Patent Publication Number: US-2010118449-A1

Title: Nulling current transformer

Description:
This invention relates to a nulling current transformer. More particularly, this invention relates to a nulling current transformer for accurately detecting current and giving an improved response, accuracy and stability using toroidal current transformer technology along with active components. This invention finds particular application in switchgear devices and metering operations. 
     Circuit protection devices, such as residual current devices are routinely used to monitor and protect against electrocution and fire risks on electrical installations. The usual technique for obtaining and processing a residual fault current is shown in  FIG. 1 . The principle of operation of these devices is well known, and a toroidal current transformer is used to measure the sum of the live and neutral currents. The current transformer detects the magnetic fields of the two supply conductors which flow in opposite directions and cancel in normal circumstances. The supply conductors form single primary turns on a magnetic toroidal core  10  and a secondary sense winding  12  of many turns is used to detect any magnetisation of the toroidal core  10 . 
     A typical fault may occur where a person touches the live conductor downstream of the residual current device allowing extra current to flow through live to ground, through the person. This current induces a fault current in the sense winding  12  which is converted to a voltage across a burden resistor  14  and this voltage is amplified  16  and fed to some further circuitry (not shown) which makes a decision as to whether the device will trip. If the outcome of this step is that a dangerous fault condition exists, then a signal can be used to energise a tripping mechanism (not shown), isolating the electrical supply. 
     As most residual current devices are electromechanical devices, they should be periodically tested, usually via a test button or switch  20  on the front of the device, to ensure reliable operation. As shown in  FIG. 1 , a test current, which simulates a fault current, is produced when the test button  20  is pressed. This is done by connecting a resistance  22  across the supply conductors, and when the test button  20  is pressed a current flows in a test winding  18  wound on the same toroidal core  10 . A fault current is then induced in the secondary winding  12  which will trip the device. 
     The magnetic detection circuit, which includes magnetic toroidal core  10  and the secondary sense winding  12 , has a low frequency cut-off and so the current transformer and burden resistor  14  values must be designed so as to ensure little filter action at the frequency of interest (50 Hz or 60 Hz). This requires a high inductance and low burden resistance, hence a large expensive inductor core  10  and large amplification gains. 
     An alternative to  FIG. 1  is to use a transresistance amplifier to convert the induced current directly to voltage, as shown in  FIG. 2 . This arrangement uses a voltage amplifier  24  with feedback resistor  26  arranged such that the output voltage is proportional to the input current and the input impedance is very low. This lessens the apparent burden resistance on the sense coil  12 , improving performance with regard to low frequency cut-off which can easily drop to 1 Hz dependent upon the toroidal core  10  used. 
     The low frequency cut-off point of the magnetic detection circuit is important to performance in many ways. It is of course important that at the working frequency (50 Hz or 60 Hz) the response is on a level plateau some way above the cut-off knee. It should also be noted that as the cut-off frequency drops the amount of magnetic field in the core  10  decreases. This is explained by transformer approaching “ideal” performance where the primary and secondary currents produce fields which exactly cancel. The device will then become less dependent of variations in the magnetic properties of the core  10  material such as saturation, permanent magnetisation and variations in permeability due to temperature and ageing. 
     The properties of the magnetic detection circuit are of course not ideal which will affect performance. That is, the input is a current (the residual) and the output is also a current whose amplitude follows the input amplitude scaled by some linear factor. However, for several reasons the system is not ideally linear. These reasons are as follows: 
     (i) Frequency response. The system is AC coupled (as are all transformers) and so rely on varying AC magnetic fields to induce signals into the sense winding  12 . This means at low frequencies the output signal amplitude will be lower than the anticipated ideal. The output drops to zero at DC. The cut-off frequency is determined by two factors, the sense winding  12  resistance and primary inductance. The size of the combined burden resistance  14  and winding resistance must be as low as possible (ideally zero ohms). The cut-off frequency increases as this resistance increases. The primary inductance is a function of the primary turns (usually just one in an residual current device), the magnetic permeability of the core  10  material and the core  10  dimensions. To achieve good response at low mains frequencies, the core  10  needs to be made of very high permeability material (10,000 to 100,000 times greater than free space) and the radius of the core  10  small but with the maximum possible cross-section of the material. Typically, a low frequency cut-off of 10 to 20 Hz is achievable such that at mains line frequencies (50 to 60 Hz) the response is reasonably flat. 
     (ii) Non-linear magnetic properties of the core  10 . As the flux density in a magnetic material increases the permeability decreases and can decrease to a point where the output is distorted. It only takes a few milliamperes of residual current to saturate a core (i.e. permeability dropped to around that of free space). However, since 1 mA of primary produces 1 uA of secondary current in a 1000-turn sense winding  12  then both currents produce the same magnetic effect in the core material but in opposing directions. Hence, no magnetic field should be present in the core material (Lenz&#39;s Law). However, some magnetic field is always present as the output current always has an error making it smaller than expected so complete cancellation does not occur. The size of this error is frequency dependent (increasing as the frequency drops) but above the cut-off frequency can drastically reduce the magnetisation of the core material  10  thus limiting non-linear effects. 
     (iii) Remanent magnetisation. The core material can become magnetised by a large fault current being suddenly disconnected as breakers trip. If this happens the core material will demonstrate low permeability and may cause the current transformer output to be attenuated to an extent that the device fails to detect a fault on reconnection of the supply. 
     (iv) Drift. The permeability of the core  10  changes with temperature and time which can shift the cut-off frequency upwards and effect performance at mains frequencies. 
     Existing residual current devices suffer from all the above effects to some degree. The present invention aims to reduce these effects so as to significantly improve the performance of existing sensors or to allow the use of lower quality sensors to achieve similar performance. This is achieved by alteration of the magnetic detection circuit. 
     In the prior art, nulling using a Hall-effect sensor placed in a gap in the magnetic core has been proposed. However, the required air gap seriously compromises the core performance, especially with regard to summing two opposite currents accurately as occurs in RCD devices. Active transformers have been described, but usually require a second core alongside the magnetic core  10  to produce a nulling field. 
     It is the object of the present invention to provide a nulling current transformer for accurately detecting current and giving an improved response, accuracy and stability using toroidal current transformer technology along with active components. 
     According to the present invention there is provided a nulling current transformer having a closed magnetic core and at least one primary winding inductively coupled thereto, comprising: 
     a secondary winding inductively coupled to said magnetic core, said secondary winding being responsive to any magnetic flux generated in said magnetic core; 
     a tertiary winding inductively coupled to said magnetic core, said tertiary winding being responsive to any magnetic flux generated in said magnetic core; and 
     nulling means for receiving the output of said tertiary winding and nulling the received output, the nulled output of said nulling means being connected to the input of said secondary winding such that it serves to cancel the magnetic flux in said magnetic core. 
     Likewise according to the present invention there is provided a method of nulling a current transformer having a closed magnetic core and at least one primary winding inductively coupled thereto, comprising: 
     monitoring the output of a secondary winding inductively coupled to said magnetic core, said secondary winding being responsive to any magnetic flux generated in said magnetic core; 
     monitoring the output of a tertiary winding inductively coupled to said magnetic core, said tertiary winding being responsive to any magnetic flux generated in said magnetic core; and 
     receiving the output of said tertiary winding and nulling the received output, the nulled output being connected to the input of said secondary winding such that it serves to cancel the magnetic flux in said magnetic core. 
     Preferably, the nulling current transformer may be incorporated as part of a residual current device. In use, the output of the secondary winding is converted to a voltage across a burden resistor and this voltage is amplified and fed to a tripping processor. 
     In one embodiment, the tertiary winding may be a test coil which is used to test the device. In use, the nulling means comprises a first stage amplifier which boosts the voltage from said tertiary winding, and which causes a current to flow in the secondary winding. Preferably, the signs of the signals are arranged such that the voltage induced in tertiary winding from the secondary winding opposes the voltage produced by the primary winding. This essentially produces negative feedback to keep the tertiary winding voltage near zero and nulls the flux in the magnetic core. 
     In use, the tertiary winding may be used in voltage mode to detect any flux present in the core but since no current flows in this winding it does not change the flux. This signal is used to create a current to cancel the flux to produce a result of near zero. The cancellation is ensured using a closed feedback loop which includes the magnetic core. Preferably, the current used to null the field will be exactly related to primary fault current by a ratio determined by the windings. 
     As both amplifiers are DC coupled and of high gain then offset voltages inherent in the amplifiers would produce large DC voltages on the amplifier outputs which wastes power and can saturate the magnetic core. In use, in order to overcome this, very low offset amplifiers may be used or a feedback system is used to produce an offset voltage to null the offset produced by the amplifiers. 
     Further according to the present invention there is provided a residual current device having a trip mechanism for isolating an electric supply to an electrical installation upon detection of a predetermined current imbalance between the line and neutral conductors of said electric supply, comprising: 
     a current transformer having a closed magnetic core and having the line and neutral conductors inductively coupled as a primary winding; 
     a secondary winding inductively coupled to said magnetic core and connectable to said trip mechanism, said secondary winding being responsive to said current imbalance on said electrical installation; 
     a tertiary winding inductively coupled to said magnetic core and responsive to said current imbalance on said electrical installation; and 
     nulling means for receiving the output of said tertiary winding and nulling the received output, the nulled output of said nulling means being connected to the input of said secondary winding such that it serves to demagnetise said magnetic core. 
     It is believed that a nulling current transformer in accordance with the present invention at least addresses the problems outlined above. The advantages of the present invention are that a nulling current transformer for accurately detecting current is provided that gives an improved response, accuracy and stability using toroidal current transformer technology along with active components. 
    
    
     
       A specific non-limiting embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: 
         FIG. 1  shows schematically the operation of a known residual current device which includes a test facility to simulate a fault condition; 
         FIG. 2  illustrates an alternative prior art residual current device; and 
         FIG. 3  shows schematically how the present invention can be implemented as a switchgear device. 
     
    
    
     Referring now to the drawings, a nulling current transformer according to the present invention is shown schematically in  FIG. 3 .  FIG. 3  shows an embodiment where the nulling current transformer is incorporated as part of a residual current device. As shown in  FIG. 3 , the phase and neutral cables from the supply to the load pass through a magnetic toroid  100 . On the toroid  100  is wound a sense coil  102 ; the toroid  100  and sense coil  102  arrangement being referred to as a current transformer. Under normal conditions, the phase and neutral currents are equal and opposite, and no flux is induced in the toroid  100  and hence no current flows in the sense coil  102 . If a fault condition occurs, and current flows through the earth path back to the electrical supply, the phase and neutral currents will no longer be balanced and flux will be induced in the toroid  100 , and a sense current will flow in the sense coil  102 . The sense current generates a voltage across a burden resistor  104  and this voltage is amplified using amplifier  106 . The output of amplifier  106  is connected to some further circuitry (not shown), which makes a trip decision and, if appropriate, open contacts in the electrical supply (not shown). 
     For the reasons previously described above, non-linearities in the magnetic detection circuit and any remanent magnetisation of the toroid  100  can seriously affect the performance and sensitivity of the current transformer, and the present invention takes the concept of cancelling the magnetic flux in the toroidal core  100  further. 
       FIG. 3  shows that a separate tertiary winding  108  is wound on the toroid  100 . In one embodiment, it is envisaged that the tertiary winding  108  could be the test coil which is used to test the device. The output of the tertiary winding  108  is taken to a first stage amplifier  110  which boosts the voltage from this coil  108 , and which causes a current to flow in the sense winding  102 . The signs of the signals are arranged such that the voltage induced in tertiary winding  108  from the sense winding  102  opposes the voltage produced by the primary. This essentially produces negative feedback to keep the tertiary winding voltage near zero thus nulling the field in the core  100 . The current in the sense winding  102  is amplified as previously described to produce an output for tripping decisions. 
     The tertiary winding  108  is used in voltage mode to detect any flux present in the core  100  but since no current flows in this winding  108  it does not change the flux. This signal is used to create a current to cancel the flux to produce a result of near zero. The cancellation is ensured using a closed feedback loop which includes the magnetic core  100 . The current used to null the field will be exactly related to primary fault current by a ratio determined by the windings. This nulling current can then be converted into a voltage using the techniques described previously (i.e., burden resistor or transresistance amplifier). 
     The effect of the feedback loop can be shown using equivalent circuit analysis to greatly reduce the sense winding burden making the frequency cut-off very low (10 mH). This gives the system excellent accuracy, stability and insensitivity to magnetic non-linearities of the core material. 
     It is noted in  FIG. 3  that an offset voltage is required. Since both amplifiers  106 ,  110  are DC coupled and of high gain then offset voltages inherent in the amplifiers  106 ,  110  would produce large DC voltages on the amplifier outputs which wastes power and can saturate the magnetic core  100 . To overcome this either very low offset amplifiers are used or a feedback system is used to produce an offset voltage to null the offset produced by the amplifiers  106 ,  110 . 
     Various alterations and modifications may be made to the present invention without departing from the scope of the invention. For example, although particular embodiments refer to implementing the present invention on a single phase electrical installation, this is in no way intended to be limiting as, in use, the present invention can be incorporated into larger installations, both single and multi-phase. 
     The circuit described in  FIG. 3  is related to an RCD switchgear device where two or more currents are summed within the current transformer and the residual is measured. The measured residual is usually zero or small in such applications. However, this technique also has merit in applications such as metering where a single conductor passes through the current transformer and the actual load current is measured. Such currents are much larger and can quickly saturate the core unless a physically large core is used. The nulling technique described greatly increases the current required to cause saturation such that the use of smaller, cheaper cores become possible.