I. Introduction Traditional electromagnetic current transformers, with their inherent drawbacks such as large size, magnetic saturation, ferroresonance, small dynamic range, and narrow bandwidth, are no longer sufficient to meet the development needs of next-generation power system automation and digitalization. With the development of fiber optic technology and optoelectronics, various electronic current transformers based on optical and optoelectronic principles have experienced rapid development. Rogowski coils, due to their unsaturation and ease of implementation, are widely used in the high-voltage side sensing units of electronic current transformers. However, the introduction of additional errors during manual winding and multi-layer winding, coupled with the significant impact of the coil's output temperature characteristics on the coil's core material, has hindered the industrial application of Rogowski coils. To address this, this paper adopts a low-power current transformer (LPCT) as the high-voltage side sensing unit. As an electromagnetic current transformer, the LPCT features high output sensitivity, mature technology, stable performance, and ease of mass production. Furthermore, due to its relatively small secondary load and the application of high-permeability magnetic core materials, it can achieve the measurement of currents with a large dynamic range. The output voltage signal of the LPCT is converted into a digital optical pulse signal by the signal processing circuit on the high-voltage side, transmitted via optical fiber to the low-voltage control room, and then the low-voltage side signal processing circuit converts the optical signal back into an electrical signal, providing output interfaces for both measurement and protection signal channels. Because optical fiber is used as the channel for connecting the high and low voltage sides, the requirements for the insulation structure of the current transformer are greatly reduced. II. Measurement Principle of LPCT As mentioned earlier, the LPCT is actually an electromagnetic current transformer with low-power output characteristics. In the IEC standard, it is listed as an implementation of electronic current transformers, representing a development direction for electromagnetic current transformers and possessing broad application prospects. Since the output of the LPCT is generally directly supplied to the electronic circuit, the secondary load is relatively small; its core generally uses high-permeability magnetic materials such as microcrystalline alloys, and the measurement accuracy requirements can be met with a relatively small core cross-section (core size). Furthermore, a voltage sampling resistor with a small resistance value is connected in parallel to the LPCT secondary circuit. This resistor is a component of the LPCT, and the equivalent circuit is shown in Figure 1. As shown in Figure 1, the output of the LPCT is a voltage signal proportional to the primary current, expressed as follows: Where: Us is the LPCT voltage output; Ip is the primary current; Rsh is the sampling resistor; Np is the number of turns in the primary winding; NS is the number of turns in the secondary winding. The LPCT core material designed in this paper is a microcrystalline alloy, with a turns ratio of 6000:1. The sampling resistor of the LPCT has a resistance of 15Ω and a temperature coefficient of less than 5×10⁻⁶. The rated primary current is 100A, and the secondary load is approximately 4mW at the rated primary current. The IEC standard imposes strict accuracy requirements on the measurement channel within a range of 5% to 120% of the rated primary current variation; for the 5P20 protection channel, accuracy requirements are imposed within a range of 100% to 2000% of the rated primary current variation. Therefore, the LPCT must maintain good linearity within a current variation range of 400 times. Assuming a rated primary current of 100A, the primary current variation range is 5A to 2000A, and the secondary output voltage variation range of the LPCT is 0.0125V to 5V. III. System Composition The design concept of the electronic current transformer is to perform local A/D conversion on the high-voltage side using the analog signal obtained from the LPCT, and then convert the digital signal into an optical signal. The transformer body provides a standard optical communication interface, through which the optical signal is transmitted via optical fiber to the signal receiving unit in the control room. In the control room, the low-voltage side signal processing unit restores the optical signal to a digital electrical signal, and separates the signal according to the requirements of relay protection and measurement equipment, providing output interfaces for protection and measurement channels. Figure 2 is a schematic diagram of the electronic current transformer system. The system uses only one LPCT as the sensing element for both the measurement and protection channels. Therefore, firstly, it is essential to ensure good linearity of the LPCT over a wide primary current variation range; secondly, because the secondary output voltage variation range of the LPCT is relatively large, achieving accurate measurement of the full range of signals is challenging, which places higher demands on the high-voltage side signal processing circuit. In Figure 2, the high-voltage side signal processing circuit mainly consists of an instrumentation amplifier, a 4th-order Bessel anti-aliasing filter, a 14-bit A/D converter, a complex programmable logic device (CPLD), and an electro-optical conversion module. The CPLD is responsible for generating the control signals required by the A/D converter and encoding the digital pulses. When the input analog signal is small, the A/D conversion error will increase due to quantization errors. In actual measurement, the input of the A/D converter is generally adjusted to near its full-scale value. The high-voltage side signal processing circuit in this paper has three analog input channels. One channel is used to measure the temperature at the sensor head (provided by an analog signal from a temperature sensor installed on the sensor head side); the other two are used to measure the output signal of the LPCT, with amplification gains set to 1 and 10 respectively. The 1x gain loop serves as a protection channel signal, and the 10x gain loop serves as the measurement channel signal. This is because the secondary output voltage of the LPCT is small within the range of 500–120% of the rated primary current. Amplifying it by 10 times before A/D conversion can reduce conversion errors and improve measurement accuracy. In Figure 2, information transmission between the high-voltage and low-voltage sides is accomplished via optical fiber. Optical fiber possesses excellent insulation properties, making it an advantageous medium for signal transmission between the high-voltage and low-voltage ends, offering advantages such as simple insulation, low cost, and mature technology. Since optical signals are transmitted through the fiber, an electro-optical conversion module must be added at the high-voltage end, and a photoelectric conversion module at the low-voltage end. To improve measurement reliability and enhance anti-interference capabilities, the analog signal output by the LPCT is converted into a digital signal before electro-optical conversion. This results in digital optical pulse signals being transmitted through the fiber, further improving anti-interference capabilities. After the optical signal is transmitted to the low-voltage end, the signal processing circuit converts the optical signal back into an electrical signal and separates the measurement channel signal and protection channel signal according to the requirements of relay protection and measurement equipment. The low-voltage side signal processing circuit mainly consists of a photoelectric conversion module, an FPGA, and a D/A converter. This circuit primarily performs signal separation, phase shifting, and necessary gain adjustment for each signal. In electronic current transformers, a certain amount of energy is required to ensure the normal operation of the high-voltage side signal processing circuit; this paper employs a laser-powered approach. Figure 3 shows a laser-powered electronic current transformer based on an LPCT. IV. Experiment (I) Experimental Scheme The test scheme for the electronic current transformer is shown in Figure 4. In the figure, A is the current source, the current sensing head is composed of an LPCT, and the high-accuracy current transformer is of class 0.1. Its secondary output signal is used as a standard signal and compared with the output signal of the electronic current transformer. This paper uses a transformer calibrator based on virtual instrument technology to calibrate the electronic current transformer. The output results of the transformer test system and the high-accuracy current transformer are both sent to the virtual instrument for processing. The output result of the high-accuracy current transformer is considered the standard value, while the output result of the electronic current transformer test system is considered the measured value. The virtual instrument calculates the measurement error of the electronic current transformer test system by comparing the magnitudes of the two signals. (II) Accuracy Test When the primary current varies from 500 to 12000 ohms (5A-120A) of the rated current (100A), the ratio error and phase angle error of the electronic current transformer measurement channel are shown in Figure 5. As can be seen from Figure 5, the ratio error and phase angle error measurements of the electronic current transformer measurement channel within the range of 5% to 120% of the rated primary current meet the accuracy requirements of a 0.2 class electronic current transformer. In addition, the protection channel was tested at 20 times the rated current, and the test results are shown in Table 1. As can be seen from Table 1, the electronic current transformer protection channel meets the accuracy requirements of a 5P20 class electronic current transformer. (III) Temperature Characteristic Test The electronic current transformer actually operates within a temperature range of -30℃ to 70℃, therefore, a temperature characteristic test must be performed on the electronic current transformer. Due to limitations, this paper uses the equal ampere-turn method to conduct a temperature test on the measurement channel under the rated primary current. The LPCT, high-voltage side signal processing circuit, and high-voltage side power supply were placed successively in an oven and a refrigerator, with the temperature ranges adjusted to 70℃～20℃ and -30℃～20℃, respectively. The measured system ratio error and phase angle error curves as a function of temperature are shown in Figure 6. Figure 6 shows that the electronic current transformer exhibits excellent temperature characteristics, with a system ratio error variation of less than ±0.1% and a phase angle error variation of less than ±2′ within the temperature range of -30℃ to 70℃. IV. Conclusion This paper utilizes the mature, accurate, and easy-to-implement LPCT technology to develop an LPCT-based electronic current transformer. This electronic current transformer has the following characteristics: 1. Only one LPCT is used for the sensing head, implementing both measurement and protection channels, further simplifying the design of the electronic current transformer. 2. Mature electronic technology and fiber optic transmission technology are adopted, ensuring reliable and simple insulation. 3. The entire system has high measurement accuracy and a wide measurement range, meeting the IEC requirements for 0.2-class and 5P20-class electronic current transformers, respectively. 4. Temperature tests show that the system's specific error variation is less than ±0.1% and the angle error variation is less than ±2′ within the temperature range of -30℃ to 70℃, verifying that the device has strong practicality.