EHV transmission line refers to the use of 500 kV - 1000 kV voltage level to transmit electric energy. If the 220 kV power transmission index is taken as 100%, the relative investment per kilometer of EHV transmission line, the relative cost per kWh power transmission of 100 kilometers, and the consumption of metal materials will be greatly reduced, and the utilization rate of the line corridor will be significantly improved. improve.
1. Introduction to EHV transmission line
Power is delivered using extra high voltage levels. extra high voltage refers to the voltage level of 330 kV to 765 kV, that is, 330 (345) kV, 400 (380) kV, 500 (550) kV, 765 (750) kV and other voltage levels. Extra high voltage transmission is an inevitable requirement for the increase of power generation capacity and power load, and the extension of transmission distance. EHV transmission line is one of the important symbols of the development level of the electric power industry. With the extensive development of electric energy utilization, many countries are building large-capacity hydropower plants, thermal power plants, nuclear power plants, and power plant groups, and the power resources are often far away from the load center. Only by using extra high voltage power transmission can the power transmission task be effectively and economically realized. EHV transmission line can increase the transmission capacity and transmission distance, reduce the project cost of power transmission per unit power, reduce line loss, save the area of line corridors, and have significant comprehensive economic and social benefits. In addition, the interconnection between large power systems also requires extra high voltage transmission to complete. If the 220 kV power transmission index is taken as 100%, the relative investment per kilometer of ultra-high voltage power transmission, the relative cost per kWh power transmission of 100 kilometers, and the consumption of metal materials will be greatly reduced, and the utilization rate of line corridors will be significantly improved. improve.
2. History of EHV transmission line
In 1952, Sweden first built a 380 kV EHV transmission line from Hashpronge to Halsberg, with a total length of 620 kilometers and a transmission power of 450,000 kilowatts. In 1956, the Soviet Union's 400 kV line from Kuibyshev to Moscow was put into operation, with a total length of 1,000 kilometers. It was boosted to 500 kV in 1959, and 500 kV power transmission was used for the first time. In 1965, Canada first built a 735 kV EHV transmission line. In 1969, the United States achieved 765 kV extra high voltage transmission. In terms of DC transmission, the Soviet Union built a ±400 kV EHV DC transmission line in 1965, and then the United States, Canada and other countries built a ±500 kV DC transmission line. China's first ±500 kV direct current transmission line - Geshang Line - was put into operation in 1989. In 1985, the Soviet Union built a ±750 kV line, from Ekibastuz to Tambov, with a transmission distance of 2,400 kilometers and a transmission power of 6 million kilowatts. It is the largest ultra-high voltage direct current transmission in the world.
The realization of extra high voltage power transmission needs to solve the following many technical issues:
2.1 Research on the dielectric strength characteristics of air and other media under extra high voltage operating conditions.
2.2 Reasonable design of insulation coordination and insulation level of transmission lines and transmission equipment.
2.3 Overvoltage (including internal and external overvoltage) prediction and protection.
2.4 Solve the stability problem of keeping synchronous generators running in parallel.
2.5 Voltage regulation and reactive power compensation under various operating modes.
2.6 Electromagnetic environment interference caused by EHV transmission lines, such as radio interference, TV interference, audible noise interference caused by corona discharge, and the impact of ground electric field strength on the human body. At present, extra high voltage transmission technology has matured and is widely used in many countries.
China first applied 330 kV transmission in 1972, and built 500 kV transmission line for the first time in 1981. As of 1987, more than 5,000 kilometers of EHV transmission lines had been built, and an ultra-high-voltage power system with 500 kV transmission as the backbone was gradually formed.
3. Relay protection of EHV transmission line
3.1 The importance of relay protection
EHV transmission lines are an important part of the power grid system. As the voltage level increases, the factors affecting the relay protection of EHV transmission lines will also increase. This is also the content that needs to be paid attention to in the relay protection of EHV transmission lines. Do a good job of relay protection. If a fault occurs, the relay protection device can cut off the connection with the fault area by itself and report the problem to the control center. If the fault does not occur in the zone, the design can be completed by taking no action. In general, after the relay protection of EHV transmission lines is realized, no matter what kind of operation state the power system is in or what kind of fault occurs during operation, the relay protection device can make correct judgments and minimize losses , to ensure the safe and stable operation of the power system.
3.2 Methods of relay protection
3.2.1 Power signal processing
For power grid protection, there is a certain relationship between it and related transient signals, and these signals have nonlinear and unstable characteristics. Before the relay protection is realized, the power grid protection needs to be processed under the action of Fourier. state signal, but in the process of using Fourier, it is found that this transformation method has certain defects and deficiencies, so it is necessary to complete the signal processing under the action of high resolution. In order to further improve the relay protection work, HH1 is applied, which effectively strengthens the transient signal processing capability. Through practice, it is known that with the application of the HHT method, not only can the judgment ability of the fault signal of the EHV transmission line be effectively improved, but also the noise can be eliminated in time, and the relevant staff can also know the fault in time.
3.2.2 Current differential protection
Through the research, it is found that various faults will be found during the operation of the power system. After the power system fault occurs, fault information will inevitably appear. The reason why using current differential to complete the relay protection of EHV transmission lines is mainly because it can protect more complex topological structures, and can also eliminate current components and obtain useful fault information from them. To use current differential to realize relay protection of EHV transmission lines is to install appropriate current sensing devices at both ends of the line and complete the connection. Normally, after a fault occurs in a protected circuit, the current in the normal part is the same as the fault current. Through the application of current differential protection, it can be found that the device not only has rich experience, but also can protect the current in zero-sequence state. Generally, after a fault occurs, the load current will bring certain negative effects. For example, after a short circuit occurs, a line fault will occur, and protection refusal will also occur.
In order to play the role of current differential protection, the protection scheme design should be done well. Because the fault component has high sensitivity, it is necessary to pay attention to the protection scheme design. In order to obtain the component signal for a long time, zero-sequence current can be used as backup protection way, and integrate it with the full current to realize the complementarity of the two. Only in this way can the shortcomings of various protections be effectively reduced. In addition, in order to understand the actual situation of the fault, it is necessary to focus on the full current protection. Only in this way can the relay protection work of EHV transmission lines be done well and the losses of power enterprises be reduced.
3.2.3 Adaptive current protection
To do a good job in the relay protection of EHV transmission lines, it is necessary not only to understand the fault type, but also to master the power operation mode. Only in this way can the current protection goal be realized. For grid operation, EHV transmission lines and power facilities are interrelated, and the equivalent impedance is relatively small. If the electromotive force is in a constant state, the load current value at the same point on the line will increase accordingly. Therefore, only after mastering the type of operation mode can the line current be detected, and only in this way can the current protection work be done well. In adaptive current protection, it is also necessary to clarify the type of fault and compare the fundamental waves before and after to determine the secondary value of the current. If a single-phase short circuit occurs, the current value of some phases may increase, while the current value of the remaining phases will not change. After a two-phase short circuit occurs, their current values ??will also increase, and the increase range will be the same. In addition, other Some will not change. Generally speaking, after the fault type is clarified, the faults that occur in the system will be positive and negative, that is to say, when the fault current passes through the place where the relay protection device is located, there will be a contrast in the direction, so the direction should be well controlled. Only then can the relay protection work be done well.