What is an Excitation System?
The excitation system is responsible for providing the electrical current that is needed to generate the magnetic field in an alternator (a type of electrical generator). This is necessary because the alternator needs a magnetic field to produce electricity.
The amount of excitation required depends on several factors, including the load current (the amount of electrical current being used by the system), the load power factor (a measure of how efficiently the system is using electricity), and the speed of the alternator. In general, more excitation is needed when the load current is high, the speed is low, and the power factor is lagging (not as efficient).
An alternator can have its own exciter (a smaller generator that provides the necessary excitation) or it can be part of a centralized excitation system, which has multiple exciters that feed a common bus bar (a conductor that distributes electrical power). A centralized excitation system is usually cheaper, but if there is a fault in the system, it can affect all of the alternators in the power plant.
Types of Excitations System:
The excitations system classifies mainly three types:
- DC Excitation System
- AC Excitation System
- Static Excitation System
1. DC Excitation System:
A DC excitation system is a type of system that is used to provide the electrical current needed to generate the magnetic field in an alternator. It has two exciters: a main exciter and a pilot exciter. The main exciter is responsible for providing most of the excitation, while the pilot exciter helps to regulate the output.
The output of the exciters is controlled by an automatic voltage regulator (AVR), which helps to maintain the proper output terminal voltage of the alternator. A current transformer is also used to limit the alternator's current during a fault (a malfunction or failure in the system).
The main exciter and the pilot exciter can be driven either by the main shaft of the alternator or by a separate motor. Direct-driven exciters (exciters that are driven by the main shaft) are usually preferred because they preserve the unit system of operation and are less likely to be affected by external disturbances.
The voltage rating of the main exciter is typically around 400 volts, and its capacity is around 0.5% of the capacity of the alternator. Exciters in turbo alternators (high-speed alternators) can sometimes experience problems because of their high speed, so it is common to have a separate motor-driven exciter as a standby.
Advantages:
- More reliable: DC excitation systems are generally more reliable than AC excitation systems because they are less affected by power fluctuations and have simpler control systems.
- Compact size: DC excitation systems are typically smaller in size than AC excitation systems, making them easier to install and maintain.
Disadvantages:
- Higher cost: DC excitation systems are generally more expensive to purchase and maintain than AC excitation systems.
- Complex voltage regulation: The voltage regulation of DC excitation systems can be more complex than that of AC excitation systems, requiring more advanced control systems.
- Slow response: DC excitation systems can have slower response times than AC excitation systems, which can limit their ability to quickly adjust to changes in load demand.
2. AC Excitation System:
An AC excitation system is a type of system that is used to provide the electrical current needed to generate the magnetic field in an alternator. It consists of an alternator and a thyristor rectifier bridge that are directly connected to the main shaft of the alternator.
The main exciter in an AC excitation system can either be self-excited or separately excited. A self-excited exciter is one that generates its own excitation, while a separately excited exciter receives its excitation from an external source.
In an AC excitation system, the alternator and the thyristor rectifier bridge work together to convert the AC power generated by the alternator into DC power, which is then used to create the magnetic field in the alternator. The thyristor rectifier bridge consists of a series of thyristors (semiconductor devices that act like switches) that are used to convert AC power into DC power.
Type of AC Excitation System:
The AC Excitation System is classified into two types:
- Rotor Excitation System
- Brushless Excitation System
1. Rotor Excitation System:
The rotor excitation system is a type of system that is used to provide the electrical current needed to generate the magnetic field in an alternator. It consists of an AC exciter, a stationary field, and a rotating armature. The AC exciter generates the electrical current that is needed to create the magnetic field, while the stationary field and the rotating armature help to convert this electrical current into a magnetic field.
The output of the AC exciter is rectified (converted from AC to DC) by a full-wave thyristor bridge rectifier circuit and supplied to the main alternator field winding. The alternator field winding is also supplied with power through another rectifier circuit. The exciter voltage can be increased by using the residual flux (the residual magnetic field that remains in the system after the power has been turned off).
The power supply and rectifier control generate the controlled triggering signal (a signal that tells the rectifier when to switch on and off). In the "auto" mode of operation, the alternator voltage is averaged and compared directly with the operator voltage adjustment (the desired voltage level set by the operator). In the "manual" mode of operation, the excitation current of the alternator is compared with a separate manual voltage adjustment.
Advantages:
- Fast response: The rotating thyristor excitation system has a fast response time, which means it can quickly adjust to changes in the system.
- Simple: The system is relatively simple, which makes it easy to install and maintain.
- Low cost: The rotating thyristor excitation system is generally less expensive than other types of excitation systems.
Disadvantages:
- Low thyristor response rate: One of the main disadvantages of the rotating thyristor excitation system is that the response rate of the thyristors (semiconductor devices that act like switches) is relatively slow. This can limit the system's ability to quickly adjust to changes in load demand.
2. Brushless Excitation System:
The brushless excitation system is a type of system that is used to provide the electrical current needed to generate the magnetic field in an alternator. It consists of an alternator, a rectifier, a main exciter, and a permanent magnet generator alternator. The main and pilot exciters are driven by the main shaft of the alternator.
The main exciter has a stationary field and a rotating armature that are directly connected to the field of the main alternator through silicon rectifiers (semiconductor devices that allow electricity to flow in only one direction). The pilot exciter is a shaft-driven permanent magnet generator that has rotating permanent magnets attached to the shaft and a 3-phase stationary armature. It feeds the main exciter field through silicon rectifiers and 3-phase full-wave phase-controlled thyristor bridges (semiconductor devices that act like switches).
The brushless excitation system eliminates the need for a commutator (a device that helps to maintain the proper current flow in an electrical system) and brushes (components that help to transfer electrical current between stationary and moving parts). It has a short time constant (a measure of how quickly a system can respond to a change) and a response time of less than 0.1 seconds. This short time constant improves the system's small signal dynamic performance (its ability to quickly adjust to changes) and allows for the application of additional power system stabilizing signals.
Advantages:
- Excellent reliability: The brushless excitation system is generally very reliable because it has no moving parts, such as brushes or a commutator, that can wear out or fail.
- Good flexibility of operation: The brushless excitation system has a good range of operating conditions and can be easily adjusted to meet changing load demands.
- Good system responses: The system has good response times, which means it can quickly adjust to changes in the system.
- Low maintenance: Because there are no moving contacts in the brushless system, maintenance is generally low.
Disadvantages:
- Slow response: The brushless excitation system has a slower response time compared to some other types of excitation systems.
- No fast de-excitation: The brushless excitation system does not have the ability to quickly reduce the excitation level, which can be a disadvantage in some situations.
3. Static Excitation System:
A static excitation system is a type of system that uses static (non-moving) components to provide the electrical current needed to generate the magnetic field in an alternator.
In a static excitation system, the supply for the system is taken from the alternator itself through a 3-phase star/delta connected step-down transformer. The primary of the transformer is connected to the alternator bus, and the secondary supplies power to the rectifier and other electrical equipment. This system has a very small response time, which means it can quickly adjust to changes in the system. It also has excellent dynamic performance, which means it can handle a wide range of operating conditions.
The static excitation system reduces operating costs by eliminating the exciter windage loss (the loss of energy caused by wind resistance on the exciter) and winding maintenance. It is generally a reliable and efficient way to provide the excitation needed to generate electricity in an alternator.
Advantages:
- Good reliability: The static excitation system is generally reliable because it has no moving parts that can wear out or fail.
- Very good flexibility of operation: The static excitation system has a wide range of operating conditions and can be easily adjusted to meet changing load demands.
- Excellent system responses: The system has excellent response times, which means it can quickly adjust to changes in the system.
- Small size: The static excitation system is typically smaller in size than other types of excitation systems, making it easier to install and maintain.
- Low loss: The static excitation system has low energy loss, which makes it more efficient than some other types of excitation systems.
- Simple: The system is relatively simple, which makes it easy to install and maintain.
- High performance: The static excitation system has good performance characteristics and can handle a wide range of operating conditions.
Disadvantages:
- Requires a slip ring and brush: One of the main disadvantages of the static excitation system is that it requires a slip ring (a device that allows electrical current to be transferred between a stationary and a rotating component) and brush (a component that helps to transfer electrical current between stationary and moving parts). These components can wear out over time and may need to be replaced, which can increase maintenance costs.