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What is an excitation system?
The system that is utilized to supply the required field current to the synchronous machine's rotor winding is known as an excitation system. In other terms, an excitation system is described as a system that generates flux by flowing current through a field coil.
A generator's field windings can receive regulated DC current from an excitation system to produce generator output voltage. A prime mover's mechanical energy is converted into electrical energy by the generator for delivery to the customers. The prime mover, which could be a hydro, gas, steam, or wind turbine, provides the generator's megawatt load.
The armature of the generator, also known as the stator winding, is fixed and carries the generator's output power to the step-up transformer. The field winding is connected to the generator's rotor, which fits into the armature's center. It transports the current that the generator is excited with using as given by the excitation system.
To produce electrical energy, all electrical generators need to be excited. The armature is excited by the excitation system, which uses a DC current to induce a magnetic field on the rotor. The magnetic field's intensity affects the armature's output voltage. As a result, by regulating the DC current to the generator field winding, the excitation system regulates the output voltage of the generator.
The amount of excitation required depends on the machine's speed, load current, and load power factor. When the load current is high, the speed is lower, and the system's power factor lags, the system requires more excitation.
Rotating exciters and static exciters are the two main categories for excitation systems. |
| Fig: Typical Excitation System of a Generator(Ref- emerson.com) |
Generator Excitation System.
Automatic Voltage Regulator (AVR)
DC Excitation System
The main exciter and the pilot exciter are the two exciters that make up the DC excitation system. An automatic voltage regulator (AVR) adjusts the exciter output in order to regulate the alternator's output terminal voltage. The alternator current is limited when there is a fault considering the current transformer input to the AVR.
The field discharge resistor is connected across the field winding while the field breaker is open in order to disperse the stored energy in the highly inductive field winding.
Either the main shaft or the motor can independently drive the main and the pilot exciters. Since the unit system of operation is maintained and the excitation is not triggered by outside disturbances, direct driven exciters are typically favored.
AC Excitation System
The alternator and thyristor rectifier bridge that are directly attached to the main alternator shaft make up the AC excitation system. The main exciter may be separately or self-excited. The two general types into which the AC excitation system can be divided are described in more detail below.
B. Brushless Excitation System.
A permanent magnet generator, rectifier, main exciter, and other components make up the brushless excitation system. The main shaft drives the main exciter as well as the pilot exciter. The main exciter is directly connected to the field of the main alternators through the silicon rectifiers and has a stationary field and moving armature.
The pilot exciter is a shaft-driven permanent magnet generator having a stationary three-phase armature and revolving permanent magnets attached to the rotor shaft. the pilot exciter feeds the main exciter field winding through silicon rectifiers controlled by AVR.
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Fig: Typical Brushless Excitation System(researchget.net)
The pilot exciter, which feeds the main exciter through 3-phase full wave phase regulated thyristor bridges, is a shaft-driven permanent magnetic generator with rotating permanent magnets attached to the shaft and a 3-phase fixed armature.
The system does not require a commutator, collectors, or brushes, and it responds in less than 0.1 seconds with a minimal time constant. Improved tiny signal dynamic performance and easier deployment of supplemental power system stabilizing signals are two benefits of the short time constant.
Static Excitation System
In this setup, a 3-phase star/delta linked step-down transformer draws the supply directly from the alternator. The transformer's secondary feeds power to the rectifier as well as the grid control circuit and other electrical equipment, while its primary is connected to the alternator bus.
This system offers exceptional dynamic performance with a very fast response time. Through the elimination of exciter windage loss and winding maintenance, this technology decreased operational costs.
FAQ of Generator excitation system.
1. How to excite a generator?
Electric currents excite generators, which rotates the magnetic field inside the stator. The magnetic field is then produced by the current that is induced by the rotating magnetic field in the rotor. The torque required to turn the output shaft of the generator is created by the interaction between these two magnetic fields.
2. What is the excitation voltage?
It takes a certain voltage to excite a field coil, and the rectifier control the voltage to that field. The two types of excitation voltage are alternating voltage and direct voltage.
3. Why DC is used for excitation?
Direct current (DC) voltage is given to a coil in order to create a constant magnetic field since an electric current can only be generated when a wire rotates in such a field.
4. Why do generator needs excitation?
For the generator to produce a magnetic field and to deliver a continuous, fixed, or stationary rotating magnetic field, excitation is required.
5. Why do we need an excitation system for alternators?
For an alternator to regulate the voltage and reactive power of the synchronous alternator or generator, this system is required.
6. What happens when generators loss excitation?
When the generator loses excitation, the rotor current falls, and due to the field time constant, the field voltage also degrades.
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