2 Research background, purpose and significance Power transformer is the main electrical equipment in power plants and substations, and its safe operation is directly related to whether the system can work stably and normally. With the improvement of power capacity and voltage level, the cost of transformer is getting higher and higher. If it is damaged due to fault, it will take a long time to overhaul and cause great economic losses. Therefore, a safe, reliable and sensitive transformer protection scheme has always been a hot topic for power system scholars at home and abroad. The key problem of transformer differential protection is how to distinguish inrush current from internal fault. Many experts and scholars at home and abroad have done a lot of research on this and achieved many beneficial results.
In recent years, the misoperation of transformer differential protection caused by operation has attracted wide attention. On June 7, 2003, an unplanned power outage occurred in Huaneng Jinggangshan Power Plant. In the process of closing the outlet circuit breaker of #2 main transformer, the differential protection action of #2 main transformer led to the disconnection of # 1 generator and system, which was later found to be caused by the misoperation of transformer differential protection caused by harmonic inrush current. At present, due to the gradual operation of large-capacity transformers in different grades and regions, the local power grid structure has undergone fundamental changes, and the misoperation accidents of transformer differential protection caused by neutral point inrush in power system have been increasing, and the problem of neutral point inrush has attracted people's attention.
The characteristics of harmonic inrush current and closing excitation inrush current are not exactly the same, and the operating transformer itself is not faulty, and the direction is opposite to that of airdrop transformer. The peak value of harmonic inrush current first increases and then decreases, and the peak value appears at the same time as the adjacent transformer, which belongs to misoperation.
Born after the airdrop of adjacent transformers is completed for a period of time, it will not decay for a long time, which will easily lead to transient saturation of current transformers, and the cause of misoperation is more hidden. Previous studies have mainly focused on the generation mechanism, waveform characteristics and variation characteristics of transformer inrush current when voltage is restored after no-load closing or external fault removal, but rarely analyzed the influence of transformer inrush current on transformer differential protection when running in parallel or series. Therefore, it is necessary to deeply study the mechanism and characteristics of harmonic inrush current, reveal its essence, and then put forward practical measures to eliminate hidden dangers and improve the reliability of power supply.
summary data
History of transformer protection, 193 1 year r? e？ Cordray puts forward the ratio differential transformer protection, which indicates that differential protection is the main protection of transformer. 194 1 year, c? d？ Hayward first proposed the differential protection with harmonic braking. 1958, r.l.sharp and w.e.glassburn put forward the method of identifying transformer inrush current by using second harmonic, and realized it in analog protection. At the same time, they also put forward a differential acceleration scheme, which consists of differential acceleration, ratio differential and second harmonic braking, and continues to this day. The research of microcomputer transformer protection began in the late 1960s and early 1970s. 1969 rockerfeller put forward the concept of digital transformer protection for the first time, which opened the prelude to the research of digital transformer protection, and then o.p.malik and degens studied the digital processing and digital filtering analysis of transformer protection; 1972, skyes published the harmonic braking scheme of microcomputer transformer, which made the development of microcomputer transformer protection take a step forward. In recent years, the appearance of digital signal processor dsp not only improves the speed and accuracy of data sampling and calculation of microcomputer protection, but also changes the design scheme of microcomputer protection device and realizes complex algorithms in the protection device.
Power transformer is one of the most important electrical main equipment in power system, and it is the hub of power transmission. Large transformers are complex in structure and expensive in cost. Once damaged, the maintenance work is difficult and the economic loss is great. In recent years, with the development of power system and the improvement of voltage level, the application of large-capacity transformers is increasing. High-capacity transformer adopts twisted winding, which is prone to turn-to-turn short circuit, so the failure rate is relatively high. In order to ensure the safe and reliable operation of transformers, electric workers constantly analyze their operating characteristics, study new principles and methods, and improve the performance of transformer protection. Aiming at the inrush current problem in differential protection, various methods are actively studied at home and abroad to solve it, such as second harmonic braking, discontinuous angle, voltage braking, magnetic flux characteristic principle and equivalent circuit method. There are also some new disciplines and methods applied to transformer protection. With the rapid development of computer and network technology and the continuous production of high-performance microprocessor chips, the performance of microcomputer transformer protection devices is constantly improving, and the whole microcomputer protection system is developing towards integration, artificial intelligence, networking, protection, control, measurement and data communication integration and standardization.
3. The main research content of this paper.
1 expounds the basic principle of transformer differential protection, analyzes the reasons that may cause misoperation of differential protection relay, and briefly introduces some preventive measures.
The mechanism and characteristics of transformer inrush current are analyzed and studied, and the present situation and development trend of transformer differential protection are summarized. Various identification methods of transformer inrush current are studied, and analyzed and evaluated. The methods of eliminating inrush current and restraining inrush current are put forward.
Based on the characteristic that the inrush current deviates from the time axis, the generation mechanism and variation characteristics of the inrush current are explained, and it is pointed out that the essential reason of the inrush current is that the inrush current of the closing transformer flows through the system resistance, which makes the voltage of the working bus of other transformers shift and leads to the saturation of the iron core. The harm of harmonic excitation inrush current to transformer differential protection is discussed, and the corresponding preventive measures are put forward.
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