Turbine Generator Stator Windings Research Articles

Relative ability of UHF antenna and VHF capacitor methods to detect partial discharge in turbine generator stator windings

On-line partial discharge (PD) monitoring is widely used to detect stator winding insulation deterioration. The most common type of on-line PD sensor for turbine generators is the 80 pF capacitive coupler installed at the machine terminals which detects the PD in the very high frequency (VHF) range. More recently, ultra high frequency (UHF) antennae have also been used to detect stator winding PD in large turbine generators. Both types of sensors have been installed in 16 turbine generators. This paper discusses the results from near simultaneous measurements from both types of sensors on the same machines. It was expected that the UHF sensors would have stronger discrimination of the electrical disturbances internal to the machine compared to the VHF sensors, but the VHF sensors would be more sensitive to PD. As expected, the VHF sensors were generally more corrupted by electrical disturbances. However, in about half of the machines investigated here, the UHF sensors were significantly more sensitive than the VHF sensors. Many examples of phase-resolved PD patterns that illustrate these findings are presented.

Characteristics of Insulation Diagnosis and Failure in Gas Turbine Generator Stator Windings

In order to evaluate the insulation deterioration in the stator windings of five gas turbine generators(137 MVA, 13.8 kV) which has been operated for more than 13 years, diagnostic test and AC dielectric breakdown test were performed at phases A, B and C. These tests included measurements of AC current, dissipation factor, partial discharge (PD) magnitude and capacitance. ΔI and Δtanδ in all three phases (A, B and C) of No. 1 generator stator windings showed that they were in good condition but PD magnitude indicated marginally serviceable and bad level to the insulation condition. Overall analysis of the results suggested that the generator stator windings were indicated serious insulation deterioration and patterns of the PD in all three phases were analyzed to be internal, slot and spark discharges. After the diagnostic test, an AC overvoltage test was performed by gradually increasing the voltage applied to the generator stator windings until electrical insulation failure occurred, in order to determine the breakdown voltage. The breakdown voltage at phases A, B and C of No. 1 generator stator windings failed at 28.0 kV, 17.9 kV, and 21.3 kV, respectively. The breakdown voltage was lower than that expected for good-quality windings (28.6 kV) in a 13.8kV class generator. In the AC dielectric breakdown and diagnostic tests, there was a strong correlation between the breakdown voltage and the voltage at which charging current increases abruptly (Pi₁, Pi₂).

Assessment of Insulation Condition in Gas Turbine Generator Stator Windings

The results of off-line and on-line diagnostic tests performed on the stator winding of an air-cooled gas turbine(G/T) generator are reported in this paper. Off-line diagnostic tests included measurements of the ac current, dissipation factor(tan), and partial discharge(PD). Six epoxy-mica capacitors were installed in the three phases of G/T generator for performing on-line diagnostic testing with the turbine generator analyzer(TGA). The TGA showed that the normalized quantity number(NQN) and the PD magnitude() were high in phase A of the stator winding. Internal discharges were generated in phases B and C, and slot discharge occurred in phase A. According to the trend analyses of the NQN and values available for insulation condition assessment for G/T generator stator windings, it was concluded that phases B and C were in good condition, whereas phase A has been significantly deteriorated.

Propagation of Partial Discharge Signals in Stator Windings of Turbine Generators

We studied the propagation of partial discharge pulses in a stator winding by means of measurements on a dismantled 35-MW generator and found that a stator winding acts as a transmission line. Therefore, a partial-discharge (PD) signal manifests itself at the generator terminals after a transit time that depends on the location of the discharge. Due to capacitive and inductive couplings in the end-winding region, a second propagation mode is present for higher frequencies. This "fast mode" manifests itself at the terminals without appreciable time delay. The capacitive and inductive couplings also cause crosstalk between the phases. As a consequence, a signal measured in one phase does not necessarily originate from a discharge in that phase. The amplitudes of the fast mode and, to a lesser extent, the slow mode (or transmission-line mode) are heavily damped when the discharge occurs further away from the measuring terminal. The consequence is that only discharges close to the measuring terminal can be observed. The propagation of a PD signal is not only influenced by the construction of the generator but also by its external connections.

Preventative maintenance of turbine-generator stator windings

The authors discuss stator winding deterioration mechanisms and their consequences. Inspection and test procedures which will detect these mechanisms, as well as programs to arrest them, are also discussed. The analysis is broken down into sections on the slot region of the stator winding and on the end winding region. Particular emphasis is directed to issues unique to liquid cooled stator windings. A simple test is described which can be used to detect the presence of excessive amounts of water in the insulation of a liquid cooled winding.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radio Frequency Response of a Large Turbine-Generator Stator Winding

Radio Frequency Response of a Large Turbine-Generator Stator Winding

Radio Frequency Response of a Large Turbine-Generator Stator Winding

Long term experience with on-line radio frequency monitoring of large turbine generators has shown a need for increased understanding of the propagation of arc generated radio frequency signals in a stator winding. This paper presents the results of an on going research program designed to obtain an increased understanding of radio frequency arc detection in large turbine generators. Test results from AC arcing response and sinewave response measurements made on a generator stator are presented in addition to the results obtained from a computer circuit analysis performed on a radio frequency circuit model of a portion of the same generator stator. These results led to a better understanding of RF arc signal propagation and helped to identify improved detection techniques. Three RF measurement techniques were studied for both the measurements and calculations, resulting in good agreement. Arc voltage detection sensitivity calculations give a minimum detectable level of arc voltage for each RF measurement technique.

On Line Incipient ARC Detection in Large Turbine Generator Stator Winding

Early detection of developing malfunctions within large generator stator windings, of the type due to conductor fatigue failure, can result in a significant improvement in unit availability. This paper describes a radio frequency (RF) method of on-line monitoring which detects the arcing associated with conductor strands which have fractured from copper fatigue.

Water Cooling of Turbine-Generator Stator Windings

THIS ARTICLE presents the results of engineering and laboratory studies on the use of water as a coolant for direct cooling of the copper conductors of turbine-generator stator windings. Table I compares the relative rates of heat removal by several fluids under similar conditions that are practical for employment in turbine-generators.

Eddy currents in end windings

THE PROBLEM of calculating eddy-current loss in the end portion of turbine-generator stator windings involves a study of the three-dimensional field of the end region of the machine. This field may be subdivided into components, and each component determined by two-dimensional methods. The components are combined to give the total flux density at any point. Once the flux density is known, eddy-current loss may be calculated by methods previously developed for bus bars and laminated cores.

Destructive Breakdown Tests on a Large Turbine-Generator Stator Winding

Destructive Breakdown Tests on a Large Turbine-Generator Stator Winding

Eddy Currents in the End Portion of Turbine-Generator Stator Windings

Eddy Currents in the End Portion of Turbine-Generator Stator Windings

Turbine Generator Stator-Winding Temperatures at Various Hydrogen Pressures [includes discussion

The power industry has for years accepted certain general principles upon which temperature limits are based in the rating of electrical machines and apparatus. AIEE Standard No. 11 sets forth the philosophy to be followed in selecting the permissible total temperatures at various locations within machines, dependent upon the class of insulation employed. The particular values of allowable temperatures and the locations at which they shall be measured in cylindrical rotor machines are stated in the American Standards Association Standard (ASA) C50.2 Both of these standards established their recommendations many years ago, when turbine generators were all air-cooled machines and the industry's experience on the performance and life of such machines was a dominant factor in the selection of allowable temperatures. In 1937, when turbine generators began to be hydrogen-cooled, the allowable temperatures established for air-cooled machines and the temperature differences between the measurable temperatures and the hot-spot temperatures were tacitly assumed to be correct for the new type of cooling at a hydrogen pressure of 1/2 pound per square inch gauge (psig) in spite of a load increase which was realized with hydrogen cooling. With time, the practice of operating machines at hydrogen pressures higher than 1/2 psig has become quite general. Also for some years a number of machines have had their ratings based on capacities associated with increased hydrogen pressures, and there is no industry standard which recognizes the proper allowable measurable temperatures and temperature rises.

Temperature Drop to Resistance Temperature Detector in Stator Windings of Turbine Generators [includes discussion

Temperature Drop to Resistance Temperature Detector in Stator Windings of Turbine Generators [includes discussion

A New High-Voltage Insulation for Turbine-Generator Stator Windings

This paper reports the development of a new synthetic resin bonded mica tape insulation for high-voltage stator windings. It reviews the problems associated with earlier forms of high-voltage machine insulation. The general construction and processing of the insulation are described briefly. Comprehensive test data are presented on the characteristics of the materials employed and the resultant physical and electrical properties obtained on the composite insulation. This new insulation is now being applied to turbine generators for central station service.

Forces in Turbine Generator Stator Windings

Experience has shown that the forces on turbine generator armature windings during short-circuit conditions are large. The windings may be seriously damaged if adequate consideration is not given to the design of the coil bracing and support. In this paper the forces are calculated for a particular case by the use of images, by the change of flux interlinkages, and by the ∫ H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> d s over some surface in air which surrounds the conductor under consideration. A new and relatively simple proof is given for the validity of the latter method. The limitations and desirable applications of each method are considered. Forces on conductors in armature slots are computed when saturation is considered, and when neglected. Approximate formulas are derived for the forces on coil sides within one layer of the conical end winding, and for the straight coil extensions from the slots. A numerical example is given in each of those cases. The applicability of certain of these formulas to the phase connecting rings is pointed out. The force on the entire conical surface of the end winding is discussed qualitatively. Certain desirable features of end winding bracing are mentioned. In most of these cases, the instant considered is that following a line-to-neutral short circuit when the maximum possible instantaneous current is flowing.

Abridgment of forces in turbine generator stator windings

Abridgment of forces in turbine generator stator windings