Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2002-05-29
2003-08-26
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S127000, C363S054000
Reexamination Certificate
active
06611443
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a high voltage power distribution and collection system and more particularly to a high voltage converter system for distributing and collecting power to and from a set of isolated, geographically-scattered, or generally inaccessible loads and sources, interconnected by one or more high voltage power lines or cables.
BACKGROUND OF THE INVENTION
In typical signal-carrying cable installations, repeater stations are required at various intervals, typically ten or more miles apart, to restore and amplify attenuated and time-smeared signals. These repeater stations are electronic devices which require electric power to operate.
Operating power for repeater stations is typically supplied by conventional power supplies connected to a local power grid at each repeater station, but, if grid power is not available or the repeater station is not readily accessible, such as repeater stations distributed at various intervals along the length of an intercontinental undersea cable, power for these repeater stations, or any other power consuming electronic devices, must be supplied at the cable ends and transmitted through the cable itself to the repeater stations, typically by means of a high voltage conductor (high voltage line) incorporated in the cable.
In the generation, transmission, and distribution of conventional electrical power, power is transmitted over long distances at high AC voltages, with the voltage step-down for local use or distribution done by AC transformers. High DC transmission voltages are also used, with down-conversion to AC being done by elaborate regional terminal installations. Conventional, long-distance, AC transmission techniques can not be used with undersea cables and similar applications for a variety of reasons, such as the capacitance effects and interference with signals. In applications like the powering of undersea repeaters, the power is most efficiently transmitted as low-current, high voltage DC introduced at the accessible ends of the cable, even though this creates a difficult voltage down-conversion or limitation problem at each of the remote locations which use the transmitted power. For ordinary electronic circuitry, the transmitted power must be converted (or limited) to low voltage at each individual locality or site where it is to be used.
Unfortunately, the existing and historical methods used for down-converting high voltage DC power to low DC or AC voltage are not at all appropriate for low or moderate power applications, since these methods typically involve the use of large SCR or mercury vapor rectifier tube installations requiring constant attention, or (for very low power installations) the use of power-wasting dropping resistors. A self-contained, reliable, high voltage DC-to-low voltage DC (or HVDC to AC) power conversion device able to run unattended is needed.
Likewise where power is to be collected from inaccessible, geographically-scattered, low-voltage sources of power (such as windmills, tidal generators, solar cells, and the like), an AC- or DC-to-HVDC converter capable of stepping up the low voltage power to high voltage before placing it on the high voltage cable is necessary, and this converter must be able to run unattended.
Furthermore, in a situation where any given device might act as either a source or a sink of power depending on local load or generating conditions, a converter readily capable of operation as either a voltage-converting step-down device or “in reverse” as a voltage step-up device in response to the direction of power flow would be very desirable.
At modest high voltages (less than 1000 VDC) and at the few-watt to several-kilowatt power levels, the power conversion or power conditioning functions are readily accomplished by conventional switching-type power supplies commonly used to convert high voltage DC to low voltage DC (or high voltage DC to low voltage AC). These prior art techniques are, however, unavailable for high voltage service above 1000-3000 volts due to the voltage limitations of the opening and closing semiconductor switches (IGBTs) used as choppers or synchronous rectifiers. Obtaining the required voltage holdoff for high voltage operation by connecting two or more switches of lesser holdoff capability in series and then switching them on and off simultaneously is notoriously difficult to do, because it requires high stability in voltage division across the switching devices, and great accuracy in the timing of their operation. If the voltage divides unevenly, or if one switch lags even slightly, to where the entire voltage appears across that switch, damage to the switch occurs instantly. Once damaged, it does not recover. For this reason, series-connected stacks of semiconductor switch devices are notoriously subject to so-called “zipper” failure, wherein the collapse of one device can set off a progressive and catastrophic destruction of all the others in the stack.
Other prior art switches have similar limitations. Sustained, repetitive opening and closing cannot be done efficiently with high voltage closing switches, such as thyratrons, spark gaps, or SCRs, since these devices, once closed, lose control and have to be shut off by other means. Some of these prior art devices also have negative current-voltage characteristics, uncertain triggering delays, and unstable holdoff recovery characteristics, making a non-wasteful, controlled, and reliable switch operation with them extremely difficult to accomplish. Repetitive high voltage switching by vacuum tubes (hard tubes) can also be done, but the technique is wasteful and inefficient due to the low current-emitting capability of the cathodes, the filament power required, the power lost in anode dissipation, and the size, fragility, and relatively short life expectancy of the tubes. In short, in the prior art, it has not been possible to switch high voltage reliably enough above 1000-3000 volts DC to make suitable high voltage switching supplies for converting high DC voltages (3000-100,000 volts or more) to low voltage DC or AC.
In the absence of suitable supplies able to be connected independently and directly to the high voltage line, other means have been used to supply power to devices to be operated in geographically isolated and inaccessible locations. One typical prior art technique of providing power to operate remotely located repeater stations or electronic devices is to connect them in series along the cable, and then force the maximum DC current needed by any given remote device through the entire system. This is typically accomplished by connecting a high voltage DC supply of one polarity at one end of the cable and another high voltage supply of opposite polarity at the other end of the cable to establish a current flow, with current return taking place by conduction through the earth. These cable-feeding, main DC power supplies are connected this way to obtain the required current at only half the voltage stress on the cable insulation with respect to ground that would otherwise occur if power were being fed from only one end. The current is passed through all the repeater stations located on the cable. The result is a series configuration of repeater stations on the high voltage power line with the necessary operating voltage at each remote location being developed by the voltage drop obtained across the equivalent input resistance of each successive load (e.g., series-connected street-lights or Christmas tree lights are powered in exactly the same way).
While the series connection assures that for a given wire size and power supply voltage, all devices will have the same current available (since current in a series circuit is the same everywhere), this prior art technique causes all the voltage drops due to cable and the power supplies of each repeater station to add along the cable length. Where a number of separate repeaters are to be powered, and the intervening cable resistance voltage drops overcome, very high voltages may be required to force the maximum desi
Biversfied Technologies, Inc.
Iandiorio & Teska
Patel Rajnikant B.
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