The changes in my country's power supply and demand relationship and the establishment of the State Power Corporation mark a new historical stage in the development of my country's power industry. The changes in power supply and demand relationship have led to the transformation of the power market from a seller's market to a buyer's market, providing a material basis for the power operation to move from monopoly to openness. The open power market will inevitably promote the construction and transformation of urban power grids, including distribution system automation, as well as the application of power electronics technology (user power). The establishment of the State Power Corporation, implementing "corporatization, commercial operation, and legal management", has timely adapted to this development trend. However, due to the long-term habit of power shortages and monopoly operation, the sudden appearance of the above-mentioned new situations has resulted in insufficient preparation in all aspects. In particular, the automation of distribution systems has been heavily lagging behind in the past, and the current situation is very strong. Moreover, it is necessary to consider the service life of the equipment and systems invested, which means that all aspects that may affect it within a certain period of time must be taken into account in order to avoid repeated construction or even reconstruction in the future. [b]1 Areas involved in urban power grid construction and transformation[/b] The areas involved in urban power grid construction and transformation include the following two aspects. (1) In terms of voltage level, urban power grids cover three voltage levels: high, medium and low. The high-voltage main grid belongs to the sub-transmission system category, while the medium and low-voltage grids are distribution systems, managed by the municipal-level SCADA/EMS system (usually without AGC) and the county-level SCADA/LM/DMS system, respectively. It should be noted that the municipal-level SCADA/EMS system is a fully remote system based on the substation RTU, while the county-level SCADA/LM/DMS cannot be fully automated, at most it can only achieve medium-voltage automation, and low voltage mainly relies on the prepaid smart card/automatic remote meter reading (AMR) electricity billing system and "complaint hotline handling" to establish communication between the supply and demand sides. (2) From the perspective of equipment configuration, it will involve the four areas shown in Figure 1. [img=327,223]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zgdl/zgdl99/zgdl9910/image10/1701.gif[/img][align=left] Figure 1 shows the relevant areas of urban power grid construction and renovation. The primary system includes high-voltage transmission systems and medium- and low-voltage distribution systems; secondary automation includes automation at the prefecture-level and county-level; the electricity market mainly refers to the "distribution market"; and power electronics technology mainly refers to "user electricity" oriented towards distribution. Of course, the most closely related are the primary and secondary systems shown in the solid lines, but the potential impact of electricity market development and power electronics applications must be considered. Furthermore, due to the significant "backlog" in distribution network automation, the construction of medium-voltage automation and "one meter per household" with two-way communication in the distribution network has naturally become a current hot topic. [b]2 Goals and Planning for Urban Power Grid Construction and Renovation[/b] The goals proposed by the State Power Corporation for urban power grid construction and renovation are: (1) to increase power supply capacity by 10%; (2) to achieve power supply reliability of 99.90% to 99.99%; (3) to achieve a voltage qualification rate of 98%; (4) to reduce network losses by 10%; and (5) to meet environmental protection requirements. Compared with the goals of power grid management during the Ninth Five-Year Plan period, there are three obvious characteristics: First, it emphasizes the specific indicators of commercial operation and increasing power supply capacity by 10%; second, the requirements for power supply reliability, power quality, and economic operation are higher (the goals of the Ninth Five-Year Plan period were: power supply reliability of 99.7% to 99.9%, voltage qualification rate of 95%, and network loss reduction of 1.2% to 2.0%); and third, environmental protection requirements have been added. Obviously, in order to achieve these goals, it is necessary to simultaneously start the construction and renovation of the urban power grid's primary system and secondary automation. Due to the different situations of each urban power grid and the limited investment level, the problems to be solved and the order of priority are also different. Therefore, the current urban power grid construction and renovation can only adopt the policy of "unified planning and phased implementation". The main contents of primary system planning include: (1) load forecasting (annual error required ≤5%); (2) voltage level (high/medium/low voltage level required ≤4); (3) grounding method (medium voltage); (4) network wiring (including feeder load rate/short-circuit capacity); (5) substation (including transformer capacity ratio); (6) reactive power compensation (including voltage/reactive power optimization); (7) power flow and reliability analysis (only considered in primary planning); (8) network loss analysis (only considered in primary planning). The main contents of secondary automation planning include: (1) distribution management system (SCADA/DMS); (2) load management system (LM); (3) electricity billing system (AMR); (4) geographic information system (GIS); (5) management information system (transmission/transformation/distribution/consumption management); (6) substation/switching station automation (including unmanned operation); (7) feeder automation (including ring network control); (8) communication network (vertical/horizontal system integration). The current task is to achieve the goal of urban grid construction and renovation by unifying the planning and phased implementation of the primary and secondary systems, based on the current status and development of the urban grid. Planning inevitably involves investment. Although the investment in urban grid construction and renovation is substantial, there is still a problem of limited investment due to the large backlog. This requires close integration, mutual support, and comprehensive optimization between primary and secondary systems, without giving up any opportunity to save or postpone investment, and taking into account the potential impact of power market development and the application of power electronics technology, so that the invested systems and equipment have a long life cycle. [b]3 Mutual Support and Comprehensive Optimization of Primary and Secondary Systems[/b] In order to achieve the expected goals of urban grid construction and renovation, some aspects of the primary and secondary systems are interrelated and require or can support each other and be comprehensively optimized. 3.1 Load Forecasting Load forecasting is the basis of urban grid construction and renovation planning. It is the basis for planning the construction and renovation of the primary system (the number, location, and scale of new or renovated substations, switching stations, lines, etc. within the planning period) and also the basis for carrying out secondary automation work (the mode, steps, and scale of new or renovated load management systems, electricity billing systems, etc.). Unlike short-term load forecasts used for dispatching, load forecasts for urban network planning are for a short period of 1 year and a long period of 5–10 years, employing rolling corrections to ensure accuracy (generally requiring an annual error ≤5%). The forecasts primarily predict the annual load growth rate of feeders and future load levels determined by land use (residential, commercial, industrial, government, school, etc.) and socioeconomic factors (city zoning size and number, number of saturated zones, density, energy factor, load factor, and diversity, etc.). Here, the urban network load forecasting zoning, the regular data entry of land use and socioeconomic factors, the offline equipment management in secondary automation, and the online geographic wiring are all related to geographic information. Therefore, the use of geographic information systems (GIS) in automation to provide urban network load forecasting services will significantly improve its usability. The integration of urban network load forecasting software with GIS allows for the statistical retrieval and modification of relevant data within any closed polygon, enabling load analysis and forecasting, greatly improving the convenience of rolling corrections, real-time forecasting, and load retrieval. 3.2 Saving or Postponing Investment in Urban Power Grid Construction and Renovation: The largest investment in urban power grid construction and renovation is in the construction and renovation of substations, switching stations, and lines based on load forecasting. 3.2.1 Substations: Most high-voltage substations in urban power grids are terminal substations of the sub-transmission system. Traditional substation designs are characterized by large land area, heavy infrastructure tasks, low automation levels, and are mostly manned. With the promotion of integrated automation and unmanned operation technologies, the performance-price ratio has significantly improved. However, most currently installed integrated automation unmanned substations still feature a centralized design with a large control room, failing to fully utilize the advantages of integrated automation unmanned operation technology, such as the decentralized placement of secondary automation with primary equipment, the replacement of a large number of signal cables with a small number of communication cables, the elimination of large control room buildings and cable trenches, and significant savings in land use and related infrastructure work. It should be noted that integrated automation systems relying on a small number of communication cables for decentralized placement replace the point-to-point debugging of a large number of cumbersome signal cables on-site with software configuration. Therefore, factory acceptance testing (FAT) of the automation system and the installation of primary equipment on-site can be carried out simultaneously, accelerating the overall substation project progress. In addition, for backup switching during maintenance or faults, substations are often equipped with two transformers with a capacity-to-load ratio of 2. When combined with automation, some load can be transferred to an adjacent substation during switching, saving or postponing necessary capacity expansion investments. 3.2.2 Switching Stations Medium-voltage switching stations in urban power grids fall under the category of distribution systems and are much simpler to construct and upgrade than high-voltage substations. They have now entered the stage of promoting integrated automation and unmanned operation technologies. In my country's distribution network switching stations, which are mainly 10 kV, secondary automation has been able to integrate feeder measurement, control, protection, and communication interfaces onto the switchgear, achieving a "zero breakthrough" in the integration of primary and secondary equipment in the distribution system. Therefore, it is best to select switchgear with integrated primary and secondary equipment and serial communication interfaces for remote monitoring to simplify investment and construction/installation. Alternatively, requirements can be specified for the shock resistance and spacing dimensions of the switchgear doors during selection to facilitate the installation of integrated measurement, control, and protection automation equipment through drilling. 3.2.3 Network Wiring: The construction and renovation of urban power grids include high-voltage main grids, medium-voltage ring networks, and low-voltage distribution networks. Among these, the construction and renovation of medium-voltage ring network lines are closely related to investment savings through comprehensive optimization of primary and secondary systems. Currently, medium-voltage ring networks are a hot topic in distribution network construction and renovation. This is not only necessary to improve power supply reliability but also simplifies wiring and saves investment. Traditionally, to ensure the reliability of radial feeder power supply, complex wiring such as double busbars and bypasses are usually used at the substation side. After conversion to a ring network, this can be fully guaranteed by a ring network with dual-sided power supply capacity and load transfer to the feeder outlet side. Furthermore, after the same radial feeder is converted to a ring network and segmented control of the ring network is achieved through automation, the number of users supplied can be multiplied, provided that the wire diameter allows, while still meeting the 99.99% reliability requirement, thus saving or postponing the required capacity expansion investment. Therefore, primary network wiring should be considered in conjunction with secondary ring network (or grid) control, and the load rate of relevant feeders should be carefully calculated. 3.3 Reliable Power Supply Power supply reliability is a crucial objective in urban power grid construction and renovation. It involves the high-voltage main grid, medium-voltage ring network, and low-voltage distribution network. The high-voltage main grid is generally planned according to the "N-1" reliability principle. The safety control of the medium-voltage ring network is inextricably linked to the primary and secondary systems. The safety control of the distribution system differs from that of the transmission system. In the transmission system, emergency control during system failures and restoration control after system disconnection are implemented separately by dedicated relay protection, reclosing, and SCADA/EMS fault diagnosis and recovery control application software. However, in the distribution system, fault identification and power restoration during faults are uniformly accomplished by the ring network-integrated "fault location, isolation, and automatic power restoration system." Here, the controller (FTU) controlling the ring network sectionalizing switches not only integrates measurement, control, and protection functions but also includes the operating power required for switch switching. This is a major characteristic of distribution network safety control and another example of the close integration of primary and secondary equipment in the distribution system, achieving "integration." The ring network control of the medium-voltage distribution network involves issues such as load, routing, sectionalizing, overhead lines, and cables. Therefore, the "fault location, isolation, and automatic power restoration system," which closely integrates primary and secondary systems, must be designed with overall consideration and optimization. For overhead bare lines, the characteristics of 70% instantaneous faults also need to be considered, requiring the addition of necessary reclosers and the provision of fault location automation to facilitate fault finding and elimination, aiming to quickly restore power supply to the faulty section. Another issue closely related to the safe power supply of medium-voltage distribution networks and closely linked to primary and secondary systems is the grounding of the neutral point. In low-current grounding systems, grounding line selection has always been a difficult problem. In recent years, arc suppression coil grounding, which has emerged in China, can clear 70% of instantaneous grounding faults on overhead bare lines; however, the issue of selecting the line for permanent faults—whether to leave it to the automatic compensation arc suppression coil controller or still rely on automation—needs to be properly resolved through coordination between primary and secondary systems, taking into account the actual distribution network situation. 3.4 Power Supply Quality The power supply quality of the distribution network is mainly reflected in the voltage level. The urban grid construction and renovation goals set a clear requirement of 98% voltage qualification rate. The qualification rate of high, medium, and low voltage levels in urban power grids involves the matching of load and grid transmission, distribution, and generation capabilities, and is the responsibility of the transmission, sub-transmission, and distribution systems at different levels. Control capability is determined by the primary system, and control measures are implemented through automation. The control capability of urban power grids is mainly reflected in the on-load tap changer and capacitor compensation of transformers at the substation level, the distributed capacitors and voltage regulators at the feeder level, and the various load transfer capabilities between substation transformers and feeders. Control measures are implemented by the SCADA/EMS of the sub-transmission system and the SCADA/LM/DMS of the distribution system, respectively. It should be noted that some important users have high requirements for power supply quality and are not satisfied with a 98% voltage qualification rate. This involves the application of "power electronics" technology, which will be discussed in Section 5 of this paper. 3.5 Reducing Network Losses Reducing network losses is a major goal of the economic operation of urban power grids, which mainly relies on the rational planning and operation of the primary system. However, effective coordination between the primary and secondary systems, through automation, can further reduce network losses. According to foreign literature, distribution network losses typically account for about 5% of the total power grid energy output. Automation can further reduce this by 10% (i.e., 0.5% of the total distribution network). Therefore, given the high distribution network losses in my country, automation should play a more significant role. Reducing network losses through automation systems requires primary system support. For example, when transferring loads automatically and reducing network losses based on load balancing principles, the relevant transformers and feeders must have high capacity-to-load ratios and load factors. Foreign experience data also shows that load transfer using "one-on-one" switching on ring networks and "two-on-two-on" switching on networks can potentially increase the reduction in network losses through load balancing from a few percent to 10%. Local compensation of distributed capacitance at the feeder level and comprehensive optimization of voltage and reactive power at the substation level are powerful measures to reduce network losses, improve power quality, and enhance distribution capacity through automation, but these also rely heavily on primary equipment support. 3.6 Improving Power Supply Capacity Improving power supply capacity by 10% is an ultimate goal of urban power grid construction and transformation, and a driving force for national economic development. This involves not only the construction and renovation of urban high- and medium-voltage systems according to the primary system plan, but also the construction and renovation of low-voltage systems with a large scale and the necessary rewiring and meter replacement for increasing capacity for thousands of households. Electricity is characterized by the simultaneous completion of generation, supply, and consumption; therefore, improving power supply capacity and improving services to increase electricity consumption are inevitably two sides of the same coin. This provides a broad prospect for the integration of load management, electricity billing, and other automation systems with power supply and consumption systems. Of course, the comprehensive optimization of the entire urban power grid construction and renovation should also include the comprehensive optimization of the primary system and distribution system automation itself. This includes the selection of primary system voltage levels, the matching of reactive power compensation and active power capacity, the use of energy-saving transformers, the shift of distribution system automation from "multi-island automation" to system integration, FTUs supporting SCADA functions, and electricity billing systems. [b]4 Challenges of the Electricity Market[/b] The launch of my country's electricity market marks a new stage in the development of my country's power industry: from long-term power shortages to improving power supply capacity, and from administrative management to "corporate restructuring, commercial operation, and legal management." Although it will take a considerable amount of time for the electricity market to transition from the current generation market to a distribution market geared towards large users, some legacy systems and operating mechanisms from long-term power shortages and monopolistic practices need necessary transformation to adapt to the changing supply and demand dynamics and future market development. The systems and equipment currently invested in urban grid construction and renovation must also be designed to avoid duplication or even complete rebuilding due to future system development or technological advancements. This primarily involves load management systems, electricity billing systems, and management information systems. 4.1 Load Management System During periods of prolonged power shortages, load control measures involving planned power outages and power cuts for users should be promptly shifted to load management (LM) that ensures users do not experience power outages. Modern load management primarily involves implementing pre-prepared load management plans, such as reducing feeder voltage and load, or periodically switching on and off controllable loads (air conditioners, water heaters, etc.) according to grouping cycles, to achieve load management that ensures users do not experience power outages. Furthermore, demand-side management (DSM) based on time-of-use pricing should be actively promoted to incentivize demanders to participate in load management. The integration of supplier load management and demand-side electricity management will significantly improve the load curve of the distribution system and lay the foundation for the subsequent development of the electricity market. 4.2 Electricity Billing System: In the past, during the period of monopolistic operation, electricity billing primarily involved a one-way communication method: supplier meter reading, user payment, and overdue penalties. During urban grid construction and renovation, a large-scale "one meter per household" system is underway. Whether considering the current "commercial operation" or the future "distribution market," two-way communication of billing information (remote meter reading for large and medium-sized users and smart cards for small and medium-sized users) and the relationship with bank settlements should be considered as much as possible. "Complaint hotline handling" is another channel for establishing two-way communication between suppliers and users. Therefore, when constructing and renovating low-voltage lines and "one meter per household," the primary and secondary systems should be uniformly optimized to establish a hierarchical and segmented two-way communication mechanism for users (including property management), thereby improving the level of power supply services. 4.3 Management Information System To ensure fairness, impartiality, and transparency in electricity market transactions, the current energy trading system used for power generation and transmission, and the future system used for power transmission and distribution, must be separate from the traditional energy management or distribution management systems that ensure the safe operation of the power grid. However, the simultaneous nature of electricity production and consumption dictates the inseparability of energy trading and safe production. Therefore, an energy trading system based on Intranet/Internet and characterized by a browser, termed e-commerce or online trading, will be connected to the real-time operating energy management or distribution management system that ensures power grid safety via a network switch, establishing an "Open Access Simultaneous Information System (OASIS)." Clearly, the traditional Management Information System (MIS), characterized by dedicated networks, dedicated channels, and graphical user interfaces, is unsuitable for electricity trading. This raises the question: should the traditional management information system and the future trading information system develop in parallel, or should the existing MIS be modified to adapt to the future development of the electricity market? In fact, some provincial dispatch centers facing pressure in the power generation market have already adopted the latter approach and are ready for modification. [b]5 Application of Power Electronics Technology[/b] The improvement of power shortages and the rise of the electricity market inevitably lead to higher requirements for power supply quality from users. In urban power grid construction and renovation, a voltage qualification rate of 98% is required, which can meet the requirements of most general users. However, under normal conditions, the distribution network still has some potential instantaneous disturbances or waveform distortion problems, such as instantaneous power loss caused by reclosing during transient faults, instantaneous undervoltage caused by adjacent line faults or impact loads, instantaneous overvoltage caused by out-of-phase faults, transient processes caused by lightning or capacitor switching operations, and harmonic pollution caused by nonlinear loads. These are intolerable for some important users with high power supply quality requirements, such as banks, hospitals, airports, and data processing centers. Especially with the increasing development and popularization of information technology (such as modern production "assembly lines"), the impact and harm caused by these disturbances are even greater. At this time, we need to turn to "user power" in power electronics technology, which is geared towards power supply and consumption. The problems of instantaneous disturbances and waveform distortion in distribution network power supply can be solved by either the user side or the power supply side. Currently, the main approach to solving the problem is to install uninterruptible power supplies (UPS), voltage regulators, surge arresters, filters, static compensators, etc., on the user side according to specific needs. However, analysis and practice show that as the number of such users continues to increase, the total investment and losses will rise significantly, while the improvement in quality will be limited. It would be better to have the supplier provide a unified solution. This is why the concept of "Custom Power," proposed by Dr. N. G. Gingorani of the Electric Power Research Institute in 1988, has attracted increasing attention in recent years. "Custom Power" refers to the provision of more reliable and higher-quality electricity to users by the power supply department through technical means, thereby increasing the added value of electricity (such as the quality-based pricing in the electricity market). Currently, foreign countries are researching and developing: distribution network static synchronous compensators (DSTATCOM) and dynamic voltage restorers (DVR) based on energy storage inverters, solid-state circuit breakers (SSCB), fault current limiters (FCL), and active power regulators (APLC) that integrate filtering, voltage regulation, and surge protection. Domestic power supply departments are currently collaborating with universities to conduct research and development in this area. Because distribution network equipment requires both high reliability and low cost, this is a major challenge currently facing the research and development of user power equipment. However, with the improvement of the "performance-price ratio" of electronic components and the growth in demand for "user power" equipment, it should be said that the application and popularization of user power is only a matter of time. [b]6 Conclusion[/b] In the past, due to limited investment, the issue of urban power grid construction and renovation was never seriously put on the agenda. Even when planning, the requirements were low, mostly focusing on emergency measures. As for the automation of the distribution system, it was generally only mentioned in the primary system planning, or even not prioritized, and only implemented occasionally when there was an opportunity. This may be one of the reasons for the long-standing "multi-island automation" of distribution. This time, the urban power grid construction and renovation is very aggressive, with high requirements and a long service life. Although the investment is large, the overall investment is still limited due to the large amount of "backlog". Therefore, unified planning, mutual support and comprehensive optimization of primary and secondary systems should be carried out, and various possible influencing factors during the service life should be taken into account to ensure that the expected goals are achieved with limited investment. **References** 1. Chen Zhangchao, Tang Deguang. Urban Power Grid Planning and Transformation. Beijing: China Electric Power Press, 1998. 2. Wang Mingjun, Yu Erkeng, Liu Guangyi. Distribution System Automation and Its Development. Beijing: China Electric Power Press, 1998. 3. Yu Erkeng, Han Fang, Xie Kai, et al. Electricity Market. Beijing: China Electric Power Press, 1998. 4. NG Hingorani. Introducing Custom Power. IEEE Spectrum. 1995(6)