Environmental Engineering Reference
In-Depth Information
Compared with the three-phase V/V scheme in (Luo et al. 2011), the YNvd transformer in
the co-phase system is much more complicated than the single-phase transformers connected
in the three-phase V/V scheme. Moreover, the APC adopted in the co-phase system is a single-
phase back-to-back converter, which requires one more converter leg than the three converter
legs needed by the three-phase V/V scheme.
In order to combine the advantages of the co-phase system and the three-phase V/V scheme, a
new topology for traction power systems is presented in this chapter. It adopts a three-phase V/V
transformer and a three-phase converter running as a static power conditioner (SPC). Moreover,
it provides a single feeding wire without the need for a neutral section at each substation. The
three-phase SPC is controlled to balance the three-phase grid currents and to compensate for
the reactive and harmonic currents. The case when the power factor of the load is not unity
is discussed in detail and the case with a unity power factor is presented as a special case.
The harmonic components are compensated without any extra cost. A compensation strategy
is presented so that all the harmonics and reactive power caused by the load are injected into
the SPC. Hence, the grid currents are balanced and in phase with the corresponding phase
voltages. It is worth noting that the SPC also maintains the DC-bus voltage and there is no
need for an external power supply. The ripple voltage at the double frequency in the controller
maintaining the DC-bus voltage is removed to make sure that the reference currents generated
for the SPC are purely sinusoidal, which improves the THD of the grid currents.
9.2 Description of the Topology
The topology for traction power systems with a single feeding wire is shown in Figure 9.1.
It adopts a three-phase V/V transformer to reduce the grid-side three-phase high voltage, e.g.
220 kV, to the track-side voltage, e.g. 27.5 kV, for traction. The turns ratio of the transformer is
K V . One open end of the secondary V windings, Terminal b in Figure 9.1, is connected to the
track (earth) and the other open end of the secondary V windings, Terminal a in Figure 9.1, is
connected to the catenary. A three-phase converter (called the static power conditioner, SPC)
is connected to the two open ends of the secondary V windings and the common point, via two
step-down single-phase transformers with a turns ratio of K D in Figure 9.1. The SPC maintains
the DC-bus voltage by itself and there is no need to provide an external power supply. The
leakage inductances of the step-down transformers on the SPC side are denoted as L a and L b ,
respectively. Since the traction voltage is the grid line voltage divided by K V , which is the same
as the one in the conventional two-phase systems equipped with V/V transformers, an SPC can
be easily retrofitted into existing two-phase traction power systems to improve power quality.
9.3 Compensation of Negative-sequence Currents, Reactive Power
and Harmonic Currents
9.3.1 Grid-side Currents before Compensation
Assume the RMS value of the grid voltage is U and the phase angle of Phase A grid voltage
is 0. Then the three phase grid voltages can be denoted as
U A =
U
0
,
U B =
2
U
3 π,
(9.1)
U C =
2
U
3 π.
 
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