In this chapter, a modified structure for the IOPPC downlead cable is proposed to reduce the electric field intensity in the vicinity of polyvinyl chloride (PVC) and to decrease or eliminate the risk of discharge. By combining the traditional structure and the modified structure, corresponding electric field models are established in this chapter. What’s more, the electric field intensity of both structures under a 110 kV AC voltage is calculated. Based on the discharge situation of the two electric field models, the effectiveness of the modified structure is preliminarily discussed.
3.1 Modified structure of IOPPC downlead cable
Based on the fault description in Chapter 1, the IOPPC downlead cable is modified in three aspects:
(1) The photoelectric separator entrance is now sealed with insulating material to prevent the ingress of rainwater.
(2) In order to reduce the electric field intensity in the air domain around the IOPPC downlead cable, a 1.5m long semiconductor shielding layer is wrapped between the silicon rubber and polyethylene, starting from the outlet of the photoelectric separator. The semiconductor shielding layer has the function of uniforming electric field [10]. The presence of the semiconductor shielding layer can effectively reduce the electric field intensity inside the optical cable.
(3) Umbrella skirts below the polyvinyl chloride tube outlet have been introduced to prevent flashover incidents.
Fig.2 illustrates the modified structure of the IOPPC downlead cable mentioned above. To validate these modifications,the finite element (FEM) was employed to assess electric field intensity in both traditional and modified structures. This study is aimed at demonstrating the effectiveness of these alterations in achieving the desired outcomes.
3.2 Geometric model and material settings
In the structure of the IOPPC downlead cable discussed in Chapter II, a considerable disparity exists between the maximum dimension and minimum dimension. It leads to the dissimilarity results in a high mesh density within the geometric model, particularly in the three-dimensional representation. Such a scenario adversely affects the convergence of the model. Additionally, even if the model manages to converge, the computational time required is excessively long [11]. To alleviate the modeling burden and enhance model convergence, a two-dimensional model is established for both the traditional and modified structures of the IOPPC downlead cable to calculate electric field intensity results.
Fig.3 illustrates the geometric model of the traditional structure of the IOPPC downlead cable. In this traditional configuration, a 4 mm- thick polyvinyl chloride tube is installed at the parallel groove clamp. The end of the polyvinyl chloride tube is appropriately bent downward to facilitate the downward extraction of the optical cable. The specific geometric parameters are detailed in Table 1. It is crucial to note that the accuracy of the calculation results and the computation time are influenced by the size of the geometric parameters. If the geometric parameters are too small, the calculation results may be inaccurate. Conversely, if the parameters are too large, it could significantly increase the computation time and potentially lead to model convergence failure. Therefore, this chapter iteratively determines the optimal values for the geometric parameters of the model.
TABLE 1
GEOMETRICAL PARAMETERS
Project
|
Size/mm
|
Length of aluminum stranded conductor
|
1169
|
Diameter of aluminum stranded conductor
|
25
|
Distance from photoelectric separator to parallel channel clamp
|
310
|
Diameter of optical cable
|
15
|
Thickness of parallel groove clamp
|
87
|
Length of parallel groove clamp
|
42
|
Spacing of umbrella skirt
|
90
|
Diameter of umbrella skirt
|
51
|
The geometric model corresponding to the modified structure of the IOPPC downlead cable is presented in Fig.3. The methodology for determining the geometrical parameters of the modified structure model aligns with that of the traditional structure. Fig.3 also delineates the enhancements made to the IOPPC downlead cable structure, encompassing the sealing of the photoelectric separator entrance with insulating material, the incorporation of a semiconductor shielding layer between the silicon rubber and polyethylene, and the installation of an umbrella skirt.
Fig.3 further elucidates the materials utilized for various components in both the traditional and the modified structures of the IOPPC downlead cable. Specifically, in the geometric model, the IOPPC is enveloped in an air domain. The conductor and parallel groove clamp are composed of aluminum, while the photoelectric separator is fabricated from aluminum alloy. Silicone rubber is employed for the umbrella skirts and the insulation at the entrance of the photoelectric separator. The numerical values of the relevant material parameters are detailed in Table 2. It is imperative to note that the semiconductor shielding layer exhibits conductivity under high-voltage conditions. As the model is computed under high-voltage conditions, the conductivity of the semiconductor shielding layer adopts the numerical value corresponding to these conditions, as determined from the literature[12].
TABLE 2
NUMERICAL VALUES OF THE MATERIAL PARAMETERS
Material
|
Conductivity/(S*m-1)
|
Relative dielectric constant
|
Air
|
0
|
1
|
Aluminium
|
3.77*107
|
1
|
Aluminum alloy
|
3.03*107
|
1
|
Silicon
|
1.00*10-12
|
11.7
|
Semiconductor shielding layer
|
6.00*107
|
1
|
Polyvinyl Chloride
|
1.00*10-2
|
2.9
|
Polyethylene
|
1.00*10-2
|
2.3
|
3.3 Boundary condition settings
In consideration of the actual operating conditions of the IOPPC downlead cable, the boundary conditions of the established model have been defined. During the operational phase, the conductor experiences a line voltage of 110 kV, resulting in the phase voltage of 63.5 kV. The boundaries at both ends of the conductor (as illustrated by boundaries S1 and S2 in Fig.3) have been designated as voltage excitation boundaries with a value of 63.5 kV. Given the direct contact between the parallel groove clamp and the conductor, the potential of the parallel groove clamp is also set at 63.5 kV. It is worth noting that in practical applications, the optical cable length from from the polyvinyl chloride tube to the grounding point tends to be relatively long. When globally modeling the IOPPC downlead cable under such circumstances, the computational workload becomes substantial. To mitigate the computational workload, it is essential to shorten the optical cable length in the model. However, to ensure the fidelity of simulation results, the potential value at the end of the optical cable in the model must be determined post-shortening. In this chapter, the potential value at the end of the optical cable is calculated based on the ratio of the model length to the actual length, following the equation (1):
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Where U represents the potential at the end of the optical cable in the model, kV. Lmodel is the length of the optical cable established in the model, m. Lreal is the actual length of the optical cable in the field, m, and the actual potential at the end of the optical cable is 63.5 kV. Utilizing equation (1), the potential value at the end of the optical cable in different models can be determined. The calculated results are employed to set the voltage excitation at boundary S3 in Fig. 3. In addition, it is pertinent to note that the IOPPC downlead cable structure in the model operates under power frequency voltage conditions. Consequently, the frequency of the voltage is set to 50 Hz in the model.
3.4 Simulation results and analysis of traditional structure and modified structure
Fig.4 illustrates the electric field distribution of the traditional IOPPC downlead cable. In this figure, region A provides a localized magnification of the photoelectric separator, while region B presents a localized magnification of the parallel groove clamp. Examining Fig.4 reveals a notable increase in electric field intensity beneath the parallel groove clamp in the traditional configuration. The insulation strength at this location is compromised due to the thinness of the polyvinyl chloride, silicon rubber, and polyethylene layers, making it susceptible to corona discharge. In severe instances, this may lead to insulation layer breakdown. Compounding the issue, the electric field strength at the outlet of the photoelectric separator is also elevated. Fig.5 depicts the electric field distribution of the modified IOPPC downlead cable. In this representation, region A provides a localized magnification of the downlead cable section between the photoelectric separator and the parallel groove clamp, while region B presents a localized magnification of the parallel groove clamp. Notably, Fig.5 highlights a significant reduction in electric field intensity at the outlet of the photoelectric separator and beneath the parallel groove clamp in the modified structure, as compared to the traditional arrangement. The overall electric field intensity in the modified structure is markedly lower.
Based on the foregoing analysis, it is evident that the electric field intensity is notably high at the outlet of the photoelectric separator and beneath the parallel groove clamp in the traditional structure, both situated below the downlead cable. To delve deeper into the electric field dynamics of both the traditional and modified structures, this section undertakes an assessment of the electric field intensity beneath the downlead cable in both structures, illustrating the findings through plotted curves. These curves are delineated in Fig.6, with the sampling paths outlined in Fig.4 and Fig.5.
Fig.6 reveals that the electric field intensity in the traditional structure experiences a marginal increase near the outlet of the photoelectric separator, with no abrupt fluctuations in intensity. However, in the airspace proximate to the bottom of the parallel groove clamp, the electric field exhibits distortion, escalating sharply and predisposing to corona discharge. Conversely, in the airspace adjacent to the bottom of the parallel groove clamp, the electric field intensity in the modified structure is diminished. Along the sampling path, no distortion in the electric field is observed, and the field strength remains low beneath the downlead cable of the modified structure. In summation, the electric field intensity in the airspace beneath the parallel groove clamp of the modified IOPPC downlead cable diminishes following the installation of the semiconductor shielding layer and the umbrella skirt structure. To further scrutinize the impact of the internal structure of the two downlead cables on electric field distribution, this section computes the potential values within the internal structure of both downlead cables and presents them as curves. These curves are depicted in Fig.7.
In Fig.7, it is evident that along the sampling path, the potential variation trend inside the traditional IOPPC downlead cable is upward. Furthermore, the maximum potential value is reached when approaching the parallel groove clamp. Given the direct connection between the parallel groove clamp and the aluminum stranded conductor, they share the same potential. As the parallel groove clamp guides the high potential (63.5kV) downward, a substantial potential difference arises between the downlead cable and the parallel groove clamp, leading to localized electric field distortion. This distortion intensifies in the air gap, potentially resulting in corona discharge. To address this issue, the modified structure incorporates a semiconductor shielding layer between the silicone rubber and polyethylene. It is essential to note that the silicone rubber and polyvinyl chloride (PVC) tube between the semiconductor shielding layer and the parallel groove clamp are too thin to significantly influence the electric field and can be disregarded. Consequently, the semiconductor shielding layer and the parallel groove clamp can be considered approximately equipotential. Additionally, the length of the semiconductor shielding layer exceeds its radius, and the interior of the downlead cable remains uncharged. Following the principle of electrostatic equilibrium, the segment of the downlead cable enveloped by the semiconductor shielding layer can be treated as an equipotential body. By neglecting the impact of silicone rubber and polyethylene, the downlead cable and the parallel groove clamp can be approximated as equipotential, leading to a reduced potential difference between them and consequently lowering the electric field intensity in the air gap.
In summary, under the influence of 110 kV alternating voltage, the traditional structure has a higher electric field intensity at the output of the photoelectric separator and below the parallel groove clamp. The modified structure, however, incorporates the semiconductor shielding layer, which makes the potential inside the downlead cable approximately equal to that of the parallel groove clamp. As a result, the potential difference between the downlead cable and the parallel groove clamp is reduced, leading to a decrease in the electric field intensity in the air gap between them.