Patent Publication Number: US-2023152375-A1

Title: Deterioration estimation device and deterioration estimation program for power conversion device

Description:
TECHNICAL FIELD 
     The present invention relates to a deterioration estimation device and a deterioration estimation program that estimate the deterioration of a switching element used in a power conversion device such as an inverter or a converter. 
     BACKGROUND ART 
     In a power conversion device such as a motor drive inverter or a converter for power transmission, power is controlled by controlling the switching width of a switching element for power conversion. The switching element is a key component of a power conversion device, and trouble occurring in the switching element has a large impact, such as causing a device that obtains power from the power conversion device to stop, or causing the entire system to stop. 
     To prevent trouble from occurring, measures such as replacing semiconductor switching elements for power conversion in an aging device with new switching elements are taken, but there are problems of not only heavy maintenance costs but also a shortage of maintenance personnel. Consequently, it is desirable to provide means for detecting the signs of trouble in a switching element for power conversion in advance and issuing an alarm before the trouble occurs. 
     In relation to detecting the signs of trouble in advance, the technology disclosed in, for example, Patent Literature 1 is known. The power conversion device disclosed in Patent Literature 1 is provided with leakage current detecting means for detecting a gate leakage current in a semiconductor switching element for power conversion, and element abnormality detecting means for outputting an element abnormality detection signal indicating signs of abnormality when the magnitude of the leakage current changes by a fixed value or greater with reference to the leakage current when the device began to be used. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2003-70231 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the power conversion device disclosed in Patent Literature 1 is newly provided with, as the leakage current detecting means between the gate and the emitter of the semiconductor switching element, a VGE detection circuit that detects the gate voltage applied between an interconnection point between the gate resistor and the gate of the semiconductor switching element, and the emitter of the semiconductor switching element. 
     In a power conversion device, even if means for measuring the input voltage and input current, the output voltage and output current, and the like is provided for conversion control, means for measuring the characteristics of a switching element and providing feedback is not usually provided. Consequently, with the power conversion device disclosed in Patent Literature 1, it is necessary to provide a new, special sensor for the switching element. 
     There is also a simple method of connecting a resistor and measuring the voltage to measure the leakage current between the gate and the emitter, but if a current flows from the gate control signal, the conditions change between when a switching element is on and when the switching element is off. In this case, the only option is to use special measuring means such as a clamp-on ammeter. 
     Accordingly, an objective of the present invention is to provide a deterioration estimation device and a deterioration estimation program for a power conversion device that can estimate the deterioration of a switching element without providing a special sensor. 
     Solution to Problem 
     A deterioration estimation device for a power conversion device of the present invention includes: calculating means for calculating a variation of characteristics of a switching element for power conversion in the power conversion device from a voltage command treated as a target for an output voltage of the power conversion device and an output voltage value of the power conversion device; determining means for determining whether the variation of the characteristics calculated by the calculating means has changed from an initial state by a threshold value or greater; and notifying means for issuing a warning when the determining means has determined that the change in the variation of the characteristics is equal to or greater than the threshold value, wherein the calculating means includes polarity determining means for determining a polarity of current according to an output current from the power conversion device formed from an upper arm and a lower arm by the switching element, and switching means for switching the variation of the characteristics according to a switching signal from the polarity determining means, and the determining means is provided in a pair for respectively determining the variation of the characteristics switched by the switching means. 
     Also, a deterioration estimation program for a power conversion device of the present invention causes a computer to function as: calculating means for calculating a variation of characteristics of a switching element for power conversion in the power conversion device from a voltage command treated as a target for an output voltage of the power conversion device and an output voltage value of the power conversion device; determining means for determining whether the variation of the characteristics calculated by the calculating means has changed from an initial state by a threshold value or greater; and notifying means for issuing a warning when the determining means has determined that the change in the variation of the characteristics is equal to or greater than the threshold value, wherein the calculating means includes polarity determining means for determining a polarity of current according to an output current from the power conversion device formed from an upper arm and a lower arm by the switching element, and switching means for switching the variation of the characteristics according to a switching signal from the polarity determining means, and the determining means is provided in a pair for respectively determining the variation of the characteristics switched by the switching means. 
     According to the present invention, by having the calculating means calculate the variation of the characteristics of the switching element from the voltage command for the power conversion device and the output voltage value of the power conversion device, and by having the determining means determine whether the change in the variation of the characteristics is equal to or greater than a threshold value, the notifying means issues a warning when the determining means has determined that the change in the variation of the characteristics is equal to or greater than the threshold value. The voltage command for the power conversion device and the output voltage value or output current of the power conversion device can be acquired from DC voltage measuring means provided in an inverter and output voltage measuring means or output current measuring means, and therefore it is not necessary to provide a special sensor. 
     Furthermore, since the switching means can switch between a pair of integrating means depending on the polarity of current determined by the polarity determining means, the change in the variation of the characteristics of each of the upper arm and the lower arm can be calculated. Consequently, signs of trouble in the upper arm and the lower arm can be estimated by the pair of determining means. 
     The determining means may store and treat the variation of the characteristics corresponding to the output current of the power conversion device as an initial state, and determine the threshold value on the basis of the initial state. With this configuration, the threshold value for the variation of the characteristics can be determined according to the output current, and therefore the deterioration of a switching element can be estimated accurately. 
     The calculating means may include subtracting means for calculating a difference between the voltage command and the output voltage value, and integrating means for outputting, to the determining means, a differential integral value obtained by integrating the difference calculated by the subtracting means. Since the difference between the voltage command and the output voltage value calculated by the subtracting means is integrated by the integrating means, tiny fluctuations or the like in the voltage value can be ignored, and since the difference is accumulated, signs of deterioration can be magnified. 
     The integrating means may output an integral value from a definite integral over a predetermined interval as the differential integral value. By calculating the differential integral value with a definite integral over an interval, divergent integral values can be suppressed. 
     The calculating means may include first averaging means for calculating an average value of an input voltage value into the power conversion device at respective time intervals synchronized with a carrier signal, multiplying means for multiplying the average value of the input voltage value from the first averaging means by a modulation ratio to calculate and output the voltage command to the subtracting means, and second averaging means for calculating and outputting, to the subtracting means, an average value of the output voltage value of the power conversion device at respective time intervals synchronized with the carrier signal. By averaging the input voltage value with the first averaging means and averaging the output voltage value with the second averaging means, during maintenance, a worker can observe the waveforms to grasp the operating state of the power conversion device. 
     Advantageous Effect of Invention 
     According to the present invention, the deterioration of a switching element can be estimated without providing a special sensor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating an example of a power conversion device. 
         FIG.  2    is a diagram illustrating a deterioration estimation device according to Embodiment 1 of the present invention. 
         FIG.  3 ( a )  is a diagram illustrating an example of waveforms indicating the average value of a U-phase voltage command from the multiplying means and the average value of the output voltage value from the second averaging means when the polarity of the output current is positive, and  FIG.  3 ( b )  is a diagram illustrating an example of waveforms indicating the average value of the U-phase voltage command from the multiplying means and the average value of the output voltage value from the second averaging means when the polarity of the output current is negative. 
         FIG.  4    is a diagram illustrating an example of waveforms of an integral value from the integrating means when a three-phase AC motor is driven by an inverter, in which  FIG.  4 ( a )  is a diagram illustrating an example of the waveform of the output from one integrating means when the delay of the upper and lower arms is in an initial state and the polarity of the output current is positive, and  FIG.  4 ( b )  is a diagram illustrating an example of the waveform of the output from the other integrating means in the delay state of  FIG.  4 ( a )  when the polarity of the output current is negative. 
         FIG.  5    is a diagram illustrating an example of waveforms of an integral value from the integrating means when a three-phase AC motor is driven by an inverter, in which  FIG.  5 ( a )  is a diagram illustrating an example of the waveform of the output from one integrating means when the off-delay of the upper arm has increased, the delay of the lower arm is still in the initial state, and the polarity of the output current is positive, and  FIG.  5 ( b )  is a diagram illustrating an example of the waveform of the output from the other integrating means in the delay state of  FIG.  5 ( a )  when the polarity of the output current is negative. 
         FIG.  6    is a diagram illustrating an example of waveforms of an integral value from the integrating means when a three-phase AC motor is driven by an inverter, in which  FIG.  6 ( a )  is a diagram illustrating an example of the waveform of the output from one integrating means when the delay of the upper arm is still in the initial state, the off-delay of the lower arm has increased, and the polarity of the output current is positive, and  FIG.  6 ( b )  is a diagram illustrating an example of the waveform of the output from the other integrating means in the delay state of  FIG.  6 ( a ) . 
         FIG.  7    is a diagram illustrating an example of waveforms of the differential integral value over an interval from the integrating means when a three-phase AC motor is driven by an inverter, in which  FIG.  7 ( a )  is a diagram illustrating an example of the waveform of the output from one integrating means when the delay of the upper and lower arms is in an initial state and the polarity of the output current is positive, and  FIG.  7 ( h )  is a diagram illustrating an example of the waveform of the output from the other integrating means in the delay state of  FIG.  7 ( a )  when the polarity of the output current is negative. 
         FIG.  8    is a diagram illustrating an example of waveforms of the differential integral value over an interval from the integrating means when a three-phase AC motor is driven by an inverter, in which  FIG.  8 ( a )  is a diagram illustrating an example of the waveform of the output from one integrating means when the off-delay of the upper arm has increased, the delay of the lower arm is still in the initial state, and the polarity of the output current is positive, and  FIG.  8 ( b )  is a diagram illustrating an example of the waveform of the output from the other integrating means in the delay state of  FIG.  8 ( a )  when the polarity of the output current is negative. 
         FIG.  9    is a diagram illustrating an example of waveforms of the differential integral value over an interval from the integrating means when a three-phase AC motor is driven by an inverter, in which FIG.  9 ( a ) is a diagram illustrating an example of the waveform of the output from one integrating means when the delay of the upper arm is still in the initial state, the off-delay of the lower arm has increased, and the polarity of the output current is positive, and  FIG.  9 ( b )  is a diagram illustrating an example of the waveform of the output from the other integrating means in the delay state of  FIG.  9 ( a )  when the polarity of the output current is negative. 
         FIG.  10    is a diagram for explaining that the output voltage of the power conversion device is treated as a line voltage rather than a phase voltage. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     A deterioration estimation device for a power conversion device according to Embodiment 1 of the present invention will be described on the basis of the drawings. 
     In Embodiment 1, the power conversion device is an inverter  10 , as illustrated in  FIG.  1   . First, the configuration of the inverter  10  will be described. 
     The inverter  10  according to Embodiment 1 accepts direct current and outputs three-phase alternating current. As illustrated in  FIG.  1   , a three-phase AC motor M is connected to the inverter  10 . 
     The inverter  10  includes a first arm  11  to a third arm  13 , in which upper arms  111 ,  121 ,  131  and lower arms  112 ,  122 ,  132  are respectively connected in series. 
     The upper arms  111  to  131  are connected to a positive power line P by first wiring lines  141   a  to  141   c.  The lower arms  112  to  132  are connected to a negative power line N by second wiring lines  142   a  to  142   c.  The upper arms  111  to  131  and the lower arms  112  to  132  are connected to each other by third wiring lines  143   a  to  143   c.    
     The upper arms  111  to  131  and the lower arms  112  to  132  are configured by a switching element and a freewheeling diode. The switching element is formed from a semiconductor device. For example, a bipolar transistor, a metal-oxide-semiconductor field-effect transistor (MOSFET), an insulated-gate bipolar transistor (IGBT), or the like can be used as the switching element. In Embodiment 1, an IGBT which allows large currents to flow through and which has a fast switching speed is used as the switching element. 
     The inverter  10  is provided with controlling means for controlling the switching of the upper arms  111  to  131  and the lower arms  112  to  132  through pulse-width modulation (PWM). 
     The controlling means is provided with voltage commanding means  15  for outputting a voltage command as a target voltage for the U to W phases, modulation ratio calculating means  16  for outputting a modulation ratio signal from the voltage command from the voltage commanding means  15  and an input voltage value indicated by an input voltage value signal, oscillating means  17  for outputting a carrier signal, comparing means  18  for comparing the modulation ratio signal to a triangle wave signal and outputting a switching signal (gate signal), and inverting means  19  for inverting the switching signals to the upper arms  111  to  131  for the lower arms  112  to  132 . 
     The voltage commanding means  15  is provided with U-phase voltage commanding means  151  for the U phase, V-phase voltage commanding means  152  for the V phase, and W-phase voltage commanding means  153  for the W phase. 
     From the U-phase voltage commanding means  151 , V-phase voltage commanding means  152 , and W-phase voltage commanding means  153 , a U-phase voltage command, V-phase voltage command, and W-phase voltage command are outputted. 
     The modulation ratio calculating means  16  is a divider that outputs the modulation ratio by dividing the voltage command from the voltage commanding means  15  by an input voltage value indicated by an input voltage value signal measured by DC voltage measuring means described later. The modulation ratio can be set from −1.0 to 1.0. 
     The modulation ratio calculating means  16  is provided with U-phase modulation ratio calculating means  161  for the U phase, V-phase modulation ratio calculating means  162  for the V phase, and W-phase modulation ratio calculating means  163  for the W phase. 
     From the U-phase modulation ratio calculating means  161 , V-phase modulation ratio calculating means  162 , and W-phase modulation ratio calculating means  163 , voltage commands (U-phase voltage command, V-phase voltage command, W-phase voltage command) are outputted to the comparing means  18 , and in addition, modulation ratios are outputted through a U-phase modulation ratio signal, a V-phase modulation ratio signal, and a W-phase modulation ratio signal. 
     The oscillating means  17  outputs a triangle wave signal as a carrier signal. 
     The comparing means  18  is provided with U-phase comparing means  181  for the U phase, V-phase comparing means  182  for the V phase, and W-phase comparing means  183  for the W phase. 
     The inverting means  19  is provided with U-phase inverting means  191  for the U phase, V-phase inverting means  192  for the V phase, and W-phase inverting means  193  for the W phase. 
     Additionally, the inverter  10  is provided with DC voltage measuring means  21  for measuring the input voltage value, a voltage divider  22  for acquiring the reference potentials of the output voltages (U-phase voltage, V-phase voltage, W-phase voltage) from a connection point between a resistor  22   a  and a resistor  22   b  connected in series between the positive power line P and the negative power line N, output voltage measuring means  23  for measuring the output voltage of the inverter  10 , and output current measuring means  24  for measuring the output current of the inverter  10 . 
     The DC voltage measuring means  21  illustrated in  FIG.  1    outputs the voltage value of the input voltage as the input voltage value signal. 
     The resistors  22   a  and  22   b  in the voltage divider  22  are set to the same resistance value for treating a medium voltage of the input voltage as a reference potential. 
     The output voltage measuring means  23  measures the output voltage of each phase. The output voltage measuring means  23  is provided with U-phase output voltage measuring means  231  for the U phase, V-phase output voltage measuring means  232  for the V phase, and W-phase output voltage measuring means  233  for the W phase. 
     From the U-phase output voltage measuring means  231 , V-phase output voltage measuring means  232 , and W-phase output voltage measuring means  233 , output voltage values are outputted through a U-phase output voltage value signal, a V-phase output voltage value signal, and a W-phase output voltage value signal. 
     The output current measuring means  24  measures the output current of each phase. Any of various types of current sensors can be used as the output current measuring means  24 . For example, various types of sensors such as a CT sensor, a Hall effect sensor, or a Rogowski coil sensor can be used as the output current measuring means  24 . The output current measuring means  24  is provided with U-phase output current measuring means  241  for the U phase, V-phase output current measuring means  242  for the V phase, and W-phase output current measuring means  243  for the W phase. From the U-phase output current measuring means  241 , V-phase output current measuring means  242 , and W-phase output current measuring means  243 , output current values are outputted through a U-phase output current signal, a V-phase output current signal, and a W-phase output current signal. 
     Next, the configuration of the deterioration estimation device that estimates the deterioration of the IGBTs (upper arms  111  to  131  and lower arms  112  to  132  of the first arm  11  to the third arm  13 ) which are the switching elements in the inverter  10  will be described on the basis of the drawings. 
     The deterioration estimation device  30  illustrated in  FIG.  2    is achieved by causing a computer to run a deterioration estimation program. 
     In addition, the deterioration estimation device  30  is provided with calculating means  31 , determining means  32 , and notifying means  33 . 
     The calculating means  31  calculates the delay, which is one example of a variation of the characteristics of a switching element, from a voltage command treated as a target for the output voltage of the inverter  10  illustrated in  FIG.  1    and an output voltage value from the inverter  10 . 
     The calculating means  31  is provided with U-phase calculating means  311  for the U phase, V-phase calculating means  312  for the V phase, and W-phase calculating means  313  for the W phase. The U-phase calculating means  311 , the V-phase calculating means  312 , and the W-phase calculating means  313  have the same configuration. 
     Each calculating means  31  is provided with first averaging means  31   a,  second averaging means  31   b,  multiplying means  31   c,  subtracting means  31   d,  integrating means  31   e,  polarity determining means  31   f,  and switching means  31   g  for switching. 
     The first averaging means  31   a  averages the input voltage value when the first arm  11  to the third arm  13  are on and off at respective time intervals synchronized with a triangle wave signal (carrier signal). 
     The second averaging means  31   b  averages the voltage value indicated by the output voltage value signals (U-phase output voltage value signal, V-phase output voltage value signal, W-phase output voltage value signal) when the first arm  11  to the third arm  13  are on and off at respective time intervals synchronized with the triangle wave signal (carrier signal). 
     The multiplying means  31   c  multiplies the voltage value from the first averaging means  31   a  by the modulation ratio signals (U-phase modulation ratio signal, V-phase modulation ratio signal, W-phase modulation ratio signal) from the voltage commanding means  15  (U-phase voltage commanding means  151 , V-phase voltage commanding means  152 , W-phase voltage commanding means  153 ). 
     The subtracting means  31   d  subtracts the voltage value from the second averaging means  31   b  from the voltage value from the multiplying means  31   c.    
     The integrating means  31   e  outputs the differential integral value obtained by integrating the voltage value from the subtracting means  31   d,  and is provided in a pair. 
     The polarity determining means  31   f  determines the polarity of current according to the output current indicated by the output current signal from the output current measuring means  24  (see  FIG.  1   ) for each phase. 
     The switching means  31   g  switches the signal from the subtracting means  31   d  to one of the integrating means  31   e  or the other of the integrating means  31   e  according to a switching signal from the polarity determining means  31   f.    
     The determining means  32  detects whether the voltage value from the calculating means  31  has changed from the initial state by a threshold value or greater, and thereby detects a delay abnormality of the upper arms  111  to  131  and the lower arms  112  to  132  in the first arm  11  to the third arm  13  which are the switching elements. The threshold value can be set to any value according to the individual characteristics, individual differences, and loads of the switching elements. 
     The determining means  32  is provided with U-phase determining means  321  for the U phase, V-phase determining means  322  for the V phase, and W-phase determining means  323  for the W phase. 
     The U-phase determining means  321  for the U phase, V-phase determining means  322  for the V phase, and W-phase determining means  323  for the W phase are provided in pairs for each phase to determine the differential integral value from each of one of the integrating means  31   e  and the other of the integrating means  31   e.    
     The notifying means  33  notifies a user, a worker, or a person in charge of maintenance about the detection of an abnormality from the determining means  32 . 
     The operations of the deterioration estimation device  30  according to Embodiment 1 of the present invention configured as above will be described on the basis of the drawings. Note that a description of the U phase will be represented for the description of Embodiment 1. 
     Description of Operations of Inverter  10   
     First, the operations of the inverter  10  illustrated in  FIG.  1    will be described. 
     A voltage command signal is outputted from the U-phase voltage commanding means  151 . On the basis of the voltage command designated by the voltage command signal, the U-phase modulation ratio calculating means  161  outputs a U-phase modulation ratio signal indicating the modulation ratio. 
     The U-phase comparing means  181  compares the voltage according to the triangle wave signal from the oscillating means  17  to the U-phase voltage command from the U-phase voltage commanding means  151  (U-phase modulation ratio calculating means  161 ), and in the period in which the U-phase voltage command is higher than the triangle wave signal, outputs an upper-arm PWM signal for turning on the upper arm  111  of the first arm  11 . Also, in the period in which the U-phase command voltage is lower than the triangle wave signal, the U-phase inverting means  191  outputs a lower-arm PWM signal for turning on the lower arm  112  of the first arm  11 . By repeating the above operations, the three-phase AC motor M can obtain alternating current as the U-phase output. 
     Description of Operations of Deterioration Estimation Device  30   
     As an advance sign of trouble in an IGBT (switching element), it is anticipated that the output voltage of the inverter  10  will no longer follow the voltage command (the error will be larger than normal). 
     Accordingly, first, as illustrated in  FIG.  2   , the deterioration estimation device  30  uses the first averaging means  31   a  to average the input voltage value indicated by the input voltage value signal from the DC voltage measuring means  21  (see  FIG.  1   ) in a half cycle of the triangle wave signal from the oscillating means  17 . Additionally, the multiplying means  31   c  multiplies the average value of the input voltage value from the first averaging means  31   a  by the modulation ratio indicated by the U-phase modulation ratio signal from the U-phase voltage commanding means  151 . Through the multiplication by the multiplying means  31   c,  the average value of the U-phase voltage command is calculated. 
     Also, on the basis of the triangle wave signal from the oscillating means  17 , the second averaging means  31   b  averages the output voltage value indicated by the U-phase output voltage value signal from the U-phase output voltage measuring means  231  (see  FIG.  1   ) in a half cycle of the triangle wave signal. 
     In Embodiment 1, the first averaging means  31   a  and the second averaging means  31   b  use a half cycle of the triangle wave signal, but it is sufficient for the timing to be synchronized with the triangle wave signal, and therefore a full cycle may also be used. 
       FIG.  3 ( a )  illustrates the average value of the U-phase voltage command from the multiplying means  31   c  and the average value of the output voltage value from the second averaging means  31   b.  In this way, by averaging the U-phase voltage command and averaging the output voltage value, the respective waveforms of the voltage command and the output voltage value can be easily understood visually. 
     Next, the subtracting means  31   d  calculates the difference by subtracting the average value of the U-phase output voltage value from the second averaging means  31   b  from the average value of the U-phase voltage command from the multiplying means  31   c,  and thereby calculates the delay of the upper arm  111  of the first arm  11  which is a switching element. 
     Thereafter, the polarity determining means  31   f  determines the polarity of current according to the output current indicated by the U-phase output current signal, and switches the switching means  31   g  according to the switching signal. 
     According to the switching signal, when the polarity of the output current is positive, the difference from the subtracting means  31   d  is integrated by one of the integrating means  31   e  (in  FIG.  2   , the integrating means  31   e  on the top). Also, when the polarity of the output current is negative, the difference from the subtracting means  31   d  is integrated by the other of the integrating means  31   e  (in  FIG.  2   , the integrating means  31   e  on the bottom). 
     Here,  FIG.  3 ( a )  illustrates the average value of the U-phase voltage command from the multiplying means  31   c  and the average value of the output voltage value from the second averaging means  31   b  when the polarity of the output current is positive. 
     Also,  FIG.  3 ( b )  illustrates the average value of the U-phase voltage command from the multiplying means  31   c  and the average value of the output voltage value from the second averaging means  31   b  when the polarity of the output current is negative. 
     In addition, the output (variation of characteristics) from one of the integrating means  31   e  (in  FIG.  2   , refer to the integrating means  31   e  on the top) when the off-delays of the upper arm  111  and the lower arm  112  are in an initial state (4 μs) is illustrated in  FIG.  4 ( a ) . 
     Moreover, the output from the other of the integrating means  31   e  (in  FIG.  2   , refer to the integrating means  31   e  on the bottom) when the delay is the same as the delay state in  FIG.  4 ( a )  is illustrated in  FIG.  4 ( b ) . 
     The waveforms illustrated in  FIGS.  4 ( a ) and  4 ( b )  are obtained in a state in which the three-phase AC motor M (see  FIG.  1   ) has a fixed number of revolutions and a fixed load, and are obtained by starting the integrations by the integrating means  31   e,    31   e  from the time 10 s. 
     As  FIGS.  4 ( a ) and  4 ( b )  demonstrate, the outputs from the integrating means  31   e,    31   e  rise or fall on a prescribed slope. 
     For example, if both the upper arms  111  to  131  and the lower arms  112  to  132  illustrated in  FIG.  1    deteriorate, the characteristics change, and the off-delay increases, the slopes of both waveforms illustrated in  FIGS.  4 ( a ) and  4 ( b )  will be gentle. At this time, if there is no switching means  31   g  and the difference from the subtracting means  31   d  is integrated by singular integrating means  31   e,  the positive and negative values will be added together and cancel each other out, and consequently, signs of abnormality cannot be detected. 
     However, in the deterioration estimation device  30 , it is determined whether the upper arms  111  to  131  or the lower arms  112  to  132  have deteriorated on the basis of the direction of the current outputted from the first arm  11  to the third arm  13 . 
     As an example of a change in the characteristics of the upper arm  111 , assume that the delay when being switched off has increased from 4 μs to 8 μs. Note that the off-delay of the lower arm  112  is still in the initial state (a delay of 4 μs). 
     The output from one of the integrating means  31   e  at this time is illustrated in  FIG.  5 ( a ) . Also, the output from the other of the integrating means  31   e  is illustrated in  FIG.  5 ( b ) . 
     The waveforms illustrated in  FIGS.  5 ( a ) and  5 ( b ) , similarly to  FIG.  4   , are also obtained in a state in which the three-phase AC motor M (see  FIG.  1   ) has a fixed number of revolutions and a fixed load, and are obtained by starting the integrations by the integrating means  31   e,    31   e  from the time 10 s. 
     In  FIG.  5 ( b ) , there is no change from  FIG.  4 ( b ) , but in  FIG.  5 ( a )  it can be seen that the slope of the off-delay has become gentler from the initial state. 
     Consequently, the variation of the characteristics is such that the difference from the initial state is larger, and one of the U-phase determining means  321  can determine that the upper arm  111  has deteriorated by detecting that the change from the initial state is equal to or greater than a threshold value. Moreover, one of the U-phase determining means  321  can inform the notifying means  33  of a U-phase first alarm signal. 
     Next, assume that, for example, the off-delay of the upper arm  111  is still in the initial state (a delay of 4 μs), but the delay of the lower arm  112  when being switched off has increased from 4 μs to 8 μs. The output from one of the integrating means  31   e  at this time is illustrated in  FIG.  6 ( a ) . Also, the output from the other of the integrating means  31   e  is illustrated in  FIG.  6 ( b ) . 
     The waveforms illustrated in  FIGS.  6 ( a ) and  6 ( b ) , similarly to  FIG.  4   , are also obtained in a state in which the three-phase AC motor M (see  FIG.  1   ) has a fixed number of revolutions and a fixed load, and are obtained by starting the integrations by the integrating means  31   e,    31   e  from the time 10 s. 
     In  FIG.  6 ( a ) , there is no change from  FIG.  4 ( a ) , but in  FIG.  6 ( b )  it can be seen that the slope of the delay has become gentler from the initial state. 
     Consequently, the difference from the initial state is larger, and the other of the U-phase determining means  321  can determine that the lower arm  112  has deteriorated by detecting that the change from the initial state is equal to or greater than a threshold value. Moreover, the other of the U-phase determining means  321  can inform the notifying means  33  of a U-phase second alarm signal. 
     Furthermore, in the case where both off-delays of the upper arm  111  and the lower arm  112  have increased from 4 μs to 8 μs, the output voltage of the upper arm  111  for which the current is a positive value becomes the waveform illustrated by  FIG.  5 ( a ) , and the output voltage of the lower arm  112  for which the current has a negative value becomes the waveform illustrated by  FIG.  6 ( b ) . 
     Consequently, since it is understood that a change has occurred from the waveforms illustrated in  FIG.  5 ( b )  and  FIG.  6 ( a )  which illustrate the normal state, by detecting that the change has become equal to or greater than a threshold value, the notifying means  33  can be informed of a U-phase first alarm signal with which the deterioration of both the upper arm  111  and the lower arm  112  can be estimated. 
     Also, by using the integrating means  31   e,    31   e  illustrated in  FIG.  2    to integrate the on-delays and off-delays (difference) from the subtracting means  31   d,  tiny fluctuations or the like in the voltage value can be ignored, and since the difference is accumulated, signs of deterioration can be magnified. 
     In addition, by informing the notifying means  33  of the abnormality, a user, an administrator, a person in charge of maintenance, or the like can be informed of the possibility of trouble in the inverter  10  (see  FIG.  1   ). The notification can be issued according to various methods, such as with light from a lamp or the like, with sound from a speaker, or through email over the Internet. 
     As above, according to the deterioration estimation device  30  according to Embodiment 1, by having the calculating means  31  calculate the variation of the characteristics of an IGBT from the voltage command treated as a target for the output voltage of the inverter  10  and the output voltage value from the inverter  10 , and by having the determining means  32  determine whether the change in the variation of the characteristics is equal to or greater than a threshold value, the notifying means  33  can issue a warning when the determining means  32  has determined that the change is equal to or greater than the threshold value. 
     The voltage command with respect to the inverter  10  and the output voltage value and output current of the inverter  10  can be acquired from the DC voltage measuring means  21 , the output voltage measuring means  23 , and the output current measuring means  24  provided in the inverter  10 , and therefore the deterioration estimation device  30  can estimate the deterioration of an IGBT which is a switching element without providing a special sensor. 
     Also, by switching the integrating means  31   e  and making respective determinations when the polarity of the output current is positive and when the polarity is negative, it is possible to determine whether the upper arm  111 , the lower arm  112 , or both have deteriorated. 
     Furthermore, since the calculating means  31  and the determining means  32  are provided respectively for each of the U, V, and W phases, the deterioration in each of the phases can be detected. 
     In the above description, an example of an increase in the off-delay is described in regard to detecting the variation of the characteristics through subtraction of the voltage command to the inverter  10 , which is one example of a power conversion device, and the output voltage value from the inverter  10 . However, it is also possible to detect the on-delay, the off-delay, the on-voltage, the on-resistance, dv/dt (rate of voltage change), or the like as a change in the characteristics of a switching element. 
     In Embodiment 1, the calculating means  31  uses the first averaging means  31   a  to average the input voltage value indicated by the input voltage value signal, and also uses the second averaging means  31   b  to average the output voltage value. With this configuration, the waveforms illustrated in  FIGS.  3 ( a ) and  3 ( b )  can be obtained, and therefore a worker can observe the waveforms during maintenance or the like and easily grasp the operating state of the inverter  10 . 
     However, if it is not necessary to obtain the waveforms illustrated in  FIGS.  3 ( a ) and  3 ( b ) , the first averaging means  31   a  and the second averaging means  31   b  can be omitted since the difference is integrated by the integrating means  31   e  downstream from the subtracting means  31   d.    
     Embodiment 2 
     Next, a deterioration estimation device according to Embodiment 2 of the present invention will be described. 
     In the pair of integrating means  31   e  illustrated in  FIG.  2   , since the difference is integrated, the differential integral value increasingly diverges over time. 
     Accordingly, in the integration by the integrating means  31   e,  a definite integral over a predetermined interval (interval integral) is evaluated to obtain the differential integral value. Hereinafter, this operation is referred to as the interval integral. 
     The interval integral can be found by the following expression (1) when the interval is set to 5 s, for example. 
     [Expression 1] 
       ∫ T−5   T e(t)dt   (1)
 
     Here, T is any time and e(t) is the integral value from the integrating means  31   e.    
       FIGS.  7  to  9    illustrate waveforms when the three-phase AC motor M illustrated in  FIG.  1    is driven by the inverter  10  and the differential integral value from the integrating means  31   e  is calculated by setting the predetermined interval to 5 s, for example. 
     The waveforms illustrated in  FIGS.  7 ( a ) and  7 ( b )  correspond to  FIG.  4    which illustrates the waveforms when the off-delay of the upper arm  111  and the off-delay of the lower arm  112  are in the initial state (4 μs), and illustrate the case where the integral of the difference is replaced by the interval integral. 
     Also, the waveforms illustrated in  FIGS.  8 ( a ) and  8 ( b )  correspond to  FIGS.  5 ( a ) and  5 ( b )  which illustrate the waveforms when the off-delay of the upper arm  111  is increased from 4 μs to 8 μs and the off-delay of the lower arm  112  is in the initial state (4 μs), and illustrate the case where the integral of the difference is replaced by the interval integral. 
     Moreover, the waveforms illustrated in  FIGS.  9 ( a ) and  9 ( b )  correspond to  FIGS.  6 ( a ) and  6 ( b )  which illustrate the waveforms when the off-delay of the upper arm  111  is in the initial state (4 μs) and the off-delay of the lower arm  112  is increased from 4 μs to 8 μs, and illustrate the case where the integral of the difference is replaced by the interval integral. 
     In Embodiment 2, the integral is over a 5 s interval and therefore becomes a constant value 5 s after the integral start (time 10 s) as illustrated in  FIGS.  7  to  9   . 
     In both  FIGS.  8  and  9   , the slopes of the waveforms become gentler with increasing delay, similarly to the waveforms illustrated in  FIGS.  4  to  6   , and therefore by detecting that the change from the initial state is equal to or greater than a threshold value, the deterioration of the delay characteristics of a switching element can be detected. 
     Also, as illustrated in  FIGS.  8  and  9   , by having the integrating means  31   e  evaluate an interval integral to obtain the differential integral value, divergent integral values can be suppressed. 
     Embodiment 3 
     Next, a deterioration estimation device according to Embodiment 3 of the present invention will be described. 
     The delay of the IGBTs constituting the first arm  11  to the third arm  13  in the inverter  10  illustrated in  FIG.  1    is calculated by the differential integral value from the pair of integrating means  31   e  illustrated in  FIG.  2   . However, the differential integral value is influenced by the load current. For example, the delay of the IGBTs constituting the first arm  11  to the third arm  13  is different between the unloaded state and the state in which a predetermined current is flowing as the output current. 
     Accordingly, in the determining means  32 , the output voltage from the first arm  11  to the third arm  13  of each phase is inputted, the differential integral value indicating the delay corresponding to the output voltage is stored and treated as an initial state, and a threshold value is determined on the basis of the initial state. With this configuration, the threshold value for the variation of the characteristics can be determined according to the load current (output current), and therefore the deterioration of a switching element (IGBT) can be estimated accurately. 
     Note that Embodiments 1 to 4 describe an example of a two-level inverter  10  including upper arms  111  to  131  and lower arms  112  to  132 , as illustrated in  FIG.  1   , but the deterioration estimation device of the present invention can also be applied to a three-level inverter. 
     Also, in the embodiments, as illustrated in  FIG.  1   , the output voltage measuring means  23  including the U-phase output voltage measuring means  231  for the U phase, the V-phase output voltage measuring means  232  for the V phase, and the W-phase output voltage measuring means  233  for the W phase measures and outputs the output voltage of each phase as a U-phase output voltage value signal, a V-phase output voltage value signal, and a W-phase output voltage value signal. 
     However, the output voltage may not only be treated as a phase voltage but also as a line voltage. 
     For example, that which outputs the output voltage to the U-phase calculating means  311  illustrated in  FIG.  2    can be configured to be a U-V output voltage value signal from U-V output voltage measuring means  251  as illustrated in  FIG.  10    instead of the U-phase output voltage value signal. In this case, a signal of “U-phase modulation ratio signal-V-phase modulation ratio signal” is used instead of the U-phase modulation ratio signal of the U-phase calculating means  311 . 
     For example, that which outputs the output voltage to the V-phase calculating means  312  illustrated in  FIG.  2    can be configured to be a V-W output voltage value signal from V-W output voltage measuring means  252  as illustrated in  FIG.  10    instead of the V-phase output voltage value signal. In this case, a signal of “V-phase modulation ratio signal-W-phase modulation ratio signal” is used instead of the V-phase modulation ratio signal of the V-phase calculating means  312 . 
     Moreover, that which outputs the output voltage to the W-phase calculating means  313  illustrated in  FIG.  2    can be configured to be a W-U output voltage value signal from W-U output voltage measuring means  253  as illustrated in  FIG.  10    instead of the W-phase output voltage value signal. In this case, a signal of “W-phase modulation ratio signal-U-phase modulation ratio signal” is used instead of the W-phase modulation ratio signal of the W-phase calculating means  313 . 
     Even if such output voltage measuring means  25  (U-V output voltage measuring means  251 , V-W output voltage measuring means  252 , W-U output voltage measuring means  253 ) for measuring line voltages is used, the output voltage can be measured and the calculating means  31  can calculate the delay which is one example of a variation of the characteristics of a switching element. 
     INDUSTRIAL APPLICABILITY 
     The present invention can estimate the deterioration of a power conversion device and is therefore suitable for a power supply that supplies power to a mission-critical system from a simple power supply or the like. 
     REFERENCE SIGNS LIST 
       10  inverter 
       11  first arm 
       12  second arm 
       13  third arm 
       111 ,  121 ,  131  upper arm 
       112 ,  122 ,  132  lower arm 
       141   a  to  141   c  first wiring line 
       142   a  to  142   c  second wiring line 
       143   a  to  143   c  third wiring line 
       15  voltage commanding means 
       151  U-phase voltage commanding means 
       152  V-phase voltage commanding means 
       153  W-phase voltage commanding means 
       16  modulation ratio calculating means 
       161  U-phase modulation ratio calculating means 
       162  V-phase modulation ratio calculating means 
       163  W-phase modulation ratio calculating means 
       17  oscillating means 
       18  comparing means 
       181  U-phase comparing means 
       182  V-phase comparing means 
       183  W-phase comparing means 
       19  inverting means 
       191  U-phase inverting means 
       192  V-phase inverting means 
       193  W-phase inverting means 
       21  DC voltage measuring means 
       22  voltage divider 
       22   a,    22   b  resistor 
       23  output voltage measuring means 
       231  U-phase output voltage measuring means 
       232  V-phase output voltage measuring means 
       233  W-phase output voltage measuring means 
       24  output current measuring means 
       241  U-phase output current measuring means 
       242  V-phase output current measuring means 
       243  W-phase output current measuring means 
       25  output voltage measuring means 
       251  U-V output voltage measuring means 
       252  V-W output voltage measuring means 
       253  W-U output voltage measuring means 
       30  deterioration estimation device 
       31  calculating means 
       311  U-phase calculating means 
       312  V-phase calculating means 
       313  W-phase calculating means 
       31   a  first averaging means 
       31   b  second averaging means 
       31   c  multiplying means 
       31   d  subtracting means 
       31   e  integrating means 
       31   f  polarity determining means 
       31   g  switching means 
       32  determining means 
       321  U-phase determining means 
       322  V-phase determining means 
       323  W-phase determining means 
       33  notifying means 
     P positive power line 
     N negative power line 
     M three-phase AC motor