Power Quality Issues in Traction Power Systems - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 167

9
Power Quality Issues in Traction
Power Systems
There are serious power quality issues in traction power systems, including negative-sequence
currents, current harmonics and low power factor, in addition to voltage harmonics. In this
chapter, these issues are dealt with. A topology for traction power systems with a single
feeding wire is implemented with a three-phase V/V transformer and a three-phase converter.
Compared to the traditional scheme with two feeding wires (in two phases) with three-phase
V/V transformers, this topology improves the system reliability and has the potential for the
traction of high-speed trains. Compared to the co-phase system proposed in the literature,
this topology adopts a simple normal transformer instead of a complicated YNvd transformer
and a three-phase converter instead of a back-to-back single-phase converter, which saves
one converter leg. A strategy is then presented to control the three-phase converter so that all
harmonic, negative-sequence and reactive currents generated by the non-linear single-phase
load of locomotives are compensated. As a result, only balanced real power is drawn from the
grid. Simulation results are provided to illustrate the performance of the system.
9.1 Introduction
In recent years, high-speed electrified trains have been rapidly developed all over the world and
this is the future trend for railway transport. But for traditional traction power systems, there
are some power quality problems such as low power factor, a significant amount of harmonics
and negative-sequence currents caused by locomotives, which are present as single-phase
non-linear loads (Chen et al. 1998). As a result, the grid currents are unbalanced and contain a
lot of harmonics and reactive power (Chang et al. 2004; Ledwich and George 1994; Lee et al.
2006; Tan et al. 2003).
The problem of reactive power and harmonics is partially solved nowadays because
high-speed locomotives are driven by four-quadrant PWM converters (Brenna et al. 2011;
Busco et al. 2003; Chen et al. 2004). However, the problem with negative-sequence currents
becomes more and more serious because the power of locomotives is increasing. An overview