Power Quality Issues in Traction Power Systems - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 168

of the imbalance in traction power systems is presented in (Kneschke 1985) and the unbalanced
currents in different kinds of power supply schemes are compared. The amount of negative-
sequence currents are determined by the topology of the traction power system, especially
the type of transformers adopted, and the power of locomotives. In traditional traction power
systems, the topology with two-phase feeding wires is widely used (Chen et al. 2004). There
are two main schemes in this category according to the transformers used: (i) with three-phase
V/V transformers; and (ii) with some balancing transformers such as Scott transformers
(Horita et al. 2010; Ming-Li et al. 2008), Woodbridge transformers (Morimoto et al. 2009) and
Le Blanc transformers (Huang and Chen 2002). The detailed evaluation of negative-sequence
currents injected into the grid from different traction substations equipped with different
transformers is given in (Chen and Guo 1996; Wang et al. 2009). If a balancing transformer is
used, the two-phase secondary currents result in balanced three phase currents on the grid side
under some specific load conditions. However, since the speed and load of locomotives change
frequently, the grid currents are normally unbalanced. In order to solve this problem, some
active power compensators (APC) can be adopted on the three-phase grid side or on the two-
phase track side. For example, an APC was proposed in (Sun et al. 2004) for a traction power
system equipped with a Scott transformer to compensate for the negative-sequence currents.
Compared to balancing transformers, three-phase V/V transformers have a simple structure.
However, since the V/V connection scheme is inherently unbalanced, its performance in
reducing the three-phase imbalance is essentially worse than that of balancing transformers.
In order to deal with this problem, a three-phase V/V transformer with a railway static power
conditioner (RPC) is proposed in (Luo et al. 2011) and a strategy to compensate the negative-
sequence and harmonic currents is explained. Some improved strategies are then proposed in
(Wu et al. 2012), where a three-phase converter is used to replace the single-phase back-to-back
converter.
In comparison with the traditional two-phase systems, the topology with a single feeding
wire has some obvious advantages. First of all, the neutral section needed for each substation
to separate the two phases can be removed. Secondly, the voltages on the two adjacent sections
are nearly of the same phase so the insulation requirement between two adjacent sections is
considerably reduced and the neutral sections needed by two-phase systems can be replaced
with section insulators. Thirdly, the insulation/neutral sections for two-phase systems are
quite long and the speed loss of locomotives when passing through neutral sections is quite
significant. Compared to two-phase schemes, the number and length of neutral sections in
single-feeder systems are considerably reduced. Therefore, the topology with a single feeding
wire is more appropriate to provide power to high-speed trains.
Systems with a single feeding wire are explored in the literature. Such a system can be
achieved by using a Steinmetz transformer (Driesen and Craenenbroeck 2002), which is a
three-phase transformer with an extra power balancing load composed of a capacitor and an
inductor rated proportional to the single-phase load. An obvious drawback is that the capacitor
and the inductor need to be changed when the load changes. As a result, the capacitor and
inductor can be replaced with static var compensators (SVC) (ABB 2010). A co-phase power
traction system is proposed in (Shu et al. 2011; Zhao et al. 2010), where a complicated
YNvd balancing transformer and an APC are used. After compensation, only the active power,
including the load active power and system losses, is provided by the grid and in a balanced
manner. All the active power is provided through the YNvd transformer and half of it flows
through the APC. Some further optimised design and performance evaluation of the co-phase
system are reported in (Chen et al. 2009).