Power quality - Knowing the facts for rail feeder systems - Part 1

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The increase in traffic on existing tracks combined with new high-speed rail projects means that rail traction is fast becoming an important load on electric supply grids. This in turn is focusing attention on voltage stability as well as the power quality of the surrounding grids. Rolf Grünbaum, Per Halvarsson, and Björn Thorvaldsson of ABB explain how FACTS (Flexible AC transmission systems) can enhance power quality in rail feeder systems

There are several ways to feed rail traction systems with electric power. One scheme used in many traditional electrification systems is to supply it directly using the fundamental frequency main power, ie, 50/60 Hz. The transmission or sub-transmission voltages are then directly transformed by a power transformer to the traction voltage.

On the traction side any one of two transformer schemes can be used to supply high and efficient power: the booster transformer and auto-transformer  schemes. In the booster transformer scheme, the main voltage is transformed into a single-phase catenary voltage. One end of the power transformer traction winding is grounded and the other is connected to the catenary wire. In the auto-transformer scheme, the traction winding is grounded at its midpoint. One end of the winding is connected to the catenary wire while the other end is linked to the feeder wire. In both schemes the grounded points are connected to the rail.

On the transmission network side the power transformer is connected between two phases. Frequently, two isolated rail sections are fed from the same feeder station, and in this case the power transformers are then connected between different phases.

Nowadays, the traction load tends to be relatively large, often with power ratings between 50MW and 100MW per feeding transformer. These loads will create imbalances in the supply system voltage if they are connected between two mains phases. A common requirement is that the negative phase sequence voltage resulting from an unbalanced load should not exceed one percent. Assuming loads of between 50 MW and 100 MW, the feeding system must have a short-circuit level of at least 5,000 MVA to 10,000 MVA if it is to stay within the imbalance requirements. In many cases the traction system is relatively far from strong high-voltage transmission lines. Weaker sub-transmission lines, however, normally run somewhere in the vicinity of the rail and can therefore be used to supply the rail in cases where an imbalance caused by the traction load can be mitigated.

Flexible AC transmission systems
Flexible AC transmission systems (FACTS) is a family composed of static devices that are controlled using state of the art computerized control systems in conjunction with high power electronics. One of these devices, the conventional static var compensator (SVC) as well as the more recently developed SVC Light (STATCOM) can be used for imbalance compensation, ie, they serve as load balancers when used with special control algorithms. Load balancing is concerned with transferring active and reactive power between different phases.
SVC and SVC Light devices can also be used to dynamically support sagging catenary voltages and mitigate harmonics emanating from thyristor locomotives. In the case of SVC Light, a certain number of these harmonics can be removed by active filtering.

FACTS in rail traction
Power grids feeding railway systems and rail traction loads can benefit enormously by using SVC and STATCOM to reduce, if not eliminate, the investments needed to upgrade the railway power feeding infrastructure.

FACTS devices in a system also enable adequate power quality to be achieved with in-feed at lower voltages than would otherwise be possible. This means, for example, it may be sufficient to feed a railway system at 132 kV rather than at 220 kV or even 400 kV.

Load balancing by means of SVC
An SVC is a device that provides variable impedance, which is achieved by combining elements with fixed impedances (eg, capacitors) with controllable reactors. Surprisingly, this combination is capable of balancing active power flows. The reactors also have fixed impedances but the fundamental frequency component of the current flowing through them is controlled by thyristor valves, which results in apparent variable impedance. This type of reactor is known as a thyristor controlled reactor (TCR).

In the conventional SVC, load balancing is achieved when, by controlling the reactive elements, active power is transmitted between the phases. In its simplest form the load balancer consists of a TCR connected between two power supply phases and a fixed capacitor bank in parallel with a TCR connected between two other phases. Power factor correction is obtained by a fixed capacitor bank in parallel with a controlled reactor between the remaining two phases. Harmonics are normally suppressed by the addition of filters. These can be connected either in a wye (Y) formation or directly in parallel with the reactors.

SVC and High Speed 1
A total of seven SVCs have been installed on High Speed 1 (HS1), the 108km high-speed rail line between London St Pancras and the Channel Tunnel at Dover that now enables trains to travel between London and Paris in just over two hours at a maximum speed of 300km/h.

Even though it is primarily designed for high-speed trains, HS1 also accommodates slower freight traffic. As modern trains have power ratings in the range of 10 MW, the power feeding system has to cope with large fluctuating loads. The HS1 traction feeding system is a modern direct supply of 25 kV with a mains frequency of 50 Hz, and each of the three traction feeding points between London and Dover is supported by SVCs. Direct transformation from the power grid via transformers connected between two phases is used, and the auto-transformer scheme is implemented to ensure a low voltage drop along the traction lines.

Dynamic voltage support
Six of the SVCs are used mainly for dynamic voltage support and are connected on the traction side of the power transformers. A seventh SVC is used for load balancing. At three of the feeding points, one of two identical single-phase SVCs is connected between the feeder and earth and the other is connected between the catenary and earth.

There were three main reasons for HS1 to invest in SVCs. The first and primary reason is to support the railway voltage in case of a feeder station trip. When this happens, two sections have to be fed from one station. It then becomes essential to keep the voltage up in order to maintain traction efficiency.

The second reason is to maintain unity power factor seen from the supergrid transformers during normal operation. This ensures a low tariff for the active power consumed. And finally, the SVCs are installed to mitigate harmonic pollution. SVC filters are designed not only to accommodate the harmonics generated by the SVC but also those created by the traction load. There are stringent requirements on the allowed contribution from the traction system to the harmonic level at the connection points to the supergrid.

The SVCs operate in a closed-loop power factor control; an outage at a feeder station automatically changes operation to closed-loop voltage control.

Part two of this article will appear in Electrical Review November 2010 or can be viewed at www.electricalreview.co.uk