What is the application analysis of a small current transformer based on the principle of zero magnetic flux in a power system?
At present, there are many types of current sensors, such as Hall sensors, coreless current sensors, high-permeability amorphous alloy multi-resonance current sensors, and electronic spin resonance current sensors. Due to the particularity of the environment in which the power system is used, many sensors have their own limitations.
At present, the current sensors used in power systems mostly use electromagnetic coupling as the basic working principle. In terms of sampling methods, these sensors mainly have three types: direct string type, clamp type, and closed-loop core-piercing type. In order to ensure the accuracy of sampling, the ratio difference and phase angle difference between the output and input signals should be as small as possible. The error compensation methods used by researchers include: short-circuit active compensation method, pure resistance error compensation method, and secondary impedance complete compensation method. , Self-balancing electronic compensation method, etc. A large number of research tests have shown that the small current transformer based on the "zero magnetic flux principle" is more suitable for the requirements of on-line detection of power system insulation.
The small current sensor described in this article is based on this basic principle, coupled with the application of adaptive dynamic tracking electronic circuits, so that the small current sensor has the advantages of high accuracy, high stability, and strong anti-interference ability.
2 Basic requirements of current sensors for on-line detection of power system insulation
The power system insulation online detection system has been working in a strong electromagnetic field environment for a long time, and most of them are outdoor environments. As its sampling input, the small current sensor must be able to complete the sampling work with high accuracy and high stability. However, because the collected signal is small, it is extremely susceptible to interference from electromagnetic fields, temperature, humidity and other factors. In order to be able to accurately sample under the strong noise interference environment of the power system, the small current sensor used for online detection should meet the following conditions:
The sampling range is from several hundred μA to several mA. High sensitivity, the output can sensitively respond to small changes in the input amount; the output signal is as large as possible.
In the measurement range, the linearity is good, the output waveform is not distorted, the ratio difference and angular difference between the output signal and the measured signal are small, and the difference is stable and does not change with changes in temperature and other factors. Strong anti-interference ability and good electromagnetic compatibility.
3 Principle of small current sensor
Let ND be the detection winding, D be the dynamic detection unit, and G be the active network that generates the secondary current. The magnetic potential balance equation of this loop is: I1N1 + I2N2 + IDND = —I0N1
The exciting magnetic flux generated by I1 generates an induced potential at both ends of ND, and is added to the input terminal of dynamic detection unit D. The secondary current I2 generated by G is provided to the secondary winding. Make the iron core reach magnetic potential balance. Therefore, in an ideal state, the secondary winding current I2 of the sensor is all supplied by the active network G, without taking current from the induced potential. D high-speed dynamic detection of the potential difference across ND. When the potential difference is small enough (approximately zero allowable value), the magnetic flux in the iron core is approximately zero magnetic flux. If the detected value deviates from the allowable value, G is automatically adjusted at high speed. Such high-speed tracking adjustment enables the iron core to always be kept close to the zero magnetic flux state, and the sensor achieves high accuracy.
4 "Zero magnetic flux principle" of current transformer
The principle of the core-type small current transformer is as follows:
Let I1 be the primary current of the small current transformer, I2 be the secondary current, and I0 be the exciting current. N1 and N2 are the number of turns of the primary and secondary windings, respectively. Therefore, the magnetic potential balance equation of this small current transformer is: I1N1 + I2N2 = -I0N1 When the excitation ampere-turn I0N1 is zero, I1N1 = -I2N2, the change of the side-ampere turn can fully reflect the change of the original side-ampere turn, and the error is zero. I0N1 is generally called absolute error, and I0N1 / I1N1 is relative error.
The error of the current transformer is a complex error, which can be expressed by the ratio difference f and the angle difference δ. ε = -I0N1 / I1N1 = f + jδ where f = (I2N2 / I1N1) / I1X100%, δ is the angle between I2 and CC1 after 180 °.
It can be seen that, due to the presence of I0N1, there is an angular difference δ and a ratio difference f between I2N2 and I1N1. If I0 = 0, the magnetizing potential is 0 and the error is zero. At this time, the iron core is in a "zero magnetic flux" state, and it works at the beginning (linear segment) of the magnetization curve. At this time, the output waveform of the current transformer will not be distorted, and good linearity is maintained. This is the "zero-flux principle." Therefore, if the core of the transformer is always in the state of zero magnetic flux, the error of the current transformer can be eliminated fundamentally.
However, according to the working principle of the transformer, it is impossible to achieve zero magnetic flux by the transformer itself, and it must be compensated or adjusted by external conditions. For this reason, the dynamic balance electronic circuit is used to dynamically adjust it, so that the iron core is always in a "dynamic zero magnetic flux state".
5 Principle of small current sensor
Let ND be the detection winding, D be the dynamic detection unit, and G be the active network that generates the secondary current. The magnetic potential balance equation of this loop is: I1N1 + I2N2 + IDND = —I0N1
The exciting magnetic flux generated by I1 generates an induced potential at both ends of ND, and is added to the input of dynamic detection unit D. The secondary current I2 generated by G is provided to the secondary winding. The magnetic flux generated by I2 demagnetizes the core. Make the iron core reach magnetic potential balance. Therefore, in an ideal state, the secondary winding current I2 of the sensor is all supplied by the active network G, without taking current from the induced potential. D high-speed dynamic detection of the potential difference across ND. When the potential difference is small enough (approximately zero allowable value), the magnetic flux in the iron core is approximately zero magnetic flux. If the detected value deviates from the allowable value, G is automatically adjusted at high speed. Such high-speed tracking adjustment enables the iron core to always be kept close to the zero magnetic flux state, and the sensor achieves high accuracy.
6 Error analysis
The error of the current sensor includes three parts: capacitive error, magnetic error, and sensitivity error of the detection and adjustment electronic circuit. The so-called capacitive error refers to the measurement error caused by the capacitive leakage current between each side coil itself and the coil. For power frequency signals, when N2 <1000, this error can be controlled within 10-5. Since the number of turns of the primary and secondary windings of this device are very small, the capacitive error can be ignored. Although the detection winding has a relatively large number of turns, its potential difference is dynamically approaching zero, so its capacitive error can still be ignored.
After the aforementioned high-speed dynamic adjustment, I0 → 0, the iron core approaches zero magnetic flux, and the magnetic error is small. However, in fact, a completely zero magnetic flux state cannot be achieved, and there must be a little weak magnetic flux in the iron core to make G output I2, which makes the magnetic error still exist. It can be seen from the magnetic potential balance equation of this sensor that the magnetic error is mainly composed of two parts: one is the error caused by the residual magnetic potential brought by I0, and the other is the error caused by the additional magnetic potential brought by the detection winding ID, namely:
ε = (I0N1 / I2N2) + (IDND / I2N2) = (108 EDl / 222μ0NDSI2N2) + (EDND / RiI2N2)
Among them: ED is the induced potential of ND, l is the length of the magnetic circuit, S is the cross-sectional area of the core, μ0 is the initial permeability of the core, and Ri is the input impedance of the detection unit.
It can be seen that to reduce the magnetic error, one should choose an iron core with a higher μ0 value and a suitable number of detection winding turns. This sensor selects an ultra-microcrystalline iron core with μ0 of 6 X 104 and an ND of 100 ~ 500 turns; It is necessary to have a large input impedance of the detection unit. ED and I2 can be controlled to the required range through an active dynamic balancing network.
In addition, it is necessary to use high-conductivity, high-permeability materials as shielding to eliminate electromagnetic field interference. Ultrafine crystal alloy can be used as magnetic shielding material.
6 Conclusion
The small current sensor uses an ultra-micro crystal as the iron core, and adopts an active electronic circuit network to directly connect to the secondary winding. After forming an adaptive dynamic adjustment loop, the measurement accuracy of the small current sensor can be greatly improved, while maintaining High stability.
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