Abstract:
A power conversion device provided with a DC power source, a voltage boost converter, an inverter, a primary smoothing capacitor, a secondary smoothing capacitor, an intermediate capacitor provided in the voltage boost converter and three voltage sensors which measure the voltages of the capacitors, further including an abnormality diagnosis controller which makes a diagnosis of the presence/absence of an abnormality in the detection characteristics of the voltage sensors, on the basis of the state of transition of the detection results of the voltage sensors which are monitored during a discharge operation by the respective capacitors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0001]    The present invention relates to a power conversion device provided with a voltage boost converter and an inverter, and more particularly, to a power conversion device which diagnoses the presence or absence of a failure in the voltage detection function of the voltage sensor, and a method for diagnosing an abnormality in the voltage sensor characteristics, on the basis of monitoring results of the voltage detection values of a plurality of voltage sensors which are used for voltage conversion control. 
       2. Description of the Related Art 
       [0002]    A power conversion device having a DC/DC voltage conversion function is generally provided with a voltage sensor which monitors a plurality of voltages including an input voltage and an output voltage, and voltage conversion control is carried out by using the signal detected by these voltage sensors. 
         [0003]    If there is a failure in the voltage detection function of the voltage sensors, then an operation such as stopping the converter operation is carried out. A method commonly used for failure detection is one in which the presence or absence of an abnormality in the voltage detection function is diagnosed while applying a certain voltage (see, for example, Japanese Patent Application Publication No. 2010-008153 and Japanese Patent Application Publication No. 2007-282299). 
       SUMMARY OF THE INVENTION 
       [0004]    The prior art involves the following problems, however. 
         [0005]    The method disclosed in Japanese Patent Application Publication No. 2010-008153 determines that the voltage detection function has failed, when an abnormal voltage is applied from an external source but an abnormality is not determined. This method, however, requires means for generating an abnormal voltage from an external source and, moreover, does not diagnose the voltage detection function directly. Therefore, Japanese Patent Application Publication No. 2010-008153 has a problem in that the diagnosis accuracy is poor. 
         [0006]    The method described in Japanese Patent Application Publication No. 2007-282299 calculates a motor output from two different parameters and determines a failure of the voltage detection function from the deviation in the calculation result. However, this method does not diagnose the voltage detection function by direct calculation. Therefore, Japanese Patent Application Publication No. 2007-282299 also has a problem in that the diagnosis accuracy is poor. 
         [0007]    The present invention has been devised in view of the conventional problems described above, an object thereof being to achieve a power conversion device and a method for detecting an abnormality in voltage sensor characteristics, with which the presence/absence of an abnormality in a voltage detection function of a voltage sensor can be diagnosed in a broad voltage range, easily and inexpensively by a simple configuration, and it can be diagnosed with high accuracy whether or not unexpected variation has occurred in the detection characteristics of the voltage sensor. 
         [0008]    The power conversion device according to the present invention is a power conversion device, provided with: a voltage boost converter which performs DC/DC conversion of a DC voltage supplied from a DC power source and outputs a boosted DC voltage by switching of a first power semiconductor element group; an inverter which is connected to a subsequent stage of the voltage boost converter, performs DC/AC conversion of the boosted DC voltage by switching of a second power semiconductor element group, and outputs an AC voltage; a primary smoothing capacitor which is connected between the DC power source and the voltage boost converter and smoothens the DC voltage supplied to the voltage boost converter; a secondary smoothing capacitor which is connected between the voltage boost converter and the inverter and smoothens the boosted DC voltage supplied to the inverter; an intermediate capacitor provided inside the voltage boost converter; a primary-side voltage sensor which measures the voltage of the primary smoothing capacitor; a secondary-side voltage sensor which measures the voltage of the secondary smoothing capacitor; and an intermediate voltage sensor which measures the voltage of the intermediate capacitor; further including an abnormality diagnosis controller which carries out a process of measuring a detected voltage value of each of the three voltage sensors, i.e. the primary-side voltage sensor, the secondary-side voltage sensor and the intermediate voltage sensor, and selecting detected voltage values of two of the three voltage sensors, multiple times over the passage of time during implementation of an operation of discharging electrical charge inside the primary smoothing capacitor and electrical charge inside the secondary smoothing capacitor, and makes a diagnosis of the presence/absence of an abnormality in detection characteristics of the three voltage sensors on the basis of comparison results of the detected voltage values of the two voltage sensors. 
         [0009]    Furthermore, the method for diagnosing an abnormality in voltage sensor characteristics according to the present invention is a method for diagnosing an abnormality in voltage sensor characteristics implemented in an abnormality diagnosis controller of a power conversion device provided with: a voltage boost converter which performs DC/DC conversion of a DC voltage supplied from a DC power source and outputs a boosted DC voltage by switching of a first power semiconductor element group; an inverter which is connected to a subsequent stage of the voltage boost converter, performs DC/AC conversion of the boosted DC voltage by switching of a second power semiconductor element group, and outputs an AC voltage; a primary smoothing capacitor which is connected between the DC power source and the voltage boost converter and smoothens the DC voltage supplied to the voltage boost converter; a secondary smoothing capacitor which is connected between the voltage boost converter and the inverter and smoothens the boosted DC voltage supplied to the inverter; an intermediate capacitor provided inside the voltage boost converter; a primary-side voltage sensor which measures the voltage of the primary smoothing capacitor; a secondary-side voltage sensor which measures the voltage of the secondary smoothing capacitor; an intermediate voltage sensor which measures the voltage of the intermediate capacitor; and an abnormality diagnosis controller which makes a diagnosis of the presence/absence of an abnormality in detection characteristics of the three voltage sensors, i.e. the primary-side voltage sensor, the secondary-side voltage sensor and the intermediate voltage sensor; wherein the method includes: a first step of halting switching operations of the voltage boost converter and the inverter; a second step of disconnecting the DC power source which is supplied to the voltage boost converter; a third step of implementing a discharge operation based on control of the voltage boost converter and a discharge operation based on control of the inverter, and measuring a detected voltage value of each of the three voltage sensors, i.e. the primary-side voltage sensor, the secondary-side voltage sensor and the intermediate voltage sensor; a fourth step of successively sampling and gathering the detected voltage values of the three voltage sensors during the discharge operation based on control of the inverter, upon halting the discharge operation based on control of the voltage boost converter when the deviation in the detected voltage values of two voltage sensors selected from among the three voltage sensors becomes equal to or lower than a predetermined deviation threshold value, and selecting and successively storing, in a storage unit, the detected voltage values of the two voltage sensors which indicate a maximum value and a minimum value, among the three voltage sensors, for each successive sampling; a fifth step of halting the discharge operation based on control of the inverter, when all of the detected voltage values of the three voltage sensors which are successively gathered in the fourth step become less than a predetermined threshold value; and a sixth step of making a diagnosis of the presence/absence of an abnormality in the detection characteristics of the three voltage sensors, on the basis of comparison results between the deviation between the maximum value and the minimum value successively stored in the storage unit during implementation of the fourth step, and a predetermined determination threshold value. 
         [0010]    According to the present invention, a configuration is achieved in which it is possible to make a diagnosis of the presence/absence of an abnormality in the detection characteristics of a voltage sensor, from the state of transition in detection results from each voltage sensor as monitored during a discharge operation. Therefore, it is possible to achieve a power conversion device and a method for detecting an abnormality in voltage sensor characteristics, whereby the presence/absence of an abnormality in a voltage detection function of a voltage sensor can be diagnosed in a broad voltage range, easily and inexpensively by a simple configuration, and it can be diagnosed with high accuracy whether or not unexpected variation has occurred in the detection characteristics of the voltage sensors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic drawing showing a power conversion device relating to the first and second embodiments of the present invention; 
           [0012]      FIG. 2  is a flowchart illustrating a processing sequence relating to the method for diagnosing abnormalities in a voltage detection function according to the first embodiment of the present invention; and 
           [0013]      FIG. 3  is a flowchart illustrating a processing sequence relating to the method for diagnosing abnormalities in a voltage detection function according to the second embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    A power conversion device and a method for diagnosing an abnormality in voltage sensor characteristics according to the first and second embodiments of the present invention are now described in detail with reference to  FIGS. 1 to 3 . 
       First Embodiment 
       [0015]      FIG. 1  is a schematic drawing showing a power conversion device relating to the first embodiment of the present invention. The power conversion device illustrated in  FIG. 1  comprises an abnormality diagnosis control unit  10  (an abnormality diagnosis controller  10 ), a DC power source  20 , a relay  30 , a primary smoothing capacitor  41 , a primary-side voltage sensor  42 , a voltage boost converter  50 , a secondary smoothing capacitor  61 , a secondary-side voltage sensor  62 , an inverter  70  and an electric motor  80 . 
         [0016]    The DC power source  20  is chargeable/dischargeable, and exchanges power with the electric motor  80  via the voltage boost converter  50  and the inverter  70 . 
         [0017]    The voltage boost converter  50  is connected between the DC power source  20  and the inverter  70 . The voltage boost converter  50  is configured from a reactor  51 , semiconductor switch elements  52   a  to  52   d,  and an energy transfer capacitor  53 . 
         [0018]    In the description given below, the energy transfer capacitor  53  is called an “intermediate capacitor  53 ”. Furthermore, the semiconductor switch elements  52   a  to  52   d  in the voltage boost converter  50  correspond to a first power semiconductor element group. 
         [0019]    The voltage boost converter  50  which is provided with a configuration of this kind boosts the DC voltage supplied from the DC power source  20  to the primary-side terminals P 1 , N 1 , by DC/DC conversion, and outputs the resultant voltage to the secondary-side terminals P 2 , N 2 . 
         [0020]    Moreover, an intermediate voltage sensor  54  which measures the voltage between a high-voltage node and a low-voltage node of the intermediate capacitor  53  (this voltage is called “voltage value V0” below) is provided in the vicinity of the intermediate capacitor  53 . 
         [0021]    The primary smoothing capacitor  41  is connected between the DC power source  20  and the voltage boost converter  50 , and smoothens the voltage between the DC power source  20  and the voltage boost converter  50 . Moreover, a primary-side voltage sensor  42  which measures the voltage between a high-voltage node and a low-voltage node of the primary smoothing capacitor  41  (this voltage is called “voltage value V1” below) is provided in the vicinity of the primary smoothing capacitor  41 . 
         [0022]    The inverter  70  is configured from semiconductor switch elements  71   a  to  71   f.  The semiconductor switch elements  71   a  to  71   f  in the inverter  70  correspond to a second power semiconductor element group. The inverter  70  which is provided with a configuration of this kind converts the high-voltage DC voltage output from the secondary-side terminals P 2 , N 2  of the voltage boost converter  50 , to an AC voltage, by DC/AC conversion. 
         [0023]    The secondary smoothing capacitor  61  is connected between the voltage boost converter  50  and the inverter  70  and smoothens the voltage between the voltage boost converter  50  and the inverter  70 . A secondary-side voltage sensor  62  which measures the voltage between a high-voltage node and a low-voltage node of the secondary smoothing capacitor  61  (this voltage is called “voltage value V2” below) is provided in the vicinity of the secondary smoothing capacitor  61 . 
         [0024]    The electric motor  80  is provided with three terminals corresponding to the coil phases: terminal Uac, terminal Vac and terminal Wac. The terminal Uac is connected to a connection node Mu between the semiconductor switch element  71   a  and the semiconductor switch element  71   b  of the inverter  70 . 
         [0025]    Similarly, the terminal Vac is connected to a connection node Mv between the semiconductor switch element  71   c  and the semiconductor switch element  71   d,  and the terminal Wac is connected to a connection node Mw between the semiconductor switch element  71   e  and the semiconductor switch element  71   f.    
         [0026]    The abnormality diagnosis control unit  10  is configured from a discharge determination instruction unit  11 , a converter control unit  12  and an inverter control unit  13 . In a specific hardware configuration, the abnormality diagnosis control unit  10  is provided with a microcomputer which is not depicted in  FIG. 1 , and executes the functions of the various component elements. 
         [0027]    The abnormality diagnosis control unit  10  is included in an integrated control unit, which is not depicted in  FIG. 1 . During normal operation, the integrated control unit controls the electric motor  80  in accordance with an input signal, by operation of the microcomputer. 
         [0028]    On the other hand, the abnormality diagnosis control unit  10  executes a diagnosis function for verifying an abnormality in the voltage detection characteristics, as described below with reference to  FIG. 2 , by operation of the microcomputer. 
         [0029]    The inverter control unit  13  adjusts the amplitude and frequency of the AC voltage output from the inverter  70 , by switching the semiconductor switch elements  71   a  to  71   f  in the inverter  70  on and off in accordance with the gate drive signal. In this way, the inverter control unit  13  controls the drive power and braking power of a vehicle, by adjusting the drive power and braking power of the electric motor  80 , via the inverter  70 . 
         [0030]    The relay  30  is set to a closed state during operation of the vehicle, whereby the DC power source  20  is electrically connected to the primary smoothing capacitor  41  and the voltage boost converter  50 . 
         [0031]    Meanwhile, the relay  30  is set to an open state in the event of a collision of the vehicle, or when the vehicle has stopped operation, whereby the DC power source  20  is electrically disconnected from the primary smoothing capacitor  41  and the voltage boost converter  50 . 
         [0032]    The open state and closed state of the relay  30  are switched in accordance with an instruction signal which is output from a control unit included in the abnormality diagnosis control unit  10 , or an instruction signal which is output from a separate electronic control device (not illustrated). 
         [0033]    The discharge determination instruction unit  11  outputs an instruction to discharge the electric charge stored in the primary smoothing capacitor  41 , the secondary smoothing capacitor  61  and the intermediate capacitor  53  in the voltage boost converter  50 , in the event of a collision of the vehicle or when the power of the vehicle has stopped, with the object of avoiding the risk of electric shock. More specifically, the discharge determination instruction unit  11  executes a discharge operation by outputting a discharge instruction to the converter control unit  12  and the inverter control unit  13 . 
         [0034]    The converter control unit  12  and the inverter control unit  13  which have received a discharge instruction perform a discharge of the primary smoothing capacitor  41 , the secondary smoothing capacitor  61  and the intermediate capacitor  53  in the voltage boost converter  50 . 
         [0035]    Here, the method of discharging the electrical charge can employ, for example, passive discharge by a discharge resistance or discharge by a switching operation of a semiconductor switch element, as described in Japanese Patent Application Publication No. 2015-033153. Furthermore, the discharge method employed may also be another method apart from this. This point is similar to the second embodiment indicated below. 
         [0036]    Furthermore, the discharge determination instruction unit  11  is able to detect “collision of the vehicle” or “stopping of the power” as described above, in the following manner, for example. In other words, the discharge determination instruction unit  11  is able to detect a “collision of the vehicle” from a sudden change in acceleration based on an acceleration sensor, which is not illustrated. Furthermore, the discharge determination instruction unit  11  is able to detect “stopping of the power”, when the use of the vehicle has ended and it is detected that the power has stopped. 
         [0037]    Next, a method for diagnosing an abnormality in a voltage detection function according to the first embodiment will be described. In a power conversion device which is provided with the configuration in  FIG. 1 , the converter control unit  12  controls the switching operation of the semiconductor switch elements  52   a  to  52   d  in the voltage boost converter  50 . Similarly, the inverter control unit  13  controls the switching operation of the semiconductor switch elements  71   a  to  71   f  in the inverter  70 . 
         [0038]    By controlling these switching operations, it is possible to discharge the electrical charge in the primary smoothing capacitor  41 , the secondary smoothing capacitor  61  and the intermediate capacitor  53  in the voltage boost converter  50 . Furthermore, the voltage values in the respective capacitors  41 ,  53 ,  61  is detected by the respective voltage sensors  42 ,  54 ,  62 . 
         [0039]    The abnormality diagnosis control unit  10  in  FIG. 1  executes a switching operation suited to the diagnosis of an abnormality in the voltage detection function, and in this case, carries out abnormality diagnosis in each of the voltage sensors on the basis of the transitions in the voltage values of the capacitors which are detected by the respective voltage sensors.  FIG. 2  is a flowchart illustrating a processing sequence relating to the method for diagnosing an abnormality in a voltage detection function according to the first embodiment of the present invention. 
         [0040]    The abnormality diagnosis control unit  10  also has a microcomputer, as well as being provided with a configuration such as that illustrated in  FIG. 1 . The description given below in relation to the flowchart in  FIG. 2  is simplified by supposing that all of the operations are executed by the abnormality diagnosis control unit  10 . 
         [0041]    Firstly, in step S 101 , the abnormality diagnosis control unit  10  stops switching of the voltage boost converter  50  and also stops switching of the inverter  70 . 
         [0042]    Next, in step S 102 , the abnormality diagnosis control unit  10  opens the relay  30 . Thereby, the DC power source  20  is disconnected electrically from the primary smoothing capacitor  41  and the voltage boost converter  50 . 
         [0043]    By the processing thus far, the abnormality diagnosis control unit  10  enters into a diagnostic mode for verifying an abnormality in the voltage detection characteristics. Thereupon, in step S 103 , the abnormality diagnosis control unit  10  respectively acquires the voltage value VO from the intermediate voltage sensor  54 , the voltage value V1 from the primary-side voltage sensor  42 , and the voltage value V2 from the secondary-side voltage sensor  62 . 
         [0044]    In this case, the abnormality diagnosis control unit  10  performs A/D conversion of the acquired three voltage values V0, V1, V2, by substantially simultaneous sampling to acquire the voltage values as numerical data. 
         [0045]    Moreover, the abnormality diagnosis control unit  10  uses a conversion table in which individual variations in the voltage sensor circuits are stored as characteristics, to carry out physical quantity conversion of the acquired data from the A/D converted values to voltage values. 
         [0046]    As described above, the abnormality diagnosis control unit  10  respectively acquires the voltage values V0 to V2, based on the transmission of electrical signals to the microcomputer. In this, when short circuit, ground fault or disconnection of the signal wire occurs in the transmission path of the electrical signals to the microcomputer, the voltage value which is subjected to physical quantity conversion indicates a markedly high voltage value or low voltage value. 
         [0047]    In this case, an abnormality such as short circuit, ground fault or disconnection of the signal wire can be identified from the voltage value characteristics, and is detected by a separate abnormality diagnosis processing method to the processing sequence relating to the method for diagnosing an abnormality according to the embodiment of the present invention illustrated in  FIG. 2 . Therefore, a voltage value abnormality caused by short circuit, ground fault or disconnection of the signal wire in the transmission path of the voltage detection signal to the microcomputer are excluded from the processing sequence described below. 
         [0048]    Next, in step S 104 , the abnormality diagnosis control unit  10  determines whether or not the maximum value of the voltage values V0 to V2 obtained by physical amount conversion is equal to or greater than a predetermined first threshold value Vth1. 
         [0049]    If the maximum value of the voltage values V0 to V2 is equal to or greater than the first threshold value Vth1, then the abnormality diagnosis control unit  10  transfers to the processing from step S 105  on in order to start discharge of the electrical charge in the capacitors  41 ,  53 ,  61 . On the other hand, if the maximum value of the voltage values V0 to V2 is less than the first Vth1, then the abnormality diagnosis control unit  10  terminates the processing sequence without discharging electrical charge. 
         [0050]    Upon advancing to step S 105 , the abnormality diagnosis control unit  10  respectively switches the semiconductor switch elements  52   a  to  52   d  in the voltage boost converter  50 , and the semiconductor switch elements  71   a  to  71   f  in the inverter  70 , to a discharge switching mode. 
         [0051]    Next, in step S 106 , the abnormality diagnosis control unit  10  determines whether or not the deviation between the maximum value and the minimum value of the voltage values V0 to V2 is equal to or less than a predetermined deviation threshold value ΔVt. If the determined deviation is equal to or less than the deviation threshold value ΔVt, then the abnormality diagnosis control unit  10  transfers to the processing from step S 107  on. Meanwhile, if the determined deviation is greater than the deviation threshold value ΔVt, then the abnormality diagnosis control unit  10  repeats the processing in step S 105 , until the condition in step S 106  is established. 
         [0052]    Upon advancing to step S 107 , the abnormality diagnosis control unit  10  stops the discharge switching of the voltage boost converter  50  and continues only the discharge switching of the inverter  70 . 
         [0053]    Moreover, in step S 108 , the abnormality diagnosis control unit  10  successively calculates the deviation between the maximum value and minimum value of the voltage values V0 to V2, and stores the deviation in the storage unit, for each predetermined deviation point, and for each predetermined timing. 
         [0054]    Subsequently, in step S 109 , the abnormality diagnosis control unit  10  determines whether or not the maximum value of the voltage values V0 to V2 is less than a second threshold value Vth2 which is set previously as a value equivalent to the voltage at the end of discharge. If the maximum value is less than the second threshold value Vth2, then the abnormality diagnosis control unit  10  transfers to the processing from step S 110  on. Meanwhile, if the determined maximum value is equal to or greater than the second threshold value Vth2, then the abnormality diagnosis control unit  10  repeats the processing in step S 108 , until the condition in step S 109  is established. 
         [0055]    Upon advancing to step S 110 , the abnormality diagnosis control unit  10  stops the discharge switching of the inverter  70 . 
         [0056]    Next, in step S 111 , the abnormality diagnosis control unit  10  determines whether or not the sum of the deviation stored in the previous step S 108  exceeds a predetermined first determination threshold value FTH_Vsns1. If the sum of the stored deviation values exceeds the first determination threshold value FTH_Vsns1, then the abnormality diagnosis control unit  10  advances to step S 112  and diagnoses that the voltage detection characteristics are abnormal, then terminates the processing sequence. 
         [0057]    On the other hand, if the sum of the stored deviation values does not exceed the first determination threshold value FTH_Vsns1, then the abnormality diagnosis control unit  10  diagnoses that the voltage detection characteristics are normal and terminates the processing sequence. 
         [0058]    As described above, according to the first embodiment, the voltage boost converter and the inverter are driven in a discharge switching mode and when diagnosis conditions have been established, a discharge switching mode is continued by the inverter alone. During the continuation of the discharge switching mode by the inverter alone, the presence/absence of an abnormality in the voltage detection function is diagnosed by successively comparing the voltage detection values of the respective capacitors. 
         [0059]    As a result of this, it is possible to diagnose the presence/absence of an abnormality in the voltage detection function, with good accuracy, inexpensively and easily, by a simple configuration, in a broad voltage range from a voltage sensor input equivalent to the power source voltage, up to the discharge lower limit. In particular, even if unexpected variation occurs in the detection characteristics of the voltage sensor, it is possible to diagnose the presence/absence of an abnormality in the voltage detection function with high accuracy by carrying out measurement in a broad voltage range during a discharge operation. 
       Second Embodiment 
       [0060]    In the first embodiment above, an abnormality in the voltage detection characteristics is diagnosed by comparing and determining whether or not the sum of stored deviation values exceeds a first determination threshold value. On the other hand, in the second embodiment, an abnormality in the voltage detection characteristics is diagnosed in a simpler fashion, without determining the sum of the stored deviation values. 
         [0061]    The configuration of the power conversion device according to the second embodiment is the same as the configuration in  FIG. 1  relating to the first embodiment above and description thereof is omitted here. The process for diagnosing the voltage detection characteristics in the second embodiment is described with reference to a flowchart. 
         [0062]      FIG. 3  is a flowchart illustrating a processing sequence relating to the method for diagnosing an abnormality in a voltage detection function according to the second embodiment of the present invention. In comparison with  FIG. 2 , in  FIG. 3 , steps S 101  to S 110  and step S 112  are the same, and the processing in step S 111   a  is carried out instead of step S 111  in  FIG. 2 . Therefore, the description given below centers on step S 111   a , which is the point of difference. 
         [0063]    In the flowchart in  FIG. 3  of the second embodiment, steps S 101  to S 110  are executed in a similar manner to the flowchart in  FIG. 2  relating to the first embodiment above, and the abnormality diagnosis control unit  10  then advances to step S 111   a.    
         [0064]    Next, in step S 111   a , the abnormality diagnosis control unit  10  determines whether or not the any of the deviation values stored in the previous step S 108  exceeds a predetermined second determination threshold value FTH_Vsns2. If any of the stored deviation values exceeds the second determination threshold value FTH_Vsns2, then the abnormality diagnosis control unit  10  advances to step S 112  and diagnoses that the voltage detection characteristics are abnormal, and terminates the processing sequence. 
         [0065]    On the other hand, if none of the deviation values stored in the storage unit exceeds the second determination threshold value FTH_Vsns2, then the abnormality diagnosis control unit  10  diagnoses that the voltage detection characteristics are normal and terminates the processing sequence. 
         [0066]    In this way, according to the second embodiment, the presence/absence of an abnormality in the voltage detection function is diagnosed by determining whether or not any of the stored deviation values exceeds the second determination threshold value, rather than using the sum of the stored deviation values. By processing of this kind, it is also possible to obtain similar beneficial effects to the first embodiment described above. 
         [0067]    In the first and second embodiments described above, the deviation is determined by finding the deviation between the maximum value and minimum value of the detection voltage values V0, V1, V2 of each of the three voltage sensors, but the present invention is not limited to calculation of deviation of this kind. By setting a suitable threshold value, it is also possible to carry out diagnosis by using the deviation between the maximum value and the intermediate value, or the deviation between the intermediate value and the minimum value, and similar beneficial effects can be obtained. 
         [0068]    Furthermore, according to the present invention, it is possible to diagnose an abnormality over a broad voltage range, from a voltage equivalent to the DC power source voltage to a voltage band below the discharge lower limit. It is not necessary to carry out diagnosis up to the discharge lower limit voltage, and diagnosis may also be carried out up to the lower limit of the normal power conversion operating range, in accordance with the application.