Patent Application
20050218906
The present invention relates to insulation condition monitoring, and more specifically to a technique for monitoring of the state of insulation, such as for stator winding insulation condition monitoring.
Many different types of electrical machines employ windings and other conductors that are insulated by various insulation systems. For example, motors and generators commonly include wound rotors and stators, or may include preformed windings or coils. The conductors typically serve either simply to conduct electrical current, or to produce magnetic fields by virtue of the flow of such current. Insulation systems separate conductors and windings from one another, and from adjacent components in the assembled system. Such insulation systems may include various varnish systems, tapes, coatings, sleeves, and so forth, or combinations of these. The integrity of the insulation systems is important to the reliability and life expectancy of the electrical equipments in which they are installed.
Insulating systems may break down for many reasons, jeopardizing the continued operation of electrical equipment. The winding insulation, for example, for electric machines is subject to damage and deterioration caused by thermal, electrical, mechanical, chemical and environmental stresses. Typical insulation failure occurs in the slot section between turns or between the coil and ground, and at end windings between coils of adjacent phases. Winding insulation degradation can result in acceleration of machine failure, which decreases the service life of the machine and results in increased costs due to repair or replacement cost and loss of revenue due to machine outage. Therefore it is desirable to monitor the insulation condition for scheduling repair or replacement of winding insulation to prevent such a failure, or at least to anticipate when maintenance or service may become in order.
Off line methods for evaluating the insulation condition include over-voltage hi-pot or high voltage ramp tests, insulation resistance or polarization index tests, surge tests, dissipation factor tests, also known as “tan delta” or power factor tests, and off-line partial discharge tests. Such tests have been extensively used and accepted over many years in providing the condition of the winding insulation. However, these conventional off-line techniques can be intrusive and costly since it is required that the electrical machine be shut down and taken out of service to perform the required diagnostic test and/or measurement. The regular machine maintenance is typically performed once every 3-6 years; therefore, the off-line stator insulation condition cannot be evaluated frequently enough to guarantee reliable operation of the machine until the next outage.
Several on-line measurement techniques are also available for monitoring the winding insulation condition. These include vibration measurement and monitoring, temperature measurement, and differential current measurement. One major drawback of these methods is that the monitoring system detects severe fault conditions only after the fault has occurred, making proactive maintenance and servicing difficult. The on-line partial discharge detects early symptoms of insulation degradation; however, it requires expensive specialized equipment and accurate interpretation of the measurements relies on the skill of an operator.
Accordingly, there is a need for a low cost, simple, and reliable on-line solution for assessing the groundwall insulation condition.
Briefly, in accordance with one aspect of the technique, an insulation condition monitoring system is provided for a 3 phase rotating electric machine. The technique may be also used in a single phase rotating electric machine. The system includes a differential current sensor and a voltage sensor coupled to each phase of the machine for measuring the instantaneous differential current and the instantaneous phase voltage, respectively. A processing module is configured for converting the values of the instantaneous differential current and the instantaneous phase voltage to respective phasor quantities, i.e. magnitudes and phase angles. The processing module further calculates an angular relationship between phasor current and phasor voltage and generates an output based on the calculated angular relationship, as an indication of insulation condition.
In accordance with another aspect of the technique, a winding insulation condition monitoring method is provided. The method includes measuring a first set of values for an instantaneous differential current and an instantaneous phase voltage during operation of the rotating machine, and calculating a second set of values for a phasor current and a phasor voltage based upon the first set of values of the instantaneous differential current and the instantaneous phase voltage, respectively. An angular relationship between the phasor current and phasor voltage is calculated and insulation condition is determined based on the angular relationship.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Referring now to
The signals from the sensors 14 and 16, measured by the data acquisition systeml8, are applied to a processing module 20. Module 20 will typically include hardware circuitry and software for performing computations indicative of insulation condition as described below. Module 20 may thus include a range of circuitry types, such as, a microprocessor based module, and application-specific or general purpose computer, programmable logic controller, or even a logical module or code within such a device. The module 20, is configured to convert the values for the instantaneous differential current and the instantaneous phase voltage to respective values for phasor current and phasor voltage. The processing module 20 fuirther calculates an angular relationship between phasor current and phasor voltage and generates an output based on the calculated angular relationship, as an indication of insulation condition. A memory module 22 is used for storing the output generated from the processing module 20. The same, or a different memory module may also store programming code, as well as parameters and values required for the calculations made by the processing module 20. An indicator module 24 compares the output of the processing module 20 to a predetermined threshold value and generates an indication signal 26 based on the comparison. In general, the indication signal 26 may provide a simple status output, or may be used to activate or set a flag, such as an alert when the output of the processing module 20 exceeds the threshold value, indicating that the insulation is in need of attention or will be in need of attention based upon its current state or a trend in its state.
The present technique provides for determining the condition of insulation systems of each of the windings or winding groups 36, 38, 40, as well as of the entire machine 12. As will be appreciated by those skilled in the art, such insulation systems may include varnishes, tapes, sleeves, or combinations of such materials. The breakdown in such systems may result in partial or total failure in one or more conductors, windings, or groups of windings. Moreover, it should be noted that various types of machines may benefit from the present techniques, including motors and generators. Such machines may be wound in a variety of patterns, and interconnected in various manners, such as in wye and delta configurations, in various numbers of poles and in various numbers of winding groups. Similarly, insulation conditions may be detected and monitored both after degradation has occurred, as well as before significant degradation has occurred, such as by trending. Finally, as discussed below, the present techniques may be used to identify and even to localize insulation conditions in individual windings, groups of windings and in the entire machine.
In the embodiment illustrated in
In accordance with the present techniques, and with the diagram of
Based upon the sensed instantaneous differential currents and voltages, then, these currents may be computed and separated, with phasor angular relationships between them used to provide an indication of the state of the winding insulation system.
A dissipation factor may be defined as the measure of the degree of electrical loss due to imperfect nature of the insulation condition of an electrical system. The dissipation factor, which may be computed as the tangent of the angle δ, is determined from the ratio of the resistive current component 84 to the capacitive current component 86. Degradation, damage, or contamination of the insulation results in changes of the desired parameters, culminating in an increase in angle δ, and a decrease in angle θ, resulting from an increase in differential current and an increase in resistive current loss. Unlike known systems, however, the present techniques permit monitoring of such parameters and conditions during operation of the machine, without requiring that the machine be shutdown.
One advantage of the present technique is that the insulation condition monitoring system does not require specialized equipment and does not depend for interpretation on the skill of an operator. The monitoring system could be based on utilizing current and voltage sensors, thereby enabling easy retrofitting to existing machines. Another advantage of the monitoring system is that the system could be used “online,” while the machine is under operation. Other benefits of the present insulation monitoring techniques includes non-invasive measurement, using trending to detect incipient winding failures for condition based maintenance, application to all single phase or three phase electric machines including motors, generators, and power transformers for insulation condition monitoring.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
