Major electrical equipment associated with advanced traction system are given below:
1. Over Head Equipment (OHE) system to provide 25 kV, 50 Hz AC power supply
2. Pantograph
3. Lightning Arresters
4. OHE voltage sensing unit or Potential Transformer
5. Vacuum Circuit Breaker
6. Traction Transformer
7. Current Transformer
8. Single Phase Four Quadrant Traction Converter
9. DC link & Capacitor bank
10. Three Phase Four Quadrant Traction Converter
11. Three Phase Induction Motor
12. Auxiliary Converter to supply auxiliary load of train
13. Microprocessor based train control system
14. Wheel & Rails provide return path for OHE current
1. Over Head Equipment (OHE) system:
It consists of power sub-station, switchgear & protection system and over-head line ( i.e. mast & foundation, arm assembly, tensioning pulley, insulators and conductors etc. ). Generally, two conductors are used in over head line; upper one is called messenger wire or catenary and lower one is called contact wire. To achieve good current collection at higher speed, it is necessary to keep the contact wire straight. It is achieved by supporting the contact wire from catenary wire. In physics and geometry, a catenary is the curve that an idealized hanging chain or wire assumes under its own weight when supported only at its ends. This curve superficially similar in appearance to a parabola, but it is not a parabola; it is a graph of the hyperbolic cosine. Contact wire is supported from the catenary wire at regular intervals by vertical wires known as "droppers" or "drop wires". Catenary and contact wires are also connected in parallel by separate jumpers for proper distribution of electric current in both wire. The whole system is then subjected to a mechanical auto tensioning device. The function of auto tensioning device (ATD) is to maintain a constant tension in OHE conductors i.e. contact wire and catenary's wire under varying temperature conditions, so that profile of OHE can be maintained for better current collection. On straight track, the contact wire is staggered or zigzagged slightly to the left and right of center at each successive support so that the pantograph wears evenly. Contact wire is made of hard-drawn copper with small amount of silver(Cu-Ag), cadmium, magnesium, tin etc. On a larger electrified railway, different areas of track are fed from different traction substations which are normally connected to different phases of power grid supply for proper load balancing. Generally, each traction sub-station covers a distance of about 30-40Km for feeding all the trains in that zone. Short circuit between two different phase supply of adjacent section fed from separate traction sub-station can be eliminated by providing isolation between them. For this purpose, neutral section is provided between two adjacent sections. Neutral section is defined as short insulated dead overhead equipment separating sections fed by two adjacent traction sub-stations.
2. Pantograph:
A pantograph is a device mounted on the roof of an electric train to collect electrical power (current) through contact with an OHE system. The most common type of pantographs have a 'Z' style shape. The pantograph is held up by compressed air pressure or spring pressure and also applies pressure on the contact wire. Current is collected from overhead lines by pantographs and feed to traction transformer through vacuum circuit breaker. For smooth operation of electrical equipment, pantograph must maintain good electrical contact with overhead line under all running conditions. At higher speed, it is more difficult to sustain a good electrical contact with contact wire. Good electrical contact between pantograph and overhead line at higher speed can be assured by increasing upward pressure applied by pantograph on the overhead line. On the other hand, the wear & tear of pantograph and overhead line also increase with rise in upward pantograph pressure. Current collecting portion of pantograph used on 25 KV OHE system, wherein amplitude of pantograph current is around 300A, is usually made of graphite / metallized carbon. Graphite is a good dry-lubricant in presence of moisture content. It is a soft material as compared to OHE contract wire material which reduces the wear & tear of OHE contact wire.
3. Lightning Arresters:
A lightning arrester or surge diverter is a device used on electrical power systems to protect the insulation and equipment of the system from the damaging effects of lightning. One terminal of lightning arrester is connected to high-voltage terminal and other with the ground terminal. Lightning arrester is used on power systems above 1000V to protect other parallel connected equipment from lightning and switching surges. It does not absorb the lightning; however, it diverts the lightning to ground terminal. All lightning arresters consist of metal oxide varistor (MOV) disk. Basically the MOV disk is a semiconductor that is sensitive to the voltage. At normal voltage, the MOV disk works as an insulator, but at higher voltage, it becomes a conductor. The MOV disk is a very fast acting semiconductor switch. Generally, gap less zinc oxide surge arresters are mainly used for surge protection. The micro structures of zinc oxide (ZnO) block highly doped with antimony represents a collection of billions of microscopic junctions of ZnO grains in series and parallel arrangement and separated by inter granular junctions. Each boundary wall between two ZnO grains works as a voltage sensitive switch that turn on at a voltage of 3V. If there are 1000 series junctions from top to bottom of a zinc oxide (ZnO) disk, it will turn on at voltage of about 3000 volts. In a lightning arrestor, many solid zinc oxide discs are arranged one by one to form a cylindrical stack.
4. Potential Transformer:Potential transformer (PT) is a voltage step-down instrument transformer of low power rating which reduces the voltage of a high voltage circuit to a lower level for the purpose of measurement. It is used to measure the voltage of the system and connected in parallel to the line. They are designed to have an accurate voltage ratio, minimum phase error and negligible loading on the system to enable accurate secondary connected metering.
5. Vacuum Circuit Breaker:A vacuum circuit breaker (VCB) is used at the output of pantograph to isolate the main traction transformer from the OHE supply in case of internal fault or for maintenance work. In vacuum circuit breaker, vacuum is used as the arc quenching medium which offers high insulating strength and superior arc quenching properties. With 25KV AC OHE system, VCB are usually installed on the roof near the pantograph. Due to less maintenance requirement and capability of repeated operation, VCB is suitable for traction purpose.
1. Over Head Equipment (OHE) system to provide 25 kV, 50 Hz AC power supply
2. Pantograph
3. Lightning Arresters
4. OHE voltage sensing unit or Potential Transformer
5. Vacuum Circuit Breaker
6. Traction Transformer
7. Current Transformer
8. Single Phase Four Quadrant Traction Converter
9. DC link & Capacitor bank
10. Three Phase Four Quadrant Traction Converter
11. Three Phase Induction Motor
12. Auxiliary Converter to supply auxiliary load of train
13. Microprocessor based train control system
14. Wheel & Rails provide return path for OHE current
1. Over Head Equipment (OHE) system:
It consists of power sub-station, switchgear & protection system and over-head line ( i.e. mast & foundation, arm assembly, tensioning pulley, insulators and conductors etc. ). Generally, two conductors are used in over head line; upper one is called messenger wire or catenary and lower one is called contact wire. To achieve good current collection at higher speed, it is necessary to keep the contact wire straight. It is achieved by supporting the contact wire from catenary wire. In physics and geometry, a catenary is the curve that an idealized hanging chain or wire assumes under its own weight when supported only at its ends. This curve superficially similar in appearance to a parabola, but it is not a parabola; it is a graph of the hyperbolic cosine. Contact wire is supported from the catenary wire at regular intervals by vertical wires known as "droppers" or "drop wires". Catenary and contact wires are also connected in parallel by separate jumpers for proper distribution of electric current in both wire. The whole system is then subjected to a mechanical auto tensioning device. The function of auto tensioning device (ATD) is to maintain a constant tension in OHE conductors i.e. contact wire and catenary's wire under varying temperature conditions, so that profile of OHE can be maintained for better current collection. On straight track, the contact wire is staggered or zigzagged slightly to the left and right of center at each successive support so that the pantograph wears evenly. Contact wire is made of hard-drawn copper with small amount of silver(Cu-Ag), cadmium, magnesium, tin etc. On a larger electrified railway, different areas of track are fed from different traction substations which are normally connected to different phases of power grid supply for proper load balancing. Generally, each traction sub-station covers a distance of about 30-40Km for feeding all the trains in that zone. Short circuit between two different phase supply of adjacent section fed from separate traction sub-station can be eliminated by providing isolation between them. For this purpose, neutral section is provided between two adjacent sections. Neutral section is defined as short insulated dead overhead equipment separating sections fed by two adjacent traction sub-stations.
2. Pantograph:
A pantograph is a device mounted on the roof of an electric train to collect electrical power (current) through contact with an OHE system. The most common type of pantographs have a 'Z' style shape. The pantograph is held up by compressed air pressure or spring pressure and also applies pressure on the contact wire. Current is collected from overhead lines by pantographs and feed to traction transformer through vacuum circuit breaker. For smooth operation of electrical equipment, pantograph must maintain good electrical contact with overhead line under all running conditions. At higher speed, it is more difficult to sustain a good electrical contact with contact wire. Good electrical contact between pantograph and overhead line at higher speed can be assured by increasing upward pressure applied by pantograph on the overhead line. On the other hand, the wear & tear of pantograph and overhead line also increase with rise in upward pantograph pressure. Current collecting portion of pantograph used on 25 KV OHE system, wherein amplitude of pantograph current is around 300A, is usually made of graphite / metallized carbon. Graphite is a good dry-lubricant in presence of moisture content. It is a soft material as compared to OHE contract wire material which reduces the wear & tear of OHE contact wire.
3. Lightning Arresters:
A lightning arrester or surge diverter is a device used on electrical power systems to protect the insulation and equipment of the system from the damaging effects of lightning. One terminal of lightning arrester is connected to high-voltage terminal and other with the ground terminal. Lightning arrester is used on power systems above 1000V to protect other parallel connected equipment from lightning and switching surges. It does not absorb the lightning; however, it diverts the lightning to ground terminal. All lightning arresters consist of metal oxide varistor (MOV) disk. Basically the MOV disk is a semiconductor that is sensitive to the voltage. At normal voltage, the MOV disk works as an insulator, but at higher voltage, it becomes a conductor. The MOV disk is a very fast acting semiconductor switch. Generally, gap less zinc oxide surge arresters are mainly used for surge protection. The micro structures of zinc oxide (ZnO) block highly doped with antimony represents a collection of billions of microscopic junctions of ZnO grains in series and parallel arrangement and separated by inter granular junctions. Each boundary wall between two ZnO grains works as a voltage sensitive switch that turn on at a voltage of 3V. If there are 1000 series junctions from top to bottom of a zinc oxide (ZnO) disk, it will turn on at voltage of about 3000 volts. In a lightning arrestor, many solid zinc oxide discs are arranged one by one to form a cylindrical stack.
4. Potential Transformer:Potential transformer (PT) is a voltage step-down instrument transformer of low power rating which reduces the voltage of a high voltage circuit to a lower level for the purpose of measurement. It is used to measure the voltage of the system and connected in parallel to the line. They are designed to have an accurate voltage ratio, minimum phase error and negligible loading on the system to enable accurate secondary connected metering.
5. Vacuum Circuit Breaker:A vacuum circuit breaker (VCB) is used at the output of pantograph to isolate the main traction transformer from the OHE supply in case of internal fault or for maintenance work. In vacuum circuit breaker, vacuum is used as the arc quenching medium which offers high insulating strength and superior arc quenching properties. With 25KV AC OHE system, VCB are usually installed on the roof near the pantograph. Due to less maintenance requirement and capability of repeated operation, VCB is suitable for traction purpose.