The power grid structure refers to the layout of power plants, substations and switching stations in the power grid, as well as the connection methods of the voltage power lines connecting them at all levels. The strength of the power grid is related to the safety and stability of the power grid operation, the quality of power supply and economic benefits.
1. Introduction to grid structure
The increase in demand for electricity requires an increase in power supply and a balance between supply and demand of electricity through the grid structure. Only through the power grid can the contradiction between the geographical distribution of energy resources and loads be unbalanced, and the power resources far away from the load center can be more rationally utilized. The large power grid can make different power plants such as hydropower, thermal power and nuclear power operate in combination, give full play to their respective advantages, and achieve greater economic benefits; the large power grid can stagger peaks, use complementary load characteristics to reduce the total installed capacity, and is conducive to the use of high-efficiency and large-capacity power generation unit, speed up the progress of power supply construction, and save construction and operating costs. In short, planning and constructing a good power grid structure and giving full play to the technical advantages and economic benefits of large power grids are the direction of the development of the electric power industry.
The expansion of grid scale, especially the emergence of high-voltage, large-capacity, and long-distance power transmission systems, has significantly increased the technical complexity of grid construction and operation. The power grid is a capital and technology-intensive comprehensive project, and its economic service life is as long as decades. Whether the grid structure is reasonable or not will have a long-term impact on the technical performance and economic benefits of the grid. In short, grid structure planning must be carried out 15-25 years in advance, and implemented step by step according to the objective development of the grid, and constantly revised.
2. Tasks of grid structure planning
The task of power grid structure planning is based on the premise of future load forecast level and power supply expansion status, to determine the expanded transmission lines and the number of circuits, so that the power grid structure can meet the required transmission capacity in the specified planning year, ensure the load power consumption, and the operation performance can meet the requirements. Some technical standards, while minimizing construction and operating costs, the best economic benefits.
3. Specific requirements for grid structure planning
The conditions for the normal development of the power grid are: long-term power supply planning; a safe, economical, and flexible power supply network structure; active power and frequency control, reactive power and voltage control to ensure power quality; Protection and safety automatic devices; monitoring system and command system adapted to the normal production of the power grid. The tasks and technical issues of grid structure planning mainly include:
3.1 To meet the reliability of power supply to the load. Transmission lines, power transmission and transformation equipment, etc. are not under load during normal operation, and can still provide continuous power supply if a single component fails.
3.2 Satisfy the joint operation of water, thermal and nuclear power, rationally utilize hydropower, reduce waste water, give full play to the benefits of power generation equipment that has been put into operation, and do not waste electricity when each plant is fully generating power.
3.3 The voltage level is reasonable. The required high-level voltage appears in a timely manner, the power transmission capacity of the network is improved, and the short-circuit current is limited within the range compatible with the equipment.
3.4 The network structure is reasonable. It has sufficient stability, can effectively isolate faulty components, and reasonably stratifies and partitions according to voltage level and partition according to power load demand.
3.5 Necessary Flexibility. It is convenient for staged construction and provides adaptability to unforeseen changes in load and power supply. Adapt to the needs of networking, and provide convenience for the interconnection grid to exchange power, electricity, accident support, meet the stability requirements of the interconnection system and the development of interconnection.
3.6 It is convenient for scheduling and management, reflects the requirements of the scheduling management system, and provides favorable conditions for the modernization of scheduling management.
3.7 Optimizing the scheme, trying to save network construction investment, reduce loss and operating costs.
4. Transmission planning depends on the results of power planning
Power supply planning is the core of power system development. According to the national energy policy, development plans for various power sources should be formulated and optimal choices should be made. Hydropower stations with good economic and technical indicators and excellent construction conditions should be developed first. In particular, hydropower stations close to the load center should be developed first, because hydropower is a primary renewable energy source, has good performance in frequency regulation and peak regulation, and is an important backup power source in case of accidents. According to different periods and tasks, the power supply can be divided into base load, waist load, peak load, frequency modulation, etc. Build large-scale thermal power plants at mine ports, ports, and intersections according to energy supply channels, and connect them with high-voltage power grids.
At present, my country's power construction lags behind the needs of economic development, energy supply is tight, and funds are in short supply. Therefore, it is economical and reasonable to build large-scale hydrothermal power plants, use high-efficiency large units, and use high-voltage transmission to speed up power construction and save infrastructure investment. Therefore, it is very important to make an optimized power supply development plan from the perspective of overall load growth forecast and stage-by-stage balance of power supply. Only when the power supply plan is optimized can a corresponding optimized power transmission plan be made.
According to the horizontal year of load forecasting and power supply planning, the horizontal year of transmission planning is determined, usually 15-30a is selected as the horizontal year of planning, and the transmission network is planned and laid out according to the load development level and power supply construction progress of the horizontal year. The benefits of choosing a longer planning level year are:
4.1 It can facilitate the step-by-step transition and implementation of the transmission network during the development process, and maintain the optimal structure.
4.2 It can be foreseen whether and when a higher level of voltage should appear during the planning period, so as to propose topics and requirements to the scientific research and equipment manufacturing departments in advance.
4.3 It can make the network construction have a realistic arrangement.
5. The voltage and transmission mode of the transmission line
The transmission line adopts DC or AC transmission, how high the voltage is, whether a higher voltage is required, and whether one or two voltages are used for the outgoing lines of the new power plant. This is an inevitable problem in the transmission planning work, and it is also closely related to the network structure. Related questions. For long-distance and large-capacity power transmission, the outgoing lines of extra-large hydropower plants and large thermal power plants or hydropower plant groups in energy bases should be analyzed and compared with multiple voltage transmissions, such as the Three Gorges Hydropower Station on the Yangtze River to the East China Power Grid, and the Zhungeer Coal Power Base to the East China Power Grid. North China and Northeast power grids have compared schemes for 500kV and 750kV AC transmission and HVDC transmission. Generally, the difference between the higher level voltage and the highest voltage level of the existing network is 2-3 times more appropriate. If the level difference is too small, the increase in line transmission capacity is also small, and the benefits obtained from the increase in power transmission capacity may not be enough to compensate for the cost of the additional substation for the interconnection of the two levels of voltage; The power is very large, and the number of low-voltage substations matching the transmission capacity of high-voltage lines will inevitably increase, and the cost will also be expensive. Only when the level difference is selected reasonably can better economic benefits be obtained.
The selection of transmission network voltage and when higher voltages appear should also consider the following limiting factors:
5.1 The power transmitted by the primary line should not be too large, especially the line that transmits power to the receiving-end grid with a single-circuit line should generally not exceed 10%-15% of the load power of the receiving-end system, so as to avoid endangering the receiving-end grid due to line fault tripping Safe operation, in special cases, corresponding safety measures should be planned.
5.2 There should not be too many outgoing circuits of main large power plants. If there are too many outgoing lines, it means that the voltage of the outgoing lines is selected too low, and the voltage suitable for the capacity of the power plant should be selected as much as possible.
5.3 There should not be more than 2 voltages for the outgoing lines of a power plant. The highest level voltage of the power grid of a large power plant should be directly sent to the main network as much as possible to avoid 2 levels of voltage boosting.
5.4 Limitation of line corridors and short-circuit current. Raising the voltage level of transmission lines is usually an effective measure to save corridors and limit short-circuit current, which needs to be determined according to economic and technical rationality.
Selection of transmission mode. With the development of DC transmission technology, DC transmission has gradually become an integral part of the transmission network. AC and DC transmission have their own characteristics, and should be selected and applied according to their characteristics.
The main way of power transmission is three-phase AC transmission, but in some occasions, such as the interconnection of two power grids with different frequencies, only DC transmission can be used. For long-distance high-power transmission, technical and economical arguments are required to determine the transmission method. The capacity and voltage of the DC line can generally be determined according to the conditions of the connected AC system, but in the AC-DC parallel transmission system, the optimal capacity of the single-circuit line of the DC transmission line is determined by the stability of the system operation. Therefore, after the preliminary completion of the planning scheme, it is necessary to further use stability calculations to check whether its capacity is reasonable, and to make the best capacity selection. In the AC-DC parallel system, the relationship between the modulation effect of the DC line and the network status is more complicated, so Its modulation effect is generally not included in the planning stage, and the effect of DC modulation on system stability is checked in the operation stage.
6. Planning principles of grid structure
The grid structure is the material basis to ensure the safe and economical production of the grid. Power grid layering and partitioning is the principle that must be followed for reasonable production management and scheduling automation. It is a network condition for reasonable voltage level coordination and power supply and load balance. The structure of the backbone network should be comprehensively planned and arranged. Formation, so that the layout is reasonable, layered and partitioned, and the primary and secondary are clearly defined.
With the development of the economy, it is required to balance the power supply and load demand. The grid structure should be planned according to several planning levels and implemented step by step. For some key technologies, transmission voltage levels, transmission methods and UHV transmission issues, etc., it should also be implemented in the grid. be taken into account in structural planning. The power grid structure is based on the power supply planning, and implements the principle of stratification and division according to different periods. Various power sources are connected to power grids with different voltage levels. And due consideration shall be given to the power consumption of tie lines and accident support.