Whether the transformer operates normally or not directly affects the production and living power consumption of users, and relates to the safety of electrical equipment. In order to ensure users to use high-quality and safe electricity, it is necessary to ensure the normal operation of distribution transformers. This paper discusses in detail the protection measures and precautions of distribution transformers in daily use.
Distribution transformer is a kind of static electrical apparatus which transfers AC electric energy by transforming AC voltage and current according to the law of electromagnetic induction in distribution system. It is usually installed in the electric pole, platform or distribution substation, and generally reduces the voltage of 6-10 kV to about 400 V for input to the user. Therefore, it is necessary for us to talk about the protection measures and precautions of distribution transformers from the perspective of protection configuration technology and daily operation management, so as to ensure the normal operation of distribution transformers and the safe use of electricity by users.
1. Protection configuration technology
1.1 Install lightning arrester protection to prevent lightning overvoltage
Distribution transformer is a very important equipment in the distribution network. Once a lightning stroke damage accident occurs, it will cause power failure, which directly affects industrial and agricultural production and people's lives. Therefore, if conditions permit, it is better to use lightning arrester for protection. In the system with ungrounded neutral point, two-phase valve type lightning arrester phase protection gap can also be used for protection. However, in the same distribution network, all gaps must be installed on the same phase conductor, which can save a valve type arrester, while not increasing the number of line trips.
The valve type arrester, pipe type arrester or protection gap protecting the transformer shall be installed as close to the transformer as possible, and the closer the distance is, the better the protection effect will be. Generally, it is required to be installed inside the fuse at the high-voltage side of the transformer. The grounding wire shall be connected to the metal shell of the distribution transformer and the neutral point at the low-voltage side for common grounding. When the transformer capacity is 100kY ˙ When A and above, the grounding resistance shall be reduced to less than 4 Ω as far as possible; When the transformer capacity is less than 100kV ˙ A, the grounding resistance is 10 Ω or less. When these two points are connected together, lightning strikes on the high-voltage side, and the arrester or gap discharges, the transformer insulation bears the residual voltage of the valve type arrester, while the voltage drop on the grounding device does not act on the transformer insulation, which is very beneficial to the transformer protection and can reduce the risk of insulation breakdown between the high-voltage and low-voltage windings and between the high-voltage windings and the transformer core and shell. However, in order to prevent the impact of the transient rise of the neutral point potential at the low-voltage side of the transformer on the safety of users, auxiliary grounding wires can be installed near users.
There are two possible overvoltages of distribution transformers: (1) positive transformation: when the lightning wave reaches the low-voltage winding of the transformer with Y, y connection, the breakdown fuse installed at the neutral point is broken down, or when the lightning wave reaches the low-voltage winding of the transformer with Y, Y connection, the impact current of the low-voltage winding of the transformer will induce electromotive force according to the transformation ratio under the action of external voltage, which will increase the neutral point voltage of the high-voltage winding. (2) Inverse transformation: when 10kY side is struck by lightning, large lightning current will pass through the arrester and produce voltage drop on the grounding device. This voltage drop will also act on the neutral point of the low-voltage winding and be added to the low-voltage winding. High voltage will also appear on the high-voltage side through electromagnetic induction. For star connected transformers, high voltage that is dangerous to insulation will also appear on the neutral point of the high-voltage side. In addition, lightning arrester or breakdown protector can also be installed on the low-voltage side of the transformer according to specific conditions, which can further improve the safety and reliability of the transformer.
The operation experience has proved that although the distribution transformer located in the thunderstorm area is equipped with valve type arrester protection, there are still many damages caused by lightning stroke. According to the lessons of the accident, in order to reduce the accident of the distribution transformer, the following technical measures should be taken according to the specific situation: (1) Eliminate the insulation weak points of the distribution transformer itself. The test and analysis of transformer insulating oil shall also be strengthened during operation. Because the deterioration of insulating oil will directly lead to the reduction of winding insulation, problems must be handled or replaced in a timely manner. A wide range of impact tests and turn to turn tests on distribution transformers can effectively find the insulation weaknesses on the transformers, arrange maintenance in time, and reduce lightning damage accidents. (2) For distribution transformers installed on wooden pole lines, protective gaps can be installed in the incoming section of the transformer, or the iron feet of top phase insulators with triangular conductors can be grounded, To reduce the steepness of lightning intrusion wave and the current flowing through valve type arrester. (3) It can also be considered to install a group of inductance coils (about 30 turns, 24cm long, 20cm in diameter, about 100 inductance) between the distribution transformer and the valve type arrester μ H) To limit the steepness of lightning intrusion wave, so as to reduce the over-voltage on the interlayer insulation of transformer windings.
HENGFENGSHUAI ELECTRIC, is an electrical enterprise with transmission and distribution industry as its main business, integrating product research and development, manufacturing and trade. Designated suppliers for the world's top 500 enterprises .Its main products include power transformers, medium and low voltage switchgear, distribution panel ,circuit breakers, transmission tower ,Cable and other power distribution equipment .The group has five department: Transformer department, Panel department, power transmission tower department, cable department and electrical engineering department. HENGFENGSHUAI ELECTRIC, has been working hard to provide the best transmission, distribution solutions for users all over the world.【WhatsApp：+86158 5325 2696（Jack） +86158 5326 5269（Alisa）hfs@hengfengshuai.com https://www.hengfengshuai.com 】
