Abstract:
A multi-level power converter is disclosed. The converter includes a DC power source having N (N is an Integer more than two)-level voltage potentials, and a plurality of switching legs connected in parallel to the DC power source. The switching leg has switching elements in series connection, and clamp diodes connected between intermediate DC potential terminals to middle points of the switching legs. The converter is provided with a control means for supplying gate pulses to the switching elements, and a supervising means for supervising every switching element to output a failure signal. The converter further includes a logic operating means for detecting an over-current failure of main circuit elements to output a device fault signal using the failure signals An operation of the logic operating means at least includes AND condition of at least two failure signals to output the device fault signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-002375, filed on Jan. 10, 2006, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a multi-level power converter which has an improved detection function of an over-current failure.  
         [0004]     2. Description of the Background  
         [0005]     A two-level inverter has been used as a power converter to change a DC power into an AC power. Recently, needs of large capacity power converter with high voltage are expanded. So-called multi-level inverter equipment having three or more voltage potentials become to be used for example, for a motor drive application.  
         [0006]     Three-level inverter equipment is one which outputs three DC voltage potentials. On the other hand, the conventional two-level inverter equipment outputs two DC voltage potentials. In the two-level inverter, switching legs in each of which two switching elements are in-series connected, are connected in parallel to the DC power supply. An AC output is obtained from central points of the switching legs. In three-level inverter, four switching elements connected in-series constituting switching legs are connected to a DC power supply with positive, zero, and negative voltage potentials. An AC output is obtained from central points of the switching legs. The both ends of the switching legs of the three-level inverter are connected to positive and negative terminals of DC power supply, and negative terminal of positive end of the switching element and positive terminal of negative end of the switching element are clamped in zero voltage potential through a clamp diode, respectively. A flywheel diode is anti-parallel connected to every switching element constituting a switching leg.  
         [0007]     By making a parallel connection of two switching legs, a single-phase AC output can be obtained. A three phase AC output can be obtained by making a parallel connection of three switching legs at either of two-level or three-level inverter.  
         [0008]     In a conventional two-level inverter equipment, when any one of the voltages between positive and negative terminals of switching elements exceeds a threshold voltage level during a period of its ON-state, a method of performing reduction of gate voltage or blocking gate signal has been used for the purpose of over-current protection of a switching element.  
         [0009]     That is, when an on-pulse is given to a gate terminal of a switching element in the state of normal operation, the voltage between positive and negative terminals of the switching element decreases to saturation voltage with some delay time. Since there is an internal resistance of the switching element during a period of its ON-state, the voltage between positive and negative terminals of the switching element does not decrease when an over-current flows by a short circuit failure. Therefore, an over-current protection of the equipment can be attained by supervising the voltage of positive and negative terminals of the switching element after the predetermined delay time when an on-pulse is given to the switching element.  
         [0010]     A proposal, which applied above mentioned viewpoint to three-level inverter equipment, is made in Japanese Patent Publication (Kokai) No. 2000-354383.  
         [0011]     In the equipment shown in the above patent publication, a simplified circuit constitution is obtained because any over-current detectors is not used. But the delay of time is needed to accomplish an over-current protection of the equipment as mentioned above. Further, it is difficult to adjust the delay time. Because a decreasing speed of the voltage between positive and negative terminals of the switching element might be changed or affected by a transient characteristic of the switching element and a load characteristic. Furthermore, in a free-back mode at a middle part of the switching leg, it is difficult to detect an abnormally high voltage between positive and negative terminals of a switching element during a period of its ON-state, because positive current does not flows through the switching element at that time.  
       SUMMARY OF THE INVENTION  
       [0012]     An advantage of an aspect of the present invention is to provide a multi-level power converter that can detect an over-current failure caused by main circuit elements more rapidly and with high reliability.  
         [0013]     According to an aspect of the invention, a multi-level power converter is provided, which includes a DC power source having N terminals with N-level voltage potentials, wherein N is an Integer more than two; a plurality of switching legs connected in parallel to the DC power source having, 2(N- 1 ) switching elements connected in series, and 2(N- 2 ) clamp diodes connected between (N- 2 ) intermediate DC potential terminals and middle points of the switching legs respectively, in order to clamp the potential of the middle points of the switching legs; a control means for supplying gate pulses to the switching elements so that a N-level phase voltage may be obtained from a central point of each of the switching legs; a supervising means for supervising the switching elements to output a failure signal when a voltage between positive and negative terminals of each of the switching elements exceeds a predetermined threshold voltage level during a period of its ON-state; and a logic operating means for detecting an over-current failure of main circuit elements to output a device fault signal using the failure signals from the supervising means; wherein an operation of the logic operating means at least includes AND condition of at least two failure signals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  shows a circuit diagram relating a first embodiment of a multi-level power converter of the present invention.  
         [0015]      FIG. 2  shows an internal circuit diagram of a voltage detector applied to the first embodiment of a multi-level power converter according to the present invention.  
         [0016]      FIG. 3  shows a circuit diagram of a logic operation circuit applied to the first embodiment of a multi-level power converter according to the present invention.  
         [0017]     FIGS.  4  to  7  show short circuit modal analysis diagrams relating the first embodiment of a multi-level power converter of the present invention.  
         [0018]      FIG. 8  shows an internal circuit diagram of a voltage detector applied to a second embodiment of a multi-level power converter according to the present invention.  
         [0019]      FIG. 9  shows a circuit diagram of a logic operation circuit applied to the second embodiment of a multi-level power converter according to the present invention.  
         [0020]     FIGS.  10  to  11  show short circuit modal analysis diagrams relating the second embodiment of a multi-level power converter of the present invention.  
         [0021]      FIG. 12  shows an internal circuit diagram of a voltage detector applied to a third embodiment of a multi-level power converter according to the present invention.  
         [0022]      FIG. 13  shows a circuit diagram of a logic operation circuit applied to the third embodiment of a multi-level power converter according to the present invention.  
         [0023]      FIG. 14  shows a circuit diagram relating a fourth embodiment of a multi-level power converter of the present invention.  
         [0024]     FIGS.  15  to  24  show short circuit modal analysis diagrams relating the fourth embodiment of a multi-level power converter of the present invention.  
         [0025]      FIG. 25  shows a circuit diagram of a logic operation circuit applied to the fourth embodiment of a multi-level power converter according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Embodiments of the present invention will be described below in detail with reference to the drawings.  
       FIRST EMBODIMENT  
       [0027]     With reference to FIGS.  1  to  7 , a multi-level power converter of the first embodiment according to the invention is explained.  FIG. 1  shows a circuit diagram of the power converter relating to the first embodiment of the present invention.  
         [0028]     A DC voltage obtained from a DC power supply  1  is supplied to a series circuit consists of DC capacitors  2   p  and  2   n  having same capacitance. The DC power supply  1  has the voltage of 2E. Therefore, when positive side voltage potential of DC capacitor  2   p  is +E, negative side voltage potential of DC capacitor  2   n  is −E, and a voltage potential of central terminal of the DC capacitors  2   p  and  2   n  is 0 potential.  
         [0029]     These three voltage potentials +E, −E and 0 are supplied to the switching legs  3   u ,  3   v , and  3   w . In  FIG. 1 , the internal composition of switching leg  3   u  is illustrated. Since the internal composition of other switching legs  3   v  and  3   w  are fundamentally same composed as that of switching leg  3   u , those illustrations and explanations are omitted.  
         [0030]     Switching leg  3   u  is composed of four switching elements Q 1 , Q 2 , Q 3  and Q 4  connected in series by which diodes D 1 , D 2 , D 3 , and D 4  are anti-parallel connected respectively, a clamp diode DP connected between a central point of the switching elements Q 1  and Q 2 , and a central terminal of the capacitors  2   p  and  2   n , and a clamp diode DN connected between a central point of the switching elements Q 3  and Q 4 , and a central terminal of the capacitors  2   p  and  2   n . A positive terminal of the switching element Q 1  and a negative terminal of the switching element Q 4  are connected to +E potential terminal and −E potential terminal of the DC power supply  1 , respectively.  
         [0031]     Since the switching element Q 1  and the switching element Q 4  form ends of switching leg  3   u , they are called the switching elements of both ends. On the other hand, since the switching element Q 2  and the switching element Q 3  form the middle part of switching leg  3   u , they are called the switching elements of a middle part. And the switching elements, flywheel diodes, and clamp diodes constituting the switching leg are called main circuit elements.  
         [0032]     Gate pulses are given to gate terminals of the switching elements Q 1 , Q 2 , Q 3 , and Q 4  constituting switching leg  3   u  from a control circuit  6 . And the switching elements Q 1 , Q 2 , Q 3 , and Q 4  output desired U-phase output voltage at a central point of the switching elements Q 2  and Q 3 , for example by PWM control. This U-phase output voltage is supplied to a primary winding of an AC motor  4 . Similarly, V-phase output voltage and W-phase output voltage are supplied to the primary winding of the AC motor  4  from the switching legs  3   v  and  3   w , respectively.  
         [0033]     The voltages Vce between positive and negative terminals of the switching elements Q 1 , Q 2 , Q 3 , and Q 4  are given to the voltage detectors  51 ,  52 ,  53 , and  54 , respectively. Gate pulses for the switching elements are also supplied to these voltage detectors  51 ,  52 ,  53 , and  54  from the control circuit  6 . Outputs from the voltage detectors  51 ,  52 ,  53 , and  54  are given to a logic operation circuit  7 . In the logic operation circuit  7 , over-current failure of the switching elements and the diodes in switching leg  3   u  is detected by performing an adequate logic operation mentioned later. A voltage Vce of each switching element constituting switching legs  3   v  and  3   w  also is supervised with another voltage detector, but illustration is omitted.  
         [0034]      FIG. 2  shows an internal circuit diagram of the voltage detector  51 . A positive potential side of the voltage Vce between positive and negative terminals of the switching element Q 1  is connected to one input terminal of a comparator  512  through the diode  511  for reverse current protection. A negative potential side of voltage Vce is connected to another input terminal of the comparator  512  through voltage bias  513  for setting predetermined threshold voltage level. When voltage Vce becomes high than the threshold voltage level, the comparator  512  outputs  1 . The output of the comparator  512  is given to one input terminal of AND circuit  514 . The gate pulse for switching element Q 1  obtained from the control circuit  6  is given to another input terminal of AND circuit  514 . Thus, a “high” signal of the voltage Vce between positive and negative terminals of the switching element Q 1  during a period of its ON-state is detected.  
         [0035]     In this case of the first embodiment, a composition of the voltage detector  51  should be applied to all switching elements in the switching legs. Therefore, the voltage detector  52 ,  53 , and  54  also serve as the same composition as the voltage detector  51 .  
         [0036]      FIG. 3  shows an internal circuit diagram of the logic operation circuit  7  shown in  FIG. 1 . Output signals from the voltage detector  51 ,  52 ,  53 , and  54  are given directly to input terminals of AND circuits  71 ,  72 ,  73 , and  74 , respectively. Output signals from OR circuits  81 ,  82 ,  83 , and  84  are given to other input terminals of AND circuits  71 ,  72 ,  73 , and  74 , respectively. Three signals other than the output signal from the voltage detector  51  i.e., output signals from the voltage detector  52 ,  53 , and  54 , are given to input terminals of OR circuit  81 . Three signals other than the output signal from the voltage detector  52 ,  53 , and  54  are similarly given to input terminals of OR circuit  82 ,  83 , and  84 , respectively. And the output signals from AND circuits  71 ,  72 ,  73 , and  74  is given to OR circuit  75 . Thus, when any of at least two signals are set to 1 among the output signals from the voltage detector  51 ,  52 ,  53 , and  54 , the output from OR circuit  75 , i.e., an output from the operation circuit  7 , is set to 1 as the Device Fault Signal.  
         [0037]     Hereafter, with reference to short circuit modal analysis diagrams of FIGS.  4  to  7 , operation principle of the first embodiment will be explained.  
         [0038]      FIG. 4  shows a current route of a short-circuit current when the flywheel diode D 1  in a switching leg changes into a short circuit state. Since a parallel connection of the flywheel diode D 1  and the switching element Q 1  is made, the following arguments are applied also when the switching element Q 1  changes into a short circuit state.  
         [0039]     As shown in  FIG. 4 , when the switching elements Q 2  and Q 3  are in there ON-states simultaneously, a short circuit current flows by a route of +E potential terminal to D 1  (or Q 1 ), Q 2 , Q 3 , DN, and 0 potential terminal. Therefore, when the voltages Vce between positive and negative terminals of both switching element Q 1  and Q 2  exceed a threshold voltage level during a period of there ON-states, it can be judged with the flywheel diode D 1  or the switching element Q 1  having changed into the short circuit state.  
         [0040]     Here, the reason why a time delay element which was conventionally necessary becomes unnecessary in the circuit of the voltage detector  51  shown in  FIG. 2  is explained. As mentioned above, to detect an over-current failure of the flywheel diode D 1  or the switching element Q 1 , both voltages Vce of switching elements Q 2  and Q 3  exceeding a threshold voltage level during a period of there ON -state are needed. Since the switching elements Q 2  and Q 3  do not change from an OFF-state to an ON-state simultaneously, even if any one of the switching elements Q 2  and Q 3  is at the beginning of its ON-state and the voltage Vce exceeds the threshold level, the voltage Vce of another switching element will not exceed a threshold voltage level. Therefore, a time delay element may be omitted and the over-current failure can be detected quickly.  
         [0041]      FIG. 5  shows the current route of a short-circuit current when the flywheel diode D 2  or the switching element Q 2  in a switching leg changes into a short circuit state. As shown in  FIG. 5 , when the switching elements Q 3  and Q 4  are in there ON-states simultaneously, a short circuit current flows by a route of 0 potential terminal. to DP, D 2  (or Q 2 ), Q 3 , Q 4 , and −E potential terminal. Therefore, when the voltages Vce between positive and negative terminals of both switching element Q 3  and Q 4  exceed a threshold voltage level during a period of there ON-states, it can be judged with the flywheel diode D 2  or the switching element Q 2  having changed, into the short circuit state.  
         [0042]      FIG. 6  shows the current route of a short-circuit current when the flywheel diode D 3  or the switching element Q 3  in a switching leg changes into a short circuit state. As shown in  FIG. 6 , when the switching elements Q 1  and Q 2  are in there ON-states simultaneously, a short circuit current flows by a route of +E potential terminal to Q 1 , Q 2 , D 3 (or Q 3 ), DN, and 0 potential terminal. Therefore, when the voltages Vce between positive and negative terminals of both switching element Q 1  and Q 2  exceed a threshold voltage level during a period of there ON-states, it can be judged with the flywheel diode D 3  or the switching element Q 3  having changed into the short circuit state.  
         [0043]      FIG. 7  shows the current route of a short-circuit current when the flywheel diode D 4  or the switching element Q 4  in a switching leg changes into a short circuit state. As shown in  FIG. 7 , when the switching elements Q 2  and Q 3  are in there ON-states simultaneously, a short circuit current flows by a route of 0 potential terminal to DP, Q 2 , Q 3 , D 4  (or Q 4 ), and −E potential terminal. Therefore, when the voltages Vce between positive and negative terminals of both switching element Q 2  and Q 3  exceed a threshold voltage level during a period of there ON-states, it can be judged with the flywheel diode D 4  or the switching element Q 4  having changed into the short circuit state.  
         [0044]     As explained above, when any one of the flywheel diodes or the switching elements in a switching leg change into a short circuit state, any two of the voltages Vce between positive and negative terminals of switching elements surely exceed a threshold voltage level during a period of there ON-states. Therefore, if it uses combining the voltage detector shown in  FIG. 2  and the logic operation circuit shown in  FIG. 3 , detection of the over-current failure of the flywheel diode or the switching element in a switching leg could be attained.  
         [0045]     The logic operation circuit  7  of  FIG. 3  was constituted so that the over-current failure could be detected when any two or more voltages Vce in the switching elements exceed a threshold voltage level during a period of there ON-states. If it constitutes so that the over-current failure may be detected when two voltages Vce of switching elements adjoined in a switching leg exceed a threshold voltage level during a period of there ON-states, as the failure mode analysis was performed in FIGS.  4  to  7 , it is clear that a more reliable detection of the over-current failure could be attained.  
       SECOND EMBODIMENT  
       [0046]     With reference to FIGS.  8  to  11 , a multi-level power converter of a second embodiment according to the invention is explained.  FIG. 8  shows an internal circuit diagram of a voltage detector  51 A applied to the second embodiment of the multi-level power converter according to the present invention.  
         [0047]     To the same parts of the second embodiment as those shown in the circuit diagram of the voltage detector  51  relating to the first embodiment shown in  FIG. 2 , the same numerals are assigned and the explanation thereof will be omitted. In the second embodiment, a time delay circuit  515  is inserted at the output side of the comparator  512  in the voltage detector  51 A and the output of the time delay circuit  515  might be given to AND circuit  514 .  
         [0048]     As mentioned above, the time delay circuit  515  protects an incorrect detection during a transient state when the switching element Q 1  turns on. Since voltage Vce is over the threshold level irrespective of an existence of the over-current failure in the transient state, it causes error detection in the case of second embodiment.  
         [0049]     In the case of the second embodiment, a composition of the voltage detector  51 A should be applied to the voltage detector  51  and  54  for both end switching elements of the switching leg in  FIG. 1 , and a composition of the voltage detector without delay shown in  FIG. 2  should be applied to the switching elements of middle part.  
         [0050]      FIG. 9  shows an internal circuit diagram of a logic operation circuit  7 A used for a power converter concerning the second embodiment of this invention. As shown in  FIG. 9 , output signals from the voltage detector  51  and  54  are given to an OR circuit  75 A directly. Output signals from the voltage detector  52  and  53  are given to an AND circuit  76 , and an output signal from the AND circuit  76  is given to OR circuit  75 A. It is constituted so that the output from the OR circuit  75 A, Device Fault Signal may turn into an output from the logic operation circuit  7 A.  
         [0051]     Hereafter, with reference to the short circuit modal analysis figure of  FIGS. 10 and 11 , operation principle of the second embodiment will be explained.  
         [0052]      FIG. 10  shows the current route of a short circuit current when the clamp diode DP in a switching leg changes into a short circuit state. As shown in  FIG. 10 , when the switching element Q 1  turns on, a short circuit current flows by the route of +E potential terminal to Q 1 , DP, and 0 potential terminal. Therefore, when the voltage Vce of the switching element Q 1  exceeds a threshold voltage level during a period of its ON-state, it can be judged with the clamp diode DP having changed into the short circuit state.  
         [0053]      FIG. 11  shows the current route of a short circuit current when the clamp diode DN in a switching leg changes into a short circuit state. As shown in  FIG. 11 , when the switching element Q 4  turns on, a short circuit current flows by the route of 0 potential terminal to DN, Q 4 , and −E potential terminal. Therefore, when the voltage Vce of the switching element Q 4  exceeds a threshold voltage level during a period of its ON-state, it can be judged with the clamp diode DN having changed into the short circuit state.  
         [0054]     According to the short circuit modal analysis shown by  FIGS. 10 and 11 , what is necessary to detect the over-current failure of a clamp diode, is just to detect that any one of the voltage Vce of both ends of switching elements exceeding the threshold voltage level during a period of there ON-states.  
         [0055]     Therefore, when any one of all main circuit elements containing clamp diodes in the switching leg makes over-current failure, it turns out that the voltage Vce of switching element Q 1  or Q 4  becomes high during a period of its ON-state, or both voltages Vce of switching elements Q 2  and Q 3  become high simultaneously during a period of there ON-states.  
         [0056]     Using the circuit  7 A shown in  FIG. 6 , it will become possible to detect an over current failure more quickly when main circuit element having changed into the short circuit state. When the voltage Vce of the switching element Q 1  becomes high at its ON-state, it might be occurred that the switching element Q 3  or the flywheel diode D 3 , or the clamp diode DP changed into the short circuit state. When the voltage Vce of the switching element Q 4  becomes high at its ON-state, it might be occurred that the switching element Q 2  or the flywheel diode D 2 , or the clamp diode DN changed into the short circuit state. And when the voltages Vce of the switching elements Q 2  and Q 3  become high at there ON-states simultaneously, it might be occurred that the switching element Q 1  or the flywheel diode D 1 , or the switching element Q 4  or the flywheel diode D 4  changed into the short circuit state.  
       THIRD EMBODIMENT  
       [0057]     With reference to  FIGS. 12 and 13 , a multi-level power converter of a third embodiment according to the invention is explained.  FIG. 12  shows an internal circuit diagram of a voltage detector  51 B applied to the third embodiment of the multi-level power converter according to the present invention.  
         [0058]     To the same parts of the third embodiment as those shown in the circuit diagram of the voltage detector  51 A relating to the second embodiment shown in  FIG. 8 , the same numerals are assigned and the explanation thereof will be omitted. In the third embodiment, an AND circuit  514 A is added to the voltage detector  51 A relating to the second embodiment shown in  FIG. 8 . A direct output signal from the comparator  512  and the gate signal are inputted to the AND circuit  514 A.  
         [0059]     Therefore, a voltage detector  51 B outputs two signals with time delay and without time delay as the voltage Vce “high” signal of the switching element during a period of its ON-state. In the case of the third embodiment, a composition of the voltage detector  51 B which outputs these two signals should be applied to the voltage detector  51  and  54  for both end switching elements of the switching leg shown in  FIG. 1 .  
         [0060]      FIG. 13  shows an internal circuit diagram of a logic operation circuit  7 B used for the power converter according to the third embodiment of this invention. To the same parts of the third embodiment as those shown in the circuit diagram of the logic operation circuit  7  relating to the first embodiment shown in  FIG. 3 , the same numerals are assigned and the explanation thereof will be omitted. In the third embodiment, output signals with time delay of the voltage detector  51 B and  54  are added to the input of OR circuit  75 B.  
         [0061]     Although the logic operation circuit  7 A shown in  FIG. 9  of the second embodiment is easily composed, since the voltage detector  51 A with time delay is used for the both ends of a switching leg, time delay by the delay circuit  515  occurs when a switching element or a flywheel diode of the middle part of a switching leg made an over-current failure. If it is used as a failure detection combining the voltage detector  51 B and the logic operation circuit  7 B, it will become possible to detect an over-current failure without time delay even when a switching element or a flywheel diode of the middle part of switching leg made a short circuit failure.  
       FOURTH EMBODIMENT  
       [0062]     With reference to FIGS.  14  to  25 , a multi-level power converter of a fourth embodiment according to the invention is explained.  FIG. 14  shows a circuit diagram of the power converter relating to the fourth embodiment of the present invention.  
         [0063]     To the same parts of fourth embodiment as those shown in the circuit diagram of the power converter relating to the first embodiment shown in  FIG. 1 , the same numerals are assigned and the explanation thereof will be omitted. As shown in  FIG. 14 , DC power supply  1  is divided into four-level DC voltage potential +E, +E/3, −E/3, and −E by capacitors  2 P,  2 C, and  2 N. DC voltages which have potentials of four obtained levels are supplied to the switching legs  3   u   1 ,  3   v   1 , and  3   w   1 . In  FIG. 14 , an internal composition of switching leg  3   u   1  is illustrated. Since internal compositions of other switching legs  3   v   1  and  3   w   1  are fundamentally the same as that of switching leg  3   u , those illustrations and explanations are omitted.  
         [0064]     Switching leg  3   u   1  is composed of six switching elements Q 1 , Q 2 , Q 3 , Q 4 , Q 5  and Q 6  connected in-series by which diodes D 1 , D 2 , D 3 , D 4 , D 5 ,and D 6  are anti-parallel connected respectively, a clamp diode DP 1  connected between a central point of the switching elements Q 1  and Q 2 , and a central terminal of the capacitors  2   p  and  2   c , a clamp diode DN 1  connected to between a central point of the switching elements Q 4  and Q 5 , and the central terminal of the capacitors  2   p  and  2   c , a clamp diode DP 2  connected between a central point of the switching elements Q 2  and Q 3 , and a central terminal of the capacitors  2   c  and  2   n , and a clamp diode DN 2  connected between a central point of the switching elements Q 5  and Q 6 , and the central terminal of the capacitors  2   c  and  2   n.    
         [0065]     A positive terminal of the switching element Q 1  and a negative terminal of the switching element Q 6  are connected to +E potential terminal and −E potential terminal of the DC power supply  1 , respectively. The voltages Vce between positive and negative terminals of the switching elements Q 1 , Q 2 , Q 3 , Q 4 , Q 5  and Q 6  are given to the voltage detectors  51 ,  52 ,  53 ,  54 ,  55  and  56 , respectively. Gate pulses for the switching elements are also supplied to these voltage detectors  51 ,  52 ,  53 ,  54 ,  55  and  56  from the control circuit  6 . Outputs from the voltage detectors  51 ,  52 ,  53 ,  54 ,  55  and  56  are given to a logic operation circuit  7 C.  
         [0066]     The circuit composition figure of  FIG. 14  changed three-level inverter equipment shown in  FIG. 1  into four-level inverter equipment. In  FIG. 10 , the gate signal to each switching element given from the control circuit  6  is illustrated as a single track.  
         [0067]     Hereafter, with reference to  FIG. 14  and short circuit modal analysis figures of FIGS.  15  to  24 , operation principle of the fourth embodiment will be explained.  
         [0068]      FIG. 15  shows the current route of a short-circuit current when the flywheel diode D 1  in a switching leg changes into a short circuit state. Since a parallel connection of the flywheel diode D 1  and the switching element Q 1  is made, the following arguments are applied also when the switching element Q 1  changes into a short circuit state.  
         [0069]     As shown in  FIG. 15 , when the switching elements Q 2 , Q 3 , and Q 4  are in there ON-states simultaneously, a short circuit current flows by the route of +E potential terminal to D 1  (or Q 1 ), Q 2 , Q 3 , Q 4 , DN 1 , and +E/3 potential terminal. Therefore, when the voltages Vce between positive and negative terminals of the switching elements Q 1 , Q 2  and Q 3  exceed a threshold voltage level during a period of there ON-states simultaneously, it can be judged with the flywheel diode D 1  or the switching element Q 1  having changed into the short circuit state.  
         [0070]      FIG. 16  shows the current route of a short-circuit current when the flywheel diode D 2  or the switching element Q 2  in a switching leg changes into a short circuit state. As shown in  FIG. 16 , when the switching elements Q 3 , Q 4 , and Q 5  are in there ON-states simultaneously, a short circuit current flows by the route of +E/3 potential terminal to DP 1 , D 1  (or Q 1 ), Q 3 , Q 4 , Q 5 , DN 2 , and −E/3 potential terminal. Therefore, when the voltages Vce between positive and negative terminals of the switching elements Q 3 , Q 4  and Q 5  exceed a threshold voltage level during a period of there ON-states simultaneously, it can be judged with the flywheel diode D 2  or the switching element Q 2  having changed into the short circuit state.  
         [0071]      FIG. 17  shows the current route of a short-circuit current when the flywheel diode D 3  or the switching element Q 3  in a switching leg changes into a short circuit state. As shown in  FIG. 17 , when the switching elements Q 4 , Q 5 , and Q 6  are in there ON-states simultaneously, a short circuit current flows by the route of −E/3 potential terminal to DP 2 , D 3  (or Q 3 ), Q 4 , Q 5 , Q 6 , and −E potential terminal. Therefore, when the voltages Vce between positive and negative terminals of the switching elements Q 4 , Q 5  and Q 6  exceed a threshold voltage level during a period of there ON-states simultaneously, it can be judged with the flywheel diode D 3  or the switching element Q 3  having changed into the short circuit state.  
         [0072]      FIG. 18  shows the current route of a short-circuit current when the flywheel diode D 4  or the switching element Q 4  in a switching leg changes into a short circuit state. As shown in  FIG. 18 , when the switching elements Q 1 , Q 2 , and Q 3  are in there ON-states simultaneously, a short circuit current flows by the route of +E potential terminal to Q 1 , Q 2 , Q 3 , D 4  (or Q 4 ), DN 1 , and +E/3 potential terminal. Therefore, when the voltages Vce between positive and negative terminals of the switching elements Q 1 , Q 2  and Q 3  exceed a threshold voltage level during a period of there ON-states simultaneously, it can be judged with the flywheel diode D 4  or the switching element Q 4  having changed into the short circuit state.  
         [0073]      FIG. 19  shows the current route of a short-circuit current when the flywheel diode D 5  or the switching element Q 5  in a switching leg changes into a short circuit state. As shown in  FIG. 19 , when the switching elements Q 2 , Q 3 , and Q 4  are in there ON-states simultaneously, a short circuit current flows by the route of +E/3 potential terminal to DP 1 , Q 2 , Q 3 , Q 4 , D 5  (or Q 5 ), DN 2 , and −E/3 potential terminal. Therefore, when the voltages Vce between positive and negative terminals of the switching elements Q 2 , Q 3  and Q 4  exceed a threshold voltage level during a period of there ON-states simultaneously, it can be judged with the flywheel diode D 5  or the switching element Q 5  having changed into the short circuit state.  
         [0074]      FIG. 20  shows the current route of a short-circuit current when the flywheel diode D 6  or the switching element Q 6  in a switching leg changes into a short circuit state. As shown in  FIG. 20 , when the switching elements Q 3 , Q 4 , and Q 5  are in there ON-states simultaneously, a short circuit current flows by the route of −E/3 potential terminal to DP 2 , Q 3 , Q 4 , Q 5 , D 6  (or Q 6 ), and −E potential terminal. Therefore, when the voltages Vce between positive and negative terminals of the switching elements Q 3 , Q 4  and Q 5  exceed a threshold voltage level during a period of there ON-states simultaneously, it can be judged with the flywheel diode D 6  or the switching element Q 6  having changed into the short circuit state.  
         [0075]     According to the short circuit modal analysis shown by FIGS.  15  to  20 , to detect the over-current failure of arbitrary switching elements or flywheel diodes in a switching leg, what is necessary is just to detect that any three of voltages Vce of adjoining switching elements exceeding the threshold voltage level during a period of there ON-states simultaneously. Moreover, if it detects strictly in this way, the part which changed into the short circuit state may be specified. When this is extended to N-level inverter, wherein N is an Integer more than two, it turns out that what is necessary to detect the over-current failure of arbitrary switching element or flywheel diode in a switching leg is just to detect that the voltage Vce of (N- 1 ) adjoining switching elements exceeding the threshold voltage level during a period of there ON-states simultaneously.  
         [0076]     Although the above is the case of strict over-current failure detection, as the case of the first embodiment described, the detection could be simplified by supervising the voltage Vce of arbitrary two switching elements exceeding the threshold voltage level during a period of there ON-states simultaneously.  
         [0077]      FIG. 21  shows the current route of a short circuit current when the clamp diode DP 1  in a switching leg changes into a short circuit state. As shown in  FIG. 21 , when the switching element Q 1  turns on, a short circuit current flows by the route of +E potential terminal to Q 1 , DP 1 , and +E/3 potential terminal. Therefore, when the voltage Vce of the switching element Q 1  exceeds a threshold voltage level during a period of its ON-state, it can be judged with the clamp diode DP 1  having changed into the short circuit state.  
         [0078]      FIG. 22  shows the current route of a short circuit current when the clamp diode DP 2  in a switching leg changes into a short circuit state. As shown in  FIG. 22 , when the switching elements Q 1  and Q 2  are in there ON-states simultaneously, a short circuit current flows by the route of +E potential terminal to Q 1 , Q 2 , DP 2 , and −E/3 potential terminal. Therefore, when the voltages Vce of the switching elements Q 1  and Q 2  exceed a threshold voltage level during a period of there ON-states simultaneously, it can be judged with the clamp diode DP 2  having changed into the short circuit state.  
         [0079]      FIG. 23  shows the current route of a short circuit current when the clamp diode DN 1  in a switching leg changes into a short circuit state. As shown in  FIG. 23 , when the switching elements Q 5  and Q 6  are in there ON-states simultaneously, a short circuit current flows by the route of +E/3 potential terminal to DN 1 , Q 5 , Q 6 , and −E potential terminal. Therefore, when the voltages Vce of the switching elements Q 5  and Q 6  exceed a threshold voltage level during a period of there ON-states simultaneously, it can be judged with the clamp diode DN 1  having changed into the short circuit state.  
         [0080]      FIG. 24  shows the current route of a short circuit current when the clamp diode DN 2  in a switching leg changes into a short circuit state. As shown in  FIG. 24 , when the switching element Q 6  turns on, a short circuit current flows by the route of −E/3 potential terminal to DN 2 , Q 6 , and −E potential terminal. Therefore, when the voltage Vce of the switching element Q 6  exceeds a threshold voltage level during a period of its ON-state, it can be judged with the clamp diode DN 2  having changed into the short circuit state.  
         [0081]     As mentioned above, it becomes possible like the case of three-level inverter in the second embodiment, what is necessary to detect the over-current failure of a clamp diode, is just to detect that any of the voltage Vce of both ends of switching element exceeding the threshold voltage level during a period of there ON-states.  
         [0082]     If the voltages Vce of adjoined switching elements of both ends are also supervised during a period of there ON-states, the part which changed into a short circuit state may be specified. In the case of N-level inverter, the clamp diode which has made a short circuit failure may be specified by supervising voltages Vce of (N- 3 ) adjoined switching elements of both ends.  
         [0083]      FIG. 25  shows one example of the internal circuit diagram of a logic operation circuit  7 C used for the power converter concerning the fourth embodiment of this invention. To the same parts of the fourth embodiment as those shown in the circuit diagram of the logic operation circuit  7  relating to the first embodiment shown in  FIG. 3  the same numerals are assigned and the explanation thereof will be omitted.  
         [0084]     In the forth embodiment, output signals from voltage detectors  52  to  54  are inputted to AND circuits  71  to  74  and OR circuits  81  to  84 , and output signals from voltage detectors  51  and  56  are directly inputted to an OR circuit  75 C.  
         [0085]     A composition of the voltage detector  51 A with time delay shown in  FIG. 8  should be applied to the voltage detectors  51  and  56  for the switching element of both ends of a switching leg, and a composition of the voltage detector  51  without time delay shown in  FIG. 2  should be applied to the voltage detectors  52  to  55  for the switching element of middle part of the switching leg.  
         [0086]     According to the logic operation circuit  7 C, same as the logic operation circuit  7 A in three-level inverter shown in  FIG. 9 , it is possible to detect an over-current failure caused by any short circuit trouble among all main circuit elements.  
         [0087]     If this is extended to a N-level inverter, detection of any over-current failure among main circuit elements will be attained by the detection of any of the voltages Vce of both ends switching element in a switching leg exceeding the threshold voltage level during a period of there ON-states, or any two or more voltages Vce of N middle part switching elements in a switching leg exceeding the threshold voltage level during a period of there ON-states.  
         [0088]     Moreover, if the case of the operation in three-level inverter is extended to N-level inverter, over-current failure detection will be obtained more quickly by supervising any one of the voltage Vce of both ends of switching elements in a switching leg exceeding the threshold voltage level during a period of its ON-state with time delay detection, or two or more voltages Vce among middle part of the switching elements in a switching leg exceeding the threshold voltage level during a period of there ON-states without time delay detection.  
         [0089]     Usually, when the over-current failure is detected, a gate pulse of the power converter is blocked, the breaker of the main circuit of the power converter may be switched off, and the power converter should be stopped running. If output signals from voltage detectors are memorized by certain memory means at this time, it might become possible to specify the part of main circuit element in the switching leg which might be changed into the short circuit