Since the 12th Five-Year Plan, smart grids have received more and more attention from the state. As an important part of the smart grid, digital smart substations have given new vitality to traditional substations. At present, China has mastered the construction of intelligent substations with multiple voltage levels such as 110kV, 220kV, 330kV, 500kV and 750kV. Since 2009, China has begun to pilot digital intelligent substation in China; in 2012, it has entered the stage of comprehensive construction of intelligent substation; it is planned that by 2015, the intelligentization of new substation will reach 40%, and 10% of the original substation will be transformed into Digital intelligent substation.
1. Overview of transformer protection system
Compared with the traditional power station, the digital intelligent substation realizes the integrated management of the substation by using electronic communication and artificial intelligence technology, and can complete a series of functions such as fault diagnosis and decision analysis of the equipment, for the state evaluation and diagnosis of the power system, solar wind energy. The introduction of etc. provides a strong support. From the perspective of system composition, digital intelligent substation can be divided into station control layer, interval layer, process layer, interval communication network, process communication network, and five parts form a three-layer two-network system. Transformer relay protection system is an important part of substation relay protection system. It is usually a digital circuit based on microcomputer. The core component is CPU and the software system is real-time processing program.
2, transformer fault diagnosis research
In the case of ignoring the transformer loss, it is known from Kirchhoff's law that the current flowing into each node should maintain the vector and constant zero, but the internal fault of the transformer is equivalent to the internal increase of a faulty branch, the current vector sum of the faulty node. Not at zero, this should be diagnosed.
The fault of the intelligent transformer can be divided into two parts: internal fault and external fault. The internal fault refers to the fault in the transformer tank, which mainly includes: phase-to-phase short circuit, turn-to-turn short circuit, single-phase grounding and other faults; external fault refers to faults on the insulating sleeve and the lead-out line. The internal fault diagnosis of digital intelligent transformers is mainly focused on transient analysis. The key to using transient analysis of transformer internal faults is the determination of the short-circuit leakage inductance parameters.
3. Transformer relay protection system
3.1, primary protection
Digital intelligent substation transformer main protection is divided into differential protection and gas protection. According to Kirchhoff's law, when the fault occurs inside the transformer, the differential current is very large. When there is power supply on each side of the transformer, the differential current is very small. When the differential current is greater than the unbalanced current, the circuit breaker is open and the protection starts; the transformer externally occurs. When the fault occurs, the differential current is small, the unbalanced current is greater than the differential current, and the protection does not start. Therefore, the operating current of the differential element is generally greater than 4 to 8 times the rated current of the transformer.
3.2, backup protection
The digital intelligent substation transformer backup protection can be divided into four types: composite voltage overcurrent protection, zero sequence overcurrent protection, neutral point gap protection and overload protection. Microcomputer protection uses deadband-free, memory positive-sequence voltage directional components to control the positive direction of the entire protection process. If the protection is adjacent, the positive direction is the transformer pointing bus; if it is the backup protection of the transformer, the positive direction is opposite.
The zero-sequence overcurrent protection is generally installed at the neutral point of the transformer above 110kV. The zero-sequence overcurrent protection of the large transformer is generally three-stage protection, and only the last segment is non-directional. Neutral point gap protection is generally used in transformers where the neutral point is not grounded. Overload protection is generally divided into three steps: sending a warning signal, turning on the cooling fan, and turning off the on-load voltage regulation.
3.3. Substation on-site commissioning
Transform the traditional substation transformer and obtain the modified secondary circuit wiring of the digital intelligent transformer. During the on-site commissioning process, attention should be paid to verifying and testing the protection and testing the drive with the switch. The protection action time is an important indicator to measure the performance of the protection device. The modified system is tested for protection action time to see if it meets the requirements. Compared with the traditional short circuit breaker, the intelligent circuit breaker reduces some intermediate links, greatly shortens the protection action time and makes the transformer differential protection more rapid.
4, the conclusion
As an important part of the smart grid, the digital intelligent substation has given new vitality to the traditional substation. It minimizes the number of transformer failures and reduces the workload of the central control personnel. Starting from the difference between digital intelligent substation and traditional substation, this paper first introduces the working process of transformer relay protection system. The steps to determine the leakage inductance parameter of the transformer between turns are discussed, and several limitations of differential protection are discussed. Then the transformer relay protection system was discussed. The main protection, backup protection and on-site debugging of the digital intelligent substation transformer were analyzed. It is hoped that it will play a positive role in the future transformation of the digital intelligent substation.
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