[b]1 Various Operating States and Safety Controls of Power Systems[/b] Power systems operate under two sets of constraints: equality constraints and inequality constraints. (1) Equality Constraints [img=101,24]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9907/image7/148.gif[/img] Where P is power; G is generation; L is load (including network loss). (2) Inequality Constraints Vmin≤V≤Vmax fmin≤f≤fmax PG≤PG.max PL≤PL.max Where V is voltage; f is frequency. Therefore, due to different combinations of the two sets of constraints, the three operating states in Table 1 are formed. Table 1. Operation states formed by equality and inequality constraints. [table][tr][td] Serial Number [/td][td] Equality Constraints [/td][td] Inequality Constraints [/td][td] Operation State [/td][/tr][tr][td] 1 [/td][td] 1 [/td][td] 1 [/td][td] Normal State [/td][/tr][tr][td][size=3]2 [/size][/td][td][size=3]1 0[/size][/td][td][size=3]0 0[/size][/td][td][size=3]Emergency State (Unstable)[/size][/td][/tr][tr][td] 3 [/td][td] 0 [/td][td] 1 [/td][td] Recovery State [/td][/tr][/table] Note: 1 means satisfied; 0 means not satisfied. The relationship between the three safety controls corresponding to the above three operating states, namely, preventive control in normal state, emergency control in emergency state and recovery control in recovery state, is shown in Figure 1. [img=236,182]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9907/image7/149.gif[/img] Fig.1 Operation state & security control of power system The dispatch automation system and safety devices adopted by the transmission and distribution systems to ensure normal operation and safety control are shown in Table 2. The dispatch automation and safety devices of the distribution system and the transmission system are mainly different in the following aspects: (1) Control function The transmission system controls power generation, and the distribution system controls load. (2) Geographic information is crucial to the distribution system, but optional for the transmission system. Table 2. Safety Operation Configuration of Transmission and Distribution Power System [table][tr][td] Safety Controls [/td][td] Transmission System [/td][td] Distribution System [/td][/tr][tr][td][size=3]Normal Operation and Preventive Controls[/size][/td][td][size=3] · SCADA/AGC/EMS · MIS/AMR[/size] [sup]＊[/sup] [/td][td][size=3] · SCADA/LM/DMS · MIS/GIS/AMR/DSM/DA[/size][/td][/tr][tr][td][size=3]Emergency Controls[/size][/td][td][size=3] · Relay Protection · Online Stability[/size][/td][td][size=3] · Fault Identification (Instantaneous/Permanent) · Fault Location and Isolation[/size] [/td][/tr][tr][td][size=3]Recovery Control[/size][/td][td][size=3] · Recovery Control[/size][/td][td][size=3] · Automatic Power Restoration · Fault Location[/size] [/td][/tr][/table] Note: SCADA—Monitoring and Control and Data Acquisition; EMS—Energy Management System; DMS—Distribution Management System; AGC—Automatic Generation Control; LM—Load Management System; MIS—Management Information System; GIS—Geographic Information System; DSM—Demand-Side Power Management; AMR*—Power Billing System for Generation and Transmission; AMR—Power Billing System for Distribution; DA—Distribution Automation. (3) Power Billing Transmission System is geared towards Generation and Tie Line Gateways, while Distribution System is geared towards Large, Medium and Small Users. (4) Characteristics of the Power Grid Transmission System: The transmission system has stability issues with a wide impact, while the distribution system faces various power quality issues. (5) Emergency control transmission systems rely on dedicated relay protection and automatic devices, while distribution systems are completed by integrated distribution automation. (6) Recovery control transmission systems utilize EMS recovery control software containing expert systems, while distribution systems are completed jointly by distribution automation and emergency control. It should be noted that in my country's urban power grids composed of high voltage (220/110 kV), medium voltage (35/10 kV), and low voltage (380/220 V), the distribution system mainly refers to medium and low voltage. As for high voltage, like most regional or municipal main grids, it falls under the category of sub-transmission systems that do not generate electricity. 2 Normal operation of the power distribution system Under normal conditions, the main tasks of the power distribution control system are: (1) Data acquisition and monitoring control through SCADA to ensure that the operating parameters of the power distribution system meet the two constraints of normal conditions; (2) LM and DSM are used to improve the load curve under normal conditions and avoid overload during peak periods, which would lead to violation of inequality constraints; (3) Under the condition of meeting the constraints of normal conditions, DMS reduces network losses through load balancing and voltage/reactive power optimization to achieve economic operation of the power distribution system; (4) AMR for large, medium and small users is combined with the bank settlement system to realize the commercial operation of the power distribution system; (5) GIS-based MIS realizes offline management of the power distribution system, such as substation, distribution, power consumption, retrieval and decision-making, through automatic mapping (AM)/equipment management (FM). Here, there are several issues under development that deserve attention. (1) System integration The current power distribution system automation is following various international open standards and moving from the traditional "multi-island automation" to system integration. my country's power distribution system automation is in the initial stage of construction and should be implemented in one step to adapt to this development trend. The main advantage of using open systems to achieve system integration is that the effective use and high sharing of data and methods greatly improves the "performance-price ratio" of the entire system. This is especially important for the automation of power distribution systems with many individual automations, such as the real-time information of SCADA and the short-term load forecast of DMS, which can support the compilation and implementation of load management schemes by LM. GIS, which is traditionally used for offline management, can provide online services: display the real-time information of SCADA on the geographic wiring diagram, support the "complaint hotline processing" of DMS, etc. (2) Load management With the development of my country's power industry, the long-term power shortage situation has been basically changed, and the construction and transformation of urban power grids has been launched, and the target of increasing electricity sales has been proposed. Therefore, the load control of planned electricity consumption implemented by the power rationing and power outage of users in the past should be promptly transferred to load management (LM) based on the insensible power outage. Modern load management mainly uses a pre-prepared load management scheme to reduce the voltage and load of feeders or to start and stop the controllable loads (air conditioners, water heaters, etc.) of users in turn according to the grouping cycle to achieve load management that users do not feel the power outage. The implementation of LM and Demand-Side Electricity Management (DSM) plans will inevitably improve the load curve of the distribution system under normal conditions and lay the foundation for the subsequent development of the electricity market. (3) Electricity Market At present, my country has entered the stage of starting the power generation market. Although it will take a long time to open up the distribution market to large users, the construction of a distribution system automation with a certain service life (such as 10 to 15 years) must take into account the requirements that the development of the electricity market during the service life may put forward on the electricity billing system and information system under construction, so as to avoid repeated construction or even demolition and reconstruction in the future. In the construction and transformation of urban grid, the construction of "one meter per household" is being carried out in a large number of areas. Whether from the current "commercial operation" or the future "distribution market", the communication of billing information (remote meter reading for large and medium users and smart cards for small and medium users) and the connection with banks should be considered. As for the information system, the traditional management information system (MIS) characterized by dedicated network, dedicated channel and graphical user interface is no longer suitable for the contemporary e-commerce and network transactions based on Intranet/Internet and characterized by browser. This raises the question: should the traditional management information system and the future transaction information system develop in parallel, or should the existing MIS be modified to adapt to the future development of the power market? In fact, some provincial dispatch centers facing pressure from the power generation market have already acted in the latter way and are ready to be modified. (4) The improvement of the power shortage situation and the rise of the power market will inevitably lead to the increase of users' requirements for power supply quality. In the construction and transformation of urban power grids, the voltage qualification rate is required to reach 98%, which can meet the requirements of most general users. However, under normal conditions, the distribution network still has some possible instantaneous disturbances and waveform distortions. For example, instantaneous power loss caused by reclosing during instantaneous faults, instantaneous undervoltage caused by adjacent line faults or impact loads, instantaneous overvoltage caused by phase faults, transient processes caused by lightning or capacitor switching operations, and harmonic pollution caused by nonlinear loads. This is intolerable for some important users with high requirements for power supply quality, such as banks, hospitals, airports, and data processing centers. In particular, with the increasing development and popularization of information technology, the impact of these disturbances and the harm they cause will become greater and greater. This necessitates the use of "custom power" technology in power electronics, specifically designed for power supply and consumption. Disturbances in distribution networks can be addressed by either the user side or the power supplier. Currently, the main approach is to install uninterruptible power supplies (UPS), voltage regulators, surge arresters, filters, and static compensators on the user side, tailored to specific needs. However, analysis and practice show that as the number of such users increases, total investment and losses will rise significantly, with limited improvement in quality, making a unified solution by the supplier far preferable. Therefore, the concept of "custom power," proposed in 1988 by Dr. N. G. Lingorani of the Electric Power Research Institute (EPR), has attracted increasing attention in recent years. "Custom power" refers to the provision of more reliable and higher-quality electricity by the power supply department through technological means, thereby increasing the added value of electricity (such as 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) integrating filtering, voltage regulation, and surge protection. Domestic power supply departments are already 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 of demand for power user equipment, it should be said that the application and popularization of user power is only a matter of time. [b]3 Safety Control of Distribution Systems[/b] The reliability of urban power grid supply includes three voltage levels: high, medium, and low. High voltage belongs to the sub-transmission system, and its safety control will be handled by automatic devices such as SCADA/EMS and relay protection equipped in the prefecture-level and city-level main grids. The safety control of the distribution system mainly refers to the medium-voltage system, including those directly feeding power to large and medium-sized users. As for the low-voltage section, due to the large number of small and medium-sized users involved, full automation is impossible. Issues involving violations of constraints (power outages, low voltage, etc.) below the transformer level and before the property management of small and medium-sized users can only be handled through a "complaint hotline." Safety control of the medium-voltage distribution system includes preventative control under normal conditions, emergency control under emergency conditions, and recovery control under restoration conditions. Since the distribution system primarily handles safety control under steady-state conditions, unlike the transmission system which has issues such as "stability" and "frequency," as shown in Figure 2, the distribution system automation can be integrated with automatic devices such as relay protection reclosing to uniformly achieve preventative control under normal conditions, emergency control under emergency conditions, and recovery control under restoration conditions. [img=279,252]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9907/image7/150.gif[/img] Fig.2 Operation state & security control of distribution system 3.1 Preventive control is mainly implemented by DMS application software to preventive control under normal distribution network conditions. Here, the main application software used are: (1) Load forecast Distribution network load forecast is divided into regional load forecast and bus load forecast. Regional load forecast is the total load forecast of the distribution network from one day to one week, or the load forecast of a certain area, mainly used for power purchase plan and power supply plan. Bus load forecast is the total load forecast of the distribution network or the load points (buses) of a certain area, mainly used for state estimation or power flow calculation. (2) Power flow calculation: The power flow of the distribution network is calculated based on the bus load forecast to verify whether the distribution network meets the constraints under normal conditions under the load forecast. (3) Safety analysis: Sometimes called "accident scenario" or "what if". The safety analysis is performed according to the "N-1" principle or a pre-arranged "disconnection table" to verify whether the distribution network will lead to line overload or abnormal voltage under the accident scenario. (4) Safety countermeasures: Based on the events that violate the constraints obtained from the power flow calculation or safety analysis, measures to prevent overload or abnormal voltage are proposed. (5) Short circuit calculation: This is an online three-phase short circuit procedure used to determine possible circuit breaker failures in distribution systems with frequent switch operations. 3.2 Emergency control and recovery control: The emergency control and recovery control of the distribution system are mainly completed by the integrated fault location, isolation and automatic power restoration system installed on the ring network. Unlike transmission systems, which rely on separate automatic devices such as relay protection and reclosing for emergency control, this system utilizes EMS alarm analysis, fault diagnosis, and recovery control software for restoration control. A typical configuration of an overhead line ring network fault location, isolation, and automatic power restoration system integrating switching, control, remote control, protection, and communication is shown in Figure 3. [img=311,124]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9907/image7/151.gif[/img] Fig.3 Fault positioning, isolating, and service restoration system. In Fig.3, the recloser has short-circuit current interruption and reclosing functions. Its FTU, in addition to remote communication, also has a protection function that operates according to the ampere-hour curve. When a fault occurs on section II/III, it can coordinate with the substation's feeder protection to selectively trip and automatically reclose to eliminate 70% of transient faults on the overhead line, returning to normal operation. When a transient fault occurs on section I, the substation's feeder protection and reclosing operation will disconnect the feeder circuit breaker to eliminate the fault. Load switches can only interrupt load current and close fault current; they have no reclosing function. When there is no voltage, they can be delayed in release to avoid the reclosing time required to clear momentary faults. Their FTUs (Functional Units) also only have remote communication functions and lack protection, but their overall price is lower than reclosers, making them suitable for large-scale use. Reclosers and load switches, which also function as segmentation switches, are controlled by the substation RTU or distribution network master station via remote communication. Together with substation protection and feeder circuit breakers controlled by the RTU, they achieve one-time location, isolation, and automatic restoration of power supply to non-faulty sections for permanent ring network faults. In the event of a permanent fault, reclosing fails, the circuit breaker (for section I fault) or the recloser (for other sections fault) trips, and the line loses power. The recloser (for section I fault) and load switch release after a delayed power loss, isolating the fault. Simultaneously, based on telemetry data and the unequal current levels on both sides of the segmented line, the fault section is determined, completing fault location. Based on this, power supply to non-faulty sections is automatically restored via remote control (including normally open open-loop load switches). Regarding the safety control of the distribution network, there are two issues worth noting: (1) The fault location problem must be taken into account. The overhead line ring network fault location, isolation and automatic power restoration system shown in Figure 3 only restores power to the non-faulty sections. Therefore, only after the faulty sections are identified and eliminated and all power is restored can the distribution network's equality constraints be satisfied and it enter a normal state. The impact of the time for eliminating this fault on the reliability level of the distribution network should not be underestimated. This is best illustrated with numbers. The radial feeder (solid line) shown in Figure 4 has an average of one permanent fault every 3 years, and the time for eliminating the fault and restoring power is 4 hours. That is, each of the 1600 households experiences a power outage of 1.33 hours per year (excluding instantaneous power loss due to reclosing), and the power supply reliability is 99.985%. After adopting a dual-power ring network and operating it in 4 segments, each segment has 400 households (dashed line), and the calculation is still based on an average of one permanent fault every 3 years and a time for eliminating the fault and restoring power being 4 hours. At this point, each of the 1600 households experiences a power outage of 1.33/4 = 0.33 hours per year, increasing the power supply reliability to 99.996%. [img=271,52]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9907/image7/152.gif[/img] Fig.4 An example of redial distribution. Clearly, if the time to restore power after a fault is delayed to 16 hours, the benefit of using a dual-power ring network operating in four segments will be zero. Conversely, if the time to restore power after a fault is reduced to 1 hour, the original radial feeder can still achieve the same high power supply reliability of 99.996%. This is why fault location in distribution networks has always been a focus. Of course, increasing the number of segments and reducing the distance between segments can also speed up the time for fault elimination and power restoration in the faulty segment. Obviously, the cost of this is too high and not practically advisable. However, this has led to the development of a "segmented, measured-only" fault location telemetry device to speed up the time for fault elimination and power restoration. In medium-voltage systems with low-current grounding, when developing such fault location devices, the fault location function of grounding line selection can also be included. If the system shown in Figure 3 is a cable ring network system, the problem of fault distance measurement is not so urgent. Because the ring network fault location, isolation and automatic power restoration system used for underground cable systems generally adopts multi-unit ring network cabinet wiring with sectional switches and load outgoing lines combined, which can achieve one-time fault location, isolation and automatic power restoration, and immediately enter the normal state. After the faulty cable section is isolated (including the fault isolation caused by human fault selection during grounding line selection), special instruments are used to eliminate the fault and restore the ring network operation. (2) Complaint hotline handling The complaint hotline handling (TC, Trouble Call) is sometimes called the Outage Management System (OMS). It mainly establishes two-way communication between the power supply department and the vast number of small and medium-sized users through telephone hotlines, and collects and handles problems that violate the constraints of the distribution network low-voltage system. For example, Southern California Edison (SCE), which includes power generation, transmission, and distribution, has four Distribution Control Centers (DCCs) managing 4.2 million customers (one meter per customer). There are 150 hotline numbers for the DCCs, staffed by 150 people during the day and 30-40 people at night. Each DCC is equipped with an Outage Management System (OMS) that can serve as a backup for each other. Each OMS has a large display screen and three control consoles, staffed by 2-3 people during the day and 1 person at night. From the moment a complaint is received, the customer must be informed within 4 hours when power will be restored. Even if power cannot be restored within 4 hours, the problem must be identified and a restoration time estimated; otherwise, fines will be imposed according to regulations. The procedure for handling the complaint hotline of the system is as follows: ——After receiving the complaint call, the TC duty officer immediately logs the complaint information on the event table of the terminal and transmits it to the power distribution control center under his/her charge via Intranet; ——After receiving one or more event tables, the power outage management system automatically generates an event summary table to realize fault location and displays the power outage area and the fault source on the screen; ——According to the fault information displayed on the screen, the DCC duty personnel use a wireless telephone to direct maintenance personnel (1-2 people) to the fault location for repair. The maintenance vehicle is equipped with a global positioning system (GPS) and displays its location on the DDC geographic wiring diagram, which facilitates cooperation between the two parties, speeds up the fault handling speed, and automatically replies to the user through voice synthesis. 4 Conclusion (1) The safety and quality of power supply are the two main indicators of the operation of the power distribution system. The equality constraint of the power system operation reflects the safety of power supply, and the inequality constraint reflects the quality of power supply. (2) The premise of ensuring that the power distribution system operates without violating the two constraints is that the generation, transmission and distribution capacity must meet the needs of the power load. Obviously, in the case of long-term power shortage, the power distribution system will inevitably suffer from frequent power outages that violate the equality constraint and poor power quality that violates the inequality constraint. In addition, even if the power generation capacity is much greater than the power demand, if the transmission and distribution capacity is mismatched, it will also lead to power shortages in the relevant voltage levels or areas of the power distribution system. (3) The normal operation and safety control of the transmission system is monitored and controlled by a single SCADA/AGC/EMS energy management system. However, the normal operation and safety control of the urban grid involves two sub-transmission and distribution systems and three voltage levels (high, medium and low), which will be monitored, controlled and handled by the main grid's SCADA/EMS, the distribution network's SCADA/LM/DMS and "complaint hotline handling" respectively. (4) The current construction of power distribution system automation in my country should move from the traditional "multi-island automation" to system integration; from load control based on power cut-off during power shortages to load management based on imperceptible power outages; from monopolistic user payment to a two-way electricity billing system, and consider the Intranet/Internetization of MIS (Management Information System) to connect with the future power distribution market; when solving the power supply quality problems of important users, full attention should be paid to the development of "user power" technology. (5) Unlike the transmission system, the ring network fault location, isolation and automatic power restoration system in the medium voltage distribution system integrates monitoring, control and protection, and uniformly solves the problems of emergency control in emergency situations and restoration control in recovery situations. For underground cable systems, it is possible to achieve "automatic power restoration", but for overhead line systems, there is still a time problem related to the reliability level of fault location, fault elimination and power restoration of fault sections. In this regard, the development of fault segment location telemetry devices with grounding line selection function is worth noting. (6) “Complaint hotline handling” sounds easy, but its implementation involves issues such as market development, electricity regulations, operating mechanisms, and technical equipment. It is not simply a matter of installing a few telephones to communicate with the geographical information of the distribution network. It needs to be gradually improved according to the national conditions to achieve practical results. [b]References[/b] 1 Wang Mingjun, Yu Erkeng, Liu Guangyi. Automation of Distribution Systems and Its Development. Beijing: China Electric Power Press, 1998 2 Yu Erkeng, Han Fang, Xie Kai et al. Electricity Market. Beijing: China Electric Power Press, 1998 3 Hingoran N G. Introducing custom power. IEEE Spectrum June, 1995 4 Fu Shuti. Report on Visit to the United States. Electric Power Research Institute, Ministry of Electric Power, 1998