Research on high-precision micro-flux current transformer

 

Abstract: The development of high-precision micro current transformers is a common topic of concern in the current power instrumentation industry. This project uses the micro-flux principle, passive error compensation method, and double-core double-winding structure to manufacture high-precision micro current transformers; using micro-flux within the allowable error range is conducive to simplifying the structure; the CT series micro current transformers designed in this project have passed the test and the measurement accuracy has reached 0.02 level.

Keywords: current transformer, high precision, micro flux, double core, passive compensation

introduction

China is the world’s second largest energy consumer after the United States, and the central government is calling for the implementation of a number of projects to improve the national power distribution network. The overall development trend of the power grid management system is intelligent, miniaturized and networked. The micro current transformer is the front-end sampling device of the power meter, which directly affects the measurement accuracy of the intelligent power instrument. For this kind of current transformer, on the one hand, it is required to have high measurement accuracy, and on the other hand, it must have good anti-electromagnetic interference ability and long-term stability. The high-precision current transformer series products have a wide measurement range, small output current, no need for electronic compensation circuits, and a small size. They are very suitable for high-precision standard watt-hour meters, power meters, power transmitters and other power instruments. However, the domestic research and development in this area started relatively late. At present, many power meter companies have invested a lot of manpower and material resources to study high-precision micro current transformers. At present, the accuracy of domestic micro current transformers is about 0.1. This project is mainly aimed at the industrial research and development project to solve this problem.

The error of the current transformer is caused by the alternating excitation current that provides the magnetic flux. If there is no excitation current, there will be no error. According to the literature [2], a zero-flux current transformer is a transformer with no magnetic flux in the core. In the “zero-flux” state, there is no angle difference or ratio difference, and the primary and secondary turns have a strict turns ratio relationship. However, it is idealized and difficult to achieve in practice. Without excitation current, there is no magnetic flux in the core, and the primary and secondary energy cannot be transferred, so the current transformer cannot work normally.

This project adopts micro-flux principle, passive error compensation method, double core double winding structure to manufacture high-precision micro current transformer. The measurement accuracy of the CT series micro current transformer designed in this project reaches 0.02 level (ratio difference: ≤±0.02%; angle difference: ≤±0.6 points, in accordance with national standard GB1208-2006).

1 Theoretical Analysis of Current Transformer Error Generation

The phasor relationship between the main electromagnetic physical quantities in the single-core current transformer is shown in Figure 1. For the convenience of drawing, the amplitudes of the excitation current I and the secondary current I are appropriately exaggerated.

in:

n: secondary winding turns/primary winding turns;

I: primary winding current (A);

I: component of primary winding current on the horizontal axis (A);

I: component of primary winding current on the vertical axis (A);

I: Secondary winding current in actual state (A);

nI: Secondary winding current converted to the actual state of the primary winding (A);

: Secondary winding current under ideal conditions (A);

φ: working magnetic flux of current transformer (Wb);

I: excitation current (A);

I: component of the excitation current on the horizontal axis (A);

I: component of the excitation current on the vertical axis (A);

: Excitation current converted to secondary (A);

α: Angle between working magnetic flux φ and excitation current I (rad);

θ: the angle between the secondary induced potential and the secondary current (rad);

E: Induced electromotive force of secondary winding (V).

Figure 1 Single core current transformer phasor diagram

 

 

As shown in the picture .

(1) The components on the coordinate axis are shown in the figure. It can be seen that

I=I+nISinθ, I=Ie+nIcosθ, (2)

I=I+I（3）

=(I+nISinθ)+(Ie+nIcosθ)

=(Icosα+nISinθ)+(ISinα+nIcosθ)

=nI+2nIISin(α+θ)+I

≈nI+2nIISin(α+θ)(where I＜＜nI),

Approximately: I = nI + ISin (α + θ), (4)

, (5)

Ideal state (no excitation current, I=0): , (6)

Actual status: ,(7)

Actual error: =△I= — = （8）

△I is the error of the current transformer, which is converted to the secondary excitation current. We must find the current that is reduced in the secondary winding and make up for it back to the secondary. This is our idea of designing a high-precision miniature current transformer.

2 Analysis of micro-flux passive compensation principle

According to the measured data of core performance, the best ultra-microcrystalline core in China can meet the needs of making 0.1-level single-core double-winding precision current transformers with sufficient turns. It is impossible to make a 0.02-level micro current transformer with a single-core structure.

As shown in Figure 2, this project uses the micro-flux principle, passive error compensation method, and double-core double-winding structure to design a high-precision current transformer. Micro-flux is different from zero flux, which is actually difficult to achieve; the use of electronic active compensation method has complex circuits and high costs. Using micro-flux within the allowable error range is conducive to simplifying the structure. The main core C bears most of the flux, and the secondary core C uses a high-permeability core to bear the task of weak flux detection; the secondary winding N superimposes the main winding N in a passive manner to compensate for the error generated by the main winding to achieve high measurement accuracy.

Figure 2 Schematic diagram of double-core double-winding micro-flux passive compensation

As shown in the figure, define:

n: N winding turns/primary winding turns;

I: Current of the secondary N winding in actual state (A);

: converted to the excitation current of N winding;

I: current of secondary winding N (A);

I: current of load R (A);

As can be seen from Figure 2: I = I + I. (9)

According to formula (8), when I= , (10)

There is I = = . (11)

Usually I is very small, and the secondary winding n is generally hundreds or thousands of turns, so the compensation amount I is weak.

 

 

3 Design of micro-flux current transformer According to the above-mentioned micro-magnetic flux passive compensation principle, we designed a double-core double-winding single-turn through-type current transformer CT19-5A/5mA, with an inner diameter of 4mm, an outer diameter of 19mm, a core height of 16mm, and a weight of 12g. In order to measure mA-level current, it is hoped that the number of secondary turns is as small as possible. After experiments, the wire specification is Φ0.1mm, the main winding N is 1000T, the secondary winding N is 1000T, and the double core is double-layered. In the case of detecting weak magnetic flux, in order to achieve spatial stray electric field shielding and magnetic field shielding, a shielding shell made of 1mm thick low-carbon steel plate with high magnetic permeability and electrical conductivity can be used on the outside. 4 Accuracy level test data of micro-flux current transformer There are difficulties in the detection of high-precision current transformers, and it is difficult to implement the detection equipment of high-precision current transformers. The comparison method can be used: use a 0.005-level ultra-high-precision standard current transformer (the standard should be two accuracy levels higher than the current transformer being tested; its actual error should not exceed 1/5 of the error limit of the current transformer being tested) in parallel with the current transformer being tested, apply the same standard sine wave current signal, and then compare their output difference, which is the absolute error. The rated primary current is 5A, the primary current range is 250~6000mA, the secondary load is 1Ω, the experimental temperature is room temperature 26 degrees, the relative humidity is 46%, and there is no electromagnetic field around the calibration site that is not related to the calibration work. Shanxi Transformer Electrical Measurement Equipment Co., Ltd. mA-level transformer calibrator. Sampling number: 5 micro-magnetic flux current transformers CT19 and CT04 (single core) each, take a group of ratio difference and angle difference tests, the data is shown in Table 1, and the ratio difference and angle difference curves are shown in Figures 3 and 4. Table 1 CT19 and CT04 current transformer test data table Tab1TestdatasheetofCT19andCT04currenttransformer Primary current (%) 5 20 50 100 120 CT19 Ratio difference (%) 0.029 0.007 0.000 -0.001 -0.001 Angular difference (′) -1.3 -0.6 -0.3 0.0 0.1 CT04 Ratio difference (%) 0.480 0.380 0.300 0.280 0.290 Angular difference (′) 18.0 18.0 12.6 7.4 6.3 Figure 3 Current transformer ratio curve Figure 4 Current transformer angle difference curve   Comparing the test data and curves of the micro-flux current transformer CT19 and the single-core current transformer CT04, it can be seen that the design effect is achieved after adopting the micro-flux passive compensation, and the measurement accuracy is greatly improved to 0.02 level (ratio difference: ≤±0.02%; angle difference: ≤±0.6 points, in accordance with the national standard GB1208-2006). 5. Load capacity test of micro-flux current transformer Through the load capacity test, the error curves of CT04-5/5 and CT19-5/5 under different loads are plotted, as shown in Figures 5 and 6. Figure 5 Error curve of current transformer under 10Ω load   Figure 6 Error curve of current transformer under 20Ω load Fig6Errorcurveof20Ωloadcurrenttransformer The load test shows that when the CT19 load is 10Ω, the maximum ratio difference is -0.04%, and the maximum angle difference is 7.4′; when the load is 20Ω, the maximum ratio difference is -0.078%, and the maximum angle difference is 12.6′. As the load increases, the measurement accuracy becomes relatively worse. Comparative conclusion of load capacity test: When the load is ≤20Ω, the angle difference of CT19 under different rated currents is significantly smaller than that of CT04, and the relative linearity is good. The ratio difference linearity of CT19 is better than that of CT04. 6 Conclusion This project adopts the principle of micro-flux passive compensation and successfully develops a high-precision current transformer. The front-stage sampling transformer designed for high-precision instruments does not require electronic compensation, has high accuracy, and the data is long-term stable and reliable. It is suitable for gateway electric energy meters, power transmitters and other instruments. Test data shows that the high-precision micro-flux current transformer has good ratio difference and angle difference characteristics, which greatly improves the accuracy of the transformer. It has the advantages of small size, simple structure, high accuracy, and easy engineering production, saving a lot of man-hour costs for senior technicians to adjust the overall after installation.

 

--- END ---