16-01-2013, 10:11 AM
Fault-Tolerant Operation of a Battery-Energy-Storage System Based on a Multilevel Cascade PWM Converter With Star Configuration
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Abstract
This paper focuses on fault-tolerant control for a
battery-energy-storage system based on a multilevel cascade
pulsewidth-modulation (PWM) converter with star configuration.
During the occurrence of a single-converter-cell or single-batteryunit
fault, the fault-tolerant control enables continuous operation
and maintains state-of-charge balancing of the remaining healthy
battery units.This enhances both system reliability and availability.
A 200-V, 10-kW, 3.6-kW·h laboratory system combining a threephase
cascade PWMconverter with nine nickel-metal-hydride battery
units is designed, constructed, and tested to verify the validity
and effectiveness of the proposed fault-tolerant control.
INTRODUCTION
A. Multilevel Cascade Converters
MULTILEVEL cascade converters have become an attractive
topology for medium-voltage high-power applications
[1]–[8]. They are simple and modular in structure, and can
reach medium/high voltages with low voltage/current harmonics
without using step-up transformers or switching devices connected
in series. Although the concept of cascade converters has
existed for more than three decades [9], it was in the mid-1990s
that they grabbed the attention of power electronics research
scientists and engineers. In 1997, two major patents [10], [11]
indicated the advent of the cascade converters intended for motor
drives and utility applications.
Reliability and Fault Tolerance
A cascade converter requires a large number of power switching
devices. A faulty switching device can potentially lead to
expensive downtime. Therefore, reliability and availability of
the converter are of prime importance [18], [19]. The cascade
converter has the potential of maintaining continuous operation
at a reduced or the rated voltage and power even when either
open- or short-circuit fault occurs in a switching device.
The so-called “fault tolerance” for multilevel converters has
been investigated in literatures [20]–[26]. For a cascade converter,
fault-tolerant control can be achieved:
1) by providing a redundant converter cell in each phase, and
bypassing not only the faulty converter cell, but also two
healthy converter cells in the other two phases;
2) by bypassing the faulty converter cell only, without providing
redundant converter cells.
Bypassing a Faulty Converter Cell
In the battery-energy-storage system shown in Fig. 1, when
either a power switching device or a battery unit is detected to
be faulty, or the fuse in series with the battery unit (not shown
in Fig. 1) blows, the corresponding converter cell is bypassed
immediately by short-circuiting its ac terminals.
Fig. 3 shows two methods for bypassing the faulty converter
cell, where Fig. 3(a) uses a set of two thyristor switches [28],
while Fig. 3(b) uses a bypass contactor [29] to short-circuit its ac
terminals. Note that it does not matter which power switching
device in the converter cell gets faulty, as long as the faulty
converter cell can be detected.
With Neutral Shift
This subsection describes a neutral shift to share the burden
of all the 3N − 1 healthy battery units equally, balancing
their SOC values. The neutral shift is based on the injection
of a fundamental-frequency zero-sequence voltage to each ac
voltage of the healthy converter cells. The zero-sequence voltage
is chosen to be out of phase by 180◦ with the u-phase
cluster voltage, where a single-converter-cell fault occurs. Note
that the idea of zero-sequence voltage injection itself is not
new. For example, Betz et al. [34] investigated the injection of
zero-sequence voltage/current for capacitor-voltage balancing
in a cascade-converter-based STATCOM intended for negativesequence
compensation of three-phase line currents. Moreover,
the authors of this paper used zero-sequence voltage injection
for clustered SOC-balancing control of a battery-energy-storage
system based on a cascade PWM converter with star configuration
[16].
EXPERIMENTAL RESULTS DURING NORMAL OPERATION
Fig. 8 shows the experimental waveforms during normal operation
when the energy-storage system was being charged and
discharged at the rated power of 10 kW. The waveforms of
vSuO and iu were in phase during charging and out of phase
by 180◦ during discharging because this system was operated
at a reactive-power command of q∗ = 0. The current total harmonic
distortion (THD) was 3.5% at a battery-unit dc voltage
of 78 V during charging and 5.4% at the battery-unit dc voltage
of 72 V during discharging. The THD value is expected
to be improved in a practical medium-voltage system, because
it will have a higher equivalent carrier frequency and a lower
ratio of an insulated gate bipolar transistor (IGBT)/diode saturation/
forward voltage with respect to a battery-unit dc voltage.
Although both charging and discharging waveforms were observed
around a mean SOC value of 42%, the battery-unit dc
voltages in Fig. 8(a) were at 78 V, whereas the battery-unit dc
voltages in Fig. 8(b) were at 72 V.
SIMULATED RESULTS
Figs. 16–18 show simulated results obtained by using the
software package “EMTDC/PSCAD.” Simulation was carried
out under the same operating conditions as the experiment.
Fig. 16(a) shows simulatedwaveforms during charging at 10kW
before and after the u-phase converter cell numbered 1 was bypassed,
while Fig. 16(b) shows the waveforms before and after
the converter cell was restored to the normal operation. Losing
the converter cell in the u-phase brought an increase in switching
ripples to the three-phase line currents iu , iv , and iw. This
is in agreement with the experimental results in Fig. 11.
CONCLUSION
This paper has described fault-tolerant control for a batteryenergy-
storage system based on a cascade PWM converter with
star configuration. It is based on the combination of bypassing a
faulty converter cell with executing the so-called “neutral shift.”
During a single-converter-cell fault, it enables the cascade converter
to maintain continuous operation, producing a three-phase
balanced line-to-line voltage at the ac side and achieving SOC
balancing of the remaining healthy battery units. Experimental
results based on a 200-V system have verified the effectiveness
of the proposed fault-tolerant control.