Electric system is one of the earliest platform recognized the importance of telecommunication. To ensure a highly reliable power supply, we allocate wireless and wired private network, such as 3D Stereo Communication System, including microwave(in the air), power line carrier (on the ground), and leased telecommunication lines (commonly underground).
Since the invention of optical fiber, the power communication department has had a great interest on it. At first, telecom monoply allows them to lease free ‘dark fibers’around the world. With the time went by, some of them became new telecom operator. This is because they have valuable tower resources. Thus, more and more ADSS and OPGW optical cable is installed on electrical poles. The power grid is developing towards the comnication network.
Structure of OPGW
OPGW should have all the functions and preference of overhead lines and optical cable. OPGW also integrated the advantages of mechanical, electrical and transmission.
OPGW mainly includes fiber optical cable (optical units)and twisted metal wire (aluminum clad steel wire, galvanized steel wire, aluminum alloy wire and hard aluminum wire). There are hundreds of structures until now. In this passage, we will demonstrate three kinds of structure.
Aluminum Tube
The aluminum tube is also know as aluminum protective tubular. The aluminum tubes are divided into three types: seankess, welded and longitudinally wrapped. The optical fiber unit placed in the aluminum tube (a part of the conductive cross). The tube can be a single loose tube, a layered loose tube, a primary coated fiber bundle of SZ twisted yam or a tightly wrapped fiber. The tube is filled with water-blocking ointment or wrapped with heat insulation. As for a standard structure, the inner layer usually is stranded steel wire (aluminum clad), while the outer layer is stranded aluminum (alloy) wire. The two layers are stranded in opposite directions with the outer layer towards right.
Aluminum Framework
Aluminum framework is also known as framework structure. Slot a spiral shape in the aluminum framework (a part of the conductive cross). Place the optical fiber tube, the primary coated optical fiber bundle of SZ twisted yam, the tightly wrapped optical fiber or the ribbon fiber in the groove. As for a standard structure, there is also a welded or longitudinally clad aluminum tubes ouside the aluminum framework. However, sometime we do nothing to the aluminum framework to minimize the diameter. The wire should match with the steel tube.
Steel Tube
Steel tube is also known as stainless steel structure. Place the optical fiber into the stainless steel tube filled with ointment with a certain excess length. The stel tube can replace at least one (less than 3 by now) monofilaments, and put them into the inner strand layer. This is the standard structure. The wire should match with the steel tube.
Structure of ADSS
ADSS is a kind of additive to the original tower structure. Thus, the ADSS cable should try its best to addapt the situation of the original electric tower and find an optimal or a compromised solution. The conditions includes system voltage, phase sequence, head shape of the tower, sag and safety distance.
ADSS is mainly made of cable core, reinforced aramid yarn (or other suitable materials, and outer sheath. There are two representative structure among all kinds of ADSS cable.
Tube in Center
The picture shows on the left is known as a bundled tube or central tube structure. Place the optical fiber in a PBT (or other suitable material) tube filled with ointment with a certain excess length, then wrap the suitable aramid yarn according to the required tensile strength. There are two kinds of outer sheath which are PE and AT. It is suitable for installation where the field strength is <12 kV or ≤20 kV respectively.
Layer Twist
The picture shows on the right is known as the layered type. The fiber optic loose tube is twisted on the central strength member (usually FRP) and then covered with the inner sheath. Wrap the suitable aramid yarn according to the required tensile strength. The selection of the outer sheath should match with the central tube.
Differences Between Main Specifications
Rated Tensile Strength (RTS)
Also known as ultimate tensile strength or breaking force. It is the calculated value of the sum of the strengths of the bearing section (ADSS mainly calculates aramid). During the breaking force test, the breakage of any element in the optical cable all judge as broken.
RTS is the important specification of the hardware’s (especially the tension clamp’s)
Maximum Allowable Tensile Strength (MAT)
It is the maximum tension obtained by OPGW or ADSS when the total load is theoretically calculated under the designed meteorological conditions. Under this tension, the fiber should be free of strain and no additional attenuation. Mat is usually about 40% of RTS.
MAT is the important basis for calculation and control of sag, tension, span and safety factor.
Daily Average Operation Strength (EDS)
It is also known as average operation tension. It is the average tension obtained by OPGW and ADSS during the long term operation when the total load is theoretically calculated under the annual average temperature with no wind and no ice. EDS is usually about 16%-25% of RTS.
Under this tension, the fiber in the cable should be very stable.
Direct Current Resistance
It is the calculated value of the parallel resistance of all conductive elements in the OPGW at 20℃, which should be as close as possible to the ground wire on the opposite side. ADSS do not have this specification.
Short Circuit Current
It is the maximum current the OPGW can bear in the curtain time of the short circuit (usually one side). When doing the calculation, the time span of short circuit and the value of start and end temprature may affect the result. Thus, this two factor should be approach to the actual condition. ADSS do not have this specification.
Short Circuit Current Capacity
It is the product of the square of short circuit current and the time span, which is I2t. ADSS do not have this specification.
Conclusion
OPGW is suitable for new lines or replace the original ground wire, while ADSS is suitable for adding on old lines. The main technical specification of different structure OPGW and ADSS should meet the requirement of electrical system and the related regulations.
