Excitation Systems: Exciting Advancements in Power Generation New Developments in Excit Systems

Excitation Systems: Exciting Advancements in Power Generation New Developments in Excit Systems

The excitation system provides the electric current to the generator's field windings to create the magnetic fields necessary for operation. Proper excitation is crucial for reliability and preventing damage to equipment.

Components of an Excitation System

A basic excitation system contains several key components:

- Rectifier: This device converts the alternating current from the generator's auxiliary winding to direct current needed for the field windings. Earlier systems used mercury arc rectifiers while modern excitation uses silicon controlled rectifiers (SCRs).

- Voltage Regulator: Excitation Systems  Also known as an automatic voltage regulator (AVR), this component monitors the generator's output voltage and controls the field current to keep voltage steady despite varying loads. Early governors used analog controls while microprocessor-based digital controls are now common.

- Sensing Circuits: These circuits measure voltage from the generator terminals and send the error signal to the AVR. Potential transformers (PTs) and summing circuits compare regulated voltage to actual voltage.

- Field Circuit: This takes the controlled DC current from the rectifier and sends it to the generator's field windings located on the rotor. Brushes and slip rings are required to transfer current to the spinning rotor.

- Protective Devices: Circuit breakers, fuses or other overcurrent protection safeguard the excitation system components. Reverse power, differential and other relaying also provide fault detection.

Principle of Operation

In operation, the generator's rotor and auxiliary winding produce alternating currents that are rectified. The rectified DC current is sent to the generator's field winding via the slip rings and brushes. Magnetic fields created by the field current induce voltage in the main generator windings on the stator.

The AVR monitors this generator output voltage and compares it to a setpoint. It adjusts the field current based on any deviation detected. More current strengthens the field to boost voltage as needed. Less current weakens the field to reduce voltage if it rises too high. This negative feedback loop keeps the generator's terminal voltage nearly constant.

Modern Digital Excitation Control

Analog excitation had limitations in responding rapidly to disturbances. Modern digital excitation controls use microprocessors for improved regulation. Additional functionality includes:

- Voltage boost during starting to help large motors start smoothly.

- Demand from the electric grid for reactive power/var control to aid grid stability.

- Sophisticated limiters and protections against over-excitation, field overcurrent and pole slipping.

- PSS (power system stabilizer) functions that dampen oscillations to counter torsional vibrations.

- Communications interfaces allowing remote monitoring and configuration from SCADA systems.

- Advanced modeling and parameter auto-tuning for better matching excitation to generator characteristics.

Digital controls provide faster response, multi-feature capability and interoperability in modern grid operations. Microprocessor flexibility also enables self-diagnostics and troubleshooting functions.

Ongoing Development Excitation Systems

Research continues aiming to enhance excitation system performance:

- Wide area monitoring allows networked units to coordinate responses for grid-wide damping of inter-area oscillations.

- Condition monitoring extracts diagnostic data to assess circuit health and predict maintenance needs before failures occur.

- Optimized algorithms explore ways to balance regulation requirements more intelligently based on dynamic priority settings.

- Stator transient control reduces oscillations during large disturbances to minimize potential damage.

- New semiconductor devices like IGBTs and SiC materials may allow higher switching frequencies and response times in future rectifiers.

- Integration with wind and solar inverters presents opportunities as well as challenges for excitation to aid renewable energy interfacing.

Proper excitation system design and operation underpins generator and power system stability. Continuous engineering work keeps excitation technology advancing to meet the control demands of modern grids with high penetrations of renewable and distributed energy resources. Close coordination between excitation and other grid control functions will grow increasingly important.
 
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