ADSS Cables Recognizing the Problems and Solutions
The installation of fiber-optic cables on towers that support extra-high-, high- and medium-voltage (EHV, HV, MV) transmission lines provides an effective communications web on electric networks. Electric utilities have used fiber-optic cables for several years to provide a communications link between neighboring substations or between substations and the control desk.
Progress in the design of optical switches opens new perspectives for the future to create “all optical” Internet and other communications systems. An increasing number of electric utilities around the world offer a fee-based licence to companies seeking access to the telecommunications channels in fiber-optic cables.
Israel Electric Corp. (IEC, Tel-Aviv, Israel) began installing all-dielectric self-supporting (ADSS) fiber-optic cables on the 161-kV and 110-kV transmission lines in the early 1990s. Having experienced a number of faults, IEC conducted an investigation that confirmed the need to establish strict installation standards to ensure a fault-free fiber-optic-cable network.
IEC's Fiber-Optic Cable System
The technical problem with the fiber-optic-cable system is the leadin and termination at the EHV/HV substation, service purchaser's or consumer's premises. Electric utilities often select ADSS as it offers the best solution. The two alternative systems are not often applicable:
Fiber-optic cable inside ground wire (OPGW) is not usable because the majority of HV and MV transmission lines have no lightning protection ground wires.
Winding the fiber-optic cable on the phase conductors of HV or MV OHTL (WRAP) is not possible because the conductor diameter is generally too small and of insufficient mechanical strength for the wind loading.
ADSS fiber-optic cables have several principle advantages:
Many more fibers and channels per cable
Lower price per channel
Easier and cheaper montage
Easier to access ADSS cable (mostly without transmission circuit outage)
Easier and faster to repair in the event of damage or breakage of the cable
The ADSS does not experience temperature rise in the event of a lightning stroke and short-circuit current, as in the case of ground wires.
However, based on IEC's experience and utility experience around the world, the use of ADSS cables on transmission lines — at 161 kV and higher — requires a careful approach. It is necessary to determine the optimal position of the ADSS cable on the tower to ensure safe distance from ADSS cable to phase conductors and to ground; to provide safe access to ADSS cable for personnel; and to decrease the electric field, voltage drop and leakage currents along ADSS cable to avoid corona sources and partial discharges on its surface that can damage the polyethylene (PE) cable jacket.
| Line No. | Line Voltage | Date ADSS Cable Installed | Line Length km (miles) | Towers Checked No. | Suspension Clamp Ends Checked No. | Damage by Severity Levels | Damaged Clamps No. Percent | |
|---|---|---|---|---|---|---|---|---|
| 1 | 2 | |||||||
| 3 | ||||||||
| 1 | 161 | 1995 | 51 | 90 | 180 | 71 | 6 | 108 |
| (32) | 31 | 60% | ||||||
| 2 | 161 | 1994 | 3.2 | 13 | 26 | 3 | 6 | 10 |
| (2.0) | 1 | 38% | ||||||
| 3 | 161 | 7/1993 | 4.5 | 16 | 32 | 3 | 2 | 3 |
| (2.8) | - | 9% | ||||||
| 4 | 161 | 1996 | 18 | 22 | 44 | 7 | 1 | 10 |
| (11.2) | 2 | 23% | ||||||
| 5 | 110 | 2/1996 | 4.5 | 3 | 6 | 0 | 0 | 0 |
| (2.8) | 0 | 0 | ||||||
In subtropical areas, such as Israel, these phenomena are especially harmful due to the specific climate conditions: an eight- to nine-month dry period followed by a shorter rainy period in winter. Additionally, there are more than 100 nights with dew during the dry season that moistens the pollution layer transforming it into an electrolytic layer. The cable surface pollution layer after the dry period is hard and adhesive, and particularly thick in the regions nearing clamps.
Within two years of commissioning the ADSS cable installed on the four 161-kV lines (77 km [48 miles] total length), the utility found damage (tracking and erosion) with different severity levels. In most instances, the damage was located close to the clamps. The severe instances of damage included two ADSS cable failures (falling). The most severe damage caused holes in the PE housing and, sometimes, an outbreak of the aramide layer (yellow lock strands). However, the ADSS cable installed on 110-kV line was in perfect condition.
Table 1 shows the number of the towers supporting IEC's 161-kV and 110-kV transmission lines strung with ADSS cable, and the number, levels and details of the recorded damage.
The ADSS cable that has an overall diameter of 14.2 mm (0.56 inches) has a PE sheath (1 to 1.5 mm thick) with a 4% carbon content that is supposed to offer good UV resistance. The cable installation clamps that are designed to safely transfer the cable weight and tension to the pole are clamped on to the cable's PE sheath. The clamps comprise aluminium alloy wires, the wires being 4.4 mm diameter for the suspension clamp and 1.8 mm for tension clamps. The assembly and installation of these clamps is critical.
Cable Failures on 161-kV Lines
One of the 161-kV transmission lines (No. 4 in Table 1) is supported by 35 suspension towers and 13 tension towers. Following two faults in which the ADSS fiber-optic cable completely failed, IEC removed the cable for laboratory investigation and analysis. The damage analysis included checking for correlation with the span length, wind direction and strength with the clamp edge configuration. Inspection confirmed that the mid-span cable was sound, but evidence of tracking and erosion were present near the majority of clamps, 87% of the suspension clamps. The cable damage was erosion of the cable's PE sheath near the clamps (evidence of tracking along the PE cable a few meters from the clamp).
The figure on page 56 shows examples of tracking and erosion damage identifying the three levels of severity detailed in Table 1. These findings can be explained by the following two physical phenomena:
Electric field intensity on the edge of the clamp exceeds the corona onset value, hence corona forms on the clamp edge. As a result, the combined effect of the products of corona and high humidity cause the PE sheathing material to erode.
Induced current is a minimum in mid-span rising to a maximum value of leakage current at the end of the clamp. In this region, the cable surface is subject to the maximum temperature creating dry ring zones and partial arcing. Damage in the form of rings is created by partial arcing across the dry band, and damage to the PE with small holes and spongy residues similar to the classical tree pictures found in many tracking and erosion tests.
| Clamp Height Above Ground Level m (ft) | Lateral Distance of Hanging Point from Tower Vertical m (ft) | * Max. Surface Electric Field E |
# Voltage Drop ADSS Cable Attached to Clamp (V eff) | Presence of One Protrusion 50-mm long from Clamp Edge |
|---|---|---|---|---|
| 16.5 (54) | 0 | 19,650 | 859 | No |
| 16.5 (54) | 0 | 47,200 | 1,038 | Yes |
| 10.0 (33) | 1.65 (5.4) | 8,620 | 290 | Yes |
| 17.7 (58) | 0 | 50,100 | 1,850 | Yes |
| 16.3 (53) | - 0.5 (-1.6) | 117,800 | 1,750 | Yes |
| 15.5 (51) | - 1.65 (-5.4) | 251,500 | 2,500 | Yes |
| * Maximum surface electric field near the clamp. | ||||
| # Voltage drop along last meter of ADSS cable attached to clamp. | ||||
To avoid corona, the maximum electric field intensity (E
IEC's computational studies that took into account the clamp, one wire protrusion from the edge of the clamp grounded tower (pillar and arms) and the dielectric constant of the cable PE sheath confirmed that the potential and electric field levels depend on:
Nominal voltage of the transmission line
Phase conductor parameters (diameter, number of conductors per phase and conductor spacing)
Phase sequence (phase voltage angles)
Position of ADSS cable, (distance from clamps and fiber-optic cable to phase conductors and tower)
Irregularities of wires at the edge of the clamp.
IEC also conducted computational studies to determine the optimum position for the ADSS cable on a typical suspension tower. IEC performed these computations for dry environmental conditions as well as for a clean or slightly polluted ADSS cable surface. In these conditions, IEC found the highest levels of voltage drop and electric field along the PE jacket of the ADSS cable near the clamp (in the severely electrically stressed region).
The computations confirmed that the maximum electric field intensity on the PE cable sheath is 13-kV/cm near the clamp edge and decreases rapidly along cable while the voltage increases to some kilovolts at a distance of 3 to 5 m (10 to 50 ft) from the clamp. The location of the most severe damage to the ADSS cables installed on the 161-kV lines supports these findings. Furthermore, IEC examined the effectiveness of clamps insulated from the grounded tower and the use of longer clamps (2 m [7 ft] longer), and found these design and installation alternatives to be ineffective.
ADSS Cables on EHV/MV Lines
IEC also considered using ADSS cable on a range of transmission lines with 3D models as follows:
33-kV and 22-kV MV Lines: These systems offer advantages as electric fields, voltage drops and induced currents along the cable are lower, even though the conductor spacing is much smaller. Since the span lengths are shorter, the mechanical strength of the cable is lower and, therefore, the load and tension on terminal clamps is much lower. MV circuits at this voltage are used for distribution so that the ADSS cable will be installed in locations that will be much closer to industrial, commercial and domestic properties.
The figure above shows the 22-kV angle lattice tower and the computational model showing the ADSS cable. To determine the optimal position for the ADSS cable on the tower, IEC studied the cable in five positions on the angle tower.
The results shown in Table 2 include the maximum surface electric field and voltage drop.
Note that one 50-mm wire protrusion leads to a 21% increase in voltage drop and the maximum field increases by 240%. The results show that the optimum position for the ADSS cable is 10 m (33 ft) hung at 1.65 m (5.4 ft) from the tower vertical center.
Table 3. Maximum Electric Field and Voltage Drop. Height of ADSS Cable Above Ground m (ft) Distance from Tower Symmetry Plane m (ft) Presence of 50-mm Wire Protrusion from Clamp # Voltage Drop along ADSS Cable (kV eff) ## Voltage Drop Along ADSS Cable (kV eff) *Maximum Electric Field on ADSS Cable (kV eff/cm) 12.2 (40) 1.7 (5.6) Yes 2.87 3.6 5.51 12.2 (40) 1.7 (5.6) No 2.87 3.5 4.10 17.2 (56) 1.7 (5.6) Yes 5.3 - 8.11 17.2 (56) 1.7 (5.6) No 5.3 - 5.57 18.62 (61) 0.95 (3.1) Yes 4.61 5.83 7.48 22.2 (73) 0.95 (3.1) Yes - 9.1 16.2 22.2 (73) 1.7 (5.6) Yes 6.84 - - 22.2 (73) 1.7 (5.6) No 8.41 - 9.81 27.2 (89) 1.7 (5.6) Yes 16.5 18.9 23.6 27.2 (89) 1.7 (5.6) No 16.5 - 16.9 60 (197) 0 Yes 9.6 11.2 17.1 * Maximum surface electric field near the clamp # Voltage drop along last half-meter of ADSS cable attached to clamp. ## Voltage drop along last meter of ADSS cable attached to clamp. Following the studies on 22-kV lines with concrete poles that gave similar results, IEC erected short lengths on ADSS cable on two 22-kV overhead line circuits and performed inspection after one year's service. IEC found no evidence of surface disturbance, tracking or erosion.
400-kV Transmission Lines: The figure to the left shows constructional features of the suspension tower used by IEC to support 400-kV double-circuit transmission lines. As each circuit is protected by an individual ground wire, it is also possible to install the ADSS cable in or near the symmetry plane of the tower (including the top of the tower).
Table 3 includes the results of the computational studies conducted for a range of ADSS cable positions on the tower. The results confirm that the minimum values of voltage drop and electric field will occur when the ADSS cable is attached at 12 m (40 ft) above ground level. Although a single wire protrusion from the clamp has little effect on the calculated voltage drop, the electric field strength on the cable surface increases by 30% to 40%.
Results
IEC's computational models confirmed that the damage to ADSS cable was the result of high values of electric field on the cable sheath, use of cable sheath materials with inadequate anti-tracking properties and the presence of protrusions at the edge of the hanging clamps. This detailed investigation also evaluated the following characteristics that utilities should consider in the design of ADSS fiber-optic-cable networks:
To minimize the electric field on the PE cable sheath, the ADSS cable can be hung in the symmetry plane of the overhead transmission line as the value of the electric field is 50% lower in this position than in the normal installation position.
The maximum electric stress on the cable can be reduced by 40% by decreasing the stringing height used.
A toroidal shield with an outer diameter of 200 mm (8 inches) positioned near the edge of the clamp also essentially reduces the electric field on the cable.
The use of longer clamps or insulated clamps will not effectively reduce the maximum electrical stress at the clamp edge.
The study shows that the generally accepted rule that ADSS cable can only be used on overhead transmission systems up to 150 kV is not accurate.
The use of ADSS cable can be safely used on all EHV and HV overhead transmission systems provided three conditions are fulfilled.
First, the cable sheath materials must have anti-tracking properties.
Second, the selection of the hanging or stringing position of the cable should consider the influence of all the details of the cable and tower structure. This exercise requires the use of 3D models to determine the electric field and potential distribution and the optimum ADSS hanging point.
Proper grading rings must be installed on the clamp's extremities.
Acknowledgment
The investigation described in this article was performed in the Electrical R&D Laboratory of Israel Electricity. The author wishes to express his appreciation for the substantial contribution made by his colleagues in the analysis of the faulted ADSS cables on the 161-kV overhead transmission lines of Israel Electric.
Felix G. Kaidanov received the MS and PhD degrees from the Leningrad Polytechnic Institute in 1962 and 1969, respectively. From 1962 to 1990, he worked in the High-Voltage Laboratory of the Direct Current Institute in Leningrad, Russia. His career in research has been in the areas of electric and magnetic fields influence, corona and partial discharges, radio interference, insulators for AC-EHV lines, ADSS fiber-optic cables and problems associated with AC-EHV cables and joints. Kaidanov joined Israel Electric in 1990 where he continued his research work in the R&D Laboratory until 2000. Currently, Kaidanov is responsible for research in the EHV line design department of Israel Electric.
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