ANALYSIS OF LARGE HYDROGENERATORS OPERATING AT CONTINUOUS OVERLOADS

Abstract

The megawatt hydrogenerators, especially of the run-of-the-river type of hydropower plant, are usually operated at over-loads during the period when excess water flow is available in the river. Similarly, the generators of reservoir based plants are also operated at overload when excess inflow is to be discharged to maintain reservoir level within limits or to sustain frequency support to the grid at peak hours. Therefore, hydro generating units need to be operated beyond their normal rated outputs. Installation of an overrated generator only during a brief high inflow period of the year is not a solution due to techno-commercial considerations. Instead, operating the existing generator to increased power delivery is considered to be a wise and favorable option. Generally, the salient pole synchronous generators (SPSGs) dedicated to hydropower applications are feasible to operate 10% more than their rated capacity in extensive cases. In case of a hydropower project in India with 1000MW comprising 4 hydrogenerating units with 250 MW capacity each made a provision for 20% continuous overload, during abundant water availability. Successful operation has been recorded with the power generation up to 20% uprated operation of each hydrogenerating unit for the past five years. It is noted that the efficiency of generator is slightly reduced at 120% load which has not received much attention as it operates only few weeks per year. Various tests were performed and performance related parameters of generating unit during commissioning were observed. However, few tests were not performed: (i) runaway speed at sudden load rejection when the generator operated at 120% load, (ii) thermal behavior and performance of cooler at continuous overloads, (iii) impacts on the thermal behavior of generator during unbalanced currents at 120% loads. In view of the above, adequacy check of a 250 MW SPSG and its accessories operate at 20% overloads is evaluated in this research. Considering the observations from the case study and to check the viability with all other SPSGs, the vibration phenomenon when subjected to sudden load rejection is taken for consideration in the present work. At overloads, when the machine is subjected to grid isolation vibration transients will be even more severe due to drop in current and internal impedance. The simulation results corresponding to the onsite measurements are validated through SIMSEN software for a 250 MW SPSG. The observed vibration transients during runoff state are compared with the maximum vibration allowance for Class III machine as per ISO 10816 standard limits. In general, high-power, low-speed, direct-driven SPSGs are having resistance in rising temperature and self-cooling techniques to withstand the multiple loading points. Therefore, a 250 MW SPSG with CB 870/300-28 stator type is electromagnetically designed and modelled through a three-dimensional, multi-physics, finite-element analysis solver package called MagNet. The temperature distribution of the various parts of machine at 10% and 20% overloads, are explored via a coupled thermal and fluid-dynamical analysis. The thermal distribution and analysis are performed through a coupled additional software package, ThermNet for providing accurate electro-thermal analysis. The simulated thermal results at continuous overloads are validated with the case plant’s test report during its commissioning time and operational data. Moreover, the International Electrotechnical Commission under 60034-1 permits the existence of negative sequence current (I2) up to 10% of its rated current for large capacity generators, irrespective of their rotor arrangement. The negative sequence current capability of the SPSG is explored in this thesis. An elaborate discussion is made on the influential stator current with a proportional increase in the effective intrinsic range of I22t values. The flow of stator current, crucially over the damper bars is computed through of three-dimensional Finite Element Model. The induced thermal stress over the rotor wedges at uprated unbalanced loads are presented utilizing the simulated results. The interpretations of the standard response value are compared with IEC 60034-1 standard for the value of I22t allowances. The survivability check of the SPSG at load disturbances when operating at 20% overload in the existing hydropower plant (250 MW) is practically not possible. Hence, to validate the impact of the load disturbances, experimental verification is conducted with a 2.2 kW SPSG prototype model at the laboratory testbed. The experimentation is conducted by subjecting the prototype to sudden load rejection and unbalance at rated and 20% above rated loads. The vibration signature is sensed using fiber-optic accelerometers supported by a data acquisition system and are signal processed by the inbuilt module. Similarly, the rise in temperature over the surface and the thermal stress in the windings are captured by a high-resolution thermal image scanner. The obtained results are substantiated and compared with the corresponding industrial standard limits i.e., ISO 10816 and IEC 60034-1 for vibration and temperature, respectively. From the test results, it is observed that the selection of generator and associated equipment has been satisfactory for maximum continuous output of 300 MW. Hence, the practical implementation of the proposed continuous overloaded operation technology in large scale SPSG is discussed in detail. The anticipated study on the effect of machine’s ageing and expected life span due to the overloaded operation is derived. The economic study when the uprated operation is implemented is analyzed along with the overall payback benefits. From the analysis and derived outcomes, some of the strong recommendations are also suggested during the field execution of this operating technology. The solution would be overpriced, but the expected payback over the period would be significant.