1. Overview Power grid energy loss rate (i.e., line loss) is one of the important economic and technical indicators for evaluating power supply companies. Reducing line loss is a crucial measure to improve economic efficiency. For example, if the annual power supply of the grid is 2700 GWh, a 1% reduction in the line loss rate can save more than 15 million RMB in electricity, demonstrating significant economic benefits. In summary, the line loss of a regional power grid mainly consists of three parts: fixed losses, namely the iron losses in all transformers, measuring instruments, secondary circuits, and other excitation circuits; variable losses, namely the copper losses in lines and transformers that are proportional to the square of the current; and management losses, namely losses caused by meter reading errors, metering instrument errors, electricity theft, and poor management. This paper, based on the specific characteristics and examples of the regional power grid, proposes some measures to reduce line loss through analysis and calculation, focusing on grid operation modes, economic dispatch, grid transformation, and line loss management. 2. Optimize the operation mode (1) Rationally arrange the power generation plan and output curve of local power plants, do a good job of active and reactive power balance in different zones and layers, and avoid long-distance power flow and multi-voltage level transformation transmission. In the power grid of the region, some local power plants are often connected to the grid via 110 kV, 35 kV or 10 kV lines. According to the needs of grid operation and configuration simplification, the electromagnetic ring network of 220 kV and 110 kV should normally be in open-loop operation. If the local power plants connected to the power supply area of ​​the 220 kV substation generate excess power, they will inevitably send the excess power to the 220 kV grid through the 220 kV main transformer, and then supply the load after the voltage is reduced by the main transformer of the adjacent 220 kV substation. This operation mode will undoubtedly increase the line loss. For areas with heavy load, if the output of local power plants is insufficient, they need to input power from the 220 kV main transformer or the tie line, which will also increase the line loss. For example, a 220 kV substation is connected to a local power plant with an installed capacity of 200 MW, while the maximum power supply load in the area is about 70 MW. In summer, when the local power plant outputs a lot, the line loss index will increase significantly. In another area, due to insufficient installed capacity of the local power plant, the power is mainly supplied by a long distance (29 km) of 110 kV line with thin conductors (LGJ-150), resulting in a large line loss. Both of these situations affect the reduction of power grid line loss. (2) Conduct line loss calculation and analysis to optimize the power supply mode. If there are two or more power supply lines and multiple power supply modes in the regional power grid, power flow and line loss analysis should be carried out. Calculate several possible power supply modes and comprehensively determine the operation mode based on the premise of taking into account reliability and the automatic device meeting the requirements, so as to achieve the purpose of reducing line loss. For example, a power grid was originally supplied by line A as the main power supply and line B as a backup. After power flow and line loss calculation, it was found that if line B is changed to be the main power supply and line A is the backup, the line loss will be reduced by 0.06%. Only a small amount of modification is required for a 110kV automatic transfer switch. (3) Reasonably arrange temporary operation modes, optimize power outage maintenance plans, and reduce repeated power outages. Power outages of transmission and transformation equipment change the normal operation mode of the power grid. Under the same power supply load, they increase the power flow and line loss of transmission and transformation equipment, which is also detrimental to the safety and reliability of power supply. Therefore, the dispatching and operation department should optimize power outage plans, shorten the power outage time of transmission and transformation equipment, and reduce the repetition of power outages. This can be achieved by optimizing monthly power outage maintenance plans, determining and refining weekly power outage plans in advance, and strictly approving temporary power outage plans to minimize the number of power outage operations, improve power supply reliability, and reduce line losses. [b]3 Strengthen Economic Dispatch[/b] (1) Realize local reactive power compensation and reduce line losses of transmission and transformation equipment caused by reactive power flow. In terms of economic dispatch of reactive power flow, the regional power grid should adhere to the principle of layered, graded and zoned compensation of reactive power output and load according to voltage level and power supply area. Reactive power compensation devices should be reasonably configured and put into operation at each level and zone. Power supply enterprises with the necessary conditions can adopt voltage and reactive power integrated adjustment devices to realize the integrated automatic adjustment of reactive power compensation devices and on-load tap changer taps, effectively improve the 10kV bus load power factor of substations, improve voltage qualification rate, avoid long-distance and repeated back-and-forth transmission of reactive power, and thus reduce line losses. (2) Use peak-valley electricity price policy to increase load rate and reduce peak-valley difference. During peak load period, the transmission current of transmission and transformation equipment increases, and the line loss, which is proportional to the square of the load current, also increases. When the load decreases, the transmission current decreases, and the line loss also decreases. Analysis of the relationship between load rate and line loss shows that if the power grid load rate is 100%, the variable line loss in the line loss composition will be the minimum. Therefore, increasing the load factor and reducing the peak-valley difference of the load have a positive effect on reducing line loss. If the peak-valley electricity price policy is actively promoted and implemented, and the electricity price leverage is used to make users consciously adjust their electricity load and electricity consumption time, the effect of avoiding peak and saving electricity can be achieved. (3) Appropriately increase the supply voltage of the transmission line. Under the premise that the operating voltage meets the power supply quality requirements, appropriately increasing the operating voltage can reduce the transmission current, thereby reducing line loss. According to preliminary calculations, if the operating voltage is increased by 5%, the line and transformer losses in the transmission link can be reduced by about 9%. The main methods to appropriately increase the operating voltage are: adjusting the tap position of the on-load tap-changing transformer, increasing the generator terminal voltage, putting into operation the reactive power capacitor compensation device, and taking out the reactor that absorbs capacitive reactive power. (4) Shut down seasonal operating transformers to reduce no-load losses. Substations with two or more transformers should, according to the actual situation of seasonal and regular loads, transfer the load to a certain transformer for power supply in an appropriate period of time, and put the other transformers into standby status to reduce the no-load losses of the transformers. This measure has a very obvious effect on reducing line loss for power grids with relatively high capacitive loads. For example, the maximum load of a certain power grid is about 140 MW, and the substation capacities of 220 kV and 110 kV are 300 MVA and 400 MVA respectively. Its maximum load in winter is 110 MW. From October to March of the following year, if some substation transformers are rotated regularly, the reduced no-load loss is greater than the increased variable loss, and the power loss can be reduced by 1 GWh in half a year. However, it should be noted that after the number of transformers in operation is reduced, the dual-circuit power supply of the transformer becomes a single-circuit power supply. The "N-1" mode cannot meet the requirements, and the power supply reliability is reduced. At the same time, in order to ensure that the main transformer in the standby state can be put into operation at any time, the regular rotation system will also increase the number of switching operations of the main transformer. [b]4 Strengthen equipment and power grid transformation[/b] (1) Strengthen line transformation and reduce line loss: The power loss of transmission lines is proportional to the line resistance and the square of the transmission current. Reducing the line resistance and current can reduce line loss. The main methods to reduce line resistance are to increase the cross-sectional area of ​​conductors and shorten the line length. Therefore, in the planning, design and renovation of the power grid, it is beneficial to reduce line losses by rationally selecting the conductor cross-section and path of the transmission line, making the substation location as close as possible to the load center to shorten the power supply radius, and selecting conductors with low resistivity materials. For example, a certain 110 kV transmission line (LGJ-150/29 km) transmits 16 GWh and 7.5 Gvar of electricity in a certain month. Its theoretical line loss is 322 MWh/month and the line loss rate is 2.01%. If the LGJ-150 conductor is replaced with the LGJ-240 conductor, its theoretical line loss is 196 MWh/month and the line loss rate is 1.23%. After the transformation of the transmission line, the annual power loss can be reduced by about 900,000 yuan. (2) Replace the transformer to reduce transformer loss. Selecting energy-saving transformers with lower no-load loss and rationally adjusting the load rate of the transformer can achieve the purpose of reducing transformer loss. Replacing an SFSL-20000 kVA transformer (no-load loss of 44 kW) with an SFZ8-20000 kVA transformer (no-load loss of approximately 18 kW) can save approximately 227 MWh of electricity per year. Selecting the appropriate transformer capacity based on the main transformer load is also a measure to reduce losses. Analysis shows that when the transformer's no-load loss and load loss are equal, the total loss is minimized and the efficiency is maximized, representing the theoretically optimal load condition for the transformer. (3) Reduce contact resistance and leakage current in power transmission networks. The electrical equipment connections in power transmission networks, including the connections of various leads such as transmission lines, circuit breakers and disconnect switches, and current transformers, have varying degrees of contact resistance. Poor contact can cause the connections to heat up, which not only leads to power loss but also threatens the safe operation of the power grid and equipment. Due to insulation aging and cracking, and severe pollution of the lines, leakage current can also cause power loss. When conducting nighttime inspections of substation power distribution equipment and devices with lights off and infrared temperature measurement, it is often found that the connections of some equipment are red, hot or have excessively high temperatures. These are all specific manifestations of power loss. Therefore, strengthening the operation and maintenance of power transmission and transformation equipment, replacing damaged equipment, and tightening equipment connections to ensure that the contact resistance meets the operating requirements will have a positive effect on reducing line loss. (4) Implementing power grid voltage upgrade: If the existing power transmission and transformation equipment can meet the insulation coordination requirements, the power grid can be upgraded by utilizing existing line corridors and transformer equipment or by partially replacing them. The operating voltage of the power transmission and transformation system can be increased by 1 to 2 levels, which can not only meet the requirements of saving money, reducing construction work and shortening power outage time, but also greatly reduce the transmission current of the line and reduce line loss. For example, in a certain power grid, two substations are supplied by 35 kV lines. Due to the large transmission power and power supply, the line loss is as high as 3.1%, and the monthly power loss is 114 MWh. According to calculation, if the line voltage is upgraded to 110 kV, the line loss will be reduced by about 90%. (5) Applying line loss calculation technology to decide on power grid transformation schemes: In the process of power grid planning, construction and transformation, not only should the safety and reliability of power grid operation and the operational flexibility be fully considered, but also the economic efficiency of power grid operation, that is, the relatively small operating line loss. After the preliminary determination of several power grid transformation schemes, a comprehensive analysis and calculation of all schemes, including line loss, should be carried out, and then the connection scheme should be determined. The following are two examples of upgrade schemes for a certain 110 kV line. Scheme 1 involves building a new 220 kV substation as the power supply hub for a local power grid, and then constructing and improving the corresponding 110 kV transmission lines (the main transformers of the new substation will be 90 MVA domestic high-loss transformers and 150 MVA imported low-loss transformers, respectively). Scheme 2 involves improving and strengthening the 110 kV transmission lines based on the existing 110 kV power grid, with the nearby 220 kV substation as the main power supply point. It is assumed that all newly built 110 kV transmission lines will use LGJX-240 conductors. Based on a 12% increase in grid power supply and an 8% increase in local power consumption, the load and power consumption of relevant substations and transmission lines were predicted. The line losses in power grid operation for 1999, 2000, and 2001 were analyzed and calculated. The results and analysis are as follows: In Scheme 1, the line loss with one main transformer is approximately 45% lower than with two main transformers. Furthermore, as grid power supply increases, the difference gradually narrows, and it is projected to decrease to 35% by 2001. This is mainly because the proportion of transformer no-load loss in the line loss gradually decreases. Since 220kV transformers have a relatively high capacitive load, the proportion of transformer no-load loss is relatively high, making them more sensitive to line losses. Therefore, selecting low-loss transformers (or even large-capacity transformers) has a significant effect on reducing line losses. In Scheme 2, the grid power supply needs to pass through 110... When kV lines are connected in series, the losses along the lines are amplified, increasing the line loss. Furthermore, the magnitude of the load current (which is essentially equivalent to the amount of electricity supplied) is almost the sole factor affecting the line loss. Therefore, as the amount of electricity supplied increases, the line loss also gradually increases. In 1999-2000, when Scheme 1 was equipped with one 220 kV transformer, the line loss of Scheme 2 was approximately 0.1% to 0.2% higher. If Scheme 1 was equipped with two transformers, its line loss would be 0.2% to 0.4% higher, mainly because the no-load loss of the transformer has a higher sensitivity to the overall line loss. By 2001, due to the increase in electricity consumption and load to a certain level, the line loss of Scheme 1 was 0.1% to 0.5% lower than that of Scheme 2. Preliminary conclusion: Compared with Scheme 1 (with one main transformer in the first phase), Scheme 2 has a line loss rate that is about 0.1% to 0.2% higher, which translates to a loss of 500 to 1,100 MWha. The electromagnetic open-loop backup point of the 110 kV line in Scheme 2 should be improved and equipped with corresponding automatic devices to improve safety and reliability, and also take into account the support capacity between adjacent local power grids. Considering safety and economic factors, we believe that Scheme 1 can be postponed until after 2001. At present, Scheme 2 is preferred. The 110 kV line is upgraded first to save the investment in the 220 kV transmission and transformation project. [b]5 Strengthen line loss management[/b] (1) Improve the configuration of electricity metering. Installing electricity meters in strict accordance with relevant regulations and provisions is a necessary means to ensure the scientific and standardized management of line loss. When arranging and implementing power grid construction and renovation projects, relevant departments should attach great importance to the improvement of electricity meter configuration and promptly install and complete the missing meters according to the development of the power grid. (2) Control the error of electricity meters and improve the metering accuracy. There are many factors that cause the metering error of electricity meters, such as the error of the metering device itself, the error of the current transformer and voltage transformer circuit, the error caused by environmental changes, and the error caused by improper installation and use. These have a great impact on the statistical line loss of the entire power grid, especially the gate meter, the assessment meter and other meters. We should strengthen the supervision of the small indicators of line loss management, and do a good job in error analysis, control and verification of the imbalance of bus power and the completion of station power consumption indicators. (3) Unify the meter reading time. With the continuous increase in the number of unmanned substations in the regional power grid, if the meter reading time of the relevant electricity meters of each substation is inconsistent, or even if the reading interval between meters is long, it will inevitably cause the electricity statistical time to be inconsistent, resulting in a large error and losing the meaning of line loss statistics and analysis. Therefore, solving the problem of unified meter reading time is an important foundation for doing a good job in line loss measurement. Line loss statistics for the entire power transmission and transformation system can rely on the dispatch automation system and RTU device to realize communication between the electricity meter and the RTU device, freeze the electricity meter reading at the same time and then transmit it to the dispatch center, or establish a dedicated remote meter reading system to ensure the simultaneity of meter reading. If the hardware conditions are not available, the meter reading system can be formulated and standardized, and sufficient personnel can be assigned to unify the meter reading time. (4) Strengthen the classification, zoning and substation line management assessment of line loss. Classifying, zoning and substation line management of line loss statistics and analysis is the basic work for deepening line loss management. It is conducive to analyzing the reasons for line loss rise and fall, checking the effect and role of loss reduction measures, and timely discovering problems in line loss management. For example, after classifying and stratifying the line loss of a county's 110 kV power grid, it was found that the local power grid had problems such as insufficient reactive power compensation, high transformer loss, small local grid-connected electricity and large line loss of some 35 kV lines. After sensitivity calculation and analysis, the problems were found, which is conducive to taking targeted measures. [b]6 Conclusion[/b] Reducing line losses is a long-term task for power supply companies, which helps improve their economic efficiency. Comprehensive statistical analysis of line losses, relying on scientific calculation and analysis to study the specific components of line losses, identifying the main causes of high line losses, taking practical and feasible measures, establishing a sound line loss management system, and conscientiously implementing it are the main contents of reducing line losses. When carrying out power grid transformation, a comprehensive economic analysis, including line losses, should be conducted to determine the optimal solution. Under the existing power grid operation mode, priority should be given to the hierarchical, zoned, and local balancing of reactive power to reduce line losses caused by reactive power flow. At the same time, line loss management should be strengthened to reduce managed line losses. [b]References:[/b] ［1］ Chen Zhangchao, Tang Deguang. Urban Power Grid Planning and Transformation［M］. Beijing: China Electric Power Press, 1998.