Excitation Systems

 Excitation Systems

Function

✓ The alternators are provided with shaft or gear mounted exciters for providing dc excitation to their field windings.

✓ For a large alternator the main exciter is a separately excited dc machine supplied by a pilot exciter.

Exciter Design

✓ Excitation power required for large turbo-generators is of the order of 0.4-0.5% of the generator rating .

✓Usually main exciter separately excited from a pilot exciter.

✓  The advantage of using pilot exciter is to improve voltage response to changes of the field current.

Minimum Requirements for the Main Exciter

✓ Rated current must not be less than 110% of rotor current for rated generator output.

✓ Rated voltage must not be less than 110% of the slip ring voltage for rated generator output.

✓ The exciter voltage is generally 230V in some cases nominal voltage of 440V is used.

✓ Ceiling voltage must not be less than 120 percent of rated slip ring voltage .

✓ Nominal response must not be less than .5 .

Types of Excitation Systems

1) DC Excitation using DC Generators

2) AC Excitation using AC Generators along with Rectifiers

3) Static Excitation (from alternator Terminal

DC Excitation System 

✓The pilot exciter is a d.c shunt machine. 

✓The main exciter is a d.c shunt machine with a number of control field windings 

✓The main and pilot exciters are coupled to the main generator shaft.

✓A d.cmotor drives an amplidyne or rotating amplifier which is cross field machine. 

✓ It has a number of control windings.

✓ The voltage transformers secondaries supply AVR and magnetic amplifier circuits.

DC Excitation System With Amplifier 

AC Excitation System

AC Excitation System 

✓ The rectifier is on the floor. Rectifier is rotating.

✓ Cooling and maintenance problem of slip rings and brushes.

Brushless Excitation System 

✓ Rectifier is rotating

✓ No need of brushes and slip rings.

✓ however it is impossible to meter and read main generator filed quantities as it is rotating.

Static Excitation Systems

✓ The supply of power to the rectifier is from the main generator through the station auxiliary

bus, using step down transformer.

✓ The rectifier output is fed directly to the field of the main generator by means of slip rings.

There can be two arrangements:

1 Power to the excitation using voltage only.

2 Using voltage as well as current from main generator.

Field flashing: from battery bank, initial supply is given for starting up the alternator.