Generator Used in Hydro Electric Power Plants
- Generators used are three phase, alternating current 3.3 kV, 11 kV, 50 Hz synchronous generators. The armature is housed in the stator consisting of a core and a frame.
- The core acts as a magnetic circuit. It is built from thin electrical quality sheet steel laminations and is given a shape of heavy steel rings. The core is enclosed in the welded steel frame. The rotor is salient pole type and is the heaviest part of the machine.
- The rotor houses the D.C. excitation winding and consists of a rim, Salient poles, spokes and a hub.
- The rim is used for spacing and securing the poles, it is made of metal cores and carry the excitation winding coils. The spokes are welded to the inside of rim at one end and the hub at the other ends.
- The hub serves to mount the rotor on the main shaft D.C. excitation to the rotor pole coils is supplied by means of brushes which rest on slip-rings rotating with the rotor.
- The pole coils are connected in series to form the excitation winding which is excited by D.C.at 110 or 220 volts. Formerly the excitation was supplied by small D.C. generators driven through pilot exciters from the turbo generators shaft.
- In modern generators, static excitation system is frequently used. The rotor poles are spaced very close together to form a continuous external diameter.
- The thrust bearing is one of the parts of the generator. It takes up and transmits the load Imposed by rotating parts and the pressure on the turbine runner to the structural elements of the power house.
- Different types of thrust bearings may be used, it may be of plate type, roller type, spring type or kings bury type, or it may be the oil pressure type.
- Load equalization improves the performance of the bearing. Mechanical power developed by turbines is transmitted to the rotor of the generator as the shaft of turbine and rotor shafts are rigidly coupled. The rotor rotates at constant speed r.p.m.
- The stator consists of a 3-phase winding, consisting of large number of turns in each phase. The turns are formed by Insulated copper rods laid in the slots of the stator.
- The rods are connected with jumper windings to form the complete turn.
- Speed of water turbines is much less. The rotational speed of turbines is related with the frequency of induced voltage by the following relation
N=120
f/P
Where, N=speed in
r.p.m. of tubine
F=Frequency
of induced voltage which should be 50 in India
P=Number
of pols of generator
- Hence, in case of turbines running at low speed, the number of poles will be more, so that frequency of 50 Hz is maintained constant.
- The power output = √3 VL IL volt amperes and real power output –√3 VL IL cos Φ, watts, where cos is load p.f. Reactive power-√3 VL IL. I sin Φ
- Generally, the power output of such plants is measured in Mega-watt, which is equal to watt/ 106.
- These generators run at low speed. If air cooling is used, then efficiency varies from 95% to 97.5% depending upon size.
- The voltages generated by hydro generator are 3.3 kV, 6.6 kV or 11 kV at 50 Hz. Transient reactance varies from 15% for high speed to 30% for low speed machines. The short circuit ratio is between 1 and 1.2.
- A proper value of flywheel effect is very essential. In case of small systems it should have a reasonably large value for satisfactory speed regulation under sudden changes in load, while in case of a system consisting of large number of generators operating in parallel the value should be sufficient to avoid hunting and ensure stability under transient and fault conditions.
- The exciters are provided to supply the field winding of generators. They are D.C. shunt or compound generators. Their capacity is about 0.5 to 3% of the main generator capacity. In some cases, pilot exciters are used to excite the main exciter and are fixed to the same shaft of the generator. The exciter voltage is generally 110 V, 220 volts.