JPH067536B2 - Wide range current transformer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to improving the performance of a current transformer.

[Prior Art] FIG. 4 shows a principle diagram of a current transformer according to the prior art. Iron core 1
A primary winding 2 and a secondary winding 3 are applied to the secondary winding 3, and a load 4 is connected to the secondary winding 3. If the magnitude of the primary winding current 5 is I ₁ and the magnitude of the secondary winding current 6 is I ₂ , And n ₂ / n ₁ is the current ratio.

[Problems to be Solved by the Invention] FIG. 5 is an equivalent circuit of the conventional current transformer shown in FIG.
In FIG. 5, the primary winding 2'and the secondary winding 3'show an ideal current transformer having a turn ratio of 1: 1 and the current 5 flowing through the secondary winding 3'is the primary current itself. . In addition, 6 in the figure
Is a secondary current, 9 is an induced voltage required to flow the secondary current 6 in the load 4, 7 is an exciting current required to induce the induced voltage 9, and 8 is an impedance for the exciting current 7. As can be seen from FIG. 5, the secondary current 6 actually flowing through the load 4 is the primary current 5 minus the exciting current 7. Therefore, although the exciting current 7 causes an error, the exciting current 7 is so small as to be negligible with respect to the secondary current 6 in a normal use state, and this is not a problem.

FIG. 6 shows the relationship between the exciting current 7 (shown by the horizontal axis in FIG. 6) and the induced voltage 9 (shown by the vertical axis in FIG. 6) of the iron core 1 shown in FIG. In FIG. 5, when the secondary current 6 increases,
Along with this, the induced voltage 9 has to increase along the curve 10 shown in FIG. 6, but when it reaches the point 10a on the curve 10, it cannot increase any more and saturates at the maximum value 9A. In this state, even if the primary current 5 further increases, only the exciting current 7 increases and the secondary current 6 does not increase. Therefore, the relationship of the equation (1) is not established, and the function of the current transformer is lost. At this time, the magnitude of the primary current 5 at which the equation (1) is satisfied is the upper limit of the current that can be measured by the current transformer. This upper limit value can be increased by increasing the cross-sectional area of the iron core 1, but there is a limit because the size and cost of the current transformer increase.

Next, as the secondary current 6 decreases in FIG. 5, the induced voltage 9 decreases along the curve 10 in FIG.
The value of the induced voltage 9 when it goes down to the upper point 10b is 9B.
Then, the ratio of the exciting current 7 to the secondary current 6 shown in FIG. 5 increases, the error increases, and the predetermined performance of the current transformer is lost. The primary current 5 at this time is the lower limit of the current that can be measured by the current transformer.

In this way, in the current transformer using the iron core 1, it is unavoidable that the measurable current value has the upper limit and the lower limit, but in the conventional current transformer, one current transformer is used. The range of current that can be measured by the instrument is not so large. Therefore, it is necessary to standardize the type of rated primary current, and select and use the most suitable rated primary current according to the current value to be actually conducted.

The present invention drastically improves the performance by expanding the measurable upper and lower limit values and using a so-called wide range current transformer in which the same current transformer can be easily set to different rated primary currents. It was done.

[Means for Solving the Problem] The iron core is provided with a current detecting section having two sets of windings and a width increasing section, and the voltage induced in one set of windings of the current detecting section is increased by the width increasing section. Then, using that as a control signal, a current is passed from the amplification part to the remaining one set of windings of the current detection part so as to cancel the induced voltage of the first winding.

[Operation] One of the two windings provided on the iron core detects the magnetic flux in the iron core, and the magnetic flux in the iron core is canceled by the current flowing from the widening portion to the remaining one winding. To work. By increasing the degree of thickening of the thickened portion, the magnitude of the magnetic flux remaining in the iron core can be made much smaller than that of the conventional CT.

As a result, iron core saturation is less likely to occur, and the upper limit value of the measured current is expanded. Further, even in the small current region, the magnetic flux in the iron core is small, so the exciting current also has a small value, the error becomes small, and the lower limit value of the measured current is expanded.

[Embodiment] FIG. 1 shows one embodiment of a current transformer according to the present invention for one phase. In the figure, 1 is an iron core of the current detecting portion, and 2 and 3 are two sets of windings provided on the iron core.
4 is a thickening part, 41 is a control thickening device, 42 is a booster,
It is composed of a resistor 43, an output amplifier 44a, and a gain setting device 44b. Reference numeral 5 is a DC control power supply, 5a is a positive output terminal, 5b is a negative output terminal, and 5c is a zero potential output terminal. Reference numeral 6 denotes a load, which is specifically a measuring device or a protective relay device. Reference numeral 7 indicates a main circuit conductor, and 8 indicates a current flowing in the main circuit. Reference numeral 9 represents a current flowing through the winding 3.

Next, the operation will be described.

The voltage induced in the winding 2 is input to the control amplifier 41. At this time, since the input impedance of the control amplifier 41 is increased, no current is supplied from the winding 2.

When the output of the control amplifier 41 is input to the booster 42, the booster 42 controls the voltage inputs 5a, 5b, 5c supplied from the control power supply 5 and outputs the output voltage E ₀ to the output terminal 42a.

The polarity of the output voltage E ₀ is set so as to add to the induced voltage of the winding 3. Also the size thereof, the voltage induced in the winding 2 and e _1, if the increase width of the control increased width 41 and G, is E ₀ = G · e _1. Further, if the numbers of turns of the windings 2 and 3 are n ₃ and n ₂ , respectively, and the voltage induced in the winding 3 is e ₂ , then e ₁ = n ₃ / n ₂ · e ₂ , and thus E ₀
= G · n ₃ / n ₂ · e ₂ .

When the output voltage E ₀ is output to the output terminal 42a of the booster 42, the voltage e ₂ induced in the winding 3 is coupled with the winding 3
Apply a current of 9 to. The current 9 flows through the resistor 43, and the voltage drop generated across the resistor 43 causes the input terminal 44 of the output amplifier 44a.
c, 44d. The output amplifier 44a widens the input value by a value determined by the setting of the gain setting device 44b, and supplies the output to the load 6 from the output terminals 44e and 44f. In a conventional current transformer, the output is a current (eg 5
A), the amplifier for output 44a according to the invention.
The output from is given as a voltage. However, if the load 6 is an electronic device, the consumption VA for the current transformer secondary circuit is very small and only a signal is needed, which is rather preferable.

Now, assuming that the magnitude of the primary current 8 in FIG. ₁ is i ₁ and the magnitude of the current 9 flowing through the winding 3 is i ₂ , the number of turns of the winding 3 is n ₂ , so i ₁ × 1 = i ₂ × n ₂ (2), it is needless to say that satisfied. Since the equation (2) is obtained by setting n ₁ = 1 in the equation (1), in the current transformer shown in FIG. 1, the winding 3 and the current flowing therethrough are secondary in the conventional current transformer. It corresponds to the winding and the secondary current. However, the current transformer according to the present invention has the following features.

In the closed circuit in which the current detecting portion side is viewed from the output terminal 42a of the booster 42 and the input terminal 44d of the output amplifier 44a in FIG. 1, the resistance of the resistor 43 is set to R _b and the other symbols are as described above. Is used, e ₂ + E ₀ = i ₂ R _b (3) holds. As described above, the output voltage E ₀ is Gn ₃ / n ₂ ·
It is e _2, also the cross-sectional area of the iron core 1 S, since it is when the magnetic flux density _{B e 2 = kn 2 BS (} k is a constant) (3) It can be expressed as. Here, only the cross-sectional area of the iron core 1 is S '
When different conventional type current transformer is to have induced the same voltage at the same flux _{density, kn 2 BS '= i 2} R b ··· (5) (4) Equation (5) the size of the core cross-sectional area from the equation If you compare the The expression (6) means the width G of the control thickener 41 shown in FIG.
It is shown that the core cross-sectional area can be reduced by increasing the value. However, this shows the principle of the present invention, and it cannot be said that the extremely small cross-sectional area of the iron core necessarily brings advantages. Rather, it is more advantageous to increase the upper and lower limit values of the measurement current. It is shown below.

If the maximum magnetic flux density of the iron core 1 in FIG. 1 is Bmax and the current flowing through the winding 3 at that time is I ₂ max, The secondary current of a conventional current transformer at the same magnetic flux density is I ₂ m
Assuming that ax ′, I ₂ max′R _b = kn ₂ BmaxS (8) Comparing the magnitude of the secondary side current from the equations (7) and (8), Equation (9) shows that the current transformer according to the present invention can measure a larger current than that of the conventional type if the magnetic flux density is the same.

Next, the measurement of the minute current will be described.

When the minimum magnetic flux density of the iron core 1 in FIG. 1 is Bmin and the current of the winding 3 at that time is I ₂ min, The magnetic flux density when measuring the same current with a conventional current transformer is B
Letting min be, I ₂ minR _b = kn ₂ Bmin ′S ·············································· ((11) (10) and (11) is compared. Equation (12) shows that the current transformer according to the present invention can measure a current of the same magnitude with a lower magnetic flux density than the conventional current transformer, and since the magnetic flux density is low, the exciting current is also small. Well, as a result, it shows that the error is small. It also shows that smaller currents can be measured if the same error is allowed.

As described above, the current transformer of the present invention can expand both the upper limit value and the lower limit value of the measured current as compared with the conventional type current transformer. Next, the effect of the gain setting device 44b shown in FIG. 1 will be described.

FIG. 2 shows the input / output characteristics of the output amplifier 44a shown in FIG. The horizontal axis is the input 44 of the output amplifier 44a.
The magnitudes of the signals input to c and 44d are shown in terms of the primary current. The vertical axis represents the magnitude of the signal output to the output terminals 44e and 44f of the output amplifier 44a. Now, the gain setting of the gain setting device 44b is set to G ₁
Then, the input / output relationship changes along the straight line G _1, and the full-scale value VFULL is output when the primary current Imax ₁ . Similarly, when the amplification factor of the gain setting device 44b is changed to G ₂ and G ₃ , Imax ₂ and Im are respectively changed.
A full-scale value is output for ax ₃ , realizing a so-called wide-range current transformer. Third
The figure shows a concrete example of the rated primary current of 10
This is an example in which 10 steps can be set from A to 600A.

[Effects of the Invention] The current transformer according to the present invention is particularly effective when incorporated in a multi-tiered cubicle. The conventional type current transformer must have a winding structure in order to obtain good characteristics when measuring small currents. Moreover, depending on the size of the short-circuit capacity of the main circuit, the size may become large and it may be incorporated in a multi-tiered cubicle. Can be difficult. On the other hand, in the current transformer according to the present invention, only one main circuit conductor needs to be penetrated to measure a small current, and it can be easily incorporated in a multi-stage cubicle, which is effective in reducing the installation space. In addition, the cubicle can be manufactured before the details of the load equipment are determined, and the current change ratio can be set after the details are determined, so that the design and manufacturing time can be greatly shortened.

Further, the effect is extremely large, for example, the waste of charging and producing several kinds of current transformers having different ratings can be omitted.

[Brief description of drawings]

FIG. 1 is a diagram showing only one phase of an embodiment of the current transformer according to the present invention, FIG. 2 is a diagram showing input / output characteristics of the output amplifier of FIG. 44a, and FIG. 3 is a diagram showing FIG. It is a figure which shows the specific example which made it possible to set a rated primary current by the gain setting device 44b. FIG. 4 is a principle diagram of a conventional current transformer, FIG. 5 is an equivalent circuit diagram of FIG. 4, and FIG. 6 is a diagram showing an excitation characteristic of an iron core. 1 ... iron core, 2, 3 ... windings provided on the iron core, 2a, 2b
... Terminals of winding 2 3a, 3b ... Terminals of winding 3 4 ... Widening section 41 ... Control widening section 42 ... Booster 42a ... Booster output terminal 43 ... Resistor 44a ... Output Magnifier, 44b ... Gain setting device, 44c, 44d ... Input terminal of output amplifier, 44e, 44f ... Output terminal of output amplifier, 5 ... Power source, 5a, 5b, 5c ... Positive polarity of power source, Negative polarity and zero voltage output terminal, 6 ... Load.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Osamu Kajiya 1-3-1 Uchisaiwaicho, Chiyoda-ku, Tokyo Toyohiro Sakai Examiner, Tokyo Electric Power Company

Claims (2)

[Claims] 1. A current detecting section, a widening section connected to the current detecting section, a DC control power source for externally supplying a voltage to the widening section, and an output from the widening section. A load, the current detection unit is formed by applying two sets of windings to an iron core, and the widening unit is a control unit for inputting a winding output of one of the two sets of windings. The amplifier and the output of the controlling amplifier are used as control signals, and the voltage input supplied from the DC control power source is controlled so that the polarity of the remaining one of the two windings is controlled. The remaining voltage equal to the voltage induced in the winding and whose magnitude is proportional to the output of the control amplifier
A booster for outputting to one terminal of a pair of windings, a resistor connected to the other terminal of the remaining pair of windings, and a signal from both ends of the resistor as input and output to the load. A wide-range current transformer, characterized in that it is formed from an output amplifier and a power amplifier. 2. A method according to claim 1, wherein the width of the output amplifier is variable so that a full scale output can be obtained for a plurality of rated primary currents. Wide range current transformer.