Arezki Merkhouf

Performance analysis of a large hydro generator after disconnection of damaged coils

The main objective of this paper is to determine the capability of the machine to operate at nominal load when some stator coils bars that are cut. Finite element electromagnetic simulations were carried out on an existing hydro generator of 32.5 MVA, 13.2 kV, 60 Hz, 68 poles, 432 stator slots, operating at nominal load with bypassed stator coil. The obtained results in terms of flux density and currents are discussed in detail.

Electromagnetic modelling of existing large hydro generator

The electromagnetic modelling of existing large hydro generator is the main focus of this paper. Electromagnetic commercial software was used where all geometrical parameters of the generator are taking into account such as: pole face geometry, air gap size, the slots geometry and pattern winding sequence. The approach used to model the machine and some simulation results are presented in this paper. The analysis of a 122.6 MVA, 60 poles, 13.8 kV generator is presented as a case study on how advanced electromagnetic numerical field simulations are employed to evaluate generator parameters and performances.

Electromagnetic losses computation in existing large hydro electrical generators

The hydroelectric generation industry faces two related challenges: increasing the output the existing hydro electrical generators but without reducing the reliability of these aging facilities. Many aspect of hydroelectric generator should be considered during any up-rate study. Magnetic losses of the stator and rotor core may change significantly with any load increase. The open and short circuit core losses are major components in the efficiency and temperature rise calculations for any considered up-rate. Advanced numerical methods are used to determine different magnetic losses in the considered hydro electrical generator before any upgrade. Magnetic core losses results will be discussed in this present paper.

Consideration of Design and Operation on Rotational Flux Density Distributions in Hydrogenerator Stators

Core losses in an electrical machine vary depending on the type of material, frequency of operation, and flux density magnitude and waveform. Loss prediction using pulsating models is insufficient for estimating core losses in a machine, since it does not take into account rotational core losses. Moreover, superposition of pulsating losses in the radial and tangential direction overestimates losses especially at higher flux densities. The objective of this paper is to study rotational flux distribution in the stator of the generator under different operating conditions. Distribution of rotational flux in a large hydrogenerator stator is studied by plotting the aspect ratio. It was found that increasing the yoke length and reducing the air gap increases the area of the stator with rotational flux. Varying the output power showed slight changes in the rotational flux distribution. Different optimized machine designs are investigated for rotational flux distribution; moreover, rotational core losses are measured and used for a comparative study radially across the stator.

Consideration of design and operation on rotational flux density distributions in hydro generator stators

Core losses in an electrical machine vary depending on the type of material, frequency and flux density. Loss prediction using pulsating models is insufficient for estimating core losses in a machine since it does not take into account rotational core losses. Moreover the superposition of pulsating losses in the radial and tangential direction overestimates losses especially at higher flux densities. The objective of this paper is to study the rotational flux density distribution in the stator of a generator under different operating conditions. The distribution of rotational flux in a large hydro generator stator is studied by plotting the aspect ratio. It was found that increasing the yoke length increases the area experiencing rotational flux. Varying the air gap and output power showed slight changes in the rotational flux distribution. However they have an effect on the flux magnitude and harmonics which potentially increase the total losses. Dimensions and operating conditions of the machine therefore affect the total core losses.

Influence of the Variation of the Input Parameters on the Simulation Results of a Large Hydrogenerator

Electromagnetic modeling and simulation of hydroelectric generators require a large number of variables for accurate study and test result validation. Simulation results can be affected by mesh density, magnetic material properties, excitation current, and air gap. A finite-element analysis was performed to calculate the relative effect of these variables on a 122.6-MVA, 13.8-kV, 60-Hz, 60-pole, 504-stator slot design. The obtained results were compared with measured parameters such as air-gap flux density, line current, and magnetic core losses.

Free vibration analysis of a large hydroelectric generator and computation of radial electromagnetic exciting forces

Major causes of vibration in electrical machines come from electromagnetic sources. A serious problem may arise when the frequencies of the periodic exciting force are identical with, or close to, one of the natural frequencies of the machine. This paper presents a 2D modal analysis which calculates the natural frequencies and mode shapes of the stator and rotor of a large hydroelectric generator. The structural model takes into account the shape of the stator teeth and rotor poles. Moreover, a 2D electromagnetic transient simulation is performed in order to predict the frequencies and mode shapes of the electromagnetic exciting force. This advanced electromagnetic numerical simulation takes into account the winding sequence and the geometry of the rotor and stator given by the manufacturer. The purpose of this work is to validate a multiphysics tool for the future and recognize its advantages over the classical method.

Study of the impact of eccentricity in large synchronous generator with finite elements

This paper presents a numerical model for computing electromagnetic forces in an existing large hydro generator at Hydro-Québec. Due to the high mass and low rotational speed, hydro generators present static, dynamic or combined eccentricities resulting in unbalanced magnetic pull in the air gap and unbalanced stator currents caused by the variation of the magnetic flux density distribution. The model uses advanced 2-D numerical finite-element analysis to compute the electromagnetic forces based on Maxwell stress tensor and the obtained results were used later as inputs to study the dynamic behavior of hydro power turbine shaft. The results obtained for electromagnetic forces, damper bar currents and circuit currents due to any magnetic unbalance are presented and discussed in detail.

Investigation of the electromagnetic simulation results variation of a hydro electrical generator

The electromagnetic modeling and simulation of existing hydro electrical generators in operation always require a large number of variables to carry out an accurate study and to be able to properly validate the different test results. Simulation can be affected by parameters, such as: the model meshed density; the magnetic materials (magnetic proprieties are usually unknown); the air gap size (the design air gap is always different from the operation air gap due to the thermal expansion) and its uniformity. The excitation current calculated during simulation to achieve the proper line current depends on the air gap and the magnetic properties at the fundamental frequency and its harmonics. This paper reports a FEM simulation of an existing hydro generator of 122.6 MVA, 13.8 kV, 60 Hz, 60 poles with 504 slots, considering the magnetic material, mesh size, air gap length and excitation current as variable inputs. The impact of the modification of these parameters on the electromagnetic finite element simulation is presented, besides, the simulation results were compared with test ones.

Consideration of loss distribution to evaluate the hotspot temperature when up-rating generators

Some exiting generators may have more generation capability than their nameplate rating if they hold large security margin. However, before changing the limit of a generator one must do a thorough evaluation of its behavior in different conditions to extrapolate the effect of any increase. With the help of simulation models to calculate magnetic and stray losses of a machine, it is possible to couple loss distribution with the calculated I2R losses and the measured ventilation losses in a thermal model and calculate the location and the magnitude of the hotspot. These models must be validated by in situ measurements to make sure they correspond to reality. The temperature of RTD is easily accessible, but most of the time insufficient to validate detailed models. This paper presents some of the limitation of the RTD to detect axial or radial temperature distribution. Unfortunately, the copper temperature can not be measured directly, but it can be calculated by numerical models and this was done for two generators in characteristic conditions. The results are compared with temperature and airflow measurements. The models show that up-rating should not be made without understanding the exact behavior of generator as a function of load.