Principles of power transmission and transformation design

Principles of power transmission and transformation design

This chapter introduces the principles of power transmission and transformation design.
The layout of traditional power systems is largely affected by engineering decisions, which are designed to adjust the “truth” of certain transmission and distribution systems and the physical laws that determine their performance and costs, and work around them. The following are several principles of power transmission and transformation design.


1.1 Economies of scale in power generation
Without exception, all traditional power generation types have very large positive economies of scale. For any traditional power generation method and technology, a larger generator set or power station (which can generate more electricity) has more potential cost-effectiveness and economy than a smaller generator set of the same type and technology. For many types of traditional power generation, such as coal and natural gas-fueled steam power plants, diesel power plants, and nuclear power plants, the generators are basically Carnot cycle engines. The main reason for this economies of scale is that “physics supports large units.” “; Its design can reduce the proportion of heat loss. A 500MW combined cycle power plant fueled by natural gas is more efficient than a 50MW unit of the same type and technology. This natural physical advantage also applies to fuel cells, which is a real Carnot cycle device that oxidizes fuel instead of burning fuel and generates heat as a by-product of electrical products. For example, 500kW solid oxide fuel cells may be very efficient. , But the 500MW fuel cell power plant designed with the same technology is significantly more efficient. Similarly, solar thermal power generation technology, especially solar tower thermal power generation technology, also has significant economies of scale in the size of units and power stations.


Unlike other types of power generation, wind power and photovoltaic power generation have almost no physical economies of scale. Their material efficiency does not increase significantly with size changes. A 100MW photovoltaic power station is composed of about 100,000 photovoltaic panels, while a 1MW photovoltaic power station is composed of only 1,000 photovoltaic panels. All photovoltaic panels are equally efficient, so two photovoltaic power plants may be equally efficient. Similarly, a 150MW wind farm composed of 50 3.0MW wind turbines has the same natural physical efficiency as a wind farm composed of only 5 wind turbines. Nevertheless, due to the concentration of larger photovoltaic power plants and wind farms, their cost-effectiveness is actually higher than that of smaller photovoltaic power plants and wind farms. The construction and management of a wind farm consisting of 50 wind turbines is less expensive than the construction and management of 10 wind farms consisting of 5 wind turbines. The required labor and maintenance costs are slightly lower, and the resulting environmental and social aesthetics The total impact is lower. This is one of the considerations, not the only consideration for large-scale. The overwhelming factor is the combination of this factor and the thermal efficiency advantage.


However, due to the low total economies of scale (the sum of actual and physical) of wind power and photovoltaic power generation, modern power systems tend to distribute more wind power and photovoltaic power generation. Factors related to location, ownership, unique customer needs and needs, and similar issues often mean choosing a small facility—distributed generation, although it may be less than optimal. The key point about modern (distributed) power plants is that these types of power generation have economies of scale, but they are often not enough to surpass other important factors. But throughout the 20th century, this was not the case in most cases.


Therefore, the actual advantages of concentration, coupled with the advantages of physical efficiency brought about by large size, mean that the centralized construction of power generation systems in several large centralized power plants has important commercial value, even if the power is composed of scattered over a large geographical area. Thousands of thousands or even millions of users use it. Designers of traditional power systems try to incorporate the largest generator sets and power stations into the system as much as possible. Some factors do not support the construction of several centralized large-scale power plants. The first factor is reliability: so to speak, utilities don’t want to put all their eggs in one basket. Therefore, under normal circumstances, a large regional power system may have 12 or 24 power plants, all of which are large power plants, but no more than 8% of the total. As a result, the electricity produced by the power system at several “massive” outlets has to be distributed to hundreds of thousands of consumption points. The transmission and distribution system is a network composed of lines, facilities and equipment, which connect these large-scale power generation outlets to countless small consumption points to provide safe, reliable and economical sufficient electric energy.


1.2 Economic transmission of electric energy
The operating voltage cannot cope with any long-distance transmission of electric energy. The 120V/240V single-phase operating voltage used in the United States or the 250V/416V three-phase operating voltage used in European systems are not suitable for economically transmitting electrical energy for more than a few hundred yards. For any power transmission that exceeds the residential area level, these low voltages will cause unacceptably high power losses, severe voltage drops, and extremely high equipment costs.


The most economical way is to transfer large amounts of electrical energy at high voltage. The higher the voltage, the lower the cost per kilowatt, and the longer the distance of power transmission at a certain efficiency level. But the higher the voltage, the higher the capacity and cost of the transmission line. Regardless of the economic situation, high-voltage lines may be more economical than low-voltage lines in power transmission. However, people must understand that although the “huge economic scale” is always huge, if it is used to transmit a large amount of electric energy, it only needs to reach the economies of scale. Therefore, for a specific amount and distance of electrical energy, after measuring the overall material, labor, and operating costs during the trial period, there will be an optimal voltage level.


1.3 The cost of changing the voltage level
Changing the voltage level is expensive, but not too expensive, because it is carried out in the entire power system (the working content of the transformer), but the loss of voltage conversion is huge, and it is meaningless for power transmission.

Hierarchical structure of different voltage levels
Hierarchical structure of different voltage levels.


According to the principles of power transmission and transformation design, the layout of the power transmission system has been developed to best deal with demand, constraints and various factors. The overall concept is a grading system that gradually reduces the voltage level by increasing the number of components, as shown in Figure . Show. Electricity is scattered throughout the service area. It is transmitted in a smaller amount (along more separate paths) on lower-capacity devices until it reaches the user, and it is gradually transmitted toward the low-voltage direction. The key element is the concept of “low voltage and division”. For example, in a distribution substation, there may be 2 to 4 incoming lines with a voltage of 138kV, but the outgoing main feeder voltage may be 4 to 34kV.

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