Static excitation pros and cons

A static excitation system is one in which the generator field is fed with DC from a thyristor (SCR) rectifier that has no rotating parts. The power is taken from the generator terminals through an excitation transformer and reaches the rotor through slip rings and brushes.

Working

1. An excitation transformer connected to the generator terminals steps the voltage down.
2. A fully controlled 3-phase thyristor bridge rectifies it. The AVR changes the firing angle $\alpha$ to control the field voltage, $V_f = 1.35\,V_{ac}\cos\alpha$ (where $V_{ac}$ is the line voltage at the bridge input).
3. The DC goes to the field through slip rings. A field breaker and a discharge resistor/crowbar are provided for de-excitation.
4. At start the terminal voltage is zero, so field flashing from the station battery or an AC supply builds up the initial voltage.

 Gen terminals -> Exc. transformer -> Thyristor bridge
                                           |
            PT, CT -> AVR (firing angle) --+
                                           v
 Battery -> Field flashing -> Field CB -> Slip rings
                                           |
                                      Main field

Best alternative

The main alternative is the brushless excitation system: an AC exciter with rotating diodes mounted on the shaft.

Pros (compared with brushless)

• Very fast response: the field voltage changes almost at once. A high ceiling voltage (often 1.6–2 times rated or more) improves transient stability.
• Fast de-excitation: the bridge can work in inverter mode (negative field voltage) to kill the field quickly during internal faults.
• No rotating exciter, so the shaft is shorter and the machine is simpler. Easy to fit in an existing plant.
• Field voltage and current can be measured directly, and a PSS and limiters can be added easily.
• Thyristors are easy to reach for maintenance, and redundant bridges can be used.

Cons (compared with brushless)

• Needs slip rings and brushes, so there is brush wear, carbon dust and regular maintenance.
• The supply comes from the generator terminals, so during a close-in three-phase fault the terminal voltage collapses and excitation is lost unless there is compound (CT) support. The fault current may decay before the relays act.
• Needs field flashing for start.
• The thyristors produce harmonics and shaft voltages. Cooling and a large excitation transformer are needed.