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Root-cause diagnostics of generator service failures
Abstract
Root-cause diagnostics of generator service failures has become a major problem in the industry, particularly during the last 20 years. Generator maintenance specialists from OEMs and independent generator repair companies are trained to be proficient in a specific assignment - repairing of generators. Typically these specialists do not have the technical background to perform diagnosis of the root cause of a failure, nor have they been educated for such diagnostics. As a result, mis-diagnosis is often occurring and these errors in diagnosis have been resulting in major expenditures for incorrect or insufficient repairs. This paper is directed toward industry generator owners. It is intended to give guidance to user maintenance personnel toward obtaining better diagnostics of failure root cause. It is hoped that through better understanding of the difficulties of obtaining accurate generator diagnostics, maintenance costs can be reduced and generator reliability improved. The paper illustrates many typical failures, and discusses the difficulties in root-cause analysis of the failure. The paper also offers suggestions on means of obtaining necessary technical assistance to better assure accurate assessment of root cause
... The recent problems experienced with motor and generator windings were investigated by Clyde et al. (2004); Stone et al. (2009). In the stator, problems such as electric stress control problems; loose coils in slots; vibration sparking; end winding partial discharge and end winding vibrations have led to failures in as short as 5 years of operation (Stone et al., 2009). ...
... Besides, the static and the dynamic eccentricity were analyzed in a three phase induction motor (Nandi et al., 2001). The papers (Vas, 1993;Nandi et al., 2001;Clyde et al., 2004;Negrea, 2006;Stone et al., 2009;Agrawal et al., 2011;Siddiqui et al., 2014) present monitoring techniques for detection of the mechanical and electrical faults. ...
... Also, conventional motors are sometimes being supplied from voltage-source converters that may shorten life [3,4]. In the past Maughan and Bonnett have published papers on problems with stator and rotor windings [5,6]. This paper continues this tradition by reviewing recent failures and " near " failures that may have occurred as a result of higher design stresses and new processing techniques. ...
... Coil vibration in the slot has long been a problem in all nonglobal VPI stators made with thermoset insulation systems such as epoxy mica. The first instances were reported over 50 years ago [5,8]. The root cause of the problem is that at full load, the twice power frequency magnetic forces will vibrate the coils if the coils are not tightly held in the slot. ...
In many respects, large motors and generators in petrochemical plants have become a commodity product with intense competition amongst manufacturers from around the world to secure orders. This has resulted in pressure on machine designers to reduce manufacturing costs. Some of the methods employed to accomplish this include: (1) Reducing the conductor cross section (2) Reducing the insulation thickness (3) Reducing the amount of steel core material (4) Developing manufacturing methods that result in less time to manufacture. Each of these methods tends to increase the operating temperature of the windings or put additional voltage stress on the electrical insulation. Many design and processing innovations have been successfully implemented. However, there are both anecdotal and statistical data that indicates that there are more problems with machines made in the past 10 years, as compared to machines made previously. Engineering firms and end-users perhaps need to provide comprehensive, yet reasonable, purchase specifications that allow all manufacturers to compete on a level playing field. This paper is mainly concerned with stator windings rated greater than 6 kV, and rotors of various sized machines.
... The tightness can be loosened by repeatedly starting and stopping the generator. This can result in the loosened wedge no longer holding the coil firmly, causing vibration, thermal expansion, and insulation failure [2]. ...
Abstract Stator wedge tightness inspection is one of the most important periodic inspections carried out on required for large‐capacity generators. The traditional wedge tightness inspection method enforces inspectors to enter the narrow and uncomfortable inner space of a stator. As an alternative, a wedge tightness inspection mobile robot is developed. This robot has a robotic arm with a wedge tightness test module. A visual navigation system using two cameras is proposed. The proposed system controls the driving of the robot in a stator by using line detection of the bottom view camera image. The system also detects wedges and assigns a location number to them by using the inspection view camera image and an object detector based on a convolutional neural network. The detected wedges are tracked using robot motion to assign location numbers to the new wedges. The inspection points are then determined using the length and position of the detected wedges. The experimental results conducted on the stator testbed demonstrate the potential applicability of the proposed system.
... The development of electrical machines greatly influences the power utility characteristics, consisting of generation, transmission, and consumption. Power generation must have unfaulty or extremely minimum fault incidents with rotating machines [1]. Although the present-day stator insulation has quite high strength, but still machines operating above 3.3 kV experience specific deteriorations over time. ...
... However, there are significant problems directly related to electrical destruction [1][2][3]. These electrical mechanisms are often similar in their physical appearance, but are fundamentally different for reasons and corrective actions. ...
The main purpose of this work is numerical investigation of HV rotating machine insulation system. Comsol Multiphysics software was used to develop the model of Resin Rich based insulation system. The numerical simulation of electrophysical processes in insulation was conducted. The influence of insulation components' electric properties on electric field distribution was studied. The recommended range of resistivity for minimization of electric field in air gaps was obtained. The breakdown possibility due to different kinds of insulation defects was evaluated.
... If lack of maintenance and left such defect untreated, it may cause surface tracking or other PDs. Furthermore, metal particles left by engineers during maintenance or resulted from generator components becoming loose during operation may lodge in vulnerable locations of the stator winding and then cause PD, such as phase-to-phase PD and end-winding corona [22]. Other than normal stresses, PDs can be stimulated by transient stresses caused by abnormal operation of the generator or the relevant devices, such as out-of-phase synchronization, power systems transients and/or abnormal external conditions [1,14,19]. ...
Online partial discharge (PD) measurements have long been used as an effective means to essess the condition of the stator windings of large generators. An increase in the use of PD online measurement systems during the last decade is evident. Impeovements in the detection capabilities are partly the reason for the increased popularity. Another reason has been the development of digital signal processing techniques. In addition, rapid progress is being made in automated single PD source classification. However, there are still some factors hindering wider application of the system, such as the complex PD mechanism and PD pulse propagation in stator windings, the presence of detrimental noise and disturbances on-site) and multiple PD sources occurring simultaneously. To avoid repetition of past work and to provide an overview for fresh researchers in this area, this paper presents a comprehensive survey of the state-of-the-art knowledge on PD mechanism, PD pulse propagation in stator windings, PD signal detection methods and signal processing techniques. Areas for further research are also presented.
... Some of these mechanisms are due to imperfections during machine's manufacturing process, other mechanisms are due to external agents. Specifically for generators, Maughan [12] reports the most common root causes for failures on stator windings: endwinding looseness, bad electrical connections, foreign object damage and vibration. Besides these mechanical causes, thermal stresses are one of the most important factors that cause aging of insulation materials. ...
The selection technique of diagnostic parameters for the creation of fault detection system of autonomous electric power sources based on gasoline and diesel engines is given in the paper. An analysis of the design features for autonomous electric power sources based on internal combustion engines, which are the most common on the Ukrainian market, was carried out. Thanks to this, a logical model of the research object, which establishes the relation between the main structural elements of the system and determines the possible states of the system, was developed. The effect of fault state initiation for each element on the other system elements was analyzed. An informative criterion – Shannon information entropy is proposed to determine the finite number of diagnostic parameters among the infinite number of possible combinations for physical parameters that characterize the system. The equal-probable cases of exit from operational state of each system elements are considered. The residual entropies of the system at the fault state for one of the autonomous power sources assembly are determined, having applied the concept of Shannon information entropy. The residual entropy value is the informative criterion. The application of this criterion allowed to determine the system elements that most effectively reduce the system uncertainty degree. Based on the residual entropy values, the system assemblies, the state of which should be primarily monitored by diagnostic system, are selected. The diagnostic parameters are determined for such elements, and the ways to obtain them are given
The presented article shows a method for finding monitored parameters for creating a diagnostic system for autonomous power sources based on spark ignition engines and diesel engines. The classification of structures of autonomous power sources based on internal combustion engines has been carried out. The analysis of the design features of the most common back-up power sources on the market based on internal combustion engines (ICE) indicates the widespread use of generators with a synchronous alternator. The analysis of the design features of autonomous power supplies has made it possible to develop logical models for different designs. The influence of the occurrence of a faulty state of each element on other elements of the systems is analyzed, and the results of the analysis are summarized in tables. An informative Claude Chenon criterion is proposed for finding the optimal number of diagnostic parameters among an infinite number of possible combinations of physical parameters that characterize the system. When solving the problem, a hypothesis was proposed about the equiprobability of cases of exit from the working state of each of the elements of the system. The use of Claude Chenon allows you to find the parts that make up the generators, which with maximum efficiency reduce the degree of uncertainty in the system. After determining the residual entropy, the parts of the system are selected, the state of which should be monitored by the diagnostic system. For such parts of the system, diagnostic parameters are found and methods for obtaining them are indicated.
This chapter describes why there are so many failure processes and what causes one process to dominate, eventually leading to failure in a particular machine. It presents information on how to select an appropriate repair method from all the possible options. Some machine failures, which are identified by a stator ground fault, rotor ground fault or extremely high vibration, occur as a result of a catastrophic event, regardless of the original condition of the insulation. Form-wound stator windings that experience localized damage during maintenance, or fail in service, can sometimes be repaired in a short outage/turnaround, and returned to service in a few days. When one or a small number of coils or bars have been damaged as a result of manufacture, maintenance or an in-service fault, it is often not possible to repair the coil/bar. Rewinding implies that the core will be reused.
Power generator is the most important equipment in power system, and the security and reliability of power system are influenced by its running status. Generators failure or generators break down can cause high financial consequences. For this reason, several concepts for the condition monitoring of generators have been developed. Large volume of available data from monitoring systems can overwhelm personnel and require extensive interpretation. Correctly handling this information requires expertise in generator design, operating limits, alarm processing and interpretation. Because expertise is often a scarce quantity, decisions must often be made when expert group is unavailable. The purpose of this article is to collect significant generator data in order to form a comprehensive analysis by Analytical Hierarchy Process (AHP) method. The AHP is a multi criteria analysis approach, where is used in order to combine the results of diagnostic methods and draw conclusions on the most probable failure. A fault diagnosis structure has been designed and several comparison charts have been generated. As a case study characteristics of a generator have been analyzed using Expert Choice (EC) software. The EC results confirmed high performance of AHP method for utilizing in generator fault diagnosis.
In many respects, Large Motors and generators in petrochemical plants have become a commodity product with intense competition to secure orders among manufacturers from around the world. This has resulted in pressure on machine designers to reduce manufacturing costs. Many design and processing innovations have been successfully implemented. However, there are both anecdotal and statistical data that indicate that there are more problems with machines made in the past ten years when compared with machines made earlier. Engineering firms and end users perhaps need to provide comprehensive, yet reasonable, purchase specifications that allow all manufacturers to compete on a level-playing field. Stator windings rated greater than 6 kV and rotors of various-sized machines are the main topics of discussion.
Since the introduction of epoxy mica stator windings, some turbine generator stator windings have apparently failed due to a mechanism variously referred to as spark erosion or vibration sparking. The mechanism can produce relatively intense sparking between the surface of the stator bar and the core. The intensity of the sparking is such that it may erode the groundwall insulation much more quickly than slot discharges. Unlike the normal loose coil/slot discharge failure process, spark erosion can happen anywhere in the winding, and not just in stator bars that are operating at high voltage. This paper describes the vibration sparking failure process, which seems to require that the partly conductive slot coating must be much more conductive than normal; and that the coil or bar must be loose in the slot and vibrating under the 100/120 Hz magnetic forces.
Slot discharge and VS are two stator winding deterioration mechanisms acting on stator winding insulation. Slot discharge can have at least three different causes only one of which is caused by bars vibrating in the slot. Slot discharge will be significant only in bars that are operating at or near rated voltage. Vibration sparking will only occur when two conditions are met simultaneously: a) the slot conductive coating is too conductive, and b) the bars are vibrating in the slot. This can occur throughout the stator.The manifestations of the two mechanisms can be very similar. The major difference between slot discharge caused by loose coils and VS is that VS can occur on bars located anywhere within the winding. Vibration sparking is generally the more aggressive of the two mechanisms. Because the root cause and corrective actions are quite different, it is important to distinguish between the two mechanisms. The presence of VS or slot discharge in a stator winding can have significant influence on the projected life of a stator winding. Thus it is important to detect and correctly diagnose either problem in its early stages. If the amount of activity is significant and advanced, repair may be difficult or impossible, particularly in the case of VS.
In the late 1980s and again in the past few years, some turbine generator stator windings have apparently failed due to a mechanism variously referred to as spark erosion or vibration sparking. The mechanism can produce relatively intense sparking between the surface of the stator bar and the core. The intensity of the sparking is such that it may erode the groundwall insulation much more quickly than slot discharges. Unlike the normal loose coil/slot discharge failure process, spark erosion can happen anywhere in the winding, and not just in stator bars that are operating at high voltage. For this mechanism to occur, apparently (a) the partly conductive slot coating much be much more conductive than normal; and (b) the bar must be loose in the slot and vibrating under the magnetic forces.
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