This chapter introduces the transmission layer and substation layer of the power transmission system in detail. Let everyone understand the specific content of the transmission layer and substation layer of the power transmission system.
1.1 Transmission layer
The power transmission system is a network composed of three-phase lines, and the voltage of the three-phase lines during operation is usually 115~765kV. The capacity of each line is 50~2000MV.A. “Net” means that there is more than one electrical path between every two points in the system. The reason for this layout is because of reliability and work flow-if any component (line) fails, there are other alternate paths, and the power flow (probably) will not be affected.
In addition to the function of transmitting electric energy, the largest component of the power transmission system, namely the main transmission line, was designed, at least in part, to meet the needs of system stability. The transmission line establishes a strong electrical connection between the generators, and each generator can be synchronized with the system and other generators. This arrangement allows the system to operate and run smoothly when the load fluctuates, and can smoothly increase the load when the generator fails—this is called operational stability (a large number of equipment in the design of the transmission system and most of the cost are used for Maintain stability, not purely or mainly for the transmission of electrical energy).
1.2 Secondary transmission layer
The secondary transmission line in the system obtains electrical energy from the transmission exchange station or power plant, and then transmits the electrical energy to the substation along its path. A typical secondary transmission line can power 3 or more substations. Usually, part of a power transmission system-a large number of transmission lines, at least part of which is designed to meet the needs of stability and power transmission-also this is the case, the distinction between transmission lines and secondary transmission lines becomes blurred.
Generally, the capacity range of the secondary transmission line is 30~250MV, and the operating voltage is 34.5~230kV. Occasionally, abnormalities occur. The secondary transmission line is a part of the power system, and there is more than one path between any two points in the system. Usually, at least two transmission lines flow to a substation, so that when one of them fails, the other can continue to supply power to the substation.
1.3 Substation layer
The substation is the intersection of the transmission layer and the medium voltage layer. The transmission system and the secondary transmission system above the substation layer usually form a transmission and distribution network. In the grid, there is more than one power flow path between any two parts. However, it is very expensive to arrange a mesh structure between the substation and the user. Therefore, most power distribution systems are radial—only one path passes through the other layers of the system.
Normally, a substation occupies an acre or more of land, and all necessary substation equipment is assembled on this land. Substation equipment includes high-voltage racks, low-voltage racks, tidal current buses, transmission and transformation level circuit breakers, measurement equipment and relay protection equipment, measurement and control equipment control rooms, etc., but the most important equipment is the transformer, which provides rated power for the substation Capacity, which converts the incoming electrical energy from the transmission voltage level to the distribution voltage level.
The transformer capacity range is 10~150MV. It is usually equipped with a tap switching mechanism and control equipment to change the winding ratio in order to maintain the distribution voltage within a narrow range, even if there are large fluctuations at the transmission end. The transmission voltage variation can be as high as ±5%, but the distribution voltage can still be maintained in a narrow frequency band, perhaps only ±0.5%.
Usually, a substation has more than one transformer. Generally speaking, a substation has 2 transformers, some have 4 transformers, and occasionally there are substations with 6 transformers or more. Having more than one transformer can increase reliability-in an emergency, one transformer can handle a load higher than the rated load in a short time (for example, about 140% of the rated load, up to 4h). Therefore, the power transmission and distribution system can pick up the load in the power outage area during a short repair period and emergency situations.
The “scale” and capacity range of a substation equipped with 1 to 6 transformers are as follows: a small single transformer substation is 5MV.A, serving sparsely populated rural areas; a very large 6 transformer substation is 400MV.A serving large In densely populated areas of cities, the “scale” and capacity range of substations are somewhere in between.
Usually the power transmission and transformation planner talks about the transformer unit. The transformer unit here includes the transformer and all the necessary auxiliary equipment in its use—”1/4 of the equipment in the 4 transformer substations”. This is a better way to consider and estimate equipment costs in power transmission and transformation planning. The transformer itself is more expensive ($50,000 to 1 million), and the cost of the busbar components, controls, circuit breakers and other auxiliary equipment can be 2 to 3 times the cost of the transformer. Since the required equipment is directly proportional to the capacity and voltage of the transformer, and is only required for the addition of transformers, it is normal to treat the equipment and transformers together as a single planning unit, adding transformers and other equipment one by one.
Preparation is very important. Compared with the needs of electrical equipment, substations include more equipment and involve more costs. At the same time, it is necessary to purchase and prepare the site; excavate the site, lay grounding mats (to protect the underground cables of the substation from miscurrent in an emergency), and construct the power transmission towers for the installation of equipment foundations and control pipelines. It is necessary to increase the feeder out-pipes or lines to output electrical energy to the power distribution system.