Fault detection and protection strategy for a pair of complementary GTO thyristors

• US 5,057,987 A
• Filed: 11/27/1990
• Issued: 10/15/1991
• Est. Priority Date: 11/27/1990
• Status: Expired due to Fees

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Kumar, Ajith K., Stitt, Thomas D.
• Primary Examiner(s): Wong, Peter S.
• Application Number:
  US07/618,783
• Time in Patent Office: 322 Days
• Field of Search: 363/55, 363/56, 363/57, 363/58, 363/96, 363/136, 363/137, 363/138, 363/135, 361/79, 361/86, 361/87, 361/88, 361/93
• US Class Current: 363/58
• CPC Class Codes: H02H 7/1225 responsive to internal faults, e.g. shoot-through avoiding shoot-through H02M1/38

Abstract

In an electric power converter comprising at least one pair of serially-connected, alternately conducting controllable electric valves spanning a pair of d-c conductors, the conducting state of each valve is detected by comparing the actual magnitude of valve current with a predetermined low threshold level and by comparing the difference between the juncture potential of the two valves and a reference potential of one of the d-c conductors with the potential diffence between both d-c conductors. If both comparisons indicate the valve is in a conducting state and if this state is indicted concurrently for both valves, logic means is operative to indicate that a "shoot through" fault has occurred. In this abnormal event, whatever turn on and turn off signals for the two valves were extant at the time such operation of the logic means commenced will be prevented from changing while both valves are conducting.

Claims

1. Improved means for detecting the conducting state of at least one electric value in an electric power converter having first and second alternately conducting controllable electric valves connected in series with one another between relatively positive and negative d-c conductors that are adapted to be connected to a source of electric power, the juncture of the valves forming an a-c terminal adapted to be connected to an alternating current load circuit, each valve having a control electrode to which alternative turn-on and turn-off signals are periodically applied and being respectively switched by such signals between turned-on and turned-off states, the first valve when turned on being arranged to freely conduct load current from the positive d-c conductor to said a-c terminal, and the second valve when turned-on being arranged to freely conduct load current from the a-c terminal to the negative d-c conductor, wherein the improvement comprises:

(a) voltage sensing means coupled to the a-c terminal and to both of the d-c conductors for deriving two voltage feedback values respectively representative of the potential difference between said d-c conductors and the potential difference between said a-c terminal and said negative d-c conductor;

(b) first bistable value comparing means responsive to said voltage feedback values and having alternative first and second operating states, said first state occurring if the potential difference between said a-c terminal and said negative d-c conductor exceeds a predetermined first fraction of the potential difference between said d-c conductors and said second state occurring otherwise, said first fraction being greater than approximately 1/4 but less than 1/2, whereby said first state occurs when said first valve is turned on;

(c) current sensing means for deriving a first current feedback value representative of the magnitude of current conducted by said first valve;

(d) second bistable means for comparing said first current feedback value with a constant value representative of a predetermined level of current and having alternative first and second operating states, said first state of said second means occurring if the magnitude of current conducted by said first valve exceeds said predetermined level, and said second state of said second means occurring otherwise, whereby said first state of said second means occurs when said first valve is turned on; and

(e) logic means responsive to the states of both said first means and said second means for providing a signal indicative of whether or not said first valve is in a turned-on state.

2. The improvement of claim 1, in which said logic means is so arranged that the signal it provides will indicate said first valve is turned on only when said first means and said second means are concurrently in their respective first states.

3. The improvement of claim 1 for an electric power converter wherein the first and second valves are gate turnoff thyristors.

4. The improvement of claim 1, and further comprising:

third bistable means responsive to said voltage feedback values and having alternative first and second operating states, said first state of said third means occurring if the potential difference between said a-c terminal and said negative d-c conductor does not exceed a predetermined second fraction of the potential difference between said d-c conductors and said second state of said third means occurring otherwise, said second fraction being less than approximately 3/4 but greater than 1/2 whereby said first state of said third means occurs when said second valve is turned on;

second current sensing means for deriving a second current feedback value representative of the magnitude of current conducted by said second valve;

fourth bistable means for comparing said second current feedback value with said constant value and having alternative first and second operating states, said first state of said fourth means occurring if the magnitude of current conducted by said second valve exceeds said predetermined level and said second state of said fourth means occurring otherwise, whereby said first state of said fourth means occurs when said second valve is turned on; and

second logic means responsive to the states of both said third means and said fourth means for providing a signal indicative of whether or not said second value is in a turned-on state.

5. The improvement of claim 4, in which said logic means are so arranged that the signal provided by the first-mentioned logic means will indicate said first valve is turned on only when said first and second bistable means are concurrently in their respective first states and the signal provided by said second logic means will indicate said second valve is turned on only when said third and fourth bistable means are concurrently in their respective first states;

and further comprising third logic means associated with the other two logic means and operative to indicate that said first and second valves are simultaneously turned on only if all four of said bistable means are in their respective first states concurrently.

6. The improvement of claim 5 for an electric power converter wherein the timing of the alternative turn-on and turn-off signals applied to the respective control electrodes of said first and second valves is determined by associated gating means the operation of which is normally controlled by a train of discrete command signals, the improvement further comprising annuling means responsive to operation of said third logic means for suspending normal operation of the gating means and thereby preventing any further changes of said turn-on and turn-off signals in the abnormal event both valves are in turned-on states simultaneously.

7. The improvement of claim 6, in which said predetermined level of current is in a range of approximately two to three percent of the turn-off current limit of said valves.

8. The improvement of claim 6, in which said annuling means is effective to suspend normal operation of the gating means for at least a predetermined desired period of time after operation of said third logic means commences.

9. The improvement of claim 8, and further comprising means associated with said annuling means for disabling the gating means as soon as operation of said third logic means terminates and said desired period expires, the gating means when disabled being arranged to cause turn-off signals to be applied to both valves.

10. For use in an electric power converter having first and second alternately conducting controllable electric valves connected in series with one another between relatively positive and negative d-c conductors that are adapted to be connected to a source of electric power, the juncture of the valves forming an a-c terminal adapted to be connected to an alternating current load circuit, each valve having a control electrode to which alternative turn-on and turn-off signals are periodically applied and being respectively switched by such signals between turned-on and turned-off states, the first valve when turned on being arranged to freely conduct load current from the positive d-c conductor to said a-c terminal, and the second valve when turned-on being arranged to freely conduct load current from the a-c terminal to the negative d-c conductor, the converter also having control means including driver means for supplying the aforesaid turn-on and turn-off signals, gating means connected to the driver means for determining the timing of such signals, and suitable means associated with the gating means for providing a train of discrete command signals that normally control the operation of the gating means, improved means for protecting the valves in the abnormal event that both valves are in turned-on states simultaneously, comprising:

(a) first means associated with said first valve for providing a first output signal to indicate that said first valve is in a turned-on state;

(b) second means associated with said second valve for providing a second output signal to indicate that said second valve is in a turned-on state;

(c) third means coupled to said first and second means and operative to indicate that said first and second valves are simultaneously turned on only when said first and second output signals are being provided concurrently; and

(d) annuling means responsive to operation of said third means for suspending normal operation of said gating means, whereby said driver means temporarily holds whatever turn-on and turn-off signals are extant at the time operation of said third means commences.

11. The improved protecting means of claim 10 and further comprising timing means associated with said annuling means for sustaining the turn-on and turn-off signal-holding action for a predetermined desired period of time after operation of said third means commences.

12. Improved means for detecting simultaneous conduction by a pair of complementary electric valves in an electric power converter having first and second alternately conducting controllable electric valves connected in series with one another between relatively positive and negative d-c conductors that are adapted to be connected to a source of electric power, the juncture of the valves forming an a-c terminal adapted to be connected to an alternating current load circuit, each valve having a control electrode to which alternative turn-on and turn-off signals are periodically applied and being respectively switched by such signals between turned-on and turned-off states, the first valve when turned on being arranged to freely conduct load current from the positive d-c conductor to said a-c terminal, and the second valve when turned-on being arranged to freely conduct load current from the a-c terminal to the negative d-c conductor, wherein the improvement comprises:

(a) voltage sensing means coupled to the a-c terminal and to both of the d-c conductors for deriving two voltage feedback values respectively representative of the potential difference between said d-c conductors and the potential difference between said a-c terminal and said negative d-c conductor;

(b) a pair of current sensing means for deriving first and second current feedback values respectively representative of the magnitudes of current conducted by said first and second valves;

(c) means for providing a constant value representative of a predetermined level of current;

(d) first means responsive to said feedback values and to said constant value for providing a first output signal that indicates a turned-on state of said first valve if the magnitude of current conducted by said first valve exceeds said predetermined level and the potential difference between said a-c terminal and said negative d-c conductor exceeds a predetermined first fraction of the potential difference between said d-c conductors, said first fraction being greater than approximately 1/4 but less than 1/2;

(e) second means responsive to said feedback values and to said constant value for providing a second output signal that indicates a turned-on state of said second valve if the magnitude of current conducted by said second valve exceeds said predetermined level and the potential difference between said a-c terminal and said negative d-c conductor does not exceed a predetermined second fraction of the potential difference between said d-c conductors, said second fraction being less than approximately 3/4 but greater than 1/2; and

(f) third means associated with said first and second means and operative to indicate that said first and second valves are simultaneously turned on only when said first and second output signals are being provided concurrently.