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December 11, 2020
The electric generator converts the mechanical energy of the turbine into electrical energy. The two major components of the generator are the rotor and the stator. The rotor is the rotating assembly to which the mechanical torque of the turbine shaft is applied. By magnetizing or “exciting” the rotor, a voltage is induced in the stationary component, the stator. The principal control mechanism of the generator is the exciter-regulator which sets and stabilizes the output voltage. The speed of the generator is determined by the turbine selection, except when geared with a speed increaser. In general, for a fixed value of power, a decrease in speed will increase the physical size and cost of the generator.
The location and orientation of the generator is influenced by factors such as turbine type and turbine orientation. For example, the generator for a bulb type turbine is located within the bulb itself. A horizontal generator is usually required for small turbine e.g. tube turbine and a vertical shaft generator with a thrust bearing is appropriate for vertical turbine installations.
Conventional cooling on a generator is accomplished by passing air through the stator and rotor coils. Fan blades on the rotating rotor assist in the air flow. For larger generator (above 5 MVA capacity) and depending on the temperature rise limitations of the winding insulation of the machine, the cooling is assisted by passing air through surface air coolers, which have circulated water as the cooling medium. Large Generators interconnected with the grid should meet grid standards issued by Central Electricity Authority (CEA) (relevant extracts are enclosed as annexure-1).
Large salient pole hydro generators specified for installation up to 1970 were constrained by following considerations. Insulation Systems for Stator and Rotor was Class B insulation with organic binding material which permitted lower temperature rises. Material for rotor rim punching etc. required limiting the diameter of the rotor so as to permit operation at runaway speed. Bearing arrangements: Top thrust and guide bearing supported on heavy brackets, capable of supporting total generator weight was provided with a bottom guide bearing to all hydro generators including slow speed large generator which constitutes majority of large hydro generators. This resulted in high cost of machine and building.
Shaft mounted excitation systems were slow and unable to meet the requirements of quick response required from large generators feeding large modern grid systems. Stability requirements for long distance transmission lines required to feed distant load centre/grids was achieved by manipulating reactances, excitation response ratio and flywheel effect. This resulted in larges size of the machine. Grids were small and there was no stringent requirement for voltage and frequency variation.
Typical section arrangement for Bhakra Left bank machines (100 MVA; 166.7 RPM) with top thrust and guide bearing and bottom guide bearing is shown in figure 9.1 and the capability curve is shown in figure.
Small hydro were a typically installed to feed remote areas and worked in isolated mode. The hydro turbines (slow speed) were directly coupled to high cost slow speed generators. Hydro stations were manually operated. The development of load was very poor. The small hydro became highly uneconomical to operate because of low load factors, high installation cost and very high running cost.
Modern Large Hydro Generator Hydraulic turbines driven generators for hydro plant above 5 MW are salient pole synchronous alternating current machines. Large salient pole generators are relatively slow speed machines in the range 80-375 rpm
with large number of rotor poles. These generators are specifically designed. These salient pole hydro generators interconnected with large grids have undergone considerable changes over time which has resulted in reducing size of hydro generators considerably from the electrical and mechanical point of view. Development in the following areas is most prominent.
i)Insulation system for stator and rotor winding
iii)Ventilation and cooling system
iv)Advanced manufacturing technology
v)Formation of large grids requires special design consideration for operation and stability.
Site Operating Conditions
Rated operation condition be specified as follows: If site operating conditions are deviating from these values, correction may be applied.
Maximum Ambient Temperature Steady State duty: Salient-pole open ventilated air-cooled synchronous generators operate successfully when and where the temperature of the cooling air does not exceed 400 C.
Salient-pole totally enclosed water to air cooled (water) synchronous generators operate successfully when and where the secondary coolant temperature at the inlet to the machine or heat exchanger do not exceed 250 C.
If the cooling air temperature (ambient) exceeds 400 C, or cooling water temperature exceeds 250 C then maximum allowable temperature based on temperature rise on reference temperature (400 /250 C) of the insulation class be specified instead of temperature.
The minimum temperature of the air at the operating site is – 150 C, the machine being installed and in operation or at rest be de-energized.
Note: If temperatures different from above are expected. The manufacturer should be informed of actual site conditions.
Generators: Generators should operate successfully at rated MVA, frequency, power factor, and terminal voltage. Generators at other service conditions should be specified with the standards of performance established at rated conditions.
Altitude: Height above sea level not exceeding 1000 m. For machines intended for operation on a site where the altitude is in excess of 1000 m. should be specifically brought out.
Number of starts and application of load: Anticipated no. of starts and maximum MVA, power, and reactive power loading rate of change are requirements for the manufacturer to take into account in the machine design. The method of starting must be identified in the case of peaking stations.
Variation from rated voltage and frequency: Generators should be thermally capable of continuous operation within the capability of their reactive capability curves over the ranges of ± 5 % in voltage and ±2 % in frequency.
Voltage and Frequency Limits for Generators
Voltage limits ± 5% ± 5% to ± 8% Frequency limit ± 2% + 2% to + 3%;– 2% to – 5%
a) As the operating point moves away from rated values of voltage and frequency, the temperature rise of total temperatures of components may progressively increase. Continuous operation at outputs near the limits of the generator’s reactive capability curve (figure 9.3) may cause insulation to age thermally at approximately two times to six times its normal rate.
b) Generators will also be capable of operation within the confines of their reactive capability curves within the ranges of + 3 % to -5 % in frequency with further reduction of insulation life.
c) To minimize the reduction of the generator’s lifetime due to the effect of temperature and temperature differentials, operation outside the above limits should be limited in extent, duration, and frequency of occurrence. The output should be reduced or other corrective measures taken as soon as practicable.
d) The boundaries as defined result in the magnetic circuits of the generator to be over fluxed under fluxed by no more than 5%.
e) The machine may be unstable or margins of stability may be reduced under some of the operating conditions mentioned in ‘a’ above. Excitation margins may also be reduced under these operating
f) As the operating frequency moves away from the rated frequency, effects outside the generator may become important and need to be considered. For example, the turbine manufacturer will specify ranges of frequency and corresponding periods during which the turbine can operate, and the ability of the auxiliary equipment to operate over a range of voltage and frequency should be considered.
g) Operation over a still wider range of voltage and frequency, if required, should be subject to agreement between the purchaser and the manufacturer and need to be specifically brought out in tender specification.
Transient and Emergency Duty Requirements
A generator confirming to these guidelines will be suitable for withstanding exposure to transient event and emergency duty imposed on a generator because of power system faults.
Sudden short circuit at the generator terminals: A generator should be capable of withstanding, without injury, a 30 second, 3 phase short circuit at its terminals when operating at rated MVA and power factor and at 5% over voltage, with fixed excitation. The machine shall also be capable of withstanding, without injury, any other short circuit at its terminals of 30s duration or less in accordance with IEEE C 50. 12-2005. Generator circuit breaker needs to be selected accordingly.
a. Generators be designed to be fit for service without inspection or repair after synchronizing that is within the limits given below:
i) Breaker closing angle ±10%
ii) Generator voltage relative to system 0% to +5%
iii) Frequency difference ±0.067 Hz
Additional information on synchronizing practices can be found in IEEE std. C37. 102TM- 1995.
b. Faulty synchronizing is that which is outside the limits given above. Under some system conditions, faulty synchronizing can cause intense, short duration currents and torques that exceed those experienced during sudden short circuits. These currents and torques may cause damage to the generator.
c. Generators should be designed so that they are capable of coasting down from synchronous speed to a stop after being immediately tripped off-line following a faulty synchronization. Any generator that has been subject to a faulty synchronization should be inspected for damage and repaired as necessary before being judged fit for service after the incident. Any loosening for stator winding bracing and blocking and any deformation of coupling bolts, couplings, and rotor shafts should be corrected before returning the generator to service. Even if repairs are made after a severe out-of-phase synchronization, it should also be expected that repetition of less severe faulty synchronizations might lead to further deterioration of the components.
d. It should be noted that the most severe faulty synchronizations, such as 1800 or 1200 out-of-phase synchronizing to a system with low system reactance to the infinite bus, might require partial or total rewind of the stator, or extensive or replacement of the rotor, or both. Check synchronizing relay and auto synchronizing equipment need to be set accordingly. Normally synchronizing closing angle is kept ±7%. Short-time volts/hertz variations: The manufacturer should provide a curve of safe short-time volts/hertz capability. Identify the level of over flux above which the machine should never be operated, to avoid possible machine failure. Unless otherwise specified, the curve apply for time intervals of less than 10 min.