WORKING PRINCIPLE
The synchronous machine, which consists of a rotor and stator, produces AC currents using the principle of electromagnetic induction. The DC current passing through the field winding of the rotor produces a static magnetic field.
STATIC EXCITATION SYSTEMS – TERMINAL-FED
In static excitation systems, the power rectifier directly supplies the field winding of the synchronous machine. There is no pilot exciter.
A static excitation system is terminal-fed (also called bus-fed) when the supply is taken from the machine itself through a power potential transformer (PPT). The primary of the PPT is connected to the stator of the machine and the secondary supplies power to the rectifier.
Static excitation systems are not inherently self-exciting, so they require an external supply to jump-start the excitation process and build up sufficient magnetic flux. This process is called field flashing
ADVANTAGES OF STATIC EXCITATION SYSTEMS
Simple, reliable and cost-effective design
Minimal maintenance requirements
High performance and fast response characteristics
MAIN COMPONENTS
.AUTOMATIC VOLTAGE REGULATOR (AVR)
The automatic voltage regulator (AVR), commonly known simply as voltage regulator, executes all control functions of the system
Operating breakers
Sending firing pulses to bridges
Responding to operator commands or grid disturbances
Monitoring system I/Os and taking appropriate actions in response to them
Maintaining the excitation system within safety and stability limits through the use of limiters and protections
The main elements of an AVR
CLOSED LOOP CONTROL
LIMITERS AND PROTECTIONS
1: OVER EXCITATION LIMITER (OEL)
2: UNDER EXCITATION LIMITER (UEL)
POWER System STABILIZER (PSS)
POWER RECTIFIER
Static excitation systems typically use a power rectifier that converts AC to DC current and provides a controlled field current to the synchronous machine. Power rectifiers typically use thyristor or IGBT technology.
Heat generation is a concern for the power rectifier. For bridge cooling, redundant fan sets are typically provided.
CONVERTER INTERFACE
The converter interface consists of all intermediary devices between the controller and power rectifier. It converts the control signal to firing pulses and isolates the control electronics from the power section
POWER POTENTIAL TRANSFORMER (PPT)
The PPT is a 3-phase step-down power transformer used in static excitation systems.
FIELD BREAKER (AC OR DC)
DC FIELD FLASHING
The field flashing circuit is used in the start-up process, when the magnetic flux in the generator is too low. The field of the synchronous machine is temporarily connected in parallel to an external DC supply, such as a station battery, until the synchronous machine develops enough terminal voltage such that the excitation becomes self-sustaining. Afterwards, the field flashing is interrupted
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