Application of DC Sensors in Rectification Power Supply Systems of Electrolysis Industry

Large DC sensors play an important role in the accurate, stable and reliable measurement of rectifier power in the electrolytic industry. The current sensor product introduced in this article has independent intellectual property rights. At present, it can replace the products of Halma Company that were imported in the past.
The main materials of the sensor, the iron core and the gallium arsenide Hall element, are introduced from Japan, but after special processing and circuit design, the components in the design are all domestic military products. According to the voltage signal converted from the current and magnetic field collected by the Hall element in the magnetic circuit gap, the full current law and new technologies and new process methods are used to successfully develop it.
The product has been well received by users through the use of the ion membrane caustic soda device of Baling Company.
DC power supply metering and control plays a pivotal role in the management of the electrolytic industry. It is one of the main technical assessment indicators for enterprises to increase efficiency and tap potential, and it is also one of the control signal sources to ensure the smooth operation of the rectifier power supply. Therefore, the accurate, stable and reliable measurement of DC sensors in the rectifier power supply system is particularly important.
As we all know, the DC current sensors popular in the domestic market are open loop and closed loop (also known as direct measurement and zero detection), and both types of sensors have their theoretical basis in terms of measurement accuracy. They are designed and manufactured under the theoretical framework of the full current law based on the voltage signal converted from the current and magnetic field in the air gap of the magnetic circuit collected by the Hall element.
In the field of electrolysis industry, the SDA series of DC high current sensors can produce a variety of products in two major types, closed loop and open loop, according to the needs of different industries and manufacturers. It can perform online measurement and detection of 5KA~420KA DC high current, with high accuracy, good reliability and stability, fast response speed, easy installation and maintenance, etc. The products have been included in the "Torch Plan", "Science and Technology Research Plan" and "National New Product Trial Production Appraisal Plan" of Hubei Province; and won the "Second Prize for Science and Technology Progress" of Hubei Province, the "Golden Bridge Award" of the State Science and Technology Commission and the "National New Product Certificate".
Our company's products are produced in strict accordance with international standards under the ISO9001-2000 quality management system. We rely on a strong and experienced scientific and technological research and development team as a backing, and use new technologies, new materials, new processes and other means to ensure that our company's products are always at the forefront of the times in terms of performance and various technical indicators.
Our company's DC high current sensor is developed on the basis of the full current law ∫LHdL=I. The core of the full current law is: the closed line integral of the magnetic field intensity is only related to the current (measured current I) intersecting the closed integral path, and it is independent of the current position and the non-interlinked current (generated stray magnetic field). When the current passing through the sensor window is zero, regardless of whether there is current in the adjacent conductor (relative to the external magnetism), the output of the sensor must be zero. We know that the measured current I generates a magnetic flux Φ in the iron core. A uniform branchless magnetic circuit with an air gap is used to convert the measured current I into the magnetic induction intensity in the air gap. According to the loop law of the magnetic field intensity H1d+H2d+…H2Nd+∫LHid1=I and the output voltage UH=KHIHB and sensitivity of the Hall element, the relationship between the current and the magnetic field intensity is controlled. The sensor is designed to have a certain core area and a certain air gap length. In the magnetic circuit, according to the principles of central symmetry and axial symmetry, the induced potential induced by the magnetic field intensity of the Hall element in each air gap channel is added and amplified N times, that is, the output voltage proportional to the actual current is obtained. Assuming that there are 2N Hall detection channels and corresponding air gap channels (designed to be an even number and arranged in the magnetic circuit according to the principle of center and axis symmetry), the width of each air gap is d, and the total length of the iron core is L, then:
H1d+H2d+…+H2Nd+∫LHid1=Io ……………………..(1)
Wherein, Hi (i=1,2,…,2N) is the magnetic field intensity in the air gap channel, H1 is the intensity at point l in the iron core, and Io is the current in the field busbar.

For the air gap, Hi=Bi/μ0, and according to the mean value theorem,
∫LH1d1=HLL=BLL/μrμ0
, so: B1d+B2d+…+B2Nd+BLL/μr=μoIoIn
the above relationship, according to the boundary conditions of the magnetic medium, it can be seen that BL is approximately equal to Bi, and in the device design, the value range of L/d is usually around 10~30, while the value of μr is usually more than four orders of magnitude. Therefore, the value of BL/μr is very small in the entire sum, so
BLL/μrμ0=βμ0I0 (and β<<1) and then it can be concluded that:
B1+B2+…+B2N=(1-β)μ0I0/d…………………………...(2)
On the other hand, it can also be concluded that:
BL ≈μ0I0/2Nd ……………………………………….(3)
β≈L/2Ndμr………………………………………….(4)
In the debugging of the sensor,
SH1I1 Sinψ1= SH2I2 Sinψ2=…=SH2NI2N Sinψ2N =KSubstitute
VHi=SHiIiBiSinψ1 into (2) to get:
VH1+VH2+…+VH2N=K(1-β)μ0I0 /dIn
the electronic circuit, we add up the various Hall induction potentials and amplify them by Q times as the output signal Vo of the device, that is:
Vo=Qk(1-β)μ0I0/d…………………………………… (5)
This formula reflects the corresponding relationship between the output voltage Vo of the detection device and the value of the primary detection current Io.
From the above theoretical analysis, the factors affecting the measurement accuracy of the sensor depend largely on the selection of the iron core and the Hall device. Therefore, we spent a lot of effort to conduct a large number of tests on various ferromagnetic materials, and finally selected a new type of magnetic conductive material produced by Japan's Nippon Steel Corporation. In order to ensure the performance of the magnetic material as much as possible, the magnetic material was subjected to special process and structural treatment before and after it was formed. In this way, the magnetic permeability of our original magnetic conductive material of 5000-8000 Gauss was suddenly increased to the current theoretical requirement of high magnetic permeability and low coercive force of 17000-20000 Gauss, thereby effectively guaranteeing the linearity of our sensor.
Similarly, in terms of Hall elements, we also strictly followed the theoretical calculation accuracy requirements and did a lot of experiments and screening work. Finally, we determined the TO90A gallium arsenide Hall element produced by Japan's Nicera Company. Its biggest advantage is that its typical temperature stability is -300 to -400PPm/℃ when working at constant current, and its output linearity is good. Its maximum error is only 0.8%. The output voltage of 1mA1kGS can reach about 65-170mV and has good frequency characteristics. The theoretical bandwidth is above 1MHz. As long as its excitation current does not exceed 3.5mA, its service life can reach more than 200,000 hours.
In order to ensure that the sensors produced by our company have high reliability, high stability and high precision when used on site, we have changed the original industrial-grade components into military-grade components based on the use of new materials, changed the circuit board from the original copper-clad board to the current double-sided gold-plated board, and changed all the original electrical connection connectors into the current fastener connection, ensuring the high reliability of the product; in terms of manufacturing process, we have adopted a two-level automatic temperature compensation technology, and have carried out a lot of research and experimental work on the compensation process of a single Hall element. According to the physical and material properties of the Hall element, a new Hall compensation process is adopted, which greatly improves the compensation accuracy and stability of the Hall element compared with the previous thermistor compensation process. In order to prevent the influence of external temperature on the sensor, the electronic circuit part is concentrated in a self-controlled constant temperature box, avoiding the change of the system offset caused by the influence of the temperature coefficient on a certain part, thereby ensuring the high stability of the product; in terms of circuit structure, we have changed the original first-level differential amplifier circuit into the current three-level differential amplifier circuit, eliminating the error caused by the output resistance of the Hall element being greater than the input resistance of the operational amplifier. The sensor's post-amplification circuit uses multi-stage isolation amplification output, and the output signals do not interfere with each other, making the connection of the large current DC metering and control system very convenient. Among multiple parallel rectifiers, when one of them is powered off, the reverse short-circuit current generated by the short circuit on the DC grid side is very large. The sensor reverse current protection contact sets a limit value to achieve the reverse protection of the rectifier unit.

Through a lot of theoretical research and practical exploration, the large current DC metering and control system produced by our company has been continuously improved and optimized, and has won wide acclaim in the market. Especially in the application of the 50,000-ton ion membrane caustic soda DC power supply system of Baling Petrochemical Company in the chemical industry (see the figure below),
our company's products have won the favor of users with their high precision, high reliability and stability, and have eliminated Halma's products in one fell swoop, and have been praised by the chlor-alkali industry, which has greatly inspired the ambition of the national industry. The outstanding advantages of our company's products are reflected in the following aspects:
1. Simple structure: The SDA series DC large current sensor produced by our company is an open-type two-part docking method, which is very convenient to install, use and maintain, and has the advantages of small size, light weight and extremely low power consumption.
2. High precision: The DC high current sensor produced by our company adopts the secondary temperature automatic compensation technology and the optimal circuit structure design technology based on the full current law. The magnetic circuit is based on the principle of center and axis symmetry, and the latest materials and process technologies are selected to ensure the theoretical calculation accuracy requirements of the product in practical applications. The three-stage differential amplifier circuit is adopted to eliminate the error caused by the output resistance of the Hall element being greater than the input internal resistance of the op amp, so that the sampling value is reflected without distortion and meets the design requirements.
3. Good stability: The DC high current sensor produced by our company is based on the principle of symmetry, and the channel air gap in the closed magnetic circuit is evenly distributed in even multiples. The technical parameters of the Hall element are strictly matched, and a unique de-remanent magnetization circuit is adopted to improve the product's ability to resist external magnetic interference. The secondary temperature automatic compensation technology is adopted to prevent the product from being affected by the external ambient temperature and causing the offset to change. The fully enclosed anti-corrosion and dustproof measures are adopted but the Hall element in the air gap is not sealed to avoid the possibility that a certain Hall element is damaged and cannot be replaced. Even if a false value is adjusted in the circuit, the symmetry principle is completely lost, so that the influence of the eccentricity error cannot be guaranteed.
4. High reliability: Our company's products strictly use military-grade components, which greatly improves the reliability of the products. All electrical connection points are connected with solid parts, and the working power supply of the sensor adopts an independent power supply method. The various output signals of the sensor are isolated and transformed, and do not interfere with each other. The cables connected to various instruments and meters use double-shielded cables. As long as one end is reliably grounded, the interference introduced by the line can be effectively suppressed.
5. High qualifications: The company has the only set of 420KA high-current standard signal debugging sources in China that have been verified and approved by national authoritative institutions, thus ensuring that our company is the only manufacturer with standard debugging capabilities in the full range of DC high current, so that the linearity and accuracy within the full range are effectively guaranteed.