Patent Publication Number: US-2023152359-A1

Title: Electricity leakage detection device and vehicle power supply system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a leakage detecting device and a vehicle power supply system that detect an electric leakage from a load insulated from the earth. 
     BACKGROUND ART 
     Hybrid vehicles (HV), plug-in hybrid vehicles (PHV), and electric vehicles (EV) have become widespread in recent years. These electric vehicles are equipped with auxiliary batteries (which are, in general, lead batteries with power output of 12 V) and with drive batteries for high-voltage use (traction batteries), which are separated from the auxiliary batteries. To prevent an electric shock accident, a high-voltage circuit including a drive battery for high voltage use, an inverter, and a running motor is insulated from the body (chassis earth) of a vehicle. 
     A Y capacitor is inserted between the positive power supply line and the chassis earth of the high-voltage circuit on the vehicle and between the negative power supply line and the chassis earth of the high-voltage circuit on the vehicle, so that the power source supplied from the high-voltage driving battery to the load on the vehicle is stabilized. A leakage detecting device is incorporated in a vehicle to monitor insulation resistance between the high-voltage circuit and the chassis earth and detect a leakage. 
     A leakage detecting device of an AC type applies a pulse voltage to a positive-electrode terminal or a negative-electrode terminal of the drive battery via a resistance and a coupling capacitor, and measures a voltage at a node between the resistance and the coupling capacitor, thereby detecting the presence or absence of a leakage. When an earth fault occurs, the impedance at the measurement point decreases, and the voltage at the measurement point decreases. Therefore, when the voltage at the measurement point is less than or equal to a threshold, it can be determined that an electrical leakage has occurred (see, for example, PTL 1). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Unexamined Japanese Patent Publication No. H08-70503 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In this method, as the capacitance of the Y capacitor increases, the measurement accuracy of the insulation resistance connected in parallel with the Y capacitor decreases. When the capacitance of the Y capacitor increases, the impedance of the Y capacitor decreases, so that the influence of the fluctuation of the Y capacitor on the calculated value of the insulation resistance increases. 
     In order to maintain the measurement accuracy of the insulation resistance even when the capacitance of the Y capacitor is large, it is conceivable to reduce the driving frequency of the applied pulse. In this case, the impedance of the coupling capacitor increases, and the influence of the error of the coupling capacitor on the calculated value of the insulation resistance increases. In order to reduce the influence of the error of the coupling capacitor, it is conceivable to increase the capacitance of the coupling capacitor. Since the coupling capacitor is a capacitor that connects the high voltage unit and the low voltage unit, a capacitor with a high withstand voltage is required. When a capacitor having a high capacity and a high withstand voltage is used, the cost increases. 
     The present disclosure has been made in view of such a situation, and an object of the present disclosure is to provide a technique for detecting an electric leakage with high accuracy at low cost even when the capacitance of the Y capacitor is large. 
     Solution to Problem 
     In order to solve the above problem, a leakage detecting device according to an aspect of the present disclosure is a leakage detecting device that detects an electric leakage from a high-voltage unit in which a power storage and a load are connected by a power supply line in a state of being insulated from an earth. A Y capacitor is connected between a current path between the power storage unit and the load and the earth. The leakage detecting device includes: a drive unit configured to output a rectangular wave voltage to the current path between the power storage unit and the load via a driving coupling capacitor to charge or discharge the Y capacitor; and a leakage detector configured to determine presence or absence of a leakage between the current path and the earth according to a time transition in which a voltage at a measurement point of the current path changed according to charging or discharging of the Y capacitor converges to an original voltage. 
     Advantageous Effect of Invention 
     According to the present disclosure, it is possible to detect an electric leakage with high accuracy at low cost even when the capacitance of the Y capacitor is large. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram for explaining a configuration of a power supply system including a leakage detecting device according to a first exemplary embodiment. 
         FIG.  2    is a diagram illustrating an example of a voltage waveform at each point at the time of electric leakage detection by the leakage detecting device according to the first exemplary embodiment. 
         FIG.  3    is a diagram for explaining a configuration of a power supply system including a leakage detecting device according to a comparative example. 
         FIG.  4  ( a )-( b )  is diagram illustrating examples of a voltage waveform at point A before an earth fault occurs and a voltage waveform at point A after the earth fault occurs. 
         FIG.  5    is a diagram for explaining a configuration of a power supply system including a leakage detecting device according to a second exemplary embodiment. 
         FIG.  6    is a diagram for explaining a configuration of a power supply system including a leakage detecting device according to a third exemplary embodiment. 
         FIG.  7    is a diagram illustrating an example of a voltage waveform at each point at the time of electric leakage detection by the leakage detecting device according to the third exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     First Exemplary Embodiment 
       FIG.  1    is a diagram for explaining a configuration of power supply system  5  including leakage detecting device  10  according to a first exemplary embodiment. Power supply system  5  is incorporated in an electric vehicle. In the electric vehicle, power supply system  5  is provided in separation from an auxiliary battery (which is usually a lead battery with power output of 12 V). Power supply system  5  includes power storage unit  20  of a high-voltage system and leakage detecting device  10  of a low-voltage system. power storage unit  20  includes a plurality of cells E 1  to En connected in series. Lithium ion battery cells, nickel metal hydride battery cells, lead battery cells, electric double-layer capacitor cells, lithium ion capacitor cells, or the like can be used as cells E 1  to En. Hereinafter, in this specification, it is assumed that lithium ion battery cells (with a nominal voltage ranging from 3.6 V to 3.7 V) are used as cells E 1  to En. 
     The electric vehicle includes inverter  2  and motor  3  as loads for a high-voltage system. The positive electrode of power storage unit  20  is connected to one end of inverter  2  through positive power supply line Lp, and the negative electrode of power storage unit  20  is connected to the other end of inverter  2  through negative power supply line Lm. Positive power supply line Lp is provided with positive-side main relay MRp, and negative power supply line Lm is provided with negative-side main relay MRm. Positive-side main relay MRp and negative-side main relay MRm function as contactors that control electrical connection/disconnection between power storage unit  20  and the loads for a high-voltage system in the electric vehicle. In place of these relays, semiconductor switches with high withstand voltage and insulation performance may be used. 
     Inverter  2  is a bidirectional inverter connected between power storage unit  20  and motor  3 . At the time of power running, inverter  2  converts DC power, which is supplied from power storage unit  20 , into AC power and supplies the AC power to motor  3 . At the time of regeneration, inverter  2  converts AC power, which is supplied from motor  3 , into DC power and supplies the DC power to power storage unit  20 . As motor  3 , for example, a three-phase AC motor is used. At the time of power running, motor  3  rotates in accordance with AC power supplied from inverter  2 . At the time of regeneration, motor  3  converts rotational energy created by deceleration into AC power and supplied the AC power to inverter  2 . 
     power storage unit  20  is incorporated in the electric vehicle such that power storage unit  20  is kept insulated from a chassis earth of the electric vehicle. The auxiliary battery is incorporated in the electric vehicle such that the negative electrode of the auxiliary battery is electrically connected to the chassis earth. Positive power supply line Lp that is closer to inverter  2  than positive-side main relay MRp is connected to the chassis earth via first Y capacitor CY 1 . In addition, negative power supply line Lm that is closer to inverter  2  than negative-side main relay MRm is connected to the chassis earth via second Y capacitor CY 2 . First Y capacitor CY 1  and second Y capacitor CY 2  insulate positive power supply line Lp from the chassis earth and negative power supply line Lm from the chassis earth in terms of direct current, and stabilize the voltages of positive power supply line Lp and negative power supply line Lm. One of first Y capacitor CY 1  and second Y capacitor CY 2  may be omitted. 
     When power storage unit  20  is insulated from the chassis earth in an ideal manner, an intermediate potential of power storage unit  20  is kept close to a potential of the chassis earth. For example, when a voltage across both ends of power storage unit  20  is 250 V, a positive electrode potential of power storage unit  20  is kept close to +125 V as a negative electrode potential of the same is kept close to −125 V. When power storage unit  20  for a high-voltage system and the chassis earth are in a state of being electrically connected to each other, a person&#39;s touching an exposed conducting part of the electric vehicle is so dangerous that it may give an electric shock to the person. For this reason, it is necessary that the electric vehicle including power storage unit  20  for a high-voltage system be equipped with leakage detecting device  10  to monitor an insulation state between a current path of power storage unit  20 , which is connected to vehicle loads for a high-voltage system, and the chassis earth. In  FIG.  1   , the insulation state between positive power supply line Lp and the chassis earth is referred to as first leakage resistor RL 1 , and the insulation state between negative power supply line Lm and the chassis earth is referred to as second leakage resistor RL 2 . 
     leakage detecting device  10  includes drive unit  11  and leakage detector  12 . drive unit  11  includes driving coupling capacitor Cd 1 , output resistor Ro, operational amplifier OP 1 , oscillator  11   a , first protection diode D 1 , and second protection diode D 2 . 
     One end of driving coupling capacitor Cd 1  is connected to a current path between power storage unit  20  and the vehicle load. In the example illustrated in  FIG.  1   , one end of driving coupling capacitor Cd 1  is connected to positive power supply line Lp. Note that one end of driving coupling capacitor Cd 1  may be connected to any position on the current path. For example, it may be connected to negative power supply line Lm. The other end of driving coupling capacitor Cd 1  is connected to output resistor Ro. First protection diode D 1  is connected between the wiring between the other end of driving coupling capacitor Cd 1  and output resistor Ro and power supply potential Vcc of a low-voltage system, and second protection diode D 2  is connected between the wiring and the ground potential. 
     Oscillator  11   a  includes a multi-vibrator or a local oscillator, and generates a rectangular wave with a preset frequency. The rectangular wave voltage generated by oscillator  11   a  is input to a non-inverting input terminal of operational amplifier OP 1 . An output terminal of operational amplifier OP 1  is connected to output resistor Ro. An inverting input terminal of operational amplifier OP 1  is connected to the output terminal of the same. Operational amplifier OP 1  functions as a voltage follower that performs only impedance conversion with an amplification factor of 1. 
     Oscillator  11   a  outputs a rectangular wave voltage to point A via operational amplifier OP 1  and output resistor Ro. The rectangular wave voltage output to point A is applied to point B on the current path between power storage unit  20  and the vehicle load via driving coupling capacitor Cd 1 . As a result, charges are charged in first Y capacitor CY 1  and second Y capacitor CY 2 , or charges are discharged from first Y capacitor CY 1  and second Y capacitor CY 2 . 
     Leakage detector  12  includes measurement coupling capacitor Cd 2 , resistor Rf, capacitor Cf, A/D converter  12   a , voltage measurement unit  12   b , leakage determination unit  12   c , third protection diode D 3 , and fourth protection diode D 4 . 
     One end of measurement coupling capacitor Cd 2  is connected to a current path between power storage unit  20  and the vehicle load. In  FIG.  1   , one end of measurement coupling capacitor Cd 2  is connected to point B on the current path to which driving coupling capacitor Cd 1  is connected, but may be connected to any position on the current path. The other end of measurement coupling capacitor Cd 2  is connected to a low-pass filter including resistor Rf and capacitor Cf Third protection diode D 3  is connected between the wiring between the other end of measurement coupling capacitor Cd 2  and resistor Rf and power supply potential Vcc of a low-voltage system, and fourth protection diode D 4  is connected between the wiring and the ground potential. 
     The low-pass filter removes noise from the voltage at point C input from measurement coupling capacitor Cd 2 , and outputs the voltage at point D from which noise has been removed. A/D converter  12   a  converts the analog voltage at point D input from the low-pass filter into a digital value voltage and outputs the digital value voltage to voltage measurement unit  12   b . Voltage measurement unit  12   b  measures a voltage of a digital value input from A/D converter  12   a . leakage determination unit  12   c  determines the presence or absence of an electric leakage between the current path between power storage unit  20  and the vehicle load and the chassis earth based on the voltage measured by voltage measurement unit  12   b.    
     In  FIG.  1   , oscillator  11   a , voltage measurement unit  12   b , and leakage determination unit  12   c  may be configured by one IC. 
     In the first exemplary embodiment, a rectangular wave voltage is applied from drive unit  11  to point B to charge first Y capacitor CY 1  and second Y capacitor CY 2 . The electric leakage is detected based on a voltage time transition (specifically, a voltage change rate) when the electric charges charged in first Y capacitor CY 1  and second Y capacitor CY 2  are discharged via first leakage resistor RL 1  and second leakage resistor RL 2 . 
       FIG.  2    is a diagram illustrating an example of a voltage waveform at each point at the time of electric leakage detection by leakage detecting device  10  according to the first exemplary embodiment. When the voltage waveform at point A rises, charges are charged in first Y capacitor CY 1  and second Y capacitor CY 2 , and the voltage of B rises due to an increase in the capacitance of first Y capacitor CY 1  and second Y capacitor CY 2  (see W 1 ). The voltage variation at point B is transmitted to point C via measurement coupling capacitor Cd 2 . The voltage waveform at point D is obtained by removing noise from the voltage waveform at point C by the low-pass filter (see W 2 ). When viewed from leakage detecting device  10 , the voltage at point A is output voltage Vout for driving, and the voltage at point D is input voltage Vin for measurement. 
     As the charges charged in first Y capacitor CY 1  and second Y capacitor CY 2  escape through first leakage resistor RL 1  and second leakage resistor RL 2 , the voltage at point B gradually decreases and converges to the original voltage. In the first exemplary embodiment, the leakage resistance value is calculated by measuring the voltage return time at point B. 
     In the following calculation formula, the parallel resistance value of first leakage resistor RL 1  and second leakage resistor RL 2  is simply referred to as RL. The parallel capacitance value of first Y capacitor CY 1  and second Y capacitor CY 2  is simply referred to as CY. The capacitance value of driving coupling capacitor Cd 1  is simply referred to as Cd. The time constant τ at point B of the circuit illustrated in  FIG.  1    is defined as the following (Formula 1). Input voltage Vin at point D is defined as the following (Formula 2). E is a voltage applied from drive unit  11  to point B. 
       τ= RL ×( CY+Cd )  (Formula 1)
 
         V in= E×Cd /( Cd+CY )× e   −t/τ   (Formula 2)
 
     Electrical leakage determination unit  12   c  measures time t at which Vin=E×Cd/(Cd+CY)×e −a . Since a relationship of t=aτ=a×(RL×CY) holds, insulation resistance RL is defined as the following (Formula 3). 
         RL=t /( a×CY )  (Formula 3)
 
     As leakage resistor RL is smaller, the voltage at point D converges to the original voltage in a shorter time. Electrical leakage determination unit  12   c  measures time t at which voltage Vin at point D becomes E×Cd/(Cd+CY)×e −a , and calculates the value of leakage resistor RL. Electrical leakage determination unit  12   c  compares the calculated value of leakage resistor RL with a resistance threshold, and determines that an electrical leakage has occurred when the calculated value of leakage resistor RL is less than or equal to the resistance threshold. Alternatively, leakage determination unit  12   c  compares the value of voltage Vin at point D measured after the elapse of the set time from the application of driving voltage Vout with the voltage threshold. When the measured value of voltage Vin at point D is less than or equal to the voltage threshold, leakage determination unit  12   c  determines that the electric leakage occurs. The resistance threshold, the setting time, and the voltage threshold are set in advance based on the leakage resistance value for determining leakage. 
     COMPARATIVE EXAMPLE 
       FIG.  3    is a diagram for explaining a configuration of power supply system  5  including leakage detecting device  10  according to a comparative example. Hereinafter, differences from leakage detecting device  10  according to the first exemplary embodiment illustrated in  FIG.  1    will be described. In the comparative example, measurement coupling capacitor Cd 2  is not provided, and leakage detector  12  measures the voltage at point A between driving coupling capacitor Cd 1  and output resistor Ro. leakage detector  12  detects an electric leakage from an impedance ratio of output resistor Ro, driving coupling capacitor Cd 1 , and leakage resistor RL. 
       FIGS.  4 ( a ) and  4 ( b )  are diagrams illustrating an example of a voltage waveform at point A before an earth fault occurs and a voltage waveform at point A after the earth fault occurs.  FIG.  4 ( a )  illustrates a voltage waveform at point A before an earth fault occurs, and  FIG.  4 ( b )  illustrates a voltage waveform at point A after the earth fault occurs. When an earth fault occurs, the impedance at point A decreases, and the voltage at point A decreases. When the measured voltage value at point A is less than or equal to the voltage threshold Vth, leakage determination unit  12   c  determines that the electric leakage occurs. 
     However, in practice, Y capacitor CY is connected in parallel with leakage resistor RL. When the capacitance of Y capacitor CY is large (=the impedance is small), the degree of influence of the change in leakage resistor RL on the combined impedance becomes small. This makes it difficult to detect leakage resistor RL with high accuracy. 
     In order to maintain the detection accuracy of insulation resistance RL even when the capacitance of the Y capacitor is large, it is conceivable to reduce the frequency of the driving voltage. In that case, as described above, it is necessary to use a capacitor having a high capacity and a high withstand voltage for driving coupling capacitor Cd 1 , and the cost increases. 
     In this regard, in the first exemplary embodiment, the capacitance of driving coupling capacitor Cd 1  does not affect the detection accuracy of leakage resistor RL as shown in (Formula 3) above. Therefore, even when the capacitance of Y capacitor CY is large, the frequency of the driving voltage can be reduced without using high-spec driving coupling capacitor Cd 1 . As described above, according to the first exemplary embodiment, even when the capacitance of Y capacitor CY is large, the electric leakage can be detected with high accuracy at low cost. 
     Second Exemplary Embodiment 
       FIG.  5    is a diagram for explaining a configuration of power supply system  5  including leakage detecting device  10  according to a second exemplary embodiment. Hereinafter, differences from leakage detecting device  10  according to the first exemplary embodiment illustrated in  FIG.  1    will be described. In the second exemplary embodiment, adjustment resistor Rc and adjustment switch Rc are connected in series in parallel with first Y capacitor CY 1  between positive power supply line Lp and the chassis earth. Note that adjustment resistor Rc and adjustment switch Rc may be connected between negative power supply line Lm and the chassis earth. 
     In the following calculation formula, the parallel resistance value of first leakage resistor RL 1 , second leakage resistor RL 2 , and adjustment resistor Rc is expressed as RL′. In the circuit illustrated in  FIG.  5   , time constant T 1  at point B when adjustment switch SWc is turned off is defined as the following (Formula 4). Input voltage Vin at point D when adjustment switch SWc is turned off is defined as the following (Formula 5). Time constant τ 2  at point B when adjustment switch SWc is turned on is defined as the following (Formula 6). Input voltage Vin at point D when adjustment switch SWc is turned on is defined as the following (Formula 7). 
       τ1= RL ×( CY+Cd )  (Formula 4)
 
         V in= E×Cd /( Cd+CY )× e   −t/τ   (Formula 5)
 
       τ2= RL ′×( CY+Cd )  (Formula 6)
 
         V in= E×Cd /( Cd+CY )× e   −t/τ2   (Formula 7)
 
     Electrical leakage determination unit  12   c  measures time t at which Vin=E×Cd/(Cd+CY)×e −a . Time t 1  when adjustment switch SWc is off is defined as the following (Formula 8), and time t 2  when adjustment switch SWc is on is defined as the following (Formula 9). 
         t 1= aτ 1= a ×( RL×CY )  (Formula 8)
 
         t 2= aτ 2= a ×( RL′×CY )  (Formula 9)
 
     Combined resistance RL′ of leakage resistor RL and adjustment resistor Rc is defined as the following (Formula 10). 
         RL′=RL×Rc /( RL+Rc )  (Formula 10)
 
     From the above (Formula 8), (Formula 9), and (Formula 10), t 2  can be rewritten as the following (Formula 11). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           t 
                           ⁢ 
                           2 
                         
                         = 
                         
                           a 
                           × 
                           RL 
                         
                       
                       ’ 
                     
                     × 
                     t 
                     ⁢ 
                     1 
                     / 
                     
                       ( 
                       
                         a 
                         × 
                         RL 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       t 
                       ⁢ 
                       1 
                       × 
                       
                         RL 
                         ′ 
                       
                       / 
                       RL 
                     
                     = 
                     
                       t 
                       ⁢ 
                       1 
                       × 
                       Rc 
                       / 
                       
                         ( 
                         
                           RL 
                           + 
                           Rc 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                         
                     11 
                   
                   ) 
                 
               
             
           
         
       
     
     When the above (Formula 11) is solved for RL, the following (Formula 12) can be defined. 
         RL=t 1× Rc/t 2− Rc   (Formula 12)
 
     As described above, in the second exemplary embodiment, leakage determination unit  12   c  measures time t 1  at which Vin=E×Cd/(Cd+CY)×e′ when adjustment switch SWc is off and time t 2  at which Vin=E×Cd/(Cd+CY)×e′ when adjustment switch SWc is on, and calculates the value of leakage resistor RL. Electrical leakage determination unit  12   c  compares the calculated value of leakage resistor RL with a resistance threshold, and determines that an electrical leakage has occurred when the calculated value of leakage resistor RL is less than or equal to the resistance threshold. 
     As shown in the above (Formula 12), according to the second exemplary embodiment, the influence of the variation of Y capacitor CY can also be removed from the calculated leakage resistor RL in addition to the influence of the variation of driving coupling capacitor Cd 1 . Accordingly, the detection accuracy of the electric leakage can be further enhanced. 
     Third Exemplary Embodiment 
       FIG.  6    is a diagram for explaining a configuration of power supply system  5  including leakage detecting device  10  according to a third exemplary embodiment. Hereinafter, differences from leakage detecting device  10  according to the first exemplary embodiment illustrated in  FIG.  1    will be described. In the third exemplary embodiment, switch SW 1  is inserted between output resistor Ro and driving coupling capacitor Cd 1 . Measurement coupling capacitor Cd 2  is not provided, and leakage detector  12  measures the voltage at point B via driving coupling capacitor Cd 1  while switch SW 1  is off. 
       FIG.  7    is a diagram illustrating an example of a voltage waveform at each point at the time of electric leakage detection by leakage detecting device  10  according to the third exemplary embodiment. When the voltage waveform at point A rises in the state where switch SW 1  is on, charges are charged in Y capacitor CY, and the voltage at B rises due to an increase in the capacitance of Y capacitor CY (see W 1 ). Thereafter, leakage detector  12  turns off switch SW 1 . 
     When switch SW 1  is off, the voltage variation at point B is transmitted to point C via measurement coupling capacitor Cd 2 . The voltage waveform at point D is obtained by removing noise from the voltage waveform at point C by the low-pass filter (see W 2 ). As the charge charged in Y capacitor CY escapes via leakage resistor RL, the voltage at point B gradually decreases and converges to the original voltage. Also in the third exemplary embodiment, the value of leakage resistor RL can be calculated as in the first exemplary embodiment. 
     As described above, according to the third exemplary embodiment, since measurement coupling capacitor Cd 2  can be omitted, the cost and the circuit area can be reduced. 
     The present disclosure is described above according to the exemplary embodiments. It will be understood by those skilled in the art that the exemplary embodiments are merely an example, other modified examples in which configuration elements and processing processes of the exemplary embodiments are variously combined are possible, and the other modified examples still fall within the scope of the present disclosure. 
     For example, the second exemplary embodiment and the third exemplary embodiment may be combined. Specifically, in the circuit configuration of  FIG.  6   , adjustment resistor Rc and adjustment switch Rc may be connected in series between positive power supply line Lp and the chassis earth in parallel with first Y capacitor CY 1 . Note that adjustment resistor Rc and adjustment switch Rc may be connected between negative power supply line Lm and the chassis earth. 
     In the above embodiment, the charge is charged in Y capacitor CY by applying the rectangular wave voltage for charging from drive unit  11  to point B, and the electric leakage is detected based on the voltage transition (specifically, the change rate of the voltage) when the charge charged in Y capacitor CY is discharged through leakage resistor RL. In this regard, a discharging rectangular wave voltage may be applied from drive unit  11  to point B to discharge the charge from Y capacitor CY, and the electric leakage may be detected based on the voltage transition when the charge discharged from Y capacitor CY is charged via leakage resistor RL. 
     The above exemplary embodiment has been described as an example in which leakage detecting device  10  incorporated in an electric vehicle is used. leakage detecting device  10  according to the exemplary embodiment, however, may also be used in applications different from in-vehicle applications. In a configuration in which power storage unit  20  and a load supplied with power from power storage unit  20  are insulated from the earth, the load may be provided as any type of a load. For example, the load may be a load used in a railway vehicle. 
     Note that, the exemplary embodiments may be specified by the following Items. 
     [Item 1] 
     Leakage detecting device ( 10 ) that detects an electric leakage from a high-voltage unit in which power storage unit ( 20 ) and load ( 2 ) are connected by a power supply line (Lp, Lm) in a state of being insulated from an earth, in which 
     Y capacitor (CY) is connected between a current path between power storage unit ( 20 ) and load ( 2 ) and the earth, leakage detecting device ( 10 ) including: 
     drive unit ( 11 ) configured to output a rectangular wave voltage to the current path between power storage unit ( 20 ) and load ( 2 ) via driving coupling capacitor (Cd 1 ) to charge or discharge Y capacitor (CY); and 
     leakage detector ( 12 ) configured to determine presence or absence of a leakage between the current path and the earth according to a time transition in which a voltage at a measurement point of the current path changed according to charging or discharging of Y capacitor (CY) converges to an original voltage. 
     According to this, even when the capacitance of Y capacitor (CY) is large, the electric leakage can be detected with high accuracy at low cost. 
     [Item 2] 
     Leakage detecting device ( 10 ) according to Item 1, in which leakage detector ( 12 ) calculates a leakage resistance value between the current path and the earth based on the time transition of the voltage at the measurement point after Y capacitor (CY) is charged or discharged, and determines that the electric leakage occurs between the current path and the earth when the calculated leakage resistance value is less than or equal to a threshold. 
     Accordingly, the influence of driving coupling capacitor (Cd 1 ) can be removed from the leakage resistance value to be calculated. 
     [Item 3] 
     Leakage detecting device ( 10 ) according to Item 1 or 2, in which leakage detector ( 12 ) measures the voltage at the measurement point of the current path via measurement coupling capacitor (Cd 2 ). 
     According to this, the time transition of the voltage fluctuation in the current path can be measured. 
     [Item 4] 
     Leakage detecting device ( 10 ) according to Item 1 or 2, in which 
     drive unit ( 11 ) includes: 
     oscillator ( 11   a ) configured to output the rectangular wave voltage; 
     output resistor (Ro) connected to an output of oscillator ( 11   a ); and 
     switch (SW 1 ) inserted between output resistor (Ro) and driving coupling capacitor (Cd 1 ), and 
     leakage detector ( 12 ) includes voltage measurement unit ( 12   b ) that measures the voltage at the measurement point of the current path via driving coupling capacitor (Cd 1 ) while switch (SW 1 ) is off. 
     According to this, the coupling capacitor (Cd 2 ) for measurement can be omitted. 
     [Item 5] 
     Leakage detecting device ( 10 ) according to any one of Items 1 to 4, in which 
     adjustment resistor (Rc) and adjustment switch (SWc) are connected in series between the current path and the earth in parallel with Y capacitor (CY), and 
     leakage detector ( 12 ) calculates a leakage resistance value between the current path and the earth based on a voltage at a measurement point of the current path during an ON period and an OFF period of adjustment switch (SWc). 
     Accordingly, the influence of Y capacitor (CY) can be removed from the calculated leakage resistance value. 
     [Item 6] 
     Vehicle power supply system ( 5 ) including: 
     power storage unit ( 20 ) that is mounted in a state of being insulated from a chassis earth of a vehicle, and supplies power to load ( 2 ) in the vehicle; and 
     leakage detecting device ( 10 ) according to any one of Items 1 to 5. 
     According to this, it is possible to realize vehicle power supply system ( 5 ) including leakage detecting device ( 10 ) capable of detecting an electric leakage with high accuracy at low cost even when the capacitance of Y capacitor (CY) is large. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               5 : power supply system 
               20 : power storage unit 
               2 : inverter 
               3 : motor 
             Lp: positive power supply line 
             Lm: negative power supply line 
             CY: Y capacitor 
             Cd 1 : driving coupling capacitor 
             Cd 2 : measurement coupling capacitor 
             Cf: capacitor 
             RL: leakage resistance 
             Ro: output resistor 
             Rf: resistor 
             MRp: positive-side main relay 
             MRm: negative-side main relay 
             E 1 -En: cell 
               10 : leakage detecting device 
               11 : drive unit 
               11   a : oscillator 
               12 : leakage detector 
               12   a : A/D converter 
               12   b : voltage measurement unit 
               12   c : leakage determination unit 
             OP 1 : operational amplifier 
             SW 1 : switch 
             SWc: adjustment switch 
             Rc: adjustment resistor 
             D 1 -D 4 : protection diode