Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2000-12-20
2001-08-28
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C323S903000, C096S082000
Reexamination Certificate
active
06282106
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a power supply for electrostatic precipitators of the type having a power electronics stage having a rectifier, intermediate circuit and an inverter, with a transformer connected at the output side, and a high-voltage rectifier, with the transformer and the high-voltage rectifier as well as a device for measured value acquisition situated in the immediate proximity of the electric filter.
2. Description of the Prior Art
Electrostatic precipitators serve for dust removed from gaseous agents in all fields of technology. The gas from which dust is to be removed is conducted between plates that are all grounded and exhibit a spacing of, for example, 600 mm. Wire-shaped spray electrodes that exhibit a highly negative voltage of, for example, 110 kV compared to ground potential are respectively situated therebetween. Due to this high D.C. voltage, the gas molecules are ionized and transfer their charge to dust particles suspended in the gas stream when they strike them. The dust particles become positively charged due to absorbed electrons and migrate to the grounded, negatively charged separation electrode, where they collect and agglomerate into flakes of dust that are stripped from the appertaining plates with vibrators or with a brush mechanism and fall by gravity. This filter method is very efficient but has problems. As a result of the high voltage, voltage arcing regularly occur between the spray electrodes and the plate-shaped separation electrodes. This effect cannot be avoided and is more or less pronounced dependent on the type of dust. The use of electric dust filters is most difficult in steel mill plants since conductive dust particles can greatly shorten the arcing distance between the spray and separation electrodes and thus lead an accumulated occurrence of arcings.
The high-voltage for electrostatic precipitators is usually generated by rectifying the output signal of a high-voltage transformer that is driven by a mains-fed thyristor at the primary side. In the case of a voltage breakdown at the electric filter, a lightning-like discharge arc that can build up. A reliable method for quenching the arc is to wait for the next zero crossing of the primary currant and then blocking the firing pulses of the thyristor for a time interval and resupply the primary side of the high-voltage transformer only thereafter. So that the electric filter remains without high-voltage for an optimally short time span, the inverter should be immediately blocked given a breakdown, so that the current drops as fast as possible and, after the arc has been reliably quenched, can in turn build up as fast as possible by activating the inverter. For this reason, the power electronics of the inverter must be coupled over an optimally short path to the sensors that measure the voltage and the current at the electric filter in order to recognize a breakdown as soon as possible. Given a voltage breakdown at an electric filter, it has been shown that the inductances at the power supply lines can no longer be neglected due to the high currents that thereby occur and can lead to discontinuities in the voltage of the grounded potential of up to 15 kV in the region of the electric filter. Even given employment of coaxial cables, a dependable data transmission from the current and voltage sensors at the electric filter to the drive electronics of the inverter power part is no longer assured in the case of such discontinuities in potential, and the time behavior of the fast disconnect in the event of a voltage breakdown is negatively influenced as a consequence of transmission errors. These disadvantages are alleviated only slightly when the power electronics is arranged in the immediate proximity of the electric filter, since a reliable data transmission is already jeopardized even given distances of a few meters.
SUMMARY OF THE INVENTION
An object of the present invention is to fashion a power supply for electrostatic precipitators such that an optimally dependable operation is assured and such that maintenance personnel can access the power electronics without increased safety risk.
This problem is inventively in a power supply for an electric filter wherein the power electronics part is situated at a location at a distance from the electric filter and, wherein the control, particularly fast disconnect thereof given a voltage breakdown, is coupled via optical fibers to the arrangement for measured value acquisition and constantly receives measured results about the electrostatic precipitator therefrom.
The invention proceeds oppositely from earlier approaches with short connections between the electric filter and power electronics, and instead employs light waveguides for the transmission of measured results from the electrostatic precipitator to the power electronics, the fiber optic function without transmission errors given pronounced discontinuities in potential. Moreover, optical fibers have a high limit frequency and therefore can be operated with a high data transmission rate, so that a sampling of the measured results of the electric filter at time intervals of about 100 microseconds is unproblematically possible, and the data thereby acquired can be transmitted online via light waveguides to the drive circuit for the power electronics, i.e. with an optimally slight time offset. Since the power electronics is supplied from a standard 380 V network, the voltage at the intermediate circuit is barely higher than 500 V and it is therefore completely safe to install the components of the inverter in a commercially obtainable control box that, for example, is disposed in a switch room. It is completely non-hazardous to approach the closed control box and to read measuring instruments or to manually actuate switches. Of course, commands or other information also can be transmitted from such a switch or control room to a device that coordinates the measurements on site and edits the results, so that a bidirectional communication via optical fibers arises.
It has proven favorable for the communication between the power electronics part and/or the switch or control room and the device installed in the region of the electrostatic precipitator, to ensue via respective control assemblies for coordinating the measurements and for editing the measured results for data transmission. These control assemblies are preferably realized with integrated sequential circuits in the form of microprocessors or micro-controllers, with which the interface modules for coupling to the light waveguide can be a discrete component can be integrated together with the control component.
An especially important aspect of the invention is a separate light waveguide for each direction for bidirectional communication between the power electronics and/or the switch or control room, and the measured value acquisition arrangement, particularly via the respectively allocated control and/or interface modules. This has the significant advantage that commands asynchronously generated in a switch room need not wait for a gap in the data transmission of the measured results, nor does an interruption of the data transmission have to be produced, but the commands can be transmitted completely independently, and can be transmitted via their own interface modules. As a result, waiting times are not produced, and the uninterrupted data transmission of measured results to the power electronics part is assured even given transmission of control commands to the measured value acquisition arrangement.
The assemblies communicating with each other via optical fibers can have a respective serial input and a serial output. For reducing the component outlay as well as for avoiding transmission errors, in accordance with the invention only a single optical fiber can be employed for each data direction and the data be serially transmitted thereover. This can be achieved with an appropriate increase of the clock rate is correspondingly, so that the measured results c
Han Jessica
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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