Electric power conversion systems – Current conversion – Including automatic or integral protection means
Patent
1997-10-21
1999-11-23
Nguyen, Matthew
Electric power conversion systems
Current conversion
Including automatic or integral protection means
363 56, H02H 7122
Patent
active
059911760
DESCRIPTION:
BRIEF SUMMARY
SUBJECT OF THE INVENTION
The present invention relates to a method for processing PWM waves intended for an inverter or a controlled rectifier, and more particularly with a view to obtaining the reduction of harmonics.
The present invention also relates to devices for implementing the method according to the present invention.
PRIOR ART RELEVANT TO THE INVENTION
Inverters, and more particularly voltage inverters, are intended to deliver an AC voltage from a DC voltage, whereas controlled rectifiers are intended to deliver a DC voltage from an AC voltage.
In the following description, reference will more particularly be made to inverters, but exactly the same problems arise for controlled rectifiers.
By way of example, inverters can be used for variable-speed control of synchronous or asynchronous machines.
In this case, it is necessary to supply the load, and more particularly each phase of a synchronous or asynchronous motor, with a three-phase voltage system as close as possible to a balanced sinusoidal three-phase system that can vary in frequency and in amplitude.
Voltage inverters are well-known devices which generally use circuits of power components such as thyristors, GTOs, etc. which switch particularly quickly.
An inverter is controlled by a modulator and a discriminator. The role of the modulator is to create a set wave, whereas the discriminator makes it possible to split this wave into a plurality of waves which are intended for individually controlling the various switches.
The simplest inverters have two levels and are composed of two switches (thyristors, GTOs, etc.) which alternately supply the load. Since this load is of the inductive type, it is necessary for a diode referred to as a "free-wheeling diode" to be placed in parallel with the switch in order to allow the load current to flow when the corresponding switch is open.
A simple square wave is customarily used as the set wave, in order to allow switching of one or other of the switches at each edge of the pulse. This square wave should successively close one or other of the switches and simultaneously open the opposite switch.
For obvious reasons of supply short-circuit, it is essential to check that two switches are never closed at the same time. It is here that the discriminator is involved, having the purpose of introducing a delay on closure of the corresponding switches, so that it is always certain that, when the command to close one switch is given, the opposite switch is already open.
This means that there is always a slight delay, referred to as "dead time" between the instant when one switch is opened and the instant when the opposite switch is closed, these two operations being therefore never simultaneous.
This is the cause of the presence of undesirable harmonics, and more particularly of the 5th and 7th harmonics which appear in the motor currents, this generating the presence of a 6th harmonic on the supply side and the drive torque side.
In reality, it is the 6th harmonic which is the most problematic because, for a motor supplied at 1/6 of the frequency used for signalling, there will be a risk of interference. By way of example, the motor supplied at 81/3 Hz risks causing a perturbation in the signalling system which uses 50 Hz.
Another problem resides in the fact that the various switches cannot switch instantaneously from a zero value (current on, corresponding to closure) to a positive value of the voltage (current off and corresponding to opening) and vice versa. This means that, for a certain time lapse, there is simultaneously a non-zero voltage and non-zero current, and a loss of power is therefore observed each time switching takes place.
In order to solve this problem, in the case when the various components cannot sustain a power loss of this type, it is known to propose a switching-aid cell. An example of a cell of this type may consist simply of a snubber which essentially comprises a capacitor connected in parallel on each switch. The purpose of this snubber is to absorb the power losses by limiti
REFERENCES:
patent: 5072354 (1991-12-01), Katto et al.
patent: 5099408 (1992-03-01), Chen et al.
patent: 5436819 (1995-07-01), Mikami et al.
patent: 5450306 (1995-09-01), Garces et al.
patent: 5615102 (1997-03-01), Daijo
patent: 5621634 (1997-04-01), Sato
patent: 5646837 (1997-07-01), Weggel
Murai, Y., et al. PWM strategy for high frequency carrier inverters eliminating current-clamps during switching dead-time. Proceedings of the Industry Applications Society Annual Meetings vol. 1, Oct. 4-9, 1992, Houston, U.S.A., pp. 317-322.
Zargari, N., et al. Input filter design for PWM current-source rectifiers. APEC '93 Mar. 7-11, 1993, San Diego, U.S.A . , pp. 824-830.
Bou Saada Johnny
Colignon Philippe
GEC Alstrhom Acec Transport S.A.
Nguyen Matthew
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