SUMMER TRAINING REPORT PRESENTATION

SUMMER TRAINING PRESENTATIONTHDC INDIA LTD.TEHRI HYDRO POWER PLANT BY ABHILASH RAWATTHDC IHET

PRESENTATION OUTLINEINTRODUCTION TEHRI HYDRO POWER PLANTEXCITATION PRINCIPLE AND TYPESTATIC EXCITATION SYSTEMFIELD FASHINGFIELD BREAKER & DE-EXCITATIONAUTOMATIC VOLTAGE REGULATION(AVR)EXCITATION PROCESS

TEHRI HYDRO POWER PROJECT

EXCITAION SYSTEMThe functions of an excitation system areto provide direct current to the synchronous generator field winding, andto perform control and protective functions essential to the satisfactory operation of the power system

4TYPES OF EXCITAIONCOMMUTATOR TYPE DC GENERATOR: These DC generators are either driven from the shaft of the main generator or from a separate motor. AC generators used in connection with rectifiers: In this case can alternator is driven by the main generator to produce AC power which is then converted to DC by rectifiers.A static excitation system which uses the AC power generated by the main generators instead of a separate generating unit. In this type, the power generated by the main generator is stepped down to required level using rectifiers. For initial excitation, DC from station battery of DC from station auxiliary power converted by rectifiers is used.Brushless excitation

STATIC EXCITATION

The ThyristorThis device is basically a controlled rectifier. It belongs to a family of called Silicon Controlled Rectifier (SCR).Its advantage is that it can carry large currents and it can be controlled using a very small current of the order of a few milli-amperes.The conduction of Thyristor can be controlled using the control input called gate. It has 3 terminals called anode, cathode and Gate. For starting conduction, anode is to be positive w.r.t cathode and there should be gate current flowing through gate.Even if the anode is positive w.r.t cathode the conduction will not start until a gate current is supplied. So we can delay the conduction by delaying the gate signal. Once the conduction is started, the gate will not have any control (even if it is removed).The conduction is will continue until the current through the device is reduced below a particular value called the holding current. It is the minimum current required to maintain the conduction and the value is specified by the manufacturer.

INITIAL FIELD EXCITAION To overcome deficiency magnetism when starting, initial excitation is applied to field winding.The field flashing is derived by auxiliary a.c. supply via transformer and rectifier. Field flashing can also be provided by station batteries.Unit Auxiliary TransformerTotal Power- 400 kVA Primary Voltage- 11 kV Secondary voltage- 400 V

FIELD BREAKERThe field breaker connects the DC from the exciter to the field. In addition to the two main contacts, the field breaker is usually provided with a power rated auxiliary contact. This auxiliary contact is used to provide a discharge path for the voltage induced when the field is isolated using the field breaker.

SELF-EXCITATION TRANSFORMERThis self-excitation Transformer , the voltage generated is back fed to the rotor windings after being tapped down through this transformer.Primary voltage 15.75KV (generated voltage) Secondary voltage 712V. The total power is 1912 kva.

AUTOMATIC VOLTAGE REGULATION(AVR)Comparator which compares the output voltage of the generator with the set value. Any error between the set value and the output voltage is reflected as an output of the comparator.This signal is amplified and given to the gate pulse generating electronic circuit.

EXCITATION PROCESS1) Incomer 11KV line is used for the initial excitation. For this 11KV is stepped down 415V followed by stepping down to 30V DC which is used for the initial excitation.2) When 30MW is generated then 11KV line is isolated & the self-excitation is carried out. This is done automatically.3) For this 15.75KV, being generated is tapped down to 712V through the self-excitation transformer.4) This 712V AC is fed to thyristor bridges which get converted to DC and the gate pulse to thyristor bridges are provided by AVR through pulse transformer to the gate of thyristor.5) These Pulse transformer smoothen the gate pulses.6) DC from thyristor after passing through field breaker goes to the rotor windings.7) This is how the excitation process continues till the machine operates

BRAKING1) To stop the machine, first wicket gates are closed.2) It is then left in the idle condition, so that the speed reduces to 50%.3) Dynamic brakes: When the speed is reduced to 50% dynamic brakes are applied. For this, first the11KV incomer line is stepped down to 230V which after passing through circuit breakers followed by field breakers is supplied to the rotor in opposite direction (opposite to the direction of excitation) and the 3 phase AC is shorted, which applies the force in opposite direction and helps in reducing the speed.4) Mechanical brakes: - Mechanical brakes are applied when the speed reduces to 4% after application of Dynamic brakes.HAVE A NICE DAYGOOD BYETHANK YOU