1

A1-101

INFLUENCE OF SPECIAL SHORT CIRCUIT ON ELECTRICAL GENERATOR DESIGN

Ding Zhong MENG

( Hong Kong, China )

SUMMARY Refer to the IEC Standard 34-3, the generator shall be designed without failure, to withstand three-phase short circuit at its terminals while operating at 1.05 p.u. rated voltage. But in recent years, a special type of internal short circuit in generators occurred in China twice, and produced the fault current and mechanical stress much higher than the IEC Standard 34-3 for generator stator windings, thus resulted to a disastrous damage. One of the special internal single-phase to neutral short circuit occurred to Unit-2 ( 752.22MVA ) Generator of the Shajiao ‘C’ Power Station in China on 2 Oct. 1997. The analysis has shown that the fault current (mechanical stress) of fault winding and line nd are 1.45 ( 2.1 ) and 2.95 ( 8.7 ) times that of three-phase short circuit respectively. Another special internal double-phase to neutral short circuit occurred to Unit-1 ( 828.889MVA ) Generator of the Zhuhai Power Station in China on 28 Sept. 1999. In addition to the calculation based on the actual condition, the same parameters and condition as Shajiao ‘C’ Power station are used for comparison. The fault current (mechanical stress) of fault windings and neutral end are 1.35 (1.82) and 4.04 (16.33) times that of three-phase short circuit respectively. The internal special short circuit current will be significantly reduced after the tripping of generator circuit breaker ( GCB ) or main transformer H.V. breaker. The quantitative analysis during generator operation condition has shown that this special short circuit current would only increase a little when the 500 kV power system fault level increased from 20 kA to 63 kA, because it is limited by the reactance of its main transformer. According to the existing common design arrangement of the stator phase rings and their connection, flashover of adjacent rings would cause such special short circuit. The Isolated Phase Bus have to be installed to separate each phase between generator, generator circuit breaker and main transformer. With such an arrangement, it will not be possible to initiate a three-phase short circuit. Therefore, the probability of special short circuit occurrence would be much higher than three-phase short circuit. Recommendations have been accepted to rewind all the three 752.22 MVA generator stators of the Shajiao ‘C’ Power Station with a new design in Europe. The stator end winding supporting system has been greatly reinforced by cone support in stead of the previous basket support. The arrangement of phase rings has also been improved that all the high voltage line end rings are arranged on one

2

side, and the neutral end rings on another side. The rewind of those two stators had been completed in 2001 and 2002, the last one would be completed at the end of 2003. For the safe and reliable generators operation, it is recommended that the requirement for the protection against special short circuit should be included in the customer specifications. How could the stator design sustain and avoid such special short circuit in a reasonable and economical way? It is expected that a suitable study result would be supplemented in the future for the generator design standard.

3

A1-102 Modeling and Dynamic Simulation of Induction Machine under Mixed

Eccentricity Conditions using Winding Function

E. Sharifi-Ghazvini (*), J. Faiz(**), I. Tabatabaei-Ardekani(**), A. Shirani(*) (*) Niroo Research Institute

(**) Department of Electrical and Computer Engineering Faculty of Engineering - University of Tehran

Tehran, Iran

SUMMARY

Induction machines are widely used in many industrial processes because they are cost effective and mechanically robust.

Mixed eccentricity is one of the common faults happened in these machines. Thus studying the impact of this kind of fault on the machine performance is very important. The winding function theory has been used since several years ago for modeling and analysis of squirrel cage induction motors on healthy and faulty condition such as eccentricity. In this paper a comprehensive definition of the winding function for non-uniform air-gap is addressed. Thus dynamic simulation of induction machine under mixed eccentricity with a precise model can be possible for different purpose such as fault diagnosis. Also this paper shows that with Machine Current Signature Analysis (MCSA) technique, mixed eccentricity can be detected.

KEYWORDS

Induction Machine, Eccentricity, Non-uniform Airgap, Inductance, Current, Spectrum, MCSA, Diagnosis, Detection

* esharifi@nri.ac.ir

TWO-AXIS EXCITED TURBO-GENERATOR DEVELOPMENT IN RUSSIA TO MEET SOME MODERN ELECTRIC POWER INDUSTRY REQUIREMENTS

I.A.KADI-OGLI “ELECTROSILA” PLANT G.MAMIKONYANTS, YU.G.SHAKARYAN* ELECTRIC POWER RESEARCH INSTITUTE (VNIIE) YU.D.VINITZKY, CIGRE SC AL NATIONAL MEMBER FROM RUSSIA

(Russia)

Summary

Because of Russian power system operational feature changes requirement of the high stability levels in particular at reactive power consumption is very important.

Two-axes excited turbo-generators known as “Asynchronized turbo-generator” (ASTG) developed in the former USSR in 80-th could meet this technical requirement as well as to increase reliability of the generator and the whole system.

ASTG advantages have been fully confirmed by thorough tests and more than 15 years successful operation of two 200 MW ASTG at Burshtinskaya power plant (Ukraine).

ASTG design and manufacturing technology are somewhat more complicated and cost is slightly (about 20-30%) higher than those of conventional Synchronous ones (STG). That is why ASTG application in each case should be motivated technically and economically.

Investigations carried out recently highlighted a number of power plants in Russia where ASTG application could be useful.

The family of fully water cooled ASTG rated 110, 160, 220, 350 MW T3WA type has been designed. Their main design features are described in the paper.

Stator design is similar to those of the many years successfully used fully water cooled 800 MW STG of T3W-800 type. Special attention was paid to ASTG stator core end zone design to eliminate its overheating above specified limits at under excited operation.

Rotor of ASTG is the main machine component of particular design because of need to arrange two similar mutually orthogonal excitation windings. Self-pressure (or self-circular) water cooling system is recommended for the rotor cooling. Water circulation in this system is ensured by the natural centrifugal effects of rotation. Such system is considerably simpler and more reliable than conventional pressured pump-fed system. Its advantages was confirmed by the long term operation of 800 MW Turbine Generators T3W-800. One fully air cooled ASTG-110 MW has been designed, manufactured and passed factory tests recently. Its main design features and some tests results are represented in this paper. Key words: Power system – Asynchronized turbo-generator – Advantage – Operation – Application – Design feature – Water cooling – Air cooling – Test result.

E-mail: dir@vniie.ru

A1-103

5

A1-104 DESIGN AND TEST FEATURES OF GENERATORS WITH MIDDLE

OUTPUTS ZORAN MILOJKOVIĆ M. Sc. E. Eng.1 DAMIR GRUBIĆ, B. Sc. E. Eng.

KONČAR – Generators and Motors Inc., 10000 Zagreb, Fallerovo šetalište 22 Croatia

Summary The paper presents some improvements made in design and production on installed generators of medium size.

- New pole shoe shape was applied (formerly sinusoidal and tapered type of pole shoes was used).

- Holes in pole shoes for inserting of damper winding bars are completely closed (usually used with small openings 2-3 mm depth and 2-3 mm width).

- Stator windings for both generators were made as lap - bar type (usually for low speed generators wave bar type was used).

- For hydro generators, ventilating ducts in rotor laminated rim were obtained by special stacking of laminations,

- VPI system used for many years was improved, - Connections of stator bars were made by bar-to-bar hard brazing method using copper

(beside of this method for connections of bars, strands-by-strands hard brazing method is used).

Results of these improvements are presented on two generators installed in the last three years, one in HPP Plave II and the second in HPP Doblar II. Both power plants are located on the Soča river in west region of Slovenia, about 50km from Italian border. Both HPP-s are new buildings, close to existing power plants. Generator for HPP Plave II (23 MVA, 10,5 kV, 50 Hz, cos ϕ = 0.8, rated speed 166.7 rpm/ runaway speed 490 rpm, 2900 tm2) was installed and commissioned in the year 2001. Generator for HPP Doblar II (50 MVA, 10,5 kV, 50 Hz, cos ϕ = 0.8, rated speed 214.3 rpm/ runaway speed 550 rpm, 2600 tm2) was installed and commissioned in the year 2002. Both generators are running by Kaplan turbine. The generator for HPP Plave II is umbrella type and generator for HPP Doblar II is suspended type (with combined bearing above and guide bearing below the rotor). The paper describes design and structural main parts of generators. The same design intention will be given as well as essential performances through testing results of mentioned generators, as for example graph of generator voltage wave form, deviation factor of voltage wave form, reactances, temperature rises and generator power losses etc. KEYWORDS Construction, production, generator, characteristics, dielectrical losses, power losses, temperature rise, voltage waveform.

1 E-mail: zmilojkovic@koncar-gim.hr

6

A1-105 New development in the design of larger generators for nuclear

plants with reliability target. by

Michel VERRIER*, Michel Thiéry, Pascal Chay, Gilbert Martinet, michel.verrier@power.alstom.com

ALSTOM (France)

RESUME

In the field of very high rating 4-pole generators for nuclear power plants, considerable experience could be gained through the detailed knowledge of operation and maintenance of more the 70 units installed in France and in other countries since the beginning of the 70s. This well-structured and well-interpreted experience now allows us to select the best solutions in terms of reliability amongst our fleet; it also allows us to act on the most critical factors and to give a reliable diagnosis of the reliability of the new prototypes. At a very early stage, the Manufacturer, together with the Users, knew that the experience that would be gained with these units would be the determining factor for the optimization of maintenance and technical developments, with the aim of providing the best service. To that effect, this article shows how an early-stage initiative was taken by them by creating a fine data collection system, regarding both reliability information and component deterioration. For twenty years or so, this system has allowed the existing fleet to be managed in an optimized manner and the most critical components to be improved. This experienced tool could be further used to design the 1950MVA models according to the high reliability target fixed. A very ambitious reliability target was fixed for this study and the technical measures envisaged make it possible to anticipate a reliability level which is in accordance with this target. In order to make reliable forecasts of reliability during operation, this method must be developed. This implies that it is necessary to analyze, diagnose and inventory all the unavailability occurrences over long periods and to assign them to machine components or maintenance actions. A tight cooperation between the Users of the different power plants and those who designed the units in operation is necessary. The reasons why we can rely on the results of the reliability forecasts made within the framework of this study are highlighted in this article.

7

A1-106 The World’s Largest Capacity Turbine Generators with Indirect Hydrogen-

Cooling Toshio Kitajima*, Hiromichi Ito, Susumu Nagano, Yukihiko Kazao

Toshiba Corporation (Japan)

In recent years, the power generation market has been demanding simple generator systems from the viewpoint of better operability and maintainability, and lower initial cost. In order to meet such customer’s requirements, authors have endeavored to expand generator capacity range of the simplified cooling system, i.e. to develop large-capacity air-cooled generators and large-capacity indirect hydrogen-cooled generators. This paper describes the state of development of indirect hydrogen-cooled generators of up to the 600MVA class for both combined-cycle and coal-fired thermal power plants. It has been a long standing practice to use direct water-cooling of the stator coil for generators larger than 400MVA. Now, authors have successfully applied the hydrogen gas cooling, which is indirect cooling of stator conductors through the insulation, to the generators of the above-mentioned capacity range. That results in the big advantage in operation and maintenance, because the auxiliary equipment and piping required for water-cooling system can be eliminated. However, the indirect hydrogen-cooling system has the issue of the stator coil cooling enhancement, because its cooling performance is inferior compared with the direct water-cooling system. In order to solve this issue, authors have developed new insulation system with higher thermal conductivity, and studied the optimal flow distribution of hydrogen gas in the generator ventilation circuit. These activities were performed by an experimental study using some models and a computer simulation. Applying these results, a high power density and a high generator efficiency have been accomplished comparable to those of the water-cooled generator. In addition, the structural parts such as the stator frame and the stator end-winding support were designed to be highly reliable, and at the same time, size reduction and structure simplification were realized by using FEM (Finite Element Method) dynamic analysis. Based on the above, hydrogen-cooled generators of 60Hz-500MVA and 50Hz-620MVA have been developed. The first 60Hz-500MVA generator was manufactured in 2002 and it demonstrated the expected high performance and high quality in the shop test. To date, including this, three generators of the same type have been manufactured and are commercially operating. In addition, nine generators, to which the developed technologies are applied, are currently either being designed or being manufactured for domestic and overseas customers. The hydrogen-cooled generators developed this time fully satisfy the customers’ requirement in performance and quality. The authors are advancing a further capacity enlargement of indirect hydrogen-cooled generators, and the applicability to 700MVA class is just in sight. Key words: Generator, Hydrogen-cooling, Electrical insulation, Ventilation, Structural analysis

8

A1-107

Performance Evaluation and Measurement of the 250-MVA Class Air-Cooled Turbo Generator

Kenichi Hattori*, Hiroshi Okabe, Kazumasa Ide,

Keiji Kobashi, and Takashi Watanabe Hitachi, Ltd.

Japan Today’s generator market is concerned with the long-term reliability and cost performance of power plants. While our 250-MVA class air-cooled turbo-generator with an ‘Inner Cooler Ventilation System’ has an efficiency over 98.8 percent, its temperature is within the class-B limitations and has the actual insulation of a class F unit. We therefore evaluated the generator’s potential by placing more than 1000 temperature and ventilation sensors in a test generator. This paper discusses the results of those measurements of the air gap ventilation, stator strand temperature, and rotor winding temperature. The ventilation calculations showed that the key value for temperature of the stator is the flow in the air gap. Various kinds of heat sources exist in the gap; such as the stator core, the rotor surface and the exhaust. The flow and temperature distribution are quite complex in this area. To measure flow distribution in the air gap, anemometers were inserted through the ventilation ducts. Temperature distribution at that location is also measured. Over 100 temperature sensors were also embedded in the stator windings. Because the stator windings carry a high voltage, optical sensors were attached to the strands while the test generator was being manufactured, before the strands were insulated. For the rotor, a total of 166 temperature sensors were placed on the interior and exterior of the slots. The test was conducted under full-load field currents by electrically connecting the generator to a load machine with larger output range. The highest temperatures measured in the stator and rotor windings had a wide margin relative to the class B temperature limitations; which assures the long-term reliability of the generator. The measured temperatures were correlated to the temperatures calculated using design programs and confirmed that the actual performance of the 250-MVA class air-cooled generator is satisfactory. KEY WORDS: Turbo-generator – Armature – Field – Air – Cooling – Measurement – Temperature – Flow

kenichi_hattori@pis.hitachi.co.jp

9

A1-108 CONVERSION OF A COMMUTATOR EXCITER INTO A BRUSHLESS EXCITER :

BENEFITS, DESIGN AND ACHIEVEMENTS J.F. BRUDNY 2*, Th. GODIN**

(*) Laboratoire Systèmes Électrotechniques et Environnement (LSEE) Université d'Artois, FSA, Technoparc Futura, 62400 Béthune, France

(**) eng., Hydro-Québec 75, boul. René-Lévesque, Montréal (H2Z 1A4), Canada

Rotating exciters that supply the rotors of synchronous generators with DC currents still remain essential components of today’s existing electrical power generation equipment. However, the vast majority of these machines were installed before 1960, and most of them have not been refurbished yet. Consequently, if nothing changes, an increase in the forced outage rate is to be envisaged. Most commonly, a commutator exciter (DC machine) is refurbished by re-insulating the unit in class F (155ºC) and then, by restoring or simply replacing its commutator. Another very interesting solution is to replace the DC machine by a rotating diodes exciter (brushless machine) whose benefits are well known: elimination of the carbon brushes, commutator, collector rings and main field breaker, all of this leading to a substantial reduction of the maintenance and group unavailabilities, which can result in an estimated annual saving of 30 k$ per machine. Nevertheless, the cost of a new brushless exciter is far from being negligible. Also, as its dimensions are surely different from those of the original machine, it is necessary to reassess the technical devices that are required for its installation. Because of that, the concept of converting a DC machine into a brushless machine became appealing, especially if the pay off of the costs involved is within 10-years. In order to reach this target, the following constraints were imposed in the conversion of two prototypes ordered by HQ : keep the main exciter rotor and its stator frame (1), keep its time constants as much as possible (2), keep its voltage regulator which requires that the resistances and exciting currents must remain almost the same (3), and, finally, keep reasonable current densities in the new multi-phase winding (4). The originality of this report lies in the respect of all the above constraints. The optimization in the search for the best possible solution to this apparently unsolvable problem is presented. At first, the general relations describing the machines are restated by putting forward the fact that a simple transformation leads to far too high brushless excitation currents when compared to the DC machine. Then, an optimization process which allows a significant reduction in the brushless excitation current initially calculated within the imposed constraints is described. Finally the last chapter summarizes the experimental results obtained following the transformations realized on the two prototypes, of which only one had been optimized. Key words : Generator – Conversion – System - Excitation – Commutator – Brush - Diode

2 brudny@univ-artois.fr

10

A1-201

EXPERIENCES OF STATOR CONDUCTOR OXIDATION BUILD UP FOR 500 MW GENERATORS

B N OJHA, DIRECTOR (O) & D K SOOD, DGM (OS) National Thermal Power Corporation

( INDIA ) .SYNOPSIS Stator conductor oxidation build up has been the cause of concern for the utilities. Uncontrolled build up results in partial blockage of the hollow conductors in Stator windings, which can result in overheating and premature failures. Increase of differential pressures across the stator windings is the key factor, which governs the determination of oxide build up in Stator winding hollow conductors. Differential pressure across the stator windings is the average reflection of the total winding. Some conductors in the stator windings may be heavily blocked which otherwise may also be not wired up for temperature monitoring. This blockage over the period results in the lower cooling and may result in subsequent failures. Authors have experienced high differential pressures across stator windings in few 500 MW Units. The name plate details of the Generators under discussions are 588 MVA, 0.85 p.f (lagging), 21 kV & 50 Hz frequency. The Stator winding of these units is directly cooled by demineralized water, which flows through hollow conductors of stator bars. The stator core and rotor is cooled by hydrogen gas. The paper explains how the Stator conductors oxidation build up results in partial blockages and high differential pressures across the Stator windings. In two cases the mix up of the materials of stator water system and in balance cases the shift of oxygen regime resulted in build up of the oxidized deposits. The paper explains how the stator conductor oxidation build up results in partial blockages and high differential pressures across stator windings. Chemical cleaning of the stator hollow conductors was attempted with both H2SO4 and EDTA solutions. The results and effects of both the processes have been discussed in details in this paper. In Authors opinion oxygen regime is very important parameter and need to be regularly maintained to avoid corrosion/build up of the oxidation in generator stator hollow conductors. Flushing of the stator conductors with H2SO4 and subsequent alkalization of stator water has been good experience as compared to EDTA flushing without alkalization of the stator water. The units, which were treated with H2SO4 solution were subsequently fitted with alkalizers to improve the pH value of stator water system from 7.0 to 8.5 or more. The frequency of stator water sampling was also restricted to once in a month for avoiding the fresh intake of de-mineralized water. The oxygen content of stator water system is being maintained less than 100 ppb to avoid oxidation of the hollow copper conductors. Units, which were cleaned with EDTA solutions, had tendency of increase in differential pressures frequently. The reverse flushing is being resorted to whenever differential pressures reaches to the specified value. Moreover during short capital overhauls the flow in each PTFE hose is being measured and compared with the earlier records to ascertain any blockage condition. In subsequent units the monitoring of stator conductors temperature was specified for each slot rather than six slots (two/ phase). The units having stainless steel as hollow conductors have not given any problem for last seven years of continuous operation. Authors feel that the regime of operation whether high oxygen or low oxygen needs to be strictly maintained to avoid corrosion/build up of the oxidation in the stator conductors. The controlled make up of stator water is considered equally important for preventing the disturbance in shift of the oxygen regime. Key words: Generators - Hollow Conductors - Oxidation Build Up – Corrosion – Cleaning Techniques

11

A1-202 ASSET MANAGEMENT PRACTICES FOR AUSTRALIAN TURBO-GENERATORS

J. LINTON* DELTA ELECTRICITY

H.ROOKE ERARING ENERGY

(Australia) SUMMARY

A survey has been conducted of generating utilities who collectively own and operate approximately 55% of total installed turbo-generator capacity in Australia. The utilities surveyed are all members of Cigre Australian Panel A1. The aim was to document commonly used practices in Asset Management and their application to assist with ensuring continued capability of Australian generating plant in cost effectively meeting business requirements. In doing this, a summary database for members, both in Australia and internationally, is provided. The survey sought information on: − Generic organizational planning processes. − Past, current and anticipated future operating detail. − Historical type and plant specific faults and failures. − The generic asset maintenance strategy followed. − Maintenance tasks, resources and condition based diagnostics used. − Data analysis. − Links to strategy and business decision making for maintenance, upgrades or replacement.

The paper backgrounds the adoption of Asset Management in Australian utilities, and highlights the key principles as well as the distinction to, and overlap with Maintenance Management.

The information provided by the survey was analysed and compared between and across respondents. The paper summarises the key practices followed in generic planning processes, and their application to turbo-generators. It identifies the range of key operating data in the subject population, and summarises typical type faults and major maintenance tasks completed. Alternatives available for maintenance strategies and where and why they are used is reviewed.

The paper looks in general detail at maintenance practices, including outage scheduling; what resources are used; the range of condition based diagnostic tools used in predictive and preventive maintenance of generator components; and how the results are analysed. Finally the range of decision making processes used and types of intervention is reviewed, and sources of technical advice to Australian generating utilities is summarised.

KEYWORDS

Asset management; maintenance; generator; condition monitoring; analysis; strategy; decision making.

12

A1-203

DETERMINATION OF THE ACTUAL PQ DIAGRAM OF THE HYDROGENERATORS, BEING IN SERVICE, IN ORDER TO ESTABLISH THEIR MAXIMUM OPERATING DOMAINS AND

THEIR CAPACITY TO PROVIDE SYSTEM SERVICES

DAN ZLATANOVICI *, POMPILIU BUDULAN, RODICA ZLATANOVICI

ICEMENERG (Romania)

The theoretical PQ diagram represents the diagram of the reactive power versus the active power of a hydrogenerator, deduced from the phasorial diagram of the synchronous machine with salient poles. The actual PQ diagram is in fact a thermal diagram, in which the main theoretical curves represent the isotherms of the maximum admissible temperatures of the different parts of the electrical generator. Determination of the actual PQ diagram of the hydrogenerators being in service is performed in steady state operation, by measuring the maximum overheating in the stator and rotor windings, the stator core and the frontal teeth sheets of the stator core. The temperature measurements for the stator winding and core were performed using Pt 100 thermoresistances, mounted in the stator slots at generator’s manufacturing. For the field winding the temperature was measured indirectly, using the resistance variation method. Temperature measurements in the frontal zone of the stator were performed by mounting special devices fitted with temperature and flux density transducers in the first ventilation channel and on the frontal face of the first pack of sheets. Based on the overheating and flux densities measured on the boundary of the frontal pack, the losses and the overheating inside it are determined in the nodes of a discrete network by using a computer program called TEMP; the hottest point is located this way and the variations of the overheating in that point are determined as a function of the reactive power Q at different active power P levels. The thermal operating limits in the capacitive regime are determined using the ∆θ = f (P, Q) curves. In order to introduce paid voltage control system services in Romania, by producing or absorbing reactive power by the hydrogenerators, ANRE (Romanian Electricity and Heat Regulatory Authority) imposed the delimitation of two types of bands on the PQ diagram of the electric generators: a primary band where the produced / absorbed reactive energy is not paid and two secondary bands where the produced / absorbed reactive energy is paid. The limits of these bands are drawn on the actual PQ diagram. The actual PQ diagram was experimentally determined for a number of 25 hydrogenerators with powers between 27 – 170 MW. 3 classes of hydrogenerators were revealed: actual diagram identical with the theoretical diagram, actual diagram more restrictive than the theoretical one by about 10-20% and actual diagram much more restrictive than the theoretical one by 20-60%.

Thermal Evaluation of Air Cooled Generators to Investigate the Generator Rotor Overheating – A Pre-rewind Evaluation Case Study

ASHUTOSH SHARMA* JAMAL AL A’ALI NILS-IVAR LANDGREN Aluminium Bahrain Aluminium Bahrain Alstom (Bahrain) (Bahrain) (Sweden)

Summary Rewinding of generators stator and rotor is a common practice for life extension of the generators being followed by power utilities. This is the stage when the latent defects can be addressed without much of an additional cost. To address these defects strong engineering tools like thermal modeling supported with online site measurements is required to do reverse engineering. The paper presents a case study of a unique investigation carried out on a gas turbine driven air-cooled 20 MW generator to find out the cause of overheating at central part of the generator rotor in 5 generators. Generators feeds an aluminium smelter potline rectifier transformers having diode based rectification. Preliminary observations were indicating that this may not be due to steady state operation of the generator but with no certainties. The generators were marked for refurbishment hence it was necessitated to carry out investigation so that the same shall not occur once refurbished. For investigation a thermal model devised for Air Cooled generators was used. The model was developed for design development and for engineering application tool for generators used by manufacturer with design details before hand. The program of the model calculates temperature of air, stator core, stator windings and rotor body and air pressures, velocities, air flows and ventilation losses in the generator. It was re-configured for this specific application. Model was adjusted and verified on a generator, where most of the design details were available, with site measurements. The actual operating data such as static and dynamic air pressure, air temperature was collected online by installing additional measurement devices on the generator. The measurements were carried out at different operating load conditions and transient operating conditions were simulated. The synchronism between calculated and measured data on the generator under investigation under different operating load condition indicated the accuracy of model. The results are presented in graphical form in the paper. Typical calculated results from the program of the model for 100 load run is attached to the paper as Appendix. After due verification of the model it was used to extrapolate the rotor surface temperature values for analysis and investigation. The investigation indicated that the cause of overheating was of transient nature and no preventive measures for the operation was recommended but it’s out come recommended many modifications in basic components of the generator such as rotor balancing screws, generator cooling fan inlet duct design, air cooling radiators and these are to be addressed during refurbishment.

*Ashutosh.Sharma@alba.com.bh

A1-204

14

A1-205

FAILURE ANALYSIS OF A 360 MW POWER UNIT GENERATOR M. PAPADOPOULOS N. BOULAXIS D. TSANAKAS A. SAFAKAS

National Technical University of Athens University of Patras

GREECE

Keywords: Generator - Failure – Breaking – Overheating

SUMMARY

Agios Demetrios is one of the largest lignite power stations of the Public Power Corporation of Greece-PPC, situated in the North-West area of Greece. It consists of four Units of 360 MW nominal capacity (No 1 to 4) and a step-up to 400 kV substation. Late in the evening of 25-12-2001 (exactly at 21:11) explosion of the 400 kV voltage transformer occurred (in phase A of the Unit No 4) that resulted in the destruction of the generator of Unit No 2 and in extensive consumers interruption. This happened due to the fact that catapulted fragments of the exploded voltage transformer, provoked serious damage of one of the 400 kV isolators of the Unit No 2, so that the phase B of the Unit remained connected to the power system, although the main circuit breaker was opened by the operation of the protective relays. As a result, the generator of the Unit was destroyed by overheating.

In this paper, first, a short presentation of the power station and a description of the events and damages are given. Next, the detailed analysis of the failure is effectuated by simulation, using the equivalent circuits of the main components of the system. Based on this analysis the behavior of the damaged generator during the fault is assessed. The results are compared to the registration of the instruments of the power station and the distance relays of the nearby substations and a close approximation between the corresponding values is ascertained. The expected overheating of the rotor is also assessed.

In conclusion, by the analysis effectuated, all the events that followed the fault and leaded to the generator damage were confirmed and all the weak points of the connection and protection scheme were revealed.

15

A1-206

ELETRONORTE TUCURUÍ HYDRO PLANT EXPANSION AND MODERNIZATION

FRANCISCO RENNÓ NETO*

GERALDO NASCIMENTO RAMOS Rennó Tecnologia e Representações Ltda.

(Brazil)

CARMO GONÇALVES CID ANTUNES HORTA

WILSON GERALDO DO NASCIMENTOWANDYR DE OLIVEIRA FERREIRA

Eletronorte – Centrais Elétr. Norte Brasil S.A.(Brazil)

MARC BISSONNETTE VibroSystM Inc.

(Canada)

DORINATO GOMES DE LIMA MRDM Engenharia

(Brazil) Keywords: - Air Gap System - Monitoring System - Predictive Maintenance - Machine Condition The Brazilian Utility in this paper is one of the most important state owned power company in Brazil. It generates and supplies power to the North Region of Brazil, including 9 States: In order to improve power output as well as machine’s performance, the Brazilian Utility has requested several high-tech improvements in this expansion installation. As shown, machine outputs have been increased from 330 to 382 in turbine shaft. In addition to that, all the new machines will be equipped with a complete automated monitoring system. The on-line monitoring system includes: Air Gap monitoring System, Stator Bar Vibration System, Magnetic Flux Measurement, Relative and Absolutes Vibration of the Bearings, Stator core absolute vibrations, Shaft Displacement, Upper Bracket Displacement, Spiral Case and Draft Tube Pressure, Draft Tube Pulsation Pressure, Pressure Pulsation in the Francis Turbine (spiral case, draft tube and head cover), Wicket Gate Position, Active Power, Temperatures, Partial Discharge, Generators temperatures, Bearings temperatures, Level (upstream and downstream), Active Power Output, Shaft Seal Wear For the Nominal Head of the first stage, each machines supplies 330 Mw. For the second stage, each generator active output machine will supply 375 Mw. For the maximum head in the first stage, the machines supply 350 Mw per unit and in the second stage, the machines will supply 396 Mw per unit. With these monitoring system plant operation and maintenance will be very much improved. Always can be set as well as tendency curves can be obtained and full diagnosis results can be printed and visualized in order to either carry on the operation of the machines or shutting them down. In other words, a Condition Based Maintenance will be the result of this installation. All the measured parameters can be analyzed at Plant Computers as well as in remote location such as the Brazilian Utility’s Head Office. The next step will be to refurbish the existing machines (12 main machines and two auxiliaries machines – Phase I) and install the same type of monitoring system. The main information about the monitoring system are: Information about construction and installation of machines under Phase II, Cost x Benefit Analysis of the monitoring systems, Results obtained through the system, Facilities provided by the system during commissioning, Drawings, pictures and displays of plant Expansion as well as of the monitoring Systems Installed.

STUDY AND DEVELOPMENT OF ON-LINE MONITORING SYSTEM FOR A KEPCO PUMPED STORAGE GENERATOR/MOTOR

HEE-DONG KIM, YOUNG-HO JU YONG-JU KIM

KEPRI KERI

KYU-BOCK CHO* HANSEO UNIVERSITY

(KOREA)

*Hanseo University, Dept. of EE, 360 Daegok Haemi Seosan Chungnam Korea; E-mail: kbcho@hanseo.ac.kr

SUMMARY An on-line diagnostic system has been developed and verified by on-site test performance of the KEPCO hydro-generator/motor set. Scope of this study was briefly described as follows: [A]. Investigation of the technical information

(1) Investigation of on-line diagnostic technology for the pumped storage generator/motor. (2) Analysis of on-line partial discharge, shorted-turn and air-gap (3) Investigation of worldwide research activities and technical information.

[B.] Application research of partial discharge coupler, shorted-turn and air-gap sensor (1) Characteristics of partial discharge coupler, shorted-turn and air-gap sensor. (2) The method of installation for partial discharge coupler, shorted-turn and air-gap sensor. (3) Field tests for development of partial discharge capacitive couplers, shorted-turn and air-gap

diagnostic algorithm. [C.] Development of on-line partial discharge, shorted-turn and air-gap monitoring system

(1) Development of on-line diagnostic method (2) Development of on-line partial discharge, shorted-turn and air-gap monitoring system (3) Development of partial discharge, shorted-turn and air-gap diagnostic program

[D.] On-line measurement for the pumped storage generator/motor (1) Samrangjin pumped storage generator/motor #1. [E.] Field tests for the reliability of shorted-turn monitoring system

(1) The test was conducted using new monitoring systems, PDA (Partial Discharge Analyzer) and digital oscilloscope.

(2) It is confirmed that results of two systems are very same in field tests. Further on due to limit of available space reporting those concerned works here, so only some parts of development works are to be presented and discussed in this paper [1, 2].

A1-207

17

A1-208

STATOR DEFORMATION OF LARGE HYDROGENERATORS AND ITS EFFECTS ON THE

MACHINES

DR. C. AZUAJE MSC. A. MILLAN C.V.G. ELECTRIFICACIÓN DEL CARONI, C.A. (EDELCA)

(Venezuela)

SUMMARY

This paper is about the evaluation of a stator roundness on hydrogenerators in some of the EDELCA´s hydroelectric complexes in Venezuela.

Large hydrogenerators frequently have several parallel groups on stator windings. These generators also have a neutral unbalance current and a phase unbalance current protection.

During the operation of these units, high values of current unbalance between neutrals/groups in parallel per phase were found, reaching alarm and sometimes trip values causing the shut down of operation of the generators. Complete vibration measurements of units were performed and normal values were found; all vibration measurements taken on bearings and static parts had acceptable values. Air-gap measurements on static conditions showed that there was a lack of roundness on the stator of one of these units. Stator behavior of a 200 and a 360 MVA machines was evaluated during the process of heating and cooling, and it was found that some of the sliding bases that were suppose to slide in the radial direction during heating expansions and contractions were stacked, not allowing the stator to keep its round shape for the operation. In this way the air-gap was then not uniform around the rotor causing an unequal distribution of the magnetic flux and consequently, the induced voltages on the parallel groups of the stator winding were unequal too. As a result, an unbalance current between neutrals or a current circulation between groups of the same phase was caused.

Since sometimes this effect cannot be observed on vibration measurements from static parts, on-site measurements were done on rotating parts of a 700 MVA generator. Due to magnetic unbalance caused by a non uniform air-gap, the effect on the rotor structure was investigated on a generator of 250 MVA at Macagua Power Plant using the Finite Elements Method. As a conclusion of this study, limits for stator roundness were established to avoid fatigue on rotor spider structure as a consequence of the cycling forces caused on the rotor structure at the rotating speed frequency.

KEYWORDS

Hydrogenerators - Stator Deformation - Synchronous Machine - Magnetic Force - Air-gap.

18

A1-209

EXPERIENCES IN IDENTIFICATION OF PARTIAL DISCHARGE

PATTERNS IN LARGE HYDROGENERATORS

DR. C. AZUAJE* W. TORRES

C.V.G. ELECTRIFICACIÓN DEL CARONI, C.A. (EDELCA)

(Venezuela)

SUMMARY

In this paper some experiences on partial discharge (PD) pattern identification in hydrogenerators from 103 to 805 MVA at the Guri and Macagua II power stations in Venezuela are presented. After a PD analysis on a 805 MVA generator, a complete inspection was performed during the disassembling process when stator rewinding was scheduled; it was found a very advanced damages on the surface of stator bars due to slot discharges, confirming the conclusion reached after the partial discharge analysis. Visual inspections and measurements have been carried out for partial discharge pattern identification in several different generators. In addition to these results, simultaneous measurements of on-line partial discharges were performed with vibration measurements on the stator core and frame under different operating conditions. Ozone concentration was also evaluated. Since slot discharges are directly related with movement in slots and to determine how load conditions would affect magnetic forces over stator bars and their oscillatory movement on the slot, simulations by MEF in electromagnetic fields were performed. In the simulations, the effect of power factor of the load and increasing load were evaluated.

Keywords: Hydrogenerator – Stator winding – Electrical Insulation – Partial discharge –

TVA antenna –MEF.