Transposition (telecommunications)
Encyclopedia
Transposition is the periodic swapping of positions of the conductors of a transmission line
, in order to reduce crosstalk and otherwise improve transmission. In telecommunications this applies to balanced pairs whilst in power transmission
lines three conductors are periodically transposed.
For cables, the swapping is gradual and continuous; that is the two or three conductors are twisted around each other. For communication cables this is called twisted pair
. For overhead power line
s or open pair communication lines, the conductors are exchanged at pylons, for example at transposing pylons or at utility pole
s, respectively.
The mutual influence of electrical conductors is reduced by transposition. Transposition also equalizes their impedance relative to ground, thus avoiding one-sided loads in three-phase systems. Transposing is an effective measure for the reduction of inductively linked normal mode interferences.
For longer powerlines without branches, wires are transposed according to the transposing scheme. At closely branched grids and where several electric circuits share a route (in particular when the lines operate at different voltages) on the same pylons the outside unbalance of the line, which is caused by the other electric circuits, dominates. In these cases one finds large deviations from the transposing schemes. For example, in some such transpositions, only two of the three conductors on the pylons change their place. Also transpositions on pylons near power substations are used to get an optimal arrangement of the feeding system without crossing of conductors.
As the mutual influence of electric circuits can change after new lines are installed or old lines dismantled, certain transpositions may disappear or be added after new construction in electricity mains. In the case of a twisted line the individual conductors of an electric circuit swap places, either in their whole course (at cables) or at certain points (at overhead lines). The mutual influence of electrical conductors is reduced by transposing. The unbalance of the line, which can lead to one-sided loads in three-phase systems, is also reduced. Transposing of overhead lines is usually realized at so-called transposing pylons. Transposing is an effective measure for the reduction of inductively linked normal mode interferences.
A transposing scheme is a pattern by which the conductors of overhead power line
s are transposed at transposing structures. In order to ensure balanced capacitance of a three-phase line, each of the three conductors must hang once at each position of the overhead line.
At a transposing structure, the conductors change their relative places in the line. A transposing structure may be a standard structure with special cross arms, or may be a dead-end structure. The transposing is necessary as there is capacitance between conductors, as well as between conductors and ground. This is typically not symmetrical across phases. By transposing, the overall capacitance for the whole line is approximately balanced. Transposings also reduce effects to communiciations circuits.
between the conductors is thus substantially higher than with Dieselhorst Martin (DHM) - stranding in which the situation of the conductors to each other in the cable changes repeatedly. Because of the smaller work capacity of the DHM stranding it is possible to form additional electric circuits with the help of a phantom circuit
. Since the phantom transducers are turned on to in the middle of the master transducers, the currents of the phantom circuit on the two coming Rome circles compensate themselves.
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
, in order to reduce crosstalk and otherwise improve transmission. In telecommunications this applies to balanced pairs whilst in power transmission
Power transmission
Power transmission is the movement of energy from its place of generation to a location where it is applied to performing useful work.Power is defined formally as units of energy per unit time...
lines three conductors are periodically transposed.
For cables, the swapping is gradual and continuous; that is the two or three conductors are twisted around each other. For communication cables this is called twisted pair
Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
. For overhead power line
Overhead power line
An overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...
s or open pair communication lines, the conductors are exchanged at pylons, for example at transposing pylons or at utility pole
Utility pole
A utility pole is a pole used to support overhead power lines and various other public utilities, such as cable, fibre optic cable, and related equipment such as transformers and street lights. It can be referred to as a telephone pole, power pole, hydro pole, telegraph pole, or telegraph post,...
s, respectively.
The mutual influence of electrical conductors is reduced by transposition. Transposition also equalizes their impedance relative to ground, thus avoiding one-sided loads in three-phase systems. Transposing is an effective measure for the reduction of inductively linked normal mode interferences.
Power lines
As the mutual influence of electric circuits can change after new lines are installed or old lines dismantled, certain transpositions may disappear or be added after new construction in electricity mains. In the case of a twisted line the individual conductors of an electric circuit swap places, either in their whole course (at cables) or at certain points (at overhead lines). The mutual influence of electrical conductors is reduced by transposing. The unbalance of the line, which can lead to one-sided loads in three-phase systems, is also reduced. Transposing of overhead lines is usually realized at so-called transposing pylons. Transposing is an effective measure for the reduction of inductively linked normal mode interferences.
A transposing scheme is a pattern by which the conductors of overhead power line
Overhead power line
An overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...
s are transposed at transposing structures. In order to ensure balanced capacitance of a three-phase line, each of the three conductors must hang once at each position of the overhead line.
At a transposing structure, the conductors change their relative places in the line. A transposing structure may be a standard structure with special cross arms, or may be a dead-end structure. The transposing is necessary as there is capacitance between conductors, as well as between conductors and ground. This is typically not symmetrical across phases. By transposing, the overall capacitance for the whole line is approximately balanced. Transposings also reduce effects to communiciations circuits.
Telecommunication
In communications cables, transposition is used to reduce coupling between circuits in the same cable. The principle measure is the pitch, or the distance over which the pairs of a circuit are twisted. By twisting, the wires become longer than the cable. The stranding factor indicates the relationship of single wire length to cable length; it amounts to with communication cables about 1.02 to 1,04.Kinds of stranding
In practice the following kinds of stranding occur more frequently:- Pair stranding: Two single wires are stranded to twisted pairTwisted pairTwisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
transmission line. - Three-stranding: Three single wires are stranded to a tripartite group.
- Four-stranding: This group is essentially divided into the two special kinds of stranding:
- Star quad twisting: Four single wires maintain the same relation to each other in the quad, whereby the members of a twisted pair face each other each other diagonally.
Transmission technique
The kinds of stranding have different transmission characteristics. The capacity of a stranding affects itself, for example the two conductors of a quadruple run over the entire cable length parallel during star quad twisting. CapacitanceCapacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
between the conductors is thus substantially higher than with Dieselhorst Martin (DHM) - stranding in which the situation of the conductors to each other in the cable changes repeatedly. Because of the smaller work capacity of the DHM stranding it is possible to form additional electric circuits with the help of a phantom circuit
Phantom circuit
In telecommunication and electrical engineering, a phantom circuit is an electrical circuit derived from suitably arranged wires with one or more conductive paths being a circuit in itself and at the same time acting as one conductor of another circuit....
. Since the phantom transducers are turned on to in the middle of the master transducers, the currents of the phantom circuit on the two coming Rome circles compensate themselves.