Abstract: Due to their harmful effects on the power grid, harmonic measurement plays an important role in power systems. This paper introduces a hardware implementation of harmonic measurement in power systems based on the basic requirements of harmonic measurement, and analyzes the methods applied to harmonic measurement. Keywords: Harmonic, Online measurement, Power quality analysis 1 Introduction With the development of power electronics technology, harmonic problems caused by power electronic devices pose a potential threat to the safe, stable, and economical operation of power systems, and have a significant impact on the surrounding electrical environment. Harmonics reduce the efficiency of power production, transmission, and utilization, cause electrical equipment to overheat, generate vibration and noise, and cause insulation aging, shorten service life, or even cause failure or burnout. They can also cause local parallel or series resonance in the power system, amplifying the harmonic content and causing equipment such as capacitors to burn out. Harmonics can also cause malfunctions in relay protection and automatic devices, leading to confusion in power metering. Externally, harmonics can cause serious interference to communication and electronic equipment. Ideally, the voltage provided by the public power grid should be a single and fixed frequency with a specified voltage amplitude. The occurrence of harmonic currents and harmonic voltages is a form of pollution to the public power grid. It deteriorates the environment in which electrical equipment is located and also affects the surrounding environment. Before the widespread use of power electronic equipment, people had conducted some research on harmonics and their hazards and had some understanding of them, but at that time, harmonic pollution still needed to be severely punished and did not receive enough attention. In the past three or four decades, the rapid development of various power electronic devices has made the harmonic pollution of the public power grid increasingly serious. Various faults and accidents caused by harmonics have also occurred continuously, and the severity of harmonic hazards has attracted people's high attention. The hazards of harmonics to the public power grid and other systems are roughly as follows: (1) Harmonics cause additional harmonic losses in the components of the public power grid, reducing the efficiency of power generation, transmission and consumption equipment. When a large number of third harmonics flow through the neutral line, it will cause the line to overheat or even cause a fire. (2) Harmonics affect the normal operation of various electrical equipment. In addition to causing additional losses, harmonics also cause mechanical vibration, noise and overvoltage in motors, and cause local overheating of transformers. Harmonics cause overheating, insulation aging, shortened lifespan and even damage to equipment such as capacitors and cables. (3) Harmonics can cause local parallel resonance and series resonance in the public power grid, thereby amplifying the harmonics, which greatly increases the harms of (1) and (2) mentioned above, and may even cause serious accidents. (4) Harmonics can cause malfunctions of relay protection and automatic devices, and make electrical measuring instruments inaccurate. (5) Harmonics can interfere with nearby communication systems, causing noise and reducing communication quality in mild cases; and causing loss of residence and making the communication system unable to work normally in severe cases. In order to reduce the harms caused by harmonics, a measurement method that measures harmonics continuously for a long time, analyzes the collected measurement data through analysis instruments, and takes appropriate compensation measures has become urgent. 2 Online measurement of harmonics using ION7550/7650 equipment 2.1 Introduction to the ION7550/7650 distributed power quality monitoring device The ION7550/7650 distributed power quality monitoring device is a multi-functional distributed online power quality monitoring device developed by POWER MEASUREMENT of Canada for the needs of online power quality monitoring. The powerful I/O functions of the ION7550/7650 can be applied to online harmonic detection and analysis, voltage overshoot and sag recording and waveform sampling, flicker analysis, voltage imbalance measurement, event logging, control output, power factor adjustment, demand monitoring, centralized data management and analysis, and other functions. The ION7550/7650 can select multiple communication methods, with a maximum configuration of 5 communication ports, including 1 RS-232/RS485 port, 1 RS-485 port, 1 infrared communication port, 1 fully integrated Ethernet port, and optionally 1 fully integrated industrial-grade modem. Due to its unique multi-communication port and multi-communication protocol, the ION7550/7650 distributed power quality monitoring device can be connected to any manufacturer's integrated automation system, power quality monitoring system, or power metering system, greatly improving the cost-effectiveness of the device. 2.2 Structure Diagram of the ION7550/7650-based Distributed Power Quality Monitoring System Figure 2-1 clearly shows the entire information transmission process of this online harmonic monitoring system. It is divided into three levels: the substation level under the power supply bureau, the regional power supply bureau level, and the provincial power bureau level. The principle is that we install ION7650 devices at each substation where power quality monitoring is needed, i.e., the substation level under the power supply bureau. The ION7650 collects harmonic signals, analyzes, classifies, and charts them to generate specialized files, which are then transmitted via Ethernet to the monitoring center at the regional power supply bureau level. The regional power supply bureau monitoring center then compares, analyzes, and summarizes the harmonic information from all its subordinate substations, generating another file for relevant personnel at the provincial power bureau level to review and analyze, so as to take appropriate harmonic suppression measures. To avoid data loss during transmission, the ION7650 devices installed at each grassroots substation also directly transmit the analyzed harmonic data to the provincial bureau's monitoring center via Ethernet or the power/telecommunications network. Similarly, relevant personnel at the provincial power bureau level can also view and analyze the data and take appropriate measures. Since the ION7650 installed at the substation base has a sampling point of 512 points/cycle and a maximum of 1024 points/cycle, it can capture sub-cycle transients of 20µs, thus realizing the online monitoring of harmonics. 2.3 Principle Diagram of Harmonic Measurement by ION7550/7650 2-2 The internal functional logic of the ION series products adopts an integrated mesh structure composed of multiple ION modules. Each module performs a specific function, just like a conventional power meter. By connecting multiple ION modules according to different logical combinations, different functions are realized. We can consider each ION as a functional box, as shown in Figure 2-2: In Figure 2-2, the input can be data signal, Boolean signal, or pulse signal, and the output is the converted data signal, pulse signal, and event signal. The algorithm of the functional box adopts harmonic measurement based on Fourier transform. Harmonic measurement based on Fourier transform is the most widely used and applied method today. It is based on the basic principle of transitioning from Discrete Fourier Transform to Fast Fourier Transform. This method of measuring harmonics is highly accurate, has many functions, and is easy to use. Its disadvantages are that it requires a certain amount of current value and two transformations, resulting in a large amount of calculation and a long calculation time, which leads to a long detection time and poor real-time performance of the detection results. Moreover, during the sampling process, when the signal frequency and the sampling frequency are inconsistent, that is, when equation (1) is not valid, the method will produce spectral leakage and picket fence effects, making the calculated signal parameters (i.e., frequency, amplitude, and phase) inaccurate, especially the phase error, which is very large and cannot meet the requirements of measurement accuracy. Therefore, the algorithm must be improved. In the equation, T0 is the signal period; Ts is the sampling period; fs is the sampling frequency; f0 is the signal frequency; and L is a positive integer. The method of correcting the fast Fourier algorithm by using the windowed interpolation algorithm. This method can reduce leakage and effectively suppress the interference between harmonics and the interference of clutter and noise, so that the amplitude and phase of each harmonic voltage and current can be accurately measured. Reference [4] gives interpolation algorithms for different window functions (such as rectangular window, Henning window, Blackman window, Blackman window-Harris window). In actual measurement, the rectangular window interpolation algorithm and the Henning window interpolation algorithm can meet the requirements of measurement accuracy. Equations (2) and (3) are the formulas for calculating the complex amplitude Am and phase angle jm by the rectangular window interpolation algorithm [5]. Equations (4) and (5) are the formulas for calculating the complex amplitude Am and phase angle jm by the Henning window interpolation algorithm. 2.4 Usage of ION7550/7650 distributed power quality monitoring device In each basic substation, an ION7550/7650 distributed power quality monitoring device can be installed in one metering loop to monitor harmonics. A typical four-wire star connection diagram is shown in Figure 2-3. Figure 2-3 3 Typical Cases To gain a deeper, more comprehensive, and systematic understanding of the characteristics of Hong Kong's power grid and its various harmonic sources, and to analyze the patterns of pollution and harm caused by various nonlinear users to the power system's power quality, so as to take targeted measures to achieve power quality monitoring and comprehensive management of the power grid and users, and to obtain first-hand power quality characteristic data to provide a reliable guarantee for future comprehensive management work, CLP Power Hong Kong introduced and implemented an online power quality monitoring system. CLP Power Hong Kong's online power quality monitoring system is currently the largest power quality monitoring system in Asia and even the world, mainly composed of more than 300 ION7550/7650 and ION EEM units manufactured by POWER MEASUREMENT of Canada. CLP Power Hong Kong selected monitoring points covering various voltage levels from transmission to distribution, including 380V, 11kV, 132kV, and 400kV. It can collect power quality data such as voltage dips and voltage harmonics, and issue power quality event alarms within 10 minutes via SMS messages and email, providing convenient and fast online power quality monitoring services for key users. Furthermore, the system integrates nearly 190 fault recorders and 160 RTUs, consolidating the data from these third-party devices into the EEM system to provide a unified analysis platform. Based on the network and data management technology of the power quality monitoring center, CLP enables online analysis of PQ parameter reports and graphs for all locations within the network through a user-friendly web interface. Data from all databases can be accessed immediately. Features include: allowing customers to configure web display settings; creating windows for comparing data from different data sources; clearly displaying the location of installed instruments (substation name, voltage level, etc., to help trace relationships between instruments); allowing users to easily add, delete, or relocate PQ devices and manage database changes; supporting all text and graphical data output functions; and querying various daily reports, special reports, and PQ indicator reports of the power system. (See Figure 3-1). ION power quality monitoring devices are widely used by foreign power companies, such as Singapore Electricity Company (Singapore), Malaysia Electricity Company (MAS), Thailand Electricity Company (TEB), and Korea Electric Power Corporation (KEPCO). Large enterprises with high requirements for harmonic control, such as Motorola, Hewlett-Packard, and Oracle, have also chosen ION power quality monitoring devices. Therefore, the introduction of the ION power quality online monitoring system is of epoch-making significance for the key task of harmonic online monitoring in the Guizhou power grid. 4. Conclusion Since harmonics are unavoidable, in order to minimize their damage to the power system, we must take measures to eliminate them before they cause damage. Therefore, online monitoring of harmonics provides real-time and reliable data, facilitating power quality monitoring for decision-makers and enabling them to formulate specific measures. The construction of the ION power quality online monitoring system also allows our management departments to quickly and intuitively understand the necessary power quality harmonic data and information online, strengthening the supervision and management of power quality and providing real, accurate, and real-time information on power quality. With the development of computer technology and the progress of database technology, the improved ION power quality monitoring and data processing system will inevitably make power quality management work more standardized, intelligent, scientific and modern. References [1] Wang Zhaoan, et al. Harmonic suppression and reactive power compensation [M]. Beijing: Machinery Industry Press, 1998. [2] Lü Runru, et al. High harmonics in power systems [M]. Beijing: China Electric Power Press, 1998. [3] Sutherland PE. Harmonic measurements in industrial power system [J]. IEEE Trans on IA, 1995, 31(1): 175-183. [4] Li Gengyin, et al. Two improved algorithms for fast Fourier transform [J]. Automation of Electric Power Systems, 1997, 21(12): 37-40. [5] Mao CS. A digital measurement scheme for timevarying transient harmonic [J]. IEEE Trans on Power Delivery, 1995, 10(3). [6] Yang Hua, et al. A new method for detecting harmonics based on wavelet transform [J]. 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