Electric Field Distribution Characteristics and Optimization of High-Voltage Disconnector Gaps: Analysis of Grading Ring Parameters, Shielding Electrode Configuration, and Breakdown Risk

1. Introduction

High-voltage disconnectors are essential components in substations and transmission lines, providing visible isolation under no-load or very low current conditions. However, the open contacts create a relatively large air gap with complex geometry, often leading to highly non‑uniform electric field distribution. Localized field enhancement at sharp edges, tips, or irregular surfaces may initiate corona, produce audible noise, radio interference, and over time degrade insulating surfaces. Under transient overvoltages (lightning or switching surges), the concentrated field can even cause dielectric breakdown, threatening system security.

To mitigate these risks, grading rings (also known as corona rings) are widely applied to smooth the electric field around the energized contact. Meanwhile, auxiliary shielding electrodes can be added near the grounded side to further balance the potential distribution. Nevertheless, improper selection of ring parameters or electrode placement may aggravate field distortion rather than relieve it. Therefore, a systematic optimization study is necessary.

This article quantifies the influence of three key grading ring parameters—major radius (R), tube diameter (d), and vertical position (H) relative to the contact tip—on the maximum electric field intensity (Emax) in the disconnector gap. Additionally, the effect of an auxiliary shield electrode mounted on the moving contact is evaluated. Based on the simulation results, we propose an optimized configuration and analyze the residual breakdown risk under both power-frequency and switching impulse conditions.

2. Electric Field Characteristics of the Disconnector Gap

A typical horizontal-break or vertical-break disconnector consists of a fixed contact (often energized) and a rotating/moving contact (connected to load or ground when open). The open gap distance ranges from 300 mm to over 1 m for 110–550 kV applications. The most critical region for field concentration is the tip of the moving contact, followed by the edges of the grading ring.

Without any grading device, the electric field along the gap exhibits a highly asymmetric distribution: the maximum field may reach 10–15 kV/mm at the contact tip under nominal phase-to-ground voltage, far exceeding the corona inception threshold (~3 kV/mm for smooth conductors at 1 atm) and approaching the breakdown strength of air (~30 kV/mm for very small gaps, but in large gaps the average breakdown strength is lower due to streamer dynamics). The field non-uniformity factor (Emax / Eavg) can be as high as 8–12. This condition is unacceptable for reliable long-term operation.

The introduction of a properly designed grading ring creates a large-diameter smooth surface that intercepts the field lines emanating from the contact tip, spreading them over a wider area. Consequently, the peak field is relocated from the tip to the surface of the grading ring, and its magnitude is significantly reduced.

3. Simulation Methodology

Three-dimensional electrostatic simulations were performed using the finite-element method (FEM) in a commercial software package (ANSYS Maxwell/Electrostatic). The model represents a 252 kV single-column vertical-break disconnector with an open gap distance of 500 mm. The fixed contact is a flat blade, and the moving contact is a cylindrical rod with a hemispherical tip (radius 8 mm). The grading ring is modeled as a torus attached to the moving contact side. The following parameters were varied:

· Major radius (R) of the grading ring: 80 mm, 100 mm, 120 mm, 140 mm, 160 mm.

· Tube diameter (d) : 20 mm, 25 mm, 30 mm, 35 mm, 40 mm.

· Vertical offset (H) from the contact tip to the ring center plane: positive means the ring is located below the tip (toward the insulator base), negative means above the tip. H ranged from −50 mm to +80 mm.

A voltage of 146 kV (peak phase-to-ground for 252 kV rms) was applied to the moving contact and grading ring, while the fixed contact and tank/enclosure were grounded. The surrounding air domain was assigned relative permittivity εr = 1. The maximum electric field magnitude was extracted along the contact surface and the grading ring. For the shielding electrode study, an additional torus (R = 100 mm, d = 20 mm) was placed concentrically around the fixed contact, and its position was optimized.

4. Results and Discussion

4.1 Influence of Grading Ring Parameters

Major radius (R): As R increases from 80 mm to 140 mm, the peak field Emax (located on the grading ring surface facing the grounded contact) decreases monotonically from 4.8 kV/mm to 3.1 kV/mm. Further increase to 160 mm gives only marginal reduction (to 3.0 kV/mm) but significantly expands the physical dimensions, which may conflict with insulation distances and mechanical stability. The recommended R is 120–140 mm for 252 kV, providing Emax ≈ 3.3–3.2 kV/mm, slightly above the corona inception threshold. With a safety margin, R = 140 mm is preferable.

Tube diameter (d): Increasing d reduces surface curvature, thereby lowering Emax on the ring itself. For d = 20 mm, Emax = 3.8 kV/mm; at d = 35 mm, Emax drops to 2.9 kV/mm. However, a larger tube diameter increases weight and wind load. The optimal d is 30–35 mm, balancing field reduction and practical constraints.

Vertical offset (H): This parameter has a non‑monotonic effect. When the ring is placed too high (H > +40 mm, i.e., above the contact tip), the field at the tip remains high because the ring does not effectively “shield” the tip. When the ring is too low (H < −30 mm), the ring moves toward the insulator, and the field concentrates on the edge of the ring nearer to the ground potential. The optimum H is between −10 mm and +10 mm (ring center approximately aligned with the contact tip). At this position, the combined field of tip and ring is smoothest, giving the lowest Emax = 3.1 kV/mm.

4.2 Shielding Electrode Configuration

Adding a second toroidal shield around the fixed contact further homogenizes the field across the gap. In the optimized configuration (R = 140 mm, d = 30 mm on moving side; R_shield = 100 mm, d = 20 mm on fixed side), the peak field on the moving side ring decreases to 2.8 kV/mm, and the field on the fixed contact edge (previously 4.0 kV/mm without shield) drops to 2.9 kV/mm. The field distribution becomes nearly symmetrical, with a maximum of 2.9 kV/mm – well below the corona inception threshold. The breakdown risk under AC voltage is negligible under clean and dry conditions.

4.3 Breakdown Risk Analysis

Under power-frequency voltage, the maximum electric field in the optimally configured gap is about 2.9 kV/mm. Even considering surface roughness and environmental factors (humidity, dust), this value is below the typical corona onset of 3.0–3.5 kV/mm for smooth electrodes. The average gap field (146 kV / 500 mm = 0.29 kV/mm) is far lower than the breakdown strength of a 500 mm air gap (≈2.2 kV/mm for a uniform field; for a weakly non‑uniform field, the critical value is about 1.5–1.8 kV/mm). However, due to the remaining slight non‑uniformity, a switching impulse of 1050 kV (1.2/50 µs) would produce a transient Emax ~ 20 kV/mm, which is close to the streamer inception. Therefore, for impulse conditions, the design should incorporate a larger grading ring (R ≥ 160 mm) or additional shielding to keep Emax below 15 kV/mm under the maximum rated switching overvoltage. Alternatively, increasing the gap distance to 600 mm would provide greater safety.

5. Practical Recommendations and Conclusion

Based on the simulation results, the following optimized parameters are recommended for a 252 kV disconnector open gap of 500 mm:

· Grading ring on the moving contact: major radius = 140 mm, tube diameter = 30 mm, vertical alignment with the contact tip (H = 0 ± 10 mm).

· Auxiliary shield on the fixed contact: major radius = 100 mm, tube diameter = 20 mm, positioned coaxially with the contact blade.

· Maximum steady‑state electric field < 3.0 kV/mm, eliminating continuous corona.

· Under switching impulses, a safety margin can be achieved by either increasing the gap to 600 mm or enlarging the grading ring to 160 mm major radius.