The dynamic connection of the generator field winding, carbon brushes and slip rings during operation is very detrimental to the synchronous generator. The carbon powder worn by the carbon brushes and slip rings is dirty and will cause the insulation of the generator to decrease. In severe cases, it will affect the safe operation of the generator, so frequent maintenance and upkeep is required.
When the electrical connection between the carbon brush and the slip ring is in motion, sparks will appear to generate interfering electromagnetic waves, which will not only affect radio communications, but also be one of the main interference signal sources for false alarms and malfunctions. In order to solve these problems, people have been trying to improve or cancel the connection method of carbon brushes and slip rings in synchronous engines. An effective measure to solve the above problems is to use a brushless excitation system.
Basic ideas of brushless excitation
Change the direct electrical connection between the stator and rotor of a conventional generator to a magnetic connection. This requires an exciter (Exciter) to cooperate with the generator. The generator adopts the rotating magnetic field type, and its field winding is assembled on the rotor; while the AC exciter adopts the rotating armature type, and its field winding is assembled on the stator. The field winding of the generator and the armature winding 3 of the exciter are fixed on the same rotating shaft. There is also a rotating rectifier on the rotating shaft. In this way, the rotor part forms a closed circuit by itself. The excitation current of the exciter is provided by the generator through the automatic voltage regulator (AVR) 6.
This structure does not have a commutator and a carbon brush slip ring. The excitation current of the generator is provided by the AC exciter. If the exciter is regarded as a component that amplifies the excitation current, various excitation adjustment devices in the self-excitation mode can still be used. When used in brushless excitation methods, the most typical one is the phase compound excitation device. Since the regulating device adjusts the excitation current of the exciter, its output power is significantly reduced compared with the self-excited device.
Due to the introduction of AC exciters, a large electromagnetic inertia link is added to the excitation regulation system, and the dynamic performance of the system becomes worse. To improve dynamic performance, the following measures can be taken.
(1) The motor uses a hidden pole rotor to reduce the rotor leakage reactance, thereby reducing the transient reactance and shortening the voltage recovery time. However, the manufacturing process is more complex than that of salient pole machines and requires larger excitation current.
(2) Install a damping winding on the generator rotor to reduce the sub-transient reactance.
(3) Appropriately increase the frequency of the AC exciter to reduce its time constant. However, too many magnetic poles will increase manufacturing difficulties.
(4) Use an excitation device with good strong excitation performance.
(5) Change the rotating rectifier to a rotating thyristor. Although the exciter still provides the excitation current for the generator, the excitation current adjustment loop excludes the exciter. The adjustment device obtains the voltage, current signals and voltage deviation signals from the generator circuit, and converts the thyristor required by the rotary transformer through the rotary transformer. The trigger pulse signal is transmitted to the rotor. The resolver has small inductance and high signal frequency, so its electromagnetic inertia is much smaller than that of the exciter. Trigger pulses can also be delivered through structures such as rotating capacitors and halo rings.
The use of thyristor components will affect the starting voltage of the generator. Therefore, an auxiliary exciter must be equipped in the practical device. The auxiliary exciter EP (Pilot Exciter) is a small permanent magnet generator. When the rotor reaches the rated speed, its stator armature winding establishes the rated voltage and supplies power to the excitation winding and regulating device of the exciter so that they can operate normally. Therefore, the starting voltage of the generator has nothing to do with its own residual magnet voltage.
The rotating rectifier in the brushless excitation generator rotor has to withstand large mechanical stress, surge voltage during transient processes and overcurrent during strong excitation, so it has become a key component for the reliable operation of the generator system. If the quality of the rotating diode or rotating thyristor cannot be guaranteed, the advantage of brushless excitation in improving the operating reliability of the generator will no longer exist. With the continuous improvement and development of power electronic device manufacturing technology and processes, the quality of rotating rectifier devices has been guaranteed, and the advantages of brushless excitation have become increasingly apparent and their applications have become more and more widespread.
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