Descriptor: AC circuit breaker arc extinguishing voltage 1 rumor In order to quickly destroy the generator's magnetic field when the synchronous generator and its unit are connected to the main transformer, the arc at the fault point is extinguished as soon as possible or when the generator is normally shut down. The magnetic field energy of the field winding is quickly absorbed by the demagnetizing resistor, and magnetic field circuit breakers are widely used at home and abroad. Its function is to quickly cut off the path of the excitation winding and the excitation power of the generator, and quickly transfer the excitation current of the generator to the attenuation of the de-excitation resistor, and the magnetic field energy is absorbed by the de-excitation resistor. The magnetic field breaker is usually connected in series in the excitation DC loop as shown. Due to the small amount of large DC magnetic circuit breakers dedicated to the rapid demagnetization of synchronous generator rotors (only one for one generator and an average life of 10 years), the development of this product is limited. At present, only domestic DDM, DM4, DM3 and other DC magnetic field circuit breakers are used. Foreign magnetic field circuit breakers are mostly modified by AC circuit breakers, such as the EX74 series of the British RR company and the 3WF series and 3WV series magnetic field circuit breakers of the German SIEMENS company. This not only greatly reduces the development and production costs, but also meets the urgent requirements of the development of the power industry, and is a shortcut to how fast and better.
Recently, generators have widely used AC rectification static excitation systems to create conditions for AC demagnetization. If the AC circuit breaker is used to break on the AC side of the rectifying excitation power supply, the effect of rapid demagnetization can also be achieved. Now foreign countries have developed in this direction. In this paper, the feasibility and advantages of the 3WE circuit breaker after its improved design are discussed.
2 The working principle of demagnetization The transfer of the excitation current of the generator to the de-excitation resistor is called commutation. At present, large and medium-sized generators widely use zinc oxide (ZnO) varistor Rv as the de-excitation resistor. At lower voltages, ZnO is in a high-resistance (MD) state. When the voltage rises to a certain value, it is converted to a low-resistance edge. status. Rv is connected in parallel with the excitation winding. When the generator is in normal operation, the rotor voltage Ul is low, Rv is high impedance, and the leakage current is only on the order of micro-ampere. When the magnetic field is interrupted, the magnetic circuit breaker MK is disconnected, and the arc voltage Uk rises, causing U1 to reverse and rise to a certain value, so that ZnO turns into conduction, the excitation current 1l is transferred into Rv, and the MK is extinguished. The MK can be mounted either on the AC side (see) or on the DC side (see ). Regardless of whether the MK is on the AC side or the DC side of the system, the key to ensuring normal commutation of the magnetic field circuit breaker is that its breaking arc voltage needs to be high enough. If the MK is installed on the AC side, the thyristor full-controlled rectifier bridge must be used, and all trigger pulses of the thyristor rectification must be blocked while the MK is disconnected. The advantage is that the arc voltage value requirement for the circuit breaker can be reduced. It is possible to apply an improved AC circuit breaker to the current part of the generator.
Test and improvement of 33WE circuit breaker demagnetization application According to the above-mentioned demagnetization mechanism, we chose to use the 3WE circuit breaker for the demagnetization commutation test. The product is manufactured by the technology imported from SffiMENS, Germany. It has strong breaking capacity, reliable operation, simple operation, small size and light weight. The most outstanding advantage is that the rated voltage of the 1604000A full range of products can reach 1 140 (1000). V, its breaking capacity is 20kA200V, and the arcing time is about 20ms. From the perspective of structural performance, 3WE circuit breakers are more complete and mature in terms of product design, process and manufacturing than some special magnetic field circuit breakers. It can be seen from the mechanism of demagnetization condition that the key factor for smooth commutation to achieve rapid demagnetization is that the magnetic circuit breaker must generate a high arc voltage when breaking, and the voltage value should be maintained for about 20 ms to make the ZnO pressure The varistor is switched from a high-resistance state to a conducting state, thereby rapidly absorbing the magnetic field energy.
In view of the performance of the smooth breaking short-circuit current under the test voltage of 200V, the product should have a high and stable arc voltage at the time of breaking, which is an important factor to ensure the breaking performance. Therefore, first of all, no changes were made to the product, and the arc voltage test of the 3WE circuit breaker was performed on the AC power supply state. The arc voltage of the fracture was measured to be about 570V (the product is in the middle MK position and the magnetic resistance is not connected). Rv) It can be seen from the test results that the amplitude and stability of the arc voltage established by the product when breaking the simulated inductive load of the generator is ideal, and the arc voltage value of the special magnetic field breaker is further improved. The first step of improvement is to increase the number of deionization grids. From the theoretical point of view of arc extinction, under certain conditions, the arc voltage value at the time of breaking is proportional to the number of deionization grids in the arc chute, and is generated on each grid. The arc voltage value is about 30 35V. The test shows that the arc voltage value of the 3WE circuit breaker not only conforms to the above rule, but also the arc voltage value of the single grid plate reaches the upper limit of the theoretical value. Then, the British RR company EX74 magnetic field circuit breaker arc-extinguishing hood was analyzed. The product has 120 pieces of deionization grid inside the arc-extinguishing hood. It can be seen that increasing the number of deionization grids is one of the important ways to obtain high arc voltage values.
However, in order to increase the number of deionization grids, it is more difficult to increase the outer dimensions of the arc-extinguishing hood: first, it is limited by the structure of the circuit breaker itself, and there is almost no room for increase in size; High, long manufacturing cycle. Therefore, we increase the number of deionization grids while maintaining the same dimensions of the original arc-extinguishing chamber of the 3WE circuit breaker. It has been found through experiments that the arc voltage value increases with the increase of the number of grids when the number of grids is increased by a small amount. However, when the number of grids exceeds a certain value, the arc pressure value does not rise any more, but instead occurs in the arcing time multiple times. A fall cannot stabilize the phenomenon of building pressure.
The reason is that the original arc extinguishing hood has the same volume of the inner cavity, and the increase of the number of deionizing grids causes the gas pressure in the cavity to change during the breaking, which leads to the gas pressure and arc motion in the cavity when the number of the grid is certain. The best fit between them was destroyed. Therefore, while increasing the number of grids, it is necessary to adjust the relative position between the grid and the fracture, and change the arc motion space to regain the best fit between the pressure in the chamber and the arc motion. In order to have more effective space for adjustment, adapt to the space requirements of arc build-up under different magnetic field energy conditions, and remove the flame-extinguishing piece in the original arc shield. The maximum breaking current of the 52-channel device due to the magnetic field is actually the strong magnetic current of the excitation system, generally only a few kA. It can be seen from the test that the original flame-extinguishing piece does not actually work, so the special magnetic field circuit breaker (application) The flame shield is also not included in the arc cover of the short arc cathode pressure drop type. After repeated trials, the appropriate fit space between the fracture and the grid under different magnetic field capacity conditions is obtained. In this state, the arc voltage value is further increased to 1600V in a relationship of about 30V per chip and proportional to the number of grids (see), which is about 180% higher than the improvement before the improvement. However, at this time, a new problem arises. As the number of grids increases, the thickness of the grid is reduced and the grid spacing is reduced, resulting in a decrease in the heat capacity of the grid and a deterioration in the cooling conditions, and the bottom edge of the grid is arced. The phenomenon of melting and even welding the edge between the grids after multiple breaks. After analysis, this burning phenomenon may be caused by the large resistance that the arc enters when entering the grid, so that the arc stays at the bottom end of the grid for a long time and the heat is excessively concentrated. In order to reduce the resistance of the arc to enter the grid, the current gap of the current AC circuit breaker (including the 3WE breaker 3WE breaker DC breaking arc voltage measurement test waveform diagram) is completely consistent and the gap is staggered. It is conducive to the upward attraction of the arc, speeding up the arc movement speed, and the phenomenon of melting is significantly improved.
In order to make the arc generated by the circuit breaker breaking DC high-sensitivity load in the commutation, the arc has better orientation, and the arc voltage with higher amplitude and relatively stable in the required time is obtained, and the number of times of breaking the strong excitation current is high. We have improved the arcing angle of the arc-striking plate and the static and dynamic contact at both ends of the arc-extinguishing hood, which makes the product one step further in practicality.
4 Conclusions The improved 3WE circuit breaker simulated commutation demagnetization test results show that the AC circuit breaker has been properly improved, mainly to redesign and manufacture its arc extinguishing cover and contacts, which can be used in the excitation system of the generator. Used as a de-excitation switch. This scheme has the following advantages: Compared with the special magnetic field circuit breaker, the 3WE circuit breaker has the advantages of low price, compact structure, reliable operation, high technical performance index, small volume and mature manufacturing process. Under the premise that the system meets certain conditions, the selection of this product can greatly reduce the cost of the equipment for degaussing, reduce the workload for on-site maintenance of circuit breakers, and expand the user's choice for product selection. However, there are still problems in that the use of the product is limited, and the arc voltage superposition is not ideal when the multiple fractures are connected in series, and further improvement and improvement are needed in the future.
(Finish)
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