s1 = C_s2 = … = C_sn = … = C_s0; the ground capacitance is C_g1, C_g2,…, C_gn…; the capacitance to the transmission line is C_l1, C_l2…, C_ln…; and the voltage at the high end of the insulator string is set to U. If the voltage borne by m insulator(s) is V, Kirchhoff’s law is used for any insulator C_sn as follows:

$$C_{\mathrm{s}0}(e_{n+1}-e_n)-(C_{gn}+C_{ln})V_n+C_{ln}V=0$$

(6)

where V_n and e_n represent the potential and voltage borne of the nth insulator.

Suppose the sum of C_gn and C_ln is independent of n. In case of an assumption of Equation (7), Equation (6) is solved [28]:

$$\{\begin{array}{l}\frac{C_{En}}{C_0}=\alpha \\ \frac{C_{Ln}}{C_0}=\beta \\ \alpha +\beta ={\gamma }^2\end{array}$$

(7)

so the potential distribution of insulator string can be obtained, as shown in Equation (8):

$$V_n=V\frac{\sinh n\gamma }{\sinh m\gamma }+\frac{V\beta }{{\gamma }^2}\left[1-\frac{\sinh n\gamma +\sinh (m-n)\gamma }{\sinh m\gamma }\right]$$

(8)

According to Equation (8), if one of the insulator strings is a degraded insulator, its capacitance C₀ decreases, resulting in a significant decrease in its voltage, which is bound to redistribute the voltage in the whole insulator string, thus causing a difference between the voltage borne by all insulators and the voltage with no degradation and an increase in the space electric field strength of other insulators. At the same time, if degraded insulators appear at different positions in the insulator string, the difference between voltages is also varying.

Therefore, in the case of degradation in the insulator string, regardless of the number of degraded insulators, the electric field around some insulators always changes greatly due to degradation, which is the most sensitive from the whole insulator string, so these insulators are called sensitive insulators. If the space electric field of a sensitive insulator is measured and compared with the space electric field at the position without degradation, the insulation performance of the insulator string can be judged. According to Equation (8), when the sensitive insulator itself is deteriorated, the voltage borne by it decreases, resulting in the decrease of the space electric field; if the degraded insulator is located in other positions, the voltage distributed by the sensitive insulator will increase to a certain extent; therefore, the location of the degraded insulator can be accurately determined whether or not it is in the sensitive insulator group.

3.2. Detection and Test Platform of Degraded Insulators

In order to explore the electric field characteristics of sensitive insulators and improve the contactless detection method of degraded insulators, a 220 kV line insulator detection platform was built. The insulator string was composed of 15 insulators. An optical electric field sensor based on the Pockels effect was installed at the front end of the insulating rod to detect the spatial electric field strength of the insulator string [21]. The schematic diagram of the detection site and test are as follows Figure 12.

The insulators were numbered from the conductor side to the cross arm side by #1–#15. During the test, the #1, #3, #5, #7, #9, #11, #13, and #15 insulators were replaced with zero-value insulators with a resistance of 2 MΩ. The horizontal distance between the electric field probe and the insulator string was 30 mm, and the space electric field of the insulator string was measured. Measurement results are shown in the table below.

According to

Table 3. Spatial electric field distribution when zero-valued insulators are included.

Zero-Value Position		Space Electric Field Strength (kV/m)
	No Degradation	#1	#3	#5	#7	#9	#11	#13	#15
1	88.7	83.5	88.5	88.0	89.5	89.7	89.9	90.8	88.6
2	76.6	82.2	70.6	75.0	77.8	78.2	78.8	78.6	76.0
3	66.5	73.2	61.6	66.4	67.0	68.1	68.8	68.2	66.5
4	58.3	65.2	58.8	60.1	59.1	59.9	60.7	60.9	58.3
5	50.5	54.9	50.8	45.3	51.9	52.0	52.5	52.1	51.5
6	42.3	46.9	43.5	45.1	38.9	43.3	44.8	44.6	42.8
7	36.9	41.3	38.6	39.6	31.3	36.6	39.2	38.8	37.9
8	30.8	35.4	33.5	32.4	31.5	30.0	32.4	31.6	29.7
9	26.9	29.7	28.7	29.4	26.6	22.1	27.1	27.0	26.4
10	25.8	27.5	27.2	26.6	25.1	27.4	29.3	25.6	25.8
11	25.1	24.8	25.3	25.7	24.9	26.3	20.3	22.5	24.1
12	27.8	28.4	28.5	27.0	29.4	28.6	27.5	28.5	27.8
13	31.5	33.1	34.4	33.3	33.4	33.1	33.2	30.1	33.6
14	35.4	39.3	37.3	37.2	37.5	38.0	36.4	37.4	38.3
15	40.4	44.5	42.6	42.5	42.8	42.7	42.5	43.0	38.1

, in the case of degradation in the insulator string, it will affect the space electric field of degraded insulators and other insulators. Moreover, in the case of degraded insulators at the high-voltage end, due to the large voltage borne by the high-voltage end, the voltage on the insulator string will be redistributed after degradation, resulting in obvious changes in the space electric field at all insulators in the insulator string. In the case of degraded insulators at the middle-voltage end and the low-voltage end, the space electric field at other insulator positions had few changes due to small voltage borne by degraded insulators.

To intuitively reflect the change of an insulator electric field, the influence of degraded insulator position on space electric field distribution of insulators was analyzed through the change rate of space electric field strength σ_i,j:

$${\sigma }_{i,j}=\left|\mathsf{\Delta }{E}_{i,j}^f-\mathsf{\Delta }{E}_i^s\right|/\mathsf{\Delta }{E}_i^s$$

(9)

where i represents the position of insulators, j represents the position of degraded insulators (i, j = 1–15), f represents the insulator string with degraded insulators, and S represents the insulator string with good insulation performance.

After many tests, the change rate of space electric field of several insulators near the low-voltage side was more stable and closer to meeting the conditions of sensitive insulators. The test results are shown in the

Table 4. The rate of change in the field strength of the low-voltage end space when 1 zero-value insulator was included.

Zero-Value Position j	Rate of Change of the Space Electric Field of Insulators at a Low-Voltage End
	σ13,j	σ14,j	σ15,j
1	0.051	0.110	0.101
2	0.083	0.078	0.088
3	0.092	0.054	0.055
4	0.078	0.054	0.064
5	0.057	0.051	0.052
6	0.054	0.054	0.058
7	0.060	0.059	0.059
8	0.057	0.041	0.061
9	0.050	0.073	0.057
10	0.037	0.053	0.051
11	0.054	0.028	0.052
12	0.057	0.016	0.053
13	0.076	0.056	0.064
14	0.044	0.071	0.062
15	0.067	0.082	0.057

below.

According to Table 4, no matter where (any position of #1–#15 insulator strings) the zero-value insulator is set, there shall be at least two σ_i,j with more than 5.0% in the last three insulators at the low-voltage end. Based on the criterion for the sensitive insulator method in Section 2.1, three insulators of the insulator string close to the cross arm side can be judged as sensitive insulators. Therefore, under a voltage level of 220 kV, the sensitive insulator of the insulator string is generally at the low-voltage end, and during detection, the space electric field strength of the sensitive insulator set can be collected first to judge whether this insulator string is degraded.

4. Detection Process of Degraded Insulators Based on Time-Domain Waveform of Space Electric Field

Based on the above simulation and test results, this paper improves the traditional insulator electric field detection method and puts forward the optimization strategy for spatial electric field detection under the actual operating conditions of insulators.

Firstly, by analyzing the test results in Section 3, it can be seen that the deteriorated insulator will have an impact on the electric field distribution of the insulator string. For the 220 kV insulator string, the electric field distribution of the last three insulators is the most sensitive to the perception of insulator deterioration at any position in the string, so it is defined as a sensitive insulator set. The pre-inspection of the sensitive insulator set can effectively identify whether there is a deteriorated insulator in the insulator string. If the pre-inspection determines that there is a deteriorated insulator in the string, the next step is to inspect the insulator string piece by piece. If the pre-inspection determines that there is no deteriorated insulator in the string, the piece-by-piece inspection operation is not required. The degradation probability of insulators in actual lines is only 1% [29]. The vast majority of insulator strings in the operating line are good insulator strings without deteriorated insulators. If the electric field strength of all insulator strings is detected piece by piece, it will not only greatly affect the patrol efficiency of patrol inspectors, but also put forward strict requirements for the endurance ability of patrol UAVs. The pre-inspection based on the sensitive insulator criteria can avoid the redundant detection of good insulator strings and effectively improve the efficiency of non-contact electric field detection methods. The following Figure 13 is the pre-inspection flow chart.

E_av in the figure is the average electric field of two insulator strings at the same position corresponding to two circuits of the same tower.

When the degraded insulator is detected in the insulator string through pre-inspection, it is necessary to further detect the insulator string piece by piece to identify the position of the degraded insulator in the string. At this time, it is necessary to consider the impact of complex working conditions on the electric field distribution of the insulator string and put forward corresponding optimization strategies.

Based on the simulation results in the second section, an optimization method of increasing path interest points is proposed for the actual working condition of insulator string sag. This method can reduce the measurement error from 4.8% to 0.4% and effectively improve the accuracy of measurement results. For the measurement error caused by UAV attitude disturbance, this paper proposes an optimization strategy combining hardware and software: in terms of hardware, it is recommended to use the UAV equipped with an RTK module as the carrying platform for an electric field detection device. At present, the mainstream UAV with an RTK function in the market can achieve centimeter-level positioning accuracy and meet the requirements of electric field detection; in terms of software, as shown in Figure 14, the electric field information obtained by the traditional detection method based on the UAV platform is only related to the time information and cannot perceive the path offset and attitude disturbance. The measured electric field data will have certain errors compared with the expected electric field data on the path, which will lead to false detection and the missed detection of degraded insulators. In this paper, an optimization strategy of spatio-temporal information fusion is proposed, which transforms the corresponding relationship between the electric field intensity of the sampling point and the time of the sampling point into the corresponding relationship between the electric field intensity of the sampling point and the coordinates of the sampling point and then obtains the electric field information of the corresponding spatial coordinates and determines the effectiveness of the sampling point according to whether the coordinate error corresponding to the electric field intensity meets the detection requirements.

The specific implementation process of the spatio-temporal information fusion optimization strategy is as follows:

(1)

The position of the center of the electric field sensor is calculated and set as the target point by using the UAV RTK main antenna position, the structure, and installation position of the optical electric field sensor and other information;

(2)

When the spatial electric field data of the insulator associated with time starts to be recorded, a positioning request is triggered to record the position information of the target point associated with time;

(3)

The clock difference between the local time of the electric field detection device and the time of the UAV system is calculated by using the underlying processing program of PSDK to realize the synchronization of the electric field detection device and the time of the UAV system;

(4)

The time request position of the UAV system where the locating event occurred is used to obtain the position coordinates corresponding to the measured spatial electric field information;

(5)

The relative distance between the position coordinates corresponding to the spatial electric field information and the route position coordinates are compared, and the electric field data of the coordinate points whose relative distance is greater than 5 cm are eliminated. Then the spatial electric field distribution curve of the target path is generated.

Through the above pre-detection method based on sensitive insulator criteria and the optimization strategy for piece-by-piece detection under complex working conditions, not only can the efficiency of insulator detection be greatly improved, but also the measurement error under actual working conditions can be effectively reduced, which has a certain guiding significance for actual field detection.

5. Conclusions

In this paper, the electric field characteristics of composite insulators under complex working conditions were explored by simulation, and optimization strategies were proposed for detection methods. A pre-detection method based on sensitive insulator criterion was proposed by combining theoretical derivation and experimentation, and the following three conclusions were obtained.

(1).

The contactless detection means of insulators are easily affected by complex site operating conditions. The influence of the surface state of an insulator on its space electric field is less than 4%; the attitude disturbance of UAV can easily affect the detection results of the electric field of insulators at the high-voltage end, 8.6% at most; the natural falling of insulators can also cause a deviation of about 4.8% for the insulator electric field. The optimization strategy proposed in this paper can effectively reduce such errors.

(2).

It was found by the degraded insulator detection that: If the zero-value insulator is set at any position, the changes of electric fields of the last insulators close to the low-voltage side are the most sensitive, and the rate of change of space electric field is basically more than 5.0%. Therefore, the sensitive insulator set under a voltage level of 220 kV is generally at the low-voltage end.

(3).

Based on the criterion for sensitive insulators, and combined with the contactless electric field detection method, the insulation performance of the insulator string can be accurately judged by measuring the space electric field of the sensitive insulator set. This method combines the advantages of the electric field distribution measurement method and sensitive insulator method, which can improve the detection efficiency and safety and be of great significance for the promotion of the contactless detection method of insulator electric fields in practical applications.

(4).

The detection method proposed in this paper is mainly for insulator strings at the 220 kV voltage level. The next step is to determine the sensitive insulator groups at different voltage levels and then propose corresponding detection strategies.