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Em-Ii Assignment: Excitation of Synchronous Generators

This document discusses different methods of excitation for synchronous generators, including direct current excitation, alternating current excitation, excitation with brushes/static excitation, and brushless/dynamic excitation. Direct current excitation is the most commonly used brushless excitation method, where a separate excitation machine supplies DC current to the rotor. Alternating current excitation also uses an excitation machine but supplies AC current which is then rectified. Excitation with brushes uses a prime mover exciter with its own control systems. Brushless excitation uses an AC generator with rectification to supply DC current to the field windings.

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Zain Ur Rahman
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156 views3 pages

Em-Ii Assignment: Excitation of Synchronous Generators

This document discusses different methods of excitation for synchronous generators, including direct current excitation, alternating current excitation, excitation with brushes/static excitation, and brushless/dynamic excitation. Direct current excitation is the most commonly used brushless excitation method, where a separate excitation machine supplies DC current to the rotor. Alternating current excitation also uses an excitation machine but supplies AC current which is then rectified. Excitation with brushes uses a prime mover exciter with its own control systems. Brushless excitation uses an AC generator with rectification to supply DC current to the field windings.

Uploaded by

Zain Ur Rahman
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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M.

Iftikhar Raza EE-055

EM-II ASSIGNMENT
EXCITATION OF SYNCHRONOUS GENERATORS
INTRODUCTION:
All the Generators are run by Gas turbines and these generators are a synchronous machine that is they have field circuit supplied by a DC Supply. As we know that this provision of DC Supply is of two types: Brushed Supply Brushless Supply DC field excitation is an important part of the overall design of a synchronous generator. The field excitation must ensure not only a stable AC terminal voltage, but must also respond to sudden load changes. Rapid field excitation response is very important.

METHODS OF EXCITATION:DIRECT CURRENT EXCITATION:


Direct current supplied excitation (DC-AC excitation) is the most commonly used brushless excitation method in which the excitation power to the rotor is supplied by a separate excitation machine. The excitation machine is usually installed in the main frame and on the same shaft as the main machine. The DC-AC excitation machine is a salient-pole external-pole generator that has its field winding in the stator and armature winding in the rotor. As the rotor rotates, the direct current supplied to the stator winding induces a three-phase alternating current into the rotor winding. This current is rectified into direct excitation current using a diode bridge installed in the rotor. The frequency of the rotor current before the rectifier is r m f = np , where n is the mechanical rotation frequency of the rotor and pm is the number of pole pairs in the excitation machine. The pole-pair number of the excitation machine is typically 13 or 17 which is larger than that of the main machine. For the control of the voltage or reactive power of the main machine, the stator current is controlled by an automatic voltage regulator (AVR). The excitation power can be obtained using voltage and current transformers, an auxiliary winding installed in the stator slots of the main machine or a separate permanent magnet generator (PMG). In the first two cases, one of the stator poles must be a permanent magnet to enable self-excitation. In each case, instrument transformers for voltages and currents are needed for the regulator to control the voltage and power. The regulator and supply of the excitation power are considered later in this work.

M. Iftikhar Raza EE-055

Alternating-current excitation:
In alternating current supplied excitation (AC-AC excitation), the rotor of the excitation machine is similar to the rotor in DC-AC excitation. The stator has a three phase winding which is supplied with alternating current using a supply transformer and a frequency converter. As the rotor rotates, a three-phase alternating current is induced into the rotor winding. Correspondingly to DC-AC excitation, this current is rectified into direct excitation current. Figure 3.2 shows a schematic of AC-AC excitation.

AC-AC excitation is used in variable-speed synchronous-motor drives to produce the needed excitation power also when operating at low speeds or when the rotor is at rest.

Excitation With Brushes/Static Excitation:


Static machines have a field circuit which is supplied through brushes and slip rings from an external source or exciter. Prime mover is used as an exciter for these machines which has its own control line diagram and controlling relays to run the exciter

Brushless/Dynamic Excitation :
It is an an ac generator with fixed field winding and a rotor with a three phase circuit. Diode/SCR rectification supplies dc current to the field windings. A brushless type machine is a machine which has its provision of DC supply right over the shaft of the generator often through a brushless exciter which is an AC generator whose field windings are at the stator and armature windings on the rotor. The output of this generator is rectified using three phase rectifier to give a rectified DC output which serves to supply the

M. Iftikhar Raza EE-055


rotor windings of the generator or the main field windings. The excitation of this generator is operated by Prismic A10.

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