IEEE Electrification Magazine - September 2014 - 43

Inverter Circuit and Corresponding Dual
Closed-Loop Control Strategy for Voltage
and Current in the Proposed Scheme
When generative braking occurs, the third-rail voltage will be
raised instantaneously, which is due to the large inertia of
trains. If the interval time between train arrivals at the station is short, the regenerative energy generated by incoming
trains could also feed trains that are departing the station. In
this case, the third-rail voltage will return quickly to its rated
value. However, if the interval is long, the regenerative energy
of the incoming trains could only be fed back to supply other
loads at the train station. Meanwhile, the third-rail voltage
would decrease slowly. However, since inverters are connected to the power distribution grid, high-quality voltage
waveforms with smaller distortions would be required by
inverters. In this case, if the third-rail voltage fluctuations are
excessive, as inverters would turn on/off for generative
braking, the proposed control system for the inverters would
have to provide a fast dynamic response to feed a highquality regenerative energy back to the distribution grid.
Figure 11 shows the dc/ac inverter circuit. This is a threephase full bridge inverter, which is composed of six IGBTs
(T1-T6) and six diodes (D1-D6). The proposed scheme offers
several advantages: 1) the inductance located at the ac side
filter would reduce the voltage distortion and current harmonics, 2) the proposed scheme requires a small capacity
filter with minute losses and a fast dynamic response

Grid

Freewheel

Bidirection
Converter

Control Unit

Figure 8 shows the proposed regenerative energy system
for feeding the energy back to the distribution grid at train
stations. Figure 9 shows the speed curve of a train. When a
train departs a train station, it accelerates to its maximum
speed 1 and then keep a constant speed 2 . During this
time, trains absorb electric energy from the third rail provided by either traction transformers or the regenerative
energy of incoming trains. For incoming trains, the trains
decelerate by regenerative braking 3 , and the air braking
makes the train stop when its speed is close to zero. When
the train is slowing down, the voltage at the third rail rises
quickly and, as the voltage at the third rail exceeds its
upper limit, the three-phase inverter would start supplying regenerative energy to the distribution grid. The regenerated electric energy could either be absorbed by other
trains (region 1 or 2 ) or be fed back to the distribution
grid through dc/ac inverters. The electric energy flow is
shown with red arrows in Figure 10. Electric generators in
trains convert the kinetic energy (3) to electric energy as
the train is slowing down.
The proposed scheme would reduce the peak voltage at
the third rail.

-
+ Third Rail
Figure 7. The flywheel energy storage system of a commuter train.

Low-Voltage Grid

High-Voltage Grid

Control Unit
dc/ac
Inverter

Topological Structure and Electric Energy Flow
for the Proposed Scheme

because it uses high-frequency power switches, and 3) the
scheme increases the rate of utilization of regenerative
energy and reduces the size of the braking resistor used on
the trains. The proposed design considers the voltage-current dual close-loop control strategy for the inverter, as

conditioning, and other stationary applications. In addition,
we present analyses for such regenerative energy applications in train stations.

-
+ Third Rail
Figure 8. The regenerative energy system feeding back to the distribution grid.

Speed
Maximum
Speed

0
Station 1

Station 2

1 Accelerating Region
2 Constant Speed Region
3 Regenerative Braking Region
Figure 9. The speed curve of a running train.
IEEE Elec trific ation Magazine / s ep t em be r 2 0 1 4

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014