Abstract:
In a method and a system for terminating inverter operation of a converting device connected to a DC power transmission system and having a bridge connection of thyristor valves, absence of a current flowing through the converting device is detected to provide an absence detection signal indicative of such absence, and the absence detection signal is used as one condition for effecting termination of the inverter operation. According to the invention, the absence is found when all the valves in at least one of the lower potential side and the higher potential side are found concurrently non-conducting, and each valve is found non-conducting when a voltage is found present across each valve.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and a system for terminating inverter operation of a converting device having a bridge connection of thyristor valves and connected to a DC power transmission system, particularly a DC power transmission system having more than two terminals. 
     FIG. 1 shows an example of a multi-terminal DC power transmission system, i.e., a DC power transmission system having more than two terminals. In the illustrated example, there are four terminals or converting stations 8a, 8b, 8c, 8d. The converting stations 8a-8d are interconnected by a pair of DC lines 7a, 7b forming a DC power transmission system, and are respectively connected to AC power transmission systems 1a, 1b, 1c, 1d. Each converting station comprises a converting device 4a, 4b, 4c, 4d. The AC terminals of each converting device are connected to the AC power transmission system 1a, 1b, 1c, 1d through a transformer 3a, 3b, 3c, 3d and an AC circuit breaker 2a, 2b, 2c, 2d. The DC terminals of each converting device are connected through a reactor 5a, 5b, 5c, 5d to the DC lines 7a and 7b. 
     Each of the converting devices 4a-4d comprises, as shown in FIG. 2, a bridge connection oil thyristor valves 6u, 6v, 6w, 6x, 6y, 6z. The valves 6u, 6v, 6w have one end connected to a negative line 7b, so that they are called valves of lower potential side. The valves 6x, 6y, 6z have one end connected (through a reactor 5a, 5b, 5c, 5d) to a positive line 7a so that they are called valves of higher potential side. FIG. 2 also shows AC terminals R, S, T for connecticn with three-phase AC power transmission system (1a-1d in FIG. 1), and DC terminals P, N for connection to the DC lines 7a, 7b through a reactor (indicated by 5 as representative of 5a-5d in FIG. 1). In FIG. 2, v d  denotes DC voltage across the DC terminals P, N and i d  denotes DC current through the DC terminals P, N. 
     Assume that the converting device in one of the converting stations, say 8d, has been in the inverter operation and it is desired that the inverter operation of the converting device be terminated. Conventionally, the DC current through the converting apparatus 4d is reduced to zero, and upon detection or confirmation of the current being zero, gate pulses are blocked (in other words the converting device is gate-blocked). 
     However, DC current id flowing through the converting device contains, as shown in FIG. 3, ripples because of the DC voltage v d  of the converting device and the DC line voltage v dL  beyond the reactor 5d, so that it is very difficult to detect the current being zero. 
     In addition, if the gate pulses are blocked while the DC current i d  is still flowing, as shown in FIG. 3, the interruption of the current need to be completed during the period of t 1  -t 2  when the DC voltage v d  is greater (in magnitude) than the line voltage v dL  as shown by hatching in FIG. 3. After t 2 , v d  becomes smaller than v dL . If the interruption is not completed by t 2 , a very large current, shown by dotted line, flows through the converting device whose gate pulses are blocked. 
     It is thus seen that it is vital to make sure that the DC current is zero prior to blocking the gate pulses for termination of the inverter operation of a converting device in a DC power transmission system. If the gate pulses are blocked while the DC current is still flowing, an excessive current flows, which may cause a serious breakdown necessitating interruption of the operation of the entire DC power transmission line. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method and a system for terminating the inverter operation of a converting device, which enable more reliable detection of the DC current being zero, and minimize the risk of an excessive current flowing through the converting device. 
     According to one aspect of the invention, there is provided a method for terminating inverter operation of a converting device connected to a DC power transmission system and having a bridge connection of thyristor valves, comprising the steps of: 
     detecting absence of a current flowing through the converting device to provide an absence detection signal indicative of such absence, and 
     using said absence detection signal as one condition for effecting termination of the inverter operation, 
     wherein said step of detecting absence comprises detecting non-conduction of all the valves in at least one of the lower potential side and the higher potential side to find said absence of a current when said all the valves are found concurrently non-conducting, and 
     the detection of non-conduction of each valve comprises detecting a voltage across the valve to find the non-conduction when the voltage is found present. 
     According to another aspect of the invention, there is provided a system for terminating inverter operation of a converting device connected to a DC power transmission system and having a bridge connection of thyristor valves, comprising: 
     means for detecting absence of a current flowing through the converting device to produce an absence detection signal indicative of such absence, 
     means using said absence detection signal as one condition for effecting termination of the inverter operation, 
     wherein said means for detecting absence comprises means for detecting non-conduction of all the valves in at least one of the lower potential side and the higher potential side to find said absence of a current when said all the valves are found concurrently non-conducting, and 
     said means for detecting non-conduction comprises means for detecting a voltage across each valve to find the non-conduction when the voltage is found present. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic diagram showing an example of a DC power system having more than two terminals; 
     FIG. 2 is a circuit diagram showing a converting device usable in the system of FIG. 1; 
     FIG. 3 shows a DC current and a voltage in the converting device in operation; 
     FIG. 4 is a diagram showing voltage detection circuits incorporated in the converting device; 
     FIG. 5 is a block diagram showing an example of control circuit for blocking gate pulses; 
     FIGS. 6 through 8 show voltages and currents at the various parts of the system; 
     FIGS. 9 and 10 show modifications of the invention; and 
     FIG. 11 shows signals at various parts of the system of FIG. 10. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention will now be described with reference to FIGS. 4-11. 
     FIG. 4 shows a converting device 4 similar to that shown in FIG. 2 and the converting device 4 similarly comprises a bridge connection of thyristor valves 6u -6z. The AC terminals R, S, T and the DC terminals P, N may be similarly connected. 
     In addition, voltage detection circuits 10u-10z  are provided in association with the thyristor valves 6u-6z respectively. Each of the voltage detection circuits is provided with a first output terminal FV u  -FV z  and a second output terminal RV u  -RV z , and produces a logic signal of &#34;1&#34; or forward voltage detection signal at the first terminal FV u  -FV z  when it detects a forward voltage across the associated thyristor valve 6u-6z, and a logic signal of &#34;1&#34; or a reverse voltage detection signal at the terminal RV u  -RV z  when it detects a reverse voltage across the associated thyristor valve 6u-6z. 
     OR gates 11u-11z are provided in association with the voltage detection circuits 10u-10z, respectively. Each of the OR gates receives the forward voltage detection signal and the reverse voltage detection signal from the output terminals of the associated voltage detection circuit. Each of the OR gates 11u-11z thus serves to detect non-conduction of the corresponding valve when either the forward voltage detection signal or the reverse voltage detection signal is being produced. 
     An AND gate 12 is provided to receive signals from the OR gates 11u-11w. Another AND gate 13 is provided to receive signals from the OR gates 11x-11z. The output of the AND gate 12 is &#34;1&#34; if a voltage (forward or reverse) is present across each of the valves 6u-6w of the lower potential side. The output of the AND gate 13 is &#34;1&#34; if a voltage is present across each of the valves 6x-6z of the higher potential side. 
     A further AND gate 14 is provided to receive output signals from the AND gates 12 and 13. The output signal of the AND gate 14 indicates absence of a current flowing through the converting device and is used as a condition for effecting termination of the inverter operation of the converting device, i.e., for blocking the gate pulses. 
     In the illustrated embodiment, the output signal of the AND gate 14 is inputted, together with a termination command TER to an AND gate 15. The output signal of the AND gate 15 is used to set an S-R type flip-flop 16, which is adapted to be reset by a start command STA. 
     The output signal of the flip-flop 16 at the output terminal Q is &#34;1&#34; when the flip-flop 16 is set, and is inverted by an inverter 17. The output signal of the inverter 17 is fed to first terminals of AND gates 18u-18z provided to receive, at second terminals, gate signals GS u  -GS z  provided by a converting device control circuit 9 for the respective phases or valves. The output signals of the AND gates 18u-18z are amplified by respective pulse amplifiers 19u-19z and are then applied as gate pulses GP u  -GP z  to the gates of the respective valves. 
     When the flip-flop 16 is set, the output signal of the inverter 17 becomes &#34;0&#34;, so that gate signals are blocked by the AND gates 18u-18z and no gate pulses (GP u  -GP z ) are produced. 
     Now the operation of the above-described circuitry is described. 
     Generally, when the DC current i d  of the converting device 4 is intermittent as shown in FIG. 6(a), the voltage V A-K  across the anode and the cathode of a valve (any one of 6u-6z) is as shown in FIG. 6(b). The various valves 6u-6z have similar voltage waveforms V uA-K  -V zA-K , but of different phases, as shown in FIGS. 7(a)-(f). FIG. 7 also shows, at (g)-(r), logic signals outputted by the voltage detection circuits 10u-10z. 
     FIG. 8 shows signals at various parts of the circuitry of FIG. 5, before and after the production of termination command TER. 
     In FIG. 8, it is assumed that a termination command TER is issued at a time point t 3 , when the W-phase valve 6w has just been fired and the W-phase valve 6w and the X-phase valve 6x are conducting, while the remaining valves are non-conducting. 
     At a time point t 4 , the DC current becomes zero, and a reverse voltage is applied on the W-phase valve 6w and the X-phase valve 6x, so that the signals at the terminals RV w  and RV x  become &#34;1&#34;. 
     Before the time point t 4 , the U-phase valve 6u is not conducting and a forward voltage is applied on it, so that the signal on the terminal FV u  is at &#34;1&#34;, and the signal at the terminal FV v  for the V-phase valve 6v is also at &#34;1&#34;. As a result, all the inputs to the AND gate 12 become &#34;1&#34; at the time point t 4 , meaning that all the valves 6u-6w of the lower potential side are non-conducting, and the output of the AND gate 12 becomes &#34;1&#34; indicating such a fact. Similarly, the signals at the terminals RV x , FV y  and FV z  are at &#34;1&#34;, and the output of the AND gate 13 becomes &#34;1&#34;. This in turn causes the output of the AND gate 14 to become &#34;1&#34;, indicating that all the valves are non-conducting. Since, as assumed, the termination command is now being applied, the output of the AND gate 15 also becomes &#34;1&#34;, and the flip-flop 16 is thereby set. 
     When flip-flop 16 is set, a logic signal &#34;0&#34; is applied to the first inputs of the AND gates 18u-18z. The gate pulses, which would otherwise be continued to be produced thereafter, e.g., at t 5  for the Y-phase valve, are blocked. 
     In summary, in the embodiment described above, non-conduction of each of the valves is ascertained by detection of the presence of either forward or reverse voltage across the valve, and when all the valves are found to be non-conducting, absence of a current flowing through the converting device is detected or recognized, and gate pulses are blocked. Thus, it is ensured that the valves are kept non-conducting thereafter. Accordingly, the danger of an excessive current flowing through the converting device is minimized. 
     In the embodiment described, detection is made to ascertain the presence of a forward or reverse voltage on all the valves in the lower potential side as well as all the valves in the higher potential side. But, the current must flow through at least one of the valves in the lower potential side and through at least one of the valves in the higher potential side. So that it is, in practice, sufficient to ascertain the presence of a voltage on all the valves of either the lower potential side or of the higher potential side. The AND gate 14 may therefore be replaced by an OR gate, or either the voltage detection circuits 10u-10w for the lower potential side or the voltage detection circuits 10x-10z for the higher potential side may be omitted. 
     As a modification, an ON-delay timer 20 may be inserted, as shown in FIG. 9, between the output terminal RV of the voltage detection circuit 10 (as representing 10u-10z) and the OR gate 11 (as representing 11u-11z). This is to make sure that the reverse voltage is applied on the thyristor for longer than the turn-off time of the thyristor, and the turn-off of the thyristor is completed, before the signal is actually applied to the OR gate 11 and the gate pulses are actually blocked. Such an ON delay timer may alternatively be inserted at the output of the OR gates 11u-11z or at the output of the AND gates 12, 13. 
     FIG. 10 shows a different arrangement for detecting the non-conduction of a valve. As shown, an S-R type flip-flop 22 is used for each valve, and the signal at the terminal RV is applied to a set input terminal S of the flip-flop 22. Applied to the reset input terminal R is a gate signal GS to be supplied to the associated valve. The OR gate 11 is connected to receive the signal at the terminal RV and the signal at the terminal Q of the flip-flop 22 which is &#34;1&#34; when the flip-flop 22 is set. 
     The signals at various parts of the circuitry of FIG. 10 as connected to most of the circuitry of FIGS. 4 and 5 are shown in FIG. 11(a)-(e). As shown, when a gate pulse is applied, the forward voltage disappears (or is kept absent). During such time that the forward voltage is not present, it is sufficient to detect presence of a reverse voltage to ascertain non-conduction of the valve. The embodiment of FIG. 10 is based on this concept, and utilizes a flip-flop 22 which is reset by the gate signal. The ouput of the OR gate 11 therefore indicates, when it is &#34;1&#34;, that the associated valve is non-conducting. 
     For simplicity of illustration, the voltage detection circuit has been shown to detect a voltage across the entire valve. But each valve may consist of a number of thyristors connected in series with each other. In such a case, a voltage across one or some of thyristors may be detected and used for the detection of the presence of the voltage across the valve. 
     In the illustrated embodiments, the converting device comprises only one bridge connection of thyristor valves. But it may alternatively comprise a plurality of bridge connections connected in series with each other.