Kraków 2021-03-03
Electric traction in Poland in PKP.
3,000 V DC.
In 1932, a cross-city railway line was commissioned in Warsaw, connecting the western and eastern sides of our capital city. Previously, it was necessary to detour the trains through the Dworzec Gdański station, which is located in the northern part of Warsaw. The cross-city railway line partially runs in the tunnel. Train tunnels were not new. However, a steam locomotive produces a lot of smoke. According to the regulations at that time, one train pulled by a steam engine could pass through the tunnel every 30 minutes to ventilate the tunnel. Such a limitation contradicted the sense of this investment. A decision was made to electrify the Warsaw railway junction (WWK). By September 1939, approximately 149 km of railway lines were electrified, including a section of 40 km of the EKD (Electric Access Railways) / WKD (Warsaw Commuter Railways) Warsaw - Milanówek / Grodzisk Maz.
In the 1930s, many cities in Poland had electric trams: Lviv (1896), Kraków (1901). The trams were powered by direct current, usually 600 V DC. Unfortunately. It was impossible to transfer solutions directly from trams to railways. A tram is a relatively light vehicle that covers distances of up to several kilometers. Trains are much heavier and cover very long distances. In such an arrangement, electrical substations should be made every few kilometers. Increasing the voltage to 1,500 V DC made the power grid thicker and heavier. The poles had to be placed more densely. In cities, tram companies had their own power plants or contracts with municipal power plants. Railways had to be provided with electricity from the national power grid. 1 500 V DC direct current electrification began to be introduced in, for example, France, Great Britain, Denmark and the Netherlands.
Initially, the electrification of railways in the world relied solely on direct current, simply because there were no suitable AC motors at that time.
Direct or alternating electric current?
Which current is better? The arguments were very difficult to discuss among engineers. In the US, it even led to the construction of an electric chair. Today, no one remembers with what electric shock the first convict was killed. But many remember that he was not killed, but only fried.
The power industry professionally produces 50 Hz alternating current, because it is easier to transmit it over long distances. Nowadays, alternating current is easy to convert to direct current with the help of a transformer. At the turn of the 19th and 20th centuries, before the invention of semiconductors, this posed a significant problem. In practice, rotating converters were used, i.e. a combination of a three-phase AC motor powered from the domestic network with a DC generator.
Alternating current was a good proposition. The traction network can be lighter, which guarantees a higher voltage (V), a lower current (A). As a result, substations can be set up every 40 - 60 km, much less often than with direct current.
Three-phase AC motor.
The most optimal type of AC motors are three-phase motors. They do not have a commutator. The disadvantage of these motors is a very high current consumption during start-up.
Historically, the first three-phase AC electric motors were very large and heavy. Only two such engines could be installed in the locomotive, at both ends of the locomotive. To transfer power to more drive axles, classic steam locomotive trusses were used. The problem was to deliver the three phases to the locomotive. Three phases or wires. The Germans experimented with the traction of three cables next to the tracks at different heights and three current collectors. This complicates the design of the overhead contact line. Nevertheless, as early as 1899, an experimental line with three-phase traction was built in Germany. On this line, in 1903, the tested train reached a speed of 210 km / h. Even so, the system was so complex that the Germans gave up further experiments and began promoting a single-phase system.
In the 1920s, Italians began electrifying their railways in a three-phase system. Their idea was to put two wires on top, to which the pantograph with two sliders touched. Rails were the third conductor. In practice, however, this system also caused a lot of problems. Three-phase motors draw a lot of current during start-up, so the network had to be heavy, and the supply substations more often than in a single-phase system. However, the substations were simple in structure and only had a transformer.
In the 1930s, Italians were leaders in electrification. They had the most electrified lines of any country in the world; 5,170 km, which accounted for 22% of all Italian rail routes. The Italians tried to export their three-phase technology, offering it, for example, for PKP and the cross-city line. But the service and repair costs were high. As a result, in the 1950s, the Italians changed the electrification system to a direct current of 3,000 V DC, such as in Poland.
Single-phase AC motor.
In practice, it was not possible to build a good single-phase AC electric motor of high power. At 50 Hz, there were considerable problems with commutation and switching of windings in the motor. An effective solution was to lower the mains frequency to 25 Hz, and even to 16 2/3 Hz. However, other problems then arose. Transformers at reduced frequency must be larger and heavier. There was a problem with the imperfection of the commutator electric motor at that time. The big challenge was to change the frequency from the industrial 50 Hz to a lower one, with a frequency of 16 2/3 Hz or at most 25 Hz. This change of frequency was associated with high costs of electro-energetic devices.
Railway electrification with 15kV 16 2/3 Hz alternating current has been implemented and has been used in Germany, Austria, Switzerland, Norway and Sweden. Electrified routes in the Sudetes in Germany (1930s) had their own power plants. There were about 400 km of electrified routes in the Sudetes, which in 1945 were dismantled by the Russians and taken to Moscow. By stealing the infrastructure, the Russians said that the poles were going to be galvanized.
Single-phase DC motor.
A commutator is present in a DC motor and its task is to change the direction of current flow through the frame. Commutators are the weakest element in a DC motor. But technical and material progress has meant that DC motors have been operating without failure for decades. As a result, all electric locomotives in the world have DC motors. Diesel locomotives that have an electric transmission also have such engines; internal combustion engine - generator - electric motor. In the past, such a system was called a three-machine.
Electricity 25 kV 50 Hz.
Several years before the Second World War, Germany attempted to use alternating current of normal frequency (50 Hz) for rail traction. They advanced so far that they launched a small section of the railway line in the Black Forest. They built four different electric locomotives and began their trials. They did not finish the tests because the war broke out. After the war, France continued these tests and concluded that it had advantages. They decided to build their railways with the support of 25 kV 50 Hz alternating current. In addition, it was possible to change the already built 1,500 V DC traction into 25 kV 50 Hz. During the tests, the French used their own locomotives and their own test section in Haute-Savoie.
In the 1950s, the concept of single-phase alternating current with an industrial frequency of 50 Hz and a voltage of 25 kV was returned to. This time, it was planned to use well-mastered DC motors in the electric locomotives, and the locomotive, apart from the transformer, had a mercury rectifier on board. This system has grown significantly in the 60's with the advancement of semiconductor technology. Rectifiers and silicon inverters were used. Currently, more and more railway lines in the world are electrified in this system. The cost of construction and operation is the lowest. The overhead contact line is light and there is a long distance between the substations, which are of simple construction. This system is used to build, among others, the French high-speed TGV railways.
In Poland, direct current.
In the 20-years of the 20th century, mercury rectifiers were developed, which eliminated the need to build special power plants or use troublesome and inefficient rotating converters. The locomotives did not have to carry large and heavy transformers, which was especially important when using the so-called ETZ - Electric Multiple Units to support suburban traffic. Electric commuter railways have gained great popularity.
As a result, for example in Germany, two different rail systems have emerged. The commuter railways for Berlin have a direct current of 800 V DC.
In Poland, after discussions between professors and engineers, it was decided to electrify with 3 kV DC. At that time, such a fragmented network was installed in the USA for a distance of 700 km, between Chicago and St. Paul. The operational results confirmed the great technical advantages of this solution. Poland modeled the systems used in England, where 1,500 V DC were used, but decided to use a higher voltage of 3,000 V DC.
In Poland, in 1928, it was assumed that we would electrify the cross-diameter line by November 1931. However, the global crisis meant that the first stage of electrification was completed in 1936. Originally, it was assumed that an electric traction would be built between the Warszawa Wschodnia and Warszawa Zachodnia stations in order to drag classic trains between them. However, it was decided to extend the scope of works to Żyrardów, Otwock and Mińsk Mazowiecki in order to handle local traffic by means of EZT trains. A total of 106 km of lines. The contract for electrification was concluded with two English companies: The English Electric Company and Metropolitan Vickers Electric Company. Part of the rolling stock was to come from Poland. Among others, 76 EZT sets, the production of which was undertaken by the Lilpop, Ran and Loewenstein plants in Warsaw, H. Cegielski - Poznań and Zieleniewski, Fitzner - Gamper in Sanok.
On December 15, 1936, electric trains were officially launched on the section to Pruszków, and a year later, on December 15, 1937, the trains were already reaching Mińsk Mazowiecki.
Single electric locomotives were remembered for pulling ordinary trains. Two EL100 series locomotives manufactured by Metropolitan Vickers were delivered from Great Britain. Based on a license, documentation and electrical equipment, four more EL100 electric locomotives were built in FabLok in Chrzanów. After the Second World War, only one locomotive, designated EP01, returned to service. In 1968, it was scrapped. In addition to these six heavy electric locomotives, four EL200 light locomotives were also built, built in 1937 at the HCP plant, from the same electrical components as the EMU trains under construction. Unfortunately, no copy of it has survived.
Post-war electrification at PKP.
The period of World War II completely destroyed the WWK (Warsaw Railway Junction) devices. A large part was also taken away by the occupiers, mainly trains and locomotives. On January 25, 1945, the overhead contact line was rebuilt. The power plant at the Grochów station and at the Warszawa Wschodnia station was rebuilt. Movement was gradually restored. On September 14, 1948, electric trains returned to the route Warszawa Wschodnia - Mińsk Mazowiecki: electric locomotives and EMU. Decisions were made to electrify the country's main railway lines.
The first electrified route to be rebuilt was the section between Warszawa Wschodnia - Otwock. The first test train traveled this route on July 14, 1946. In April 1947, contracts were concluded with Sweden for the supply of electrical components for the railways, EMU and electric locomotives. In 1948, renovation and electrification of the Warsaw-Katowice route began. Many stations along the route had to be rebuilt. The plan also provided for the electrification of the Koluszki - Łódź branch and Wejherowo - Pruszcz line. On June 3, 1956, the electrified route from Warsaw to Łazy near Zawiercie was put into use, with a total length of 281 km. The route led through: Skierniewice - Koluszki - Piotrków Trybunalski - Częstochowa - Zawiercie. In 1957, electric trains were already commuting to Katowice and Gliwice. In 1959, the Silesia - Krakow route was electrified, via: Szczakowa - Trzebinia - Krakow - Nowa Huta. In 1964, the Kraków - Rzeszów route was electrified. In 1961, we already had about 1,200 km of routes electrified in Poland. It was 25% of the assumptions at that time.
At that time, the discussion about the type of electric traction began anew. After substantive discussion, it was decided to stick to the pre-war 3 kV DC system. Upgrading to 25 kV 50 Hz was then an economic nonsense. The contribution of the pre-war pioneers of PKP electrification, who carried out a lot of research and development work, also in the field of nomenclature and technical regulations, was appreciated.
Mercury rectifier.
Mercury rectifier otherwise called ignitron and also called Hewitt rectifier. A rectifier is a component or set of electronic components that converts an AC voltage to a single character voltage that can be converted to DC voltage after further filtering.
These rectifiers were used in all devices requiring high power from a few kilowatts to a few megawatts. The operating voltages ranged from 110 V to 30 kV. The method of operation is based on the discovery that the electric arc between liquid mercury (cathode) and a metal electrode (anode) allows the current to flow in one direction. They usually had several anodes powered from a multiphase transformer, where the electric arc jumped from the cathode (mercury pool) to individual anodes. This allowed for more precise and continuous operation of the rectifier. Six-phase and even twelve-phase systems have often been used using star-connected three-phase transformers with phase-to-phase transformers at the connections.
The use of electricity with a value of 3 kV DC to drive electric locomotives and EMUs will be possible thanks to an alternating current to direct current rectifier in the form of a mercury rectifier. A mercury rectifier is generally a glass bulb containing approximately 1 liter of mercury. Mercury is on the electrode side of the cathode. The mercury rectifier is made as a vacuum or gas lamp (inert gas). The rectifier has an additional ignition electrode called ignitor and therefore this is the name of the rectifier.
The mercury rectifier was the basic device installed in PKP traction substations. Often these were mercury rectifiers manufactured by the EAW plant in the GDR. There was a vacuum inside the rectifier and it could not be dismantled. This made handling easier and reduced the number of parts that could be damaged. Two such devices worked in one team, connected in parallel. The substation was supplied with three-phase alternating current from the 110 kV grid. On the other hand, there was a direct voltage of up to 3,300 V DC and a current of 330 A (from one mercury rectifier). There were six phases in total, which were distributed by cables to different traction sections. The mercury rectifier had safety features. This rectifier required a very stable anode temperature and therefore was either heated or cooled. The disadvantage of mercury rectifiers is the mercury itself. There was a concern about the emission of small mercury vapor into the environment.
This was a time when semiconductors were not yet used. In the following years, the glass bulb was replaced with a box made of special metal alloys, and its walls were a mercury rectifier cooler. Currently, mercury rectifiers are no longer used in industry. They have been completely replaced by cheaper, more efficient and smaller semiconductor rectifiers, silicon diodes. Semiconductor technology made it possible to produce silicon rectifiers. The simplicity and durability of their construction made it possible to put them in the box of an electric locomotive.
Discussion on the electrification of PKP in 1956 year.
Discussion about what current; permanent or commutative, it began again around 1956. Opinions were divided. Supporters of single-phase alternating current even used the argument that PKP is against technical progress. Nevertheless, the decision of PKP was preceded by long-term and comprehensive research and calculations, opinions of scientific institutions and professional scientific and technical associations NOT. The results and opinions were clear and unanimous; It is not profitable to introduce alternating current to Polish railway lines. The discussion that started again after 2015 did not change the conclusions. For the railroad, the matter is quite clear; We stay at 3 kV DC.
The voltage level determines the cross-section of the conductors. The greater the tension, the smaller the cross section. It must be remembered that 100 m of a copper wire with a cross-section of 100 mm2 weighs about 1 ton. In Poland, we have usually used and still use two contact wires with a cross section of 250 mm2. But in France, when it used a voltage of 1,500 V, the conductor cross-sections were as high as 800 mm2. It is easy to calculate how many tons of copper had to be suspended in the air and how densely the traction poles were.
In the 40s of the twentieth century, all over the world, Engineers worked intensively on a single-phase AC motor that could work at an industrial frequency of 50 Hz. At that time, the problem was not resolved satisfactorily.
A few years before the Second World War, Germany attempted to apply alternating current of normal frequency (50 Hz) to the railroad traction so much that they put in operation a small section of the railway line in the Black Forest. They built four different electric locomotives and began their trials. They did not finish the tests because the war broke out. After the war, France continued these tests and concluded that it had advantages. They decided to build their railways with the support of 25 kV 50 Hz alternating current. In addition, it was possible to change the already built 1,500 V DC traction into 25 kV 50 Hz. During the tests, the French used their own locomotives and their own test section in Haute-Savoie.
In 1953, France decided to electrify exclusively with alternating current. From that moment on, the trend of electrification with single-phase alternating current dates back.
Various decisions were made in other countries. Countries that were at the beginning of electrification opted for alternating current. Countries that already had partial electrification remained with direct current: Belgium, Italy, Poland. Poland was justified by the mastery of the 3 kV DC technology and the lack of 25 kV 50 Hz technology. The English went the farthest, and even converted some electrified sections with direct current of 1,500 V DC into high voltage alternating current traction. Of course, this caused disputes between professionals, the echo of which continues to this day (2021).
However, the problem was not completely resolved. Alternating current does not matter in all conditions. It is economical on long distances and on routes with a high volume of transport. Then freight trains weighing 3,000 - 4,000 tons can be assembled. But such a train is very long and will not fit on the sidetrack at every station. Alternating current is not suitable for local EOD trains, because I have no place for a large and heavy transformer.
In 1960, Polish engineers simulated the Poznań - Szczecin route with a current of 25 kV 50 Hz. (Comparative study of the electrification of the Poznań - Szczecin line with 3 kV direct current and 25 kV 50 Hz alternating current. May 1961). With alternating current, the investment cost would be 5-7% lower. Operating costs would also be lower by around 10%. It would be an anachronism to electrify only some of the routes. There would have to be different locomotives. There would be no reduction in travel times
In addition, a new electricity grid would be necessary. The 110 kV grid is too weak. A 220 kV or even 400 kV network was suitable, and there was and is not much of such a network in Poland.
The conducted research showed that in the current conditions, despite some savings on power devices, the total expenditure on electrification and associated works for the new system would be greater than the corresponding expenditure related to electrification of the DC system.
In the 1960s, there was a problem for the rolling stock industry, especially the PaFaWag factory, which built electric locomotives. For them, the foreign market with 25 kV 50 Hz was closed. It was planned to build a test track for this type of electric locomotive, but it was not done. Only in 2017, such a test track was built by NEWAG near Nowy Sącz.
In 1965, the discussion was concluded by the Ministry of Communications by issuing an order to continue electrification with 3 kV DC. In Poland, the discussion started in the mid-1980s. But the Polish economy was in a serious crisis and the discussion was only paper-based.
The planned Central Communication Port once again provoked a discussion: 3 kV DC or 25 kV 50 Hz. Another report appeared, titled "3 or 25 kV?" It was another substantive discussion. The obtained results showed that it is not possible to clearly state which of the traction power supply systems is better in Polish conditions. Especially if we are talking about passenger trains with a running speed of 200 - 250 km / h.
In 2020, there are approximately 12,000 km of railway lines electrified in the 3 kV DC system in Poland. It is about 60% of all railway lines in Poland. These are lines of mainly national and transit importance. They are powered by over 500 traction substations, mostly new or modernized in recent years.
25 kV 50 Hz alternating current system currently. 2020 year.
In the block diagram, both systems are similar: 3 kV DC and 25 kV AC. The traction energy flow sequence is as follows: AC power line - step-down transformer - rectifier - voltage regulator - DC traction motors. Even in the AC system, DC series motors are used because their traction characteristics best match the required operating conditions in an electric and diesel locomotive with an electric transmission.
The difference is a different location of the contact point in this line - the current receiver (pantograph). In the DC (direct current) system, the step-down transformer and the rectifier are located in the traction substation building. As a result, the pantograph (current collector) is the weakest link in the system. Therefore, in the 3 kV DC system, there are two traction cables, not because of the wire cross-section, but because of the largest possible contact area between the collector and the wires.
The 25 kV AC system has substations spaced less frequently, every 50 - 60 km, much less often than at 3 kV DC (every 15 - 25 km). Thinner traction cables mean a lighter network and less frequent traction poles. There is only one contact wire. The higher the voltage, the lower the amperage, and as a result, less voltage drops along the overhead contact line, and therefore less energy transmission losses.
The disadvantage of the 25 kV 50 Hz system is the high requirements for the public supply network. One phase loads the supply network more and asymmetrically. Therefore, a 220 kV or even 400 kV line is preferred. The 110 kV network can be used, but requires the use of additional devices that will reduce the phase asymmetry. In addition, 25 kV 50 Hz interferes with devices used in PKP, for example SRK. Axle counters are already used on modernized railway lines, which are insensitive to traction return and stray currents, but a whole range of sensors and controllers operate at the industrial frequency of 50 Hz. The 3 kV DC system does not have these drawbacks.
Many years of operation have shown that up to a train speed of up to 200 km / h, there is no problem with energy transmission through the pantograph. The problem occurs at speeds over 250 km / h. In Italy, the high-speed train is scheduled to run at 220 km / h. In Poland, the Pendlino runs 200 km / h and a bit more.
At 3 kV DC, however, it is easier to build an electric locomotive, and therefore a lower cost of purchasing rolling stock for servicing specific trains.
Technological progress made it possible to send energy through high voltage overhead lines, while its lowering and straightening took place in the locomotive itself. The problem, however, was the step-down transformer, the dimensions of which did not allow it to be placed under the vehicle's body or on its roof. Therefore, EMU structures have not developed on AC rails. As a half-measure, variable-direction trains, also now known in Poland as "push-pull", were developed.
The current technology enables the construction of multi-system locomotives. Such locomotives are built in Poland. The cost of a multi-system locomotive is not as drastically higher than a vehicle with one power system as it was in the 20th century. All that is needed is a safety system so that the wrong pantograph is not lifted to the wrong net.
In Poland, the standard is 440 mm2, i.e. two wires of 220 mm2. There were economically important railway sections where two 250 mm2 cables were installed. For example, in Italy there is 620 mm2 on the Rome - Florence route. Here, trains run at a speed of 220-250 km / h. In Polish conditions, the nominal speed is 220 km / h.
The current discussion is pointless, because billions of zlotys have already been spent on modernizing the railway infrastructure. Many routes have already been adapted to the running speed of passenger trains up to 160 km / h.
Polish railways have electric locomotives that can already run at a speed of 200 km / h. Polish new EMUs run up to 160 km / h as standard. Similarly, classic trains already reach 160 km / h. Many electric locomotives already have asynchronous electric motors which have better parameters. The new PCS trains normally run at speeds of up to 120 km / h. This is due to the physics of the electric transmission used in these trains and the economics.
What was planned for the electrification of 25 kV AC at PKP?
The Central Railway Main Line was typed to change the current to 25 kV AC. However, the venture is wasteful.
The newly built route "Y" Warsaw - Łódź with departures to Poznań and Wrocław was selected. The trail has not yet been built.
The route No. 203 Kostrzyn nad Odrą - Gorzów Wielkopolski - Krzyż Wielkopolski - Piła - Chojnice - Tczew was selected. Here, the problems of crossing the 25 kV 50 Hz route with the 3 kV DC network were encountered at the stations: Kostrzyn (here there is less problem, because the main routes cross on the railway viaduct), Krzyż Wielkopolski, Piła, Tczew.
In 2010, the Management Board of PLK S.A. decided to prepare the planned section of the "Rail Baltica" line from the Ełk station through Suwałki to the border in Trakiszki, treating this section as a testing ground.
From an economic point of view, only the electrification of the LHS line with a current of 25 kV 50 Hz makes sense. This broad-gauge railway network is independent of the standard-gauge railway network. The more that there is a 400 kV power grid nearby.
Written by Karol Placha Hetman