A useful result of the hierarchical structure of the traditional power transmission system is that it can be seen as consisting of several different levels or levels of power equipment, as shown in Figure . Each level contains many basically similar equipment groups-the same nominal voltage, roughly the same capacity, performing roughly the same work-located in different parts of the public utility service area to cover the entire public utility service area, and in this The levels or the next highest level are interconnected. For example, all substations are planned and designed in roughly the same way and undertake roughly the same tasks. All substations are composed of roughly similar equipment that undertakes the same tasks: a large amount of electrical energy enters the substation, is stepped down by the transformer of the substation, and is output to nearby residential areas and districts. The line voltage of the output electrical energy is lower than the line voltage of the electrical energy entering the substation. Some substations may be “larger” than other substations in terms of book management and equipment-one substation may have a 450MV.A transformer and another substation has two, but fundamentally, all substations are all in the same way for the same reason Play the same function and hope to get the same result (economical, safe, and reliable local power transmission). These substations constitute the substation layer of the system. Their service areas are spliced together, each covering a piece of service area.
Similarly, the power input feeder of the substation is similar in equipment type, layout and tasks. All the power input service transformers serve the same basic tasks similarly and are designed with similar planning goals and similar engineering standards.
Therefore, electrical energy can be regarded as flowing “downward” through these different levels, from the generation level and large-scale grids to users. When electric energy is transmitted from the power plant (Power generation layer) to the user, the electric energy passes through the transmission layer, reaches the secondary transmission layer, then reaches the substation layer, reaches the medium voltage layer, and passes through the medium voltage layer to the secondary service layer, and finally at the secondary service layer Reach the user. Each layer transfers the electric energy obtained from the upper layer of the system to the lower layer of the system. In all cases, each transmission of electrical energy splits into several paths when transitioning to the next layer, or immediately after the transition.
Each layer is powered by the upper layer. In a sense, the upper layer is closer to electricity generation.
In the process of electric energy flowing from the power generation site to the user, the rated voltage level and the average capacity of the equipment decrease layer by layer. When the transmission line is in operation, the voltage is 69~1100kV, and the capacity is 50~2000MW. In contrast, when the distribution feeder is running, the voltage is 2.2~34.5kV and the capacity is 2~35MW.
Each layer has more equipment than the previous layer. A system serving hundreds of users may have 50 transmission lines, 100 substations, 600 feeders, and 40,000 service transformers.
Therefore, the net capacity of each layer (the number of units multiplied by the average size) rises in the process of power transmission to users. It is possible that the substation capacity of a power system is 4500 MV.A, but the feeder capacity is 6200 MV.A, and the installed service transformer capacity is 9000 MV.A. In order to meet the reliability and adapt to the diversity of local maximum demand, this feature of lower level, greater capacity is deliberately designed by most power systems. This is the Hierarchical structure of the traditional power transmission system.
The closer the electrical energy is to the user, the lower the reliability, because a large number of devices between the power source and this point may fail. Most power outages are caused by equipment failures that are relatively close to the customer (whether due to aging or damage from bad weather).
The net effect of the change in average size and number of units is that the total capacity of each layer is higher than the previous layer-in the service transformer layer of any utility system, the installed capacity (number of units multiplied by the average capacity) is relatively higher than that of the feeder System or substation system. In the process of power transmission to users, the total capacity rises because of the maximum load and the guidelines for manufacturing equipment to adapt to this feature.