A traction substation, traction current converter plant, rectifier station or traction power substation (TPSS) is an electrical substation that converts electric power from the form provided by the electrical power industry or railway owned traction power network to an appropriate voltage, current type and frequency to supply trains, trams (streetcars) or trolleybuses with traction current. A traction power substation may also refer to a site that supplies a railway traction power network with power from the public electricity utility.
Function
The exact functions and power conversions made by a traction substation depends on the type of electrification system in use. Broadly there are three categories of electrification system each with different system architectures: Low Voltage DC Electrification (using conductor rail of overhead line), High Voltage Low Frequency AC Electrification with overhead line, and 25kV mains frequency AC with overhead line.
However, across all systems, traction substations can be defined as any site where multiple overhead line or conductor rail circuits are connected to through (usually) circuit breakers to a busbar. This allows for two functions.
To provide the capability to isolate specific sections of electrification either automatically (due to fault conditions detected by the breaker or other equipment) or remotely by the action of an electrical control operator for the purposes emergency situations of engineering work. To reduce overall volt drop of the system by paralleling electrification circuits together.
Beyond this, many substations will have additional functions for providing new power to the electrification circuits (from grid connections, other high voltage cables, a stepdown transformer or transformer with rectifier) or for separating different supplies apart from each other by way of circuit breakers fitted between sections of the busbar that are normally open (from now on "normally open points").
But at their core all substations will at least perform those initial two functions, and will revert to that basic state in degraded operations, such as if a particular supply or step-down transformer at a specific site falls offline. Because of these two basic principles, substations are often located at or near major stations and junctions between multiple electrified lines so as to allow for flexibility in switching and feeding arrangements. But also for regular substations to be spaced across long distances of plain line to reduce volt drop.
In terms of busbar configuration, railway substations are almost always radial, the simplest type.
600–3,000V DC Overhead Line or Conductor Rail Low voltage DC systems were the first kind of electrification to emerge in the late 1800s using both overhead line and conductor rail collection. Although many countries (especially in Japan and across Europe) implemented 1.5kV or 3kV DC overhead line networks across mixed traffic mainline networks, the system is mostly deployed nowadays on rapid transit, or short distance/high frequency suburban systems as well as light rail, trams and trolleybuses. 750V is the standard for metros using conductor rail collection and tram or light rail (using overhead line) while 1.5kV is the standard for metros and commuter systems using overhead line collection.
The main benefit of these systems is the relative lack of on-board conversion equipment required by the train with it possible to connect the traction current directly to the traction motor control equipment. This is opposed to AC electrification which requires voltage transformation and (typically) rectification on board every train. However, the lower line voltage means a greater number of tractions substations must be built with the DC current needing to be supported at regular intervals of between depending on exact system voltage and traffic levels/line speed. Although a higher line voltage would decrease the number of substations needed, the maximum traction voltage of a DC system is limited by
the physical size on-board traction motors can be (higher voltages require larger insulation components) the expense and complexity of rectification equipment for higher voltages (especially historically speaking) the difficulty in breaking DC fault currents if conductor rail electrification is preferred, this limits voltage to a maximum of 1,200V due to insulation requirements
The frequent substation spacing and relative lack of on-train equipment required means low-voltage DC system see regular use on short distance/high frequency railways (like metros) because the number of trains running will be greater than the number of transformers and rectifiers installed in traction substations when route distances are short and train frequencies high.
Architecture Source:
This need for many regular DC supply points means low voltage DC systems are likely to utilise a high voltage 3 phase AC traction power network, although this isn't always the case. Broadly speaking, DC electrification that uses traction power networks are typically larger metros and suburban networks started in the early 20th century when public electricity infrastructure was not as readily available (since DC electrification dates back to the late 19th century). While DC electrification installations that connect to the public utility grid at every traction substation were typically installed more recently or are tram systems since public electricity infrastructure is most dense in urban areas.
Traction power networks for DC electrification utilise high voltage 3 phase AC current at utility frequency (50 or 60Hz) and at common voltages of 11, 22, 33 or 66kV although lower voltages have been used historically. 50–60Hz 3-phase AC is not only used for its efficient power transmission properties but also for easy integration with the local electricity grid made possible by the fact that AC/DC rectifiers can easily be fed with 3-phase AC. The railway traction power network primarily supports multiple DC Traction Substations (TSS) where the current is converted to low voltage DC for the railway electrification system. Trains then collect the low voltage DC current which can be fed directly to a motor control system, either resistor banks for DC motors or an inverter motor drive for 3 phase AC motors.
Types of substation
Grid Supply Point (GSP) This is a substation where the railway's traction power network is supplied by the local electricity grid. Since traction power networks for DC railways can be three-phase and can be rated at voltage levels similar to that of the electricity grid in that jurisdiction, grid supply points are often simply switching stations where incoming grid circuits are collected to a busbar and then distributed on the railway side. Or, if the railway load is especially high, the electricity supplier can provide dedicated transformers for railway use supporting the railway traction power network from a higher grid voltage (like ~132kV). Typically, the supplies from different grid supply points are kept separated within the traction power network by using normally open circuit breakers. This is done to limit the impact of a HV fault and to ensure the correct load is being drawn from each grid supply point to avoid over currents (each grid site in a railway system is normally rated for a different power draw).
Traction Substation (TSS) This is where the high voltage 3-phase AC of the traction power network is first stepped down to the traction current voltage (600–3,000V) using a power transformer then rectified to DC before being distributed to the immediate overhead line or conductor rail circuits. The AC switchboard of a typical DC traction substation normally connects to multiple AC circuits allowing for the substation to be fed from two different points for resiliency, and the ability to move the normally open point between different grid supplies around the traction power network. It may also support other auxiliary transformers to supply railway signalling systems, tunnel drainage pumps or station domestic supplies. Where a traction power grid is not used, the three-phase voltage from the local utility is stepped down and rectified in the traction substations to provide the required DC voltage. The typical physical spacing of DC traction substations depends primarily on the traction power voltage supplied to the trains but also the traffic demand and average line speed. +DC Substation SpacingDC Electrification VoltageSpacing of Traction Substations (TSS)600 V750 VAround 1,500 V3,000 V
Signalling Power Railway signalling and telecoms systems generally have their own LV supplies off of the local electricity grid. However, for added supply security, it's common for signalling supplies to also draw from the 3-phase AC traction power network at traction substations with either one or two transformers dedicated to signalling supply.
Track Paralleling Hut (TPH) The distance between traction substations can be increased by installing one or more track paralleling huts between adjacent traction substations. A track paralleling hut provides means for each overhead line or conductor rail circuit to be electrically paralleled at a shared DC busbar. This allows for more efficient use of available power by connecting more trains in parallel to supply points reducing volt drop.
Tee'd Substation This is similar to a TSS but refers to a situation where the DC substation has no ability to switch high voltage AC circuits and is permanently attached to a particular circuit of the traction power network as opposed to a normal traction substation which can switch AC circuits to some extent. Tee'd substations have less operational resiliency than normal TSSs but save in installation and operational cost since no AC switchboard is built other than a single circuit breaker. Track Paralleling Huts are often converted into tee'd substations as a cost effective way to increase the available power to a railway line.
12–15kV AC overhead line
Higher voltage AC electrification at low frequency is almost as old as low voltage DC systems being implemented as early as 1904. Originally, the low frequency AC system fundamentally relied on the utilisation of universal motors with AC current of a frequency between 16 and 25Hz. In the early 1900s, the technology did not exist to implement rectifiers or power electronics on board trains so engineers were limited in methods to run traction motors. Higher frequencies (like those which came to be used by the electrical supply frequency) were found to create intolerable arcing between motor components but at lower frequencies these problems were resolved. This allowed for trains to be fed at a high line voltage which can be then stepped down by an onboard transformer to a voltage appropriate for a traction motor, then fed directly as AC to the motors. Almost all AC systems that have been implemented at scale use single-phase AC due to the mechanical and practical complexities of duplicating catenary systems as is required by 3-phase AC electrification
In the early years of railway electrification, the main advantage of low frequency AC systems was that AC can be fed to trains at a much higher voltage than that which is optimal for the traction motors since the AC voltage can be stepped down on board the train rather than matching exactly what is required by the traction motor (like on DC electrification since DC cannot be stepped up or down by transformers). This leads to several further advantages of an AC electrification system
Line voltage can be set much higher allowing for far fewer substations to be built as high voltage transmission of electricity is more efficient over distance. The higher line voltage can allow for much more power to be delivered to trains allowing for high speed rail and heavy freight traffic