A high precision measurement scheme for 20uA DC small current (leakage current)

At present, DC power supply equipment in power engineering such as state grid and inverter station, as the security power supply and control signal power supply of main electrical equipment, is a very large multi-branch power supply network. Under normal circumstances, one point of grounding does not affect the operation of the DC system. However, if the ground fault cannot be quickly found and repaired, and another point of grounding fault occurs, a major fault may occur.

At present, the leakage current detection of feeder branches is mostly used to detect ground fault points (leakage current), as shown in the figure below:

In the process of customer use, the following important deficiencies were found:

1) Poor accuracy and linearity

2) High temperature drift, narrow working temperature range

3) When the line is aging and the leakage current is small, the small current signal cannot be measured.

To this end, our company recently developed a high-precision DC/leakage current sensor based on fluxgate principle with a resolution of up to 20uA, namely: CHDC-LTHA series high-precision DC/leakage current sensor.

Advantages of products:

1: The product improves the magnetic core of fluxgate current sensor

Requirements for core performance of fluxgate current sensors are as follows:

1) High permeability; 2) Low coercivity; 3) The hysteresis loop area should be as small as possible;

4) Low magnetostriction; 5) Low Barkhausen noise; 6) Smooth surface;

7) The same cross-sectional area and uniform magnetic parameters; 8) Low saturation magnetization

A high precision measurement scheme for 20uA DC small current (leakage current)

Permalloy materials and amorphous materials meet the above conditions. Both soft magnetic materials have been used in large quantities in fluxgate current sensors.

The magnetic core of the existing leakage current sensor continues the core technology of the leakage current transformer. Although the fluxgate sensing and measurement can be realized, there will be some deviations from the requirements of the fluxgate sensor in several important performances, resulting in the difficulty of achieving high accuracy of fluxgate sensing.

According to the characteristics of runway fluxgate current sensing, our company has improved the process of nano amorphous materials, with a smaller hysteresis loop area, so that the magnetic core is easier to enter the saturation state, and there is a smaller residual magnetic and noise, from the most fundamental solution to the problem of small current measurement inaccurate.

2. The product optimized the excitation circuit of the sensor

Fluxgate sensors require high magnetic permeability of the magnet core, and the core is periodically saturated under the action of the excitation signal. In this way, the measured signal will be modulated, and then demodulated and processed through the circuit. In practical application, the core of the circuit that can affect fluxgate current sensor is the excitation oscillation circuit and signal demodulation circuit. Excitation oscillation circuit and signal demodulation circuit have very many choices, including zui is suitable for the way flux-gate current sensor is the average of the square wave self-excited oscillation circuit and the demodulation circuit, but the existing self-excited oscillation circuit has poor stability, poor symmetry of disadvantage, lead to the sensor exists relatively large zero offset, and zero temperature performance and stability.

The product uses a self-excited oscillation circuit, with the same reference voltage as the positive and negative excitation signal voltage reference, without other isolation devices, to ensure the symmetry of the positive and negative excitation signal. The stability of the reference voltage is far better than the regulator tube and diode, to ensure the stability of the oscillation circuit. The average demodulation circuit is a highly stable low-pass filter circuit,

3, better shielding means

Because the current measured by the leakage current is very small and reaches UA level, the sensitivity of the sensor will be very high, which will also lead to the sensor very easily being interfered by the external magnetic field. The shielding shell is also indispensable to shield the external magnetic field.

The existing leakage current sensor mostly uses silicon steel shield piece and shield ring splicing into a shield shell. This way will have more gaps. The shielding effect is greatly reduced.

This product is made of permalloy material with magnetic permeability dozens of times higher than the silicon steel sheet, which reduces the gap of the shielding shell, and the high magnetic permeability also improves the shielding effect.​

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