The thyristor-type static excitation system, due to its many advantages, excellent response characteristics,easy maintenance and simplified main machine construction, is now extensively used for medium-and large-capacity hydro-or steam-turbine generators.

The voltage regulator and potential source static excitation system functions to control the voltage of an synchronous  generator by directly controlling the generator's DC field current. The static excitation system is composed of the followings:

Thyristor rectifier bridge and thyristor elements:

The 3phase full bridge rectifier circuit has fast response characteristics. A compact cubicle design is realized with the large on-state current and high reversed voltage flapack type thyristor elements, and forced air cooling. The thyristor elements are installed in a tray, and can be exchange during operation. For better cost performance, a tray-less-type can also be manufactured.

Field Flashing

The field flashing circuit is necessary when a generator is started, because of self excitation system. A DC battery is usually used as the initial excitation power supply. An AC power supply can also be adopted by means of rectifiers and a transformer.

Field suppression

The de-excitation function is to reduce rapidly field energy when needed and also to separate the rotor circuit from the excitation system. The DC field circuit breaker is generally used. For better cost performance, a static field circuit breaker system can be supplied. This system reduce the field energy by reversing the excitation voltage by rectifier gate controls.

Over voltage protections

The C-R absorbers and varisters are installed in each AC and DC circuit for over voltage protections of thyristor elements. In large capacitance system, a crowbar circuit is adapted on DC circuit.

Excitation transformer

The excitation transformer reduces the supply voltage to the level required for excitation. A dry-type for small capacity or a oil-type for large capacity is generally used.

Monitoring devices

The alarms for thyristor fuse blown, cooling fan failure and air temperature high are available. A rotor temperature converter and field earth detector can be installed in excitation cubicles optionally.

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source: excitations system

Excitation systems have a powerful impact on generator dynamic performance and availability, it ensures quality of generator voltage and reactive power, i.e. quality of delivered energy to consumers. Following types are common:

• Brushless excitation systems, with rotating exciter machines and Automatic Voltage Regulator (AVR), or
• Static excitation systems (SES), feeding rotor directly from thyristor bridges via brushes.

Main functions of excitation system are to provide variable DC current with short time overload capability, controlling terminal voltage with suitable accuracy, ensure stable operation with network and/ or other machines, contribution to transient stability subsequent to a fault, communicate with the power plant control system and to keep machine within permissible operating range.