Abstract:
Aspects of the invention are directed to a protection control system of the invention is a protection control system for a multilevel power conversion circuit of a flying capacitor type, the power conversion circuit including six semiconductor switches of first through sixth semiconductor switches sequentially connected in series from a positive terminal to a negative terminal of a DC power supply, a first capacitor connected between the connection point of the second and third semiconductor switches and the connection point of the fourth and fifth semiconductor switches and a second capacitor connected between the connection point of the first and second semiconductor switches and the connection point of the fifth and sixth semiconductor switches, wherein the protection control system of the invention turns ON the second semiconductor switch or maintains the second semiconductor switch in an ON state, in the case of short-circuit fault of the third semiconductor switch.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a protection control system for a multilevel power conversion circuit of a flying capacitor type for AC motor driving and other applications. 
     2. Description of the Related Art 
       FIG. 4  shows a general type of power conversion circuit for converting a DC power to an AC power. A DC power supply DP delivers a voltage Ed between the positive electric potential terminal P and a negative electric potential terminal N. The DC power supply can be generally composed of an AC power supply system using a rectifier and a capacitor with a large capacitance, though those components of the AC power supply system are not shown in the figure. 
     The power conversion circuit comprises semiconductor switches Q 1  through Q 6  each composed of an IGBT and a diode, gate driving circuits GD 1  through GD 6  for driving the IGBTs, and a controller CNT. An example of a load on this power conversion circuit is an AC motor ACM. The semiconductor switches Q 1  through Q 6  are ON/OFF controlled by the gate driving circuits GD 1  through GD 6  according to the ON/OFF command of the controller CNT. The power conversion circuit of this construction can deliver an output voltage of the electric potential P or N of the DC power supply DP to the AC output terminal by the switching operation of the IGBTs. Thus, this converter is a two-level power conversion circuit. 
       FIG. 5  shows a four-level power conversion circuit of a flying capacitor type. This circuit comprises: six semiconductor switches, which are IGBTs, T 1  through T 6  connected in series between the positive electric potential terminal P and the negative electric potential terminal N of the DC power supply DP, a capacitor C 1  connected between the collector of the IGBT T 3  and the emitter of the IGBT T 4 , and a capacitor C 2  connected between the collector of the IGBT T 2  and the emitter of the IGBT T 5 . The capacitors C 1  and C 2  are called flying capacitors. The voltages across the capacitors C 1  and C 2  are controlled to be, in average, ⅓ Ed and ⅔ Ed, respectively, where Ed is the output voltage of the DC power supply DP. The connection point between the emitter of the IGBT T 3  and the collector of the IGBT T 4  is an AC output terminal in this circuit construction. This circuit is a four-level power conversion circuit that delivers the four levels of electric potentials given in the following. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Mode (1) 
                 Ed when T1, T2, and T3 are ON 
               
               
                   
                 Mode (2) 
                 Ed - 1/3 Ed when T1, T2, and T4 are ON 
               
               
                   
                 Mode (3) 
                 Ed - 2/3 Ed when T1, T5, and T4 are ON 
               
               
                   
                 Mode (4) 
                 0 + 2/3 Ed when T6, T2, and T3 are ON 
               
               
                   
                 Mode (5) 
                 0 + 1/3 Ed when T6, T5, and T3 are ON 
               
               
                   
                 Mode (6) 
                 0 when T6, T5, and T4 are ON 
               
               
                   
                   
               
             
          
         
       
     
     The modes (2) and (4) give an equal voltage, and the modes (3) and (5) give an equal voltage as well. But the capacitors C 1  and C 2  undergo either charging or discharging action. Thus, selectively controlling the output time allows the voltages across the capacitors C 1  and C 2  maintained, in average, at ⅓ Ed and ⅔ Ed, respectively. 
     Each IGBT is subjected to a voltage of Ed/3 in a steady state. Considering a surge voltage emerging in a transient phenomenon at a switching action, a substantially required withstand voltage for the IGBTs should be about ⅔ Ed, which is twice the steady state value. 
       FIG. 6  shows a five-level power conversion circuit of a flying capacitor type, which is an advanced form of the circuit of  FIG. 5 . Controlling the voltage across the capacitor C 1  at Ed/4, the voltage across the capacitor C 2  at Ed/2, and the voltage across the capacitor C 3  at 3Ed/4, the power conversion circuit gives five levels of electric potential of Ed, 3Ed/4, Ed/2, Ed/4, and 0 (zero) at the AC output terminal. 
       FIG. 7  is a six-level power conversion circuit. Controlling the voltage across the capacitor C 1  at Ed/5, the voltage across the capacitor C 2  at 2Ed/5, the voltage across the capacitor C 3  at 3Ed/5, and the voltage across the capacitor C 4  at 4Ed/5, the power conversion circuit gives six levels of electric potential of Ed, 4Ed/5, 3Ed/5, 2Ed/5, Ed/5, and 0 (zero) at the AC output terminal. 
       FIG. 8  shows a power conversion circuit composed by blending a neutral point clamped type (an NPC type) conversion circuit with a flying capacitor type conversion circuit. This power conversion circuit comprises, in addition to the four-level power conversion circuit of a flying capacitor type shown in  FIG. 5 , a pair of series-connected IGBTs T 7  and T 8  in parallel to the capacitor C 2 , and a bidirectional switch composed of a pair of antiparallel-connected reverse blocking IGBTs T 9  and T 10 , the bidirectional switch being connected between the point of series-connection of the IGBTs T 7  and T 8  and the point of series connection of a DC power supply DP 1  and a DC power supply DP 2 . The power conversion circuit of  FIG. 8  is a seven-level power conversion circuit. The examples of power conversion circuits mentioned above are disclosed in a Japanese Translation of PCT International Application No. 2009-525717, and Technical Report of The Institute of Electrical Engineers of Japan, No. 1,093 (in Japanese), FIGS. 2.2 and 2.3 in particular. 
     A short-circuited state usually occurs when a semiconductor switch composing a power conversion circuit breaks down for some reason.  FIG. 9  shows short-circuit current running in a short-circuit fault in a two level inverter circuit. When a short-circuit fault occurs at the IGBT T 2  in the circuit of  FIG. 9  and then an ON command is given to the IGBT T 1 , a DC short-circuit current Ist flows in the path designated by the broken line in the figure. If this state continues for a certain period of time, the IGBT T 1  also breaks down leading to a complete DC short-circuit state, increasing the damage of the power conversion system. In order to avoid such a situation, the gate driving circuit GD for each IGBT is usually provided with an arm short-circuit detecting circuit and a short-circuit protecting circuit that forcedly interrupts the IGBT upon detection of a short-circuit event. 
       FIG. 10  shows such a gate driving circuit. The gate driving circuit gives an ON/OFF signal for a gate-emitter voltage of the IGBT T 0  through electrical insulation by a photo-coupler PC 1 . When an ON signal is given through the photo-coupler PC 1 , a transistor Qa turns ON and the positive side power supply Ep performs forward bias driving the gate-emitter voltage of the IGBT T 0  through a resistance RG. Thus, the IGBT T 0  is turned ON. When an OFF signal is given through the photo-coupler PC 1 , a transistor Qb turns ON and the negative side power supply En performs reverse bias driving the gate-emitter voltage of the IGBT T 0  through a resistance RG. Thus, the IGBT T 0  is turned OFF. A short-circuit protection circuit that forcedly interrupts the IGBT T 0  in the event of overcurrent is composed of a diode Dc, a resistor R 1 , a capacitor Cd, a Zener diode ZD, a transistor Qc, and a diode Dd. If an overcurrent flows through the IGBT T 0  in the period of ON signal, the collector-emitter voltage of the IGBT T 0  rises causing a non-conducting state of the diode Dc. Consequently the transistor Qc turns ON and then the transistor Qb turns ON to interrupt the IGBT T 0  forcedly. A photo-coupler PC 2  is a short-circuit detecting circuit that feeds back information of overcurrent interruption event to a control circuit. 
     Although the example of  FIG. 9  is a two-level circuit, a multi-level circuit as shown in  FIG. 5  is also operated similarly. When the IGBT T 3  (or T 4 ) undergoes a short-circuit fault, if another normal IGBT T 4  (or T 3 ) turns ON, the capacitor C 1  becomes short-circuited. Accordingly, the IGBT T 4  (or T 3 ) is forcedly turned OFF with the gate driving circuit thereof. When the IGBT T 2  (or T 5 ) undergoes a short-circuit fault, if another normal IGBT T 5  (or T 2 ) is in an ON state, the capacitor C 1  and C 2  becomes short-circuited. Accordingly, the IGBT T 5  (or T 2 ) is forcedly turned OFF with the gate driving circuit thereof. When the IGBT T 1  (or T 6 ) undergoes a short-circuit fault, if another normal IGBT T 6  (or T 1 ) is in an ON state, the capacitor C 2  and the power supply DP becomes short-circuited. Accordingly, the IGBT T 6  (or T 1 ) is forcedly turned OFF with the gate driving circuit thereof. The circuits of  FIG. 6  and  FIG. 7  are similarly operated. 
     Now, the operation on the short circuit fault of the IGBT T 3  in the circuit of  FIG. 5  is more closely considered. Referring to  FIGS. 11(   a )- 11 ( d ), from the state (a) in which electric current from the DC power supply DP flows through the path of IGBT T 1 →T 2 →T 3  to a load, the state transfers to the state (b) in which the IGBT T 3  is short-circuited. When an ON signal is given to the IGBT T 4  in this state (b), short circuit current Ist flows as indicated by the broken line. In that state, the gate driving circuit for the IGBT T 4  detects the short-circuit event and forcedly interrupts the IGBT T 4 . At the same time, a short-circuit fault detection signal is transmitted through the photo-coupler PC 2  in  FIG. 10  to the control device, which delivers an interruption signal to every IGBT. As a result, the current running through the load flows in the state (c) of  FIGS. 11(   a )- 11 ( d ) through the path of: the diode of the IGBT T 6 →the diode of the IGBT T 5 →the capacitor C 1 →the IGBT T 3 . Since the IGBT T 3  is in the short-circuited state in this time, the current flows through the capacitor C 1  and the capacitor C 1  continues to discharge until the voltage V C1  across the capacitor C 1  decreases to zero volts at which the diode of the IGBT T 4  turns to a conductive state. Thus, the current flows in the path indicated in the state (d) of  FIGS. 11(   a )- 11 ( d ). At this state, the voltage V C2  across the capacitor C 2  is about 2Ed/3, and so the IGBT T 2  is subjected to the voltage V T2 =V C2 ≈2Ed/3. 
       FIGS. 12(   a )- 12 ( d ) shows operation in the case of a fault of the IGBT T 4 , which is basically similar to the one shown in  FIGS. 11(   a )- 11 ( d ). Referring to  FIGS. 12(   a )- 12 ( d ), from the state (a) in which electric current from the load flows through the path of IGBT T 4 →T 5 →T 6 , the state transfers to the state (b) in which the IGBT T 4  is short-circuited. When an ON signal is given to the IGBT T 3  in this state (b), short circuit current Ist flows as indicated by the broken line. In that state, the gate driving circuit for the IGBT T 3  detects the short-circuit event and forcedly interrupts the IGBT T 3 . At the same time, a short-circuit fault signal is transmitted through the photo-coupler PC 2  in  FIG. 10  to the control device, which delivers an interruption signal to every IGBT. As a result, the current running through the load flows in the state (c) of  FIGS. 12(   a )- 12 ( d ) through the path of: the IGBT T 4 →the capacitor C 1 →the diode of the IGBT T 2 →the diode of the IGBT T 1 . Since the IGBT T 4  is in the short-circuited state in this time, the current flows through the capacitor C 1  and the capacitor C 1  continues to discharge until the voltage V C1  across the capacitor C 1  decreases to zero volts at which the diode of the IGBT T 3  turns to a conductive state. Thus, the current flows in the path indicated in the state (d) of  FIGS. 12(   a )- 12 ( d ). At this state, the voltage V C2  across the capacitor C 2  is about 2Ed/3, and so the IGBT T 5  is subjected to the voltage V T5 =V C2 ≈2Ed/3. 
     In these cases, a withstand voltage of at least 2Ed/3 is required by the IGBT T 2  in the case of fault of T 3  and by the IGBT T 5  in the case of fault of T 4 . 
     Actually, these IGBTs need a withstand voltage of about Ed that is the voltage of the DC power supply DP. In the normally operating state mentioned earlier, these IGBTs need only about 2Ed/3 that is twice the voltage steadily subjected to. However, the IGBTs are required to exhibit a withstand voltage higher than the value as described above. This leads to an enlarged size and an increased cost. Thus, there is a need in the art for an improved protection control system. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention address these and other needs. Some embodiments provide such a protection control system for a multilevel power conversion circuit that when an IGBT become accidentally short-circuited, the voltage experienced by a normally operating IGBT is suppressed low without unnecessarily increasing the withstand voltage of the IGBT, thereby providing a system with a reduced size and at a low cost. 
     Some embodiments provide a protection control system for a multilevel power conversion circuit of the first aspect of the present invention is a protection control system for a multilevel power conversion circuit of a flying capacitor type for converting a DC power into an AC power or converting an AC power into a DC power, one phase portion of the power conversion circuit comprising: six semiconductor switches of first through sixth semiconductor switches sequentially connected in series from a positive terminal to a negative terminal of a DC power supply circuit, each semiconductor switch having an antiparallel-connected diode; a gate driving circuit with an arm short-circuit protection circuit connected to a gate of each semiconductor switch; a first capacitor connected between a connection point of the second and third semiconductor switches and a connection point of the fourth and fifth semiconductor switch; a second capacitor connected between a connection point of the first and second semiconductor switches and a connection point of the fifth and sixth semiconductor switches; and an AC terminal at a connection point of the third and fourth semiconductor switches; wherein the protection control system turns ON the second semiconductor switch or maintains the second semiconductor switch in an ON state, in a case of short-circuit fault of the third semiconductor switch. 
     A protection control system for a multilevel power conversion circuit of a second aspect of the present invention is a protection control system for a multilevel power conversion circuit of a flying capacitor type for converting a DC power into an AC power or converting an AC power into a DC power, one phase portion of the power conversion circuit comprising: six semiconductor switches of first through sixth semiconductor switches sequentially connected in series from a positive terminal to a negative terminal of a DC power supply circuit, each semiconductor switch having an antiparallel-connected diode; a gate driving circuit with an arm short-circuit protection circuit connected to a gate terminal of each semiconductor switch; a first capacitor connected between a connection point of the second and third semiconductor switches and a connection point of the fourth and fifth semiconductor switch; a second capacitor connected between a connection point of the first and second semiconductor switches and a connection point of the fifth and sixth semiconductor switches; and an AC terminal at a connection point of the third and fourth semiconductor switches; wherein the protection control system turns ON the fifth semiconductor switch or maintains the fifth semiconductor switch in an ON state, in a case of short-circuit fault of the fourth semiconductor switch. 
     A protection control system for a multilevel power conversion circuit of a third aspect of the present invention is the protection control system for a multilevel power conversion circuit according to the first or second aspect of the invention, wherein the second or fifth semiconductor switch is turned OFF when the voltage across the second capacitor decreases to a predetermined voltage value after the second or fifth semiconductor switch has been turned ON. 
     A protection control system for a multilevel power conversion circuit of a fourth aspect of the present invention is the protection control system for a multilevel power conversion circuit according to the third aspect of the invention, wherein the predetermined voltage value is approximately 50% of a voltage value of the DC power supply. 
     A protection control system for a multilevel power conversion circuit of a fifth aspect of the present invention is the protection control system for a multilevel power conversion circuit according to the third aspect of the invention, wherein the multilevel power conversion circuit is a multilevel power conversion circuit of a flying capacitor type in combination with a neutral point clamped circuit that is additionally provided and composed of a semiconductor switch for clamping the electric potential of the second capacitor at a neutral point electric potential of the DC power supply, and the predetermined voltage value is approximately ⅙ of a voltage value of the DC power supply. 
     A protection control system for a multilevel power conversion circuit of a sixth aspect of the present invention is a protection control system for a five-level or more levels of power conversion circuit of a flying capacitor type for converting a DC power into an AC power or converting an AC power into a DC power, one phase portion of the power conversion circuit comprising: 2 N pieces of semiconductor switches, N being an integer of four or larger, sequentially connected in series from a positive terminal to a negative terminal of a DC power supply circuit, each semiconductor switch having an antiparallel-connected diode; a gate driving circuit with an arm short-circuit protection circuit connected to a gate terminal of each semiconductor switch; and (N−1) pieces of capacitors connected between a connection point of the (N−I+1)-th and (N−I)-th semiconductor switches and a connection point of the (N+I)-th and (N+I+1)-th semiconductor switches, I being an integer from 1 to (N−1); wherein when a J-th semiconductor switch, J being an integer from 3 to (2N−2), becomes in a short-circuit fault state, short-circuit current is interrupted by a normally operating semiconductor switch in which the short-circuit current running through the semiconductor switch in the short-circuit fault state flows, and the protection control system turns ON, or maintains in an ON state, the (J−1)-th semiconductor switch if J≦N or the (J+1)-th semiconductor switch if J≧(N+1), which is then turned OFF when the voltage across the (J+1)-th capacitor decreases to a predetermined voltage value. 
     A protection control system for a multilevel power conversion circuit of a seventh aspect of the present invention is the protection control system for a multilevel power conversion circuit according to the sixth aspect of the invention, wherein the predetermined voltage value is approximately a middle voltage value between a final voltage value that the (J−1)-th capacitor reaches and a voltage value of the (J+2)-th capacitor. 
     A protection control system for a multilevel power conversion circuit of a seventh aspect of the present invention is the protection control system for a multilevel power conversion circuit according to the sixth aspect of the invention, wherein the multilevel power conversion circuit is a multilevel power conversion circuit of a flying capacitor type in combination with a neutral point clamped circuit that is additionally provided and composed of a semiconductor switch for clamping the electric potential of either one of the (N−1) pieces of capacitors at a neutral point electric potential of the DC power supply, and the predetermined voltage value is approximately an average voltage value of the (J−2)-th capacitor in a normally operating state. 
     In a protection control system for a multilevel power conversion circuit of a flying capacitor type of embodiments of the present invention, in the case of a short-circuit fault of a semiconductor switch, another semiconductor switch through which discharge current from a flying capacitor flows is interrupted by a short-circuit protection circuit of a gate driving circuit and still another semiconductor switch that is connected to the flying capacitor and disposed in the power supply side is turned ON or maintained in an ON state. This semiconductor switch in an ON state is turned OFF when the voltage across another flying capacitor that is connected in the side of power supply with respect to this semiconductor switch is decreased to a predetermined value. 
     Consequently, such a protection control system for a multilevel power conversion circuit is provided that even when an IGBT become accidentally short-circuited, the withstand voltage of the IGBT need not to be unnecessarily high. Therefore a system with a reduced size and at a low cost is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of protection operation algorithm according to an embodiment of the present invention; 
         FIGS. 2(   a )- 2 ( d ) show an example of protection operation mode in the case of a short-circuit fault of IGBT T 3 ; 
         FIGS. 3(   a )- 3 ( d ) show an example of protection operation mode in the case of a short-circuit fault of IGBT T 4 ; 
         FIG. 4  shows a general inverter system; 
         FIG. 5  shows a four-level power conversion circuit of a flying capacitor type; 
         FIG. 6  shows a five-level power conversion circuit of a flying capacitor type; 
         FIG. 7  shows a six-level power conversion circuit of a flying capacitor type; 
         FIG. 8  shows a seven-level power conversion circuit of a flying capacitor type in combination with a neutral point clamped circuit; 
         FIG. 9  illustrates operation in an event of arm short-circuit; 
         FIG. 10  shows a gate driving circuit having an arm short-circuit detecting circuit and a protection circuit; 
         FIGS. 11(   a )- 11 ( d ) show a first operation mode in arm short-circuit protection according to a conventional system; and 
         FIGS. 12(   a )- 12 ( d ) show a second operation mode in arm short-circuit protection according to a conventional system. 
     
    
    
     DETAILED DESCRIPTION 
     In some embodiments, in a protection control system for a multilevel power conversion circuit of a flying capacitor type, in the case of a short-circuit fault of a semiconductor switch, another semiconductor switch through which discharge current from a flying capacitor flows is interrupted by a short-circuit protection circuit of a gate driving circuit and still another semiconductor switch that is connected to the flying capacitor and disposed in the power supply side is turned ON or maintained in an ON state. This semiconductor switch in an ON state is turned OFF when the voltage across another flying capacitor that is connected in the side of power supply with respect to this semiconductor switch is decreased to a predetermined value. 
     Embodiment Example 1 
     The following describes a protection control system according to an embodiment of the present invention for a four-level power conversion circuit of a flying capacitor type.  FIG. 1  shows a flow of an operation to shut down the main circuit at an event of arm short-circuit from a normally operating state;  FIGS. 2(   a )- 2 ( d ) shows a flow of protection operation in the case of a short-circuit fault of IGBT T 3 ; and  FIGS. 3(   a )- 3 ( d ) shows a flow of protection operation in the case of a short-circuit fault of IGBT T 4 . 
     Referring to  FIG. 1 , when a short-circuit fault has occurred at the IGBT T 3  and an arm-short-circuit is detected with the IGBT T 4  in the block  28 , the IGBTs T 1 , T 5 , and T 6  are turned OFF in the block  29 . In the blocks  30  and  31 , the IGBT T 2  is turned ON or, if in the ON state already, it is held ON. In the blocks  32  and  33 , the IGBT T 2  is turned OFF when the voltage V C2  across the capacitor C 2  decreases down to a predetermined voltage value. 
     Similarly, when a short-circuit fault has occurred at the IGBT T 4  and an arm-short-circuit is detected with the IGBT T 3  in the block  34 , the IGBTs T 1 , T 2 , and T 6  are turned OFF in the block  35 . In the blocks  36  and  37 , the IGBT T 5  is turned ON or, if in the ON state already, it is held ON. In the blocks  38  and  39 , the IGBT T 5  is turned OFF when the voltage V C2  across the capacitor C 2  decreases down to a predetermined voltage value. 
     It is desirable to set the predetermined voltage value for the voltage V C2  across the capacitor C 2  to be about half the voltage Ed of the DC power supply DP. Explanation is omitted here about a well known circuit of voltage detection across the capacitor C 2 . 
     The voltage V C1  across the capacitor C 1 , which is connected outside, or in the side toward the DC power supply terminals, of the faulted switching element is zero volts. And the voltage V C2  across the capacitor C 2 , which is connected outside this capacitor C 1 , is made at a voltage in the middle between the voltage Ed of the DC power supply DP and the final voltage zero volts of the capacitor C 1 . Thus, the maximum voltage undergone by the switching elements T 1 , T 2 , T 5  and T 6  is restricted to one and a half of the normal value of Ed/3. In the conventional system as described previously, the IGBTs undergo a voltage of 2Ed/3, which is twice the normal value. Therefore, the semiconductor elements in the system of the embodiment of the invention can be with a smaller size and at a lower cost. 
     The operation described above is the one in an embodiment for a four-level power conversion circuit of a flying capacitor type shown in  FIG. 5  or an embodiment for a seven-level power conversion circuit of a flying capacitor type in combination with a neutral point clamped circuit shown in  FIG. 8 . However, the protection control system of the invention can be applied to multi-level power conversion circuits of flying capacitor type of more than four levels as shown in  FIG. 6  and  FIG. 7 , as described in the following. 
     Embodiment Example 2 
     The Embodiment Example 2 is an embodiment for a five-level power conversion circuit of a flying capacitor type as shown in  FIG. 6 . The Embodiment Example 2 is the case of N=4 in the following general expression for a circuit having 2 N semiconductor switches. Of the 2 N semiconductor switches connected in series between the positive terminal and the negative terminal of the DC power supply, the third semiconductor switch from the positive terminal or the negative terminal is supposed to be short-circuited, or a semiconductor switch at the side of the middle connection point of the third semiconductor switch is supposed to be short-circuited. The short-circuit current through the short-circuit fault semiconductor switch flows through a normally operating semiconductor switch, and the short circuit current is interrupted by this normally operating semiconductor switch. A semiconductor switch disposed at just outside of the fault semiconductor switch is turned ON. The outside here means the side toward the positive terminal or toward the negative terminal of the DC power supply. The semiconductor switch in the ON state is turn OFF when the voltage across the capacitor connected outside of the semiconductor switch in the ON state decreases down to a predetermined voltage value. 
     In this Embodiment Example 2, the semiconductor switches concerned that become into short-circuit fault state are IGBTs T 4 , T 5 , T 3 , and T 6 . The following describes operation and setting of the capacitor voltages in the cases of short-circuit of one of these four IGBTs. 
     (1) The Case of Short-Circuit Fault of IGBT T 4   
     The short-circuit fault is detected by the IGBT T 5 . The IGBTs other than the IGBT T 3  are turned OFF. The IGBT T 3  is turned OFF when the voltage V C2  across the capacitor C 2  becomes about half the voltage V C3  across the capacitor C 3 , that is, 3Ed/8. 
     (2) The Case of Short-Circuit Fault of IGBT T 5   
     The short-circuit fault is detected by the IGBT T 4 . The IGBTs other than the IGBT T 6  are turned OFF. The IGBT T 6  is turned OFF when the voltage V C2  across the capacitor C 2  becomes about half the voltage V C3  across the capacitor C 3 , that is, 3Ed/8. 
     (3) The Case of Short-Circuit Fault of IGBT T 3   
     The short-circuit fault is detected by the IGBT T 6 . The IGBTs other than the IGBT T 2  are turned OFF. The IGBT T 2  is turned OFF when the voltage V C3  across the capacitor C 3  becomes about half the sum of the voltage Ed of the DC power supply and the voltage V C1  across the capacitor C 1 , that is, 5Ed/8. 
     (4) The Case of Short-Circuit Fault of IGBT T 6   
     The short-circuit fault is detected by the IGBT T 3 . The IGBTs other than the IGBT T 7  are turned OFF. The IGBT T 7  is turned OFF when the voltage V C3  across the capacitor C 3  becomes about half the sum of the voltage Ed of the DC power supply and the voltage V C1  across the capacitor C 1 , that is, 5Ed/8. By controlling as described above, the maximum voltage undergone by the IGBTs is 3Ed/8. 
     Embodiment Example 3 
     The Embodiment Example 3 is an embodiment for a six-level power conversion circuit of a flying capacitor type as shown in  FIG. 7 . The Embodiment Example 3 is the case of N=5 in the following general expression for a circuit having 2 N semiconductor switches. Of the 2 N semiconductor switches connected in series between the positive terminal and the negative terminal of the DC power supply, the third semiconductor switch from the positive terminal or the negative terminal is supposed to be short-circuited, or a semiconductor switch at the side of the middle connection point of the third semiconductor switch is supposed to be short-circuited. The short-circuit current through the short-circuit fault semiconductor switch flows through a normally operating semiconductor switch, and the short circuit current is interrupted by this normally operating semiconductor switch. A semiconductor switch disposed at just outside of the fault semiconductor switch is turned ON. The outside here means the side toward the positive terminal or toward the negative terminal of the DC power supply. The semiconductor switch in the ON state is turned OFF when the voltage across the capacitor connected outside of the semiconductor switch in the ON state decreases down to a predetermined voltage value. 
     In this Embodiment Example 3, the semiconductor switches that can become into short-circuit fault state are IGBTs T 5 , T 6 , T 4 , T 7 , T 3  and T 8 . The following describes operation and setting of the capacitor voltages in the cases of short-circuit fault of one of these four IGBTs. 
     (1) The Case of Short-Circuit Fault of IGBT T 5   
     The short-circuit fault is detected by the IGBT T 6 . The IGBTs other than the IGBT T 4  are turned OFF. The IGBT T 4  is turned OFF when the voltage V C2  across the capacitor C 2  becomes about half the voltage V C3  across the capacitor C 3 , that is, 3Ed/10. 
     (2) The Case of Short-Circuit Fault of IGBT T 6   
     The short-circuit fault is detected by the IGBT T 5 . The IGBTs other than the IGBT T 7  are turned OFF. The IGBT T 7  is turned OFF when the voltage V C2  across the capacitor C 2  becomes about half the voltage V C3  across the capacitor C 3 , that is, 3Ed/10. 
     (3) The Case of Short-Circuit Fault of IGBT T 4   
     The short-circuit fault is detected by the IGBT T 7 . The IGBTs other than the IGBT T 3  are turned OFF. The IGBT T 3  is turned OFF when the voltage V C3  across the capacitor C 3  becomes about half the sum of the voltage V C4  across the capacitor C 4  and the voltage V C1  across the capacitor C 1 , that is, Ed/2. 
     (4) The Case of Short-Circuit Fault of IGBT T 7   
     The short-circuit fault is detected by the IGBT T 4 . The IGBTs other than the IGBT T 8  are turned OFF. The IGBT T 8  is turned OFF when the voltage V C3  across the capacitor C 3  becomes about half the sum of the voltage V C4  across the capacitor C 4  and the voltage V C1  across the capacitor C 1 , that is, Ed/2. 
     (5) The Case of Short-Circuit Fault of IGBT T 3   
     The short-circuit fault is detected by the IGBT T 8 . The IGBTs other than the IGBT T 2  are turned OFF. The IGBT T 2  is turned OFF when the voltage V C4  across the capacitor C 4  becomes about half the sum of the voltage Ed of the DC power supply and the voltage V C2  across the capacitor C 2 , that is, 7Ed/10. 
     (6) The Case of Short-Circuit Fault of IGBT T 8   
     The short-circuit fault is detected by the IGBT T 3 . The IGBTs other than the IGBT T 9  are turned OFF. The IGBT T 9  is turned OFF when the voltage V C4  across the capacitor C 4  becomes about half the sum of the voltage Ed of the DC power supply and the voltage V C2  across the capacitor C 2 , that is, 3Ed/10. 
     In similar ways, the protection control system of the invention can be applied to all multilevel power conversion circuits of the flying capacitor type excepting a three-level circuit. The maximum voltage undergone by the semiconductor switches can be restricted within one and the half of the average voltage in the steady state. 
     The present invention is applied to power inverter circuits for obtaining multilevel AC voltage from a small number of DC power supplies and also to rectifying circuits that performs reversed power conversion. Actual applications include high voltage motor driving equipment and power converter equipment for grid connection. 
     Examples of specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the above description, specific details are set forth in order to provide a thorough understanding of embodiments of the invention. Embodiments of the invention may be practiced without some or all of these specific details. Further, portions of different embodiments and/or drawings can be combined, as would be understood by one of skill in the art. 
     This application is based on, and claims priority to, Japanese Patent Application No. 2012-134361, filed on Jun. 14, 2012, the contents of which are incorporated herein by reference.