Current situation and development trend of equipme

Current situation and development trend of power plant equipment fault diagnosis

Abstract: This paper introduces the typical faults of main equipment in thermal power plants and the fault diagnosis methods adopted, expounds the existing fault diagnosis system and the problems existing in the fault diagnosis of thermal power plant equipment, points out the development trend of fault diagnosis system, and puts forward the research direction

the main equipment of large thermal power plants includes boilers, steam turbines and generators, which complete the conversion process from heat energy to mechanical energy and then to electric energy. The coupling between equipment, the complexity of the system, and the special working environment in which the equipment rotates at high temperature, high pressure and high speed determine that the thermal power plant is a production site with high failure rate and great hazard of failure. These failures will cause significant economic losses and social consequences. Therefore, it is necessary to monitor and analyze the equipment status parameters through advanced technical means to judge whether there is abnormality or failure, the location and cause of the failure, and the deterioration trend of the failure, so as to determine the reasonable maintenance opportunity

I. typical faults of main equipment in thermal power plants and their diagnosis methods

1 Main faults and diagnosis methods of boiler

(1) main faults

① superheater leakage. The leakage and tube burst area of the superheater is concentrated on the fireward side of the outer ring of the elbow under the high-temperature superheater. The main reason is that the furnace height is low, which causes overheating. There are also insufficient material selection margin and welding quality problems here

② economizer leakage. The main reason is that the pipe wall is thinned due to the wear of fly ash, especially at the through wall pipe, furnace wall air leakage and elbow

③ water wall leakage. The main reasons are local overheating and corrosion. Local overheating is caused by the destruction of water circulation and the scaling in the pipe, while the flame deflection or the high smoke temperature in the combustion area will cause the high temperature corrosion of the water wall

④ dust collector failure. The main reasons are that the flow rate of flue gas is too fast, the particle size of ash particles is large, the dust concentration is large, and all plastics in flue gas exhaust have limitations, and the temperature is lower than the dew point temperature

(2) diagnostic methods

in boiler fault diagnosis, physical diagnostic methods include: infrared temperature measurement technology, the specific application scope includes the identification and control of boiler flame and combustion state, fatigue damage of thermal equipment, thermal mechanical characteristics of thermal equipment, diagnosis and evaluation of thermal system leakage and insulation, boiler thermal pollution control, etc; Ultrasonic diagnosis method can be used to monitor the flue gas temperature in the upper area of the furnace, decide when to conduct soot blowing operation, maintain good operating performance of the boiler, monitor the flue gas temperature near each burner area of the furnace, help to identify and eliminate the abnormal combustion conditions caused by burner failure, and optimize the control of the temperature that has an important impact on the generation of pollutants, so as to realize clean combustion; Nondestructive testing technology refers to the nondestructive testing of materials and components in order to find surface and internal defects. Mathematical diagnosis methods include fault tree diagnosis method, fuzzy diagnosis method and so on. In terms of diagnosis system, it mainly includes the large-scale power station boiler remote monitoring and fault diagnosis system developed by Tsinghua University, and the circulating fluidized bed boiler monitoring and condition diagnosis expert system developed by Huazhong University of science and technology

2. Main faults and diagnosis methods of steam turbine unit

(1) main faults of steam turbine

① imbalance. It is mainly caused by thermal deflection caused by machining error and permanent bending of rotor, blade falling off, coupling drift, poor manufacturing of extension shaft, uneven heating or material defects

② misalignment. The main reasons are that the sliding pin system is blocked, the temperature difference of the cylinder exceeds the limit, the unit speed rises too fast, the vacuum drops too much, the foundation sinks unevenly, etc., which makes the unit expand poorly and the bearing seat expand unevenly, as well as the coupling is eccentric, etc

③ rotor rubbing. The main reasons are improper design and adjustment of steam seal clearance, poor expansion of cylinder, excessive thermal deformation of cylinder, excessive vibration of unit and increased axial displacement, etc

④ leaves fall off. It is mainly due to the insufficient strength of the design, the large fluctuation of steam parameters, the long-term operation of the unit under the large change of electric cycle and low load, and the long residence time of the unit at a speed below the rated speed

⑤ steam excitation. The main reason is that the critical speed of the high and intermediate pressure rotor is low, and the deflection of the high and intermediate pressure rotor changes greatly in the hot state, the bearing elevation changes greatly, the processing and installation quality of the extended shaft is poor, the radial or axial error of the coupling center is too large, and the shafting balance and alignment state deteriorate, which makes the high and intermediate pressure rotor unstable

(2) main faults of generator

① oil film oscillation. This is mainly because the first critical speed of the generator rotor is too low, and the stability of the three shaft wedge is poor. When the load is reduced, the unstable speed is too low

② stator coil insulation failure. It is mainly caused by insulation failure due to wear, aging, pollution, corrosion and other reasons, resulting in partial discharge and temperature rise, as well as insulation layer damage due to loose binding, cooling water leakage, fatigue wear and other reasons, resulting in bar displacement and inter turn short circuit

③ the stator coil is overheated. It is mainly due to some defects in the manufacturing or installation process that cause inter turn short circuit and local overheating

④ rotor winding failure. Including grounding, inter turn short circuit and wire breaking fault. Grounding and inter turn short-circuit obstacles are mainly caused by insulation reduction and damage. Open welding and thermal deformation of the joint will cause wire breakage

⑤ cooling water system failure. It is mainly due to cooling water leakage, blockage of foreign matters in the pipeline, malfunction, etc., which reduces and interrupts the cooling water flow, as well as stator water leakage due to material and installation defects

(3) diagnosis method

in the fault diagnosis of steam turbine units, vibration method is the most widely used and relatively mature method. Thermodynamic analysis is also an important means to diagnose the performance faults of steam turbine units. In addition, there are oil analysis, acoustic emission method, nondestructive testing technology and so on. Acoustic emission method is mainly used for dynamic and static rubbing fault detection, leakage detection, etc. Hitachi set test pieces on the high and intermediate pressure rotors of 350MW steam turbines, and carried out acoustic emission and recording at the bearing pads of bearings at both ends to diagnose the rubbing of rotors. In the life diagnosis of steam turbine units, the application of nondestructive testing technology is very important. The non-destructive evaluation methods currently used mainly include hardness measurement method, electrical resistance method, ultrasonic method 1, installation of tensile testing machine, tissue comparison method, crystal particle deformation method, microscopic observation and determination method, X-ray analysis method, etc

3. Main faults and diagnosis methods of transformer

(1) main faults

① coil turn to turn short circuit. The main reasons are insulation aging, poor heat dissipation or long-term overload, interturn insulation damage due to short-circuit electrodynamic force, defects in winding materials or processes, water ingress and moisture, atmospheric or operating overvoltage attack. Then within a period of time

② the winding is broken. The main reasons are short-circuit electric power, coil disconnection, poor welding and inter turn short circuit

③ winding breakdown to ground. The main reasons are the aging of the main insulation, the moisture of the insulating oil, the entry of impurities in the winding, the deformation and damage of the coil during overvoltage and short circuit, and the insulation damage caused by overall or local overheating due to the failure of the cooling system, the blockage of the cooling oil duct, and the protection failure

④ phase to phase short circuit of winding. The main reason is similar to the ground breakdown, but it may also be the short circuit between leads or bushings, and the oil level is too low

⑤ local short circuit or local melting of iron chip. The main reason is that the insulation of the screw of the iron core or choke is damaged, there are metal sheets at the fault to short circuit the iron chips, the insulation between the sheets is seriously damaged, and the grounding method is incorrect to form a current loop

⑥ the tap changer is in poor contact and the contact surface is melted. The main reason is that there are defects in the structural assembly. After switching the tap, the contact is not reliable, the spring pressure of the moving contact is not enough, and the on load voltage regulating device is installed and adjusted improperly

⑦ casing flashover or explosion to the ground. The main reasons are that the surface area is gray, dirty, cracked, the seal is not tight, and the respirator is not configured properly

⑧ the oil temperature rises under normal load. The main reasons are winding inter turn short circuit, increased loss, poor contact at high current connection, low oil level and poor cooling effect

(2) diagnostic methods

in transformer fault diagnosis, the commonly used methods include vibration analysis, gas in oil analysis, partial discharge method, recovery voltage method, frequency response analysis and infrared diagnosis technology. At present, infrared diagnosis technology is mainly used. In terms of diagnosis system, scholars and research units at home and abroad have done a lot of work in this regard, and have developed expert systems with fault detection and preliminary diagnosis functions, such as the microcomputer expert system for fault diagnosis of power transformers developed by Henan electric power test and Research Institute. In addition, many famous universities in China are engaged in this research and have made great theoretical achievements

II. Problems existing in the current fault diagnosis of thermal power plant equipment

at present, although many diagnostic methods and diagnostic systems have been applied to the fault diagnosis of thermal power plant equipment and achieved good application results, there are also many problems in practical application, mainly in the following aspects

1. Detection means

the reasoning mechanism of fault diagnosis has reached a high level, but the acquisition of symptoms has become a bottleneck, that is, the biggest problem is that the detection means can not meet the needs of diagnosis and can not truly reflect the characteristics of faults

2. Complex fault mechanism

understanding the fault mechanism is the premise of accurate fault diagnosis. At present, it is difficult to explain the complex faults of some equipment in power plants theoretically, and the understanding of its mechanism is not deep

3. The price of each bag made of artificial intelligence application

expert system should also rise by at least 0.5 yuan. The application of artificial intelligence in the fault diagnosis of main equipment in power plants has been successful, but there are still some key problems of artificial intelligence application that need to be solved, mainly including knowledge expression and acquisition, self-learning, intelligent identification, information fusion, etc

4. At present, the diagnosis methods used in the fault diagnosis system of thermal power plant equipment include fuzzy logic method, fault tree analysis method, expert system, artificial neural network and so on. However, a single diagnostic method is often difficult to achieve the desired diagnostic effect

5. Fault location

the current fault diagnosis system often only carries out the part of fault type identification, which can not determine the specific location of the fault, and the functional research of predicting the state of the equipment is not enough