[Abstract]: distributed access and centralized access also play an important role in the wind power well network. In this paper, two kinds of access modes are analyzed effectively, and the influence of wind power access on the relay protection of power grid is studied effectively.
[Key words]: wind power access; power grid construction; relay protection; impact strategy
1 research background
Wind power generation can not only adjust the structure of power supply, but also reduce the emission of pollution gas, and then effectively reduce the pressure on energy import, and improve the security of energy supply in China. However, due to the high cost of wind power generation, there is no way to achieve the same good results as thermal power generation. The proportion of wind power generation in China's power grid will be higher and higher. However, the randomness and uncontrollability of wind power itself have a great impact on the operation and management of power grid [1]. In order to better improve the ability of the grid to accept wind power, it is very important to effectively study the impact of wind power access on the grid relay protection.
2 overview of wind power access in China
China's wind power installation ranks first in the world's total wind power installed capacity, including seven provinces and cities including Gansu, Inner Mongolia, Hebei and Xinjiang, and more than eight ten million watt wind power institutions have passed the national review, and are developing with the development of the times. According to the statistics of the total installed capacity of wind power in China, the total installed capacity of wind power over 80 million kilowatts has accounted for four fifths.
With the increasing capacity of wind power plants, the whole wind farm will have an important impact on the operation of the grid. More and more experts have begun to pay attention to the study of power, voltage, power and other aspects of the wind power system, which will ultimately have an important impact on the stability of the whole system. There are still many problems in the process of using the relay protection in the wind farm access system. If necessary, it is necessary to analyze the internal problems and choose the right setting mode.
3 main ways of wind power grid connection
3.1 distributed access
Distributed access can connect the wind turbine to the load center in the nearby distribution network, which is the earliest traditional grid connection mode. Distributed access is often used in small wind farms [2]. At present, more than 80% of the wind power in Denmark is connected in the distribution network below 20kV. More than 70% of the distribution networks in Germany are connected to the power grid below 110KV scale. The distributed access mode is shown in Figure 1.
Figure 1 distributed access of wind power
3.2 centralized access
Centralized access also plays an important role in the development of power grid. High voltage from a long distance will be directly incorporated into the high-voltage transmission network. At present, China's wind energy resources are mainly concentrated in the remote western and northern regions, including Xinjiang, Inner Mongolia and the Qinghai Tibet Plateau. Not only the power load distribution of wind energy resource itself is very unbalanced, but also most areas will bear smaller power load and have smaller consumption capacity. In the process of practical operation, the development mode of "large scale, high concentration, high voltage and long distance" can be used to concentrate large-scale wind energy in the long-distance transportation of power grid.
Hundreds of wind turbines are effectively distributed in large wind farms, and their installed capacity can be hundreds of megabytes. In the process of planning the wind farm, the layout of the internal wind turbine, the capacity of a single wind turbine and the internal topography should be taken into account effectively. In addition, 7 or so wind turbines are combined into a group [3], and the power output from the wind farm will be directly delivered to the low-voltage bus of the wind farm through the wind farm. Figure 2 shows the main structure of the transmission network through the wind farm step-up transformer.
Figure 2 centralized access mode of wind power
5.1 the fault point occurs at the downstream of wind power connection
Protection 1. Due to the shunt effect of wind power, the fault current flowing through protected 1 will be reduced. The larger the wind power capacity is, the current will also be reduced, and the sensitivity of current protection will also be reduced.
Protection 2. Due to the influence of wind power augmentation, if protection 2 flows through the fault, the current will increase accordingly. As the capacity of wind power increases, so does the current flowing through it. After that, the fault current can be larger than the setting value of the current quick break protection in a short time. At this time, the current quick break protection will have problems in the use process.
Protection 3. The increase of wind power will have an impact, and protection 3 will flow through the fault, so that the current will increase, and the wind power capacity will also increase. The protection sensitivity of the whole device will be further increased.
Protection 4. Because it is not affected by the internal current of wind power failure, its protection action will not be affected.
5.2 location of fault point upstream of wind power connection
Protection 1. Because of the influence of wind power shunt, the fault phase current flowing through protection 1 will be reduced. If the wind power capacity is increasing, the current flowing through it will be reduced, and finally the time limit current at protection 1 will fail to operate.
The positions of protection 2, protection 3 and protection 4 will not be affected by the current generated in case of wind power failure, so the protection action will not be greatly affected.
6 impact of wind power access capacity on distribution network protection
When analyzing the existing distribution network system with wind power connected, it is also necessary to analyze the impact of wind power connected capacity on distribution network protection. If the amount of wind power capacity in the system is small, then the protection degree of the relay protection system will hardly change, so it is necessary to control the capacity in a small range for further study. However, if the number of grid capacity itself is large, when the wind power is connected to the grid, the system cannot ignore the fault current that may be generated in the middle [4]. Once the fault occurs, different short-circuit current can be felt at different protection points, and the output capacity will change constantly. It is necessary to discuss the impact of different capacity fans on the whole power grid according to the actual situation.
Using the internal power supply of the system to measure the whole short-circuit current will not have a great impact on the subsequent wind power connection. In the actual operation process, the short-circuit current of protection 2 can be generated according to the wind power connection. With the increasing of capacity and a certain range of values, protection 2 can misoperate.
7. Influence of line length on distribution network protection
It is also assumed that each point in the line is ABCD and the protection points are 1, 2 and 3. The different length of the line will affect the distribution network.
Changes in the length of the upstream line. First, determine the size of the internal capacity of the system and the amount of wind power access, and better determine the size value. At this time, the line length of BC section will change, and the setting value of protection 3 and the short-circuit current will change accordingly. When the length is increased, the setting range of three quick break protections will be continuously reduced, and the short-circuit current through protection 3 will be reduced accordingly. If the length of BC section continues to increase, under the condition of wind power connection, the value of fault current caused will remain at the original value level. If the value set by protection 3 is further reduced, the line system will malfunction. From the above situation, it can be seen that if the length of the line is always longer, the degree of protection misoperation will become more obvious.
Changes in the length of the downstream line. If the line length of CD section changes, the quick break setting at protection 3 will also be reduced. If there is a fault in the use process, the short-circuit current after the wind power is connected will be further reduced. If the line length at CD increases gradually, the fault current through protection 3 will be less than its fast cut-off fault setting value. In such a case, the protection process will make the system circuit avoid mal operation.
4 principles of distribution network relay protection
Time limited current quick break protection. This is a fast protection method with time delay. Generally, the safety of the whole line shall be ensured by limiting the fault of quick cut-off of current. At present, Section 1 of line current which plays the role of protection can also play a protective role, in order to cut off the fault beyond the range.
Definite time overcurrent protection. After the device is started, the action of limit protection will occur. The flowing current can effectively set the protection mode by avoiding the current within the maximum bearing limit, which is also a three-stage current section protection mode. In general, not only the grid protection action is small, but also can effectively maintain the length of the whole line, and then maintain the whole length of the next line, even in the whole grid system can play a good role in protection.
5 impact of centralized access on distribution network protection
When the wind power is connected to the distribution network, the initial set of operation will have different range of impact on the basis of current protection settings. In the process of actual operation, if the location of wind power access is not the same, different fault problems will occur, which will also have different impact on the power grid later. As shown in Figure 3, if the assumed wind power is connected at 10kV bus B, the impact of wind power connection on the internal current protection of the distribution network is analyzed.
Figure 3 Schematic diagram of influence of wind power analytical access on Protection
