Online power quality monitoring equipment is the most basic and essential equipment in the power grid's power quality supervision and monitoring network. Currently, most domestic and internationally manufactured power quality monitoring equipment sold and used in the market for certain indicators (such as harmonics and imbalance) cannot fully meet the actual needs of power quality supervision and management in my country's power grid. The southern Hebei power grid began installing online harmonic monitoring devices in 1996. The initial devices had many problems in data storage, data transmission, and background statistical analysis. To meet the growing needs of power quality supervision, based on several years of operational experience, Baoding Guodian Zhongke Electric Co., Ltd. developed the GDDN-500 series digital online power quality monitoring terminal. The new GDDN-500 series digital online power quality monitoring device features the ability to collect various power quality parameters according to national standards, high reliability for long-term online operation, convenient and practical on-site operation, and communication with the central station. It also allows for long-term data recording and storage with convenient data retrieval. This device adopts the same DSP digital signal processor and high-speed multi-channel AD simultaneous acquisition technology as the latest foreign products. It uses a PC104 industrial control computer for data processing, display, and storage, offering powerful functions and ease of operation and software upgrades. 1. Functions and Composition The power grid power quality monitoring system consists of a power quality monitoring terminal, a central station, and analysis software. 1.1 Power Quality Monitoring Terminal The terminal processes input three-phase voltage (100V) and three-phase current (5A or 1A) for data signal processing, using FFT to calculate the amplitude and phase angle of each harmonic voltage and current. It calculates unbalanced voltage and current, and calculates and displays technical data such as three-phase voltage, current, voltage qualification rate, frequency, active power, reactive power, and power factor. It is responsible for data processing, storage, communication connection and data transmission with the central station, generating substation reports. The main functions of the power quality monitoring terminal are as follows: a. Input signals are TV and TA secondary side three-phase voltage (100V) and three-phase current (5A or 1A). b. Equipped with a public telephone modem interface, allowing convenient dial-up connection and data reception at the central station. c. Large-screen (320×240) backlit LCD graphic display. d. Chinese graphical user interface (spectrum graph, waveform graph, curve graph, vector graph). e. The terminal can store more than one year's worth of data, stored in 3-minute or 5-minute data packets. f. Equipped with a LAN connection interface, allowing for on-site data recording using a laptop. g. Multi-parameter integrated measurement, real-time fixed-point alarm, with configurable parameter values ​​and alarm states. h. Harmonic voltage and current, negative sequence voltage and current over-limit alarm output relays. 1.2 Central Station and Analysis Software The central station receives data from the processor via a modem or network, performs statistical analysis, generates files, reports, and curves, and can display data and graphs (such as spectrum graphs, waveform graphs, curve graphs, vector graphs, etc.). It can manage multiple power quality monitoring terminals, analyze and process the collected data, analyze the power quality of a certain period or event period, generate reports, automatically generate daily, monthly and yearly reports, automatically identify the periods and lines with excessive harmonic content, and calculate voltage compliance rate and power supply reliability. The central station adopts a client-server model, and the data is stored in the server's database, which can be easily accessed and queried. 2 Main Technical Indicators 2.1 Measurement Items The device uses (220±15%)Vac or [(220 10%)～(220-15%)]Vdc power supply. The measurable items include: voltage, current, frequency, voltage compliance rate, active power, reactive power, apparent power, power factor, voltage imbalance, current imbalance, harmonic voltage, harmonic current (up to 31/50 or higher), harmonic phase, harmonic power, distortion rate, etc. 2.2 Measurement Accuracy Voltage Measurement: ±0.2% Current Measurement: ±0.2% Voltage Unbalance Measurement Error: ≤0.2% Current Unbalance Measurement Error: ≤1% Frequency Measurement: 47～53 Hz, accuracy ±0.01Hz (50Hz) Signal Conversion Accuracy: 14bit Sampling Frequency: 8kHz/channel 3 Power Quality Monitoring Terminal Hardware / Software Composition [b] The hardware of the power quality monitoring terminal consists of TA/TV and signal preprocessing, DSP processor, PC104 industrial computer, parallel expansion of PC104 and DSP parallel communication ISA bus, modem, LCD display (VGA monochrome with backlight), network adapter, power supply, etc. The software of the power quality monitoring terminal consists of DSP software and PC104 software. 3.1 DSP Software 3.1.1 DSP Principle The monitoring terminal uses the TI 320C2XX series TMS320F240 chip. Considering the limited internal storage capacity of this chip, high-speed SRAM and EEPROM are expanded in the DSP section. The final system design requires the acquisition of 1024 points (6 channels simultaneously) in each power frequency cycle, and requires 6-channel radix-2 FFT transformation calculation of 1024 points and transmission to the PC104 processing unit. This requires a relatively fast clock frequency; in this device, the internal clock of the DSP is close to 40MHz. A fast 14-bit high-precision AD converter is added to the DSP processing section. This AD converter can perform 6-channel simultaneous sampling, ensuring accurate calculation of active and reactive power, and positive/negative sequence. 3.1.2 DSP Composition and Function a. Data acquisition section, including frequency sampling and calculation, and 6-channel simultaneous sampling of the AD converter. b. Data processing: Convert the format of the acquired data. c. FFT transformation calculation. d. Data transmission: Transmit the DSP data to PC104. 3.1.3 Input and Operation: Input three-phase voltage and current, measure frequency, perform 1024 or 512-point AD conversion (where the AD uses a dual 6-channel simultaneous high-speed AD converter), perform FFT transformation, calculate the root mean square value, and then upload the data. As needed, only transmit the 31st or 61st harmonic or higher harmonics during data transmission. Perform FFT calculation, taking the 31st (or 61st) harmonic every 0.5s, and taking 6 harmonics every 3s to calculate the root mean square value. The formula is: Where U[sub] hk [/sub]——the root mean square value of the hth harmonic measured within 3s. 3.1.4 Data transmission is based on a pulse given by the host computer every 0.5s, and data is uploaded every 3s. Taking the 31st harmonic as an example, each group of data is as follows. a. Frequency f. Each harmonic is divided into real and imaginary parts, with the phase of Ua as the reference phase. 3.2 PC104 section [b] [/b] The PC104 industrial control board uses the highly integrated PCM-3336 board, which has floppy disk and hard disk interfaces, can directly drive a 320×240 LCD monochrome display, 2 RS232C serial interfaces, 1 printer parallel interface, and can directly drive a keyboard and ordinary display. The board's BIO design can connect to hard disks up to 15G. For ease of use and to ensure reliability, the hard disk uses an electronic disk or a laptop hard disk. The industrial control board has a WATCH-DOG function, which automatically resets when the operation is abnormal. The PC104 board is responsible for data processing, storage, display, communication connection and data transmission between the power quality monitoring terminal and the central station, forming substation reports. Send 0.5 to DSP 3.2.1 PC104 Software Structure a. Calculates and processes various data, including voltage, current, active power, reactive power, positive and negative sequence, voltage unbalance, voltage qualification rate, harmonic content, etc. b. Displays the amplitude and phase angle of the fundamental and harmonic voltage and current, vector diagrams of voltage and current, and voltage and current waveforms on the LCD in a graphical manner. c. Communication transmission functions, including communication with the DSP, communication with the MODEM, and network communication. d. Parameter input, including voltage and current ratio, voltage upper and lower limits, and over-limit settings for harmonic content, etc. 3.2.2 Receiving DSP Data The data received from the DSP is temporary data, including frequency, three-phase voltage, three-phase current, and corresponding positive and negative zero sequence components and harmonic components (divided into real and imaginary parts, a total of 2×3×64 data). 3.2.3 Calculation of Harmonic and Unbalance Indicators The calculation of harmonic and unbalance related indicators is based on GB/T According to GB/T 14549-1993 "Power Quality - Harmonics in Public Power Grids" and GB/T 15543-1995 "Power Quality - Permissible Unbalance of Three-Phase Voltage", the specific formulas are as follows: 3.2.3.1 Harmonic Calculation (Calculated once for each set of data) a. H-th harmonic voltage content: Where U[sub] h [/sub]——h-th harmonic voltage (RMS value); U[sub] 1 [/sub]——fundamental voltage (RMS value). b. H-th harmonic current content: Where I[sub] h [/sub]——h-th harmonic current (RMS value); I[sub] 1 [/sub]——fundamental current (RMS value). c. Harmonic voltage content f. Total harmonic distortion rate of current g. H-th harmonic power and phase 3.2.3.2 Calculation of Harmonic Maximum Value and Probability Value: a. Calculation of Harmonic Maximum Value (each order and total distortion rate); b. Calculation of 95% probability value. Calculate the 95% probability value of each phase's measured value and the value of the largest phase within the measurement period, and store them. 3.2.3.3 Harmonic Over-Limit Alarm: Compare the measured value with the allowable value to determine if it exceeds the limit. If it does, an alarm is issued. 3.2.3.4 Voltage and Current Imbalance Calculation: Calculate the voltage and current imbalance (read a set of data every 3 seconds), and calculate the 95% probability value of the voltage and current imbalance. a. Take the maximum imbalance value; b. 95% probability value. Calculate the 95% probability value within the measurement period (statistical cycle). 3.2.3.5 Imbalance Over-Limit Alarm: Compare the measured value with the allowable value to determine if it exceeds the limit. If it does, an alarm is issued. 3.2.4 Voltage Qualification Rate: 3.2.4.1 Calculate the voltage (read a set of data every 3 seconds). Calculate the over-limit rate and over-limit rate, and statistically analyze the cumulative time of over-limit and over-limit; calculate the voltage qualification rate; store the recorded data of the previous month and the current month, the previous day and the current day; record the maximum value, minimum value and average value. It can set the rated value and limit value of the monitored voltage. The voltage quality monitoring statistical time is in minutes, and the average voltage value of 1 minute is taken as a statistical unit. The monitored voltage is displayed in real time, with a refresh cycle of 2 seconds. 3.2.4.2 Calculate the voltage qualification rate 3.2.5 Frequency Using a zero-crossing detection circuit and DSP capture function, the width of the integer cycle is accurately measured to calculate the frequency. 3.2.6 Display Voltage/current waveforms, voltage/current vector diagrams, amplitude and phase angle of the fundamental and harmonic waves of voltage/current are displayed graphically and in Chinese characters. The amplitude and phase angle of each harmonic are displayed digitally and in bar graphs with angle pointers. 3.3 ISA parallel expansion unit for communication between PC104 and DSP [b] [/b] To facilitate communication between the DSP and the PC, a parallel interface with interrupts was added, occupying the peripheral address and interrupts of the PC104. This parallel communication is an 8-bit bidirectional, interrupt-enabled communication. 3.4 MODEM and LAN Communication Management The MODEM is connected to the RS232C serial interface, and several additional control lines were added for real-time monitoring and control of the MODEM to ensure normal communication over extended periods. The extended network card allows LAN network communication. 4 Conclusions a. The power quality monitoring terminal can monitor the power supply and consumption status of the power grid in real time and accurately, especially to monitor the exceedance of harmonics, asymmetry, and voltage compliance, providing convenient monitoring equipment for power supply and consumption companies. b. The power quality monitoring terminal features high sampling frequency, accurate measurement, and fast processing speed, and its measurement indicators meet the requirements of national power quality standards. c. The Chinese and graphical display interfaces of the power quality monitoring terminal make it more convenient and intuitive for users. d. The power quality monitoring terminal adopts a DSP and PC104 industrial control board design, which is technologically advanced, highly accurate, and allows for convenient software maintenance and upgrades of the DSP and PC104. e. The power quality monitoring terminal can form a power quality monitoring network in regional power grids, provincial grids, or inter-grid grids. Through dedicated central station software, it can perform statistical analysis of large amounts of historical data, generate various statistical reports, and draw harmonic spectrum diagrams and distribution maps of various indicators, providing advanced means for power quality supervision.