Large hydrogenerator design using advanced tools

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LARGE HYDROGENERATOR DESIGN USING ADVANCED TOOLS A. Merkhouf, L. Benchaita GE Hydro, 107 Park St. N, Peterborough, Ontario, K 9 J 7 B 5 , Canada

Keywords: Maximum optimisation, finite element, Electrical machines, Permeance program.

new analysis tool has also proven to be very powerful in analyzing the sources of vibration frequencies and mode shapes and,in the negative sequence heating studies.

Abstract A more powerful optimization technique has recently been developed that enlarges the design space, optimizes the design, and further automates the design process. The new process is used to optimize the total losses in generator design with constraints on the stator temperature rise. Using this new technique it was possible to reduce the total losses by 20 %. Automatic finite element mesh generators for both integral and fractional slot machines have been developed to model single and multiple pole pitches for electromagnetic field solutions. The permeance model developed recently by using combined analytical and magneto-static finite element solutions with one pole pitch model has proven to be very powerful in analyzing the sources of vibration frequencies and mode shapes.

1. Introduction In the design process of large electrical machines, numerous tradeoffs between conflicting requirements are involved. Therefore, modem computer tools and clear design practices are of paramount importance in achieving competitive robust designs. Recently many modem design tools were introduced with embedded optimization techniques that allow engineers to explore a large number of design possibilities in a short time optimize the design for each customers' needs. For more accurate prediction of the electromagnetic parameters, advanced electromagnetic analysis software tools have been developed. Automatic finite element mesh generators for both integral and fractional slot machines have been developed to model single and multiple pole pitches for electromagnetic field solutions. The geometry and mesh density are modeled for the desired analysis application using the design parameters obtained by the new process. Both steady state and transient electromagnetic field problems with balanced and unbalanced loads have been solved using timestepping formulations with actual winding topology, load conditions and kinematic equation of rotor motion [1-7]. The permeance model has been developed recently by using combined analytical and magneto-static finite element solutions with one pole pitch model [12,1 I]. The method has been applied to large hydro-generators with fractional slot stator windings. This novel method makes the computational algorithm very fast compared to time-stepping solutions. This

2. Hydroelectric generator design optimisation GE Hydro is a world leader in the design and manufacture of large hydro-generators in the range of 10 to 1000 MVA. Each machine is optimised to meet each customers particular contract specifications. The designs are generally optimised for minimum cost and maximum efficiency. GE Hydro's primary electromagnetic design program has sizing and optimisation capabilities that scan the design space and pick the best design. A more powerful optimisation technique has recently been developed that enlarges the design space, optimises the design, and further automates the design process. Because of the large number of input variables and generator performance parameters, the overall generator optimisation problem has been implemented in a multi-task approach. The overall generator optimisation is broken up into a sequence of smaller optimisation problems that address individual generator components. The optimisation of individual components is performed in subtasks for a given set of main performance requirements. The new optimisation process then performs trade-off and design sensitivity studies to determine the benefit of modifying design constraints and improving overall design. 7m

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The new process is used to optimise the total losses in generator design with constraints on the stator temperature rise. Fig. 1 shows the evolution of total losses and the stator temperature rise in hydroelectric generator as a function of the number of iterations of the process. It can be observed that the use of this optimisation procedure can results in a reduction of the total losses by 20 % (compare to the first design) while maintaining the stator temperature rise specification of 82 degree Kelvin.

the flux linking the stator is trapped in the armature winding at the instant when the short circuit is applied. Due to large armature fault currents, the magnetic flux in the stator teeth is predominantly tangential with flux-density levels around 2 T. As the rotor moves out of the initial position, the currents induced in the damper bars lead to strong saturation in the bridge above the damper slots, which can he recognized by field lines circling around individual bars and escaping into the air gap.

3. Advanced electromagnetic using TSTEP i

Due to the advent of powerful computer workstations, it is now feasible to use time-stepping finite element methods for many design applications. Since the time-stepping FE (TSTEP) formulation models the dynamic behaviour of the machine rigorously, the simulation results have been proven to be more accurate and reliable than those obtained by conventional analysis tools, such as MMF-permeance or equivalent circuit models. The application of the FE timestepping analysis to the control of acoustic noise of large motors for variable speed operation has been demonstrated in an earlier paper [I]. Other applications of this new software tool have also been reported in previous publications [2-61. The present capability of the software package includes the following analysis and post-processing options: Open circuit analysis at rated terminal voltage or at any other point on the saturation curve; load analysis in generator or motor mode; steady state short circuit analysis such as tbree-pha:e or phase to phase and sudden short circuit (three phases or phase to phase) analysis from open circuit or load pre-fault conditions. Fig. 2 shows the field distribution in a generator (263 MVA, 32 poles, 15.7kV, 50 Hz and 4 damper bars per pole) at open circuit operating condition and rated voltage. The armature winding has a fractional number of 4.5 slots per pole per phase, which allows a two pole-pitch model to be used in the time stepping simulation. -.

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Fully automated data reduction programs based on the graphical procedure of IEEE Std. 115-1995 [9] are used to determine the values of x ' ~x:'~, , TldrTIld from the wave forms of instantaneous phase currents during the sudden short circuit. The obtained results for this particular hydroelectric generator are summarized in Table 1. The predicted reactances and time constants in this particular case are compared to the correlated test data.

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TSTEP was also used in a specific study of the evaluation of generator continuous negative sequence capability [7]. In this particular operating condition, the thermal analysis of the damper winding requires an accurate prediction of the current distribution in the damper circuits. The damper current distribution for unbalanced steady-state operation has been traditionally calculated with d- 'and q-axis synchronous machine equivalent circuits [IO]. These analysis methods have provided qualitative information on the distribution of

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damper bar currents. The bars in the tip of the pole, predominantly on the trailing edge, are exposed to larger currents than in the center of the pole. This distribution pattem has been confirmed by test results [IO]. The accuracy of the solution for the bar currents is, however, limited by the accuracy with which the d- and q-axis circuit parameters can be determined. Table 2 displays the damper bar current distribution obtained from FE time stepping simulation, when the stator winding canies 20 % continuous negative sequence current for an hydroelectric generator with rating of 80.6 MVA, 28 poles, 1IkV, 50 Hz with 7 damper bars per pole, more details on this simulation study is given in [5]. The currents decay from the maximum values in the outermost bars of the pole tip towards the center of the pole. At twice line frequency, the bar currents in the trailing edge are higher than in the leading edge, and the bar current in the trailing edge is almost three times the bar current in the center of the pole. The current in the three bars in the pole center (bar 3 to 5 ) at frequency of 100 Hz are almost uniform with an average magnitude of 180 A. BarNo 100Hz 200Hz 1200Hz I 757 124 124 2 154 58 58 3 186 60 60 4 181 60 60 5 141 59 59 6 93 36 36 7 394 19 16

TABLE 2: CALCULATE0 DAMPER BAR CURRENT

4. Advanced electromagnetic using WAVE It is important that designers of large synchronous machines be able to evaluate their designs in terms of various performance parameters. However, an accurate prediction of the electromagnetic performance, such as, negative sequence rotor heating, requires prohibitive computing resources for time-stepping finite element modeling and solution. This is especially true for machines with fractional slot windings where the field solution requires modeling of large number of pole pitches. Recently, a paper was presented [ 1 I ] describing the development of the software package WAVE. The same software tool was also for the negative sequence heating studies [12]. These studies show the potential of the software for applications that have not been explored previously. To verify the calculated results obtained using WAVE

software, experiments were performed on a synchronous machine (0.654 MVA, 1 kV, 1200 rpm 60 Hz) by simulating the negative sequence fields using phase-phase short circuit [ 121. Table 3 shows the calculated damper bar currents by the WAVE software for 20% negative sequence current and the frequencies at which significant currents are induced. The predicted dampers bars currents are in good agreement with tests results [12].

The software tool based on a permeance and MMF model require considerably less CPU times than the full timestepping finite element simulations. The new tool shows promising results for applications to machines with fractional slot windings requiring prohibitive amounts of computing resources for time-stepping finite element solutions. Bar

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5. Conclusion The new automated design optimisation process introduced by GE Hydro is able to perform trade-off and design sensitivity studies to determine the benefit of modifying design constraints and improving enabling technologies. The use of electromagnetic finite element codes for the design phase of hydroelectric generators has been described. Special emphasis has been laid on time step finite element program and on the new software tool for steady state time-harmonic field solution in salient pole synchronous machines based on a permeance and MMF model. The latter requires considerably less CPU time than the full time-stepping finite element simulations. The wave program shows promising results for applications to machines with fractional slot windings requiring Prohibitive amounts of computing resources for time-stepping finite element solutions.

References H. Karmaker and K. Weeber, “Time-stepping Finite Element Analysis of Large Synchronous Motors with AC Adjustablespeed Drives for Acoustic Noise Control, Studies”, Proceeding of the international Electric Machines and Drives Conference, Milwaukee, 1997. 2. K. Weeber, “Design of Amortisseur Windings of Single-phase Synchronous Generators Using Time-Stepping Finite Element Simulations”, Proceedings ICEM. 1998. 3. K. Weeber, “Determination of Dynamic Parameters of Large Hydro-Generators by Finite-Element Simulation of ThreePhase Sudden Short Circuit Tests”, IEMDC, 1997. 4. I.A. Tsukerman, A. Konrad, J.D. Lavers, K. Weeber, H. Karmaker, “Finite Element Analysis of Static and Timedependent Fields and Forces in a Synchronous Motor”, Proceedings of the International Conference on Electric Machines. Vol. 2, pp. 27-32. Paris, 1994. 5. H. Karmaker, “Time-Stepping Finite Element Analysis of Starting Performance of Large Salient-Pole Synchronous Machines”. IEMDC, 2003. I.

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6.

A. Merkhouf, B. F. Boueri and H. Karmaker “Generator End

Windings Forces and Natural Frequency Analysis”. IEMDC, 2003. 7. A. Merkhouf, “Evaluation of the Generator Continuous Negative Sequence Capability Using Time Stepping Finite Element Simulations”. 15’ International Conference on Electrical Machines (ICEM), Belgium, 2002. 8. “International standard, Rotating electrical machines part I”, IEC 34-1 1994 9. “Test Procedures for Synchronous Machines”, lEEE Std.1151995.

10. H. VBgele, M. Xuan, .I. Simond, “Modelling of a Single-Phase Generator Equipped With a Damper Winding Having Solid Lateral Bars’’, Vol. 3, pp. 266-271, Proceedings ICEM 1998. II. A.M. Knight, H. Karmaker and K. Weeber, “Use of a Permeance Model to Predict Force Harmonic Components and Damper Winding Effects in Salient Pole Synchronous Machines” Proceedings of the International Electric Machines and Drives Conference, MIT, p. 179, June 17,2001. 12. H. Karmaker, “Combined Analytical and Finite Element Modeling for Negative Sequence Studies in Salient Pole Synchronous Machines, Proceedings of the 2002 lEEE Winter Power Meeting, New York, Jan. 2002

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