Static excitation system

A static excitation system has no rotating exciter. The field current is supplied by thyristor (SCR) rectifiers fed from the generator terminals through an excitation transformer, and is given to the rotor through slip rings and brushes.

 Gen terminals --+--------------------------> to grid
                 |
            +----+----+
            | Excit.  |   step-down
            | transf. |
            +----+----+
                 | 3-ph AC
            +----+----+     firing   +-----+
            | Thyristor|<-----------| AVR |<-- PT, CT
            | bridge   |            +-----+
            +----+----+
                 | DC (field breaker)
              slip rings
                 |
            Rotor field winding

Working

1. A part of the generator output is stepped down by the excitation transformer.
2. A fully controlled 3-phase thyristor bridge rectifies it to DC.
3. The AVR compares the terminal voltage (from the PT) with the reference and changes the firing angle of the thyristors. A smaller firing angle gives a higher field voltage.
4. The DC is fed to the field through slip rings.
5. Field flashing: at start-up there is no terminal voltage (only a small residual voltage), so the field is first energised from the station battery or station supply until the generator voltage builds up.
6. A field breaker and a crowbar/discharge resistor de-excite the field on faults. The bridge can also invert (negative field voltage) for fast de-excitation.

Types

• Potential-source (bus-fed) static exciter: supply from a terminal transformer only.
• Compound-source static exciter: supply from both a PT and a series CT, so excitation is kept during a close-in fault.

Advantages

• Very fast response (time constant of a few ms) and high ceiling voltage; improves transient stability.
• No rotating exciter, so a shorter shaft and less maintenance.
• Fast field suppression by inverting the bridge.

Disadvantages

• Slip rings and brushes are still needed.
• The field supply depends on the terminal voltage; a close-in fault reduces excitation (unless compounded).
• Thyristors produce harmonics and need cooling.