Abstract:
A direct current detection circuit has a zero-phase current transformer with source lines inserted therethrough for detecting current differences among them and generates a comparison voltage value based on a divided voltage value obtained between the zero-phase current transformer and a voltage divider resistor according to a change in the self-impedance of the zero-phase current transformer. An offset current is passed through the zero-phase current transformer to make it possible to detect on the basis of the comparison voltage value a direct current value in a range which is otherwise difficult to detect accurately on the basis of the comparison voltage value because of influence of hysteresis characteristic of the zero-phase current transformer. A control circuit detects a present direct current value based on the comparison voltage value and the value of the offset current.

Description:
[0001]     Priority is claimed on Japanese Patent Application 2003-290269 filed Aug. 8, 2003.  
       BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to a direct current detection circuit for detecting a direct current value by using a zero-phase current transformer (hereinafter referred to as ZCT) making use of a magnetic material like as Permalloy, such as a direct grounding current detecting circuit for detecting the occurrence of a direct grounding current used, for example, for linkage protection of a linkage apparatus.  
         [0003]     As an example of a direct grounding current detecting circuit serving as a direct current detecting circuit, Japanese Patent Publication Tokkai 11-122819 has disclosed the technology applicable to a linkage apparatus for converting direct-current power from a direct-current source into alternate-current power by using a current transformer to detect the difference between the current values flowing for the positive and negative terminals of the direct-current source and to judge according to the level of this current difference whether a direct grounding current is being generated.  
         [0004]     With such a detection circuit, a direct grounding current accident of a system linkage apparatus can be prevented on the basis of the presence or absence of a direct grounding current because the occurrence of such a direct grounding current may be detected if the current difference detected by the current transformer exceeds a certain level while it is concluded that such a direct grounding current is absent if the detected current difference is found not to exceed this level.  
         [0005]     The manner in which such a direct grounding current detecting circuit may be used in a linkage system is explained next with reference to  FIG. 10  which is a block diagram showing the structure of a general linkage system  100  comprised of a system linkage apparatus  103  disposed between a direct current source  101  such as a solar battery and an alternate current system  102  such as a single-phase three-line type for converting the direct current power of the direct current source  101  into alternate current power. The system linkage apparatus  103  comprises an inverter  104  for converting the direct current power from the direct current source  101  into alternate current power, a linkage relay  105  for the ON/OFF control on the linkage with the alternate current system  102 , a ZCT  10  for magnetically detecting the current difference between the U-phase source line  110 A and the W-phase source line  110 B between the inverter  104  and the alternate current system  102 , a direct grounding current detection circuit  107  for detecting the presence or absence of a direct grounding current on the basis of the current difference detected by this ZCT  10  and a CPU  108  for controlling the inverter  104 , the linkage relay  105 , etc. on the basis of the detection result by the direct grounding current detection circuit  107 .  
         [0006]     The CPU  108  of the system linkage apparatus  103  serves to cut off the linkage with the alternate current system  102  by controlling the linkage relay  105  to be switched off when the direct grounding current detection circuit  107  detects the generation of a direct grounding current.  
         [0007]      FIG. 11  is a block diagram for approximately showing the inner structure of the direct grounding current detection circuit  107 .  FIGS. 12A and 12B  are waveform diagrams of various outputs for showing the operations of the direct grounding current detection circuit  107  respectively when it is judging that a direct grounding current is absent and present.  
         [0008]     The direct grounding current detection circuit  107  comprises the aforementioned ZCT  10  having the U-phase source line  110 A and the W-phase source line  110 B inserted as explained above for magnetically detecting the current different therebetween and changing its self-impedance according to the change in the magnetic field generated by this current difference, an oscillator circuit  11  for generating a specified voltage V 1 , a voltage divider resistor  12  connected in series with the ZCT  10 , a comparison voltage generating circuit  13  for generating a comparison voltage V 5  on the basis of the voltage value V 2  divided between the ZCT  10  and the voltage divider resistor  12  according to the change in the impedance of the ZCT  10  and a control circuit  14  for judging whether or not the comparison voltage V 5  generated by the comparison voltage generating circuit  13  is above a threshold value for determining the presence or absence of a direct grounding current and detecting the presence or absence of the occurrence of a direct grounding current on the basis of the result of this judgment. In the above, the voltage value V 2  is obtained as Z 1 /(Z 1 +R 1 )*V 1  where Z 1  is the impedance of the ZCT  10  and R 1  is the resistance of the voltage divider resistor  12 .  
         [0009]     As shown in  FIG. 11 , the comparison voltage generating circuit  13  comprises a rectifier circuit  131  for detecting and rectifying the voltage value V 2 , an offset amplifier circuit  132  for offset-amplifying the output voltage V 3  from this rectifier circuit  131  and a filter circuit  133  for filtering the output voltage V 4  from this offset amplifier circuit  132  and thereby outputting a comparison voltage value V 5 .  
         [0010]     The control circuit  14  judges that a direct grounding current has occurred relative to the source lines  110 A and  110 B if the comparison voltage value V 5  from the comparison voltage generating circuit  13  is above a threshold value (a reference level) as shown in  FIG. 12B , but that there is no occurrence of a direct grounding current if it is determined that the comparison voltage value V 5  is not above the threshold value, as shown in  FIG. 12A .  
         [0011]     In summary, the prior art direct grounding current detection circuit  107  serves to generate the comparison voltage value V 5  on the basis of the voltage value V 2  divided by the ZCT  10  and the voltage divider resistor  12  according to the change in the impedance of the ZCT  10 , determining the occurrence of a direct grounding current to be present or absence according to whether this comparison voltage value V 5  is above or below the threshold value, and hence is capable of dependably detecting the presence or absence of a direct grounding current.  
         [0012]     Although an example with an alternate current system with the single-phase three-line type was described above with reference to  FIG. 10  with the U-phase and W-phase source lines  11 A and  110 B inserted through the ZCT  10 , a similar effect can be obtained with a system of the single-phase two-line type by inserting the L-phase and N-phase source lines through the ZCT  10 .  
         [0013]     It is to be noted that the prior art direct grounding current detection circuit  107  employs a ZCT  10  made of Permalloy as its magnetic material for detecting the current difference between the source lines  110 A and  110 B. Since Permalloy has a hysteresis characteristic as shown in  FIG. 13 , the magnetic permeability-magnetization current characteristic of the ZCT  10  is as shown in  FIG. 14 . The prior art direct grounding current detection circuit  107  is therefore influenced by this hysteresis characteristic of its ZCT  10 . If it is desired to detect the presence or absence of a direct grounding current of a very small value in the range of −30 mA to +30 mA (such as shown shaded in the graph of  FIG. 15 ) such as 15 mA, the comparison voltage value V 5  appears at two points and it becomes difficult to detect an accurate direct current value or to judge correctly if there is a direct grounding current.  
         [0014]     This problem is not limited to the direct grounding current detection circuit  107 . Direct current detection circuits are generally influenced by the hysteresis characteristic of their ZCT  10  and are not capable of accurately detecting a direct current value.  
       SUMMARY OF THE INVENTION  
       [0015]     It is therefore an object of this invention in view of the above to provide a direct current detection circuit capable of accurately detecting the direct current value of the present moment without the influence of the hysteresis characteristic of the ZCT.  
         [0016]     It is a particular object of this invention to provide a direct grounding current detection circuit capable of accurately detecting the occurrence of a direct grounding current without the influence of the hysteresis characteristic of the ZCT.  
         [0017]     A direct current detection circuit according to this invention, in view of such objects, may be characterized not only as comprising a ZCT having source lines inserted therethrough and being adapted to magnetically detect current differences among these source lines and to vary its self-impedance according to changes in magnetic field generated by this current difference, a voltage divider resistor connected in series to the ZCT, a comparison voltage generating circuit for generating a comparison voltage value based on a divided voltage value obtained between the ZCT and the voltage divider resistor according to a change in the self-impedance of the ZCT, a control circuit for detecting a present direct current value based on the comparison voltage value generated by the comparison voltage generating circuit, an offset current line which is inserted through the ZCT and an offset current generating circuit for generating an offset current through the offset current line to make it possible to detect on the basis of the comparison voltage value a direct current value in a range which would be difficult to detect accurately on the basis of the comparison voltage value without the offset current because of the influence of the hysteresis characteristic of the ZCT, but also wherein the control circuit is adapted to detect a current value on the basis of the comparison voltage value generated by the comparison voltage generating circuit and to calculate the present direct current value on the basis of the detected value and the value of the offset current.  
         [0018]     Another direct current detection circuit according to another embodiment of this invention may be characterized not only as further comprising a compensatory ZCT having the same characteristics as the aforementioned ZCT, having the source lines inserted therethrough and having inserted therethrough the offset current line such that an opposite offset current which is opposite in direction to the offset current is detected thereby, a compensatory voltage divider resistor connected in series to the compensatory ZCT, and a compensatory comparison voltage generating circuit for generating a compensatory comparison voltage value based on another divided voltage value obtained between the compensatory ZCT and the compensatory voltage divider resistor according to a change in the self-impedance of the compensatory ZCT, but also wherein the control circuit includes a current value calculating circuit adapted to detect a current value in a positive direction based on the comparison voltage value generated by the comparison voltage generating circuit, to calculate a direct current value in the positive direction based on the current value in the positive direction and the current value of the offset current, to detect a current value in the negative direction based on the compensatory comparison voltage value generated by the compensatory comparison voltage generating circuit, and to calculate a direct current value in the negative direction based on the current value in the negative direction and the current value of the offset current and a present current value detection circuit adapted to selectively detect the direct current value in the positive direction as the present direct current value if the comparison voltage value is judged to be higher than the compensatory comparison voltage value and to selectively detect the direct current value in the negative direction as the present direct current value if the compensatory comparison voltage value is judged to be higher than the comparison voltage value.  
         [0019]     Still another direct current detection circuit according to a third embodiment of this invention may be further characterized wherein the offset current generating circuit generates an offset current with different current values for every specified period and the control circuit includes a direct current value calculating circuit for detecting current values in the positive and negative directions based on the comparison voltage value generated by the comparison voltage generating circuit and calculating a direct current value in positive direction based on the current value in the positive direction and the current value of the offset current and a direct current value in negative direction based on the current value in the negative direction and the current value of the offset current, a monitoring circuit for monitoring shift in relationship between the current value of the offset current and the comparison voltage value, and a present current value detection circuit for detecting the direct current value in positive direction as the present direct current value if the monitor circuit finds that the comparison voltage value becomes higher as the offset current value becomes higher and detecting the direct current value in negative direction as the present direct current value if the monitor circuit finds that the comparison voltage value becomes lower as the offset current value becomes higher.  
         [0020]     Still another direct current detection circuit according to a fourth embodiment of this invention may be characterized not only as comprising a ZCT having source lines inserted therethrough and being adapted to magnetically detect current differences among the source lines and to vary its self-impedance according to changes in magnetic field generated by the current difference, a voltage divider resistor connected in series to the ZCT, a comparison voltage generating circuit for generating a comparison voltage value based on a divided voltage value obtained between the ZCT and the voltage divider resistor according to a change in the self-impedance of the ZCT, and a control circuit for detecting a present direct current value based on the comparison voltage value generated by the comparison voltage generating circuit, but also wherein the ZCT has the source lines inserted therethrough a plural number of times so as to make it possible to detect on the basis of the comparison voltage value a direct current value in a range which is otherwise difficult to detect accurately on the basis of the comparison voltage value because of influence of hysteresis characteristic of the ZCT, and wherein the control circuit is adapted to detect a current value on the basis of the comparison voltage value generated by the comparison voltage generating circuit and to calculate the present direct current value on the basis of the detected value and the value of a direct current value passing through the source lines passing through the ZCT the plural number of times.  
         [0021]     A direct grounding current detection circuit embodying this invention, in view of the aforementioned object of the invention, may be characterized not only as comprising a ZCT having source lines inserted therethrough and being adapted to magnetically detect current differences among the source lines and to vary self-impedance according to changes in magnetic field generated by the current difference, a voltage divider resistor connected in series to the ZCT, a comparison voltage generating circuit for generating a comparison voltage value based on a divided voltage value obtained between the ZCT and the voltage divider resistor according to a change in the self-impedance of the ZCT, a control circuit for detecting presence or absence of direct grounding current relative to the source lines based on the comparison voltage value generated by the comparison voltage generating circuit, an offset current line which is inserted through the ZCT, an offset current generating circuit for generating an offset current through the offset current line to shift a range of current values where it is difficult to accurately detect a direct current value of the direct grounding current because of influence of hysteresis characteristic of the ZCT into a detectable range where it is possible to detect the direct current value of the direct grounding current, and a threshold memory storing an upper threshold value and a lower threshold value to be used in determining presence and absence of the direct grounding current in the detectable range, but also wherein the control circuit is adapted to conclude that there is occurrence of the direct grounding current relative to the source lines if it is judged that the comparison voltage value is equal to or above the upper threshold value or equal to or below the lower threshold value.  
         [0022]     Another direct grounding current detection circuit according to a second embodiment of the invention may be characterized as further comprising a compensatory ZCT having same characteristics as the ZCT, having the source lines inserted therethrough and having inserted therethrough the offset current line such that an opposite offset current which is opposite in direction to the offset current is detected thereby, a compensatory voltage divider resistor connected in series to the compensatory ZCT, and a compensatory comparison voltage generating circuit for generating a compensatory comparison voltage value based on another divided voltage value obtained between the compensatory ZCT and the compensatory voltage divider resistor according to a change in the self-impedance of the compensatory ZCT, and wherein the control circuit includes a judging circuit adapted to judge that a direct grounding current has occurred in a positive direction if the comparison voltage value generated by the comparison voltage generating circuit is equal to or above the upper threshold value and that a direct grounding current has occurred in the negative direction if the compensatory comparison voltage value generated by said compensatory comparison voltage generating circuit is equal to or above the upper threshold value.  
         [0023]     Still another direct grounding current detection circuit according to a third embodiment of the invention may be further characterized wherein the offset current generating circuit generates an offset current with different current values for every specified period and the control circuit includes a monitoring circuit for monitoring shift in relationship between the current value of the offset current and the comparison voltage value when it is judged that a direct grounding current has occurred relative to the source lines, and a detection circuit for detecting the direct current value in a positive direction if the monitoring circuit finds that the comparison voltage value becomes higher as the offset current value becomes higher and detecting the direct current value in a negative direction opposite the positive direction if the monitoring circuit finds that the comparison voltage value becomes lower as the offset current value becomes higher.  
         [0024]     Still another direct grounding current detection circuit according to a fourth embodiment of the invention may be characterized not only as comprising a ZCT having source lines inserted therethrough and being adapted to magnetically detect current differences among the source lines and to vary the self-impedance according to changes in magnetic field generated by the current difference, a voltage divider resistor connected in series to the ZCT, a comparison voltage generating circuit for generating a comparison voltage value based on a divided voltage value obtained between the ZCT and the voltage divider resistor according to a change in the self-impedance of the ZCT, and a control circuit for detecting presence or absence of occurrence of a direct grounding current relative to the source lines based on the comparison voltage value generated by the comparison voltage generating circuit, but also wherein the ZCT has the source lines inserted therethrough a plural number of times so as to shift a range of direct current values where it is difficult to accurately detect a direct current value of the direct grounding current because of influence of hysteresis characteristic of the ZCT into a detectable range where it is possible to detect the direct current value of the direct grounding current based on the comparison voltage value.  
         [0025]     With a circuit thus structured, an offset current is passed through a ZCT such that the range or current values in which it is difficult to accurately detect a direct current value such as a direct grounding current value based on a comparison voltage value because of the influence of hysteresis characteristic of the ZCT will be sufficiently shifted to a detectable range in which such detection is possible. As a result, a present direct current value such as a direct grounding current value can be detected accurately without being affected by the hysteresis characteristic of the ZCT. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a block diagram of the inner structure of a direct grounding current detection circuit according to a first embodiment of the invention.  
         [0027]      FIG. 2  is a graph for showing the relationship between the comparison voltage value and the direct current value for the direct grounding current detection circuit of  FIG. 1  according to the first embodiment of the invention.  
         [0028]      FIG. 3  is a block diagram of the inner structure of a direct grounding current detection circuit according to a second embodiment of the invention.  
         [0029]      FIG. 4  is a graph for showing the relationship between the comparison voltage value and the direct current value for the direct grounding current detection circuit of  FIG. 3  according to the second embodiment of the invention.  
         [0030]      FIG. 5  shows an example of relationship between the direct grounding current and the comparison voltage value for each offset current for the direct grounding current detection circuit of  FIG. 3  according to the second embodiment of the invention.  
         [0031]      FIG. 6  is a block diagram of the inner structure of a direct grounding current detection circuit according to a third embodiment of the invention.  
         [0032]      FIG. 7  is a graph for showing the relationship between the comparison voltage value and the direct current value for the direct grounding current detection circuit of  FIG. 6  according to the third embodiment of the invention.  
         [0033]      FIG. 8  shows an example of relationship between the direct grounding current and the shift in the comparison voltage value for the direct grounding current detection circuit of  FIG. 6  according to the third embodiment of the invention.  
         [0034]      FIG. 9  is a block diagram of the inner structure of a direct grounding current detection circuit according to a fourth embodiment of the invention.  
         [0035]      FIG. 10  is a block diagram of a general linkage system.  
         [0036]      FIG. 11  is a block diagram of the inner structure of a prior art direct grounding current detection circuit.  
         [0037]      FIG. 12 , including  FIGS. 12A and 12B , is a waveform diagram for the direct grounding current detection circuit of  FIG. 11 , when it is determined that there is no direct grounding current (as shown in  FIG. 12A ) and when it is determined that there is a direct grounding current (as shown in  FIG. 12B ).  
         [0038]      FIG. 13  shows a typical hysteresis characteristic of a ZCT in a prior art direct grounding current detection circuit.  
         [0039]      FIG. 14  shows the principle of how the magnetic permeability-magnetization current characteristic of a ZCT in a prior art direct grounding current detection circuit is affected.  
         [0040]      FIG. 15  shows the relationship between the comparison voltage value and the direct current value for a prior art direct grounding current detection circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]     The invention is described next by way of several embodiments.  FIG. 1  is a block diagram showing the inner structure of a direct grounding current detection circuit  1 A according to one (first) embodiment of this invention. Those of its components that are similar or equivalent to those already shown in and explained above with reference to  FIG. 10  are indicated by the same numerals.  
         [0042]     The direct grounding current detection circuit  1 A of this invention comprises not only a ZCT  10  with source lines  110 A and  110 B passing therethrough, an oscillator circuit  11 , a voltage divider resistor  12  and a comparison voltage generating circuit  13 , but also an offset current wire  21 , an offset current generating circuit  22 , a threshold memory  23  and a control circuit  14 A. The offset current generating circuit  22  is for generating an offset current such as 60 mA and causing it to constantly flow through the offset current wire  21  such that a range X (shaded in  FIG. 2 ) of current in which the value of the direct grounding current is difficult to detect from the comparison voltage value V 5  as explained above because of the influence of the hysteresis characteristic of the ZCT  10  can be shifted to another range where such detection is possible. The threshold memory  23  is a memory device for storing an upper threshold value VA and a lower threshold value VB corresponding to a direct current value related to the determination of the occurrence of a direct grounding current within the range wherein the detection is possible as explained above. The control circuit  14 A serves to judge that a direct grounding current occurred relative to the source lines  110 A and  110 B if the comparison voltage value V 5  generated by the comparison voltage generating circuit  13  is found to be above the upper threshold value VA or below the lower threshold value VB.  
         [0043]     Next, the operation of the direct grounding current detection circuit  1 A as described above will be explained. As a result, the range of current values (such as from −30 mA to +30 mA) in which the direct current value could not detected accurately because of the effect of the hysteresis characteristic of the ZCT  10  is shifted as shown in  FIG. 2  into a range from +30 mA to +90 mA which is not influenced by the hysteresis characteristic of the ZCT  10 . As a result, the direct grounding current in the range of from −30 mA to +30 mA can be accurately detected on the basis of the comparison voltage value V 5 .  
         [0044]     When the voltage value V 2  divided between the ZCT  10  and the voltage divider resistor  12  according to the change in the impedance of the ZCT  10  is detected, the comparison voltage generating circuit  13  generates the comparison voltage value V 5  on the basis of this divided voltage value V 2  and transmits this generated comparison voltage value V 5  to the control circuit  14 A.  
         [0045]     As the comparison voltage value V 5  is detected, the control circuit  14 A determines whether or not it is above the upper threshold VA. If it is determined to be equal to or above the upper threshold VA, it concludes that the direct grounding current is equal to or greater than 30 mA and hence that the direct grounding current relative to the source lines  110 A and  110 B has occurred. The control circuit  14 A also serves to determine whether or not the comparison voltage value V 5  is equal to or below the lower threshold value VB. If it is determined to be equal to or below the lower threshold value VB, it concludes that the direct grounding current is equal to or below −30 mA and hence that the direct grounding current relative to the source lines  110 A and  110 B has occurred. If the comparison voltage value V 5  is found to be neither equal to or above the upper threshold value VA nor equal to or below the lower threshold value VB, the control circuit  14 A concludes that there is no occurrence of direct grounding current relative to the source lines  110 A and  110 B.  
         [0046]     According to this first embodiment of the invention, an offset current is caused to flow through the ZCT  10  such that the minimum current range which is influenced by the hysteresis characteristic of the ZCT  10  is shifted to another range (the detectable range) where the detection of direct current value based on the comparison voltage value V 5  is possible, while the upper and lower threshold values VA and VB for the judgment of presence or absence of direct grounding current relative to the source lines  110 A and  110 B are stored in a memory. Thus, the presence or absence of the occurrence of direct grounding current can be dependably determined without the influence of the hysteresis characteristic of the ZCT  10 .  
         [0047]     Although the first embodiment of the invention was described above by way of the direct grounding current detection circuit  1 A, it goes without saying that it can be applied to a direct current detection circuit for the detection of a direct current value at the present time. In this case, too, since an offset current is used to shift the range of detection, it becomes possible to detect a current value based on the comparison voltage value V 5  and since this detected current value may be regarded as the current value at the present time, the detection can be made dependably without the influence of the hysteresis characteristic of the ZCT  10 .  
         [0048]     When the direct grounding current detection circuit  1 A according to the first embodiment of the invention finds that the comparison voltage value V 5  is equal to or above the upper threshold value VA, it is concluded that a direct grounding current has occurred, as explained above, but when the comparison voltage value V 5  is equal to or above the upper threshold value, the direct grounding current may be in the positive direction, being equal to or greater than 30 mA, or in the negative direction, being equal to or greater (in absolute value) than 150 mA, as shown in  FIG. 4 . Similarly, when the comparison voltage value V 5  is equal to or below the lower threshold value VB, the direct grounding current may be between 30 mA and 90 mA in the negative direction. In the above, a direct grounding current in the positive (negative) direction means a current in the positive (negative) direction with respect to the 0 mA of the total of the “direct grounding current” and the “offset current” as shown in  FIG. 4 .  
         [0049]     The flow direction of the direct grounding current can be determined accurately by continuing to monitor the direct grounding current based on the comparison voltage value V 5  equal to or below the lower threshold value VB. This method of determining whether the direct grounding current is flowing in the positive direction or in the negative direction by continuing to monitor the direct current value is applicable only if the variation in the direct grounding current is slow and gentle. When the direct grounding current changes suddenly from 0 mA to −150 mA, for example, it may be erroneously concluded to be a direct grounding current in the positive direction although the comparison voltage value is actually above the upper threshold value VA because it is not possible to continue monitoring the changes in the direct current value based on the comparison voltage value V 5 .  
         [0050]      FIG. 3  is a block diagram of another direct grounding current detection circuit  1 B according to a second embodiment of this invention, adapted to be able to determine whether a direct grounding current is in the positive or negative direction although its occurrence takes place suddenly. Components which are substantially the same as those shown above in  FIG. 1  are indicated by the same numerals and their structures and functions may not be described repetitiously.  
         [0051]     The direct grounding current detection circuit  1 B shown in  FIG. 3  comprises not only a ZCT  10  with source lines  110 A and  110 B and an offset current wire  21  passing therethrough, an oscillator circuit  11 , a comparison voltage generating circuit  13 , an offset current generating circuit  22  and a threshold memory  23  but also another (compensatory) ZCT  10 B, another (compensatory) voltage divider resistor  12 B, another (compensatory) comparison voltage generating circuit  13 B and a control circuit  14 B. The compensatory ZCT  10 B has the same characteristics as the first ZCT  10  and has inserted therethrough not only the source lines  110 A and  110 B but also the offset current wire  21  in such a way that an offset current in the opposite direction to the offset current detected by the first ZCT  10  is detected. The compensatory voltage divider resistor  12 B has the same characteristics as the first voltage divider resistor  12  described above with reference to  FIG. 1  and is connected in series with the compensatory ZCT  10 B. The compensatory voltage generating circuit  13 B is for generating a compensatory comparison voltage value on the basis of a divided voltage value obtained between the compensatory ZCT  10 B and the compensatory voltage divider resistor  12 B according to the change in the impedance of the compensatory ZCT  10 B. The control circuit  14 B serves to judge the presence or absence of a direct grounding current relative to the source lines  110 A and  110 B on the basis of the stored content of the threshold memory  23 , the comparison voltage value or the compensatory comparison voltage value.  
         [0052]     The offset current which flows through the compensatory ZCT  10 B will be (−60 mA) opposite in direction to the offset current (+60 mA) which flows through the first ZCT  10 .  
         [0053]     The voltage generating circuit  13  is for judging the presence of a direct grounding current in the positive direction on the basis of the comparison voltage value V 5 , while the compensatory comparison voltage generating circuit  13 B is for judging the presence of a direct grounding current in the opposite (negative) direction on the basis of the compensatory comparison voltage value V 5 ′.  
         [0054]     The control circuit  14 B comprises a judging circuit  31  for direct grounding current in the positive direction for judging whether or not the comparison voltage value V 5  generated by the comparison voltage generating circuit  13  is equal to or above the upper threshold value VA, another judging circuit  32  for direct grounding current in the negative direction for judging whether or not the compensatory comparison voltage value V 5 ′ generated by the compensatory comparison voltage generating circuit  13 B is equal to or above the upper threshold value VA, and a final judging circuit  33  for judging on the basis of the results of judgments by these two judging circuits  31  and  32  not only whether or not a direct grounding current has occurred but, if it is determined that there was an occurrence, also whether a direct grounding current in the positive direction or in the negative direction has occurred. In summary, the final judging circuit  33  concludes that a direct grounding current has occurred in the positive direction if the comparison voltage value V 5  is judged to be equal to or above the upper threshold value VA by the judging circuit  31  for the positive direction and that a direct grounding current has occurred in the negative direction if the compensatory comparison voltage value V 5 ′ is judged to be equal to or above the upper threshold value VA by the judging circuit  32  for the negative direction. In other words, the direction of the direct grounding current is determined, depending upon whether the comparison voltage value or the compensatory comparison voltage value is equal to or above the upper threshold value VA.  
         [0055]     Next, the principle of thus judging the direction of the direct grounding current (that is, depending on whether the comparison voltage value or the compensatory comparison voltage value is equal to or above the upper threshold value VA) will be explained with reference to  FIGS. 4 and 5 .  FIG. 4  shows the relationship between the comparison voltage value V 5  and the direct current values (for the direct grounding current and the sum of the direct grounding current and the offset current) in the case of the direct grounding current detection circuit  1 B according to the second embodiment of the invention.  FIG. 5  shows an example of relationship between the direct grounding current and the comparison voltage value for each offset current for the direct grounding current detection circuit of  FIG. 2  according to the second embodiment of the invention.  
         [0056]     As shown in  FIGS. 4 and 5 , it will be assumed that an offset current of +60 mA flows through the ZCT  10  and that of −60 mA through the compensatory ZCT  10 B. Let us further assume that a direct grounding current of +30 mA has occurred through the ZCT  10  and the compensatory ZCT  10 B. This means, as shown in  FIGS. 4 and 5 , a total current of +90 mA (=+30 mA of direct grounding current and +60 mA of offset current) flows through the ZCT  10  while the total current through the compensatory ZCT  10 B is −30 mA (=+30 mA of direct grounding current and −60 mA of offset current).  
         [0057]     The comparison voltage generating circuit  13  generates the comparison voltage V 5  on the basis of the voltage value V 2  divided between the ZCT  10  and the voltage divider resistor  12  according to the change in the impedance of the ZCT  10 . In this case, the value of the comparison voltage V 5  becomes the same as the upper threshold value VA, as shown in  FIG. 4 , and the sum of the direct grounding current and the offset current becomes 90 mA.  
         [0058]     The compensatory comparison voltage generating circuit  13 B generates the compensatory comparison voltage V 5 ′ on the basis of the voltage value V 2 ′ divided between the compensatory ZCT  10 B and the compensatory voltage divider resistor  12 B according to the change in the impedance of the compensatory ZCT  10 B. In this case, the value of the compensatory comparison voltage V 5  becomes the same as the lower threshold value VB, as shown in  FIG. 4 , and the sum of the direct grounding current and the offset current becomes −30 mA.  
         [0059]     In summary, although the same direct grounding current of +30 mA is flowing through both the ZCT  10  and the compensatory ZCT  10 B, the comparison voltage value V 5  becomes greater than the upper threshold value while the compensatory comparison voltage value V 5 ′ becomes below the lower threshold value. In other words, from the point of view that the actual direct grounding current value is 30 mA, it may be concluded that it is the comparison voltage value V 5  found to be above the upper threshold value VA that is correct.  
         [0060]     When a direct grounding current of −30 mA is flowing, on the other hand, a total current (with the offset current) of +30 mA flows through the ZCT  10  while the total current that flows through the compensatory ZCT  10 B is −90 mA. Thus, as shown in  FIG. 4 , a comparison voltage value V 5  below the lower threshold value VB is generated by the comparison voltage generating circuit  13  while a compensatory comparison voltage value V 5 ′ above the upper threshold value VA is generated by the compensatory comparison voltage generating circuit  13 B. From the point of view that the actual direct grounding current value is −30 mA, by contrast, it may be concluded that it is the compensatory comparison voltage value V 5 ′ found to be above the upper threshold value VA that is correct.  
         [0061]     Thus, it is concluded that a direct grounding current in the positive direction has occurred if it is the comparison voltage value V 5  that is found to be equal to or above the upper threshold value and that a direct grounding current in the negative direction has occurred if it is the compensatory comparison voltage value V 5 ′ that is found to be equal to or above the upper threshold value.  
         [0062]     Next, the operation of the comparison voltage generating circuit  1 B according to the second embodiment of the invention will be described. With reference still to  FIG. 3 , it will be assumed that an offset current of +60 mA flows through the ZCT  10  and that of −60 mA flows through the compensatory ZCT  10 B.  
         [0063]     The comparison voltage generating circuit  13  serves to generate a comparison voltage value V 5  from the voltage value V 2  obtained between the ZCT  10  and the voltage divider resistor  12  and transmits this generated comparison voltage value V 5  to the judging circuit  31  for direct grounding current in the positive direction. The compensatory comparison voltage generating circuit  13 B serves to generate a compensatory comparison voltage value V 5 ′ from the voltage value V 2 ′ obtained between the compensatory ZCT  10 B and the compensatory voltage divider resistor  12 B and transmits this generated compensatory comparison voltage value V 5 ′ to the judging circuit  32  for direct grounding current in the negative direction.  
         [0064]     The judging circuit  31  for direct grounding current in the positive direction examines whether or not the comparison voltage value V 5  is equal to or above the upper threshold value VA and transmits the result of this judgment to the final judging circuit  33 . The judging circuit  32  for direct grounding current in the negative direction examines whether or not the compensatory comparison voltage value V 5 ′ is equal to or above the upper threshold value VA and transmits the result of this judgment to the final judging circuit  33 .  
         [0065]     The final judging circuit  33  concludes that a direct grounding current in the positive direction has occurred if the judging circuit  31  for direct grounding current in the positive direction judges that the comparison voltage value V 5  is equal to or above the upper threshold value VA. The final judging circuit  33  concludes that a direct grounding current in the negative direction has occurred if the judging circuit  32  for direct grounding current in the negative direction judges that the compensatory comparison voltage value V 5 ′ is equal to or above the upper threshold value VA.  
         [0066]     In summary, the second embodiment is characterized as additionally comprising a compensatory ZCT  10 B having the same characteristic as the first ZCT  10 , connected in series with a compensatory voltage divider and having an offset wire  21  and source lines  110 A and  110 B inserted therethrough such that an offset current opposite to that detected by the first ZCT  10  will be detected thereby, as well as a compensatory comparison voltage generating circuit  13 B for generating a compensatory comparison voltage based on a voltage value obtained between the compensatory ZCT  10 B and the compensatory voltage divider resistor  12 B according to the change in the impedance of the compensatory ZCT  10 B. It is concluded that a direct grounding current in the positive direction has occurred if the comparison voltage value generated by the comparison voltage generating circuit is found to be equal to or above the upper threshold value VA but that a direct grounding current in the negative direction has occurred if the compensatory comparison voltage value generated by the compensatory comparison voltage generating circuit is found to be equal to or above the upper threshold value VA. Thus, not only the presence or absence of a direct grounding current but also its direction, even if its occurrence has been sudden, can be accurately detected without being influenced by the hysteresis characteristic of the ZCT  10 .  
         [0067]     It now goes without saying that the second embodiment of this invention described above as applied to a direct grounding current detection circuit can be equally applicable to a direct current detection circuit for detecting a direct current value in either positive or negative direction. This may be done by calculating the direct current value in the positive direction on the basis of the comparison voltage value and the direct current value in the negative direction on the basis of the compensatory comparison voltage value, detecting the direct current value in the positive direction as the present direct current value if the comparison voltage value is higher than the compensatory comparison voltage value and the direct current value in the negative direction as the present direct current value if the comparison voltage value is lower than the compensatory comparison voltage value.  
         [0068]      FIG. 6  is a block diagram of still another direct grounding current detection circuit  1 C according to a third embodiment of this invention, adapted to be able to determine whether a direct grounding current is in the positive or negative direction although its occurrence takes place suddenly. Components which are substantially the same as those shown above in  FIG. 1  are indicated by the same numerals and their structures and functions may not be described repetitiously.  
         [0069]     The direct grounding current detection circuit  1 C shown in  FIG. 6  comprises not only a ZCT  10  with source lines  110 A and  110 B and an offset current wire  21  passing therethrough, an oscillator circuit  11 , a comparison voltage generating circuit  13  and a threshold memory  23  but also an offset current generating circuit  22 C adapted to generate an offset current having different current values for each of specified periods and a control circuit  14 C which concludes that a direct grounding current has occurred relative to the source lines  110 A and  110 B if it is judged that the comparison voltage value V 5  is equal to or above the upper threshold value VA or equal to or below the lower threshold value VB.  
         [0070]     For example, the offset current generating circuit  22 C may be adapted to output an offset current of +58 mA and +60 mA alternately at a specified period such as 20 ms.  
         [0071]     The control circuit  14 C comprises a monitoring circuit  41  for monitoring the shift in the offset current value and the comparison voltage value V 5  when it is judged that a direct grounding current relative to the source lines  110 A and  110 B has occurred and a judging circuit  42  for judging the occurrence of a direct grounding current in the positive or negative direction on the basis of the result of monitoring by the monitoring circuit  41 .  
         [0072]     If the result of the monitoring by the monitoring circuit  41  is such that the comparison voltage value V 5  increases as the offset current value increases, the judging circuit  42  judges that a direct grounding current in the positive direction has occurred. If the monitoring circuit  41  detects that the comparison voltage value V 5  decreases as the offset current value increases, however, it is judged that a direct grounding current in the negative direction has occurred.  
         [0073]     The judgment process as described above is explained next with reference to  FIG. 7  which shows the relationship between the comparison voltage value V 5  and the direct current values (the direct ground current and the sum of the direct ground current and the offset current) in the case of the direct grounding current detection circuit  1 C according to the third embodiment of the invention and  FIG. 8  which shows an example of relationship between the shift and the direct grounding current.  
         [0074]     Let us assume that there was a direct grounding current of +30 mA. As shown in  FIGS. 7 and 8 , the total current (the direct grounding and offset currents) will be +90 mA when the offset current is +60 mA but the total current will be +88 mA when the offset current is 58 mA. It can also be understood for this case that the comparison voltage value V 5  when the offset current is +60 mA is higher than when the offset current is +58 mA.  FIGS. 7 and 8  also show that when there is a direct grounding current of 0 mA, the comparison voltage value V 5  when the offset current is +60 mA is higher than that when the offset current is +58 mA. In such cases, the comparison voltage value is said to shift higher as the offset current increases.  
         [0075]     Let us next assume that there was a direct grounding current of −120 mA. As shown in  FIGS. 7 and 8 , the total current that flows is −60 mA when the offset current is +60 mA but it is −62 mA when the offset current is +58 mA. It can also be understood for this case that the comparison voltage value V 5  when the offset current is +58 mA is higher than when the offset current is +60 mA. Similarly, if it is assumed that there was a direct grounding current of −90 mA, the comparison voltage value V 5  when the offset current is +58 mA is higher than that when the offset current is +60 mA. In such cases, the comparison voltage value is said to shift lower as the offset current increases.  
         [0076]     Next, the operation of the comparison voltage generating circuit  1 C according to the third embodiment of the invention will be described.  
         [0077]     When it is judged that a direct grounding current has occurred, the monitoring circuit  41  serves to monitor the shift in the comparison voltage value  5  as the offset current changes from +58 mA to +60 mA. On the basis of the monitoring by the monitoring circuit  41 , the judging circuit  42  concludes that the direct grounding current is in the positive direction if the comparison voltage value V 5  shifts higher as the offset current increases, that is, if the comparison voltage value V 5  is higher when the offset current is +60 mA than when the offset current is +58 mA. Similarly, the judging circuit  42  concludes that the direct grounding current is in the negative direction if the comparison voltage value V 5  shifts lower as the offset current increases, that is, if the comparison voltage value V 5  is lower when the offset current is +60 mA than when the offset current is +58 mA.  
         [0078]     In summary, the third embodiment is characterized as causing offset currents with different current values to flow at specified periods and monitoring the shift in the comparison voltage value V 5  when the occurrence of a direct grounding current is determined relative to source lines  110 A and  110 B such that the direct grounding current is judged to be in the positive direction if the shift is to increase the comparison voltage value as the offset current increases and it is judged to be in the negative direction if the shift is to decrease the comparison voltage value as the offset current increases. Thus, according to this embodiment, too, the direct grounding current can be detected accurately even if its occurrence is sudden without the influence of the hysteresis characteristic of the ZCT  10 .  
         [0079]     It also goes without saying that the third embodiment of this invention described above as applied to a direct grounding current detection circuit can be equally applicable to a direct current detection circuit for detecting a direct current value in either positive or negative direction. This may be done by calculating the direct current values in the positive and negative directions on the basis of the comparison voltage value and monitoring the shift in the comparison voltage value as the offset current is varied. The calculated direct current value in the positive direction is determined to be the present current value if the shift is to increase the comparison voltage value as the offset current is increased and the calculated direct current value in the negative direction is determined to be the present current value if the shift is to decrease the comparison voltage value as the offset current is increased.  
         [0080]      FIG. 9  is a block diagram of still another direct grounding current detection circuit ID according to a fourth embodiment of this invention. Components which are substantially the same as those shown above in  FIG. 1  are indicated by the same numerals and their structures and functions may not be described repetitiously.  
         [0081]     The direct grounding current detection circuit ID shown in  FIG. 9  not only comprises a ZCT  10  with source lines  110 A and  110 B, an oscillator circuit  11 , a comparison voltage generating circuit  13  and a threshold memory  23  but is also characterized wherein the source lines  110 A and  110 B are passed through the ZCT  10  more than once such that the range of direct grounding current value which is difficult to detect on the basis of the comparison voltage value V 5  is shifted sufficiently into a new range where such detection is enabled. Explained in other words, the direct grounding current is added more than once if the source lines  110 A and  110 B are passed through the ZCT  10  a plural number of times.  
         [0082]     The operation of the direct grounding current detection circuit ID according to the fourth embodiment of the invention will be explained next by way of an example wherein the source lines  110 A and  110 B are passed three times through the ZCT  10 . Thus, if there is a direct grounding current of +20 mA through the source lines  110 A and  110 B, it has the same effect as a current of +60 mA passing through the ZCT  10 . The comparison voltage generating circuit  13  generates the comparison voltage value V 5  based on the divided voltage V 2  obtained between the ZCT  10  and the voltage divider resistor  12  according to the change in the impedance of the ZCT  10  and the control circuit  14  detects the occurrence of a direct grounding current if the comparison voltage value V 5  is equal to or above the upper threshold value VA or equal to or below the lower threshold value VB.  
         [0083]     The direct grounding current detection circuit ID according to the fourth embodiment of the invention is advantageous in that additional circuits such as an offset current generating circuit can be dispensed with because the occurrence of a direct grounding current can be accurately detected without being influenced by the hysteresis characteristic of the ZCT  10  merely by changing the number of times the source lines are passed through the ZCT  10 .  
         [0084]     It also goes without saying that the fourth embodiment of this invention described above as applied to a direct grounding current detection circuit can be equally applicable to a direct current detection circuit for detecting a direct current value without providing an offset current generating circuit but merely by changing the number of times the source lines are passed through the ZCT  10 . In this case, with the source lines  110 A and  110 B being passed through the ZCT  10  a plural number of times, a current value may be calculated on the basis of the comparison voltage value V 5  and a direct current value calculated from this value and the value of the current that passes through the ZCT  10  may be detected as the present direct current value.  
         [0085]     In summary, the present invention makes it possible to accurately detect a present direct current value such as a direct ground current value that may be so small for detection on the basis of a comparison voltage value.