IEEE Electrification Magazine - September 2014 - 10

4,000
3,000
2,000
1,000

Peak Value of iin (A)

0
19
320

Peak Value of vout (V)

At the substation, a study of active and reactive energy consumption was performed over a five-month period. It was
thus demonstrated that the invoiced reactive energy could

4,000

23 24
Vline (kV)

23 24
Vline (kV)
(a)

300
280
260
240
19

be reduced from 5,000 Mvarh to 1,500 Mvarh by adding variable compensation of 3 Mvar.
The new compensation circuit is presented in Figure 5.
AC choppers are connected in series with the existing
fixed compensators. A filtered shunt capacitor bank
(L 3 - C 3) is added and sized to provide an additional reactive power of 3 Mvar at 22 kV (for a total maximum of
13 Mvar). The controlled impedance part allows reactive
power control by variation of the duty cycle according to
Figure 6 as a function of the line voltage and the maximum compensated reactive power, limited to 13 Mvar.
The peak voltage on each ac chopper is limited to
3.6 kV for a line voltage of 27.5 kV (no-load operation). As a
result, four series-connected ac choppers are required. The
advantage of the voltage divider with regard to semiconductor stress is shown in Figure 7, where the maximum
input current is reached when the output voltage is close
to 1 kV.

experimental results: reactive power
Compensation with step-Up AC Chopper

A prototype was developed to demonstrate the feasibility
of the solution presented in Figure 5. The maximum reactive power level was set to 1.2 Mvar. The ac chopper was
1,000
built at the Plasma and Conversion of Energy Research
0
Laboratory (LAPLACE) in Toulouse, France, and tested on
19
20
21 22 23
24 25 26 27
Vline (kV)
the SNCF test platform in Vitry (Paris), France. The experi240
mental setup, shown in Figure 8, is based on the series
connection of an ac chopper and an LC filter, which has a
220
capacitive response at 50 Hz. For safety reasons, resistors
200
R dis1 and R dis2 are installed to discharge the capacitors
when the circuit is turned off.
180
19
20
21 22 23
24 25 26 27
The RMS value of the ac supply used during the test is
Vline (kV)
2,450 V. The semiconductor devices used for the ac chopper
(b)
converter are 3.3-kV/1,500-A insulated-gate bipolar transistors (IGBTs) switching at 1 kHz. The maximal reactive
Figure 7. The ac chopper output voltages and input currents versus
power provided is about 1.2 Mvar, and the reactive power
line voltage for (a) bank 1 and (b) bank 2.
variation, ∆Q, is 320 kvar. An air-cooled
system based on heat pipes is used. The
control part and the generation of IGBT
L
switching patterns are achieved by using
a mixed-environment digital signal
Discharge
Discharge
processor and field-programmable gate
Contactor
Contactor R
array. The experimental setup is shown
dis1
v
C
in Figure 9, and the waveforms are preRdis2
iout
sented in Figure 10. It can be seen that
Co
the current i in is sinusoidal; voltage v cell
vout
corresponds to the voltage across capaci
in
vin
T1
T2
D1
itor C 0 when Vout is positive. The current
D2
vcell
in switch K1_C is chopped with a polariC1
C2
D1C
ty opposite to i in , and the voltage across
C 0 increases with duty cycle a. All
T1C
D2C
T2C
iK1_C
experimental measurements match well
to the previously calculated values. The
reactive power variation Q ^a h is plotted
Figure 8. The reactive power compensator based on a step-up ac chopper.
in Figure 11.
3,000

Peak Value of iin (A)

2,000

I E E E E l e c t r i f i c ati o n M agaz ine / september 2014

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