IEEE Electrification Magazine - September 2014 - 11

Chopper-Controlled steinmetz Circuit for
Voltage balancing in railway substations
Chopper-Controlled Steinmetz
Circuit for Voltage Balancing

Control System

Figure 12 shows a classical railway substation supplied by
a three-phase network. At the point of common coupling,
to avoid penalties from the utility, the railway company is
forced to meet a maximum voltage unbalance factor (UF)
averaged over 10 min. The UF is defined as the ratio of the
negative sequence component V- and the positive
sequence component V+ of the line voltages (v a, v b, and v c) .
Figure 13 shows the basic principle of the active Steinmetz compensator with ac choppers realizing controlled
impedance, both capacitive and inductive, as required.
These impedances, connected across two lines of the
three-phase network, draw currents with a negative
sequence, which compensates the current unbalance
and, consequently, the voltage unbalance produced by
two-line loading.
Only the real part of the negative sequence component
drawn by the substation is compensated, which is the
main drawback of the active Steinmetz circuit. Nevertheless, modern locomotives are equipped with active frontend rectifiers, which draw a sinusoidal current in phase
with the line voltage. In the future, locomotives using thyristor rectifiers will no longer be used; therefore, it will not
be necessary to consider low-power-factor operation during development. Moreover, the railway operator is not
interested in an instantaneous compensation since penalties are applied on the basis of a 10-min average. In this
case, a very simple control strategy can be implemented:
the duty cycle of the ac choppers will be controlled as a
function of the active power consumed by the substation.

ac Chopper
ac Power-Supply
Connection

Capacitor C0

S comp = S L - UF S cc .

T
Vcell

4 ms

500 V/div
1

Vin

500 V/div
2
iin

500 A/div

1) [Tek TDS3014B].CH1 500 V 4 ms
2) [Tek TDS3014B].CH2 500 V 4 ms
3) [Tek TDS3014B].CH3 500 V 4 ms i
4) [Tek TDS3014B].CH4 500 V 4 ms K1_C

500 A/div

Figure 10. The ac chopper waveforms (V = 2450; VRMS - a = 0.5) .

Q (kvar)

-900
-950
-1,000
-1,050
-1,100
-1,150
-1,200
-1,250

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Duty Cycle α

Figure 11. The experimental results: leading reactive power versus
duty cycle a.

(7)

Thus, the power of each CCI is equal to S comp divided by
3 and set to 3.3 Mvar. The converter is designed with
standard 3.3-kV/1.5-kA IGBT modules with a switching
frequency fsw = 1 KHz. For the design, the following specifications were developed.
xx
Transformer ratio: N T1 and N T2 limit the semiconductor voltage to 1,800 V.

Inductor L

Figure 9. The reactive power compensator under test.

Chopper-Controlled Steinmetz Circuit Design in a
Typical Substation of the French National Railways
The case study is a 16-MVA substation located in Évron,
Pays de la Loire, France. The primary of the transformer is
connected across two of the three 90-kV/50-Hz transmission lines, and a 2.7-Mvar reactive power compensation
bank is connected on the 25-kV side. The rating of the
compensator was chosen to guarantee a UF of 1.5% when
the substation is loaded at 10 MW and for the lower shortcircuit power S cc = 295 MVA. The power rating of the
unbalance compensator is given by

Capacitor C

Zcc

PCC

il Substation
itrain

Figure 12. A single-phase substation connection.

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