29-11-2012, 05:58 PM
Fault-Tolerant Voltage Source Inverter for Permanent Magnet Drives
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Abstract
In this paper, a two-level fault-tolerant voltage source
inverter (VSI) for permanent magnet drives is systematically designed
and tested. A standard two-level inverter consists of three
legs. In this case of fault-tolerant inverter, a redundant leg is added
that replaces the faulted leg. Faulted leg isolation and redundant leg
insertion are done by using independent back-to-back-connected
thyristors. The proposed inverter provides tolerance to both shortcircuit
and open-circuit faults of the switching devices. The postfault
performance is the same as the normal prefault operation and
fault compensation is fast enough such that there is negligible disturbance
in the drive operation. The fault tolerance of the inverter
is verified using field-oriented control of a permanent magnet synchronous
motor.
INTRODUCTION
TYPICALLY, a modern industrial drive consists of a power
electronic converter, a digital controller for implementing
the control algorithms, feedback sensors, and a motor. Several
faults can affect the motor drive and a fault in any of the aforementioned
will stop the drive running or at least it affects the
drive performance [1]. There are some critical applications such
as power plants, aerospace, railway locomotives, automobiles,
chemical plants, etc., where the fault tolerance of the drive is
very important. For an interlinked production process, as in modern
industrial processing plants, a fault in a single drive can result
in tremendous damages of materials and machines. Follow-up
costs due to faults with drives in modern production plants can
amount to huge sums. So the fault tolerance of adjustable speed
drives is the area of great interest for modern drive solutions.
So far, redundant or conservative design has been used in every
application, where the continuity of operation is a key feature.
Nevertheless, some applications accept short-torque transients
and even permanently reduced drive performance after a fault,
under the condition that drive continues to run.
SYSTEM DESCRIPTION OF THE FAULT-TOLERANT INVERTER
Fig. 1 shows the proposed fault-tolerant two-level VSI. A
standard two-level three-phase inverter consists of only three
legs but the fault-tolerant inverter of this topology consists of
four legs, with one leg as redundant. The redundant leg is normally
not used when the standard three legs are working without
any fault. Back-to-back-connected thyristors (ISa, ISb , and ISc )
are connected between output terminals of the inverter (Va ,
Vb , and Vc ) and corresponding motor phases. These thyristors
are used as isolating switches of faulted leg. Additional three
thyristors (THa, THb , and THc ) are connected between the output
terminal of redundant leg (Vr ) and motor phases as shown
in Fig. 1. These thyristors are used for inserting the redundant
leg in the place of faulted phase.
EXPERIMENTAL RESULTS FOR THE IGBT OPEN-CIRCUIT
FAULT
A laboratory prototype has been built for testing the faulttolerant
inverter driving a field-oriented controlled PMSM. The
PMSM is coupled with another PMSM, which is used as a load
machine. A three-phase variable resistance is connected at the
output terminals of the load machine which provides the required
load torque. The control algorithm is implemented in a
Texas Instruments based F2812 fixed-point digital signal processor
(DSP) evaluation board. A fault-tolerant controller was
presented in [19], but this is not the subject of this paper. Generation
of command signals for the converter, data acquisition,
fault insertion, and fault compensation is done through software
written in “C” language.
CONCLUSION
This paper has presented a fault-tolerant VSI that can compensate
both short-circuit and open-circuit faults in the switching
devices. It is simple in construction, modular, and easy to control.
Experimental results showthat the compensation strategy is
fast enough such that there is negligible disturbance in the drive
operation. Results show that thyristors can successfully isolate
the faulted leg in all the fault cases. The postfault performance
of the machine is the same as the prefault, and the postfault control
algorithm is the same as prefault. The achieved results show
that this inverter can fit in much safety critical and industrial
applications where fault tolerance is of prime importance.