Ground fault detection apparatus

This nonprovisional application is based on Japanese Patent Application No. 2013-122619 filed on Jun. 11, 2013 with the Japan Patent Office, the entire contents of which are hereby incorporated by reference.

The present invention relates to a ground fault detection apparatus, and in particular to a ground fault detection apparatus detecting a ground fault in an ungrounded alternating current (AC) circuit in which a single-phase three-wire AC power source is connected to a load via a first phase wire, a neutral wire, and a second phase wire.

Description of the Background Art

Japanese Patent Laying-Open No. 2012-122986 (Patent Document 1) discloses a ground fault detection apparatus detecting a ground fault in an ungrounded direct current (DC) circuit in which a DC power source is connected to a load via a positive-side DC bus and a negative-side DC bus. The ground fault detection apparatus includes first and second resistive elements having an identical resistance value. One terminals of the first and second resistive elements are connected to the positive-side DC bus and the negative-side DC bus, respectively, and the other terminals thereof are connected to each other. The ground fault detection apparatus further includes a current detector detecting a value of a current flowing between the other terminals of the first and second resistive elements and a ground point, and determines that a ground fault occurs in a case where the value detected by the current detector exceeds a predetermined threshold value.

It is also conceivable to apply the ground fault detection apparatus of Patent Document 1 to an ungrounded AC circuit in which a single-phase three-wire AC power source is connected to a load via a first phase wire, a neutral wire, and a second phase wire, and connect one terminals of the first and second resistive elements to the first phase wire and the second phase wire, respectively.

However, if the neutral wire has a ground fault when such connection is made, a current flowing from the first phase wire to the neutral wire via the first resistive element, the current detector, the ground point, and a ground fault point, and a current flowing from the neutral wire to the second phase wire via the ground fault point, the ground point, the current detector, and the second resistive element cancel each other, and thus a ground fault cannot be detected (see FIG. 8 ).

Accordingly, a main object of the present invention is to provide a ground fault detection apparatus capable of accurately detecting a ground fault in an ungrounded AC circuit.

A ground fault detection apparatus in accordance with the present invention is a ground fault detection apparatus detecting a ground fault in an ungrounded AC circuit in which a single-phase three-wire AC power source is connected to a load via a first phase wire, a neutral wire, and a second phase wire, including: a first resistive element and a second resistive element, one terminal of the first resistive element being connected to the first phase wire, one terminal of the second resistive element being connected to the second phase wire, the other terminal of the first resistive element and the other terminal of the second resistive element being connected to each other via a node; and a current detector detecting a value of a current flowing between the node and a ground point, wherein, in a case where the value detected by the current detector exceeds a predetermined threshold value, the ground fault detection apparatus determines that the ground fault occurs.

Preferably, the one terminal of the second resistive element is connected to the neutral wire, and the first resistive element and the second resistive element have an identical resistance value.

Preferably, the ground fault detection apparatus further includes a third resistive element, and a first switching element and a second switching element which are turned on alternately, wherein one terminal of the third resistive element is connected to the second phase wire, the other terminal of the third resistive element is connected to the other terminal of the first resistive element and the other terminal of the second resistive element via a predetermined node, the first switching element is connected in series with the first resistive element between the first phase wire and the node, and the second switching element is connected in series with the third resistive element between the second phase wire and the node.

Preferably, the third resistive element has a resistance value identical to those of the first resistive element and the second resistive element.

Preferably, the one terminal of the second resistive element is connected to the second phase wire, and the first resistive element and the second resistive element have resistance values different from each other.

Further, another ground fault detection apparatus in accordance with the present invention is a ground fault detection apparatus detecting a ground fault in an ungrounded AC circuit in which a single-phase three-wire AC power source is connected to a load via a first phase wire, a neutral wire, and a second phase wire, the single-phase three-wire AC power source having an inverter converting DC power supplied from a DC power source via a positive-side DC bus and a negative-side DC bus into AC power, and supplying the AC power to the load via the first phase wire and the second phase wire, and a voltage divider generating an intermediate voltage of the positive-side DC bus and the negative-side DC bus and providing the intermediate voltage to the neutral wire, the ground fault detection apparatus including: a first resistive element and a second resistive element, one terminal of the first resistive element being connected to the positive-side DC bus or the negative-side DC bus, one terminal of the second resistive element receiving the intermediate voltage, the other terminal of the first resistive element and the other terminal of the second resistive element being connected to each other via a node; and a current detector detecting a value of a current flowing between the node and a ground point, wherein, in a case where the value detected by the current detector exceeds a predetermined threshold value, the ground fault detection apparatus determines that the ground fault occurs.

Preferably, the first resistive element and the second resistive element have an identical resistance value.

Preferably, the ground fault detection apparatus further includes a third resistive element, and a first switching element and a second switching element, wherein one terminal of the first resistive element is connected to the positive-side DC bus, one terminal of the third resistive element is connected to the negative-side DC bus, the other terminal of the third resistive element is connected to the other terminal of the first resistive element and the other terminal of the second resistive element via the node, the first switching element is connected in series with the first resistive element between the positive-side DC bus and the node, and the second switching element is connected in series with the third resistive element between the negative-side DC bus and the node.

Preferably, the third resistive element has a resistance value identical to those of the first resistive element and the second resistive element.

The ground fault detection apparatus in accordance with the present invention can also detect a ground fault in the neutral wire, and can accurately detect a ground fault in the ungrounded AC circuit.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

FIG. 1 is a circuit block diagram showing a configuration of a ground fault detection apparatus in accordance with Embodiment 1 of the present invention.

FIG. 2 shows an operation of the ground fault detection apparatus in a case where a neutral wire shown in FIG. 1 has a ground fault.

FIG. 3 shows time charts showing simulation results of currents I 1 , I 2 , Ig shown in FIG. 2 .

FIG. 4 shows an operation of the ground fault detection apparatus in a case where a U phase wire shown in FIG. 1 has a ground fault.

FIG. 5 shows time charts showing simulation results of currents I 1 , I 2 , Ig shown in FIG. 4 .

FIG. 6 shows an operation of the ground fault detection apparatus in a case where a V phase wire shown in FIG. 1 has a ground fault.

FIG. 7 shows time charts showing simulation results of currents I 1 , I 2 , Ig shown in FIG. 6 .

FIG. 8 is a circuit block diagram showing a comparative example of Embodiment 1.

FIG. 9 is a circuit block diagram showing a configuration of a ground fault detection apparatus in accordance with Embodiment 2 of the present invention.

FIG. 10 shows an operation of the ground fault detection apparatus in a case where a neutral wire shown in FIG. 9 has a ground fault.

FIG. 11 shows time charts showing simulation results of currents I 3 , I 4 , Ig shown in FIG. 10 .

FIG. 12 shows an operation of the ground fault detection apparatus in a case where a U phase wire shown in FIG. 9 has a ground fault.

FIG. 13 shows time charts showing simulation results of currents I 3 , I 4 , Ig shown in FIG. 12 .

FIG. 14 shows an operation of the ground fault detection apparatus in a case where a V phase wire shown in FIG. 9 has a ground fault.

FIG. 15 is a circuit block diagram showing a configuration of a ground fault detection apparatus in accordance with Embodiment 3 of the present invention.

FIG. 16 shows an operation of the ground fault detection apparatus in a case where a neutral wire shown in FIG. 15 has a ground fault.

FIG. 17 shows an operation of the ground fault detection apparatus in a case where a U phase wire shown in FIG. 15 has a ground fault.

FIG. 18 shows an operation of the ground fault detection apparatus in a case where a V phase wire shown in FIG. 15 has a ground fault.

FIG. 19 is a circuit block diagram showing a configuration of a ground fault detection apparatus in accordance with Embodiment 4 of the present invention.

FIG. 20 is a view showing an operation of the ground fault detection apparatus in a case where a neutral wire shown in FIG. 19 has a ground fault.

FIG. 21 is a view showing an operation of the ground fault detection apparatus in a case where a U phase wire shown in FIG. 19 has a ground fault.

FIG. 22 is a view showing an operation of the ground fault detection apparatus in a case where a V phase wire shown in FIG. 19 has a ground fault.

FIG. 23 is a circuit block diagram showing a configuration of a ground fault detection apparatus in accordance with Embodiment 5 of the present invention.

FIG. 24 shows an operation of the ground fault detection apparatus in a case where a neutral wire shown in FIG. 23 has a ground fault.

FIG. 25 shows an operation of the ground fault detection apparatus in a case where a U phase wire shown in FIG. 23 has a ground fault.

FIG. 26 shows an operation of the ground fault detection apparatus in a case where a V phase wire shown in FIG. 23 has a ground fault.

A ground fault detection apparatus 1 in accordance with Embodiment 1 of the invention of the present application is an apparatus detecting a ground fault in an ungrounded AC circuit, and includes resistive elements RE 1 , RE 2 , a current detector 2 , and a determination unit 3 , as shown in FIG. 1 . The ungrounded AC circuit includes a single-phase three-wire AC power source PS, a U phase wire UL, a neutral wire OL, a V phase wire VL, and loads LD 1 , LD 2 .

Single-phase three-wire AC power source PS is formed, for example, by equally dividing a secondary winding of a transformer into two, and includes a U phase terminal TU, a neutral terminal TO, and a V phase terminal TV. The voltage between terminals TU and TO is 100 (V), the voltage between terminals TV and TO is 100 (V), and the voltage between terminals TU and TV is 200 (V). An AC voltage VAC 1 between terminals TU and TO and an AC voltage VAC 2 between terminals TV and TO are 180° out of phase with each other.

U phase terminal TU is connected to one terminal of load LD 1 via U phase wire UL. Neutral terminal TO is connected to the other terminal of load LD 1 and one terminal of load LD 2 via neutral wire OL. V phase terminal TV is connected to the other terminal of load LD 2 via V phase wire VL. None of U phase wire UL, neutral wire OL, and V phase wire VL is grounded.

Resistive elements RE 1 , RE 2 of ground fault detection apparatus 1 have an identical resistance value. Resistive elements RE 1 , RE 2 have one terminals connected to V phase wire VL and neutral wire OL, respectively, and the other terminals connected to each other via a node N 1 . Current detector 2 detects an instantaneous value of a current Ig flowing from the other terminals of resistive elements RE 1 , RE 2 (i.e., node N 1 ) to a line of a ground voltage VSS (ground point), and provides a signal indicating the detected value to determination unit 3 . The line of ground voltage VSS is, for example, an automobile chassis.

Determination unit 3 generates a ground fault detection signal φD indicating whether or not a ground fault accident occurs in the ungrounded AC circuit, based on the output signal of current detector 2 . In a case where no ground fault accident occurs and thus the absolute value of the instantaneous value of current Ig is lower than a predetermined threshold value ITH, ground fault detection signal φD is set to an “L” level, which is a deactivated level. In a case where a ground fault accident occurs and thus the absolute value of the instantaneous value of current Ig exceeds predetermined threshold value ITH, ground fault detection signal φD is set to an “H” level, which is an activated level.

Next, an operation of ground fault detection apparatus 1 will be described. A current flowing from V phase wire VL into resistive element RE 1 is referred to as I 1 , and a current flowing from neutral wire OL into resistive element RE 2 is referred to as I 2 . Further, the resistance values of resistive elements RE 1 , RE 2 are referred to as R 1 , R 2 , respectively. R 1 , R 2 are set as R 1 =R 2 , and each of R 1 , R 2 is, for example, 40 (kΩ). In addition, effective values of AC voltages VAC 1 , VAC 2 are referred to as Vac 1 , Vac 2 , respectively. Vac 1 , Vac 2 are set as Vac 1 =Vac 2 , and each of Vac 1 , Vac 2 is 100 (V).

In the case where no ground fault accident occurs, a current I=I 1 =−I 2 =Vac 2 /(R 1 +R 2 )=Vac 2 /(2·R 1 ) flows from V phase wire VL, via resistive elements RE 1 , RE 2 , to neutral wire OL. On this occasion, current Ig is set as Ig=I 1 +I 2 =0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

FIG. 2( a ) is a circuit diagram showing a path of a current flowing in a case where a given point Pg in neutral wire OL has a ground fault. In FIG. 2( a ) , when point Pg in neutral wire OL has a ground fault, a low impedance current path is formed between a negative-side terminal of current detector 2 and ground fault point Pg, and a current flows through a path from V phase terminal TV, via V phase wire VL, resistive element RE 1 , current detector 2 , the line of ground voltage VSS, ground fault point Pg, and neutral wire OL, to neutral terminal TO. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 is set as Ig=Vac 2 /R 1 .

That is, current Ig having a value up to twice the value of current I 1 , I 2 flowing through resistive element RE 1 , RE 2 under normal operation conditions flows through current detector 2 . Since current Ig is normally Ig=0 (mA), in a case where a current having a value exceeding threshold value ITH flows through current detector 2 , it can be determined that a ground fault occurs. It is noted that, normally, a current supplied to loads LD 1 , LD 2 is sufficiently larger than Ig=Vac 2 /R 1 , and thus the current supplied to loads LD 1 , LD 2 does not change even if neutral wire OL has a ground fault, and a ground fault current does not change depending on the magnitude of loads LD 1 , LD 2 .

FIG. 2( b ) and FIGS. 3( a ) to 3( c ) are time charts showing simulation results of temporal changes of currents I 1 , I 2 , Ig in the case where point Pg in neutral wire OL has a ground fault at a given time. In FIG. 2( b ) and FIGS. 3( a ) to 3( c ) , in a normal state before a ground fault accident occurs, currents I 1 , I 2 flowing through resistive elements RE 1 , RE 2 have opposite phases, and current Ig is set as Ig=I 1 +I 2 =0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

It is noted that, although threshold value ITH is set as ITH=2 (mA) in FIG. 2( b ) and FIG. 3( c ) , it is merely set as ITH=2 (mA) to avoid making the drawings complicated, and in fact, ITH may be set to any value which can avoid a malfunction due to noise and can accurately detect a ground fault accident in U phase wire UL, neutral wire OL, and V phase wire VL. Concerning the value of ITH, the same applies hereinafter.

When a ground fault occurs at point Pg in neutral wire OL at a given time, the current flows through the path shown in FIG. 2( a ) , current I 2 is set as I 2 =0 (mA) and current Ig is set as Ig=I 1 , and the absolute value of the instantaneous value of Ig exceeds threshold value ITH. Determination unit 3 in FIG. 1 sets ground fault detection signal φD to an “H” level as an activated level in response to the absolute value of the instantaneous value of Ig exceeding threshold value ITH, and notifies a user of the ungrounded AC circuit of the occurrence of a ground fault accident.

FIG. 4( a ) is a circuit diagram showing paths of currents flowing in a case where point Pg in U phase wire UL has a ground fault. In FIG. 4( a ) , when point Pg in U phase wire UL has a ground fault, a low impedance current path is formed between the negative-side terminal of current detector 2 and ground fault point Pg, and current I 1 flows through a path from V phase terminal TV, via V phase wire VL, resistive element RE 1 , current detector 2 , the line of ground voltage VSS, ground fault point Pg, and U phase wire UL, to U phase terminal TU. Further, current I 2 flows through a path from neutral terminal TO, via neutral wire OL, resistive element RE 2 , current detector 2 , the line of ground voltage VSS, ground fault point Pg, and U phase wire UL, to U phase terminal TU. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 is set as Ig=(Vac 1 +Vac 2 )/R 1 +Vac 1 /R 2 =3·Vac 1 /R 1 .

That is, current Ig having a value up to six times the value of current I 1 , I 2 flowing through resistive element RE 1 , RE 2 under normal operation conditions flows through current detector 2 . Since current Ig is normally Ig=0 (mA), in the case where a current having a value exceeding threshold value ITH flows through current detector 2 , it can be determined that a ground fault occurs. It is noted that, normally, the current supplied to loads LD 1 , LD 2 is sufficiently larger than Ig=3·Vac 1 /R 1 , and thus the current supplied to loads LD 1 , LD 2 does not change even if U phase wire UL has a ground fault, and the ground fault current does not change depending on the magnitude of loads LD 1 , LD 2 .

FIG. 4( b ) and FIGS. 5( a ) to 5( c ) are time charts showing simulation results of temporal changes of currents I 1 , I 2 , Ig in the case where point Pg in U phase wire UL has a ground fault at a given time. In FIG. 4( b ) and FIGS. 5( a ) to 5( c ) , in a normal state before a ground fault accident occurs, current I 1 and current I 2 have opposite phases, and current Ig is set as Ig=I 1 +I 2 =0 (mA).

When a ground fault occurs at point Pg in U phase wire UL at a given time, the currents flow through the paths shown in FIG. 4( a ) , and the absolute value of the instantaneous value of Ig=3·Vac 1 /R 1 exceeds threshold value ITH. Determination unit 3 in FIG. 1 sets ground fault detection signal φD to an “H” level as an activated level in response to the absolute value of the instantaneous value of Ig exceeding threshold value ITH, and notifies the user of the ungrounded AC circuit of the occurrence of a ground fault accident.

FIG. 6( a ) is a circuit diagram showing a path of a current flowing in a case where point Pg in V phase wire VL has a ground fault. In FIG. 6( a ) , when point Pg in V phase wire VL has a ground fault, a low impedance current path is formed between the negative-side terminal of current detector 2 and ground fault point Pg, and a current flows through a path from V phase terminal TV, via V phase wire VL, ground fault point Pg, the line of ground voltage VSS, current detector 2 , resistive element RE 2 , and neutral wire OL, to neutral terminal TO. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 is set as Ig=Vac 2 /R 2 .

That is, current Ig having a value up to twice the value of current I 1 , I 2 flowing through resistive element RE 1 , RE 2 under normal operation conditions flows through current detector 2 . Since current Ig is normally Ig=0 (mA), in the case where a current having a value exceeding threshold value ITH flows through current detector 2 , it can be determined that a ground fault occurs. It is noted that, normally, the current supplied to loads LD 1 , LD 2 is sufficiently larger than Ig=Vac 2 /R 2 , and thus the current supplied to loads LD 1 , LD 2 does not change even if V phase wire VL has a ground fault, and the ground fault current does not change depending on the magnitude of loads LD 1 , LD 2 .

FIG. 6( b ) and FIGS. 7( a ) to 7( c ) are time charts showing simulation results of temporal changes of currents I 1 , I 2 , Ig in the case where point Pg in V phase wire VL has a ground fault at a given time. In FIG. 6( b ) and FIGS. 7( a ) to 7( c ) , in a normal state before a ground fault accident occurs, currents I 1 and current I 2 have opposite phases, and current Ig is set as Ig=I 1 +I 2 =0 (mA).

When a ground fault occurs at point Pg in V phase wire VL at a given time, the current flows through the path shown in FIG. 6( a ) , current I 1 is set as I 1 =0 (mA) and current Ig is set as Ig=I 2 , and the absolute value of the instantaneous value of Ig=Vac 2 /R 2 exceeds threshold value ITH. Determination unit 3 in FIG. 1 sets ground fault detection signal <D to an “H” level as an activated level in response to the absolute value of the instantaneous value of Ig exceeding threshold value ITH, and notifies the user of the ungrounded AC circuit of the occurrence of a ground fault accident.

FIG. 8 is a circuit block diagram showing a comparative example of Embodiment 1, and is a view to be compared with FIG. 1 . Referring to FIG. 8 , the comparative example is different from Embodiment 1 in that the one terminal of resistive element RE 2 is connected to U phase wire UL. FIG. 8 shows a state in which point Pg in neutral wire OL has a ground fault. In this case, current I 1 flows through a path from V phase terminal TV, via V phase wire VL, resistive element RE 1 , current detector 2 , the line of ground voltage VSS, ground fault point Pg, and neutral wire OL, to neutral terminal TO. Further, a current (−I 2 ) flows through a path from neutral terminal TO, via neutral wire OL, ground fault point Pg, the line of ground voltage VSS, current detector 2 , resistive element RE 2 , and U phase wire UL, to U phase terminal TU. The current flowing through current detector 2 is set as Ig=I 1 −I 2 =0 (mA), and a ground fault cannot be detected.

As described above, in Embodiment 1, the one terminals of resistive elements RE 1 , RE 2 having identical resistance values R 1 , R 2 are connected to V phase wire VL and neutral wire OL, respectively, current detector 2 is connected between the other terminals of resistive elements RE 1 , RE 2 and the ground point, and, in the case where current Ig flowing through current detector 2 exceeds threshold value ITH, it is determined that a ground fault occurs. Therefore, in the case where any of U phase wire UL, V phase wire VL, and neutral wire OL has a ground fault, the occurrence of the ground fault can be detected.

In addition, by using resistive elements RE 1 , RE 2 having high resistance values R 1 , R 2 , even if a current flows through a human body when a one-point ground fault occurs, the ground fault can be detected without seriously affecting the human body.

That is, in Embodiment 1, when U phase wire UL has a ground fault, the largest current flows on the occurrence of a ground fault accident, and resistance values R 1 , R 2 of resistive elements RE 1 , RE 2 are set such that the current on this occasion does not cause a harmful effect on the human body. For example, when it is set that Vac 1 =Vac 2 =100 (V) and R 1 =R 2 , Ig is set as Ig=Vac 1 /R 2 +(Vac 1 +Vac 2 )/R 1 =300/R 1 (RMS: root mean square value). When the value of the current which does not cause a harmful effect on the human body is less than 10 (mA), it is satisfactory if R 1 >30 (kΩ). Further, if R 1 =30 (kΩ), in the case where V phase wire VL or neutral wire OL has a ground fault, a current of 3.3 (mA), which is one third of 10 (mA), flows through the human body.

It is noted that, although the one terminals of resistive elements RE 1 , RE 2 are connected to V phase wire VL and neutral wire OL, respectively, in Embodiment 1, it is needless to say that the same effect can also be obtained when the one terminals of resistive elements RE 1 , RE 2 are connected to U phase wire UL and neutral wire OL, respectively.

FIG. 9 is a circuit block diagram showing a configuration of a ground fault detection apparatus 5 in accordance with Embodiment 2 of the present invention, and is a view to be compared with FIG. 1 . Referring to FIG. 9 , ground fault detection apparatus 5 is different from ground fault detection apparatus 1 in FIG. 1 in that resistive elements RE 1 , RE 2 are replaced by resistive elements RE 3 , RE 4 , respectively, and one terminals of resistive elements RE 3 , RE 4 are connected to V phase wire VL and U phase wire UL, respectively. Further, a resistance value R 3 of resistive element RE 3 and a resistance value R 4 of resistive element RE 4 are different from each other. For example, R 3 is 30 (kΩ), and R 4 is 60 (kΩ).

Next, an operation of ground fault detection apparatus 5 will be described. A current flowing from V phase wire VL into resistive element RE 3 is referred to as I 3 , and a current flowing from U phase wire UL into resistive element RE 4 is referred to as I 4 . In a case where no ground fault accident occurs, when the voltage of V phase wire VL is higher than the voltage of U phase wire UL, current I=I 3 =−I 4 =2·Vac 1 /(R 3 +R 4 ) flows from V phase wire VL, via resistive elements RE 3 , RE 4 , to U phase wire UL. On this occasion, current Ig is set as Ig=I 3 +I 4 =0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

FIG. 10( a ) is a circuit diagram showing a path of a current flowing in a case where point Pg in neutral wire OL has a ground fault. In FIG. 10( a ) , when point Pg in neutral wire OL has a ground fault, a low impedance current path is formed between the negative-side terminal of current detector 2 and ground fault point Pg. Thereby, current I 3 flowing from V phase terminal TV through V phase wire VL and resistive element RE 3 is divided into a first path leading to U phase wire UL via resistive element RE 4 , and a second path leading to neutral wire OL via current detector 2 , the line of ground voltage VSS, and ground fault point Pg. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 is set as Ig=Vac 2 /R 3 −Vac 1 /R 4 =100·(R 4 −R 3 )/(R 3 ·R 4 ).

That is, when R 3 ≠R 4 , current Ig is set as Ig≠0 (mA), and the current flows through current detector 2 . Since current Ig is normally Ig=0 (mA), in the case where a current having a value exceeding threshold value ITH flows through current detector 2 , it can be determined that a ground fault occurs. It is noted that, normally, the current supplied to loads LD 1 , LD 2 is sufficiently larger than Ig=100·(R 4 −R 3 )/(R 3 ·R 4 ), and thus the current supplied to loads LD 1 , LD 2 does not change even if neutral wire OL has a ground fault, and the ground fault current does not change depending on the magnitude of loads LD 1 , LD 2 .

FIG. 10( b ) and FIGS. 11( a ) to 11( c ) are time charts showing simulation results of temporal changes of currents I 3 , I 4 , Ig in the case where point Pg in neutral wire OL has a ground fault at a given time. In FIG. 10( b ) and FIGS. 11( a ) to 11( c ) , in a normal state before a ground fault accident occurs, currents I 3 and current I 4 have opposite phases, and current Ig is set as Ig=I 3 +I 4 =0 (mA).

When a ground fault occurs at point Pg in neutral wire OL at a given time, the current flows through the paths shown in FIG. 10( a ) , and the absolute value of the instantaneous value of Ig exceeds threshold value ITH. Determination unit 3 in FIG. 9 sets ground fault detection signal φD to an “H” level as an activated level in response to the absolute value of the instantaneous value of Ig exceeding threshold value ITH, and notifies the user of the ungrounded AC circuit of the occurrence of a ground fault accident.

FIG. 12( a ) is a circuit diagram showing a path of a current flowing in a case where point Pg in U phase wire UL has a ground fault. In FIG. 12( a ) , when point Pg in U phase wire UL has a ground fault, a low impedance current path is formed between the negative-side terminal of current detector 2 and ground fault point Pg, and current I 3 flows through a path from V phase terminal TV, via V phase wire VL, resistive element RE 3 , current detector 2 , the line of ground voltage VSS, ground fault point Pg, and U phase wire UL, to U phase terminal TU. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 is set as Ig=(Vac 1 +Vac 2 )/R 3 =2·Vac 1 /R 3 .

Current Ig having a value up to three times the value of current I 3 , I 4 flowing through resistive element RE 3 , RE 4 under normal operation conditions flows through current detector 2 . Since current Ig is normally Ig=0 (mA), in the case where a current having a value exceeding threshold value ITH flows through current detector 2 , it can be determined that a ground fault occurs. It is noted that, normally, the current supplied to loads LD 1 , LD 2 is sufficiently larger than Ig=2·Vac 1 /R 3 , and thus the current supplied to loads LD 1 , LD 2 does not change even if U phase wire UL has a ground fault, and the ground fault current does not change depending on the magnitude of loads LD 1 , LD 2 .

FIG. 12( b ) and FIGS. 13( a ) to 13( c ) are time charts showing simulation results of temporal changes of currents I 3 , I 4 , Ig in the case where point Pg in U phase wire UL has a ground fault at a given time. In FIG. 12( b ) and FIGS. 13( a ) to 13( c ) , in a normal state before a ground fault accident occurs, current I 3 and current I 4 have opposite phases, and current Ig is set as Ig=I 3 +I 4 =0 (mA).

When a ground fault occurs at point Pg in U phase wire UL at a given time, the current flows through the path shown in FIG. 12( a ) , and the absolute value of the instantaneous value of Ig=2·Vac 1 /R 3 exceeds threshold value ITH. Determination unit 3 in FIG. 9 sets ground fault detection signal φD to an “H” level as an activated level in response to the absolute value of the instantaneous value of Ig exceeding threshold value ITH, and notifies the user of the ungrounded AC circuit of the occurrence of a ground fault accident.

FIG. 14( a ) is a circuit diagram showing a path of a current flowing in a case where point Pg in V phase wire VL has a ground fault. In FIG. 14( a ) , when point Pg in V phase wire VL has a ground fault, a low impedance current path is formed between the negative-side terminal of current detector 2 and ground fault point Pg, and a current flows through a path from V phase terminal TV, via V phase wire VL, ground fault point Pg, the line of ground voltage VSS, current detector 2 , resistive element RE 4 , and U phase wire UL, to U phase terminal TU. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 is set as Ig=2·Vac 2 /R 4 .

That is, current Ig having a value up to three times the value of current I 3 , I 4 flowing through resistive element RE 3 , RE 4 under normal operation conditions flows through current detector 2 . Since current Ig is normally Ig=0 (mA), in the case where a current having a value exceeding threshold value ITH flows through current detector 2 , it can be determined that a ground fault occurs. It is noted that, normally, the current supplied to loads LD 1 , LD 2 is sufficiently larger than Ig=2·Vac 2 /R 4 , and thus the current supplied to loads LD 1 , LD 2 does not change even if V phase wire VL has a ground fault, and the ground fault current does not change depending on the magnitude of loads LD 1 , LD 2 .

FIG. 14( b ) is a time chart showing simulation results of temporal changes of currents I 3 , I 4 , Ig in the case where point Pg in V phase wire VL has a ground fault at a given time. In FIG. 14( b ) , in a normal state before a ground fault accident occurs, currents I 3 and current I 4 have opposite phases, and current Ig is set as Ig=I 3 +I 4 =0 (mA).

When a ground fault occurs at point Pg in V phase wire VL at a given time, the current flows through the path shown in FIG. 14( a ) , current I 3 is set as I 3 =0 (mA) and current Ig is set as Ig=I 4 , and the absolute value of the instantaneous value of Ig=2·Vac 2 /R 4 exceeds threshold value ITH. Determination unit 3 in FIG. 9 sets ground fault detection signal φD to an “H” level as an activated level in response to the absolute value of the instantaneous value of Ig exceeding threshold value ITH, and notifies the user of the ungrounded AC circuit of the occurrence of a ground fault accident.

In Embodiment 2, the one terminals of resistive elements RE 3 , RE 4 having resistance values R 3 , R 4 different from each other are connected to V phase wire VL and U phase wire UL, respectively, current detector 2 is connected between the other terminals of resistive elements RE 3 , RE 4 and the ground point, and, in the case where current Ig flowing through current detector 2 exceeds threshold value ITH, it is determined that a ground fault occurs. Therefore, in the case where any of U phase wire UL, V phase wire VL, and neutral wire OL has a ground fault, the occurrence of the ground fault can be detected.

In addition, by using resistive elements RE 3 , RE 4 having high resistance values, even if a current flows through a human body when a one-point ground fault occurs, the ground fault can be detected without seriously affecting the human body.

That is, in Embodiment 2, when U phase wire UL has a ground fault, the largest current flows on the occurrence of a ground fault accident, and resistance values R 3 , R 4 of resistive elements RE 3 , RE 4 are set such that the current on this occasion does not cause a harmful effect on the human body. For example, when it is set that Vac 1 =Vac 2 =100 (V), Ig is set as Ig=(Vac 1 +Vac 2 )/R 3 =200/R 3 (RMS). When the value of the current which does not cause a harmful effect on the human body is less than 10 (mA), it is satisfactory if R 3 >20 (kΩ) and R 4 >20 (kW). Further, if R 3 =30 (kΩ) and R 4 =60 (kΩ), neutral wire OL has a ground fault current of 100×(60−30)/30×60=1.67 (mA).

It is noted that, when R 3 +R 4 =R 1 +R 2 , a current flowing through a resistive element in a normal state is larger in Embodiment 2, and power consumed in the resistive element is increased.

Further, since current Ig flowing through current detector 2 differs depending on the phase where a ground fault occurs, highly accurate current detector 2 is required. In the above example, current detector 2 capable of measuring a current of 1.67 (mA) is required.

FIG. 15 is a circuit block diagram showing a configuration of a ground fault detection apparatus 10 in accordance with Embodiment 3 of the present invention, and is a view to be compared with FIG. 1 . Referring to FIG. 15 , ground fault detection apparatus 10 is different from ground fault detection apparatus 1 in FIG. 1 in that resistive elements RE 1 , RE 2 are replaced by resistive elements RE 5 to RE 7 , and switches SW 1 , SW 2 and a pulse signal generator 11 are added.

Switch SW 1 is connected in series with resistive element RE 5 between V phase wire VL and node N 1 . Resistive element RE 6 is connected between neutral wire OL and node N 1 . Switch SW 2 is connected in series with resistive element RE 7 between U phase wire UL and node N 1 . Current detector 2 is connected between node N 1 and the line of ground voltage VSS. Resistive elements RE 5 to RE 7 have an identical resistance value.

Switches SW 1 , SW 2 are controlled by pulse signals φ 1 , φ 2 , respectively, from pulse signal generator 11 . Switch SW 1 is ON when signal φ 1 is at an “H” level, and is OFF when signal φ 1 is at an “L” level. Switch SW 2 is ON when signal φ 2 is at an “H” level, and is OFF when signal φ 2 is at an “L” level.

Pulse signal generator 11 sets pulse signal φ 1 to an “H” level for a predetermined time with a predetermined cycle, and sets pulse signal φ 2 to an “H” level for the predetermined time with the predetermined cycle. Pulse signal φ 1 is set to an “H” level for the predetermined time within a time period in which pulse signal φ 2 is at an “L” level, and pulse signal φ 2 is set to an “H” level for the predetermined time within a time period in which pulse signal φ 1 is at an “L” level. Switches SW 1 , SW 2 are turned on alternately, and are not turned on simultaneously. Switches SW 1 , SW 2 are turned on/off with an interval which is sufficiently shorter than a time required to detect a ground fault.

In a case where switch SW 1 is ON and switch SW 2 is OFF, ground fault detection apparatus 10 has the same configuration as that of ground fault detection apparatus 1 . In a case where switch SW 1 is OFF and switch SW 2 is ON, ground fault detection apparatus 10 has the same circuit configuration as that of ground fault detection apparatus 1 except that the U phase and the V phase of ground fault detection apparatus 1 are reversed. In order to detect a ground fault within a predetermined time from when the ground fault occurs, it is only necessary to set a time in which both of switches SW 1 , SW 2 are OFF to be sufficiently shorter than the predetermined time.

Next, an operation of ground fault detection apparatus 10 will be described. A current flowing from V phase wire VL into resistive element RE 5 is referred to as I 5 , a current flowing from neutral wire OL into resistive element RE 6 is referred to as I 6 , and a current flowing from U phase wire UL into resistive element RE 7 is referred to as 17 . Further, the resistance values of resistive elements RE 5 to RE 7 are referred to as R 5 to R 7 , respectively. R 5 to R 7 are set as R 5 =R 6 =R 7 , and each of R 5 , R 6 . R 7 is, for example, 40 (kΩ).

In a case where no ground fault accident occurs, when switch SW 1 is ON, current I 5 =−I 6 =Vac 2 /(R 5 +R 6 )=Vac 1 /(2·R 6 ) flows from V phase wire VL, via switch SW 1 and resistive elements RE 5 , RE 6 , to neutral wire OL. On this occasion, current Ig is set as Ig=I 5 +I 6 =0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

Similarly, in the case where no ground fault accident occurs, when switch SW 2 is ON, current I 7 =−I 6 =Vac 1 /(R 6 +R 7 )=Vac 1 /(2·R 6 ) flows from U phase wire UL, via switch SW 2 and resistive elements RE 7 , RE 6 , to neutral wire OL. On this occasion, current Ig is set as Ig=I 5 +I 6 ==0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

In the case where no ground fault accident occurs, when both of switches SW 1 , SW 2 are OFF, no current flows through resistive elements RE 5 to RE 7 , and I 5 to I 7 and Ig are each 0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

FIGS. 16( a ) to 16( d ) are time charts showing simulation results of temporal changes of currents I 5 to I 7 and Ig in a case where point Pg in neutral wire OL has a ground fault at a given time. FIGS. 16( e ) and 16( f ) are time charts showing ON/OFF states of switches SW 1 , SW 2 . In a case where switch SW 1 is ON, ground fault detection apparatus 10 has the same configuration as that of ground fault detection apparatus 1 , and currents I 5 , I 6 , Ig change as with currents I 1 , I 2 , Ig shown in FIGS. 2( a ) and 2( b ) and FIGS. 3( a ) to 3( c ) . Since switch SW 1 is ON for the predetermined time with the predetermined cycle, and currents I 5 , I 6 , Ig flow only when switch SW 1 is ON, each of currents I 5 , I 6 , Ig also changes in a pulsed manner in response to switch SW 1 .

In a case where switch SW 2 is ON, ground fault detection apparatus 10 has the same configuration as that of ground fault detection apparatus 1 except that the V phase and the U phase are reversed, and currents I 6 , I 7 , Ig change as with currents I 1 , I 2 , Ig shown in FIGS. 2( a ) and 2( b ) and FIGS. 3( a ) to 3( c ) . Since switch SW 2 is ON for the predetermined time with the predetermined cycle, and currents I 6 , I 7 , Ig flow only when switch SW 2 is ON, each of currents I 6 , I 7 , Ig also changes in a pulsed manner in response to switch SW 2 .

In a normal state before a ground fault accident occurs, current I 5 and current I 6 have opposite phases, and current Ig is set as Ig=I 5 +I 6 =0 (mA). Further, current I 6 and current I 7 have opposite phases, and current Ig is set as Ig=I 6 +I 7 =0 (mA). When a ground fault occurs at point Pg in neutral wire OL at a given time, current I 6 is set as I 6 =0 (mA) and current Ig is set as Ig=I 5 =Vac 2 /RE 5 or Ig=I 7 =Vac 1 /RE 7 . Therefore, the absolute value of the instantaneous value of Ig exceeds threshold value ITH, and ground fault detection signal φD is set to an “H” level as an activated level.

FIGS. 17( a ) to 17( d ) are time charts showing simulation results of temporal changes of currents I 5 to I 7 and Ig in a case where point Pg in U phase wire UL has a ground fault at a given time. FIGS. 17( e ) and 17( f ) are time charts showing ON/OFF states of switches SW 1 , SW 2 . In a case where switch SW 1 is ON, ground fault detection apparatus 10 has the same configuration as that of ground fault detection apparatus 1 , and currents I 5 , I 6 , Ig change as with currents I 1 , I 2 , Ig shown in FIGS. 4( a ) and 4( b ) and FIGS. 5( a ) to 5( c ) . Since switch SW 1 is ON for the predetermined time with the predetermined cycle, and currents I 5 , I 6 , Ig flow only when switch SW 1 is ON, each of currents I 5 , I 6 , Ig also changes in a pulsed manner in response to switch SW 1 .

In a case where switch SW 2 is ON, ground fault detection apparatus 10 has the same configuration as that of ground fault detection apparatus 1 except that the V phase and the U phase are reversed, and currents I 6 , I 7 , Ig change as with currents I 2 , I 1 , Ig shown in FIGS. 6( a ) and 6( b ) and FIGS. 7( a ) to 7( c ) . Since switch SW 2 is ON for the predetermined time with the predetermined cycle, and currents I 6 , I 7 , Ig flow only when switch SW 2 is ON, each of currents I 6 , I 7 , Ig also changes in a pulsed manner in response to switch SW 2 .

In a normal state before a ground fault accident occurs, current I 5 and current I 6 have opposite phases, and current Ig is set as Ig=I 5 +I 6 =0 (mA). Further, current I 6 and current I 7 have opposite phases, and current Ig is set as Ig=I 6 +I 7 =0 (mA).

In the case where a ground fault occurs at point Pg in U phase wire UL at a given time, current I 7 is set as I 7 =0 (mA). In addition, when switch SW 1 is ON, current Ig is set as Ig=Vac 1 /R 6 +(Vac 1 +Vac 2 )/R 5 ; when switch SW 2 is ON, current Ig is set as Ig=Vac 1 /R 6 ; and when both of switches SW 1 , SW 2 are OFF, current Ig is set as Ig=Vac 1 /R 6 . Therefore, the absolute value of the instantaneous value of Ig exceeds threshold value ITH, and ground fault detection signal φD is set to an “H” level as an activated level.

FIGS. 18( a ) to 18( d ) are time charts showing simulation results of temporal changes of currents I 5 to I 7 and Ig in a case where point Pg in V phase wire VL has a ground fault at a given time. FIGS. 18( e ) and 18( f ) are time charts showing ON/OFF states of switches SW 1 , SW 2 . In a case where switch SW 1 is ON, ground fault detection apparatus 10 has the same configuration as that of ground fault detection apparatus 1 , and currents I 5 , I 6 , Ig change as with currents I 1 , I 2 , Ig shown in FIGS. 6( a ) and 6( b ) and FIGS. 7( a ) to 7( c ) . Since switch SW 1 is ON for the predetermined time with the predetermined cycle, and currents I 5 , I 6 , Ig flow only when switch SW 1 is ON, each of currents I 5 , I 6 , Ig also changes in a pulsed manner in response to switch SW 1 .

In a case where switch SW 2 is ON, ground fault detection apparatus 10 has the same configuration as that of ground fault detection apparatus 1 except that the V phase and the U phase are reversed, and currents I 6 , I 7 , Ig change as with currents I 2 , I 1 , Ig shown in FIGS. 4( a ) and 4( b ) and FIGS. 5( a ) to 5( c ) . Since switch SW 2 is ON for the predetermined time with the predetermined cycle, and currents I 6 , I 7 , Ig flow only when switch SW 2 is ON, each of currents I 6 , I 7 , Ig also changes in a pulsed manner in response to switch SW 2 .

In a normal state before a ground fault accident occurs, current I 5 and current I 6 have opposite phases, and current Ig is set as Ig=I 5 +I 6 =0 (mA). Further, current I 6 and current I 7 have opposite phases, and current Ig is set as Ig=I 6 +I 7 =0 (mA).

When a ground fault occurs at point Pg in V phase wire VL at a given time, current I 5 is set as I 5 =0 (mA). In addition, when switch SW 1 is ON, current Ig is set as Ig=Vac 2 /R 6 ; when switch SW 2 is ON, current Ig is set as Ig=Vac 1 /R 6 +(Vac 1 +Vac 2 )/R 7 ; and when both of switches SW 1 , SW 2 are OFF, current Ig is set as Ig=Vac 1 /R 6 . Therefore, the absolute value of the instantaneous value of Ig exceeds threshold value ITH, and ground fault detection signal φD is set to an “H” level as an activated level.

As described above, in Embodiment 1, since current Ig flowing through current detector 2 differs depending on the point where a ground fault occurs, highly accurate current detector 2 capable of detecting even small current Ig is required. In contrast, in Embodiment 3, since switches SW 1 , SW 2 are turned on alternately, current Ig has a high value whenever either one of switches SW 1 , SW 2 is turned on. Therefore, highly accurate current detector 2 is not required, and thus the apparatus can be manufactured at a lower cost.

In addition, when both of switches SW 1 , SW 2 are in an OFF state, no current flows through resistive elements RE 5 to RE 7 . Therefore, power consumption during that period can be reduced, and lower power consumption can be achieved.

FIG. 19 is a circuit block diagram showing a configuration of a ground fault detection apparatus 20 in accordance with Embodiment 4 of the present invention. In FIG. 19 , ground fault detection apparatus 20 is an apparatus detecting a ground fault in an ungrounded AC circuit, and includes resistive elements RE 11 , RE 12 , current detector 2 , and determination unit 3 . The ungrounded AC circuit includes a DC power source 21 , a resistive element RE 13 , capacitors C 1 to C 4 , transistors Q 1 to Q 4 , diodes D 1 to D 4 , drivers DR 1 to DR 4 , a PWM controller 22 , reactors L 1 to L 4 , and loads LD 1 , LD 2 . The ungrounded AC circuit also includes a positive-side DC bus PL, a negative-side DC bus NL, a DC neutral point NP, U phase wire UL, neutral wire OL, and V phase wire VL.

DC power source 21 outputs a DC voltage VDC between a positive electrode and a negative electrode. Positive-side DC bus PL is connected to the positive electrode of DC power source 21 , and negative-side DC bus NL is connected to the negative electrode of DC power source 21 . Capacitor C 1 is connected between positive-side DC bus PL and DC neutral point NP. Capacitor C 2 is connected between DC neutral point NP and negative-side DC bus NL. Capacitors C 1 , C 2 constitute a voltage divider which equally divides DC voltage VDC to set the voltage of DC neutral point NP to VDC/2. Further, capacitors C 1 , C 2 stabilize the DC voltage between positive-side DC bus PL and negative-side DC bus NL. Resistive element RE 13 is connected between DC neutral point NP and negative-side DC bus NL, and is provided to adjust the voltage of DC neutral point NP to an intermediate voltage VDC/2 of DC buses PL, NL.

Transistors Q 1 , Q 3 have collectors both connected to positive-side DC bus PL, and emitters connected to output nodes N 2 , N 3 , respectively. Transistors Q 2 , Q 4 have collectors connected to output nodes N 2 , N 3 , respectively, and emitters both connected to negative-side DC bus NL. Diodes D 1 to D 4 are connected in antiparallel with transistors Q 1 to Q 4 , respectively.

PWM (Pulse Width Modulation) controller 22 performs PWM control on gate voltages of transistors Q 1 to Q 4 via drivers DR 1 to DR 4 , and converts the DC voltage between DC buses PL, NL into an AC voltage. That is, transistors Q 1 to Q 4 , diodes D 1 to D 4 , drivers DR 1 to DR 4 , and PWM controller 22 constitute an inverter which converts DC power into AC power.

Reactors L 1 to L 3 have one terminals connected to output node N 3 , DC neutral point NP, and output node N 2 , respectively, and the other terminals connected to one ends of U phase wire UL, neutral wire OL, and V phase wire VL, respectively. Capacitor C 3 is connected between the other terminals of reactors L 1 , L 2 . Capacitor C 4 is connected between the other terminals of reactors L 2 , L 3 . Load LD 1 is connected between the other end of U phase wire UL and the other end of neutral wire OL. Load LD 2 is connected between the other end of neutral wire OL and the other end of V phase wire VL. It is noted that DC neutral point NP and neutral wire OL may be directly connected to each other with reactor L 2 being removed. Simulation results in FIGS. 20 to 22 are results obtained when reactor L 2 is not provided.

Reactors L 1 to L 3 and capacitors C 3 , C 4 constitute a low pass filter which passes the AC voltage generated by the inverter to loads LD 1 , LD 2 , and prevents switching frequency noise generated in the inverter from being passed to loads LD 1 , LD 2 . Further, reactors L 1 to L 3 and capacitors C 3 , C 4 convert a pulse voltage train generated by the inverter into a sinusoidal AC voltage, and provides it to loads LD 1 , LD 2 .

Resistive elements RE 11 , RE 12 of ground fault detection apparatus 20 have an identical resistance value. Resistive elements RE 11 , RE 12 have one terminals connected to positive-side DC bus PL and DC neutral point NP, respectively, and the other terminals connected to each other via node N 1 . Current detector 2 detects an instantaneous value of current Ig flowing from the other terminals of resistive elements RE 11 , RE 12 (i.e., node N 1 ) to the line of ground voltage VSS (ground point), and provides a signal indicating the detected value to determination unit 3 . The line of ground voltage VSS is, for example, an automobile chassis.

Determination unit 3 generates ground fault detection signal φD indicating whether or not a ground fault accident occurs in the ungrounded AC circuit, based on the output signal of current detector 2 . In a case where no ground fault accident occurs and thus the absolute value of the instantaneous value of current Ig is lower than predetermined threshold value ITH, ground fault detection signal φD is set to an “L” level, which is a deactivated level. In a case where a ground fault accident occurs and thus the absolute value of the instantaneous value of current Ig exceeds predetermined threshold value ITH, ground fault detection signal φD is set to an “H” level, which is an activated level.

Next, an operation of ground fault detection apparatus 20 will be described. A current flowing from positive-side DC bus PL into resistive element RE 11 is referred to as I 11 , and a current flowing from DC neutral point NP into resistive element RE 12 is referred to as I 12 . Further, the resistance values of resistive elements RE 11 , RE 12 are referred to as R 11 , R 12 , respectively. R 11 , R 12 are set as R 11 =R 12 , and each of R 11 , R 12 is, for example, 20 (kΩ). It is noted that, when the resistance value of resistive element RE 13 is referred to as R 13 , R 13 is set as R 13 =R 11 +R 12 .

In the case where no ground fault accident occurs, current I=I 11 =−I 12 =(VDC/2)/(R 11 +R 12 )=VDC/(4·R 11 ) flows from positive-side DC bus PL, via resistive elements RE 11 , RE 12 , to DC neutral point NP. On this occasion, current Ig is set as Ig=I 11 +I 12 =0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

FIG. 20 is a time chart showing a simulation result of a temporal change of current Ig in a case where point Pg in neutral wire OL has a ground fault at a given time. In FIG. 20 , in a normal state before a ground fault accident occurs, currents I 11 , I 12 flowing through resistive elements RE 11 , RE 12 have opposite polarities, and current Ig is set as Ig=I 11 +I 12 =0 (mA).

When a ground fault occurs at point Pg in neutral wire OL at a given time, a low impedance current path is formed between the negative-side terminal of current detector 2 and ground fault point Pg, and a current flows through a path from positive-side DC bus PL, via resistive element RE 11 , current detector 2 , the line of ground voltage VSS, and ground fault point Pg, to neutral wire OL. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 is set as Ig=VDC/(2·R 11 ), and the absolute value of the instantaneous value of Ig exceeds threshold value ITH. Determination unit 3 in FIG. 19 sets ground fault detection signal φD to an “H” level as an activated level in response to the absolute value of the instantaneous value of Ig exceeding threshold value ITH, and notifies the user of the ungrounded AC circuit of the occurrence of a ground fault accident.

That is, current Ig having a value up to four times the value of current I 11 , I 12 flowing through resistive element RE 11 , RE 12 under normal operation conditions flows through current detector 2 . Since current Ig is normally Ig=0 (mA), in the case where a current having a value exceeding threshold value ITH flows through current detector 2 , it can be determined that a ground fault occurs. It is noted that, normally, the current supplied to loads LD 1 , LD 2 is sufficiently larger than Ig=VDC/R 11 , and thus the current supplied to loads LD 1 , LD 2 does not change even if neutral wire OL has a ground fault, and the ground fault current does not change depending on the magnitude of loads LD 1 , LD 2 .

FIG. 21 is a time chart showing a simulation result of a temporal change of current Ig in a case where point Pg in U phase wire UL has a ground fault at a given time. In FIG. 21 , in a normal state before a ground fault accident occurs, currents I 11 , I 12 flowing through resistive elements RE 11 , RE 12 have opposite polarities, and current Ig is set as Ig=I 11 +I 12 =0 (mA).

When a ground fault occurs at point Pg in U phase wire UL at a given time, a low impedance current path is formed between the negative-side terminal of current detector 2 and ground fault point Pg, and a current flows through a path from positive-side DC bus PL, via resistive element RE 11 , current detector 2 , the line of ground voltage VSS, and ground fault point Pg, to U phase wire UL. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 has an average value Ig=VDC/(2·R 11 ). It is noted that the absolute value of Ig may not exceed ITH, depending on the timing of the ground fault. Determination unit 3 in FIG. 19 sets ground fault detection signal φD to an “H” level as an activated level in response to the absolute value of the instantaneous value of Ig exceeding threshold value ITH, and notifies the user of the ungrounded AC circuit of the occurrence of a ground fault accident.

FIG. 22 is a time chart showing a simulation result of a temporal change of current Ig in a case where point Pg in V phase wire VL has a ground fault at a given time. In FIG. 22 , in a normal state before a ground fault accident occurs, currents I 11 , I 12 flowing through resistive elements RE 11 , RE 12 have opposite polarities, and current Ig is set as Ig=I 11 +I 12 =0 (mA).

When a ground fault occurs at point Pg in V phase wire VL at a given time, a low impedance current path is formed between the negative-side terminal of current detector 2 and ground fault point Pg, and a current flows through a path from positive-side DC bus PL, via resistive element RE 11 , current detector 2 , the line of ground voltage VSS, and ground fault point Pg, to V phase wire VL. When the ground fault path has an impedance of 0Ω, the current flowing through current detector 2 has an average value Ig=VDC/(2·R 11 ). In FIG. 22 , current Ig exceeding threshold value ITH does not flow when a ground fault occurs, and the absolute value of current Ig exceeds threshold value ITH after a predetermined time Tx elapses. It is noted that the absolute value of Ig may become considerably high, depending on the timing of the ground fault.

FIG. 23 is a circuit block diagram showing a configuration of a ground fault detection apparatus 25 in accordance with Embodiment 5 of the present invention, and is a view to be compared with FIG. 19 . Referring to FIG. 23 , ground fault detection apparatus 25 is different from ground fault detection apparatus 20 in FIG. 19 in that resistive elements RE 11 , RE 12 are replaced by resistive elements RE 21 to RE 23 , and switches SW 1 , SW 2 and pulse signal generator 11 are added.

Switch SW 1 is connected in series with resistive element RE 21 between positive-side DC bus PL and a node N 5 . Resistive element RE 22 is connected between DC neutral point NP and node N 5 . Switch SW 2 is connected in series with resistive element RE 23 between negative-side DC bus NL and node N 5 . Current detector 2 is connected between node N 5 and the line of ground voltage VSS (ground point). Resistive elements RE 21 to RE 23 have an identical resistance value.

Switches SW 1 , SW 2 are controlled by pulse signals φ 1 , φ 2 , respectively, from pulse signal generator 11 . Switch SW 1 is ON when signal φ 1 is at an “H” level, and is OFF when signal φ 1 is at an “L” level. Switch SW 2 is ON when signal φ 2 is at an “H” level, and is OFF when signal φ 2 is at an “L” level.

Pulse signal generator 11 sets pulse signal φ 1 to an “H” level for a predetermined time with a predetermined cycle, and sets pulse signal φ 2 to an “H” level for a predetermined time with a predetermined cycle. Pulse signal φ 1 is set to an “H” level for the predetermined time within a time period in which pulse signal φ 2 is at an “L” level, and pulse signal φ 2 is set to an “H” level for the predetermined time within a time period in which pulse signal φ 1 is at an “L” level. Switches SW 1 , SW 2 are turned on alternately, and are not turned on simultaneously. Switches SW 1 , SW 2 are turned on/off with an interval which is sufficiently shorter than a time required to detect a ground fault.

In a case where switch SW 1 is ON and switch SW 2 is OFF, ground fault detection apparatus 25 has the same configuration as that of ground fault detection apparatus 20 . In a case where switch SW 1 is OFF and switch SW 2 is ON, ground fault detection apparatus 25 has the same circuit configuration as that of ground fault detection apparatus 20 except that positive-side DC bus PL and negative-side DC bus NL of ground fault detection apparatus 20 are reversed. In order to detect a ground fault within a predetermined time from when the ground fault occurs, it is only necessary to set a time in which both of switches SW 1 , SW 2 are OFF to be sufficiently shorter than the predetermined time.

Next, an operation of ground fault detection apparatus 25 will be described. A current flowing from positive-side DC bus PL into resistive element RE 21 is referred to as I 21 , a current flowing from DC neutral point NP into resistive element RE 22 is referred to as I 22 , and a current flowing from negative-side DC bus NL into resistive element RE 23 is referred to as I 23 . Further, the resistance values of resistive elements RE 21 to RE 23 are referred to as R 21 to R 23 , respectively. R 21 to R 23 are set as R 21 =R 22 =R 23 , and each of R 21 , R 22 , R 23 is, for example, 40 (kΩ).

In a case where no ground fault accident occurs, when switch SW 1 is ON, current I=I 21 =−I 22 =(VDC/2)/(R 21 +R 22 )=VDC/(4·R 22 ) flows from positive-side DC bus PL, via switch SW 1 and resistive elements RE 21 , RE 22 , to DC neutral point NP. On this occasion, current Ig is set as Ig=I 21 +I 22 =0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

Similarly, in the case where no ground fault accident occurs, when switch SW 2 is ON, current I=I 22 =−I 23 =(VDC/2)/(R 22 +R 23 )=VDC/(4·R 22 ) flows from DC neutral point NP, via switch SW 2 and resistive elements RE 22 , RE 23 , to negative-side DC bus NL. On this occasion, current Ig is set as Ig=I 22 +I 23 =0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

In the case where no ground fault accident occurs, when both of switches SW 1 , SW 2 are OFF, no current flows through resistive elements RE 21 to RE 23 , and 121 to 123 and Ig are each 0 (mA). Therefore, |Ig|<ITH is obtained, and ground fault detection signal φD is set to an “L” level as a deactivated level.

FIGS. 24( a ) to 24( d ) are time charts showing simulation results of temporal changes of currents I 21 to I 23 and Ig in a case where point Pg in neutral wire OL has a ground fault at a given time. FIGS. 24( e ) and 24( f ) are time charts showing ON/OFF states of switches SW 1 , SW 2 . In a case where switch SW 1 is ON, ground fault detection apparatus 25 has the same configuration as that of ground fault detection apparatus 20 . Since switch SW 1 is ON for the predetermined time with the predetermined cycle, and currents I 21 , I 22 , Ig flow only when switch SW 1 is ON, each of currents I 21 , I 22 , Ig also changes in a pulsed manner in response to switch SW 1 .

In a case where switch SW 2 is ON, ground fault detection apparatus 25 has the same configuration as that of ground fault detection apparatus 20 except that positive-side DC bus PL and negative-side DC bus NL are reversed. Since switch SW 2 is ON for the predetermined time with the predetermined cycle, and currents I 22 , I 23 , Ig flow only when switch SW 2 is ON, each of currents I 22 , I 23 , Ig also changes in a pulsed manner in response to switch SW 2 .

In a normal state before a ground fault accident occurs, current I 21 and current I 22 have opposite polarities, and current Ig is set as Ig=I 21 +I 22 =0 (mA). Further, current I 22 and current I 23 have opposite polarities, and current Ig is set as Ig=I 22 +I 23 =0 (mA). When a ground fault occurs at point Pg in neutral wire OL at a given time, current I 22 is set as I 22 =0 (mA) and current Ig is set as Ig=I 21 =VDC/(2·R 21 ) or Ig=−I 23 =−VDC/(2·R 23 ). Therefore, the absolute value of the instantaneous value of Ig exceeds threshold value ITH, and ground fault detection signal φD is set to an “H” level as an activated level.

FIGS. 25( a ) to 25( d ) are time charts showing simulation results of temporal changes of currents I 21 to I 23 and Ig in a case where point Pg in U phase wire UL has a ground fault at a given time. FIGS. 25( e ) and 25( f ) are time charts showing ON/OFF states of switches SW 1 , SW 2 .

In a normal state before a ground fault accident occurs, current I 21 and current I 22 have opposite polarities, and current Ig is set as Ig=I 21 +I 22 =0 (mA). Further, current I 22 and current I 23 have opposite polarities, and current Ig is set as Ig=I 22 +I 23 =0 (mA). Therefore, the absolute value of the instantaneous value of Ig is lower than threshold value ITH, and ground fault detection signal φD is set to an “L” level as a deactivated level.

In the case where a ground fault occurs at point Pg in U phase wire UL at a given time, when switch SW 1 is ON, Ig has an average value of VDC/(2·R 21 ); when switch SW 2 is ON, Ig has an average value of −VDC/(2·R 23 ); and when both of switches SW 1 , SW 2 are OFF, Ig has an average value of 0. Therefore, the absolute value of the instantaneous value of Ig exceeds threshold value ITH, and ground fault detection signal φD is set to an “H” level as an activated level.

FIGS. 26( a ) to 26( d ) are time charts showing simulation results of temporal changes of currents I 21 to I 23 and Ig in a case where point Pg in V phase wire VL has a ground fault at a given time. FIGS. 26( e ) and 26( f ) are time charts showing ON/OFF states of switches SW 1 , SW 2 .

In a normal state before a ground fault accident occurs, current I 21 and current I 22 have opposite polarities, and current Ig is set as Ig=I 21 +I 22 =0 (mA). Further, current I 22 and current I 23 have opposite polarities, and current Ig is set as Ig=I 22 +I 23 =0 (mA). Therefore, the absolute value of the instantaneous value of Ig is lower than threshold value ITH, and ground fault detection signal φD is set to an “L” level as a deactivated level.

In the case where a ground fault occurs at point Pg in V phase wire VL at a given time, when switch SW 1 is ON, Ig has an average value of −VDC/(2·R 21 ); when switch SW 2 is ON, Ig has an average value of VDC/(2·R 23 ); and when both of switches SW 1 , SW 2 are OFF, Ig has an average value of 0. Therefore, the absolute value of the instantaneous value of Ig exceeds threshold value ITH, and ground fault detection signal φD is set to an “H” level as an activated level.

As described above, in Embodiment 4, time Tx is required from when a ground fault occurs to when the ground fault is determined, and a longer time may be required to detect a ground fault, depending on when the ground fault occurs. In contrast, in Embodiment 5, a ground fault can be detected in a short time determined by on/off times of switches SW 1 , SW 2 .

In addition, when both of switches SW 1 , SW 2 are in an OFF state, no current flows through resistive elements RE 21 to RE 23 . Therefore, power consumption during that period can be reduced, and lower power consumption can be achieved.

Although the embodiments of the present invention have been described, it should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the scope of the claims, and is intended to include any modifications within the scope and meaning equivalent to the scope of the claims.