IES20100254A2 - DC current detection circuit - Google Patents

Abstract

A circuit for detecting a DC current in at least one conductor comprises a current transformer having the conductor as a primary and also having at least one secondary. An oscillator supplies an oscillating signal across the secondary, and a change in the impedance of the secondary is detected.

Description

This invention relates to a circuit for detecting DC 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.

Prior art indicates that current transformers have been used for the detection of DC currents in a circuit. However, in most cases, this involves the use of two CTs used in tandem, or a CT with two windings which are used in tandem to detect the DC current. This often involves a need to saturate and desaturate the CT core so as to produce an output signal in response to a DC current. This further requires the use of sophisticated electronic circuitry to process the resultant signal so as to determine the level of the DC current.

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 comprising a current transformer having the conductor as a primary and also having at least one secondary, an oscillator for supplying an oscillating signal across the secondary, .and means for detecting a change in the impedance of the secondary.

In an embodiment a plurality of conductors form the primary, and the detecting means is arranged to detect a non-zero vector sum of DC currents flowing in the conductors.

Other embodiments may additionally include means for detecting a non-zero vector sum of AC currents flowing in the conductors.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a first embodiment of the invention.

If 10 02 Figure 2 is a circuit diagram illustrating DC currents flowing in opposite directions.

Figure 3 shows a typical hysteresis curve plotted for a ferromagnetic material.

Figure 4 are waveforms showing the effect of DC currents flowing in opposite directions.

Figure 5 is a circuit diagram of a second embodiment of the. invention.

Figure 6 shows an arrangement using inverting and noninverting amplifiers to detect currents of different polarities or direction- flowing in the conductor-.

Figure 7 shows an embodiment of the invention for detecting a residual or differential DC current in two conductors.

Figures 8 and 9 are further embodiments of the invention which can detect residual AC currents as well as DC currents.

Referring to Figure 1, a circuit for detecting DC current in a conductor Ll comprises a current transformer CT including a toroidal core 10 formed from, for example, ferromagnetic material, and a single winding W1 wound on the core. The conductor Ll passes through the core 10 and forms a primary winding for the current transformer, and the winding W1 forms a secondary winding. The winding W1 forms a reactive element whose impedance Z is determined by the 100254 characteristics of the winding and the core. The winding Wl is connected in series with a resistor R1 so that R1 and Wl form a potential divider. Alternatively, the resistor R1 could be replaced by a capacitor.

An oscillator Osc generates a preferably sinusoidal oscillatory voltage of fixed magnitude and frequency which is developed across R1 and Wl. The alternating voltage developed across Wl, i.e. the voltage at the junction 12 of Wl and Rl, is half wave rectified by a diode Dl, smoothed by a capacitor Cl, and fed to the negative input of a comparator Comp 1. A reference voltage Vref is applied to the positive input of Comp l·. Vref is chosen such that under normal . conditions the rectified voltage across Cl is greater than Vref with the result that the signal at the output Vout. of Comp 1 will be low. Rb is a bleed resistor to prevent Cl from holding a charge indefinitely, with the result that the voltage on Cl tracks the voltage on Wl.

When a DC current flows through conductor LI, the resultant magnetic field changes the characteristics of the current transformer CT such that impedance Z is reduced from its initial level, and as a result the voltage developed across Wl is reduced. When the DC current flow through Ll reaches or exceeds a certain magnitude, the voltage developed across Wl will be sufficiently reduced so as to cause the voltage level at the negative input of Comp 1 input to be less than Vref, and as a result, the output signal Vout from Compl will go high. In detecting the DC current the CT core 10 is not driven into saturation, and the basic principle of operation is .to detect an AC current whose magnitude changes in response to magnitude changes in a coincident DC current.

Thus, the simple circuit of Figure 1 provides an effective means for the contactless detection of a DC current at or in excess of a certain magnitude.

The foregoing provides a particularly simple measurement of the DC current, i.e._ simply whether or not it exceeds a certain threshold. However, since the reduction in the AC voltage developed across W1 will be proportional to the increase in the magnitude of the DC current flowing in LI until saturation of the core 10 occurs, it is possible to measure the magnitude of the DC current within a range· by measuring the W1 voltage, either directly, or by measuring.the DC voltage across Cl.

DC current can be of a positive or negative polarity, and the above circuit will detect currents of either polarity. However, the polarity of the current flow though LI can affect the symmetry of the AC signal developed across W1 with the result that the circuit's detection threshold for a DC current of positive polarity may be different to that for a DC current of a negative polarity.

For example, the circuit of Figure 1 could be fitted into a residual current monitor or a residual current device; in either case the load conductors will be passed through the CT. The direction in which the conductors are connected will determine the direction of DC current flow in the conductors, IE 1 0 0 2 5 4 as shown in Figure 2. DC1 represents a DC current flowing in one direction, and DC2 represents a DC current flowing in the opposite direction. These could be considered to be currents flowing in different directions, or currents of opposite polarity.

Figure 3 shows a typical hysteresis curve plotted for a ferromagnetic material.

The principle of how the hysteresis curve is plotted will be well known to those versed in the art. The key point is that under normal circumstances the curve is centered symmetrically around the B and H axis. However, when a DC current flows through the current transformer CT of Figure 1, the B-H curve will be repositioned .to the left or right along the H axis such that the curve is no longer symmetrical with respect to the B line. When the DC current flows in one direction, the curve will be repositioned to the right, and when the current flows in the opposite direction, the curve will be repositioned to the left. The extent to which the curve will be repositioned will depend on the magnitude of the DC current. This will result in the AC voltage developed across winding W being asymmetrical, as represented in Figure 4.

Figure 4(a) shows a typical voltage signal across W with no DC current flow through the conductor. Figure 4(b) shows that the signal has been shifted upwards with respect to the zero line, and Figure 4(c) shows that the signal has been shifted downwards with respect to the zero line, the upwards or downwards shifts being determined by the direction of DC current flow through the CT. Although in practice the magnitude of the AC signal would also· have been reduced because of the DC current flow in the CT, Figures 4(a) to 4(c) have shown signals of equal magnitude for all three cases so as to demonstrate more clearly the upward or downward shift in the zero line in the signals caused by the DC current.

In Figure 1, only the positive half cycles of the voltage across W1 are fed to Comp 1. It' follows that it would take a larger DC current to cause Comp 1 output to go high in the case of Figure 4(c) as compared to Figure 4(b)·. Thus the threshold of DC current detection would be different when the DC current flows in one. direction rather than .the other, direction, but it may be important to have a circuit which, has the same threshold of detection for the- DC current regardless of its direction or polarity.

This .object can be achieved by full wave rectifying the output from Wl, as shown in Figure 5, where the diode DI is replaced by the full wave rectifier 14.

In the arrangement of Figure 5, the full wave output is smoothed by Cl and applied to Comp 1 as before. The threshold of detection will be the same regardless of the polarity of the DC current.

Figure 6 shows an arrangement using inverting and non inverting amplifiers to detect currents of different IE 1 0 02 54 polarities or direction flowing in the conductor. Amplifier Al is a non inverting amplifier and amplifier A2 is an inverting amplifier. Al will only amplify and pass positive half cycles of the signal from Wl to the output. A2 will invert and pass only the negative half cycles of the signal from Wl to the output. Geff provides effective grounding of the Wl signal via A2. Vin is the signal presented to Al and A2 for processing. Each amplifier output can be fed to a respective smoothing capacitor Cl or C2 to produce a DC voltage Vout 1 or Vout 2 which is proportional to the voltage across Wl. The two outputs can be used separately, or combined to produce an effective full wave and amplified version of the Wl signal at Vout combined· If the gain of Al and A2 is the same, the current flow in the conductor is in the direction of DC1, and the Wl signal is offset as shown in figure 4b, then the DC voltage produced by Al will be greater than that of A2 because the positive half cycles are of greater magnitude than the negative half cycles.

Conversely, if the gain of Al and A2 is the same, the current flow in the conductor is in the direction of DC2, and the Wl signal is offset as shown in figure 4c, then the DC voltage produced by Ά2 will be greater than that of Al because the positive half cycles are of greater magnitude than the negative half cycles.

Therefore, the circuit can include means (not shown) to determine which of the Cl and C2 voltage levels is the IE 1 0 02 5 4 greater, to indicate the direction of DC current flow in the conductor Ll. Further, the Cl and C2 voltage levels can be combined to indicate the magnitude of the DC current.

The above embodiments show how the invention can be used to detect a DC current in a single conductor. The invention may also be used to detect a residual or differential DC current in two or more conductors passing through the core 10. In such a case, when the vector sum of the DC currents flowing through the core is zero there will be no change in the impedance of the secondary winding W1 and no change in the voltage at the junction 12, whereas when the DC currents in the conductors are out of balance (i.e. non-zero vector sum) this will give rise to a change of impedance which can be detected.

An embodiment for detecting residual DC currents is shown in Figure 7, in which two conductors Ll and L2 pass through the core 10. The conductor carry respective DC currents DC1 and DC2 flowing in opposite directions. The DC sensing circuit 20, which may be as constructed as described for any of Figures 1, 5 or 6, will not respond when the magnitude of DC1 is equal to DC2, but when the difference between the two exceeds a threshold set by the circuit 20 the latter will output a signal Vout as previously described.

The embodiment of Figure 7 may be modified to include circuitry for also detecting a non-zero vector sum of AC currents flowing in the conductors. One such embodiment is shown in Figure 8.

IE 1 Ο Ο 2 5 4 In Figure 8, the core has two secondary windings, W1 as before and a second winding W2. An electronic circuit 22 comprising an integrated circuit type WA050 is connected across the winding W2 via a filter 24. The WA050 is an RCD IC supplied by Western Automation Research & Development Ltd, Ireland.

The oscillator signal flows through W1 as previously described. As this signal will appear like a residual AC current within CT, it will induce a corresponding signal into winding W2 which, but. f.or the low pass filter 24, would be detected by the WA050 IC. However, the oscillator signal is at a relatively high frequency, e.g. several KHz, and the low pass filter 24 substantially blocks this high frequency, signal and prevents its detection by the WA050 IC. However, AC residual currents at mains power frequency, e.g. 50Hz, flowing in the conductors Ll, L2 will induce a current into W2 and will be passed by the filter to the WA050 for detection.

Another embodiment for detecting AC as well as DC is shown in Figure 9. In Figure 9 the secondary W1 comprises two sections Wla and Wlb having the same number of turns but wound in antiphase, such that the vector sum of the oscillator current lose flowing through Wla and Wlb as seen by the CT is zero. Therefore the filter 24 used in Figure 8 can be omitted. The voltage supplied to the DC sensing circuit 20 is taken across the section Wlb, but it could be taken across Wla, or across both Wla and Wlb, or across part IE 1 0 02 5 4 of one and/or part of the other. It is processed in the circuit 20 as previously described.

Refinements could be made to the embodiments above without departing from the scope of the invention. For example, the signal from W1 could be amplified prior to being fed to Comp 1. Thus there has been described a simple but effective contactless means of detecting and measuring a DC current, regardless of the direction of current flow through the load conductor.

Although the foregoing embodiments have triggered the comparator to generate an output signal when the voltage drop across W1 falls below a certain threshold set by Vref, the circuit could alternatively be designed to generate a signal when the voltage drop across W1 exceeds a certain threshold, corresponding to a drop in DC current in Ll below a certain level.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.

Claims (13)

Claims

1. A circuit for detecting a DC current in at least one conductor, the circuit comprising a current transformer having the conductor as a primary and also having at least one secondary, an oscillator for supplying an oscillating signal across the secondary, and means for detecting a change in the impedance of the secondary.

2. A circuit as claimed in claim 1, wherein the detecting means is arranged to detect a change in the voltage across at least a portion of the secondary.

3. A circuit as claimed in claim 2 wherein the detecting means comprises a comparator for generating a signal when the voltage change exceeds, or falls below, a given threshold.

4. A circuit as claimed in claim 3 wherein the voltage is applied to the comparator after half wave rectification.

5. A circuit as claimed in claim 3 wherein the voltage is applied to the comparator after full wave rectification.

6. A circuit as claimed in claim 2, wherein the voltage is applied to respective amplifiers arranged to pass opposite half cycles respectively of the applied voltage.

7. A circuit as claimed in claim 6 including means for indicating the direction of DC current flow according to which amplifier output is greater. IE 1 0 02 5 4

8. A circuit as claimed in claim 6 including means for indicating the magnitude of the DC current by combining the amplifier outputs.

9. A circuit as claimed in any preceding claim, in which a plurality of conductors form the primary, and the detecting means is arranged to detect a non-zero vector sum of DC currents flowing in the conductors.

10. A circuit as claimed in claim 9, additionally including means for detecting a non-zero vector sum of AC currents flowing in the conductors.

11. A circuit as claimed in claim 10, wherein the additional detecting means comprises a further secondary and a residual AC current detection circuit connected to the further secondary via a filter which substantially filters out signals at the oscillator frequency but passes signals at the frequency of the AC flowing in the conductors.

12. A circuit as claimed in claim 10, wherein the first secondary comprises two sections having the same number of turns but wound, in antiphase, and wherein the additional detecting means comprises a further secondary and a residual AC current detection circuit connected to the further secondary.

13. A circuit as claimed in any preceding claim, wherein the current transformer comprises an apertured core, the IE 1 0 0254 conductor passing through the aperture and the, or each, secondary being a winding on the core.