FACTS smooth the way for modern railways
The fast growth in traffic on existing tracks combined with new high-speed rail projects has made traction a very significant load on electrical power networks. Hence, the increased focus on voltage stability and power quality. Trains drawing power from the catenary must be sure that the supply voltage is stable and does not sag.
For the most common 50 hertz (Hz) applications power is taken between two phases of the feeding grid, this can cause substantial imbalance between phases in networks not designed for this kind of operation.
Voltage and current imbalances between phases of alternating current (AC) supply systems must be confined in magnitude and prevented from spreading through the grid to other parts of the system. Voltage fluctuations and harmonics need to be controlled if they are to stay within the stipulated limits. This is where ABB’s portfolio of FACTS (flexible AC transmission systems) technologies comes in.
Deploying FACTS in existing systems saves both time and money, rather than investing in costly and time-consuming reinforcement of the railway feeding infrastructure, such as building new transmission or sub-transmission lines, and/or building new substations and feeding points.
A London Underground SVC installation
Furthermore, FACTS can help achieve adequate power quality with lower infeed voltages than would otherwise be possible. For example, it could be sufficient to feed a railway system at 132 kV rather than at 220 kV or even 400 kV, which will of course enable a lower investment cost and also shorter implementation times.
FACTS for railways comprises SVC, SVC Light and Dynamic Energy Storage.
Connecting single-phase railway loads to a three-phase grid can result in severe imbalance conditions. Imbalance is measured in terms of negative-phase sequence voltage and current. These quantities are regulated in Grid Codes that stipulate how much negative-phase sequence can be tolerated at the point of common connection with the grid.
The relationship between the railway load and the fault level of the feeding grid is what determines the imbalance caused by the rail load. The larger the load and the lower the fault level, the worse the unbalance becomes, and thereby, the larger the negative-phase sequence components.
Besides violating grid codes, imbalance between phases induces extra losses, as well as causing wear to rotating machinery connected to the same grid. The remedy is FACTS, where SVC as well as SVC Light is capable of restoring three-phase balance.
Railway loads are highly fluctuating, since acceleration and braking lead to variation in voltage. FACTS controllers can keep the voltage levels stable, offering these benefits to grid owners and railway operators:
Dynamic mitigation of voltage fluctuations in feeding grids, helping to fulfil Grid Code conditions
Dynamic voltage support of catenaries feeding high power locomotives, maintaining traction capability despite weak feeding, and without harmful voltage drops along the catenary; furthermore enabling adequate power for locomotives during outages of feeding points, or, alternatively, with fewer feeding points required in the system. (saving investment in additional infrastructure)
Dynamic voltage control of AC supply systems for DC converter fed traction (typically underground and suburban trains).
Railway loads often generate harmonics
that enter into the feeding power grid, creating Grid Code issues. FACTS controllers mitigate these harmonics.
Suppressing the harmonics can also decrease or completely eliminate the extra losses they can induce, as well as preventing possible overloading of harmonic filters located elsewhere in the grid, and malfunction of protective devices.
FACTS case study – SVC Light® for railway load balancing
Evron is a substation in the French rail system between Paris and Rennes in Western France, fed from the RTE national power grid. An SVC Light® is operated by SNCF (the French railway company) for dynamic balancing of asymmetry between phases caused by the mode of traction feeding, take-off of power between two phases in the three-phase grid. The project was executed in collaboration between ABB, RFF (the owner of the railway infrastructure) and SNCF.
The SVC Light also performs the task of active filtering of harmonics generated by thyristor and diode locomotives up to and including the 9th harmonic without the need for passive filters. Active filtering is made possible by the high dynamic response inherent in SVC Light.
By installing the SVC Light, the conditions set out in the National Grid Code concerning power quality at the point of connection to the grid of the traction feeder have been fulfilled, i.e. requirements on limits for voltage fluctuations, phase unbalance and harmonic distortion were met. An alternative to the SVC Light would have been to build a new overhead line, to increase the fault level of the feeding grid. In feasibility studies performed before the project, it was demonstrated that the SVC Light approach was considerably less costly as well as less time-consuming than building new lines.
FACTS case study – London Underground
In the early years of the 21st century, the London Underground closed its old 180 MW oil/gas fired power plant at Lots Road to take its supply from the National Grid. Since the underground load consists mainly of diode converters that feed DC current to the trains, FACTS has been deployed to limit or prevent disturbances, such as voltage fluctuations and harmonics, from reaching the public grid.
In 2009, an ABB SVC was commissioned for the 11 kV feeding grid to work together with several other ABB SVCs and standalone harmonic filters in operation since mid 2000 at critical points of the London Underground 22 kV and 11 kV grid.