Traction Power Supply System - Traction Substations and Catenary

The traction power supply system refers to receiving electric energy from the electric power system or the primary power supply system, and providing electric energy of the required current system to the electric locomotive load after transforming voltage, changing phase or commutating (converting power frequency AC to low frequency AC or DC voltage), It is also a complete system that completes all functions such as traction power transmission and power distribution. The performance of the traction power supply system directly affects the performance of the traction power of the train and the performance of the traction drive control system.

1. Introduction of traction power supply system

It is mainly composed of traction substation and catenary. The traction substation reduces the voltage and converts the electric energy sent by the power system through the high-voltage transmission line, and then transmits it to the catenary to supply the electric locomotive running along the line. In some countries electrified railways are sometimes powered by dedicated power plants.

The electric traction power supply system is divided into DC system and AC system according to the nature of the current provided to electric locomotives, and the AC system is divided into industrial frequency single-phase AC system and low-frequency single-phase AC system. Power frequency refers to the industry standard frequency, that is, 50 Hz or 60 Hz; low frequency refers to the frequency lower than the industry standard frequency, and the most widely used frequency is 1/3 of 50 Hz. There are great differences in the equipment of the electric traction power supply system of various current systems.

2. Current-based development of traction power supply system

The application of direct current system was the earliest. When electric traction began to be used on railway trunk lines at the end of the 19th century, direct current system was applied. At present, there are still a large number of DC systems in Britain, France, Japan, the Soviet Union and other countries. The direct current system is to step down and convert the three-phase alternating current of the power system into direct current to supply the catenary. The catenary voltage is 1200 volts, 1500 volts, 3000 volts and so on. Since the voltage of electric locomotives is limited by the commutation conditions of the DC traction motor, it is difficult to increase the catenary voltage greatly, so the DC system must transmit a large amount of current along the catenary. Generally, two copper contact wires must be used on the catenary, and copper bearings should be used. Lisuo, plus some parallel aluminum reinforcement wires to shunt, consumes a large amount of non-ferrous metals. In addition, in order to maintain the voltage level of the catenary, a traction substation must be installed every 10 to 30 kilometers along the line. These weaknesses of the DC system have promoted the research of the AC system.

At the beginning of the 20th century, power frequency three-phase AC system and low frequency single-phase AC system appeared one after another. The power frequency three-phase AC system was once applied in Italy, two phases of the three phases were conveyed by catenary, and the other phase was grounded. Later, it was eliminated due to the complex structure and difficult maintenance of the two-phase catenary. The low-frequency single-phase AC system has been developed in Germany, Sweden, Switzerland and other countries. The catenary voltage of this current system is generally 15,000 volts, and the voltage is stepped down on the electric locomotive, using a single-phase commutator traction motor. The catenary of the AC system is much simpler than that of the DC system, and the setting distance of the traction substation is also lengthened. The main reason for using low frequency is that the commutator traction motor is difficult to commutate, which is not suitable for operation at industrial frequency. Low-frequency systems require low-frequency power supplies, so low-frequency electrified railways must build dedicated low-frequency power plants, or step down and transform the power-frequency current sent by the power system into low-frequency current at traction substations. In the early years, a motor generator set was used to change the frequency, and later a static frequency converter was used, and the equipment was more complicated than the DC system. Single-phase commutator traction motors are not as simple in structure and easy to maintain as DC traction motors.

In 1933, Hungary built a power-frequency single-phase AC electrified railway with a catenary voltage of 16,000 volts. The electric locomotives used rotary phase-changing frequency converters and three-phase asynchronous motors. This electric locomotive has not been popularized due to its complex structure. In 1955, France successfully adopted static rectifiers and DC traction motors on electric locomotives, and the power frequency single-phase AC system was implemented in various countries. Japan, the Soviet Union, the United Kingdom, India and other countries that originally adopted the DC system also successively adopted the power frequency single-phase AC system. The AC catenary voltage is generally 25,000 volts, the structure of the catenary is further simplified, and the distance between traction substations is expanded to 30-70 kilometers. Static rectifiers on electric locomotives initially used squibs, and later silicon semiconductor rectifiers were commonly used. The advancement of rectification technology is an important factor for the wide application of power frequency single-phase AC system. China's railways adopt a power frequency single-phase AC system, and 25,000 volts is set as the standard voltage of the catenary. The operation experience of the "Shaoshan" type electric locomotive using silicon semiconductor rectifiers has proved that this type of electric locomotive is easy to maintain and reliable in operation.

3. Electric traction power supply system

The electric traction power supply system refers to receiving electric energy from the electric power system or the primary power supply system, and providing electric energy of the required current system to the electric locomotive load after transforming voltage, phase changing or commutating (converting power frequency AC to low frequency AC or DC voltage) , and complete a complete system with all functions such as traction power transmission and power distribution. The performance of the traction power supply system directly affects the performance of the traction power of the train and the performance of the traction drive control system.

The power frequency AC single-phase electric traction power supply system is mainly composed of traction substation, traction network, partition station, switching station and other parts.

3.1 Traction substation

The main transformer used in the DC traction substation steps down and converts the three-phase AC into 6-phase or 12-phase, and then rectifies it with a rectifier. In the power frequency single-phase AC system, only step-down is performed in the traction substation, and the main equipment is a step-down transformer, called the main transformer. Traction substations are divided into three-phase, single-phase and three-phase-two-phase traction substations according to the main transformer winding connection mode.

3.1.1 Three-phase traction substation

Its main transformer structure is the same as that of a general three-phase power transformer, except that the rated voltage of the secondary side is 27,500 volts. Windings are usually connected in Y/△. Run in parallel with two main transformers. The primary Y-shaped winding is connected to the high-voltage bus of the power system, and one end of the secondary △ winding is grounded, and the other two ends supply power to the catenary on both sides. The advantage of the three-phase traction substation is that the main transformer is cheap and the power distribution equipment is simple, and the power transformer on the 27,500-volt side can be stepped down to 10,000 volts to supply power to the three-phase loads in neighboring areas and railways. The disadvantage is that the capacity utilization rate of the main transformer is low, and one of the three-phase windings cannot reach the rated load. In addition, traction substations form asymmetric loads on the power system, and usually the two phases with heavy negative charges of each traction substation must be connected to the three phases of the power system in rotation. Most of the power frequency single-phase AC electrified railways in China and the Soviet Union use three-phase traction substations.

3.1.2 Single-phase traction substation

Single-phase double-winding main transformer is adopted. There are two wiring methods: simple single-phase wiring and V/V wiring. V/V wiring is to connect the primary side of the two main transformers between two different phases of the high-voltage bus, and the secondary side supplies power to the catenary on both sides with different phase voltages. Simple single-phase wiring equipment is simple and economical, and the capacity utilization rate of the main transformer is high. However, since the traction substation constitutes a single-phase load on the power system, even if each traction substation is connected to the three phases of the power system in rotation, a large amount of negative sequence current will still be generated in the local system, so it is only suitable for power systems. It is used in areas with larger capacity. The negative sequence current generated by single-phase V/V connection in the power system is the same as that generated by three-phase traction substation, which is smaller than that generated by simple single-phase connection. This wiring can also be used on the 27500 volt side to apply a step-down transformer to supply three-phase loads in the area. However, the two main transformers are not connected in parallel, and the operation procedures and equipment are more complicated. Single-phase traction substations are generally used in power frequency single-phase AC electrified railways in France, the United Kingdom, and India, and most of them use simple single-phase wiring. China only uses single-phase V/V wiring on individual lines.

3.1.3 Three-phase-two-phase traction substation

The main transformer generally adopts Scott connection. There are two windings on the primary side, the turns ratio is 1:3, the short winding (called the high winding) is connected to the midpoint of the long winding (called the bottom winding), and the three outlets are connected to the three phases of the high-voltage bus to form "T" shaped connection. The two windings on the secondary side output symmetrical two-phase voltages, which supply power to the catenary on both sides respectively. The advantage of Scott connection is that when the catenary loads on both sides are equal, the main transformer takes symmetrical three-phase current from the power system. The disadvantage is that a special main transformer is required. In addition, like simple single-phase wiring, the regional three-phase load cannot be supplied on the 27,500-volt side. Three-phase-two-phase traction substations are the most widely used in Japan.

3.2 Division Office

In order to increase the flexibility of power supply and improve the reliability of operation on AC electrified railways, a partition station is often added between the power supply areas of the two traction substations. The functions of the partition stations can be briefly described as follows.

3.2.1 It is possible to make two adjacent power supply sections work in parallel or work alone. When the parallel operation is realized, the circuit breaker of the partition is closed, otherwise it is opened.

3.2.2 When the adjacent traction substation fails and cannot continue to supply power, the circuit breaker of the district substation can be closed, and the non-faulty traction substation will implement cross-area power supply.

3.2.3 When a short-circuit accident of the traction network occurs in the power supply area with bilateral power supply, half of the power supply area where the accident point is located can be cut off by the circuit breaker of the subdivision station, and the non-accident section can still work as usual.

3.3 Opening and closing station

The switch station of the AC electric traction system is actually a switch station for power distribution, and is generally set up in the following two situations or systems.

One situation is that in the railway hub area far away from the traction substation, due to the large number of station lines and the corresponding complexity of the catenary, the passenger and freight traffic, marshalling and locomotive maintenance operations are busy, resulting in an increase in the probability of failure in this area. In order to ensure the power supply of the hub area To improve reliability and reduce the scope of accidents, the catenary is generally divided into horizontal groups and partitions for power supply, and the multiple feeders of the switching station supply power to each group and partition of the catenary.

Another situation is to set a switching station in the middle of the power supply arm of the double-line traction network in the AT power supply mode. Due to the increase in the power supply voltage (2×25kV) in the AT power supply mode, the distance of the power supply arm increases, up to 40-50km, in order to improve the flexibility of power supply (such as catenary power outage maintenance, etc.), to reduce the scope of accidental power outages, it is necessary to set up switching stations between the traction substation and the divisional station.

3.4 Autotransformer station

If the power frequency single-phase AC electrified railway adopts autotransformer (AT) power supply mode, an autotransformer needs to be installed every 10-15km along the line. The autotransformer should be installed on each station along the railway as far as possible, which is roughly the same distance as the railway section. At the same time, it should be merged with the partition office and the opening and closing office to facilitate operation and management.

3.5 Catenary

The catenary is a transmission network laid overhead along the electrified railway. The sliding contact between it and the pantograph of the electric locomotive sends the current sent by the traction substation to the electric locomotive.

The catenary is mainly composed of contact suspension and its pillars. Commonly used are simple elastic suspension and single chain suspension.

Simple elastic suspension has only one contact wire, which is hung on the support by elastic suspension strings. The elastic suspension string can moderate the impact of the pantograph on the suspension point. This kind of suspension can adapt to the running speed of 70-90 km/h. The contact wire has good elasticity and can adapt to speeds above 100 km/h. The contact wire material has the characteristics of wear resistance, corrosion resistance, high tensile strength and good electrical conductivity. Most countries mainly use copper wires and cadmium copper wires. Steel-aluminum bimetal conductors are widely used in China. In order to make the contact wire have the necessary tension, an anchor section is set every certain length of the catenary, one end of the contact wire is anchored, and a load-bearing body is hung on the other end, which is called a compensator. The compensator automatically moves up and down when the contact wire shrinks and expands due to seasonal changes, so that the tension of the contact wire remains unchanged.

The single-chain suspension uses a catenary cable, and the contact wire is evenly suspended on the catenary cable with a suspension string. Compensation measures for catenary cables are called full-compensation single-chain suspensions. The advantage of this structure is that the contact wire is straight, and the contact suspension is evenly elastic, so the pantograph and the wire have better contact, and the current is better, and it is suitable for lines with frequent operation and high speed. DC electrified railway catenary generally adopts two contact wires and single-chain suspension. The AC catenary adopts a contact wire and single-chain suspension or simple elastic suspension. China mainly adopts single chain suspension, but also begins to adopt simple elastic suspension. There is also a double-chain (double-chain, triple-chain) suspension (Figure 7), which is to add an auxiliary catenary cable between the single-chain suspension catenary cable and the contact wire, and hang it on the bearing cable with a hanging string. Then hang the contact wire on the auxiliary catenary. This structure makes the contact suspension elasticity more uniform and adapts to higher operating speeds. Japan's Tokaido Shinkansen uses elastic double chain suspension.

Most of the early catenary used metal pillars, and later switched to reinforced concrete pillars. This pillar saves steel, is corrosion-resistant, and has a lower cost. The contact suspension is hung on the metal arm of the pillar, and the positioner is used to fix the horizontal position of the contact wire, so that the contact wire goes in a zigzag direction along the line, so as to prevent the pantograph of the electric locomotive in operation from being rubbed by the contact wire at one point. hurt.

3.5.1 Power supply mode

The DC electrified railway catenary generally adopts the power supply mode on both sides, and a partition booth is set in the middle of the catenary powered by two adjacent traction substations to connect the catenary. The electric locomotives in operation are powered by the traction substations on both sides at the same time. This power supply mode can reduce the loss of electric energy in the catenary, reduce the voltage drop of the catenary, and when a traction substation fails, the operation of the electric locomotive will not be interrupted. AC electrified railways often adopt the power supply method on one side, and the catenary is disconnected at the partition booth. The function of connecting the upstream and downstream terminals in parallel.

3.5.2 Anti-interference facilities

In order to reduce the interference of the electromagnetic induction of the catenary current on the communication circuit along the line, the catenary is divided into a current-absorbing segment every 2-4 kilometers in the section of the AC electrified railway adjacent to the town, and the return line and the current-absorbing transformer are set up. At this time, the current of the electric locomotive flows back to the traction substation along the return line, so that there is little current flowing back along the track and the ground. The currents of the return line and the catenary are approximately equal and opposite in direction, which greatly reduces the interference of the electrified railway to the communication circuit along the line. The disadvantage of this method is that the current-absorbing transformer is connected in series in the circuit, which increases the impedance of the catenary. The new power frequency single-phase AC electrified railway in Japan adopts the autotransformer method, and an autotransformer is installed every 10 kilometers along the railway. The neutral point of the autotransformer is grounded, one end is connected to the catenary, and the other end is connected to the return line, which is called a positive feed (electricity) line. The positive feeder and catenary current are equal in size and opposite in direction, which also plays a role in reducing interference to the communication circuit. On the other hand, since the voltage between the catenary and the positive feeder is twice the catenary voltage, the voltage drop along the catenary is greatly reduced.