GOINGA, Ydo (Roosjespeer 5, KG Borne, NL-7623, NL)
1. 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.
2. A circuit as claimed in claim 1, including 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.
3. A circuit as claimed in claim 2, wherein the oscillator circuit comprises two secondary windings and the capacitor is connected in series between the secondary windings.
4. A circuit as claimed in claim 3, wherein the detected non-zero voltage across the capacitor is arranged to operate a current control means.
5. A circuit as claimed in claim 4, wherein operation of the current control means allows a further capacitor to charge up, and the voltage on the further capacitor is monitored. 6. A circuit as claimed in claim 5, wherein there are a pair of current control means, preferably transistors, each of which is turned on in response to a detected non-zero voltage in a respective one of the two opposite directions across the first capacitor.
7. A circuit as claimed in claim 2, wherein the oscillator circuit comprises two capacitors and the secondary winding is connected in series between the capacitors.
8. A circuit as claimed in claim 7, wherein said detected non-zero voltage across at least one capacitor is arranged to operate a current control means.
9. A circuit as claimed in claim 8, wherein operation of the current control means allows a further capacitor to charge up, and the voltage on the further capacitor is monitored.
10. A circuit as claimed in claim 9, wherein there are a pair of current control means each of which is turned on in response to a detected non-zero voltage in a respective one of the two opposite directions across the capacitors. 11. A circuit as claimed in any preceding claim, wherein the oscillator frequency is at least an order of magnitude higher than mains supply frequency to allow the detection of mains frequency AC currents in the conductor.
12. A circuit as claimed in claim 11, wherein the value of at least one circuit component is selected so that the same DC offset is obtained in respect of AC and DC currents having different RMS values.
13. A circuit as claimed in claim 11, wherein the value of at least one circuit component is selected so that the DC offset obtained in respect of DC currents is substantially greater than that for AC currents of equivalent RMS value.
14. A circuit as claimed in claim 11, wherein the value of at least one circuit component is selected so that the DC offset obtained in respect of DC currents is substantially less than that for AC currents of equivalent RMS value.
15. A circuit as claimed in any preceding claim, wherein the current transformer comprises an apertured core, the conductor passing through the aperture and the, or each, secondary being a winding on the core.
This invention relates to a circuit for detecting DC and in some embodiments AC currents.
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.
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.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a basic circuit illustrating the principle of operation of the embodiments of the invention.
Figure 2a shows a typical hysteresis curve plotted for a ferromagnetic material.
Figures 2b to 2d are waveforms showing the effect of DC currents flowing in the primary winding of Figure 1.
Figure 3 is a circuit diagram of a first embodiment of the invention. Figure 4 is a circuit diagram of a second embodiment of the invention.
Figure 5 is a circuit diagram of a third embodiment of the invention. Figures 6 and 7 are schematic diagrams of an electric vehicle and a typical
arrangement for charging the vehicle.
Figure 1 shows a current transformer CT having a toroidal ferromagnetic core 10, a secondary winding Wl wound on the core, and a primary winding in the form of a single conductor LI passing through the core aperture. The secondary winding Wl is connected to an oscillator 12, with a capacitor CI connected in series with Wl. The oscillator 12 produces an alternating current H at frequency Fl which causes a current H to flow through Wl and CI. 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 Figure 2a.
The plot of Figure 2a 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 Fl which, in order to detect AC as will be described, will normally be substantially higher than normal mains supply frequency of 50 or 60Hz, e.g. at least an order of magnitude higher and preferably at least 1500Hz and most preferably around 3KHz. 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.
Figure 2b 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 LI), the AC current H flowing in the oscillator circuit will have a mean DC value of zero. If a DC current +ld c is passed in a certain direction through the conductor LI in Figure 1, the current flowing in the oscillator circuit will be shifted from the mean zero level to a mean positive level as indicated by Figure 2c, the magnitude of the DC offset being proportional to the DC current flow in LI. Conversely, if the DC current flow in LI is reversed to become -ld c , the DC offset will also be reversed, as shown in Figure 2d. The DC offset that occurs in the oscillator circuit can be used to detect and measure a DC current flow in the primary conductor LI, as shown in the embodiment of Figure 3.
In Figure 3 the winding Wl is separated into two secondary windings Wla, Wlb wound separately on the core 10, and the capacitor CI is connected in series between the secondary windings (in Figures 3 and 4 the toroidal core 10 is shown
schematically). As in Figure 1 the primary conductor LI passes through the core aperture. The oscillator 12 is supplied with a 15V supply from Vcc to ground. The oscillator 12, the two windings Wla, Wlb and the capacitor CI 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 Wla, CI and Wlb at the oscillator frequency F which, as mentioned, will typically be substantially higher than the normal mains supply frequency of 50Hz, for example about 3KHz.
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 LI the DC current through the secondary windings and the DC voltage across CI will be substantially zero. When a DC current +idc or -|dc flows in the primary circuit a resultant DC shift as shown in Figure 2c or 2d 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 LI.
To this end, a second circuit to ground is formed by bipolar transistors Trl and Tr2, a resistor Rl and a further capacitor C2. When a DC current id c flows in the primary conductor LI the difference in the DC voltage between points 2 and 3 will increase proportionately. When id c 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 TR2 will start to conduct. This will allow the DC current to flow to ground via resistor Rl and develop a voltage across Rl and capacitor C2 will charge up to a certain voltage. When id c 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 TR1 will conduct. The oscillator current will again flow to ground via resistor Rl and develop a voltage across C2. Thus a DC voltage will be developed across C2 which will be proportional to the DC current flowing in LI and, therefore, to a predetermined magnitude of DC offset in the oscillator circuit.
In the arrangement of Figure 3 the transistors Trl and Tr2 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 CI allowing the high frequency current produced by the oscillator 12 to bypass the transistors. To this end, Trl and Tr2 are used as current control means to control the current flow into C2. If there is a DC current in LI, the voltage across CI will always reach a level where Trl starts conducting independent of the magnitude of the current and the value of CI. The small capacitor CI is intended to absorb the AC ripple current, typically at 3KHz, produced by the oscillator 12. The ripple voltage across CI stays well below 0.7V. The value of the capacitor CI is much smaller than that of the capacitor C2 and its part in time delay is negligible. The tripping threshold is adjusted by suitable choice of the values on the components Rl, C2 and the number of turns in the windings Wla and Wlb.
Figure 4 shows a second embodiment of the invention wherein a single secondary winding Wl is used but the rectifying means is split and the single capacitor CI has been replaced by two capacitors Cla and Clb and the secondary winding Wl is connected in series between the capacitors. The oscillator currents H+ and H- now flow through Wl and capacitors Cla and Clb. Under normal conditions (no current flowing in LI) 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 Cla and Clb will be zero. When a DC current of a certain polarity flows in LI a resultant DC shift as shown in Figure 2c will occur and capacitors Cla and Clb will acquire a charge. When the differential voltage across each of them reaches about 0.7V a diode Dl and a bipolar transistor Tr2 will conduct and the DC component in the oscillator current will flow from point 1 through Dl, through Wl and into the emitter of Tr2, and the resultant collector current of Tr2 will flow into Rl and cause a DC voltage to be developed across C2.
When a DC current of the opposite polarity flows in LI a resultant DC shift as shown in Figure 2d will occur and capacitors Cla and Clb will acquire a charge. When the differential voltage across each of them reaches about 0.7V a diode D2 and a bipolar transistor Trl will conduct and oscillator current will flow from point 4 through D2, through Wl and into the emitter of Trl, and the resultant collector current of Trl will flow into Rl and cause a DC voltage to be developed across C2.
The main advantage of the arrangement of Figure 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 Figure 4 compared with 0.7 V over one capacitor in Figure 3 to achieve the same performance.
The voltage across Rl is smoothed by C2, and the DC voltage Vout developed across Rl will be proportional to the DC current flow in the primary conductor LI. 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 LI 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 Rl. The WA050 is an industry standard RCD IC supplied by Western Automation Research & Development Ltd, Ireland. The circuit arrangements of Figures 3 and 4 can also be used to detect AC current flow in the primary conductor LI at a typical mains supply frequency of 50Hz. This is because the frequency of the oscillator 12 is sufficiently high compared to the mains frequency, e.g. 3KHz as compared to 50Hz, 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 Rl. 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 lHz up to and approaching any selected oscillator frequency.
By suitable selection of component Wl and CI, the circuit can be optimised to detect DC or AC currents of selected magnitudes flowing in the primary conductor LI.
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 CI in Figure 3. For low values of capacitance, AC currents will mainly flow into the emitters of the rectifier transistors. By increasing the value of CI the circuit will become less responsive to AC currents because an increasing part of the current is needed to charge capacitor CI 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 CI, 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 Cla and Clb in Figure 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. Figure 5 shows an embodiment which achieves this.
In Figure 5, components R3, R5 and C3 (the left chain) have been inserted between CI and Trl, and components R4, R6 and C4 (the right chain) have been inserted between CI and Tr2. In the case of a DC current flow above a certain level in the primary circuit (LI) the offset current produced by CI 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 C3 and C4 will act as DC blocks, so all of the offset current will be forced to flow via R5 or R6 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 CI will have an AC component, e.g. 50Hz. 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 C3 and C4 will provide an additional path for current flow and thus for an AC current there will be less impedance between CI and the Trl or Tr2 with the result that the charge on C2 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. Figure 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 50Hz 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 Figure 6 the EV 100 is connected to a standard TN system single phase 230V/50Hz AC supply comprising of live, neutral and protective earth conductors. Once the EV is so connected, its body will be at earth potential a nd shock risks will be minimized. The DC supply within the EV is norma lly isolated from the EV body. The installation supplying the EV is normally protected by a conventiona l RCD, shown schematically at 120, which is based on I EC61008 or a GFCI to UL943 with a normal trip current level not exceeding 30mA.
An RCD based on I EC61008 is required to detect a n AC residua l current up to its rated trip level, e.g. 30mA, at rated frequency, e.g. 50Hz or 60Hz. Such RCDs are generally blind or non responsive to DC residual currents or currents at substantially higher frequencies than the norma l 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 conventiona l RCDs are accepted because residual currents at DC or at high frequencies or multifrequency residua l currents ra rely occur in domestic installations. However, the emergence of the EV in the mass market has given rise to a new and potentia lly hazardous problem.
For example, an insulation breakdown could occur between the DC supply a nd 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 I MD 118 will be activated in the event of such a fault. However, if the fa ult condition is ignored for any reason and the EV is connected to the AC supply a current id c will flow from the DC conductor through the PE to the supply live and through the RCD 120 as a differentia l 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 ca n be in the range of several hundred volts, the resultant residual current will be relatively high and in a ny event substantially higher than the 30mA rating of the conventional RCD. The effect of id c will be to desensitize the RCD 120 to such an extent that its norma l 50Hz trip level will be increased well above its rated trip level and thereby impair its ability to provide adequate shock protection against a subsequent 50Hz residual current.
A second fault condition could occur where a person touches a live part of the AC supply. The resultant residual current l ac will also flow through the RCD 120 but is highly likely to go undetected due to the presence of the first fault current id c - 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 lKHz. 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.
Figure 7 shows a simple but effective means to mitigate the above problem.
In Figure 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 SW1 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 Figure 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 Figure 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.
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