Good quality high voltage (HV) insulators help in achieving high performance of overhead Transmission lines (OHL). They isolate HV power lines from the tower and support the lines by maintaining them connected to the tower. HV outdoor insulators are facing many challenges in their life-time. One of the problems is the corona discharge that results from the high electric field stress at both ends of the insulator regardless of the type of the insulator (ceramic, non-ceramic, or glass). The electric filed distribution around the insulators, under dry condition, is mainly affected by the geometry and the capacitance of the insulator [1][2]. Large capacitance of ceramic insulators helps in grading the voltage along the insulator and thus reducing the stress around the two ends of the insulator. However, for non-ceramic isolators, which are widely used recently due to their low cost and light weight, the electric field around the two ends is high. The reason behind this is that non-ceramic insulator's shape as well as the material's low permittivity helps in creating a non-uniform potential distribution along the insulator which produces high e-filed strength in the vicinity of the two ends [1-3]. For non-ceramic insulator, the maximum electric field that is suggested by IEEE task force in [4] is 4.5 KV/cm which is the corona inception level.
Corona rings help reducing the electric fields around the two ends of the insulator. Moreover, they also grade the potential distribution along the insulator as shown in Fig. 1. Corona ring (also known as arcing ring) is a metal ring placed around the two ends of the insulator (or around the energized end only in case of voltages below 345KV [5]). Corona ring is usually
Fig.: Potential distribution of a 400 KV non-ceramic insulator with and without corona ring
manufactured out of aluminum to obtain greater weight reduction as well as higher levels of corrosion resistance [6]. Also, some utilities prefer to use steel instead of aluminum since the steel melting point is higher than that of aluminum and in case of a flashover, the aluminum may melt. The maximum electric field along the insulator surface depends on the geometry and the position of the corona ring. However, using corona rings increases the length of the insulator because they decrease the dry arcing distance of the insulators [1]. Knowing that, it is of importance to design a corona ring that will give us the desired insulation requirement with less electric field stress and with longest dry arcing distance.
ENERGIZED-END CORONA RING DESIGN
A 400 KV composite insulator used for an overhead line used in TRANSCO was simulated using COMSOL Multiphysics software. This insulator has 100 sheds and it is of 5180 mm long. The Maximum Electric Field along the insulator surface depends on the ring dimensions and its vertical position along the insulator. The ring diameter (R), diameter of ring tube (r), and vertical position of the ring along the insulator (H) are the dimensions that need to be controlled in designing the corona ring. Fig.2 shows the variables as well as the insulator and the point along the insulator (point A) where the electric field will be calculated. Fig.2-a shows a photo of the corona ring around a 400KV insulator, and fig.2-b shows the 2D model used in COMSOL Multiphysics. In the study, two variables are kept
H
A
Energized End
R
r
Corona Ring
a.
b.
Insulator
Corona ring
Energized-end fitting
Fig. : a. 400KV corona ring around energized-end; b. 2-D Model used in COMSOL Multiphysics simulation
constant while the third variable has been changed. The starting dimensions are: R=360mm, r= 200mm, and H=310mm which is used in TRANSCO.
Figs. 3 to 5 show the effect of changing R, r, and H. It is evident from Fig.3 that as the ring Radius (R) increases, the E-field at point A decreases. Also, the E-field at corona surface decreases as the corona ring diameter increases. Fig. 4 shows that as the corona ring tube radius (r) increases, the E-field at point A decreases and the E-field at corona surface decreases as well. However, as the vertical position of the corona ring increases (moving up along the insulator), the E-field at point A increases and the E-field at corona surface increases as well as shown in Fig.5. Also from fig. 5, we can see that as we move away from the energized end fitting, the curves shifts to the right which tells us that the electric field changes across the insulator depending on the position of the corona ring.
Moreover, changing R has stronger effect on the electric field
along the insulator than changing r. From the above results we
can conclude that the dimensions used by TRANSCO which
Fig. : Electric field along the insulator surface with variation of R; r=200mm, H=310mm
Fig. : Electric field along the insulator surface with variation of r; R=360mm, H=310mm
Fig. : Electric field along the insulator surface with variation of H; r=200mm, R=360mm
are: r=200mm, R=360mm and H=310mm are acceptable. This is because the resulted electric field at point A is around 0.38 KV/mm (shown in the black curve in the figures) and at the surface of the ring is around 0.5 KV/mm and they are well below the corona inception level which is 4.5 KV/mm.
GROUND END CORONA RING DESIGN
Same as the corona ring around the energized end fitting case, two variables are kept constant while the third variable is changing. Taking R as the ring diameter, r as diameter of ring tube, and H as the vertical position of the ring along the insulator, the starting dimensions are: r= 130mm, R= 130mm, and H=470 mm. Those dimensions are used in TRANSCO with the same insulator described in section II. Fig.6 shows the real photo of the corona ring around the ground end as well as the model used in COMSOL software.
It is evident from figs.7 and 8 that as we increase the ring diameter (R) and the ring tube radius (r), the electric field at point B on the insulator and on the corona ring surface decrease. However, as we increase the vertical position of the ring away from the ground end, the electric field on both point B as well as corona ring surface increase (fig.9). Also, as we can see from the figures that the vertical position has more
Corona Ring
Ground-end
r
R
B
H
Ground-end corona ring
b.
a.
Fig. : a. 400KV corona ring around ground-end; b. 2-D Model used in COMSOL Multiphysics simulation
effect on the electric field and choosing the right position will help us reduce the electric field significantly.
The resulted electric field computed around point B by using the ring dimensions used in TRANSCO is around 3.9 KV/mm which is close to the corona inception level (4.5 KV/mm). Changing only one dimension which is the vertical position H of the ring, we can reduce the eclectic filed well below the corona inception level. Taking H to be 450mm instead of H=470mm (existing design), the computed electric field at point B will be around 1.0 KV/mm which is well below the corona inception level and we will be at the safe side.
Fig. : Electric field along the insulator surface with variation of R; r=130mm, H=470mm
Fig. : Electric field along the insulator surface with variation of r; R=130mm, H=470mm
Fig. : Electric field along the insulator surface with variation of H; r=130mm, R=130mm
SINGLE CRONA RING VS. DOUBLE CORONA RING
Some utilities prefer to use two corona rings around the energized end fitting instead of one corona ring. Fig. 10 shows the two corona rings dimensions as well as the model used in the simulation where r is the ring radius, R is the ring tube radius, H is the vertical position and D is the distance between the two rings. Fig.10-a shows a real 400KV insulator were two corona rings are used. Fig.10-b shows the dimensions provided by the manufacture. Fig.10-c shows the model used in COMSOL in 2D.
The two rings usually have the same dimension with a little difference of H because they will be above each other as shown in Fig. 10. The simulation was made with the following dimensions: r=200 mm, R=360 mm, H=262 mm and D=36 mm which are an existing data used in TRANSCO.
The use of two corona rings will have the effect of having better reduction of electric field (fig.11) as well as having better potential distribution along the insulator. Not only will the electric field be reduced around the energized end-fitting, but also along most of the insulator's surface as shown in Fig.11.
However, besides having more cost, two corona rings will make the dry arcing distance shorter and hence a longest insulator is required. We can overcome these problems by having a good design of the double rings to avoid the problem of the arcing distance and by choosing a low cost material for the corona ring.
On the other hand, having a well designed single arcing ring will help us achieve the desired electric field and potential distribution with less cost and shorter insulator as well.
Corona Ring (1)
Corona Ring (2)
H
r
A
Energized End Fitting
R
DH
r=200mm
R=360mm
D=36mm
H=262mm
c.
a.
b.
Fig. : Two corona rings model used in the simulation (around energized end)
Fig. : Computed electric filed along a 400 KV insulator with single and double corona rings
CONCULSION
Designing a good corona ring can be helpful to utilities to increase the life-span of the insulators since they reduce the electric field and grade the electric potential along the insulator surface. A corona ring design for a 400KV non-ceramic OHL insulator was simulated using COMSOL Multiphysics software. Changing ring diameter (R), diameter of ring tube (r), and vertical position of the ring along the insulator (H) are the dimensions to be controlled in designing the corona ring. Also, double corona rings have better electric filed reduction and better potential distribution than a single corona ring. However, the cost and shorter arcing distance are the main disadvantages for using double corona rings.
ACKNOWLEDGMENT
The authors would like to thank TRANSCO (Abu Dhabi Transmission and Dispatch Company) for providing them with the technical data for the corona ring. Their help is highly appreciated
