Abstract: Unmanned substations with automatic recording have gradually become a trend in my country. This paper introduces the working principle of a microcomputer power transmitter based on AC sampling technology and the corresponding hardware block diagram and modular software structure designed with synchronous sampling, frequency tracking and dynamic range changing technologies. Keywords: substation, AC sampling technology, power transmitter, microcontroller With the rapid development of my country's power facilities, unmanned substations have become a trend. As a key device of the substation integrated automation and dispatch automation system, the microcomputer power transmitter based on AC sampling technology is directly related to the safe operation of the substation and the power grid. The microcomputer power transmitter based on AC sampling technology overcomes the disadvantages of conventional transmitters such as many components, high power consumption, poor stability, high cost, large size and slow response, and is widely used in the power system. However, due to various reasons, the microcomputer power transmitter based on AC sampling technology generally has the following problems: (1) Poor reliability. Since the microcomputer power transmitter is installed in a substation with severe electromagnetic interference, the environmental conditions are very harsh. It cannot operate continuously for 24 hours and often "crashes". (2) The reasons for low accuracy are multifaceted: ① The asynchronous sampling method is used. ② The fixed period algorithm is used, that is, the power grid frequency is assumed to be constant at 50Hz. ③ Due to the severe electromagnetic interference in the environment where the transmitter is located, random interference, large pulse interference and other phenomena are unavoidable. How to eliminate these interferences needs to be solved from both software and hardware aspects. The following introduces a new type of high-performance microcomputer power transmitter based on AC sampling technology. 1 Working principle The microcomputer power transmitter based on AC sampling technology uses the direct input sampling method of AC quantity to sample the AC power signals of several power lines. Then, the microcontroller calculates the effective values ​​of voltage and current, active and reactive power, power factor, and other parameters according to the sampled data and a specific algorithm. There are many algorithms for measuring AC power using AC sampling methods. This paper adopts the integral method, which starts from the definition of the effective value of continuous periodic signal and the definition of power, and uses numerical integration to approximate the continuous integration for calculation. The integral method has a small calculation amount, fast speed and good real-time performance of the transmitter. According to the basic definition of power parameters of power transmission and distribution system: where u[sub]i[/sub], i[sub]i[/sub] are instantaneous values ​​of voltage and current with period T, U and I are effective values ​​of voltage and current, and P is active power. Within one power cycle T, the microcomputer samples u[sub]i[/sub], i[sub]i[/sub] at equal intervals and continuously at N points with sampling period T (T=N*T), and obtains N A/D conversion values ​​(u[sub]o[/sub], u[sub]1[/sub]……u[sub]N-1[/sub]) and (I[sub]0[/sub], I[sub]1[/sub]……I[sub]N-1[/sub]). Discretizing (1), (2), and (3) gives: where N is 64, and u[sub]k[/sub], i[sub]k[/sub] are the instantaneous sampling values ​​of voltage and current for the kth time. U and I are the effective values ​​of voltage and current, respectively, and P is the average active power per phase. Since active power P and reactive power Q differ by only 90° in voltage phase, therefore: where i[sub][k+（N/4）][/sub] represents the instantaneous current sample value at the [k+（N/4）]th iteration (the sample value after shifting by 90°). When [k+（N/4）] is greater than N, [k+（N/4）] is taken as [k-（N/4）], and N is taken as 64. In power systems, the three-meter method and the two-meter method are commonly used to calculate active and reactive power. This paper adopts the two-meter method. The formula for calculating power using the two-table method after discretization is as follows: Where Uab(k) and Ucb(k) represent the kth instantaneous sampled values ​​of the two line voltages Uab and Ucb within one cycle; Ia(k) and Ic(k) represent the kth instantaneous sampled values ​​of the two phase currents Ia and Ic within one cycle, and N is 64. 2. Methods to Improve the Measurement Accuracy of Microcomputer Power Transmitters 2.1 Synchronous Sampling Technology In microcomputer power transmitter devices, most products use a single multi-channel A/D converter, such as the A/D converter in 80196 or an independent A/D converter. However, it can only perform A/D conversion of one signal at a time, that is, the sampling time of different input signals is not the same. It is feasible to measure voltage and current using this method according to equations (4) and (5). However, to calculate active power and reactive power according to equations (8) and (9), errors will occur due to the inconsistent sampling time of voltage and current. The magnitude of this error is also related to the programming. In order to eliminate this error, the author adopted a four-channel simultaneous sampling 12-bit data acquisition system AD7874. AD7874 contains a high-speed 12-bit analog-to-digital converter, a reference power supply, a clock generator and four-channel tracking and holding amplifiers. The sampling rate of the four channels is 29×103r/s, which effectively eliminates this error. 2.2 Frequency Tracking Technology Currently, most microcomputer power transmitters use a fixed 50H algorithm. In reality, the frequency of the electric kitchen appliance varies within the range of 45-55Hz, and using a fixed-frequency algorithm will inevitably lead to errors. Frequency tracking technology can effectively solve this problem. This involves using an LM3900 comparator to generate a TTI square wave signal from the electric kitchen voltage signal. Before each sampling program is initiated, the grid frequency (cycle) is measured, and the latest measured frequency replaces the old frequency before starting the sampling program. 3.3 Dynamic Range Changing Technology: When the measured signal is small or the CT/PT ratio is large (e.g., 2000/5), many microcomputer-controlled power transmitters exhibit significant measurement errors. This error is caused by the quantization error of the A/D converter. There are two methods to eliminate this error: ① improve the quantization accuracy of the A/D converter; ② preprocess the measured signal. Using a high-precision A/D converter significantly increases costs without achieving satisfactory results. The authors adopted a method of preprocessing the measured signal, namely, using a programmable amplifier AD526 to linearly amplify the measured signal. When a measured signal is low, the CPU automatically controls the amplifier to amplify the signal to near the overload voltage value of the A/D converter, thereby greatly reducing the quantization error. This method allows the microprocessor-based power transmitter to dynamically change its range, improving measurement accuracy. 3. Hardware and Software Design The hardware block diagram of the microprocessor-based power transmitter based on AC sampling technology is shown in Figure 1. This microprocessor-based power transmitter adopts a dual-CPU structure, with the CPU being a 16-bit high-performance microcontroller 87C196KC. The two CPUs exchange data through a dual-port RAM; CPU 1 is responsible for sampling and computation, while CPU 2 is responsible for the human-machine interface. The input signal is isolated, filtered, and limited by a high-precision intermediate voltage/current transformer before being converted into an AC signal allowed by the AD7874. All components in this circuit use high-precision, low-temperature drift devices to improve the stability of the microcomputer power transmitter. A dedicated hardware "watchdog circuit" and power-on, power-off, and power transient protection circuits are designed to enhance the transmitter's anti-interference capability. To provide a better human-machine interface, a dot-matrix LCD display, keyboard, and serial communication interface are included, capable of displaying large amounts of data, tables, and graphs; various parameters can be modified via the keyboard; and maintenance is convenient. The software adopts a modular structure, is written in assembly language, and features software traps and digital filtering functions. It mainly includes an initialization module, interrupt service module, data acquisition module, digital filtering module, calculation module, keyboard module, display module, communication module, and clock module . This paper introduces a microcomputer power transmitter based on AC sampling technology, with voltage and current errors <0.1% and power errors <0.2%. It can detect 8 voltage segments and 16 lines of P, Q, I, and U total quantities, with high accuracy and stable and reliable operation.