Poland, in pursuit of western Europe, wants to build a high-speed rail network, but a much higher speed of trains involves the consumption of significantly more power. The 3 kV DC system used by the Polish railway network is incapable of providing it in such amounts. The issue of powering high-speed rail (HSR) with 25 kV AC is a multidisciplinary problem and covers topics in the fields of mechanics, transport, construction, electrical engineering, electricity, environmental protection, spatial planning, economics, as well as climate policy. The better rail transport we have, the less traffic will burden Polish roads, which will also contribute to energy savings due to greater energy efficiency of collective transport. The new railways will get a modern power system – writes Daniel Radomski, contributor to BiznesAlert.pl.
History and specificity of the 3 kV DC system in Poland
During the communist regime, railway lines were electrified for train speeds at 120 km/h, and substations (PT) were connected to the electricity grid at medium voltage (standard 15kV, in some parts of the country 20 kV or 30 kV). In some cases the new power lines had 110 kV and were located along a specific railway live (e.g. LK 351 Poznań-Szczecin). The ongoing and already completed modernization projects of the railway lines to increase the maximum speed and capacity has on several occasions necessitated the modernization of existing PTs or the construction of new ones in order to reduce the length of the sections between existing PTs. Faster, more frequent trains need more power, and the combination of a relatively low 3kV voltage at high operating currents and long distances results in significant voltage drops. The new PTs, and most of the upgraded ones, are connected at 110kV to reduce voltage drops on the capacity side and to increase reliability.
In the 3 kV DC system, under the current standards it is necessary to use numerous PTs, even every 12 km – these standards require that the currents drawn by the train can reach values of the order of 3-4 kA, which results in a large cross-section of the traction wires, which are made of copper. The supply of new and upgraded PTs with 110 kV, with a poorly developed 110 kV network in many areas of the country, necessitates the construction of long 110 kV lines, which are connections to new PTs. Overhead power lines require an environmental decision (DŚU), which involves a lengthy administrative process (Act on Environmental Protection). In order to shorten the time of the investment, a much more expensive solution can be used in the form of a cable line, the construction of which does not require an environmental decision. The risks and delays related to powering new PTs with led to the fact that the already modernized LK No. 8 Warsaw – Radom will reach the capacity to operate trains with a projected speed of 160 km/h only in 2024. Currently, the speed limit for modernized railways can be increased only once the works on PTs end and the “Energia” sub-system is certified.
The technical limit for the use of the 3 kV DC system is train speeds of up to 250 km/h. It is not possible to achieve higher speeds in this system. Firstly, due to the significant increase in power demand – due to stronger motion resistance, and secondly, due to the physical limitations associated with the current flowing at the connecting point of the overhead contact line and the pantograph. For the new, longer railway lines, a speed of 200 km/h shall be considered as the technical and economic threshold for the use of the 3 kV DC system. Very short lines connecting the existing 3 kV DC lines are better left in this system, also because they make it possible for vehicles adapted only to 3 kV DC to use the new lines.
Additionally, the 3 kV DC system in Poland uses the non-traction power supply line (LPN), i.e. a 3-phase medium voltage line running along a railway line, often on the same poles as the overhead line. The LPN is used to power the reception related to the management of the railway line and traffic control, for example, to power the automation and traffic lights at rail/road crossings.
The 2x25kv AC system – basic conditions
The solution to most of the limitations and problems that the 3 kV DC system generates is to switch to AC with a much higher voltage. In a 2 × 25 kV AC system, substations can be located every approx. 65 km on the HSR, and on lines that are strained less, even every 100 km. A significantly lower number of the substations means considerably fewer connections will have to be built, which will greatly lower the costs in terms of the entire section. One PT in the 2 × 25 kV AC system “replaces” up to 6 PTs in the 3 kV DC system, and also achieves much higher speeds as it can power vehicles with higher performance ability. Depending on the upstream voltage and the capacity of the traction transformers, a single PT in a 2 × 25 AC system will be 1.1 to 3 times more expensive than a single PT in a 3 kV DC system.
In addition to the cost of the substation itself, it is also very important to lower the cost of power connections and (less important) to reduce the cross–section of the overhead wires-thanks to lower operating currents. The size of the cross-section is also affected by the maximum speed of the train. Mechanical resistance also comes into play, as higher speeds require larger cross-sections. An additional advantage of the AC system is the fact there is no need to use an LPN, since any non-traction offtakes can be supplied from the traction network, provided that appropriate transformers are used. Although railway/road crossings are only located on tracks where the maximum speed is 160 km/h, this feature can be useful for electrification in the system of network elements that has not yet been electrified for economic reasons.
The 2×25 kV AC system can also have the added advantage: if for whatever reason adding a high voltage supply line to the railway line is problematic (e.g. the lack of a convenient route, the need to route such a line through populated areas, which can have a negative impact on the time of implementation of the investment), it is usually technically possible to locate PT 2 × 25 kV AC at a certain distance from the railway line (a few, sometimes even several kilometers) and cover this distance already at the voltage level of 2 × 25 kV. However, each time this needs to be verified from the point of view of the distance from the substation to the farthest powered point.
Vehicles: powers and speeds
Standard long-distance trains or electric traction units (EZT) are 200 m long, while the platforms are 400 m long. Today in Poland the Pendolino EZT or Husarz-type locomotives have the capacity of 6 MW. The maximum commercial speed of the Pendolino is theoretically 250 km/h, but for a number of technical reasons trains of this type run in Poland at a speed of no more than 200 km/h. The record speed was 293 km/h, but it was broken at a railway track that was out of use – only one EZT was powered from the traction substations. If a maximum speed of 250 km/h is introduced on the Central Railway Line, then the Pendolino – type intercity EZTs currently owned by PKP, will be able to handle this. However, this line has been modernized for years and at the moment there are no plans to change the supply voltage to a 2x25kV system.
As Mateusz Malinowski, head of SRK Rail and Power Projects at the Solidarity Transport Hub, explained, “any talk of a partial conversion of the existing network to a 2×25 kV AC system is a little late, because too much money has been allocated for the modernization of the 3 kV DC system, and these expenses must first be cushioned.”
HSR trains can reach a maximum speed of up to 350 km/h. An EZT with a length of 200 m has a power of about 8-9 MW (more precisely MVA). However, in practice, the operation of HSR systems is often carried out on double traction, i.e. two EZTs with a length of 200 m each, or a single vehicle with a length of 400 m. This means the power drawn from the overhead line is doubled, up to 16-18 MW, which creates a challenge from the engineering side and is beyond the technical sensibility and feasibility in a 3kv DC system.” You have to realize that 250 km/h and 350 km/h, is twice the power,” emphasized Mateusz Malinowski.
The idea of the 2x25kv AC system
There are many implementations of the 25kV system. Poland will adopt the same system as the one in other EU countries, with an additional supply line that runs along the track, but does not transmit energy directly to the vehicles. Power is transmitted through 2 wires with voltages shifted in phase by 180°, which in fact means transmission at the level of 50 KV-this is the effective voltage between the overhead line and the additional power supply.
The main advantage of a two-phase system with a displacement of 180° is the mutual transfer of electromagnetic fields from the traction network and from the additional power supply, which greatly improves the operation of the system in terms of electromagnetic compatibility, that is, it helps to reduce interference induced by traction currents. The system is closed by means of autotransformers located every 8-20 km, which “push” some of the current from the traction network to the additional power supply, which reduces the load on the contact wires.
On the other hand, the requirement to provide a sufficiently high short circuit capacity at the connection point usually results in the need to connect to the national electricity grid at a voltage of 220kV or higher, exceptionally 110kV-provided that there is a sufficiently “strong” main power point (GPZ-power station WN/SN, belonging to the Distribution System Operator).
To put it simply, PT equipment in the 3 kV DC system acts as a transformer, rectifier and filtration system, while a PT in the 2x25kv system is just a transformer. Depending on the expected load, 2 to 4 traction transformers are used in a single PT.
Implementation of 2x25kv AC system for Poland’s HSR
As a result of the introduction of the 2 × 25 kV AC system on the new HSR lines, while leaving the existing lines and urban railway hubs at 3 kV DC, there is a need to implement two- or multi -system trains on all new lines. The difference in price between single- and multi-system trains is from several to several percent. It is technically possible to retrofit rolling stock built on modern, unified platforms such as Stadler Flirt or Siemens with a voltage transformer module – even if it is ordered as a single system.
In terms of the capacity of local governments to organize services on new lines and to have the proper rolling stock, the right investments should be made in rolling stock purchases, at least in terms of securing at the tender stage the possibility of subsequent retrofitting of the ordered vehicles with the correct power supply module (e.g. as a right of option).
Urban railway hubs will remain electrified in the 3kv DC system. This also applies to the HSR tunnel in Łódź. The newly designed railway junction of the Solidarity Transport Hub (STH) in the vicinity of the airport will be fully electrified in the 2×25 kV system, and on the connecting lines from the existing lines to this junction there will be sections with a voltage change. Therefore, only two-system trains will be able to access the Hub.
The power supply voltage is changed along a straight section or a curve with a long radius, as well as with a relatively small longitudinal slope – the HSR designs adopted a limit of 5 per milles. It is permissible to change the supply voltage in the tunnel, provided that additional space is provided for the protection and connection devices, the use of which is necessary in the vicinity of the transition section. According to the current design, this will be done in Warsaw, in the area of the proposed technical and parking station of the second metro line. Resistance in tunnels is greater than outside, therefore, for such a long tunnel as the one proposed at the exit from Warsaw (almost 10 km), the achievable speeds and the elevation led to the need to introduce a power change section approximately in the middle of the route. If the 3 kV system was left along the entire tunnel, the train speed would be limited along a large portion of this section, as well as outside of the tunnel, as right outside it, there would be no sufficiently small longitudinal decline.
The annual energy demand of the Polish railway network is 2 TWh. Preliminary estimates of the power demand for the entire HSR programme hover around 0.5 TWh per year.
Connection to the electricity grid
In the 3 kV DC system, the substations are located in the immediate vicinity of the railway line and are connected to the electricity system by medium or high voltage connections. Where the high voltage lines were built for railway lines, both the 110 / 15kV power station and the PTs are adjacent to each other and to the railway line.
The HSR tenders for designing PTs with connections to the national grid and the railway line propose a different solution. In order to avoid the implementation of separate high or high voltage subscriber lines that will power PTs, it is assumed that PTs will be located directly at PSE (state-owned grid operator – ed.) or OSD (power distribution system operator – ed.) junction stations. The 2x25kv power supply usually allows for a certain distance between the PT and the railway line, sometimes up to several kilometers, in the form of a cable. There were many reasons behind this decision. Firstly, overhead HV/LV lines have a significant impact on the landscape and may necessitate numerous demolitions. This is why such projects usually cause social conflicts, and have a bigger impact on the environment. To install overhead HV/LV lines it it necessary to produce an environmental report and acquire an environmental decision, which makes the investment process longer and more comlicated. In the case of cable lines, there are no such problems. Secondly, the routing of the overhead line in urbanized areas would, in many cases, involve a much longer route, due to the need to bypass buildings at a suitable distance. This, in turn, entails numerous collisions with protected habitats. The introduction of a cable route at a lower voltage allows for a significant “straightening” of the power supply route, and also makes it possible to omit protected areas. As a result, in addition to shorter time, lower investment risk, the cost of construction is also reduced, while maintaining full functionality.
In the case of the construction of the subscriber line, the energy customer, i.e. the STH company, would have to be the investor. However, this company does not in the specialize in the construction of high voltage lines. In cases where it is not possible to locate the PTs directly at the PSE/OSD station, the STH will seek to build as short a cable connection as possible in order to reduce investment risk, environmental impact and speed up administrative procedures.
A key technical parameter for the analysis of the impact of the substations on the electricity system is the short circuit capacity. It determines the stability of a given connection point and, to put it simply, defines whether the network at a given point is “strong” or “weak”. Transformers generating 2x25kv voltage are installed in PTs on a line to line voltage basis, and in majority of the cases they don’t take on the same load. In addition, most of the PTs will be equipped with two transformers. This results in asymmetry of the network load. For these reasons, the short circuit capacity at the connection point must be sufficiently high, but the required value depends on local conditions and varies depending on the PTs.
“When looking for a power point to connect to, one should start with the lowest possible voltage, because it is the cheapest and easiest,” explains Mateusz Malinowski. Given the density of the LV network in Poland, this is not an easy task, because most 110kV main power supply points located at a distance from the LV network have a short-circuit capacity of approx. 1000 MVA, which is not enough. After preliminary analyses and consultations with PSE (state-owned national grid operator – ed.), it was agreed that most of the PTs will be connected to the nodes of the system nodes of the LV network to voltages of 220kV or 400kV. However, 3 PTs will be connected to a voltage of 110 kV. In two cases, this is due to sufficient short-circuit capacity in the main power supply point, respectively approx. 3000 MVA at the main power supply point in Wałbrzych Podzamcze, and approx. 2300 MVA at the main power supply station Łomża. The third case is the Pisz main power supply point, which has a short-circuit capacity of only a few hundred MVA, which is far from sufficient, and therefore additional equipment for electrical energy conversion will be used there. This is due to the lack of “more powerful” main power supply points in the region.
In the immediate vicinity of the Wałbrzych Podzamcze main power supply point runs the 220kV Boguszów – Świebodzice line, and in the vicinity of Łomża there is the 400kV Łomża system station. However, in Wałbrzych it was not possible for PSE to install an electricity station dedicated to the HSR along this line, so SE Świebodzice would have to be the possible connection point. In both cases, connecting to a PSE station would require the construction of a 220/400kV subscriber line or a 2x25kv cable line several kilometres long. These investments would increase the cost and represent an additional investment risk, especially in the context of environmental impacts, which could lead to significant delays in implementation. In the absence of the need to implement these investments and thanks to sufficient short-circuit capacity in selected main power supply points a decision about a 110 kV connection was made.
In Pisz, an analysis was conducted on whether it was possible to connect the PT from a 400 kV Ełk bis station, which is located about 35 km from Pisz in a straight line. For such a significant distance, it would be necessary to stretch a 400kV subscriber line. However, between Ełk and Pisz there are numerous environmental areas that are protected, as well as a military training field. After a cost analysis, a decision was made to use a cheaper and easier connection to the main power supply point in Pisz, while equipping the PT with symmetric devices, i.e. a rectifier – inverter system. Therefore, this will be a set of devices similar to those used in the cross-border connection LitPol Link in Alytus, Lithuania, where the so-called DC insert is located, but it will operate at a much lower capacity.
The voltage of the PT connection also depends on the expected load, which depends on the assumed maximum transport offer. For example, on the Warsaw – Łódź line 2 or 3 transformers with a capacity of 60 MVA will be built, which can be effectively handled only by the highest voltage network. As a rule, 220kv is sufficient, but it is not available everywhere, and PSE is reluctant to develop a 220kV network.
The analysis of the demand for capacity includes a significant surplus and a completely busy train timetable that would strain a given substation to its limits. Substations on the “Ygrek” line (Warsaw-STH-Łódź-Sieradz-Poznań/Wrocław) will be adapted to handle trains running at a speed of 350 km/h. “We will definitely not build a substation that doesn’t have enough capacity to handle the traffic at a given railway line,” Malinowski promised.
Currently, the Solidarity Hub is not planning to generate its own power along the railway lines, it is also not planning any power banks, as there are no rectifiers in PTs, the energy from breaking may be returned by the transformer to the grid. Moreover, the PGE group will not build a gas-fired power plant in the vicinity of the Solidarity Hub. Instead, heat and cold will be generated locally through heat pumps or geothermal energy, with the support of PVs.
During the preliminary analytical work, it has been found that several areas of the railway network were not electrified yet, and it was concluded there was no economic justification for their electrification in the 3kv DC system, but such an investment would be justified in the 2x25kv AC system. These areas include the vicinity of the LK 29, Zamość and the new line of the planned eastern railway road from Lublin to Białystok (LK 30, 31, 32). In the area of LK29 it would be sufficient to build two additional PTs in the area of Szczytno and Mikołajki in order to electrify the railway lines with a total length of approx. 600 km. A proposal on this was submitted at a conference in Gdańsk on 7 April 2022, but so far PKP PLK (Polish State Railways – ed.) has not expressed any interest in it.
The substations will be maintenance-free and remotely controlled from area control centres common to rail traffic management and the internal energy network. When it comes to the transmission of control data for the HSR, optic fibers are to dominate, whereas solutions based on copper will be kept at a minimum.
Axel counters, which are widely used by rail companies today, will be used for track vacancy detection. According to the existing analyses, line-side indications (outside of the nodes) should not be used, including automatic line block systems (SBL) on the main railway lines (including Y and CMK North). Instead traffic management should be based only on solutions from the ERTMS/ETCS family. The first reason for this approach is the inability of the human eye to see the traffic light at such high speeds. The second is the need for efficient traffic management on new railway lines, including mixed traffic (i.e. HSR trains and regional express trains). In the absence of SBL traffic lights, the length of the intervals can be flexibly selected depending on traffic needs, including in the area of such stations where faster trains will overtake slower ones (eg. HSR trains going faster than regional express trains) the intervals can be maximally compacted. In the case of SBL, the intervals between the traffic lights are selected on the basis of one constant value of the braking distance, which is speed-dependent, e.g. 1300 m. In addition, the visibility of the traffic lights needs to be considered – they cannot be installed behind a barrier (e.g. a traction pole), or on some curves, as the length between the intervals would differ. The introduction of the classic SBL on HSR lines would therefore make mixed traffic more difficult, and contribute to a significant reduction in line capacity.
In both tenders for the development of the HSR power supply design, the basic range related to the power supply of the “Ygrek” line range was provided, as well as 6 options for each of the other PTs. This is an organizational procedure associated with the risk of high prices. The Solidarity Hub prefers to carry out studies for all substations, which will be similar to one another, and most of them will be consulted and agreed with PSE. Only one of the substations has a much lower priority than the others. This is the Szczawno-Zdrój PT near Wałbrzych, as it is expected that the HSR will be built in the Czech Republic much later.
This article was written on the basis of Technical Standards for the Solidarity Hub, description of the subject of the order titled “Development of design concepts for 2×25 kV traction substations (including the connection to the national grid and the railway line), materials for obtaining location decisions and connection conditions and order documents,” technical knowledge and a two-hour conversation with Mr. Mateusz Malinowski, head of SRK Rail and Power Projects at the Solidarity Transport Hub in the Office of Strategy, Planning, Design and Engineerinf of the STH Railway Sub-Program.