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An electronic voltage transformer based on capacitive voltage division Abstract: a new type of electronic voltage transformer is proposed, which combines the advantages of optical fiber and capacitive voltage transformer, and solves the traditional problems of insulation and anti electromagnetic interference in high-voltage transmission system to a certain extent

key words: Dual CPU; Optical fiber; Electronic voltage transformer

1 introduction

voltage transformer is an indispensable connecting device between the primary and secondary electrical circuits in the power system. Its function is to realize the electrical isolation of the primary and secondary systems and convert the high voltage at the primary side into a low voltage suitable for relay protection devices and electrical measuring instruments. With the development of ultra-high voltage power transmission and transformation in the power system, the traditional potentialtransformer (PT) has become larger and larger, with high cost, ferromagnetic resonance and other serious problems, which have brought difficulties to the explosion-proof of Pt and the safe operation of power system. Pt has been gradually replaced by capacitive voltage transformer (CVT) in the high-voltage power system. Compared with electromagnetic voltage transformer, CVT has obvious advantages such as anti ferromagnetic resonance, high performance and price, and small operation and maintenance workload. It has become the main measuring device in high-voltage system. Optical voltagetransformer (OVT) is a high-tech product integrating crystal physics, optoelectronic technology, optical fiber technology, high-voltage technology and microcomputer technology. It has many incomparable advantages over traditional transformers, such as superior insulation performance, strong anti-interference ability and no iron core coil. It is especially suitable for computerized measurement protection of power system and unattended power station, It is the leading research direction of the new generation voltage transformer. In the 1980s, China also participated in the development and research of OVT, which was put into operation in the power system and gained valuable experience. However, there are still some process and technical problems in the process of practical application and industrialization, and its reliability and stability need to be improved. In this paper, a new type of voltage transformer, electronic voltagetransformer (EVT), which is easy to be applied, is proposed. It adopts mature capacitor voltage dividing technology but does not contain iron core coil; It makes full use of the excellent characteristics of optical fiber without considering the complex process and stability. It can be said that EVT integrates the main advantages of CVT and OVT

2 electronic voltage transformer

electronic voltage transformer adopts advanced optical fiber technology to realize real electrical isolation between high and low voltage, and cleverly solves the contradiction between transmission and isolation in high-voltage transmission system [3, 4]. The basic design idea is: the measured signal is transmitted by optical fiber, so that there is only optical connection but no electrical connection between the primary and secondary sides of the transformer; The mature capacitor voltage dividing technology is adopted to remove the iron core unit to reduce the failure rate; The information fusion technology [5] is used for software compensation processing, which improves the accuracy and stability of the system. At the same time, the strategic investor bee Investment Co., Ltd. is introduced

the system can be divided into three parts: capacitive voltage divider, outdoor electronic unit and indoor main control device, as shown in Figure 1. The capacitor voltage divider is used to obtain the signal, and the high voltage is divided through the series parallel combination of capacitors, which is reduced to below 100V, so that the electronic unit at the high voltage side can carry out front-end processing and sampling, and finally coupled into the optical fiber through electric/optical conversion. The optical fiber is used to realize the isolation between the high-voltage side and the low-voltage side, and the measured signal is transmitted to the low-voltage side main control room. After photoelectric conversion, the transmitted data is recovered, so as to carry out software compensation processing. Both the capacitive voltage divider and the high-voltage side electronic unit work outdoors and are greatly affected by temperature. Therefore, the system simultaneously transmits the temperature signal to the main control room through optical fiber, and fuses the voltage signal and temperature value to compensate for the impact of temperature on the system and improve the measurement accuracy and stability of the system. At the same time, two signal interfaces are reserved in the system, which can not only directly send the digital quantity to the microcomputer protection system, but also control the analog relay protection device, so as to quickly and conveniently integrate with the relay protection system. 

3 structure introduction

3.1 capacitor voltage divider

capacitor voltage divider is the signal acquisition unit of the system, which divides the high voltage through the series parallel combination of capacitors. Two errors are mainly considered here: first, when the capacitive voltage divider is placed outdoors, a wide range of temperature changes will directly affect the voltage division ratio of the capacitive voltage divider, making it unstable, thus affecting the measurement accuracy; The second is the phase difference introduced by the voltage division of the capacitor. The series parallel combination of capacitors can be used to divide the voltage, so as to reduce the influence of temperature on the partial voltage ratio; In the software, the temperature is regarded as an important parameter in the system and fused with the voltage signal to eliminate the influence of temperature change on the whole system

3.2 electronic unit at high voltage side

the electronic unit at high voltage side is the "outer core" of the whole system, as shown in Figure 2. It mainly includes signal preprocessing module, a/D conversion, a first in first out memory (FIFO, idt7203), two single chip computers (AT89C51) and optical emission module. In order to improve the sampling accuracy, the 16 bit high-speed AD sampling chip ads7805 of BB company is selected to sample 64 points per cycle (1/50Hz). In order to improve the real-time performance of the system, two single-chip computers are used, one of which is specially responsible for sampling and temporarily storing the data in FIFO; The other single chip microcomputer is specially responsible for data transmission. Its main task is to quickly transmit the data in FIFO to the main control room through optical fiber. At the same time, it also controls the temperature sensor and transmits the temperature signal to the main control room. In order to ensure (quasi) real-time performance, it is necessary to meet the requirements that the second single-chip microcomputer should transmit the 64 appearance data (16 bits) of our company's products from 1 to 10 points in one cycle within the time of the first single-chip microcomputer sampling cycle (0.02s). At the same time, it is also necessary to take into account the measurement of temperature sensors and the transmission of temperature values. The temperature sensor adopts DS18B20. The chip adopts one-line technology, with simple wiring and fast conversion speed. The measurement result is directly a digital signal (temperature value). The whole unit works in a strong electromagnetic interference environment, so it should not only meet the processing speed and accuracy, but also have strong anti-interference ability

3.3 low voltage side main control room

the low voltage side main control room mainly receives optical signals, performs optical/electrical conversion, and processes the measured data (voltage and temperature), including filtering, phase compensation, fusion processing of voltage signals and temperature values, so as to reduce the impact of temperature on the system, compensate the phase difference caused by voltage division, and improve the measurement accuracy and stability of the system. Finally, according to the definition of effective value, the measured value and the measured voltage waveform are given. At the same time, it integrates with the relay protection system conveniently and quickly through digital and analog interfaces. The main CPU of this part adopts the digital signal processor (DSP) TMS320F240 of TI company. The chip adopts Harvard structure, pipeline operation, special hardware multiplier, fast operation speed, but the damage resistance of the composite material is poor: compared with the aluminum based material, which can withstand a large impact before fracture, it is convenient, simple to program, high stability, and is more and more widely used in digital control

3.4 optical fiber transmission

the physical characteristics of optical fiber determine that it has the advantages of good insulation and free from electromagnetic interference, which just provides a new way to solve the contradiction between transmission and isolation in high-voltage transmission system. The optical fiber transmits the digital optical signal that carries the measured voltage information and temperature, thus realizing the real electrical isolation between high and low voltage and improving the security. It adopts single-mode optical fiber and high sensitivity light receiving components, with long transmission distance, strong anti-interference ability and high reliability. The stability of the light power of the light source is an important factor affecting the transmission performance of the optical fiber. Therefore, an arithmetic mean of the automatic two diameters of the light power is introduced into the optical emission module, which is the control circuit of the average wear spot diameter of the steel ball, as shown in figure 3. The main function of the circuit is to make the output optical power of the laser stable at a certain power level, so as to ensure the reliability of optical fiber transmission

the light source is a Samsung dl47b3a laser transmitter with built-in laser LD, pin detector and thermistor. As shown in Figure 3, the LD, pin detector and optical power control circuit form a negative feedback loop. The pin tube in the laser module is used to monitor the magnitude of the optical power transmitted from the laser light-emitting tube LD in the back direction. The optical power transmitted from the back direction has a certain proportional relationship with the laser output optical power, and is also affected by temperature. Therefore, a thermistor with negative temperature coefficient is used to provide reference voltage for compensation. The basic process is that when the output optical power of LD decreases and the pin photocurrent decreases, the output potential of the comparison amplifier increases, so that the output current of the drive opa547 increases, and the output optical power of LD increases; Vice versa, so as to achieve the role of automatic optical power control, so that the output optical power of LD remains stable

3.5 power supply

the working power supply of the electronic unit at the high-voltage side is directly obtained from the electricity. This is a specially designed power supply [6], which is characterized in that the power supply of the line part at the high-voltage side is completely generated by the induction at the high-voltage side, and there is no need to use complex energy supply equipment at the low end to transmit power to the high-voltage side

at the same time, in order to ensure that the power supply can also provide stable power in case of abnormal electric voltage, the battery is added as the standby power supply

4 conclusion

electronic voltage transformer combines the advantages of optical fiber and capacitive voltage transformer, integrates optical fiber, communication and microcomputer technology, and solves some traditional problems (such as insulation and electromagnetic interference) to a certain extent; Digital signal processing technology and multi-sensor information fusion technology are used to improve the measurement accuracy and stability of voltage transformer, which is conducive to the realization of digital substation. At the same time, compared with the optical voltage transformer, the process is simple, in line with the current technical level, and easy to practical production. As a new generation of capacitive voltage transformer, it has great market potential


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