Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2001-04-30
2002-02-26
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S159000, C323S207000
Reexamination Certificate
active
06351397
ABSTRACT:
TECHNICAL FIELD
This invention relates to a protection apparatus and a protection method of a PWM cycloconverter for converting an AC voltage into an arbitrary AC voltage or DC voltage.
BACKGROUND ART
FIG. 8
shows a protection circuit for suppressing a surge voltage of the output side of a PWM cycloconverter in a related art. In
FIG. 8
, numeral
1
denotes a three-phase AC power supply, numeral
100
denotes a normal PWM cycloconverter, numeral
21
denotes a three-phase AC motor, numeral
110
denotes a rectification snubber circuit, numeral
120
denotes an output short circuit, numerals
111
,
112
, and
121
denote three-phase rectification bridges, numerals
113
and
122
denote uni-directional semiconductor switches,
114
and
115
denote resistors, numeral
116
denotes a capacitor, numeral
117
denotes a voltage detection circuit, and numeral
118
denotes a gate driver. In the PWM cycloconverter, the output and input sides are connected directly by bi-directional semiconductor switches for allowing a current to flow in bi-directionals and thus if two or more of three bi-directional switches connected in the same output phase are turned on at the same time at the commutating time, the input side becomes short-circuited and if the three bi-directional switches are turned off at the same time, the output side becomes open. As means for preventing the phenomenon, the load current direction is detected and the information can be used to commutate without opening the output side and without short-circuiting the input side. However, if the load current direction is erroneously detected during commutating (easy to be erroneously detected particularly in the vicinity of zero of load current), there is a possibility that the output side may be opened. When an overcurrent or an overvoltage on the input side or the load side, overheating of a motor winding or a semiconductor device of the main circuit, or the like (simply, operation anomaly) occurs, it is necessary to shut off the current of the PWM cycloconverter, namely, stop the operation abruptly. If all three bi-directional switches are turned off at the same time, the output side becomes open. To drive an induction motor, if the output side of the PWM cycloconverter becomes open during the operation, a large surge voltage occurs on the output side of the PWM cycloconverter.
When the output side of the PWM cycloconverter becomes open because of a commutation mistake during the normal operation, the load current continues to flow into the capacitor
116
via the three-phase rectification bridge
112
, and a large surge voltage does not occur. The voltage detection circuit
117
detects the voltage level of the capacitor
116
and when the voltage of the capacitor
116
becomes a certain degree or more, the voltage detection circuit
117
outputs an overvoltage signal OV. The uni-directional switch
113
is turned on by the overvoltage signal OV and the capacitor
116
is discharged. When an operation anomaly occurs, first all uni-directional switches of the PWM cycloconverter are turned off. After a given time interval (&Dgr;t) from the time, the uni-directional switch
122
is turned on. &Dgr;t should be longer than the on or off time of the semiconductor switch. The load current continues to flow into the capacitor
116
via the three-phase rectification bridge
112
during &Dgr;t and the uni-directional switch
122
is turned on and thus the load current continues to flow via the three-phase rectification bridge
121
and the uni-directional switch
122
and reduces to zero.
However, in the protection method of the PWM cycloconverter in the related art, if the uni-directional switch
122
is turned on during the operation and three phases of output are short-circuited for a long period of time (for example, until the end of shut off), an excessive motor stator current flows and there is a possibility that the facilities may be destroyed.
Next, the function of rectification snubber is to absorb a surge voltage occurring in the period of time in which the output side becomes open. The output side becomes open in either of the following two cases: One case where the load current direction is erroneously detected at the commutating time and the other case being the time interval &Dgr;t until all uni-directional switches are turned off and the uni-directional switch
122
is turned on at the shut off time. In both cases, the period of time in which the output side becomes open is almost the same; however, as compared with the fact that a detection mistake of the load current direction occurs only in the vicinity of zero of the load current, shut off caused by a fault, etc., often occurs when the current is rated current or overcurrent. Therefore, the surge voltage in the latter case is larger by far than that in the former case and as its countermeasure, the capacitor capacitance of the rectification snubber is large; this is also a problem. Further, normally the surge voltage produced by a commutation mistake is absorbed by the capacitor
116
of the rectification snubber and electric power accumulated in the capacitor
116
of the rectification snubber cannot be regenerated; this is also a problem.
DISCLOSURE OF INVENTION
It is therefore an object of the invention to provide a protection apparatus and a protection method of a PWM cycloconverter wherein when the PWM cycloconverter is shut off, a surge voltage is not produced on the output side, an overcurrent of a motor stator is not produced, an output short circuit is not required, and moreover a rectification snubber circuit does not exist or the capacitance of a capacitor of rectification snubber can be lessened exceptionally.
To solve the above-mentioned problems, according to the invention, there is provided a protection apparatus of a PWM cycloconverter of a power converter wherein phases of a three-phase AC power supply and phases of the power converter for outputting M phases (where M is an integer of two or more) are directly connected by bi-directional semiconductor switches each consisting of a pair of uni-directional semiconductor switches for allowing an electric current to flow only in uni-directional and being able to be turned on and off separately, the protection apparatus comprising:
fault detection means for detecting a fault of the above-mentioned PWM cycloconverter;
operation stop means for turning off at least some of the above-mentioned uni-directional semiconductor switches of the above-mentioned PWM cycloconverter if the above-mentioned fault signal occurs;
continuous flow means for allowing a load current of the above-mentioned PWM cycloconverter to continuously flow if at least some output phases of the above-mentioned PWM cycloconverter become open; and
protection switching means for turning on and off the above-mentioned uni-directional semiconductor switches of the above-mentioned PWM cycloconverter based on the fault signal output from the above-mentioned fault detection means, thereby allowing the above-mentioned load current to continuously flow into the above-mentioned three-phase AC power supply via the above-mentioned PWM cycloconverter.
In the invention, the above-mentioned protection switching means comprises:
input voltage information detection means for recognizing voltages of three terminals of the input side of the above-mentioned bi-directional semiconductor switches as three-phase input voltages and detecting information based on which greater-than, equal-to, less than relation of the above-mentioned three-phase input voltages can be determined as input voltage information and regenerative switching means for turning on only the above-mentioned uni-directional semiconductor switch from each output phase to an input phase which becomes the maximum voltage of the above-mentioned three-phase input voltages or only the above-mentioned uni-directional semiconductor switch from an input phase which becomes the minimum voltage of the above-mentioned three-phase input voltages to each output phase based on the input voltage informat
Hara Hidenori
Ishii Sadao
Sawa Toshihiro
Xia Xiaorong
Yamamoto Eiji
Berhane Adolf Deneke
Kabushiki Kaisha Yaskawa Denki
Sughrue & Mion, PLLC
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