Excitation system

1. Excitation Transformer: It is used to step down the generated voltage to rated voltage of excitation system.

2. Rectifier system: Converts alternating current (AC) to direct current (DC).The rectifier PIV (Peak Inverse Voltage) rating shall not be less than 4 times the maximum RMS voltage of the inputs.

3. Digital voltage regulators (DVR): It used to correct any change in generator voltage. It is used to maintain the generator voltage for steady-sates load conditions, without hunting, with 0.5% accuracy for any excitation within the operating range of the generator.

Following Features must be in DVR:

a. Stator current limiter

b. Field current limiter

c. Load angle limiter( Angle b/w the rotating field & stator field)

d. (N-1) limiter (suppose, in DVR we are using 3 thyristor & one get out of service. Thus our system must be capable of operating the associated device with 2 thyristor i.e. (3-1)=2, here ,N= No. of thyristor)

4. Power supply units:

5. Field flashing equipment: Field flashing is used to develop the residual magnetic flux in the rotor to the level where generator is capable of generating the respected voltage. Equipment used in automatic field flashing circuit breakers, Blocking Diodes, Control devices, protection etc.

6. Field circuit breaker:

7. Discharge resistors: The discharging resistor is used for discharging stored energy due to inductance (having the property of storing energy due to virtue of changing current) during

EXCITATION SYSTEM

In order to overcome the deficiency of remnant magnetism when starting the machine, initial excitation must be applied to the field winding .The respective power is then derived from the A.C. Supply via transformer & rectifier. Provision shall al/so be made for field flashing through station batteries as standby arrangement. The field flashing shall get disconnected when 70% of rated voltage is achieved at the generator terminals & normal excitation circuit takes over.

the open circuit of rotor; otherwise stored energy would lead to the heating of rotor winding.

8. Thyristor equipment: Design must provide 100% redundancy i.e. at full load operation (inc. ceiling voltage) the thyristor shall be working at 50% of their nominal rating.

IF Rotor Winding gets open circuit

Stored energy is very large due to inductance (Inductance stored energy due to virtue of changing current)

STORED ENERGY= (1/2) LI2

Stored energy discharged (Dissipated) through the DISCHARGE RESISTOR

CIRCUIRT

BREAKER GET OPEN

Suppose at the entering, rated current would be 1000A, which would further divide into three parts i.e. 1000/3 .This mean each thyristor of rating 1000 A carrying only 1/3 of rated current i.e. 333.34 A current which much lower than its rated current. Therefore, thyristor is operating in safe limit. If one of the thyristor is get out of service due to some fault or any other mean, it will not disturb the operation of associated equipment to which it is used for controlling or monitoring by dividing the current into two part i.e. each thyristor is now carrying 500 A current which again less than their rated current. Thus, device is providing 100% redundancy.

DE-EXCITATION SYSTEM

To achieve a quick suppression of the generator field and thus, the voltage ,the field winding shall be shunted by a discharge resistor ( voltage dependent type).After tripping the generator circuit breaker shall connect the discharge resistor and simultaneously the polarity of the excitation shall be reversed.

Types of Excitation System:

1. Static Excitation

2. Brushless Excitation