KR20080014295A - Evaluation device of burden for current transformer using current transformer comparator and precise shunt resistor and method thereof - Google Patents

Abstract

An apparatus and a method for estimating a burden of a current transformer using a current transformer comparator and a precise shunt resistor are provided to measure a burden based on a spot condition without dissembling the burden connected to the current transformer comparator and the current transformer. An apparatus for estimating a burden of a current transformer using a current transformer comparator and a precise shunt resistor comprises a large current generation unit(10), a standard current transformer(12), a current transformer(14) to be measured, a current comparator(20), a burden(18), and a precise shunt resistor(16). The large current generation unit outputs a uniform current. The standard current transformer and a current transformer to be measured are connected to the large current generation unit in series. The current comparator compares a secondary side current of the standard current transformer with a second side current of the current transformer to be measured. The burden is serially connected to the second side of the current transformer to be measured. The precise shunt resistor is connected to the second side of the current transformer to be measured in parallel.

Description

Evaluation device of burden for current transformer using current transformer comparator and precise shunt resistor and method

1 is an equivalent circuit diagram of a current transformer when there is an external burden (Z _b ),

Fig. 2 is an equivalent circuit diagram of a current transformer when the precision shunt resistor Z is connected to both ends of the secondary side of the current transformer.

3 is a configuration diagram of a burden measuring device using a current transformer comparator;

Figure 4 is a graph of the results of measuring the error of the current transformer according to the change in the precision shunt resistance value in the current transformer according to the present invention,

5 is a graph of the results of measuring the phase angle error of the current transformer according to the change in the precision shunt resistance value in the current transformer according to the present invention;

<Description of the symbols for the main parts of the drawings>

10: large current generating means,

12: standard current transformer,

14: current transformer to be measured,

16: precision shunt resistance (Z),

18: burden for the current transformer (Z _b ),

20: current comparator.

The present invention relates to an apparatus for evaluating the burden of a current transformer, and more particularly, to an apparatus and method for evaluating the burden for a current transformer using a current transformer comparator and a precision shunt resistor.

In general, large currents of several kA or more cannot be directly measured, and are classified into low currents using a current transformer. In order to obtain the magnitude and phase of the large current from the measured low current, the error and phase angle error of the current transformer must be known. The measurement of the non-error and phase-angle error of the current transformer under measurement is based on a standard current transformer that can ignore the non-error and phase-angle error (within 0.01% error). Measure the phase angle error with.

However, the error and phase angle error of the current transformer are measured with the load connected in series to the secondary side of the current transformer according to the KS standard, and the error of the current transformer depends on the burden value and the power factor. Therefore, accurate measurement of burden value and power factor is very important for precise measurement of non-error and phase angle error of current transformer.

Accordingly, the present invention has been made to solve the above-described problems, and a first object of the present invention is to provide a load value and a power factor connected to the secondary side of the current transformer to be measured by using a current transformer comparator and a precision shunt resistor. It is to provide an evaluation apparatus and an evaluation method which can be measured.

An object of the present invention as described above, the large current generating means for outputting a constant current (10);

A standard current transformer 12 and a measured current transformer 14 connected in series with the large current generating means 10, respectively;

A current comparator 20 for comparing the secondary side current of the standard current transformer 12 with the secondary side current of the current transformer 14 to be measured;

A burden Z _b , 18 connected in series to the secondary side of the current transformer 14 under measurement; And

A precision shunt resistor 16 connected in parallel to the secondary side of the current transformer 14 under measurement; It can be achieved by the current transformer comparator and the evaluation device of the load for the current transformer using a precision shunt resistor, characterized in that consisting of.

And, the precision shunt resistor 16 may be variable in the range between 200 Ω and 8 kΩ.

In addition, it is suitable that the standard current transformer 12 and the current transformer 14 to be measured have the same current ratio.

Resistance value calculating means for calculating a resistance value R _b of the burden on the current transformer 14 to be measured based on a change in the resistance value of the precision shunt resistor 16;

Reactance value calculating means for calculating a reactance value X _b of the burden on the current transformer 14 to be measured based on a change in the resistance value of the precision shunt resistor 16;

It is further preferable to further include calculating means for calculating the burden value and the power factor of the burden based on the calculated resistance value R _b and the reactance value X _b .

And the calculating means is based on the following equation,

It is most preferable to calculate the burden value and the power factor of the burden.

The object of the present invention as described above, as another category, the standard current transformer 12 and the measured current transformer 14 connected in series to each of the large current generating means 10, the large current generating means 10 for outputting a constant current; ), A load connected in series to the secondary side of the current comparator 20 and the current transformer 14 to compare the secondary current of the standard current transformer 12 and the secondary current of the current transformer 14 to be measured ( Z _b , 18); And a precision shunt resistor (16) connected in parallel to the secondary side of the current transformer (14) to be measured.

A change step of changing the resistance value of the precision shunt resistor 16 step by step (S10);

A calculation step (S20) of calculating a resistance value (R _b ) and a reactance value (X _b ) of the burden of the current transformer 14 under measurement according to the precision shunt resistance 16 which is changed in stages;

From the following equations based on the calculated resistance value R _b and reactance value X _b ,

Computing the burden value and the power factor of the burden (S30); can be achieved by the current transformer comparator and the evaluation method of the burden for the current transformer using a precision shunt resistor.

And the calculating step (S20),

A first calculating step (S22) of calculating a ratio error and a phase angle error of the current transformer 14 to be measured according to the precision shunt resistor 16 which is changed in stages;

A second calculation step (S24) of calculating a slope from a plurality of data pairs composed of the changed resistance value of the precision shunt resistor 16 and the corresponding error, and setting the slope to the resistance value R _b ;

And a third calculation step (S26) of calculating a slope from a plurality of data pairs consisting of a changed resistance value of the precision shunt resistor 16 and a corresponding phase angle error to make the reactance value X _b . Do.

And step (S40) of removing the burden (Z _b , 18);

A changing step (S42) of changing the resistance value of the precision shunt resistor 16 step by step;

A calculation step (S44) of calculating a pure wire resistance value r and pure pure reactance value x according to the precision shunt resistor 16 which is changed in stages;

The net resistance is calculated by subtracting the wire resistance value r from the resistance value R _b of the calculating step S20, and the pure preactance value x is subtracted from the reactance value X _b to obtain the pure load. Calculating a reactance value of S46; And

It is most preferable to further include; calculating the power factor of the burden value and the burden from the calculation step (S30) using the calculated resistance value and reactance value of the pure burden.

Other objects, specific advantages and novel features of the invention will become more apparent from the following detailed description and the preferred embodiments described in conjunction with the accompanying drawings.

The invention will now be described in detail with reference to the accompanying drawings, in which preferred embodiments are shown.

The present invention first measures the rate of change of the non-error and phase angle error with respect to the change in resistance by connecting a parallel shunt resistor in parallel with the secondary side of the current transformer to be measured, in which a DC-to-AC conversion difference can be ignored below 10 ^-5 . As a result, the burden value and power factor for the current transformer are obtained. And the uncertainty about the measured value obtained by this method was also evaluated.

(Burden definition)

In the present invention, the burden is defined as a load connected between the secondary terminals of the current transformer, and the input impedance of the ammeter and the wattmeter current terminals acts as a burden. The burden is expressed in terms of apparent power (VA) and power factor consumed by the load under conditions of rated frequency and secondary rated current. The load rating to be tested depends on the size of the load to be connected to the secondary side of the current transformer and the KS standard. For current transformers, there are 2.5, 5, 15, and 25 VA for standard (grades 0.1 to 0.2) and 5, 10, 15, 25, 40, for general instruments (0.5, 1.0, 3.0). There is 100 VA. The test of the current transformer is carried out at 25% and 100% of the rated load, but the standard current transformer is tested at power factor 1 and the general current transformer is tested at power factor 0.8.

As a specific configuration of the burden for the current transformer for general instruments other than the power factor of 1, a resistor (R) and an inductor (L) are connected in series. Of course, the burden of standard current transformers, which require the use of a power factor of 1, must be made of pure DC resistance alone.

(Measurement principle of burden)

1 is an equivalent circuit diagram of a current transformer when there is an external burden Z _b . As shown in FIG. 1, the current ratio of the primary side to the secondary side of the current transformer is the ratio of the number of turns of the secondary side to the primary side, as shown in [Equation 1].

In Equation 1, N ₁ and N ₂ are the number of turns on the primary side and the secondary side, respectively, and N is the rated turns ratio of the current transformer.

The complex ratio of the primary side (I _p ) current vector to the secondary side (I _b ) of the current transformer when there is an external burden (Z _b ) can be written as Equation (2).

Each factor in [Equation 2] is as follows.

α _b =-Re ((Z ₂ + Z _b ) / Z _m ): Error of current transformer in case of external burden Z _b

β _b =-Im ((Z ₂ + Z _b ) / Z _m ): Phase angle error of current transformer with external burden Z _b

Z ₁ : Primary leakage impedance of current transformer

Z ₂ : Secondary leakage impedance of current transformer

Zm: Magnetization impedance

Z _b = R _b + j X _b : Impedance of external burden

2 is an equivalent circuit diagram of the current transformer when the precision shunt resistor Z is connected to both ends of the current transformer secondary side. As shown in FIG. 2, the shunt precision resistor Z is connected to both ends of the secondary side of the current transformer in order to measure the burden of the current transformer Z _b .

In FIG. 2, the current I flowing to the secondary side of the current transformer is the sum of I _m and I _z , I _b , and a Since b and c and d have the same voltage drop, it can be written as in [Equation 3].

[Equation 3a] ~ [Equation 3e] to summarize the calculation can be written as [Equation 4].

Using Equation 4 and Equations 1 and 2, the complex ratio (I _p / I _s ) of the current vectors on the primary and secondary sides when there is a standard precision shunt resistor is expressed as in Equation 5 Can write

In Equation 5, α _b 'and β _b ' are the error and phase angle error when there is precision shunt resistance, respectively. The standard precision shunt resistor (Z) is Z = R with a negligible resistor with a DC-to-AC conversion difference of 10 ^-5 or less. to be. Therefore, the error (α _b ') and the phase angle error (β _b ') when there is a precision shunt resistance can be written as shown in [Equation 6] and [Equation 7] using [Equation 2], respectively. .

or

or

Equation (6) shows the difference between the difference of the error with and without the precision shunt resistance (α _b ' The difference between the phase angle error (β _b '-β _b ) with and without precision shunt resistance in -α _b ) and [Equation 7] is proportional to the inverse of the precision shunt resistance value (1 / R). . Therefore, α _b 'measured while changing the value of the inverse (1 / R) of the precision shunt resistance by [Equation 6]. When the -α _b value is fitted as a linear function of (1 / R), the slope of the straight line becomes the burdensome resistance component (R _b ). Strictly speaking, the resistance component (R _b ) obtained here is the value that contains the line resistance (r) of the measuring cable connected to the load, that is, R _b + r. Similarly, if the value of β _b '-β _b measured while changing the value of (1 / R) according to Equation 7 is fitted as the first-order function of (1 / R), the slope of the straight line is the burden reactance component. (X _b ) Strictly speaking, the reactance component (X _b ) obtained here is a value containing the reactance component (x) of the measuring cable connected to the burden, that is, X _b + x. Therefore, in order to obtain the pure burden resistance component (R _b ) and reactance component (X _b ), the resistance component (r) and reactance component (x) of the measurement cable should be obtained and subtracted.

Thus, the burden value and the power factor are obtained from Equations 8a to 8d from the pure burden of the resistive component R _b and the reactance component X _b .

In general, the secondary side current of the current transformer is measured at 5 A, so substitute 5 A in [Equation 8].

(Burden measurement system configuration)

3 is a configuration diagram of a burden measuring apparatus using a current transformer comparator. As shown in Fig. 3, the large current generating means 10 supplies the same current in series to the primary side of the standard current transformer 12 and the current transformer 14 under measurement having the same current ratio. By comparing the secondary currents of the two current transformers 12 and 14 with the current comparator 20, the error and phase angle error of the current transformer 14 to be measured are measured.

Here, the standard current transformer 12 and the current comparator 20 are manufactured by Knopp, USA, and the models are P-5000 and Knopp KVTs, respectively. The measured current transformer 14 used was a model 2261 manufactured by Yokogawa, Japan, and was measured while maintaining the primary current at 100 A and the secondary current at 5 A. As shown in FIG. 3, the precision shunt resistor Z 16 is connected in parallel to the secondary side of the current transformer 14 to measure the burden Z _b 18.

(Burden measurement result)

While changing the precision shunt resistor 16 in a range between 200 Ω and 8 kΩ, the error and phase angle error of the current transformer 14 to be measured are measured. Examples of the measurement results of the error at the current transformer strain value 10 VA, power factor 0.8, 60 Hz, and the secondary current 5 A are shown in FIG. 4, respectively.

Figure 4 is a graph of the results of measuring the error of the current transformer according to the change in the precision shunt resistance value in the current transformer according to the present invention. As shown in Fig. 4, the y-axis shows the difference in error (α _b 'with and without precision shunt resistance. -α _b ), and the x-axis represents the inverse of the precision shunt resistance value (1 / R). In FIG. 4, the straight line shows the fitting result according to [Equation 6], wherein the slope of the straight line is 0.4162 Ω and the fitting error is shown in parentheses. The slope of the line 0.4162 Ω indicates that the resistance component of the burden for the current transformer plus the resistance component of the measuring cable connected to the burden, ie R _b + r = 0.4162 Ω.

In order to find the resistance component of the pure burden not including the wire resistance of the cable, remove the burden from FIG. 3 and short-circuit the measurement cable to measure the change in the non-error while changing the precision shunt resistance as before. The wire resistance of the cable was measured. As a result of the fitting, the wire resistance r = 0.0396 kPa of the cable. Therefore, 10 VA / PF = 0.8, the pure burden resistance component is R _b = 0.4162 Ω-0.0396 Ω = 0.3766 Ω.

Similarly, the phase angle error of the current transformer is measured while varying the precision shunt resistance in the range of 200 Ω to 8 kΩ. Examples of the measurement results of the phase angle error at a load value of 10 VA, a power factor of 0.8, 60 Hz, and a secondary current of 5 A for the current transformer are shown in FIG. 5, respectively.

5 is a graph of the results of measuring the phase angle error of the current transformer according to the change in the precision shunt resistance value in the current transformer according to the present invention. As shown in FIG. 5, the y-axis represents the difference (β _b '-β _b ) between phase angle errors with and without the precision shunt resistance, and the x-axis is the inverse of the precision shunt resistance value (1 / R). Indicates. In FIG. 5, the straight line shows the fitting result according to [Equation 7]. The slope of the straight line is 0.252 Ω, and the parenthesis shows an error. The slope of the straight line 0.252 Ω represents the sum of the reactance component of the burden for the current transformer plus the reactance component of the measurement cable connected to the burden (X _b + x = 0.252 Ω).

To obtain the purely charged reactance component that does not include the reactance component of the cable, remove the burden from FIG. 3 and short-circuit the measurement cable. To obtain the reactance component of the cable. As a result of the fitting, the reactance component of the cable was x = 0.004 Hz. Accordingly, the reactance component of 10 VA / PF = 0.8 and pure burden was obtained by subtracting x = 0.004 에서 from 0.252 Ω obtained in FIG. 5 to obtain X _b = 0.248 Ω.

The burden value and power factor for the current transformer from the load resistance component (R _b ) and reactance component (X _b ) obtained using the precision shunt resistance can be obtained by Equation (8). On the other hand, in the burden in the range of 2.5 VA / 0.8 to 40 VA / 0.8, the resistance component (R _b ) and the reactance component (X _b ) of the burden were obtained in the same manner as the previous method. The power factor was calculated and shown in [Table 1].

(Uncertainty assessment and validation)

Uncertainty evaluation in the measurement of the current transformer burden finds the uncertainty factor, obtains the standard uncertainty (u ₁ , u ₂ , u ₃ ...,) and the degrees of freedom for each factor, and obtains the relative synthesized standard uncertainty and the effective degree of freedom from it. Relative expansion uncertainty (U) is obtained by finding the inclusion factor according to the effective degrees of freedom and confidence level and multiplying it by the relative composite standard uncertainty. Since the inclusion factor is 2, the relative expansion uncertainty (U) is expressed as shown in [Equation 9].

When measuring the burden for the current transformer using the current transformer comparison measuring apparatus according to the present invention, the uncertainty factors for the burden value and the power factor are summarized in the second row of [Table 2] and [Table 3], respectively. The relative composite standard uncertainties and relative expansion uncertainties obtained for the load values and power factor at various loads (2.5 VA to 40 VA) are shown in the last two lines of [Table 2] and [Table 3], respectively (unit%). Relative expansion uncertainty for burden values ranges from 1.62% to 1.83% and relative expansion uncertainty for power factor ranges from 1.68% to 2.02%.

Meanwhile, in order to verify the validity of the evaluation apparatus and the evaluation method according to the present invention, the impedance value of the current transformer burden measured using the current transformer comparator and the impedance value of the current transformer burden measured directly using a digital multimeter are measured. Table 3] was compared with each other. The last column of Table 3 shows the relative differences for the impedance values obtained for the two methods. Here, the relative difference is defined as

Relative difference (%) = (impedance value measured by multimeter-impedance value measured by current comparator) / impedance value measured by current comparator × 100 (%)

As can be seen from Table 3, the relative difference (%) was 2.7% at the maximum of 2.5 VA / 0.8 of the burden, and about 1% at the other burden. This indicates that the uncertainty for the two methods is about 2%, which allows us to verify that the two measurements agree within the mutual uncertainty.

Therefore, according to one embodiment of the present invention as described above, there is an advantage that the burden can be evaluated according to the field conditions in the direct connection state without disassembling the load connected to the current transformer comparator and the current transformer to be measured. .

In addition, it is possible to evaluate the burden from time to time using only precision shunt resistor, as well as the current transformer comparator is characterized.

In addition, the burden for the current transformer is different from the burden for the voltage transformer, so that the impedance of the burden is small as about 0.1 작아 and the impedance value of the measurement cable connected to the burden should be considered. However, in the present invention, since the burden value including the impedance value of the measurement cable as well as the pure burden value can be known, it is a convenient method that does not need correction considering the effect of the impedance of the measurement cable itself.

Although the invention has been described in connection with the preferred embodiments mentioned above, it will be readily apparent to those skilled in the art that various other modifications and variations can be made without departing from the spirit and scope of the invention. And modifications all fall within the scope of the appended claims.

Claims (8)

A large current generating means 10 for outputting a constant current; A standard current transformer 12 and a measured current transformer 14 connected in series to the large current generating means 10, respectively; A current comparator 20 for comparing the secondary current of the standard current transformer 12 with the secondary current of the current transformer 14 to be measured; A burden (Z _b , 18) connected in series with the secondary side of the current transformer 14 under measurement; And A precision shunt resistor 16 connected in parallel to the secondary side of the current transformer 14 under measurement; Apparatus for evaluating the burden for the current transformer using a current transformer comparator and a precision shunt resistor, characterized in that consisting of. The method of claim 1, The precision shunt resistor (16) is a current transformer comparator and the evaluation device for the current transformer using the precision shunt resistor, characterized in that it can be varied in the range between 200 Ω and 8 kΩ. The method of claim 1, And a current transformer comparator and a current transformer comparator, wherein the current transformer comparator and the current transformer 14 under measurement have the same current ratio. The method of claim 1, Resistance value calculating means for calculating a resistance value (R _b ) of the burden of the current transformer (14) under measurement based on a change in the resistance value of the precision shunt resistor (16); It means for calculating the reactance value based on the resistance value variation of the precision shunt resistor 16 calculates a reactance value (X _b) of the burden of the current transformer under test (14); A current transformer comparator and a current using a precision shunt resistor, further comprising calculating means for calculating a burden value and a power factor of the burden based on the calculated resistance value R _b and the reactance value X _b , respectively. Apparatus for evaluating burden for transformers. The method according to claim 4, wherein the calculating means is based on the following equation:

An apparatus for evaluating the burden for a current transformer using a current transformer comparator and a precision shunt resistor, characterized in that the burden value and the power factor of the burden are calculated. Large current generating means 10 for outputting a constant current, the standard current transformer 12, the current transformer 14 to be measured and the secondary current of the standard current transformer 12 connected in series to the large current generating means 10, respectively A current comparator 20 for comparing the secondary current of the current transformer 14 to be measured and a load (Z _b , 18) connected in series to the secondary side of the current transformer 14 to be measured; And a precision shunt resistor (16) connected in parallel to the secondary side of the current transformer (14) to be measured. Changing step (S10) of changing the resistance value of the precision shunt resistor (16) step by step; A calculation step (S20) of calculating a resistance value (R _b ) and a reactance value (X _b ) of the burden of the current transformer 14 to be measured according to the precision shunt resistor 16 which is changed in stages; From the following equations based on the calculated resistance value R _b and reactance value X _b ,

Calculating a burden value and a power factor of the burden (S30); and a method for evaluating the burden for a current transformer using a current transformer comparator and a precision shunt resistor. The method of claim 6, wherein the calculating step (S20), A first calculating step (S22) of calculating a non-error and a phase angle error of the current transformer 14 to be measured according to the precision shunt resistor 16 changed in stages; A second calculation step (S24) of calculating a slope from a plurality of data pairs formed of the changed resistance value of the precision shunt resistor (16) and the non-error corresponding thereto and making the resistance value (R _b ); A third calculation step (S26) of calculating a slope from a plurality of data pairs consisting of the changed resistance value of the precision shunt resistor 16 and the phase angle error corresponding thereto to set the reactance value (X _b ); A method for evaluating the burden for a current transformer using a current transformer comparator and a precision shunt resistor. The method according to claim 6 or 7, Removing the burden (Z _b , 18) (S40); Changing step (S42) of changing the resistance value of the precision shunt resistor (16) step by step; A calculation step (S44) of calculating a pure wire resistance value (r) and pure pure reactance value (x) according to the precision shunt resistor (16) which is changed in stages; The wire resistance value r is subtracted from the resistance value R _b of the calculating step S20 to calculate the pure burden resistance value, and the pure line reactance value x is obtained from the reactance value X _b . Subtracting to calculate a reactance value of the pure burden (S46); And Calculating a burden value and a power factor of the burden from the calculation step (S30) by using the calculated pure burden resistance value and reactance value (S48); and the current transformer comparator and the precision shunt resistance further comprising: Evaluation method of burden for current transformer used.

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