I. Several Methods of Generator Excitation Current 1. DC Generator Powered Excitation Method: This type of generator uses a dedicated DC generator, called a DC exciter. The exciter is generally coaxial with the generator. The generator's excitation winding obtains DC current from the exciter through slip rings and fixed brushes mounted on the main shaft. This excitation method has advantages such as independent excitation current, relatively reliable operation, and reduced self-consumption of electricity. It has been the main excitation method for generators for the past few decades and has mature operating experience. The disadvantages are slow excitation regulation speed and high maintenance workload, so it is rarely used in units above 10MW. 2. AC Exciter Powered Excitation Method: Some modern large-capacity generators use AC exciters to provide excitation current. The AC exciter is also mounted on the generator's main shaft. Its output AC current is rectified and supplied to the generator rotor for excitation. In this case, the generator's excitation method is a separately excited method. Because a static rectifier is used, it is also called separately excited static excitation. An AC auxiliary exciter provides the excitation current. The AC auxiliary exciter can be a permanent magnet generator or an AC generator with a self-excited constant voltage device. To improve the excitation regulation speed, the AC exciter usually uses a 100-200Hz medium-frequency generator, while the AC auxiliary exciter uses a 400-500Hz medium-frequency generator. In this type of generator, both the DC excitation winding and the three-phase AC winding are wound in the stator slots. The rotor only has teeth and slots without windings, resembling a gear. Therefore, it lacks rotating contact parts such as brushes and slip rings, offering advantages such as reliable operation, simple structure, and convenient manufacturing. Disadvantages include higher noise levels and larger harmonic components in the AC potential. 3. Excitation method without an exciter: In this excitation method, no dedicated exciter is used. Instead, the excitation power is obtained from the generator itself, rectified, and then supplied to the generator itself for excitation. This is called self-excited static excitation. Self-excited static excitation can be divided into self-shunt excitation and self-compound excitation. The self-excited method obtains excitation current through a rectifier transformer connected to the generator outlet, which is then rectified and supplied to the generator for excitation. This excitation method has advantages such as simple structure, fewer equipment, lower investment, and less maintenance. The self-resetting excitation method, in addition to lacking a rectifier transformer, also has a high-power current transformer connected in series in the generator stator circuit. The function of this transformer is to provide a larger excitation current to the generator in the event of a short circuit, to compensate for the insufficient output of the rectifier transformer. This excitation method has two excitation power sources: a voltage power source obtained through the rectifier transformer and a current source obtained through the series transformer. II. Relevant Characteristics of Generator and Excitation Current 1. Voltage Regulation The automatic excitation system can be viewed as a negative feedback control system with voltage as the controlled variable. Reactive load current is the main cause of generator terminal voltage drop. When the excitation current remains constant, the generator terminal voltage will decrease as the reactive current increases. However, to meet the user's requirements for power quality, the generator terminal voltage should remain basically constant. The way to achieve this is to adjust the generator excitation current according to the change in reactive current. 2. Reactive Power Regulation: When a generator operates in parallel with a system, it can be considered as operating on the bus of an infinitely large capacity power source. Changing the generator's excitation current will change the induced electromotive force and stator current, thus changing the generator's reactive current. When a generator operates in parallel with an infinitely large capacity system, the generator's excitation current must be adjusted to change its reactive power. This change in excitation current is not the usual "voltage regulation," but rather a change in the reactive power supplied to the system. 3. Reactive Load Distribution: Generators operating in parallel distribute reactive current proportionally according to their rated capacity. Larger capacity generators should bear more reactive load, while smaller capacity generators should provide less. To achieve automatic reactive load distribution, an automatic high-voltage regulating excitation device can be used to change the generator's excitation current to maintain a constant terminal voltage. The slope of the generator's voltage regulation characteristics can also be adjusted to achieve a reasonable distribution of reactive load among generators operating in parallel. III. Methods of Automatically Adjusting Excitation Current In changing the generator's excitation current , it is generally not done directly in the rotor circuit because the current in this circuit is very large, making direct adjustment inconvenient. The commonly used method is to change the excitation current of the exciter to achieve the purpose of adjusting the generator rotor current. Common methods include changing the resistance of the exciter's excitation circuit, changing the additional excitation current of the exciter, and changing the conduction angle of the thyristor. This section mainly discusses the method of changing the thyristor conduction angle. It adjusts the conduction angle of the thyristor rectifier accordingly based on changes in the generator's voltage, current, or power factor, thus changing the generator's excitation current. This device is generally composed of transistors and thyristor electronic components, and has advantages such as sensitivity, speed, no failure zone, high output power, small size, and light weight. In case of an accident, it can effectively suppress generator overvoltage and achieve rapid demagnetization. Automatic excitation adjustment devices typically consist of a measurement unit, a synchronization unit, an amplification unit, a differential adjustment unit, a stabilization unit, a limiting unit, and some auxiliary units. The measured signal (such as voltage, current, etc.) is transformed by the measurement unit and compared with the given value. The comparison result (deviation) is then amplified by the preamplifier and power amplifier units and used to control the conduction angle of the thyristor, thereby regulating the generator excitation current. The synchronization unit synchronizes the trigger pulse output by the phase-shifting section with the AC excitation power supply of the thyristor rectifier to ensure correct thyristor triggering. The differential unit ensures stable and reasonable distribution of reactive load among parallel-operated generators. The stabilization unit is introduced to improve the stability of the power system. The excitation system stabilization unit improves the stability of the excitation system. The limiting unit prevents the generator from operating under over-excitation or under-excitation conditions. It must be noted that not every automatic excitation control device has all of the above-mentioned units; the units a regulator device has are related to its specific task. IV. Components and Auxiliary Equipment of Automatic Excitation Regulation The automatic excitation regulation system consists of an organic terminal voltage transformer, a generator terminal current transformer, and an excitation transformer . The excitation device requires the following currents: AC380V and DC220V control power supply for the plant; DC220V closing power supply for the plant; and the following open contacts for automatic start-up, automatic shutdown, and grid connection (one normally open, one normally closed) increase/decrease. It also requires the following analog signals: generator terminal voltage 100V, generator terminal current 5A, bus voltage 100V. The excitation device outputs the following relay contact signals: excitation transformer overcurrent, loss of excitation, and excitation device malfunction. The excitation control, protection, and signal circuit consists of a demagnetizing switch, an auxiliary excitation circuit, a fan, demagnetizing switch tripping, excitation transformer overcurrent, regulator fault, generator abnormal operating condition, and a power transmitter. When a synchronous generator experiences an internal fault, in addition to disconnecting the generator, it is also necessary to demagnetize it. The primary function of the demagnetizing device is to reduce the rotor magnetic field to a minimum as quickly as possible, ensuring the rotor does not fail to pass through the grid, and to minimize the demagnetizing time. Based on the rated excitation voltage, demagnetizing can be divided into linear resistance demagnetizing and nonlinear resistance demagnetizing. In the past decade or so, the emergence and use of new technologies, processes, and devices have led to continuous development and improvement in generator excitation methods. In the area of ​​automatic excitation control devices, many new types of control devices have been continuously developed and promoted for use. Because automatic excitation control devices implemented using microcomputer software have significant advantages, many countries are currently researching and testing digital automatic excitation control devices composed of microcomputers and corresponding external equipment. Such control devices can achieve adaptive optimal regulation.