1. Field of the Invention
The present invention is directed to a method and apparatus for detecting a fault in electrical motors and, more particularly, using a power decomposition technique (PDT) to derive positive and negative sequence components of motor signals (e.g., the three-phase voltage or current) to determine the existence of an induction motor stator winding fault.
2. Description of the Related Art
Many critical industrial and related processes require reliable operation of electric rotating machinery such as an induction motor. Notably, stator winding faults most often lead to motor failure relatively soon after the fault develops, certainly within hours, but more typically within tens of minutes, or even seconds. In the past, because computing power was so limited and because motor lifetime after development of the fault is so short, such faults were not monitored. However, with the ability of current technology to process data in a cost-effective fashion, improved methods can be developed in detecting such faults. As a result, common problems associated with unexpected machinery failure, including costly repair, failure to shut down a process in an orderly fashion, extended process downtime and health and safety problems, can be avoided given advance warning of impending failure.
Known systems and methods of analyzing motor behavior during abnormal conditions to detect and predict these conditions have been generally directed to detecting faults in the stator of the motor, as such faults typically are the most common type of electrical failure. Typically, the stator of an electric machine includes windings that may experience a short circuit leading to a fault condition. Such faults are a common source of failure in induction motors and can be the result of aging and deterioration of the stator insulation which may become partially conducting (thus causing short circuits in the windings) before breakdown of the motor occurs. A short circuit in one of the three-phase windings will show up as an imbalance in the three winding impedances. When being fed by a balanced main supply, the motor fault then causes an imbalance in the three-phase currents. This imbalance in the three-phase currents is indicative of a stator winding fault.
It is known that this imbalance may be analyzed in terms of an appearance of negative sequence current in the motor. More particularly, in a three-phase system, the peaks of the currents in each phase occur in time order, for example, a, b then c. In the event that the current peaks are unequal, those currents can be expressed as a combination of positive sequence current and negative sequence current. The negative sequence component is a small component of the current that peaks in the order, for example, a, c then b, such that if it were the only component of the current applied it would turn the motor in the opposite direction, as is conventionally understood. Notably, when a fault in the motor exists, the fault causes the peaks of the current in each phase to be unequal. By measuring the negative sequence current caused by the fault, winding deterioration can be detected and subsequent motor failure can be predicted. However, a problem with detecting the appearance of negative sequence current from the motor is that an unbalanced supply will also produce negative sequence current.
Detecting negative sequence current without accounting for the component of the negative sequence current due to the unbalanced supply, and other contributions to the measured negative sequence current, thoroughly limits any such system. Most remarkably, these systems typically are able to detect only relatively large stator faults. In particular, unless the stator fault produces a negative sequence current that is 20% or more of the load current, the faults likely will go undetected, because such negative sequence currents can be caused by supply unbalance alone. Notably, however, often times the short circuit fault occurs involving only a single turn coil of the winding, or a few turns. Such a fault will produce negative sequence currents as low as 0.2% of the full load current. To detect the appearance of negative sequence current in the motor due to the winding fault with precision, it would be desirable to separate the contributions to negative sequence current due to motor faults and system unbalance.
Known methods have used variation in effective negative sequence impedance of an induction motor to detect inter-turn short circuit faults. These methods have been extended to include accounting for the effect of residual negative sequence current due to intrinsic motor asymmetry. Nevertheless, such methods are severely limited due to complex processing required and the need to gather data for multiple supply cycles to detect stator faults that are responsible for producing relatively low negative sequence current. Because even a seemingly minor stator fault, e.g., a fault in a single winding turn, will still eventually lead to motor failure, there was a need for a system capable of detecting such faults.
Along these lines, the short circuit faults may occur intermittently in the first place, before arcing fuses the shorting conductors and makes the short circuit permanent. Intermittent shorts, although not continuous, will still lead to motor failure. Notably, these faults are very difficult to detect with known methods because the fault may not appear over many of the supply cycles, yet, as stated previously, multiple supply cycles of data are typically required to perform fault detection. Presently, there are no known methods for efficiently detecting intermittent short circuit faults in-the stator windings.
As a result a method and apparatus of on-line diagnostic monitoring was desired that will detect motor faults that produce relatively small magnitude negative sequence fault current. Furthermore, the process preferably will be capable of detecting intermittent short circuit faults to gain even more advanced early detection of potential motor failure.
The, preferred embodiment of the present invention utilizes a power decomposition technique (PDT) to derive positive and negative sequence components of arbitrary three-phase signals in the time domain to detect motor deterioration, e.g., in the stator of an induction motor. The effects of variation in load, supply voltage and voltage unbalance are accounted for by utilizing a recursive least squares method, in conjunction with the PDT, to characterize the associated variation in motor reactance and the attendant residual negative sequence current.
According to one aspect of the invention, a method of detecting a fault in an induction motor includes sampling signals from the motor and then deriving sequence components of at least some of the instantaneous signals using a power decomposition technique (PDT), one of the sequence components being a total negative sequence current component. In addition, the method includes calculating an expected negative sequence current based on at least some of the sequence components. Then, the method subtracts the expected negative sequence current from the total negative sequence current to determine a fault negative sequence current, wherein the fault negative sequence current is indicative of the fault.
According to another aspect of the invention, the method includes calculating an intrinsic motor negative sequence current based on at least some of the sequence components, and subtracting the intrinsic motor negative sequence current from the total negative sequence current generated as described above to determine a modified fault negative sequence current. Notably, the method and apparatus are capable of detecting faults that cause a modified fault negative sequence current as low as approximately 0.2% of the full load current.
According to a still further aspect of the invention, an apparatus for detecting a fault in an induction motor using a power decomposition technique (PDT) includes a data collecting circuit to collect three-phase instantaneous signals from the motor. In addition, the apparatus includes a processor that determines an expected negative sequence current based on the negative sequence impedance, determines, using PDT, a total negative sequence current based on at least some of the instantaneous signals, and the main power supply unbalance, and calculates a motor negative sequence current by subtracting the expected negative sequence current from the total negative sequence current, wherein the motor negative sequence,current is indicative of the fault.
These and other objects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.
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