Patent Publication Number: US-8975890-B2

Title: DC and AC current detection circuit

Description:
This application is a 35 USC 371 national phase filing of PCT/EP2011/066450, filed Sep. 21, 2011, which claims priority to Irish national application S2010/0604, filed Sep. 21, 2010, the disclosures of which are incorporated herein by reference in their entireties. 
     FIELD OF THE DISCLOSURE 
     This invention relates to a circuit for detecting DC and in some embodiments AC currents. 
     BACKGROUND 
     It is often desirable to detect DC currents in circuits or installations. Detection of DC currents is often achieved by the use of a shunt. Shunts have to be inserted in the circuit being monitored and this involves direct contact with the DC supply. In many cases direct contact with the circuit being monitored is undesirable or even impractical. Hall Effect devices are also commonly used for detection of DC currents, but these tend to be bulky and expensive. 
     Current transformers (CTs) are not normally used to detect DC currents because CTs are only responsive to alternating currents and are not inherently responsive to a steady state current. However, current transformers have the advantage of being compact and inexpensive, and would be an attractive means for achieving contactless detection of DC currents if the above technical problem could be overcome. 
     It is an object of the invention to provide a simple circuit using a current transformer to detect a DC current. 
     SUMMARY 
     According to the present invention there is provided a circuit for detecting a DC current in at least one conductor, the circuit including a current transformer having a ferromagnetic core, a primary winding comprising the conductor and at least one secondary winding, the circuit further including an oscillator for supplying an oscillating signal across the secondary winding and means for detecting a dc offset in the current flowing in the secondary winding. 
     The term “winding” is used in relation to the primary in accordance with conventional terminology, even though the primary may constitute a single conductor passing through a current transformer core. 
     The detection circuit may be configured to detect AC currents as well as DC currents if the oscillator frequency is sufficiently high compared to the frequency of the current to be detected, preferably at least an order of magnitude higher. 
     The primary winding may comprise more than one conductor, in which case the circuit will detect the vector sum of the currents flowing in the primary conductors. In such a case the circuit may be used as a residual current device. 
     In preferred embodiments the circuit includes at least one capacitor in series with the secondary winding, wherein the detecting means is arranged to detect a non-zero voltage across the capacitor above a certain level, the non-zero voltage corresponding to a dc offset greater than a predetermined magnitude. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a basic circuit illustrating the principle of operation of the embodiments of the invention. 
         FIG. 2   a  shows a typical hysteresis curve plotted for a ferromagnetic material. 
         FIGS. 2   b  to  2   d  are waveforms showing the effect of DC currents flowing in the primary winding of  FIG. 1 . 
         FIG. 3  is a circuit diagram of a first embodiment of the invention. 
         FIG. 4  is a circuit diagram of a second embodiment of the invention. 
         FIG. 5  is a circuit diagram of a third embodiment of the invention. 
         FIGS. 6 and 7  are schematic diagrams of an electric vehicle and a typical arrangement for charging the vehicle. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a current transformer CT having a toroidal ferromagnetic core  10 , a secondary winding W 1  wound on the core, and a primary winding in the form of a single conductor L 1  passing through the core aperture. The secondary winding W 1  is connected to an oscillator  12 , with a capacitor C 1  connected in series with W 1 . The oscillator  12  produces an alternating current H at frequency F 1  which causes a current H to flow through W 1  and C 1 . As the current H will flow alternately in one direction and then in the opposite direction, the opposite directions of current flow can be represented by currents H+ and H−. When H+ is increased from zero, the core  10  will be magnetised, and this magnetisation will increase until the core reaches magnetic saturation. If H+ is initially reduced and then reversed as represented by H−, the core  10  will be brought out of saturation and then the magnetisation of the core will be reversed until the core reaches saturates again in the opposite direction. This behaviour is represented in  FIG. 2   a.    
     The plot of  FIG. 2   a  is known as a B−H (hysteresis) loop, where H is the current required to magnetise the core and B is the magnetic flux produced by current H. It can be seen that current H+ starts from zero at point  0  and is increased with a positive polarity until magnetic saturation occurs at point a (positive saturation), at which point H+ is reduced and then reversed to become H− to take the hysteresis loop from point a through point b through point c to negative saturation point d. The current is again reduced and then reversed to become H+ which takes the loop from point d through point e through point f and back to point a again. 
     This process will continue in an oscillatory manner as determined by frequency F 1  which, in order to detect AC as will be described, will normally be substantially higher than normal mains supply frequency of 50 or 60 Hz, e.g. at least an order of magnitude higher and preferably at least 1500 Hz and most preferably around 3 KHz. It can be seen that after the initial saturation point is reached, the core will have a residual magnetism when H is zero, as shown at points b and e. Magnetisation will be zero when current H has a positive or negative value as shown at points c and f. 
       FIG. 2   b  shows the corresponding waveform for the current flow H in the oscillator circuit. It can be seen that the current reaches a peak in each polarity during each cycle of the oscillator frequency. 
     Under normal conditions (no current flowing in L 1 ), the AC current H flowing in the oscillator circuit will have a mean DC value of zero. If a DC current +I dc  is passed in a certain direction through the conductor L 1  in  FIG. 1 , the current flowing in the oscillator circuit will be shifted from the mean zero level to a mean positive level as indicated by  FIG. 2   c , the magnitude of the DC offset being proportional to the DC current flow in L 1 . Conversely, if the DC current flow in L 1  is reversed to become −I dc , the DC offset will also be reversed, as shown in  FIG. 2   d . The DC offset that occurs in the oscillator circuit can be used to detect and measure a DC current flow in the primary conductor L 1 , as shown in the embodiment of  FIG. 3 . 
     In  FIG. 3  the winding W 1  is separated into two secondary windings W 1   a , W 1   b  wound separately on the core  10 , and the capacitor C 1  is connected in series between the secondary windings (in  FIGS. 3 and 4  the toroidal core  10  is shown schematically). As in  FIG. 1  the primary conductor L 1  passes through the core aperture. The oscillator  12  is supplied with a 15V supply from Vcc to ground. The oscillator  12 , the two windings W 1   a , W 1   b  and the capacitor C 1  form a loop or a first circuit for current flow from Vcc to ground. The oscillator current as represented by H+ and H− will flow back and forth through W 1   a , C 1  and W 1   b  at the oscillator frequency F which, as mentioned, will typically be substantially higher than the normal mains supply frequency of 50 Hz, for example about 3 KHz. 
     During the positive half cycles Vcc will be distributed approximately as 15V, 7.5V, 7.5V and 0V at points  1 ,  2 ,  3  and  4  respectively, and during the negative half cycles Vcc will be distributed approximately as 0V, 7.5V, 7.5V and 15V at points  1 ,  2 ,  3  and  4  respectively. So, whilst points  1  and  4  will swing fully between 15V and ground, the voltages at points  2  and  3  will remain relatively stable at 7.5V. In the absence of any current flow in the primary conductor L 1  the DC current through the secondary windings and the DC voltage across C 1  will be substantially zero. When a DC current + Idc  or − Idc  flows in the primary circuit a resultant DC shift as shown in  FIG. 2   c  or  2   d  will occur and the current in the oscillator circuit will now have a DC offset with the result that the voltages at points  2  and  3  will no longer be the same. The resultant differential voltage at those points can be used to control rectifying means (bipolar transistors in the present embodiment) and so detect the DC current in the primary conductor L 1 . 
     To this end, a second circuit to ground is formed by bipolar transistors Tr 1  and Tr 2 , a resistor R 1  and a further capacitor C 2 . When a DC current  Idc  flows in the primary conductor L 1  the difference in the DC voltage between points  2  and  3  will increase proportionately. When  Idc  is of a certain polarity and of sufficient magnitude, corresponding to a DC offset greater than a pre-determined magnitude, point  2  will reach approximately 0.7V higher than point  3  during the oscillator cycles and transistor TR 2  will start to conduct. This will allow the DC current to flow to ground via resistor R 1  and develop a voltage across R 1  and capacitor C 2  will charge up to a certain voltage. When  Idc  is of the same magnitude but of opposite polarity point  3  will reach approximately 0.7V higher than point  2  during the oscillator cycles and transistor TR 1  will conduct. The oscillator current will again flow to ground via resistor R 1  and develop a voltage across C 2 . Thus a DC voltage will be developed across C 2  which will be proportional to the DC current flowing in L 1  and, therefore, to a predetermined magnitude of DC offset in the oscillator circuit. 
     In the arrangement of  FIG. 3  the transistors Tr 1  and Tr 2  are used to “siphon off” the DC component in the oscillator current by way of a rectifying action so as to detect a DC current flow in the primary conductor, the capacitor C 1  allowing the high frequency current produced by the oscillator  12  to bypass the transistors. To this end, Tr 1  and Tr 2  are used as current control means to control the current flow into C 2 . 
     If there is a DC current in L 1 , the voltage across C 1  will always reach a level where Tr 1  starts conducting independent of the magnitude of the current and the value of C 1 . The small capacitor C 1  is intended to absorb the AC ripple current, typically at 3 KHz, produced by the oscillator  12 . The ripple voltage across C 1  stays well below 0.7V. The value of the capacitor C 1  is much smaller than that of the capacitor C 2  and its part in time delay is negligible. The tripping threshold is adjusted by suitable choice of the values on the components R 1 , C 2  and the number of turns in the windings W 1   a  and W 1   b.    
       FIG. 4  shows a second embodiment of the invention wherein a single secondary winding W 1  is used but the rectifying means is split and the single capacitor C 1  has been replaced by two capacitors C 1   a  and C 1   b  and the secondary winding W 1  is connected in series between the capacitors. The oscillator currents H+ and H− now flow through W 1  and capacitors C 1   a  and C 1   b . Under normal conditions (no current flowing in L 1 ) during the oscillator cycles Vcc will be distributed approximately as 15V, 15V, 0V and 0V at points  1 ,  2 ,  3  and  4  respectively during positive half cycles, and during the negative half cycles Vcc will be distributed approximately as 0V, 0V, 15V and 15V at points  1 ,  2 ,  3  and  4  respectively, and the effective differential voltage across each of C 1   a  and C 1   b  will be zero. 
     When a DC current of a certain polarity flows in L 1  a resultant DC shift as shown in  FIG. 2   c  will occur and capacitors C 1   a  and C 1   b  will acquire a charge. When the differential voltage across each of them reaches about 0.7V a diode D 1  and a bipolar transistor Tr 2  will conduct and the DC component in the oscillator current will flow from point  1  through D 1 , through W 1  and into the emitter of Tr 2 , and the resultant collector current of Tr 2  will flow into R 1  and cause a DC voltage to be developed across C 2 . 
     When a DC current of the opposite polarity flows in L 1  a resultant DC shift as shown in  FIG. 2   d  will occur and capacitors C 1   a  and C 1   b  will acquire a charge. When the differential voltage across each of them reaches about 0.7V a diode D 2  and a bipolar transistor Tr 1  will conduct and oscillator current will flow from point  4  through D 2 , through W 1  and into the emitter of Tr 1 , and the resultant collector current of Tr 1  will flow into R 1  and cause a DC voltage to be developed across C 2 . 
     The main advantage of the arrangement of  FIG. 4  is that the CT can now comprise a single winding rather than two windings. However, this advantage is offset to some extent by the fact that it requires a higher conducting threshold of 1.4 V over two capacitors in  FIG. 4  compared with 0.7 V over one capacitor in  FIG. 3  to achieve the same performance. 
     The voltage across R 1  is smoothed by C 2 , and the DC voltage Vout developed across R 1  will be proportional to the DC current flow in the primary conductor L 1 . This voltage can be monitored and used for measurement purposes or for detection purposes such as RCD (residual current device) applications where, for example, the DC current flowing in the primary conductor L 1  is the difference in the currents in the line and neutral conductors of a mains electricity supply. For example, an electronic circuit  14  comprising an integrated circuit type WA050 may be connected across the resistor R 1 . The WA050 is an industry standard RCD IC supplied by Western Automation Research &amp; Development Ltd, Ireland. 
     The circuit arrangements of  FIGS. 3 and 4  can also be used to detect AC current flow in the primary conductor L 1  at a typical mains supply frequency of 50 Hz. This is because the frequency of the oscillator  12  is sufficiently high compared to the mains frequency, e.g. 3 KHz as compared to 50 Hz, that each half cycle of primary current will be effectively a DC current for the half cycle period, and during this period the oscillator  12  will have completed multiple cycles during which oscillator current will flow through R 1 . The DC offset in the oscillator current will therefore vary relatively slowly during each half cycle of the primary current. It will be appreciated therefore that circuits according to the invention can be employed to detect AC residual currents or composite AC residual currents with frequencies from 1 Hz up to and approaching any selected oscillator frequency. 
     By suitable selection of component W 1  and C 1 , the circuit can be optimised to detect DC or AC currents of selected magnitudes flowing in the primary conductor L 1 . 
     The circuit can be designed to produce the same DC offset, and accordingly the same level of Vout, for AC and DC currents of approximately the same RMS value, i.e. to have in effect the same response to AC and DC currents, or the circuit can be made less responsive to AC currents without changing the DC detection threshold if discrimination between AC and DC currents is required. This can be done by changing the value of C 1  in  FIG. 3 . For low values of capacitance, AC currents will mainly flow into the emitters of the rectifier transistors. By increasing the value of C 1  the circuit will become less responsive to AC currents because an increasing part of the current is needed to charge capacitor C 1  to a voltage level Vbe of 0.7 V above which the bipolar transistors start conducting and it will take an AC current of larger magnitude to cause the same level of Vout in comparison to a given DC current. 
     By suitable selection of the value of capacitor C 1 , a threshold of detection for AC currents can be set independently of the DC current level and in this way the circuit can be made responsive to DC currents of a certain magnitude whilst being effectively blind to AC currents of a similar or higher magnitude and thereby provide a very high level of discrimination between AC and DC currents. 
     Similarly, by suitable selection of the values of capacitors C 1   a  and C 1   b  in  FIG. 4 , threshold of detection for AC currents can be set independently of the DC current level. 
     It may be desirable to set the AC current detection threshold independent of and at a lower value than a corresponding DC current.  FIG. 5  shows an embodiment which achieves this. 
     In  FIG. 5 , components R 3 , R 5  and C 3  (the left chain) have been inserted between C 1  and Tr 1 , and components R 4 , R 6  and C 4  (the right chain) have been inserted between C 1  and Tr 2 . In the case of a DC current flow above a certain level in the primary circuit (L 1 ) the offset current produced by C 1  will be DC, and a resultant current will flow through the left chain for one polarity of the primary DC current, and through the right chain for a DC current of the opposite polarity. Capacitors C 3  and C 4  will act as DC blocks, so all of the offset current will be forced to flow via R 5  or R 6  as applicable. In the case of an AC current flow above a certain level in the primary circuit, the resultant DC offset current produced by C 1  will have an AC component, e.g. 50 Hz. For one half cycle of the AC primary current the resultant offset current will flow via the left chain, and it will flow through the right chain for the other half cycle. However, in the case of the AC condition, capacitors C 3  and C 4  will provide an additional path for current flow and thus for an AC current there will be less impedance between C 1  and the Tr 1  or Tr 2  with the result that the charge on C 2  will be greater for a given AC current than it would be for the corresponding DC current. Thus the AC current threshold can be set lower than the DC current threshold. 
       FIG. 6  is a schematic diagram of an electric vehicle (EV)  100  and a typical arrangement for charging the vehicle. 
     The EV  100  comprises a body  102  shown in dashed lines having an externally accessible mains connecter  104  through which the EV can be charged from an AC mains socket  106  via a three core cable  108  comprising live L, neutral N and protective earth PE conductors. The mains socket  106  is located in a building, outhouse, garage or other fixed location, not shown. 
     The EV  100  converts the AC mains supply to DC to charge a battery  110  using an inverter  112 . The operation of inverters is well known to those familiar with the art, but basically the inverter chops up the 50 Hz AC mains current at a substantially higher frequency rate (e.g. several KHz) such that each half cycle of the mains supply becomes a high frequency signal within respective positive and negative going half cycles. This gives rise to a multifrequency or high frequency current with a DC component. When it is desired to operate the EV  100  the DC output from the battery  110  is converted back to AC using a second inverter  116  to drive a motor  114 . 
     Generally the DC supply is isolated from the protective earth PE and the EV body  102  to minimize the risk of electric shock in the event of a person touching one side of the DC supply within the EV. An insulation monitoring device (IMD)  118  is normally fitted within the EV to detect an inadvertent connection from the DC supply conductors to the PE, for example due to insulation breakdown. In such a case an audible or visible alarm will be activated. However, the IMD  118  is not a protective device and generally does not prevent the continued use of the EV after an insulation breakdown has occurred. Thus, after a first fault within the EV a shock risk can arise and the user may be exposed to such risk. 
     In  FIG. 6  the EV  100  is connected to a standard TN system single phase 230V/50 Hz AC supply comprising of live, neutral and protective earth conductors. Once the EV is so connected, its body will be at earth potential and shock risks will be minimized. The DC supply within the EV is normally isolated from the EV body. The installation supplying the EV is normally protected by a conventional RCD, shown schematically at  120 , which is based on IEC61008 or a GFCI to UL943 with a normal trip current level not exceeding 30 mA. 
     An RCD based on IEC61008 is required to detect an AC residual current up to its rated trip level, e.g. 30 mA, at rated frequency, e.g. 50 Hz or 60 Hz. Such RCDs are generally blind or non responsive to DC residual currents or currents at substantially higher frequencies than the normal mains supply. Furthermore, the operation of such RCDs may be impaired by the presence of DC residual currents or residual currents at high frequencies or at composite frequencies. Such limitations in the operation of conventional RCDs are accepted because residual currents at DC or at high frequencies or multifrequency residual currents rarely occur in domestic installations. However, the emergence of the EV in the mass market has given rise to a new and potentially hazardous problem. 
     For example, an insulation breakdown could occur between the DC supply and the earthing system or body of the EV. This condition could occur during normal everyday use of the EV or could occur during the charging process. The IMD  118  will be activated in the event of such a fault. However, if the fault condition is ignored for any reason and the EV is connected to the AC supply a current  Idc  will flow from the DC conductor through the PE to the supply live and through the RCD  120  as a differential current back to the DC conductor. This current will have DC components and will be at a relatively high frequency as determined by inverter  112 . Given that the voltage at the output of inverter  112  can be in the range of several hundred volts, the resultant residual current will be relatively high and in any event substantially higher than the 30 mA rating of the conventional RCD. The effect of  Idc  will be to desensitize the RCD  120  to such an extent that its normal 50 Hz trip level will be increased well above its rated trip level and thereby impair its ability to provide adequate shock protection against a subsequent 50 Hz residual current. 
     A second fault condition could occur where a person touches a live part of the AC supply. The resultant residual current I ac  will also flow through the RCD  120  but is highly likely to go undetected due to the presence of the first fault current  Idc . Thus, because of a fault within the EV, the normal protection provided by the RCD  120  has been compromised. A similar problem would arise in the case of a breakdown of the insulation between the output of the inverter  116  and the protective earth. The potential hazard is not confined to the immediate vicinity of the charging circuit. The EV may be connected to a charging circuit outside a house. The house and all socket outlets are protected by a conventional RCD, as is standard practice. When the EV with the insulation breakdown problem is connected to a socket outlet in a garage or car port, the RCD protecting the entire installation will be compromised, and a person touching a live part within the house during the EV charging operation may no longer have the expected shock protection previously afforded by the RCD. 
     Although an alarm may be activated by the IMD  118  for the first fault within the EV, the user would have no way of realizing that this fault could cause failure of an RCD  120  in an external installation when the EV is connected to that installation. Thus, a shock hazard could be generated within an external installation due to a fault within the EV and conventional solutions would not provide adequate protection. The EV could be left connected overnight or even over a weekend, in which case a sustained shock hazard would arise. 
     This problem could be overcome by replacing the conventional RCD  120  with a B Type RCD based on IEC62423 which is designed to detect DC residual currents and AC residual currents up to 1 KHz. However, such RCDs are substantially more expensive than conventional RCDs and are therefore unlikely to be used in residential applications. Furthermore, it would not be feasible to replace millions of conventional RCDs worldwide just to mitigate this problem. 
     The insulation breakdown as described will generally go undetected unless special equipment such as an IMD is fitted within the EV to detect it. Failure of the IMD itself could cause a resultant breakdown in insulation between the DC supply and the PE and possibly cause the very problem that compromises the external RCD. There is no way of ensuring that the user will not connect the EV to an external supply to charge the battery under a fault condition, especially if the user believes the fault and the resultant risks to be contained within the EV, or that there is simply a false alarm. 
       FIG. 7  shows a simple but effective means to mitigate the above problem. 
     In  FIG. 7  a residual current device (RCD)  122  has been fitted within the EV to monitor the current flowing in the PE conductor within the EV. In this arrangement, the RCD  122  is constructed as described in any of the preceding embodiments and uses a current transformer CT having a toroidal core  10  which surrounds the live L and neutral N supply conductors to detect a current imbalance in those conductors indicating a residual current flowing in the PE conductor. In the event that a residual current in excess of a predetermined threshold flows in the PE, the RCD IC (WA050)  14  will produce an output. This output can be used to disconnect the EV from the external supply by opening contacts SW 1  in the AC live and neutral conductors, and also advantageously the PE conductor. The shock hazard will then be removed regardless of the actions of the user. (It will be appreciated that the RCD  122  is shown in highly simplified form in  FIG. 7 , and shows only the toroidal core  10  and the IC  14 , all intermediate components being omitted). The RCD  122  is designed to detect residual currents over the range of DC to the highest frequency component produced by the inverter  112 . The CT core  10  may encompass the active L and N conductors together as shown in  FIG. 7  and detect DC or AC residual current flow in the AC supply to the EV. An alternative option would be for the CT core  10  to encompass the PE conductor alone, but if the body of the EV was externally grounded, the residual current could be split between the PE and the external return path and possibly go undetected. 
     The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.