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Mechanical Characteristics of Hydro turbine Generator

December 16, 2020

Direction of Rotation

 

The direction of the rotation of the generator should suit the prime mover requirements.

Large Generator Stator

The stator frame is designed for rigidly and strength to allow it to support the clamping forces needed to retain the stator punching in the correct core geometry. Strength is needed for the core to resist deformation under fault conditions and system disturbances. Also, the core is subjected to magnetic forces that tend to deform it as the rotor field rotates. In some large size machines, this flexing has been known to cause the core to contact the rotor during operation. In one instance, the core deformed and contacted the rotor, the 218 machine was tripped by a ground fault, and intense heating caused local stator tooth iron melting, which damaged the stator winding ground insulation. In machines with split phase windings where the split phase currents are monitored for machine protection, the variation in the air gap causes a corresponding variation in the split phase currents. If the variations are significant, the machine will trip by differential relay action, or the differential relays will have to be desensitized to prevent tripping. Desensitizing the relays will work, but it reduces their effectiveness in protecting the machine from internal faults.

 

Rotor Assembly Critical Speeds

A rotor dynamic analysis of the entire shaft system should be performed. This analysis should include the prime mover, generator, and any other rotating components. This analysis should include lateral and torsional shaft system response to the various excitations that are possible within the operational duties allowed by the standards. When the turbine generator is purchased as a set, it would be typical that the manufacturer should perform this analysis. When shaft components are purchased from different manufacturers, the purchaser should arrange to have this analysis. Critical speeds of the generator rotor assembly should not cause unsatisfactory operation within the speed range corresponding to the frequency range agreed in accordance with 9.3.1.4. The generator rotor assembly shall also operate satisfactorily for a reasonable period of time at speeds between standstill and rated speed agreed upon by the prime mover and generator designers. The turbine generator set shaft vibration at operating speed should be within limits specified by ISO: 7919-5 for machine sets in hydraulic power generating and pumping plants.

 

Bearings

The allowable hydraulic thrust provided in standard generator design is satisfactory for use with a Francis runner, but a Kaplan runner requires provision for higher-than-normal thrust loads. It is important that the generator manufacturer have full and accurate information regarding the turbine. Specifications for generators above 10 MW, and for generators in unmanned plants, should require provisions for automatically pumping oil under high pressure between the shoes and the runner plate of the thrust bearing just prior to and during machine startup, and when stopping the machine.

Noise Level and Vibration

 

Under all operating conditions, the noise level of generator should be less than 85-95 dB (A) at a distance of 1 meter radialy & 1.5 m from floor of operating. In order to prevent undue and harmful vibrations, all motors should be statically and dynamically balanced in accordance with IEEE std. C50.12-2005. Test procedure for verification should be based on ISO 3746. Acoustic treatment may be necessary to achieve decreasing sound pressure levels at 90 db.

 

Over speed withstand

It is general practice in India to specify all hydro generators to be designed for full turbine runaway conditions (IS: 4722-2001, table 3 Clause 1). The stresses during design runaway speed should not exceed two-thirds of the yield point. American practice as per Army Corps Engineers Design Manual is as follows; Generators below 360 rpm and 50,000 kVA and smaller are normally designed for 100% over speed.

 

Flywheel Effect

The flywheel effect (WR2 ) of a machine is expressed as the weight of the rotating parts multiplied by the square of the radius of gyration. The WR2 of the generator can be increased by adding weight in the rim of the rotor or by increasing the rotor diameter. Increasing the WR2 increases the generator cost, size and weight, and lowers the efficiency. The need for above-normal WR2 should be analyzed from two standpoints, the effect on power system stability, and the effect on speed regulation of the unit. Speed regulation and governor calculation are discussed in the guidelines for selection of turbines and governors.Electrical system stability considerations may in special cases require a high WR2 is only one of several adjustable factors affecting system stability, all factors in the system design should be considered in arriving at the minimum overall cost. Sufficient WR2 must be provided to prevent hunting and afford stability in operation under sudden load changes. The index of the relative stability of generators used in electrical system calculations is the inertia constant, H, which is expressed in terms of stored energy per kVA of capacity. It is computed as:

 

H =kVAkW • s=kVA0.231(WR2 )(r/min)2 ×10−6

 

The inertia constant will range from 2 to 4 for slow-speed (under 200 rpm) water wheel generators.Transient hydraulic studies of system requirements furnish the best information concerning the optimum inertia constant, but if data from studies are not available, the necessary WR2 can be computed or may be estimated from knowledge of the behavior of other units on the system. Flywheel effect is expressed as moment of inertia (GD2 ) (in India) as compared to flywheel effect WR2

 

(US/English) GD2 = weight x Diameter2 and WR2 = weight x Radius2 (lb.ft2 ). Accordingly, WR2 =4 GD2 Conversion factor for WR2 (USA) in lb.ft2 and GD2 (India) kg. m2 is as follows:

 

GD2 = x×5.9=lb. ft 2 ≈6 x lb. ft 2

The flywheel effect of the generator can be increased by adding weight in the rim of the rotor or by increasing the rotor diameter. Increasing the GD2 increases the generator cost, size and weight, and lowers the efficiency. The need for above-normal WR2 / GD2 is analyzed from two standpoints, the effect on power system stability, and the effect on speed regulation of the unit. Speed regulation and governor calculation are discussed in guidelines for turbine selection. Power system stability considerations do not arise in small hydro generators under considerations.

 

Mechanical characteristics of the generator are based on the hydraulic turbine data to which the generator will be coupled. Characteristics regarding speed, flywheel effect have been discussed in guidelines of turbine selection.

 

Cooling

Losses in a generator appear as heat which is dissipated through radiation and ventilation. The generator rotor is normally constructed to function as an axial flow blower, or is equipped with fan blades, to circulate air through the windings. Small-generators up to 5 MW may be partially enclosed, and heated generator air is discharged into the generator hall, or ducted to the outside. Adequate ventilation of the generator hall preferably thermostatically should be provided in this case.

 

Water to air coolers normally is provided for all modern hydro generators rated greater than 5 MVA. The coolers are situated around the outside periphery of the stator core. Generators equipped with water-to-air coolers can be designed with smaller physical dimensions, reducing the cost of the generator. Automatic regulation of the cooling water flow in direct relation to the generator loading results in more uniform machine operating temperatures, increasing the insulation life of the stator windings. Cooling of the generator can be more easily controlled with such a system, and the stator windings and ventilating slots in the core kept cleaner, reducing the rate of deterioration of the stator winding insulation system. The closed

 

systems also permits the addition of automatic fire protection systems, attenuates generator noise, and reduce heat gains that must be accommodated by the powerhouse HVAC system. Normally, generators should be furnished with one cooler than the number required for operation at rated MVA. This allows one cooler to be removed for maintenance without affecting the unit output.The generator cooling water normally is supplied from the penstock via a pressure reducing station or pumped from the tailrace. In either case, automatic self-cleaning filters must be provided in the cooling water supply lines to avoid frequent fouling or plugging of the water-to-air coolers.

 

Fire Extinguishing System

All hydroelectric generators greater than 25 MVA should be furnished with either a water deluge or carbon dioxide (CO2) fire extinguishing system, to minimize the damage caused by a fire inside the machine. Generators 25 MVA or below should be evaluated individually to ensure installation of a cost effective system. When total thermo setting insulation system is adopted water sprinkler system may be used. This system is safer for operating staff.

 

High Pressure Oil Injection System (For Thrust Bearing)

Modern Thrust Bearings have a high-pressure oil lift system, which injects oil into each shoe pad of the bearing during starting and stopping of the unit. This helps establish a hydrodynamic oil film between the rotating (thrust collar) and stationary (bearing shoes or pads) members of the thrust bearing. The presence of the hydrodynamic oil film minimizes bearing wear. Consequently, during a unit start the high pressure oil of lift system is turned on before the unit starts to rotate. It is usually shut off after the unit speed exceeds 75 to 90% of rated speed because, at the higher rotational speeds, the hydrodynamic oil film is self sustaining. On unit shutdown, the high pressure oil lift system is turned on when the unit speed decreases to the 75 to 90% range and maintained until the unit is at stand still. The maximum wear on the bearing pads occurs at slow speeds, when, due to hydrodynamic effects, the oil film is not maintained over the entire bearing surface.

 

Dynamic Braking System

Modern hydroelectric generators use electrical dynamic braking in addition to the mechanical friction braking system. The electrical dynamic braking system minimizes the wear on the mechanical brake ring and brake pads, prolonging their life. In addition, electrical dynamic braking reduces the duration over which the mechanical brakes are applied during a unit stop sequence, thereby minimizing the amount of brake dust produced by the mechanical brake system. The extended life and reduced brake dust are especially significant for units that are started and stopped several times a day; or units; such as those connected to pelton turbines, which operate at very high rpms.

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Electrical dynamic braking is initiated at a higher unit rotational speed, normally 50% of rated speed; and maintained until the mechanical friction brakes are applied. When dynamic braking is utilized in conjunction with the mechanical brakes, the mechanical brakes normally are applied at ten to 15% of rated speed. Modern hydroelectric generators, especially pumped storage units, also utilize a brake dust vacuum system to capture most of the brake dust produced when the mechanical friction brakes are applied.