Electrical transmission or interconnection systems – Switching systems – Condition responsive
Reexamination Certificate
1999-10-06
2001-04-24
Ballato, Josie (Department: 2836)
Electrical transmission or interconnection systems
Switching systems
Condition responsive
C320S122000, C320S120000, C200S043160, C200S290000, C200S318000, C200S323000, C307S144000
Reexamination Certificate
active
06222284
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates to a drive and specifically a drive and a method for operating a variable drive such as a variable-speed motor drive system.
II. Description of Prior Art
Variable-output drives traditionally are used in industry to provide variable electric power to AC motors. These same drives can be used in other applications not related specifically to motors but where a variable-output voltage or frequency is desired. Typical drives have an AC input power source and some type of conversion apparatus, usually using solid-state devices, for converting the fixed AC input voltage into a variable-voltage and/or variable-frequency output. One such type of drive is described in U.S. Pat. No. 5,625,545, which is incorporated herein by reference. That patent describes a power supply used as a drive which utilizes a number of power cells arranged to produce a three-phase AC output. Such multiple power cells in series can be utilized to provide higher voltage outputs than would be available with only a single cell. However, should a given cell in a series arrangement fail during operation, that current path can become open-circuited, rendering the entire drive inoperable. In many applications, especially industrial applications utilizing variable-speed drives, it is desirable to continue to operate in a reduced mode when a single cell fails. One method to do such a reduced operation is to short out the damaged or failed cell. This can then result in an unbalanced output, in that the leg having the shorted cell must operate at less than full voltage. Such operation can create imbalanced line conditions and be damaging to load elements. Therefore, when a cell fails in a series arrangement, it is usually desirable to permit operation at a reduced voltage or speed by shunting out an equal number of cells in each of the other legs. While the output from such a drive having shunted cells in each leg is reduced, it does permit the drive to operate at a reduced voltage or speed. Often times such reduced operation is highly desirable so that the process being controlled by the drive can continue until it is convenient to shut down for repairs. In addition, it may be desirable to operate the drive at a reduced-voltage condition so as to maintain a desired minimum level of production. Typically, when a drive having multiple converters arranged in series has a failure, the output voltage and power available is reduced proportional to the ratio of the number of unfailed cells in a given leg divided by the number of total cells in such leg. As an example, if one of the cells in the aforesaid patent, such as power cell A
1
were to fail, typically power cells A
1
, B
1
and C
1
would be shunted by appropriate bypass circuitry. Such circuitry is shown in FIG. 9 of the U.S. Pat. No. 5,625,545 patent. When operating under such bypass condition, the output of the three-phase drive at references 22, 25 and 24 is proportionally reduced by one-third. In such an operating condition, the output voltage available to the motor would only be 66% of the full voltage. In such a condition, the motor would only be able to reach approximately two-thirds of its rated speed. In many applications being able to reach even 66% of rated speed in a failed cell mode is desirable. However, it still represents a substantial reduction in the output speed and power available from the drive. When multiple power cells or converters are used in series, the total power delivered to the load is usually supplied generally equally by all the power cells. Thus in the example above when one of nine cells fails, 89% of rated power should still be available. By shunting out or bypassing two operational power cells such as B
1
and C
1
in the example, these power cells can no longer contribute to the output power available to the load. It would be very desirable, then, to optimize the output voltage and power available to the load during a failed cell condition. The invention provides for an apparatus and a method that permits operation during a failed mode at a power output that utilizes all of the cells or converters that are operational.
SUMMARY OF THE INVENTION
The invention is directed to a method of operating a multiphase power supply that has a plurality of cells or converters connected in series in each of a plurality of legs. This results in each leg having a leg voltage equal to the sum of connected cells in such leg. In addition, the power supply has a line-to-line voltage output between the terminal ends of each pairs of legs. This invention applies to loads that are not connected to the common leg junction, but only to the end points of the legs, so that only the line-to-line output voltage appears on the load. During a failed condition in which any one or number of cells have failed in any of the legs, the line-to-line voltage output is maximized. This is done by detecting failed cells in any of the legs and forming a current path around such failed cell by an electrical bypass. As a result no voltage component from the failed cell is included within the leg in which it previously was connected. The outputs of the unfailed cells are controlled so as to maximize the line-to-line voltage in the output of the power supply so that their magnitudes are generally equal. By so regulating their outputs, the line-to-line phase voltages of the output of the power supply can be balanced. In a three-phase output supply without failed cells, balanced voltage is maintained by having a 120° phase relationship between the leg output voltages. The line-to-line phase balance can be restored by adjusting the leg-to-leg phase relationship between the respective legs when failed cells are detected. Circuitry is provided to implement a method for reducing the peak voltage demands between legs. Voltage command signals can be generated to provide peak voltage reduction, and such signals can be modified to compensate for the detection of failed cells. The modification can be a gain control using a gain which is proportional to the ratio of the total number of cells in the leg (including failed cells) divided by the number of active cells (not bypassed) in the leg. The invention is particularly adapted to pulse width modulated serial converters in each leg. The result is balanced line-to-line output from the power supply and full utilization of each unfailed cell in the system.
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patent: 4039925 (1977-08-01), Fletcher et al.
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patent: 5625545 (1997-04-01), Hammond
patent: 5708576 (1998-01-01), Jones et al.
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patent: 5744936 (1998-04-01), Kawakami
patent: 5814970 (1998-09-01), Schmidt
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patent: 5898239 (1999-04-01), Kawam
patent: 5952815 (1999-09-01), Rouillard et al.
patent: 45-2703 (1970-01-01), None
Aiello Marc F.
Hammond Peter W.
Ballato Josie
Buchanan Ingersoll P.C.
Roberto Rios
Robicon Corporation
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