[b]1 Introduction[/b] In recent years, with the increase of nonlinear and impulsive loads, power quality problems have become increasingly prominent, and user complaints due to power quality issues are also increasing. Many new load devices with microprocessors and power electronic devices have high requirements for power quality, and some enterprises require power supply bureaus to provide high-quality and high-priced electricity when signing power supply agreements. With the deepening of the work to create a first-class power system and China's imminent accession to the WTO, power quality standards will also be aligned with international standards. 2 Power Quality Monitoring Methods and Their Limitations Currently, statistical voltmeters are widely used in China to monitor voltage quality levels. These voltage monitors can only monitor the voltage pass rate, require manual meter reading, and lack statistical analysis functions. Harmonic, voltage fluctuation, and flicker measurements are performed using portable testing instruments to measure the voltage of each level of the substation busbars, the harmonic current on each side of the main transformer, the harmonic current of the capacitor bank, and the voltage of large and medium-sized nonlinear load users, power plants, and low-voltage distribution networks. The measurement data are then summarized and statistically analyzed to assess the power quality level of the power grid. This power quality monitoring method and management model has obvious limitations: ① Poor real-time performance: long monitoring cycles and scattered monitoring points make it impossible to understand the power quality level of the power grid in a timely manner; ② Few monitoring indicators: due to the limitations of the monitoring devices, the same instrument cannot monitor multiple power quality indicators simultaneously; ③ High workload: it requires a lot of manpower and resources for measurement, statistics, and analysis; ④ Large measurement error: in actual measurements, the ideal measurement environment is often not achieved, the data is highly random, and there are even cases of missed or falsified data collection; ⑤ Low efficiency: from the discovery of power quality problems to their resolution, it often takes a long time; ⑥ Lack of judgment criteria: insufficient data and the inability to track and test monitoring points make it difficult to deeply analyze the causes of power quality issues and to propose measures to improve power quality. Based on this situation, establishing an online power quality monitoring network and a unified and open monitoring and management platform that can analyze and reflect the power quality level of the power grid in a timely manner, in order to identify the causes affecting power grid quality and take corresponding corrective measures, improve the power supply quality of the existing power supply system, reduce power loss, and ensure the safe, reliable, and economical operation of the power grid, is of great significance. This also provides accurate historical and basic data for further construction and improvement of the power grid and accident analysis. Since the 1970s, foreign countries have been conducting power quality testing, analysis, and management. Currently, power quality monitoring instruments have been installed at all power supply connection points (including some user terminals). 3. Relevant Domestic and International Standards and Development Trends Due to different understandings of the technical meaning of power quality, people also have different perspectives. For example, the power sector mainly defines power quality as the pass rate of voltage and frequency, and uses statistics to illustrate that the power system is 99.9% safe and reliable; power users simply define power quality as whether the power supply to the load is normal. The International Electrotechnical Commission (IEC) does not have a term for power quality, but instead uses the term electromagnetic compatibility (EMC), pointing out and emphasizing the interaction and influence between equipment, as well as the interaction and influence between power sources and equipment. The IEC has developed a series of electromagnetic compatibility standards. In addition, there are many advanced foreign standards, such as the power quality standard promulgated by the European Community in 1995, called "Power Supply Characteristics of Public Distribution Systems," which serves as a unified standard for medium and low voltage power quality. This standard is divided into 5 categories and contains 13 indicators. Countries around the world attach great importance to the monitoring and management of power quality. China is actively adopting international standards and advanced foreign standards, especially the EMC standard formulated by the IEC. China currently has five national standards on power quality, among which "Power Quality - Permissible Voltage Fluctuations and Flicker" (GB12326-2000) is a new standard. Other national standards that have been submitted for approval include those on temporary overvoltage and transient overvoltage, and those addressing voltage dips and short-term power outages are also under development. 4 Implementation of Power Quality Monitoring Network 4.1 Determining the Monitoring Indicators From a technical perspective, the factors affecting power quality mainly fall into three categories: ① Natural phenomena, such as lightning strikes on lines; ② Normal use and automatic protection of power equipment and devices, such as switching operations; ③ Nonlinear loads and impulsive loads at user terminals. The hazards of poor power quality include the following: ① Excessive voltage deviation increases losses, shortens the lifespan of electrical equipment, and causes malfunctions. It also disrupts the stability of the power system's synchronous operation, voltage stability, and the economical operation of the power grid. ② Voltage fluctuations and flicker are mainly caused by fluctuating loads (such as electric arc furnaces). Excessive voltage fluctuations can harm the equipment of other users connected to the same power supply point, such as causing flickering lights, uneven motor speeds, and malfunctions in computers and electronic equipment. ③ Harmonics injected into the system from harmonic sources (nonlinear equipment and loads) can damage system equipment (such as capacitors, cables, motors, voltage transformers, etc.), threaten the safe operation of the system (such as malfunctions of relay protection and automatic devices), increase power losses (such as line losses), increase the error of measuring instruments (such as electricity meters), and interfere with communications. ④ Three-phase imbalance can cause generators to malfunction, increase additional losses in transformers causing local overheating, cause malfunctions of relay protection and automatic devices, generate non-characteristic harmonics in converters, increase neutral current to generate electrical noise interference, increase transmission line losses, and interfere with communications. ⑤ When the power system is operating stably, the entire system has the same frequency. Within the allowable frequency deviation range, it mainly causes equipment efficiency problems; when the deviation exceeds the range, it will endanger the safety of the equipment, and in severe cases, even cause system collapse. ⑥ Temporary overvoltage, transient overvoltage, voltage drop, and short-term power outages are generally caused by short-circuit faults within the system or user, which will directly affect or even interrupt the user's normal power supply, causing serious economic losses. This is the issue that users are most concerned about. Based on the IEC and international content on power quality, and considering the actual situation, we determine the key points to be monitored. Voltage and frequency compliance rates have always been highly valued by the power system and are basically in line with national standards. Therefore, we believe that the power quality research should focus on four phenomena: voltage dip, transient overvoltage, harmonic distortion, and flicker. The first two are short-term transient phenomena, while the latter two are persistent events. 4.2 Selection of Measurement Points The correct selection of measurement points is directly related to the accuracy of the measurement. Based on the harmonic measurement work of Shaoxing power grid, it was found that there are many factors that affect the accuracy of harmonic measurement. The harmonics of some substations are increased due to other factors, which is not a true reflection of the system harmonics. For example: (1) In some substations, in order to avoid damage to the voltage transformer due to resonance, the neutral point of the transformer is often grounded through a harmonic suppression device, which makes the third harmonic content very distorted. For example, the total harmonic distortion rate of the 10 kV bus voltage of 110kV Yangxun substation is 8.8%, while the total harmonic distortion rate of the 10 kV bus voltage of 35 kV Sanjiang substation is as high as 25%. When the harmonic suppression device is short-circuited and the neutral point of the transformer is directly grounded, the total harmonic distortion rate of the 10 kV voltage is only 2.5%. (2) The total harmonic distortion rate of the incoming bus voltage of some substations exceeds 20%, such as 35 kV Sandu substation and 35 kV Huangshan substation, which are mainly third harmonics. This is caused by three-phase imbalance. The cause of the three-phase imbalance should be identified and eliminated by checking the power supply voltage, line impedance, and load characteristics. Additionally, some measurement points lack a neutral point in the substation's secondary circuit, resulting in the measured harmonic voltage being the line voltage, which cannot effectively reflect the voltage harmonic distortion rate. 4.3 Selection of Monitoring Instruments With the continuous development of digital signal processing (DSP) and network technologies, and the continuous improvement of online monitoring technologies, domestic manufacturers are actively developing online monitoring devices. Foreign countries started monitoring and managing power quality earlier and have some mature monitoring devices. Among them, the power quality detection series products produced by the Swiss LEM Group have wide applications globally. Generally speaking, power quality monitors should adopt DSP technology to ensure simultaneous sampling and high-speed acquisition of signals. In harsh operating conditions, susceptible to interference, a high common-mode rejection ratio is essential. Monitoring instruments typically installed on the 10 kV busbar of substations must, in addition to meeting the requirements and regulations for measuring instruments in the national power quality standards, effectively monitor all indicators of the national power quality standards (grid frequency, voltage deviation, three-phase unbalance, harmonics, voltage fluctuations, and flicker), and possess measurement functions for temporary overvoltage and transient overvoltage (the national power quality standard has been submitted for approval) and short-term power outages (currently under development). Other indicators should ideally align with international standards. Monitoring instruments installed in low-voltage distribution networks and at users can appropriately reduce the number of power quality assessment indicators, such as only voltage compliance rate, simple harmonic measurement, and power measurement functions. Alternatively, appropriate monitoring instruments can be selected based on the specific power quality requirements of the users to reduce costs. 4.4 Software The main functions the software should achieve are: ① The monitoring center can remotely set parameters for on-site monitoring points and access any real-time data; ② The monitored indicators can be recorded, statistically analyzed, and stored; ③ The monitoring center's analysis software has database management functions for the monitored data; ④ The data can be displayed in graphical or tabular form, generating trend curves for any time period, and has functions such as zooming in and out to meet analysis requirements; ⑤ Data downloaded from different time periods can be merged; ⑥ The software interface is user-friendly and easy to maintain. 4.5 Network Structure With the rapid development of communication technology and the widespread implementation of fiber optic communication in substations, a good network communication foundation is provided for the composition of the power quality monitoring network. Web technology can be used to analyze and manage the power quality of the power grid in a layered, hierarchical, and access-based manner. Each monitoring point uploads data to the web server via scheduled or scheduled access. By setting different permissions, different personnel can access different data to understand the current power quality indicators of the power grid. It is even possible to upload certain indicators to the Internet and make them public. [b]5 Main Economic and Social Benefits[/b] (1) Strengthen the supervision and management of power quality in the public power grid to ensure the safe operation of the power grid; (2) Analyze monitoring data and carry out power quality management to improve power supply quality, reduce equipment failures and power outage time, and reduce energy consumption; (3) Provide reliable data for power grid optimization and accident analysis; (4) Can replace some of the currently used related meters, saving personnel and time; (5) Is a powerful means to face the open power market and adapt to the competition mechanism; (6) Further ensure the normal power consumption order of users at all levels and provide them with high-quality power products; (7) According to the different requirements of different users for power supply quality, pilot the signing of charging standards at all levels with users (based on the signed power supply contracts and quality agreements at all levels).