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December 14, 2020
Types of Generators and Configuration (Vertical or Horizontal)
Vertical shaft generators are generally used. There are two types of vertical shaft hydro generators distinguished by bearing arrangements.
Umbrella type generators: These generators have combined bottom thrust and guide bearings and confined to low operating speeds (up to 200 rpm) are the least expensive generator design. In semi umbrella type generators a top guide bearing is added. Umbrella/Semi Umbrella design is being increasingly used for slow speed vertical generator.
Conventional generators: Prior to introduction of umbrella and semi umbrella designs conventional design comprised of top-mounted thrust and guide bearing supported on heavy brackets, capable of supporting total weight of generator. All thrust bearing supported brackets take care the weight of generator rotating parts. Turbine rotation parts and axial component of water thrust acting on turbine runner. A bottom guide bearing combined with turbine shaft is usually provided. This conventional design is used for high speeds (up to 1000 rpm) generators. Some medium size low flow turbine and tube turbine generators are horizontal shaft. Direct driven bulb turbine generators are also horizontal shaft generators located in the bulb. Pelton turbine coupled generators are mostly horizontal shaft.
Capacity and Rating
kW Rating: kW capacity is fixed by turbine rated output. In a variable head power plant the turbine output may vary depending upon available head. In general the generator is rated for turbine output at rated head. In peaking power plant higher generator kW rating could be specified to take care of possible higher turbine output. Economic analysis is required for this purpose as the cost will increase and generator capacity remains unutilized when heads are low. The kilowatt rating of the generator should be compatible with the kW rating of the turbine. The most common turbine types are Francis, fixed blade propeller, and adjustable blade propeller (Kaplan). Each turbine type has different operating characteristics and imposes a different set of generator design criteria to correctly match the generator to the turbine. For any turbine type, however, the generator should have sufficient continuous capacity to handle the maximum kW available from the turbine at 100-percent gate without the generator exceeding its rated nameplate temperature rise. In determining generator capacity, any possible future changes to the project, such as raising the forebay (draw down) level and increasing turbine output capability, should be considered. kVA Rating and power factor: kVA and power factor is fixed by consideration of interconnected transmission system and location of the power plant with respect to load centre. These requirements include a consideration of the anticipated load, the electrical location of the plant relative to the power system load centers, the transmission lines, substations, and distribution facilities involved. A load flow study for different operating condition would indicate operating power factor, which could be specified. (Turbine output in MW) x (Generator efficiency) Generator MVA =Generator power factor
Electrical Characteristics e.g. voltage, short circuit ratio, reactance, line charging capacity etc. must conform to the interconnected transmission system. Large water wheel generators are custom designed to 215 match hydraulic turbine prime over. Deviation from normal generator design parameters to meet system stability needs can have a significant effect on cost. The system stability and other needs can be met by modern static excitation high response systems and it is a practice to specify normal characteristics for generators and achieve stability requirements if any by adjusting excitation system parameter (ceiling voltage/exciter response).
Generator Terminal Voltage
Generator terminal voltage should be as high as economically feasible. Standard voltage of 11 kV or higher is generally specified for hydro generator.
Insulation and Temperature Rise
Modern hydro units are subjected to a wide variety of operating conditions but specifications are prepared with the intention of achieving a winding life expectancy of 35 years or more under anticipated operating conditions. Present practice is to specify class F insulation system for the stator and rotor winding with class B temperature rise over the ambient. Ambient temperature rise should be determined carefully from the temperature of the cooling water etc.
If may be noted that as per IS the temperature rise specified over an ambient of 400 C. Accordingly maximum temperature for the insulation class under site conditions should be specified. The class F system is known as thermo setting insulation system. The epoxy resins systems may be divided into the following two major classes.
i) Resin rich system
ii) Resin poor system
Resin poor technology needs sophisticated resin storage, transfer and impregnation plants. Most of large machines commissioned in India have class F insulation resin rich. It is generally believed that stator insulation degradation occurs due to slot discharge as well as due to hionization. Slot discharge is a result of poor contact between the conducting surface on generator coil and stator iron and built up of high surface potential. Discharges of this nature can be very severe because of
the charges current involved and cause serious damage to the insulation. At some stage slot discharges appear to or begin to increase between the surface of the insulation of the winding and laminated slot wall and provide one of the deteriorating factor. Phenomenon of slot discharges is known since long and efforts have been made to evolve suitable methods for monitoring the state of erosion of insulation in an assembled machine. With adoption of epoxy mica insulation and use of increased current densities resulting in higher electro-dynamic forces, there is an increase in the intensity of these slots discharges. Slot discharges have been reported in water wheel generators. Thermosetting insulation systems materials are hard and do not readily conform to the stator slot surface, so special techniques and careful installation procedures must be used in applying these materials to avoid possible slot discharges. Special coil fabrication techniques, installation, acceptance and maintenance
procedure are required to ensure long, trouble-free winding life. All components of stator and rotor insulation should be of class F insulation with class B temperature rise.
Short Circuit Ratio
The short circuit ratio of a generator is the ratio of field current required to produce rated open circuit voltage to the field current required to produce rated stator current when the generator terminals are short circuited and is the reciprocal of saturated synchronous reactance. Normal short circuit ratios are given below. Higher than normal short circuit ratio will increase cost and decrease efficiency. Generator Power Factor Normal Short Circuit Ratio In general, the requirement for other than nominal short-circuit ratios can be determined only from a stability study of the system on which the generator is to operate. If the stability study shows that generators at the electrical location of the plant in the power system are likely to experience instability problems during system disturbances, then higher short-circuit ratio values may be determined from the model studies and specified.
Line Charging and Synchronous Condensing Capacity
This is the capacity required to charge an unloaded line. Line charging capacity of a generator having normal characteristics can be assumed to equal 0.75 of its normal rating multiplied by its short circuit ratio. If the generator is to be designed to operate as synchronous condenser. The capacity when operating over excited as condensers can be as follows: Power Factor Condenser Capacity
The eight different reactances of a salient-pole generator are of interest in machine design, machine testing, and in system stability model studies. Lower than normal reactances of the generator and step-up transformer for system stability will increase cost and is not recommended.
Both rated voltage values of transient and subtransient reactances should be used in computations for determining momentary rating and the interrupting ratings of circuit breakers. Typical values of transient reactances for water wheel generators up to 25 MA are given below. Guaranteed values of transient reactances will be approximately 10% higher.
A short circuit grid copper conductor in face of each of the salient poles is required to prevent pulling out of step the generator interconnected to large grid. Two types of damper windings may be connected with each other, except through contact with the rotor metal. In the second, the pole face windings are connected at the top and bottom to the adjacent damper windings.
The damper winding is of major importance to the stable operation of the generator. While the generator is operating in exact synchronism with the power system, rotating field and rotor speed exactly matched, there is no current in the damper winding and it essentially has no effect on the generator operation. If there is a small disturbance in the power system, and the frequency tends to change slightly, the rotor speed and the rotating field speed will be slightly different. This may result in oscillation, which can result in generator pulling out of step with possible consequential damage. The damper winding is of importance in all power systems, but more important to systems that tend toward instability, i. e. systems with large loads distant from generation resources, and large intertie loads. In all cases, connected damper windings are recommended. If the windings are not interconnected, the current path between adjacent windings is through the field pole and the rotor rim. This tends to be a high impedance path, and reduce the effectiveness of the winding, as well as resulting in heating in the current path. Lack of interconnection leads to uneven heating of the damper windings, their deterioration, and ultimately damage to the damper bars.
The damper winding also indirectly aids in reducing generator voltage swings under some faults conditions. It does this by contributing to the reduction of the ratio of the quadrature reactance and the direct axis reactance, This ratio can be as great as 2.5 for a salient pole generator with no damper winding,and can be as low as 1.1 if the salient pole generator has a fully interconnected winding practice is to provide.
As high an efficiency as possible which can be guaranteed by manufacturer should be specified. Calculated values should be obtained from the manufacturer. For a generator of any given speed and power factor rating, design efficiencies are reduced by the following:
i.Higher Short-Circuit Ratio