A self-calibration method for low-frequency standard current transformers

Abstract

The present application belongs to the technical field of mutual inductor calibration, and particularly relates to a self-calibration method for a low-frequency standard current mutual inductor. In view of the fact that the difference measurement method used for the calibration of a power-frequency standard current mutual inductor cannot be applied to the calibration of a low-frequency standard current mutual inductor, the present application adopts the following technical solution: a self-calibration method for a low-frequency standard current mutual inductor, comprising: obtaining the secondary load resistance value of a current mutual inductor with the same transformation ratio; establishing an equivalent formula to calculate the equivalent secondary load resistance value of the current mutual inductor at another transformation ratio; using two digital multimeters to obtain the ratio error and phase error of the current mutual inductor with the same transformation ratio; and calculating the ratio error and phase error of the current mutual inductor with different transformation ratios according to the equivalent formula. The present application has the beneficial effect of realizing the calibration of low-frequency standard current mutual inductors with various transformation ratios without high-level low-frequency standard current mutual inductors and low-frequency current mutual inductor calibrators.

Description

Technical Field

[0001] This invention belongs to the field of current transformer calibration technology, specifically relating to a self-calibration method for a low-frequency standard current transformer. Background Technology

[0002] The traditional verification of power frequency current transformers is based on the differential measurement method. The principle of the differential measurement method is explained below. Figure 1 The primary sides of a standard current transformer with the same transformation ratio and the current transformer under test are connected in series and the same current flows through them. The secondary sides form a differential measurement circuit. The transformer calibrator measures the amplitude and phase of the differential current and the standard current, and calculates the corresponding amplitude difference and phase difference.

[0003] The advantage of the differential measurement method is that it does not require a highly accurate calibrator; typically, a Class 2 calibrator is sufficient. However, the transformer under test must have the exact same transformation ratio as the standard transformer. Under power frequency conditions, this is a very mature technology and the most basic and universally accepted method for calibrating current transformers internationally.

[0004] For current transformers with a 5 / 5 equal transformation ratio of 1, the error can be measured using the absolute self-calibration principle (see...). Figure 2 However, under low-frequency conditions, the lack of high-grade low-frequency standard current transformers with the same transformation ratio, as well as transformer calibrators that meet low-frequency requirements, makes it impossible to apply the traditional differential measurement method used on the power frequency to low frequencies, which brings great difficulties to the calibration and traceability of low-frequency current transformers. Summary of the Invention

[0005] This invention addresses the limitation that the differential measurement method used for calibrating power frequency standard current transformers cannot be applied to the calibration of low-frequency standard current transformers. It provides a self-calibration method for low-frequency standard current transformers that can still achieve traceability of low-frequency standard current transformers without the need for high-grade low-frequency standard current transformers and low-frequency transformer calibrators.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a self-calibration method for a low-frequency standard current transformer, the self-calibration method for the low-frequency standard current transformer comprising the following steps:

[0007] Step S1: Obtain the secondary load resistance value of the low-frequency standard current transformer under rated primary current, rated ampere-turns and rated secondary current when the transformation ratio is 1.

[0008] Step S2: Establish the equivalent formula and use the equivalent formula to calculate the equivalent secondary load resistance value of the low-frequency standard current transformer under another rated primary current, another rated ampere-turns, and another equivalent rated secondary current at another transformation ratio.

[0009] Step S3: Using two digital multimeters, the ratio error between the primary and secondary currents of the low-frequency standard current transformer with a transformation ratio of 1 is obtained by direct testing.

[0010] Step S4: Using two digital multimeters, the combined error of the low-frequency standard current transformer with a transformation ratio of 1 is measured through an absolute self-calibration circuit. The phase error is then decomposed based on the combined error and the ratio error.

[0011] Step S5: Calculate the ratio error and phase error of the low-frequency standard current transformer under other ratios using the equivalent formula, the equivalent secondary load resistance value, and the measured ratio error and phase error of the low-frequency standard current transformer when the ratio is 1.

[0012] The self-calibration method for low-frequency standard current transformers of the present invention, by discovering and establishing an equivalent formula, allows the use of two digital multimeters to first obtain the ratio error and phase error of the low-frequency standard current transformer with a transformation ratio of 1, and then obtain the ratio error and phase error of current transformers with different transformation ratios through the equivalent formula. Thus, the calibration of low-frequency standard current transformers with various transformation ratios can be achieved without the availability of high-grade low-frequency standard current transformers and low-frequency current transformer calibrators.

[0013] As an improvement, in step S2, the equivalent formula is obtained based on the equivalent circuit of the current transformer, and the equivalent formula is expressed as:

[0014]

[0015]

[0016] In the formula, f is the ratio error and δ is the phase error.

[0017] Z1—Primary winding impedance; Z2—Secondary winding impedance, Z2=r2+jx2;

[0018] I1—Primary current; N1—Number of turns in the primary winding;

[0019] N2—Number of turns in the secondary winding; I st — Total secondary current; I2 — Secondary load current;

[0020] I ex —Secondary excitation current; E2 —Secondary excitation electromotive force;

[0021] r2—DC resistance of the secondary winding; x2—leakage reactance of the secondary winding;

[0022] Z b —Secondary load impedance, Z b =r b +jx b ;

[0023] L—Average magnetic circuit length of the iron core;

[0024] μ — magnetic permeability of the iron core;

[0025] S—Effect cross-sectional area of ​​the iron core;

[0026] α — Secondary impedance angle;

[0027] θ—core loss angle;

[0028] ω — angular frequency.

[0029] As an improvement, in step S3, the two digital multimeters are self-calibrated proportionally before the direct method test.

[0030] As an improvement, in step S3, the proportional self-calibration process is as follows: connect the current terminals of two digital multimeters in series, input the same current, record the ratio of the readings of each measurement under different frequencies and currents, and obtain the self-calibration error, which is used for compensation in subsequent calculations.

[0031] As an improvement, frequency selection is available at 10Hz, 20Hz, 30Hz, 40Hz, 50Hz, and 60Hz; test points at each frequency cover 1%, 5%, 20%, 100%, and 120% of the rated current.

[0032] As an improvement, in step S3, during the direct method test, two digital multimeters are connected to the primary and secondary sides of the low-frequency standard current transformer with a transformation ratio of 1, respectively. The primary circuit inputs the aforementioned current and frequency. After deducting the self-calibration error from the readings of the two digital multimeters, the ratio error of the low-frequency standard current transformer under each current and frequency is obtained.

[0033] As an improvement, in the direct method test, the ratio of the theoretical readings of the secondary circuit meter to the primary circuit meter under certain conditions is k1, and the ratio of the actual measured readings is k2. Then, the ratio error of the current transformer under this frequency and current condition is:

[0034]

[0035] As an improvement, in step S4, during the absolute self-calibration circuit test, one digital multimeter measures the differential current ΔI, and another digital multimeter measures the primary circuit current I. The ratio of the readings of the two meters is the combined error of the low-frequency standard current transformer at that frequency and current condition, expressed as:

[0036]

[0037] Therefore, the phase error can be separated and expressed as:

[0038]

[0039] As an improvement, step S6 is also included, which evaluates the uncertainty of the calibration.

[0040] As an improvement, step S6 includes the following evaluation:

[0041] During the self-calibration process, due to the dispersion of the readings of the two multimeters, the deviation of k1 is estimated to be no greater than 0.005%, and the readings satisfy a uniform distribution, u1 = 0.005% / √3 = 0.003%.

[0042] During the direct method test, the deviation of k2 is caused by the dispersion of the readings of the two multimeters. It is estimated to be no greater than 0.005%, and the readings meet the uniform distribution. u2 = 0.005% / √3 = 0.003%.

[0043] During the test of the absolute self-calibration circuit, the measurement deviation of the primary current shall not exceed 0.05% within 24 hours after the multimeter is calibrated, satisfying the requirement of uniform distribution, u3 = 0.05% / √3 = 0.03%;

[0044] During the test of the absolute self-calibration circuit, the measurement deviation of the differential current shall not exceed 0.05% within 24 hours after the multimeter is calibrated, satisfying the requirement of uniform distribution, u4=0.05% / √3=0.03%;

[0045] The combined uncertainty of the difference measurement is expressed as: The expanded uncertainty is expressed as U f =2u f =0.008%, because the direct measurement method is used, the uncertainty of the ratio error is an absolute uncertainty;

[0046] Regarding the uncertainty of the synthesis error, i.e. The uncertainty is expressed as The expanded combination uncertainty is: U ∑ =2u ∑ =0.08%, which is the relative uncertainty according to the principle of the difference method. The low-frequency standard current transformer is in the 0.05 class, which is converted to an absolute uncertainty of 0.004%.

[0047] The beneficial effects of the self-calibration method for low-frequency standard current transformers of the present invention are as follows: by discovering and establishing an equivalent formula, two digital multimeters can be used to first obtain the ratio error and phase error of the current transformer with a transformation ratio of 1, and then the ratio error and phase error of current transformers with different transformation ratios can be obtained through the equivalent formula. Thus, the calibration of low-frequency standard current transformers with various transformation ratios can be achieved without the availability of high-grade low-frequency standard current transformers and low-frequency current transformer calibrators. Attached Figure Description

[0048] Figure 1 This is a schematic diagram illustrating the verification principle of the differential measurement method for power frequency current transformers.

[0049] Figure 2 This is the schematic diagram of the absolute self-calibration principle of a power frequency current transformer.

[0050] Figure 3 This is the equivalent circuit diagram of the current transformer according to Embodiment 1 of the present invention.

[0051] Figure 4 This is a wiring diagram of a proportional self-calibration method for two digital multimeters according to an embodiment of the present invention.

[0052] Figure 5 This is a schematic diagram of the ratio error of the current transformer when the transformation ratio is 1 in Embodiment 1 of the present invention.

[0053] Figure 6 This is a schematic diagram of the phase error of the current transformer when the transformation ratio is 1 in Embodiment 1 of the present invention. Detailed Implementation

[0054] The technical solutions of the embodiments of the present invention will be explained and described below with reference to the accompanying drawings. However, the following embodiments are only preferred embodiments of the present invention and not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments in the implementation methods without creative effort are all within the protection scope of the present invention.

[0055] See Figures 3 to 6 The present invention discloses a self-calibration method for a low-frequency standard current transformer, the self-calibration method for the low-frequency standard current transformer comprising the following steps:

[0056] Step S1: Obtain the secondary load resistance value of the low-frequency standard current transformer under rated primary current, rated ampere-turns and rated secondary current when the transformation ratio is 1.

[0057] Step S2: Establish the equivalent formula and use the equivalent formula to calculate the equivalent secondary load resistance value of the low-frequency standard current transformer under another rated primary current, another rated ampere-turns, and another equivalent rated secondary current at another transformation ratio.

[0058] Step S3: Using two digital multimeters, the ratio error between the primary and secondary currents of the low-frequency standard current transformer with a transformation ratio of 1 is obtained by direct testing.

[0059] Step S4: Using two digital multimeters, the combined error of the low-frequency standard current transformer with a transformation ratio of 1 is measured through an absolute self-calibration circuit. The phase error is then decomposed based on the combined error and the ratio error.

[0060] Step S5: Calculate the ratio error and phase error of the low-frequency standard current transformer under other ratios using the equivalent formula, the equivalent secondary load resistance value, and the measured ratio error and phase error of the low-frequency standard current transformer when the ratio is 1.

[0061] The self-calibration method for low-frequency standard current transformers of the present invention, by discovering and establishing an equivalent formula, can first obtain the ratio error and phase error of the current transformer with a transformation ratio of 1 using two digital multimeters, and then obtain the ratio error and phase error of current transformers with different transformation ratios through the equivalent formula, thereby realizing the calibration of low-frequency standard current transformers without the availability of high-grade low-frequency standard current transformers and low-frequency current transformer calibrators.

[0062] Example 1

[0063] See Figures 3 to 6 The present invention discloses a self-calibration method for a low-frequency standard current transformer according to Embodiment 1. The self-calibration method for the low-frequency standard current transformer includes the following steps:

[0064] Step S1: Obtain the secondary load resistance value of the low-frequency standard current transformer under rated primary current, rated ampere-turns and rated secondary current when the transformation ratio is 1.

[0065] Step S2: Establish the equivalent formula and use the equivalent formula to calculate the equivalent secondary load resistance value of the low-frequency standard current transformer under another rated primary current, another rated ampere-turns, and another equivalent rated secondary current at another transformation ratio.

[0066] Step S3: Using two digital multimeters, the ratio error between the primary and secondary currents of the low-frequency standard current transformer with a transformation ratio of 1 is obtained by direct testing.

[0067] Step S4: Using two digital multimeters, the combined error of the low-frequency standard current transformer with a transformation ratio of 1 is measured through an absolute self-calibration circuit. The phase error is then decomposed based on the combined error and the ratio error.

[0068] Step S5: Calculate the ratio error and phase error of the low-frequency standard current transformer under other ratios using the equivalent formula, the equivalent secondary load resistance value, and the measured ratio error and phase error of the low-frequency standard current transformer when the ratio is 1.

[0069] In this embodiment, in step S2, the equivalent formula is obtained based on the equivalent circuit of the current transformer. (See the equivalent circuit for details.) Figure 3 The equivalent formula is expressed as:

[0070]

[0071]

[0072] In the formula, f is the ratio error and δ is the phase error.

[0073] Z1—Primary winding impedance; Z2—Secondary winding impedance, Z2=r2+jx2;

[0074] I1—Primary current; N1—Number of turns in the primary winding;

[0075] N2—Number of turns in the secondary winding; I st — Total secondary current; I2 — Secondary load current;

[0076] I ex —Secondary excitation current; E2 —Secondary excitation electromotive force;

[0077] r2—DC resistance of the secondary winding; x2—leakage reactance of the secondary winding;

[0078] Z b —Secondary load impedance, Z b =r b +jx b ;

[0079] L—Average magnetic circuit length of the iron core;

[0080] μ — magnetic permeability of the iron core;

[0081] S—Effect cross-sectional area of ​​the iron core;

[0082] α — Secondary impedance angle;

[0083] θ—core loss angle;

[0084] ω — angular frequency.

[0085] In this embodiment, the equivalent rated secondary current can be obtained by the ratio of the rated secondary current to the rated ampere-turns. The following explains the error in calculating the rated 2000 ampere-turns current transformer using a 5 / 5A winding with a rated 1200 ampere-turns.

[0086] The 100% secondary current of the 2000 / 5 winding is 5A. When the 5 / 5 winding reaches 2000 amp-turns, the secondary current is already 5 × (2000 / 1200) = 8.3333A. Assuming the DC resistance of the 5 / 5 winding secondary is r2, the resistance of the 2000 amp-turn secondary winding is approximately 1.6667r2. In both cases, x2 is ignored at low frequencies, i.e., Z2 = r2. Taking the ratio difference as an example for analysis: the ratio error under the normal rated 2000 amp-turns condition is:

[0087]

[0088] The calculated ratio error when increasing the rated 1200 amp-turns 5 / 5 winding to 2000 amp-turns is:

[0089]

[0090] Only need to obtain the appropriate Z b If the ratio error under normal circumstances is equal to the calculated ratio error, then the work of calculating the error can be successful.

[0091] The theoretical basis for the above calculation is that the ratio of the excitation current to the primary current determines the magnitude of the error, that is:

[0092]

[0093]

[0094] And: Different primary ampere-turns (or secondary ampere-turns) correspond to different excitation ampere-turns and also to different errors. The number of ampere-turns can be changed by changing the number of turns or by changing the current.

[0095] Based on the above analysis, we have derived the following table as the experimental basis for calculating all turns ratio errors using the equal ampere-turn method. The leftmost column contains the basic data, while the equivalent rated current and secondary load resistance of the other columns can be calculated using the equivalent formula and the basic data.

[0096] Table 1 shows the corresponding parameters extended using the 5 / 5 absolute error.

[0097]

[0098]

[0099] In this embodiment, a 34465A six-and-a-half-digit digital multimeter is used. The 34465A six-and-a-half-digit digital multimeter has excellent AC current accuracy and frequency response. It is used to perform low-frequency performance testing on a common standard current transformer. The testing principle is to use the direct method to measure the ratio difference between the primary and secondary sides, and then use the difference method to measure the composite error and separate the phase angle difference.

[0100] In this embodiment, in step S3, the two digital multimeters are self-calibrated proportionally before the direct method test.

[0101] In this embodiment, the proportional self-calibration process in step S3 is as follows: A wide-band programmable current source (e.g., Fluke 6100A) is used to output currents of various frequencies and magnitudes. Two 34465A current terminals are connected in series, and the same current is input. The test points at each frequency cover 1%, 5%, 20%, 100%, and 120% of the rated current. The rated current is set to 5–8.3333A according to Table 1, and the frequencies are selected as 10Hz, 20Hz, 30Hz, 40Hz, 50Hz, and 60Hz. The ratio of the readings from each measurement is recorded for use in the formal compensation calculation. The calibration wiring is shown below. Figure 4 .

[0102] In this embodiment, in step S3, during the direct method test, two digital multimeters are connected to the primary and secondary sides of the current transformer with a transformation ratio of 1, respectively. The primary circuit inputs the aforementioned current and frequency. After deducting the self-calibration error from the readings of the two digital multimeters, the ratio error of the current transformer at each current and frequency is obtained. (See wiring diagram). Figure 5 .

[0103] In this embodiment, during the direct method test, the ratio of the theoretical readings of the secondary circuit meter to the primary circuit meter under certain conditions is k1, and the ratio of the actual measured readings is k2. Therefore, the ratio error of the current transformer under this frequency and current condition is:

[0104]

[0105] In this embodiment, during step S4, the wiring diagram for the absolute self-calibration circuit is shown in section 3. Figure 6 A digital multimeter is used to measure the differential current ΔI, and another digital multimeter is used to measure the primary circuit current I. The ratio of the readings of the two meters is the combined error of the standard current transformer at that frequency and current condition, expressed as:

[0106]

[0107] Therefore, the phase error can be separated and expressed as:

[0108]

[0109] In this embodiment, step S6 is also included: evaluating the uncertainty of the calibration.

[0110] In this embodiment, step S6 includes the following evaluation:

[0111] During the self-calibration process, due to the dispersion of the readings of the two multimeters, the deviation of k1 is estimated to be no greater than 0.005%, and the readings satisfy a uniform distribution, u1 = 0.005% / √3 = 0.003%.

[0112] During the direct method test, the deviation of k2 is caused by the dispersion of the readings of the two multimeters. It is estimated to be no greater than 0.005%, and the readings meet the uniform distribution. u2 = 0.005% / √3 = 0.003%.

[0113] During the test of the absolute self-calibration circuit, the measurement deviation of the primary current shall not exceed 0.05% within 24 hours after the multimeter is calibrated, satisfying the requirement of uniform distribution, u3 = 0.05% / √3 = 0.03%;

[0114] During the test of the absolute self-calibration circuit, the measurement deviation of the differential current shall not exceed 0.05% within 24 hours after the multimeter is calibrated, satisfying the requirement of uniform distribution, u4=0.05% / √3=0.03%;

[0115] The combined uncertainty of the difference measurement is expressed as: The expanded uncertainty is expressed as U f =2u f =0.008%, because the direct measurement method is used, the uncertainty of the ratio error is an absolute uncertainty;

[0116] Regarding the uncertainty of the synthesis error, i.e. The uncertainty is expressed as The expanded combination uncertainty is: U ∑ =2u ∑ =0.08%, which is the relative uncertainty according to the principle of the difference method. The low-frequency standard current transformer is in the 0.05 class, which is converted to an absolute uncertainty of 0.004%.

[0117] Through analysis of the uncertainties, it can be found that the method in this embodiment can meet the relevant accuracy requirements.

[0118] The beneficial effects of the self-calibration method for low-frequency standard current transformers in Embodiment 1 of this invention are as follows: By discovering and establishing an equivalent formula, the equivalent secondary load resistance value is calculated based on the secondary load resistance value of the low-frequency standard current transformer under rated primary current, rated ampere-turns, and rated secondary current when the transformation ratio is 1. Two digital multimeters can be used to first obtain the ratio error and phase error of the current transformer when the transformation ratio is 1, and then the ratio error and phase error of current transformers with different transformation ratios can be obtained through the equivalent formula. Thus, the calibration of low-frequency standard current transformers with various transformation ratios can be achieved without the availability of high-grade low-frequency standard current transformers and low-frequency current transformer calibrators. Proportional self-calibration before absolute self-calibration can improve accuracy.

[0119] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that the present invention includes, but is not limited to, the content described in the above specific embodiments. Any modifications that do not depart from the functional and structural principles of the present invention will be included within the scope of the claims.

Claims

1. A self-calibration method for low frequency class 2 current transformers, characterized in that: The self-calibration method of the low-frequency standard current transformer comprises the following steps: Step S1, obtaining the secondary load resistance value of the low-frequency standard current transformer at a rated primary current, a rated ampere-turn number and a rated secondary current when the transformation ratio is 1; Step S2, establishing an equivalent formula to calculate the equivalent secondary load resistance value of the low-frequency standard current transformer at another rated primary current, another rated ampere-turn number and another equivalent rated secondary current when the transformation ratio is another transformation ratio through the equivalent formula; Step S3, obtaining the ratio error between the primary and secondary currents of the low-frequency standard current transformer when the transformation ratio is 1 through direct method testing by using two digital multimeters; Step S4, measuring the combined error of the low-frequency standard current transformer when the transformation ratio is 1 through the absolute self-calibration circuit form by using two digital multimeters, and decomposing the phase error according to the combined error and the ratio error; Step S5, calculating the ratio error and the phase error of the low-frequency standard current transformer at other transformation ratios through the equivalent formula, the equivalent secondary load resistance value and the measured ratio error and phase error of the low-frequency standard current transformer when the transformation ratio is 1; The equivalent formula is obtained according to the equivalent circuit of the current transformer, and the equivalent formula is represented as: In the formula, f - ratio error; δ - phase error; Z 1 - primary winding impedance; Z 2 - secondary winding impedance, Z 2= r 2 +jx 2; I 1 - primary current; N 1 - primary winding turns; N 2 - number of secondary turns; I st - total secondary current; I 2 - secondary load current; I ex — secondary excitation current; E 2 — secondary excitation electromotive force; r 2 - Secondary winding direct current resistance; x 2 - Secondary winding leakage reactance; Z b — a secondary load impedance, Z b = r b + jx b ; L - the average magnetic path length of the core; μ - core permeability; S - the effective cross-sectional area of the core; α — secondary impedance angle; θ - core loss angle; ω - angular frequency.

2. The self-calibration method of a low-frequency standard current transformer according to claim 1, characterized in that: In step S3, the two digital multimeters are proportionally self-calibrated before direct method testing.

3. The self-calibration method of a low-frequency standard current transformer according to claim 2, characterized in that: In step S3, the process of proportional self-calibration is as follows: after the current terminals of the two digital multimeters are connected in series, the same current is input, and the ratio of the readings of each measurement at different frequencies and currents is recorded to obtain the self-calibration error, which is used for subsequent compensation.

4. The self-calibration method of a low-frequency standard current transformer according to claim 3, characterized in that: Frequency selection: 10 Hz, 20 Hz, 30 Hz, 40 Hz, 50 Hz, 60 Hz; the test points at each frequency cover 1%, 5%, 20%, 100% and 120% of the rated current.

5. The self-calibration method of a low-frequency standard current transformer according to claim 3, characterized in that: In step S3, during direct method testing, the two digital multimeters are respectively connected to the primary side and the secondary side of the low-frequency standard current transformer when the transformation ratio is 1, the current and the frequency of the foregoing size are input to the primary circuit, and the readings of the two digital multimeters after deducting the self-calibration error are the ratio error of the low-frequency standard current transformer at each current and each frequency.

6. The self-calibration method of a low-frequency standard current transformer according to claim 5, characterized in that: In the direct method test, the ratio of the secondary circuit meter to the primary circuit meter under certain conditions is k 1, the ratio of the measured readings is k 2, then the ratio error of the current transformer under the current condition at this frequency is: 。 7. The self-calibration method of a low-frequency standard current transformer according to claim 6, characterized in that: In step S4, the difference current is measured with a digital multimeter during the test in the form of an absolute self-correcting circuit ΔI , and another digital multimeter measures the current of the primary circuit I , at which time the ratio of the readings of the two meters is the combined error of the low-frequency standard current transformer at this frequency and this current, expressed as: Thus, the phase error is separated out and represented as: 。 8. The self-calibration method of a low-frequency standard current transformer according to claim 7, characterized in that: It also includes step S6, evaluating the uncertainty of calibration.

9. The self-calibration method of a low-frequency standard current transformer according to claim 8, characterized in that: In step S6, the evaluation includes: During the self-calibration process, the deviation of k1 caused by the discreteness of the readings of the two multimeters is estimated to be not more than 0.005%, the readings satisfy uniform distribution, and u1=0.005% / √3=0.003%; During the direct method testing process, the deviation of k2 caused by the discreteness of the readings of the two multimeters is estimated to be not more than 0.005%, the readings satisfy uniform distribution, and u2=0.005% / √3=0.003%; During the absolute self-calibration circuit form testing process, the measurement deviation of the primary current is not more than 0.05% within 24 hours after the multimeter calibration, the readings satisfy uniform distribution, and u3=0.05% / √3=0.03%; In the form of absolute self-circuit test process, the measurement deviation of difference current is not more than 0.05% within 24 hours after the calibration of the multimeter, which meets the uniform distribution, u4=0.05% / √3=0.03%; The combined uncertainty of the ratio difference measurement is expressed as The expanded uncertainty is expressed as U f = 2u f = 0.008%, since the direct measurement method is used, the ratio error uncertainty is the absolute uncertainty; The uncertainty of the combined error, i.e. is expressed as The expanded combined uncertainty is: According to the principle of the difference method, the relative uncertainty is 0.05 for the low-frequency standard current transformer, and the absolute uncertainty is 0.004%.

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