Essential Components of Excitation Systems

Excitation power systems play a vital role in power generation by keeping generators functioning at optimum levels. Some key components that make up excitation power systems include:

- Rotating Exciter: This component generates alternating current (AC) power to energize the generator's field windings. Rotating exciters come powered by the generator shaft. Different design variants include brushless excitation power systems or static excitation power systems.

- Rectifier: The rectifier converts the AC power from the rotating exciter to direct current (DC) power required to energize the generator's field windings. Excitation Systems Earlier systems used mercury-arc valves or thyristor valves as rectifiers. Modern excitation power systems employ silicon controlled rectifiers (SCRs) or diodes in bridge rectifier configurations.

- Regulator: The regulator controls the output voltage of the excitation power system by regulating the field current supplied to the generator. Earlier analog designs used rotating amplidynes as regulators. Digital static excitation power systems now incorporate microprocessor-based regulators with firing circuits to control SCR valves.

- Sensing and Feedback: Various sensors measure generator output parameters like voltage, current, power factor, etc. and feed this valuable feedback to the regulator. This closed-loop control ensures generator output remains stable under all load variations. Potential and current transformers are commonly used for sensing.

Modes of Operation and Control Strategies

Excitation systems operate in various modes depending on generator operational needs:

- No-Load Excitation: At no external load, the minimum field current required to maintain rated voltage is supplied. This establishes the steady-state operating point.

- Build-Up: When the generator is started from standstill, the field is progressively strengthened in controlled steps to build voltage up to rated levels before taking load. This avoids transients on switch-on.

- Voltage Regulation: Under changing load conditions, the regulator acts to keep output voltage constant by adjusting the field current in linear or quadratic response modes via thyristor firing angles.

- Stabilization: Large disturbances are counteracted by providing additional field current momentarily until normal voltage control resumes. Strict operating bandwidth and response times are crucial for grid stability.

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