Weak feeders and high harmonic characteristics of wind farms connected to the power grid affect the safe operation of the system, especially the selectivity and reliability of traditional power frequency protection, which makes the transient protection of wind farms an important issue in the wind energy development and research. This paper reviews the key technologies of transient protection of offshore wind farm transmission lines, including the analysis of the fault characteristics of wind farm transmission lines based on two types of wind turbines (i.e., doubly fed asynchronous wind turbines and direct-drive permanent magnet synchronous generators), and compares with traditional ones. The comparison of the ground fault and short-circuit characteristics of synchronous power systems and the research status of transient protection of offshore wind farm transmission lines are made. On this basis, the protection of wind farm transmission lines is discussed.
With the continuous development of renewable electricity generation technology, wind energy has occupied a vital position due to its mature technology and other advantages (O’Shaughnessy et al., 2021; Telukunta et al., 2017; Wang et al., 2022). Offshore wind farms mainly adopt centralized access methods to connect to the grid. However, after wind power is connected to the grid, its randomness, intermittency and instability will affect the safe operation of the system, including the relay protection is the most important (Gao et al., 2021). The fault characteristics of wind farm grid are very different from those of traditional power grids (Bi et al., 2014; Prasad et al., 2020; George and Ashok, 2021), which leads to abnormal operation and decreased sensitivity of traditional frequency protection. in wind farm connected systems and other issues (Jia et al., 2018; Xu et al., 2021). In addition, related studies show that the data collected as the main protection of transmission lines such as longitudinal difference protection, distance protection and directional longitudinal difference protection are usually frequency voltage and current, and the phase extraction of frequency quantities based on the Fourier equation algorithm is no longer accurate (Swetapadma and Yadav, 2018; Guillen et al., 2020). In the literature (Wang, 2016), it is shown through theoretical analysis and simulation that the protection based on distance protection and directionality of positive sequence short-circuit components is no longer suitable for large wind farm transmission lines.
In addition, the transmission lines of the offshore wind power plant are high-voltage transmission lines. If they are not eliminated in time after the fault, the system will operate with faults for a long time, which may lead to the expansion of the accident. must be eliminated quickly and reliably. In addition, after the occurrence of a fault, the high-frequency transient values contain extensive information about the characteristics of the fault, and the protection based on transient values requires a short time window and high speed of action. Therefore, the study of transient protection schemes is a modern direction of relay protection development.
This paper first introduces the main transient characteristics of doubly fed wind turbines and direct drive permanent magnet wind turbines after transmission line faults of wind power plants, and summarizes the selection principles of transient protection for wind power transmission lines, and then summarizes the characteristics of wind power plants; on the transient protection, the current research status of transmission line faults is analyzed, and the transient protection characteristics of wind power plants are analyzed, finally, suggestions are put forward for the research of transient protection of transmission lines in large wind power plants in the future;
The intermittency and randomness of wind energy cause changes in its internal control system, which affects the fault characteristics (Li et al., 2018a; Alejandro and Joaquin, 2019), and the analysis of transient characteristics after fault is the key to the transient protection of wind power systems. Research background. The main renewable energy sources currently used in the market can be divided into doubly fed sources and inverter power sources, represented by direct-drive permanent magnet wind turbines (Haj-Ahmed et al., 2018). In particular, the transient characteristics after transmission line failure of wind power plants based on these two types of wind turbines will be analyzed below.
Compared with traditional synchronous generators, doubly fed asynchronous wind turbines do not have a separate excitation winding. Research shows (Chen, 2019) that the transient electric potential remains almost unchanged for a short period of time after the fault of the doubly fed wind turbine. The fault of the doubly fed wind turbine is observed, and the rotor flux linkage remains unchanged after the fault. It satisfies the principle of flux conservation for a short time and changes slowly. In practical applications, the “crowbar” protection usually delays the input signal for a few milliseconds (usually more than 5 ms) (Zheng et al., 2014). However, after the doubly fed fan controller detects the fault, harmonics will appear in the transient current, and the waveform is relatively complex (Li et al., 2020), and the duration of the transient equivalent potential inertia is very short. Considering the converter delay on the rotor side of the dual-fed wind turbine and the response time of the control system, it can be assumed that within 2 ms after the damage, there is an inertia of the transient potential of the dual-fed wind turbine.
Compared with doubly fed wind turbines, permanent magnet synchronous generators do not have a step-up gearbox, which avoids many adverse effects and improves the stability of the system. The short-circuit current characteristics of power supplies containing inverters are significantly different from those of conventional synchronous generators (Biswas and Nayak, 2021; Dakic et al., 2021; Liu et al., 2021). After a three-phase fault occurs, the phase fault current is a superposition of non-frequency damped sine waves and fluctuating non-frequency sine waves. The transient characteristics in the initial stage of fault are related to the internal control after grid connection, while the fault characteristics after grid connection are mainly affected by the grid-connected power of the wind farm (Li et al., 2019; Zeng et al., 2019). In the AC grid of the offshore wind farm, multiple resonant frequencies may occur, resulting in increased harmonic losses in the power system, which affects the transient response of the system after fault (Wang et al., 2020). When studying the influence of the characteristics of direct drive wind turbines on the transient response of the system after grid connection, it is believed that the transient model of direct drive wind turbines is too complex and has a high order (Xu et al., 2015). Generally, the transient model of direct drive wind turbines is simplified (Kunjumuhammed et al., 2017), and the converter is replaced by a controllable source to more accurately reflect the output characteristics of the wind turbine and reduce the complexity of the analysis process.
In addition, for the connection of offshore wind power through the flexible direct-sequence converter station of the grid-connected system, both the wind power plant side and the grid side are supplied with electronic power, which cannot provide a stable fundamental frequency short-circuit current, and the internal control characteristics of the converter will determine its short-circuit characteristics when there is a fault. At this time, the wind power converter only provides positive sequence current, and the fault current generally does not exceed the current withstand limit of the amplitude-limited power electronic converter, thereby exhibiting the characteristics of a current source. The short-circuit current generated by the flexible converter contains both positive sequence and negative sequence components. The short-circuit current generated by the wind power converter is included in the short-circuit current of the flexible converter. Therefore, in order to prevent the amplitude of the short-circuit current from exceeding the capability of the power electronic device, the flexible converter must have a fast blocking capability so that the flexible converter exhibits the characteristics of a voltage source after a fault.
Therefore, the transient response of wind power transmission lines based on the analysis of two wind turbines is the basis for studying the transient protection after wind power is connected to the grid. Wind power systems contain a large number of power electronic devices, in which the control strategy and system topology inside the converter, as well as the network capacity, will affect the transient response after a fault. Moreover, the access to wind power plants complicates the electromagnetic transient process of the power system, and harmonic oscillations may occur. In traditional power systems containing only synchronous generators, there will be many problems in analyzing the electromagnetic transient processes and separating the fault processes.
The main transmission methods in offshore wind farms include high-voltage alternating current (HVAC) transmission and high-voltage direct current (HVDC) transmission (linear converter-conventional converter (LCC-HVDC) and voltage source converter-HVDC (VSC-HVDC)) (Lin and Chao 2010; Li et al. 2018b; Chang et al. 2018), as shown in Figure 1. Most types of equipment such as pumping stations of offshore wind farms are unattended. At present, the relay protection equipped with electrical equipment must be able to quickly and reliably eliminate faults to prevent them from expanding to a larger scale and causing more serious consequences.
Figure 1. Offshore wind farm using converter-type voltage source (VSC-HVDC) and high-voltage alternating current (HVAC) transmission.
All offshore wind farms in the UK currently use AC fusion and transmission to connect to the Internet, including the largest offshore wind farm London Array (630 MW) and Hornsea Project One (the largest offshore wind farm currently under construction in the world), via three 142 km long 220 kV AC submarine cables). The first real offshore wind farm in my country is the 200 MW Jiangsu Dongtai Offshore Wind Farm. Electricity is transmitted from the offshore compressor station to the central control center on the road via 220 kV AC submarine cables. It can be seen that AC cables are the main transmission lines of the offshore wind farms built so far.
Current research on the transient value-based protection of offshore wind AC cables mainly focuses on the protection of AC cables in collector systems. For example, in the literature (Li et al., 2020), the singular value decomposition method is used to protect AC lines, which can quickly identify inside and outside the zone of faults. The identification time is less than 5 ms. This method has reference value for the protection of AC cable transmission lines. fault detection (Zheng et al., 2020). There are few studies on the protection of offshore wind AC transmission lines, and some studies on the positioning methods of offshore wind AC cables can be used as reference. For example, related studies have proposed a fault location method for submarine surveillance networks based on the time difference between multiple terminals (Zheng et al., 2020). This method uses intelligent branching units (BUs) to locate the faulty line by comparing the time difference. wave arrival matrix before and after the fault, and this method is easy to implement and accurate in positioning, which ensures the safe and reliable operation of the seabed observation network power system. In order to adapt to the grid connection of large-capacity offshore wind farms, the unified power flow controller (UPFC) is often used for compensation. The performance of the traditional distance protection scheme is affected by the nonlinear fluctuation of its output power with an unpredictable nature. wind speed and different characteristics of the UPFC. The influence of the working mode in which the electrical quantity used is the three-phase transient current (Jia et al., 2019).
The proportion of offshore wind farms connected to the grid via DC transmission is 30%. Most of the offshore wind farms built in Germany use flexible DC transmission to connect to the grid. The real offshore wind power projects in China are being built in China. 2021, for example, the flexible direct transmission wind power project in the East China Sea. When the offshore wind farm transmits power via flexible direct transmission, the fault characteristics of its converter exhibit the characteristics of a controlled voltage source, and the rapid blocking of the DC converter will limit the amplitude of the fault current and change the fundamental frequency characteristics. be short-term, affecting the correct detection of faulty lines by relay protection.
Considering that the distributed capacitance of cable lines in offshore wind power systems is large, the literature (Adetokoun and Muriithi, 2021) uses the forward and backward traveling wave amplitude ratio distributed parameter model to determine the fault direction based on the traveling wave transmission characteristics. This method is not only immune to transient effects. It has the influence of distributed capacitive current and is suitable for offshore wind transmission lines. To detect the faults of multi-terminal DC systems connected to offshore wind power, the literature (Cao et al., 2020) proposed a fault detection method based on the average value of transient current, which improved the transient equivalent model of the radiating multi-terminal DC system and simplified the fault analysis scheme that calculates the short-circuit current in the initial stage of fault, and the model accuracy is high, based on which the protection scheme is realized using transient averaging. This method has lower sampling frequency requirements. Each collected sample point only needs to be added, subtracted, multiplied and divided twice, making it suitable for real-time calculations. When a fault occurs in the actual transmission of wind power over DC lines, the system typically needs to act quickly within a few milliseconds to ensure reliable fault clearance. However, it can be hampered by lightning strikes and other factors, and its reliability has yet to be tested.
For the transmission line of offshore wind power plants, whether it is AC cable or DC cable, there is little research on the corresponding transient protection. With the development of new grid connection technology, the grid connection technology of onshore wind power has been improved, while the grid connection technology of offshore wind power has not yet matured. In terms of offshore wind power, wind power plants are more susceptible to environmental influences than onshore wind power plants, have a larger installed capacity, and have larger short-circuit current fluctuations after faults. It can be seen that in order to integrate offshore wind power into the grid, it is necessary to conduct simulation and analysis based on the actual operating mode of the sea and the characteristics of its wind turbine.
This paper discusses the transient protection of offshore wind farm output lines, including the transient response of doubly fed wind turbines and direct-drive permanent magnet wind turbines after grid-related faults, combined with the transient protection research of existing wind farm feeders. Progress is made, the feeder protection of large wind farms is reviewed, and relevant research directions are suggested, so as to provide reference information for the research on relay protection technology after wind power is connected to the grid.
The protection of transient values when connecting wind power plants to the power grid requires further research. Future research could be conducted on the following aspects:
1) With the continuous increase of wind power capacity, continuous increase of transmission line length, and the influence of physical geography and other factors, it is necessary to pay attention to the nonlinear and time-varying characteristics of converters in power electronics and the wideband communication caused by interference. The oscillation problems threaten the stable operation of the system and the temporary protection of the lines from lightning interference.
2) Further strengthen the research on the control technology of flexible DC transmission or multi-pole flexible DC transmission. The input of flexible transmission equipment is a means of long-distance transmission, and reliable and flexible control technology is a prerequisite for line protection. The short-circuit current characteristics are closely related to the control response according to the control strategy; Study the short-circuit current transient characteristics of the wind turbine converter control strategy and the DC control technology under the mutual influence, and use the transient information in the whole fault process to develop new principles to meet the needs of fast line protection.
3) Study the whole process of converter control and adjustment after fault, ensure that the control is based on the principles of simplification and voltage reduction, and take into account the dynamic factors in the wind turbine that have a significant impact on electromagnetic transient processes. Building a more accurate transient process model will be a difficult task for future transient protection research, so it is especially important to build a more detailed and realistic simulation model to illustrate the reliability of the adopted protection scheme.
4) Wind power plants can explore and improve the comprehensive protection criteria based on transient protection devices and improve the existing protection. Large-scale sensing technologies, multi-agent technologies and intelligent algorithms can be used as applications in the field of comprehensive protection of power grids. When the conditions are ripe, more pilot studies should be carried out to put the developed protection prototypes into field testing. In addition, it is also necessary to consider the relevant issues of wind power feeder lines with shunt reactors and the impact of reactive power compensation devices with different performance on the fault phase recovery voltage and potential feeder current to study fault identification schemes suitable for wind power feeder lines (Wang et al., 2018).
XS: Conceptualization, Writing – review and editing; XY: Writing – Preparation and research of original draft. JC: Supervision. XJ: Visualization and participation in discussions on the topic;
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
All claims expressed in this article are solely those of the authors and do not necessarily reflect the views of their affiliates, publishers, editors, or reviewers. Any products evaluated in this article, or claims that may be made by their manufacturers, are not guaranteed or endorsed by the publisher.
The authors sincerely thank the National Natural Science Foundation of China (52037003 and 51007035) and the Yunnan Province Major Project (202002AF080001).
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Post time: Dec-13-2024