Discussion on the problems existing in the operati

2022-08-16
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Discussion on the problems existing in the operation of large self shunt excitation system

Abstract: Yiyang Power Plant adopts large self shunt excitation system. On the whole, its regulation performance can meet the technical requirements of our country. However, in the field operation, due to the lack of operation experience of corresponding equipment, there are still some problems in the auxiliary system of the equipment, so the operation is unstable. The equipment and its problems are analyzed and discussed, and some meaningful conclusions and suggestions are drawn

key words: self shunt excitation system slip overvoltage jumper machine terminal transformer 0 Introduction

Yiyang Power Plant Unit 1 and 2 adopt the self shunt excitation system produced by a foreign company. This type of equipment is applied for the first time in Hunan electric power, and there are few application examples in China, so it is necessary to discuss the problems reflected in its commissioning and operation

the traditional excitation mode of three machine system includes high-frequency generator and intermediate frequency generator. The high-frequency generator obtains excitation energy from DC power supply and supplies it to the excitation coil of intermediate frequency generator. The output of intermediate frequency generator provides bismuth iodide oxide to the generator after rectification, which can maintain a stable excitation current in the air for up to 197 days. The excitation mode of large self shunt excitation system is different from this. The system components are mainly excitation transformer, regulator and rectifier output equipment

practice has proved that the shaft length of steam turbine generator can be greatly shortened by adopting self parallel excitation mode, which is very helpful to reduce the vibration of steam turbine. At the same time, because the excitation system of Yiyang Power Plant is a microcomputer excitation system, instead of using separate components, its operation is more flexible and maintenance is more convenient. The schematic diagram of excitation system is shown in Figure 1

in principle, since the excitation current is generated by the regulator to control the conduction angle of the thyristor, during shutdown, the regulator can first close the conduction angle, invert from the DC side to the AC side, and then de excite by the DC de excitation switch and arc suppression resistance after consuming most of the energy, so as to effectively protect the de excitation switch and prolong its service life. However, there are also some problems in the self shunt excitation system. For example, when a short circuit occurs somewhere in the system, the system voltage drops. At this time, the exciter needs to provide a large amount of excitation energy to quickly increase the system voltage, and at the same time, provide a large current to the short-circuit point to facilitate the correct action of the protection device. There are also two reasons for such errors as jumping off the fault point: the first reason is in the process of the experiment. For the three machine system, because its excitation energy comes from the outside, it can quickly realize forced excitation, increase voltage and current in case of system failure. However, for the self shunt excitation system, the excitation energy comes from the generator end transformer. In the case of short-circuit fault, the generator end voltage will drop very seriously. At this time, a large amount of energy should be absorbed from the generator end for strong excitation. In a short time, the generator end voltage will fall again

1 problems reflected in field operation and analysis

the regulation accuracy of the company's microcomputer digital excitation regulator is high. Even in the case of operation mode switching, its disturbance is very small, and the error between mode switching is less than the relevant national standards. In terms of design principle, the traditional dual channel design mode is abandoned, and the "two out of three" mode is adopted for all input and output switching values and analog quantities. There are three control modules that work independently, which carry out sampling analysis and control output independently, average the analog quantity with two relatively close quantities, and control the switching quantity with two identical outputs. On the whole, its standby coefficient is much higher than that of the dual channel design, which improves the reliability of the equipment as a whole

due to the microcomputer design, the whole system adopts a modular structure, and its operation and maintenance are relatively simple. The fault components can be plugged in and out, that is, the fault can be handled quickly. This is very effective for troubleshooting in an emergency. However, in operation, it is found that there are also some noticeable problems in the self shunt excitation system. The analysis of these problems will be helpful to the commissioning and operation of new units in Liyujiang power plant and Zhuzhou power plant. It is difficult to identify the yield point

1.1 the design defects and unstable contacts of the cooling fan

first is the problem of the cooling mode. Since the self shunt excitation system obtains energy directly from the transformer at the generator end and supplies it directly to the generator excitation coil after rectification, the current passing through the rectifier device is very large. The following data can explain the problem (power factor is 0.85): when the generator is unloaded, the excitation current is 012 a; At full load, the excitation current is 2000 a

it can be seen that the power flowing through the rectifier device is very large. Therefore, two cooling fans are considered in the design. These two cooling fans have practical problems and design defects

1.1.1 contact instability problem

first, the mechanical contact of the fan is unstable, which cannot reflect the actual air flow of the device. The standby fan is often started abnormally. Moreover, there are also problems in the design logic of the fan. Under normal circumstances, the main fan should be in operation and the other one should be standby. However, it has been found many times that when the air volume of fan 1 fails to reach the design air volume, the second fan is started. When the air volume has reached the standard, the second fan does not stop running, and the signal of insufficient air volume does not disappear. Obviously, this design idea is flawed

1.1.2 processing of air flow signal

another defect of the cooling fan is to take the insufficient air flow signal as the generator trip signal. Moreover, this kind of mechanical contact is not reliable. This design is obviously unreasonable at the cost of jumping off a 300 MW generator with an unreliable air volume contact. Personally, I think we can consider changing the air volume signal into an alarm signal instead of a trip signal, especially the fast foam spraying time number, and taking the temperature protection of the rectifier device as the main protection of the device body

1.2 crowbar (jumper) action and abnormal tripping

abnormal tripping occurred on February 4, 2002. At that time, the load was 4000 kW, and the switch tripped

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