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Patent Issued for Power Converter with a Central Controller and Cell Controllers Daisy-Chained by Optical Fiber

September 10, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- Hitachi, Ltd. (Tokyo, JP) has been issued patent number 8817503, according to news reporting originating out of Alexandria, Virginia, by VerticalNews editors.

The patent's inventors are Inoue, Shigenori (Hitachi, JP); Katoh, Shuji (Hitachiota, JP); Kato, Tetsuya (Hitachiota, JP).

This patent was filed on July 13, 2010 and was published online on August 26, 2014.

From the background information supplied by the inventors, news correspondents obtained the following quote: "A modular multilevel converter (MMC) comprising an on/off-controllable switching device, such as an insulated gate bipolar transistor (IGBT), is a converter capable of outputting a voltage exceeding a breakdown voltage of the on/off-controllable switching device. The MMC is promising in applications, such as a high voltage DC power transmission system (HVDC), a static compensator (STATCOM), and a motor drive inverter.

"[Non Patent Literature 1] discloses a circuit system for the MMC.

"According to Non Patent Literature 1, the MMC comprises a plurality of unit-converters (hereinafter, referred to as cells) serially-connected (cascaded). Each cell is a bidirectional chopper circuit, for example, and includes a switching device and a DC capacitor. Each cell is coupled to the outside via at least two terminals, and can control the voltage between the two terminals to a voltage of the DC capacitor which this cell includes, or to zero.

"In PWM-control of each cell, by appropriately shifting the phase of a triangular waveform carrier supplied to each cell, the output voltage waveform of the MMC can be formed into a multilevel waveform. This can reduce the harmonic component as compared with the case of a two-level converter.

"The features of the MMC include that the potential of each cell differs from each other and that there is a cell with a high earth potential. In particular, when the MMC is applied to HVDC or the like, the earth potential of a cell reaches several tens of kV to several hundreds of kV. In addition, the earth potential of each cell momentarily varies.

"However, in Non Patent Literature 1, an isolation voltage between a controller and each switching device is not considered because the device at a laboratory level is targeted at.

"Non Patent Literature 2 discloses a configuration, wherein a signal processing circuit with the same potential as each cell is mounted in the vicinity of the each cell, and wherein a central controller at the earth potential and each signal processing circuit are coupled to each other by an optical fiber cable."

Supplementing the background information on this patent, VerticalNews reporters also obtained the inventors' summary information for this patent: "Technical Problem

"Non Patent Literature 2 discloses a configuration, wherein from the central controller to one cell, at least one optical fiber cable is coupled. That is, from the central controller to each cell, the optical fiber cable is star-connected. Accordingly, in Non Patent Literature 2, at least the same number of optical fiber cables as the numbers of cells are needed.

"In this case, all the optical fiber cables need to have a dielectric strength to withstand a potential difference between the central controller at the earth potential and each cell. That is, all the optical fiber cables need to be special optical fiber cables (hereinafter, referred to as high-voltage optical fiber cables) with a dielectric strength against creeping discharge and the like.

"Note that a special sheath material needs to be used for the high-voltage optical fiber cable, resulting in a complexity of the manufacture process.

"Furthermore, as with Non Patent Literature 2, when a controller and each cell are star-connected using an optical fiber cable, the length of each optical fiber becomes very long.

"The present invention has been made in view of the above circumstances, and provides a power conversion device comprising a plurality of cascade-connected cells, wherein the required length of an optical fiber cable used for control/communication is shortened.

"Solution to Problem

"In order to achieve the above-descried objective, the present invention provides a power conversion device comprising a plurality of cascade-connected cells, wherein a controller of the power conversion device comprises a central controller, and a cell controller with the same potential as each cell, the cell controller being installed in the vicinity of the each cell, and wherein the central controller and each cell controller are daisy-chained using an optical fiber cable.

"According to the present invention, in the power conversion device, the cell includes: a main circuit comprising on-off control switching devices and a DC capacitor; a DC voltage sensor detecting a voltage of the DC capacitor; a cell controller having a function to receive a signal from the central controller and generate a gate pulse of the on-off control switching devices and a function to transmit a signal from the DC voltage sensor to the central controller; a gate driver receiving the gate pulse from the cell controller and turning on/off the on-off control switching devices; and a boot-strap power supply supplying power to the cell controller and the gate driver.

"According to the present invention, in the power conversion device, an optical fiber cable coupling the central controller and the cell controller to each other is an optical fiber cable with a dielectric strength against a sum of the voltages at output terminals of a plurality of cells.

"According to the present invention, in the power conversion device, an optical fiber cable coupling the cell controllers of cascade-connected two cells is an optical fiber cable with a dielectric strength against the output voltage of one cell.

"According to the present invention, in the power conversion device, an optical fiber cable coupling cell controllers of two cells, in which one of two terminals of the output terminal of one of the two cells and one of two terminals of the output terminal of the other cell are electrically coupled to each other, is an optical fiber cable with a dielectric strength against the output voltage of one cell.

"According to the present invention, in the power conversion device, the optical fiber cable coupling the cell controllers of the cascade-connected two cells is supported by an electrical wiring between the cascade-connected two cells or by a member of the same potential as that of the electrical wiring.

"According to the present invention, in the power conversion device, the optical fiber cable coupling cell controllers of two cells, in which one of two terminals of the output terminal of one of the two cells and one of two terminals of the output terminal of the other cell are electrically coupled to each other, is supported by an electric wiring cascade-connecting the two cells or by a member of the same potential as that of the electric wiring.

"According to the present invention, in the power conversion device, the cell controller includes: a communication manager managing communication with the central controller; a modulation index buffer storing a modulation index for a main circuit therein, the modulation index being received from the central controller; a digital to analog converter converting an analog signal from the DC voltage sensor into a digital signal; a DC voltage buffer storing the digital signal therein; a triangular waveform carrier generator generating a triangular waveform carrier; and a gate pulse generator comparing the triangular waveform carrier with the modulation index stored in the modulation index buffer and generating a gate pulse supplied to the on-off control switching devices.

"According to the present invention, in the power conversion device, the cell controller includes a timing controller issuing an operation timing command to the modulation index buffer, the triangular waveform carrier generator, and the gate pulse generator.

"According to the present invention, in the power conversion device, the central controller transmits an optical serial signal frame comprising: a signal start mark; a synchronization carrier number; a series of signals comprising a modulation index signal with a cell number and a DC capacitor voltage dummy signal; and a signal end mark, at a substantially constant cycle.

"According to the present invention, in the power conversion device, the cell controller receives the synchronization carrier number of the optical serial signal frame which is received via other cell controller daisy-chained from the central controller, and if the synchronization carrier number coincides with its own carrier number, the cell controller, when it has received the signal end mark included in the optical serial signal frame, resets the triangular waveform carrier generator via the timing controller.

"According to the present invention, in the power conversion device, the cell controller receives the modulation index signal with a cell number included in the optical serial signal frame which is received via other cell controller daisy-chained from the central controller, and if the attached cell number coincides with its own cell number, the cell controller stores the received modulation index signal into the modulation index buffer.

"According to the present invention, in the power conversion device, the cell controller receives the DC capacitor voltage dummy signal with a cell number included in the optical serial signal frame which is received via other cell controller daisy-chained from the central controller, and if the attached cell number coincides with its own cell number, the cell controller replaces the received DC capacitor voltage dummy signal with a signal stored in the DC voltage buffer.

"Advantageous Effects of Invention

"According to an aspect of the present invention, in a power conversion device comprising a plurality of cascade-connected cells, among the optical fiber cables used in control/communication, at least the majority of high-voltage optical fiber cables with a dielectric strength against a sum of the output voltages of a plurality of cells can be eliminated, allowing for the use of an optical fiber cable with a dielectric strength against the output voltage of one cell. Furthermore, a reduction of the length required for the optical fiber cable is achieved. The other purposes, features, advantages of the present invention become clear from the following description of the examples of the present invention in conjunction with the accompanying drawings."

For the URL and additional information on this patent, see: Inoue, Shigenori; Katoh, Shuji; Kato, Tetsuya. Power Converter with a Central Controller and Cell Controllers Daisy-Chained by Optical Fiber. U.S. Patent Number 8817503, filed July 13, 2010, and published online on August 26, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8817503.PN.&OS=PN/8817503RS=PN/8817503

Keywords for this news article include: Electronics, High Voltage, Hitachi, Hitachi Ltd., Signal Processing.

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Source: Electronics Newsweekly


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