The fast-changing operational conditions of synchronous machines require a powerful system to control the stability. This is where Excitation control plays a vital role. To maintain stability, it is necessary to have very fast response excitation systems for synchronous machines operating with the grid. This means that the field current of a synchronous machine must be watched extremely fast to changing operational conditions. With the advent of fast microprocessors, it is now possible to achieve the matching response in digital control systems using a faster sampling rate, in addition to many user-friendly features.

A high control speed is achieved by using an interia free control and power electronic system. Fully controlled thyristor bridges are used to feed the generator/exciter field for controlling the generator output parameters. Any deviation in the generator terminal voltage is sensed by an error detector and caused the voltage regulator to advance or retard the firing pulses of the thyristors, which in turn controls the field supply of the alternator/exciter.

Objectives of Excitation Control

Besides maintaining the field current and steady-state operating point, the excitation system is required to improve the natural damping behavior and extend the stability limits.

The operating conditions to be taken into consideration are-

• Good response in voltage and reactive power control
• Satisfactory steady-state and dynamic stability
• Transient stability for all stated condition
• Quick voltage recovery after fault clearance

In steady-state operation, voltage response is of main importance. Under dynamic conditions damping behavior of small oscillations is of importance. Under fault conditions, transient stability i.e. ability to return to the normal operating point is the main objective and voltage response plays a secondary role.

Direct Excitation System (Static Excitation Equipment)

• Rectifier Transformer - The power transformer normally gets input supply from the generator output terminals. The secondary is connected to the thyristor bridge which delivers a variable DC output to the generator field. The transformer is housed in a ventilated cubicle. Generally, the dry-type transformer is provided devices.

• Thyristor Bridges - The thyristor bridges are assembled in one or more cubicles depending on the number of thyristor bridges connected in parallel. The number of bridges is designed that in the case one bridge fails during operation, the remaining bridges will have adequate capacity to feed the generator field for full load output.

Under such (n-1) operation, where ‘n’ specifies the total number of thyristor bridges, the converter is capable of meeting the field forcing conditions. This configuration can be varied depending on specific customer requirements. The thyristor bridges are cooled by fans mounted on the top of cubicles. Adequate protection and monitoring are provided for the thyristors and cooling fans.

• Control Electronics - The control circuits contain various electronic sub-assemblies in modular from mounted in various racks in the regulation cubicle. The racks are mounted in a swing frame in the cubicles. Other items pertaining to the control scheme like auxiliary transformers, relays, MCBs, conductors etc. are mounted in the cubicle on channels. Various features and working of the control scheme of the digital voltage regulator are explained separately.

• Field Flashing - It is not possible to start the excitation system with the residual voltage at nominal speed, hence a field flashing circuit is provided to overcome this problem. Initially, the station auxiliary supply 415V AC is stepped down by a small transformer the rectified in a rectifier bridge, and supplied to the generator field through a breaker. As soon as the generator output builds up, the excitation system starts working smoothly and the field flashing circuit is then cut off.

Field flashing can also be done by feeding the generator field from a station battery supply. The battery will be required to deliver approx. 25% of the no-load current for 20 seconds. Blocking diodes are provided to prevent any back feed from the field to the battery when the generator voltage rises under excitation control by thyristors.

• Field Suppression - For rapid de-excitation of the synchronous machine and complete isolation of the field from the thyristor bridge, a field breaker is provided. In case of severe internal faults or a three-phase short circuit at the generator terminals or short circuit on the slip rings, the field breaker provides protection by isolating the DC source from the field. The field energy is dissipated through a field discharge resistance, which gets connected across the field under such operation.

• Installation - The static excitation equipment should be normally mounted in a dust-free atmosphere on a floor, free from vibration and heat source