Autotransformer system
The autotransformer system is a special version of the traction power supply . With appropriate transformers , two alternating voltages with a phase shift of 180 ° are made available in the substation . One of the two conductors is designed as a catenary, with the second conductor carried along the route in an insulated manner. Autotransformers connected between the two conductors are located at appropriate intervals . The term "two voltage" comes about because there are two voltages out of phase by 180 ° (see also: single-phase three-wire network ). The term “multi-voltage system” is also known.
construction
Substation
The required rail voltage is transformed by means of a transformer connected to the high-voltage network. The transformers are single-phase transformers with two windings on the low voltage side. The end and the beginning of one of the windings are connected together so that theoretically a winding with a center tap is formed. Compared to the center tap, there are two voltages available, phase-shifted by 180 °. One of the two external connections is called a positive feeder (PF) and corresponds to the contact line, the other external connection is called a negative feeder (NF) and is carried along the route. The center tap is connected to the rail and corresponds to the return conductor.
The connection on the high-voltage side is a three-phase network. As a rule, the 110 kV or 220 kV network is selected as the voltage level. Due to strong unbalanced load distribution in the three-phase system with the use of only one transformer two transformers are connected in the so-called V-circuit across the three phases of the high-voltage network, however, it remains an unbalanced load are made, since the load between the conductors L1 and L3 is absent.
The undervoltage (PF and NF) provided by both transformers are not in phase with each other, so that parallel operation of the two transformers is not possible. The switches shown in the picture have the task of connecting the divided route network when using only one transformer. This option is only intended for emergency operation.
Car transformer station
The so-called auto transformer stations are located at regular intervals along the route. In an auto-transformer is an autotransformer . The "PF" and "NF" are connected to the two outer winding connections of the autotransformer, the middle winding tap is connected to the rail. Depending on the system design, symmetrical or asymmetrical winding divisions are used.
route
As a rule, the rail is insulated from the earth , so that the operating currents can flow in a defined manner via the return conductors and not via the earth. It should be noted that certain limit values for the rail potential relative to the earth must not be exceeded. The return current is conducted via the rail and so-called return conductors, which are carried along the mast.
Execution types
A distinction is made between the symmetrical and the asymmetrical autotransformer system. A symmetrical system has two voltages phase-shifted by 180 °, with the same amplitude . The asymmetrical system, on the other hand, uses two voltages phase-shifted by 180 ° with different amplitudes . The different systems differ in the current distribution between the "PF" and "NF". The following is an example of the system specification:
2AC 50 / 25kV (sometimes also indicated as AC 2x25kV )
- "2AC" - refers to a dual voltage system
- "50" - indicates the system voltage, in this case 50kV (voltage between positive feeder and negative feeder)
- "25kV" - indicates the contact line voltage, here 25kV - between overhead contact line and rail
- Since the overhead line voltage of 25kV results in half of the system voltage of 50kV, this is a symmetrical system
2AC 40 / 15kV
- "2AC" - refers to a dual voltage system
- "40" - indicates the system voltage, in this case 40kV (voltage between positive feeder and negative feeder)
- "15kV" - indicates the contact line voltage, here 15kV - between the overhead contact line and the rail
- Since the contact line voltage of 15kV does not correspond to half the system voltage of 40kV, the system is asymmetrical
principle
The locomotive on the route takes a current x from the contact line. This current flows over the contact line from the substation to the traction unit and back into the rails. Part of the return current from the rail now flows to the autotransformer in the center tap. In the autotransformer, this current is now divided between "PF" and "NF". In this case that means that the current is now divided as follows.
- Part of the current flows from the substation to the traction unit via the contact line
- the return current flows via the rail and return conductor to the autotransformer (center tap)
- the reverse current in the autotransformer is divided into "PF" and "NF"
- The "PF" current from the car transformer flows to the vehicle via the contact line
- The "NF" current from the car transformer flows back to the substation via the "NF"
The following figure illustrates the principle with ideal current distribution when the traction unit is located directly near a car transformer station and far away from the substation.
However, this representation refers to an ideal current distribution, which does not occur in reality. The current distribution is generally dependent on the prevailing impedance conditions at the respective location of the traction vehicle. In principle, all connected autotransformers are involved in the current flow, but these proportions are heavily dependent on the load location (location of the traction unit).
In the following picture, however, a real power distribution is shown in the symmetrical AT system.
In the example image it can be seen that the load currents in the overhead line are halved over long distances. This also means halving the voltage drop, whereby this voltage drop does not relate to the overhead line voltage , but to the system voltage . If the losses are neglected, the voltage drops are reduced by a theoretical ratio of 4 compared to a single-phase system without an amplification line and return conductors. In practice, ratio values of 2.5 to 3.5 can be achieved. In a single-phase system with reinforcement lines and return cables, there can also be a reduction in the load currents in the overhead line and thus also a reduction in voltage drops. However, this reduction in voltage drops is less. Theoretically, ratio values of max. 2 compared to a single-phase system without reinforcement cables and return cables. The definition of substation distances is largely determined by the voltage drops on the overhead line. This means that the UW clearances can be selected larger for the AT system. Practical systems (the Stralsund-Prenzlau stretch of 132.5 km) have shown that common designs as a single-phase system with reinforcement lines and return cables require three substations. The conversion as an AT system meant that the middle of the three substations could be saved.
Protection concept
Substation
As usual, the transformer is protected with a transformer differential protection. The currents are recorded at each winding connection of the transformer. With the V connection it is therefore necessary that the currents of the individual transformer are recorded separately. A current measurement of both transformers with only three current transformers on the high-voltage side is not sufficient, since the current of both transformers flows in phase L2 and would thus cause a differential current. The individual currents of the "PF", "NF" and the "center tap" must also be recorded on the low voltage side. In addition, overcurrent protection is usually used on the high-voltage side, which monitors both transformers. The undervoltage side of the transformers that are routed to the feeder panels of the switchgear is usually protected by a simple overcurrent protection.
Route protection
The catenary is protected by a distance protection, but some special features must be observed. Since it is a two-phase system, the total current from "PF" and "NF" is used for impedance detection. Modern traction current protection devices record the current of the "PF" and "NF" individually and form a total current within the device. It should be noted that this is not an addition of the currents with the correct sign, only the amounts are added. Due to the phase shift of 180 ° between "PF" and "NF", adding the two phases with the correct sign would result in a total current of zero if "PF" and "NF" have the same amplitudes. It is necessary because current and voltage are measured to calculate the impedance - with a total current of zero, the impedance would therefore have the value "infinite". It is also possible to obtain the total current by connecting the secondary current transformer circuits accordingly. Due to the connected autotransformers, which also have their own impedance and are connected to the catenary, the measured impedance is falsified, since the impedance of the autotransformer is also measured. This must be taken into account when designing the distance protection zones - especially in order to maintain selectivity.
Car transformer protection
Various protection concepts can be used for the auto transformer. One possibility would be not to protect the autotransformer separately, but to “protect” it solely through the route protection. This concept is known to some extent in France for AT systems. Protection by means of a transformer differential protection is also possible - protection by an UMZ would also be possible.
Advantages over single-phase systems
- the current in the contact line and thus the voltage drops along the contact line are reduced, which leads to lower power losses
- Substations can be saved by choosing a much larger distance between the substations (cost savings)
- Reduction of interference, especially with telecommunication lines
use
In Germany there has been a system between Stralsund and Prenzlau since 2001 , which is designed as a 2AC 30 / 15kV. A second application between Knappenrode and Horka was put into operation in 2018. A section from Geltendorf via Memmingen to Lindau is under construction (as of March 2020).
In Switzerland, an autotransformer system will be used for the first time from 2013: on the Cadenazzo – Luino railway in Ticino, i. H. for the single-track connection between the Gotthard Railway and northern Italy , which is mainly used for freight traffic. For this purpose , mobile autotransformers are being installed in Cadenazzo , Ranzo- Sant'Abbondio and Luino .
In Italy, the newly built high-performance lines are operated with 2 × 25 kV / 50 Hz.
In Sweden, autotrafo installations already feed the 15kV / 16⅔ Hz lines of the Erzbahn and Botniabahn .
In Finland, the autotransformer system 2 × 25 kV / 50 Hz is used on 6 route sections (over 600 km).
In Luxembourg, virtually the entire route network electrified with alternating current (25 kV / 50 Hz) is designed as an autotransformer system.
Such systems are widespread on high-speed lines with 25kV, 50 or 60Hz operation. Japanese railways in particular use car transformer systems.
Below are some routes that are designed as a two-phase autotransformer system:
- Beijing – Tianjin high-speed line - high-speed line in the PRC
- Schiphol – Antwerp high-speed line - HSL-Zuid section from Amsterdam to the Belgian border
Web links
- Description SF6 circuit breaker , application to HSL Zuid , Netherlands , English (PDF; 680 kB)
Individual evidence
- ↑ track practice E - Output: 2.1 - Publisher CEC
- ^ Zurich-Munich: electrification on track . In: Eisenbahn-Revue International 10/2019, pp. 510–511
- ^ Journal of Railway Amateur EA 5/2013, page 225
- ↑ Il Sistema di alimentazione delle linee ferroviarie Italiane ad Alta Velocità: esigenze e nuove soluzioni (PDF) Prof. Ing.Alfonso Capasso (Italian): Lecture on railway power systems, accessed on July 15, 2017
- ↑ Ratahallintokeskus (Ed.): Turvallisuusohjeita sähköradalle (PDF; 1.6 MB). (Finnish Railway Administration Center (Ed.): Instructions for occupational safety on electrified railway lines . Finnish), accessed on September 29, 2013
- ↑ Les Cheminots Philatélistes (ed.): 50 years of electrification of the CFL, Luxembourg, 2006
- ↑ Niklas Biedermann: Banmatningssystem för höghastighetsjärnvägar . KTH 2006. (Power supply systems for high-speed rail lines. Swedish), PDF; 2.6 MB, accessed September 29, 2013
literature
- Hartmut Biesenack: Energy supply for electric railways , 2006