DOC-20240122-WA001jhfjyfguyf1. (1) (1).pptx

fuyiguyuytig

1. Excitation systems NAME : VEDHANARAYANAN K REG NO : 21MT1050 DEPARTMENT : MECHATRONICS SUBJECT : POWER ELECTRONICS AND DRIVES

2. Definition:   The system which is used for providing the necessary field current to the rotor winding of the synchronous machine, such type of system is called an excitation system. In other words, excitation system is defined as the system which is used for the production of the flux by passing current in the field winding. The main requirement of an excitation system is reliability under all conditions of service, a simplicity of control, ease of maintenance, stability and fast transient response. The amount of excitation required depends on the load current, load power factor and speed of the machine. The more excitation is needed in the system W hen the load current is large, the speed is less, and the power factor of the system becomes lagging. The excitation system is the single unit in which the each alternator has its exciter in the form of generator. The centralised excitation system has two or more exciter which feeds the bus-bar. The centralised system is very cheap, but the fault in the system adversely affects the alternators in the power plant

3. Types of Excitation System The excitation system is mainly classified into three types. They are1. DC Excitation System AC Excitation System Rotor Excitation System Brushless Excitation System Static Excitation System Their types are explained below in details. 1. DC Excitation System The DC excitation system has two exciters the main exciter and a pilot exciter. The exciter output is adjusted by an the main exciter and a pilot exciter. The exciter output is adjusted by an automatic voltage regulator (AVR)for controlling the output terminal voltage of the alternator. The current transformer input to the AVR ensures limiting of the alternator current during a fault. When the field breaker is open, the field discharge resistor is connected across the field winding so as to dissipate the stored energy in the field winding which is highly inductive.

4. The main and the pilot exciters can be driven either by the main shaft or separately driven by the motor. Direct driven exciters are usually preferred as these preserve the unit system of operation, and the excitation is not excited by external disturbances.The voltage rating of the main exciter is about 400 V, and its capacity is about0.5% of the capacity of the alternator. Troubles in the exciters of turbo alternator are quite frequent because of their high speed and as such separate motor driven exciters are provided as standby exciter.

5. 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. ADVANTAGES AND DISADVANTAGES OF DC EXCITATION SYSTEM: 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.

6. 2. AC Excitation System: The AC excitation system consists of an alternator and thyristor rectifier bridge directly connected to the main alternator shaft. The main exciter may either be self-excited or separately excited. 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. The AC excitation system may be broadly classified into two categories which are explained below in details. A. ROTATING THYRISTOR EXCITATION SYSTEM The rotating portion is being enclosed by the dashed line. This system consists an AC exciter, stationary field and a rotating armature. The output of the exciter is rectified by a full wave thyristor bridge rectifier circuit and is supplied to the main alternator field winding. The alternator field winding is also supplied 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.

7. AC to DC Conversion: The main function of the rotating thyristor system is to convert the AC voltage generated in the rotor windings to a DC voltage. Thyristors are semiconductor devices that act as controlled rectifiers, allowing the conversion of AC to DC in a controlled manner: ADVANTAGES AND DISADVANTAGES: 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. A

8. B. BRUSHLESS EXCITATION SYSTEM The rotating portion being enclosed by a dashed line rectangle. The brushless excitation system consists an alternator, rectifier, main exciter and a permanent magnet generator alternator. The main and the pilot exciter are driven by the main shaft. The main exciter has a stationary field and a rotating armature directly connected, through the silicon rectifiers to the field of the main alternators. The stator and rotor are the main components of the brushless alternator system. The stator body consists of the main stator and an exciter stator similarly the rotor assembly consists of the main rotor and the exciter rotor along with a bridge rectifier assembly mounted on a plate that is attached to the rotor. The exciter stator has residual magnetism when the rotor starts rotating AC (Alternating Current) output is generated in the exciter rotor coils and this output is passed through a bridge rectifier. The output passed through a bridge rectifier is converted into DC (Direct Current) and given to the main rotor. The moving main rotor generates AC in the stationary main rotor coils. The exciter plays a key role in controlling the output of the alternator. The DC magnetization current supplied to the rotor, which is the field of the main alternator thus if we increase or decrease the amount of current to the stationary exciter field coils, the output of the main alternator can be varied. 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.

9. ADVANTAGES AND DISADVANTAGES: 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.

10. 3. STATIC EXCITATION SYSTEM This system consists of rectifier transformers, SCR output stage, excitation start-up, and field discharge equipment, and regulator and operational control circuits. In this system, there is no rotating part, so there is no windage losses and no rotational losses. In this system, the supply is taken from the alternator itself through a 3-phasestar/delta connected step-down transformer. The primary of the transformer is connected to the alternator bus and their secondary supplies power to the rectifier and also feed power to the grid control circuit 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.

11. Advantages: The advantages of the static system are Reliability is good The flexibility of operation is very good System responses are excellent Small in size Low loss Simple High performance Disadvantages: The main disadvantages of the static system are, it requires a slip ring and brush