Method for protecting thyristors of a converter and system therefor

Claim 2

A thyristor protection system for use in a converter, said converter comprising at least one arm which comprises a plurality of thyristors connected in series and/or parallel, and an a.c. circuit which includes a voltage transformer, said system comprising a first monostable circuit for receiving the phase control of the secondary side of the voltage transformer and for generating an output for a predetermined period of time upon triggering, said first monostable circuit being triggered when the phase current becomes zero, a differentiation circuit connected to the lines of the primary side of said voltage transformer for generating a pulse output when the line voltage becomes positive, and a first AND circuit connected to said first monostable circuit and said differentiation circuit for generating a pulse output when both of said circuits simultaneously give outputs, the pulse output of said first AND circuit being supplied to the thyristors to fire them.

Claim 3

A thyristor protection system according to claim 1, further comprising a second monostable circuit for receiving the phase current and for generating an output for a predetermined time upon triggering, said second monostable circuit being triggered when the phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm under protection, and a second AND circuit connected to said second monostable circuit and said terminal for generating an output when both of said second monostable circuit and said terminal simultaneously give outputs, the thyristors of the arm under protection being fired by either output of said first and second AND circuits.

Claim 4

A thyristor protection system according to claim 2, in which one of said first and second monostable circuits is further adapted to serve as the other circuit.

Claim 5

A thyristor protection system for use in a converter comprising arms, each arm including a plurality of thyristors connected in series and/or parallel, the system comprising a monostable circuit for receiving a phase current and for generating an output for a predetermined period of time upon triggering when said phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm under protection, and an AND circuit connected in said monostable circuit and said terminal for generating an output when both of said circuits simultaneously give outputs, the output of said AND circuit being sent to the thyristors of said arm to fire them.

Claim 6

A thyristor protection system for use in a converter comprising arms, each arm including a plurality of thyristors connected in series and/or parallel, the system comprising a monostable circuit for receiving a phase current and for generating an output for a predetermined period of time upon triggering when said phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm under protection, and an AND circuit connected in said monostable circuit and said terminal for generating an output when both of said circuits simultaneously give outputs, the output of said AND circuit being sent to the thyristors of said arm to fire them.

Claim 7

A thyristor protection system for use in a converter which comprises arms including a plurality of thyristors connected in series and/or parallel, said system comprising a voltage divider for detecting the voltage applied to an arm, a waveform shaping circuit connected to said voltage divider for deriving the time period when the input voltage from said voltage divider is negative, a differentiation circuit for differentiating the output of said waveform shaping circuit to generate a positive and a negative pulse at the time of the increase and reduction of the input, respectively, a first monostable circuit connected to said differentiation circuit for generating an output for a predetermined period of time upon triggering by the first pulse of the circuit, and a first AND circuit connected to said first monostable circuit and said differentiation circuit for transmitting the second pulse from differentiation circuit when said first monostable circuit supplies its output, the second pulse transmitted through said first AND circuit being sent to the thyristors of the arm under protection to fire them, and further comprising a second monostable circuit for receiving a phase current and for generating an output for a period of time upon triggering when the phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm undeR protection, and a second AND circuit connected to said second monostable circuit and said terminal for generating an output when both said monostable circuit and terminal supply outputs simultaneously, the thyristors of said arm being fired by either output of said first and second AND circuits.

Claim 8

A thyristor protection system for use in a converter which comprises arms including a plurality of thyristors connected in series and/or parallel, said system comprising a voltage divider for detecting the voltage applied to an arm, a waveform shaping circuit connected to said voltage divider for deriving the time period when the input voltage from said voltage divider is negative, a differentiation circuit for differentiating the output of said waveform shaping circuit to generate a positive and a negative pulse at the time of the increase and reduction of the input, respectively, a first monostable circuit connected to said differentiation circuit for generating an output for a predetermined period of time upon triggering by the first pulse of the circuit, and a first AND circuit connected to said first monostable circuit and said differentiation circuit for transmitting the second pulse from differentiation circuit when said first monostable circuit supplies its output, the second pulse transmitted through said first AND circuit being sent to the thyristors of the arm under protection to fire them, and further comprising a second monostable circuit for receiving a phase current and for generating an output for a period of time upon triggering when the phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm undeR protection, and a second AND circuit connected to said second monostable circuit and said terminal for generating an output when both said monostable circuit and terminal supply outputs simultaneously, the thyristors of said arm being fired by either output of said first and second AND circuits.

Claim 9

A thyristor protection system according to claim 5, in which said first monostable circuit is further adapted to serve as said second monostable circuit.

Claim 10

In a converter connected between a DC power line and a polyphase AC power line which comprises a plurality of arms corresponding to the phases of said AC power line and each having a plurality of thyristors connected together; a method for protecting said thyristors of each one of said arms comprising detecting the time when a forward current passed through said arm ceases to flow, producing a signal responding to a forward voltage when said forward voltage is imposed to said arm before a predetermined time interval elapses from said time, and firing said thyristors of said arm by said signal.

Claim 11

In a converter connected between a DC power line and a polyphase AC power line which comprises a plurality of arms corresponding to the phases of said AC power line and each having a plurality of thyristors connected together; a method for protecting said thyristors of each one of said arms comprising detecting the time when a forward current passed through said arm ceases to flow, producing a signal responding to a forward voltage when said forward voltage is imposed to said arm before a predetermined time interval elapses from said time, and firing said thyristors of said arm by said signal.

Claim 12

A method in accordance with claim 7, wherein said time when said arm current ceases to flow is detected from a corresponding phase current of said AC power line, and a forward voltage which is imposed to said arm is detected from a corresponding line voltage of said AC power line.

Claim 13

In a converter connected between a DC power line and a polyphase AC power line which comprises a plurality of arms corresponding to the phases of said AC power line and each having a plurality of thyristors connected together; a method for protecting said thyristors of each one of said arms comprising detecting the time when a reverse voltage appears across said arm, producing a signal responding to a forward voltage which appears across said arm before a predetermined time interval elapses from said time, and firing said thyristors of said arm by said signal.

Claim 14

In a converter connected between a DC power line and a polyphase AC power line which comprises a plurality of arms corresponding to the phases of said AC power line and each having a plurality of thyristors connected together; a method for protecting said thyristors of each one of said arms comprising detecting the time when a reverse voltage appears across said arm, producing a signal responding to a forward voltage which appears across said arm before a predetermined time interval elapses from said time, and firing said thyristors of said arm by said signal.

Claim 15

In a converter connected between a DC power line and a polyphase AC power line which comprises a plurality of arms corresponding to the phases of said AC power line and each having a plurality of thyristors connected in series and/or parallel; a method for protecting said thyristors of each one of said arms comprising detecting the time when a current passed through said arm becomes zero, introducing a firing pulse which is used for firing the thyristors of another arm to be fired in lag by 180* electrically from said first-mentioned arm, and firing said thyristors of said first-mentioned arm by said firing pulse when said firing pulse appears before a predetermined time interval elapses from said time.

Claim 16

In a converter connected between a DC power line and a polyphase AC power line which comprises a plurality of arms corresponding to the phases of said AC power line and each having a plurality of thyristors connected in series and/or parallel; a method for protecting said thyristors of each one of said arms comprising detecting the time when a current passed through said arm becomes zero, introducing a firing pulse which is used for firing the thyristors of another arm to be fired in lag by 180* electrically from said first-mentioned arm, and firing said thyristors of said first-mentioned arm by said firing pulse when said firing pulse appears before a predetermined time interval elapses from said time.

Claim 17

In a converter connected between a D.C. power line and a polyphase A.C. power line having a plurality of arms corresponding to the phases of said A.C. power line, each arm having a plurality of thyristors connected therein a method for protecting the thyristors in each one of said arms comprising the steps of: detecting the instant of time when the current through an arm becomes zero; generating a first signal, the duration of which extends between the instant of time when said current becomes zero and an instant of time subsequent thereto when a forward voltage may be applied across said arm without igniting one of said thyristors; generating a second signal, in response to the application of a forward voltage across the thyristors of said arm only during the duration of said first signal; and applying said second signal to said thyristors of said arm, whereby said thyristors within said arm are fired no later than said application of a forward voltage across said arm.

Claim 18

In a converter connected between a D.C. power line and a polyphase A.C. power line having a plurality of arms corresponding to the phases of said A.C. power line, each arm having a plurality of thyristors connected therein a method for protecting the thyristors in each one of said arms comprising the steps of: detecting the instant of time when the current through an arm becomes zero; generating a first signal, the duration of which extends between the instant of time when said current becomes zero and an instant of time subsequent thereto when a forward voltage may be applied across said arm without igniting one of said thyristors; generating a second signal, in response to the application of a forward voltage across the thyristors of said arm only during the duration of said first signal; and applying said second signal to said thyristors of said arm, whereby said thyristors within said arm are fired no later than said application of a forward voltage across said arm.

Claim 19

A thyristor protection system for use in a converter, said converter having at least one arm which includes a plurality of thyristors connected together and across an A.C. power line, said system comprising: first means, responsive to the current flow in one of said arms, for detecting when said current flow beComes zero; second means, responsive to said first means and to the output voltage across said one of said arms, for generating a first signal the duration of which extends between the instant when said current flow becomes zero and when said voltage becomes a forward voltage across said thyristors of said one of said arms; third means, responsive to said second means, for generating a thyristor firing signal, upon the duration of said first signal ending prior to the termination of a prescribed period of time after said current flow becomes zero; and fourth means, responsive to said third means, for applying said thyristor firing signal to be thyristors of said one of said arms.

Claim 20

A thyristor protection system for use in a converter, said converter having at least one arm which includes a plurality of thyristors connected together and across an A.C. power line, said system comprising: first means, responsive to the current flow in one of said arms, for detecting when said current flow beComes zero; second means, responsive to said first means and to the output voltage across said one of said arms, for generating a first signal the duration of which extends between the instant when said current flow becomes zero and when said voltage becomes a forward voltage across said thyristors of said one of said arms; third means, responsive to said second means, for generating a thyristor firing signal, upon the duration of said first signal ending prior to the termination of a prescribed period of time after said current flow becomes zero; and fourth means, responsive to said third means, for applying said thyristor firing signal to be thyristors of said one of said arms.

Claim 21

A thyristor protection system for use in a converter comprising at least one arm having a plurality of thyristors connected in series, said system comprising: first means for producing a first output signal corresponding to a margin angle of commutation for said arm responsive to a reverse voltage applied to said arm, second means being rendered operative simultaneously with initiating of said first output signal to produce a second output signal during a predetermined reference time longer than the longest turnoff time for the thyristors of said arm, and third means for comparing said first and second output signals to produce a firing signal when said first output signal terminates in the presence of said second output signal, said firing signal being applied to said arm for firing the thyristors thereof whereby the thyristors of said arm are refired when the time at which said arm is impressed with a reverse voltage is shorter than a predetermined reference time.

Claim 22

A thyristor protection system for use in a converter comprising at least one arm having a plurality of thyristors connected in series, said system comprising: first means for producing a first output signal corresponding to a margin angle of commutation for said arm responsive to a reverse voltage applied to said arm, second means being rendered operative simultaneously with initiating of said first output signal to produce a second output signal during a predetermined reference time longer than the longest turnoff time for the thyristors of said arm, and third means for comparing said first and second output signals to produce a firing signal when said first output signal terminates in the presence of said second output signal, said firing signal being applied to said arm for firing the thyristors thereof whereby the thyristors of said arm are refired when the time at which said arm is impressed with a reverse voltage is shorter than a predetermined reference time.

Claim 23

A thyristor protection system for use in a converter which comprises arms including a plurality of thyristors connected in series and/or parallel, said system comprising a voltage divider for detecting the voltage applied to an arm, a waveform shaping circuit connected to said voltage divider for deriving the time period when the input voltage from said voltage divider is negative, a differentiation circuit for differentiating the output of said waveform shaping circuit to generate a positive and a negative pulse at the time of the increase and reduction of the input, respectively, a first monostable circuit connected to said differentiation circuit for generating an output for a predetermined period of time upon triggering by the first pulse of the differentiation circuit, and a comparing circuit connected to said first monostable circuit and said differentiation circuit for comparing the output of said first monostable circuit and the second pulse from said differentiation circuit to transmit the second pulse from said differentiation circuit to the thyristors of the arm under protection to fire them when said second pulse occurs in the presence of the output of said first monostable circuit, and further comprising a second monostable circuit for receiving a phase current and for generating an output for a period of time upon triggering when the phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm under protection, and an AND circuit connected to said second monostable circuit and said terminal for generating an output when both said monostable circuit and terminal supply outputs simultaneously, the thyristors of said arm being fired by either output of said AND circuit and said comparing circuit.

Claim 24

A thyristor protection system for use in a converter which comprises arms including a plurality of thyristors connected in series and/or parallel, said system comprising a voltage divider for detecting the voltage applied to an arm, a waveform shaping circuit connected to said voltage divider for deriving the time period when the input voltage from said voltage divider is negative, a differentiation circuit for differentiating the output of said waveform shaping circuit to generate a positive and a negative pulse at the time of the increase and reduction of the input, respectively, a first monostable circuit connected to said differentiation circuit for generating an output for a predetermined period of time upon triggering by the first pulse of the differentiation circuit, and a comparing circuit connected to said first monostable circuit and said differentiation circuit for comparing the output of said first monostable circuit and the second pulse from said differentiation circuit to transmit the second pulse from said differentiation circuit to the thyristors of the arm under protection to fire them when said second pulse occurs in the presence of the output of said first monostable circuit, and further comprising a second monostable circuit for receiving a phase current and for generating an output for a period of time upon triggering when the phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm under protection, and an AND circuit connected to said second monostable circuit and said terminal for generating an output when both said monostable circuit and terminal supply outputs simultaneously, the thyristors of said arm being fired by either output of said AND circuit and said comparing circuit.

Claim 25

A thyristor protection system according to claim 14, in which said first monostable circuit is further adapted to serve as said second monostable circuit.

Claim 26

A thyristor protection system for use in a converter which comprises arms including a plurality of thyristors connected in series and/or parallel, said system comprising a voltage divider for detecting the voltage applied to an arm, a waveform shaping circuit connected to said voltage divider for deriving the time period wheN the input voltage from said voltage divider is negative, a first monostable circuit responsive to the output of said waveform shaping circuit to produce an output for a predetermined period of time from the occurrence of the output of said waveform shaping circuit, and a comparing circuit for comparing the output of said first monostable circuit and a first signal indicative of the expiration of the output of said waveform shaping circuit to produce a second signal when said first signal occurs in the presence of the output of said first monostable circuit, said second signal being supplied to the thyristors of the arm under protection to fire them, and further comprising a second monostable circuit for receiving a phase current and for generating an output for a period of time upon triggering when the phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm under protection, and an AND circuit connected to said second monostable circuit and said terminal for generating an output when both said monostable circuit and terminal supply outputs simultaneously, the thyristors of said arm being fired by either output of said AND circuit and said comparing circuit.

Claim 27

A thyristor protection system for use in a converter which comprises arms including a plurality of thyristors connected in series and/or parallel, said system comprising a voltage divider for detecting the voltage applied to an arm, a waveform shaping circuit connected to said voltage divider for deriving the time period wheN the input voltage from said voltage divider is negative, a first monostable circuit responsive to the output of said waveform shaping circuit to produce an output for a predetermined period of time from the occurrence of the output of said waveform shaping circuit, and a comparing circuit for comparing the output of said first monostable circuit and a first signal indicative of the expiration of the output of said waveform shaping circuit to produce a second signal when said first signal occurs in the presence of the output of said first monostable circuit, said second signal being supplied to the thyristors of the arm under protection to fire them, and further comprising a second monostable circuit for receiving a phase current and for generating an output for a period of time upon triggering when the phase current becomes zero, a terminal for introducing a firing pulse having a lag angle of 180* with respect to the firing pulse for the arm under protection, and an AND circuit connected to said second monostable circuit and said terminal for generating an output when both said monostable circuit and terminal supply outputs simultaneously, the thyristors of said arm being fired by either output of said AND circuit and said comparing circuit.

Claim 28

A thyristor protection system according to claim 16, in which said first monostable circuit is further adapted to serve as said second monostable circuit.

Claim 29

In a converter connected between a D.C. power line and a polyphase A.C. power line which comprises a plurality of arms corresponding to the phases of said A.C. power line and each having a plurality of thyristors connected together; a method for protecting said thyristors of each one of said arms comprising detecting the time when a forward current passed through said arm ceases to flow producing a signal responding to the expiration of a reverse voltage when said reverse voltage expires before a predetermined time interval elapses from said time, and firing said thyristors of said arm by said signal.

Claim 30

In a converter connected between a D.C. power line and a polyphase A.C. power line which comprises a plurality of arms corresponding to the phases of said A.C. power line and each having a plurality of thyristors connected together; a method for protecting said thyristors of each one of said arms comprising detecting the time when a forward current passed through said arm ceases to flow producing a signal responding to the expiration of a reverse voltage when said reverse voltage expires before a predetermined time interval elapses from said time, and firing said thyristors of said arm by said signal.

Claim 31

A method in accordance with claim 18, wherein said time when said arm current ceases to flow is detected from a corresponding phase current of said A.C. power line, and the expiration of a reverse voltage which has appeared across said arm is detected from a corresponding line voltage of said A.C. power line.

Claim 32

In a converter connected between a D.C. power line and a polyphase A.C. power line which comprises a plurality of arms corresponding to the phases of said A.C. power line and each having a plurality of thyristors connected together; a method for protecting said thyristors of each one of said arms comprising detecting the time when a reverse voltage appears across said arm, producing a signal responding to the expiration of said reverse voltage when said reverse voltage expires before a predetermined time interval elapses from said time, and firing said thyristors of said arm by said signal.

Claim 33

In a converter connected between a D.C. power line and a polyphase A.C. power line which comprises a plurality of arms corresponding to the phases of said A.C. power line and each having a plurality of thyristors connected together; a method for protecting said thyristors of each one of said arms comprising detecting the time when a reverse voltage appears across said arm, producing a signal responding to the expiration of said reverse voltage when said reverse voltage expires before a predetermined time interval elapses from said time, and firing said thyristors of said arm by said signal.