0 Introduction Among the existing generating units in the Inner Mongolia West Power Grid, the generator-terminal self-excited excitation system and the three-generator excitation system are the two most widely adopted excitation methods. Haibowan Power Plant is a large pithead power plant with an installed capacity of 600MW in its first and second phases, and its third phase is currently under expansion. The two 100MW units put into operation in the first phase use a three-generator excitation system consisting of a main excitation, auxiliary excitation, and generator. Initially, it used the GLT-4A type simulated excitation control system produced by Beizhong Machinery Plant, which was later replaced by the GEC-1 type microcomputer excitation system developed by Tsinghua University, using NEC control logic. The two newly put into operation in the second phase use the GES-3223 type generator-terminal self-excited excitation control system produced by Dongfang Electric Machinery Plant, using PID control logic. This paper analyzes and compares the performance of the excitation systems of the first and second phases, and proposes some views and suggestions regarding some problems existing in the field excitation systems. 1. Performance Characteristics Analysis and Comparison of Two Typical Excitation Systems 1.1 Composition of the Excitation System The self-excited static excitation system consists of an excitation transformer, a thyristor power rectifier, an automatic excitation regulator, a generator demagnetization and overvoltage protection device, excitation starting equipment, and excitation operation equipment. The three-machine excitation system consists of a main exciter, an auxiliary exciter, two sets of excitation regulators, three power cabinets, one demagnetization switch cabinet, and one overvoltage protection device. 1.2 Unit Investment and Engineering Cost Compared with the three-machine excitation system, the self-excited static excitation system, by eliminating the main and auxiliary exciters, significantly shortens the unit length (approximately 6-8m per unit). This not only reduces the number of shaft connection links, shortens the shaft system length, and improves shaft system stability, but also significantly reduces the length of the main plant for the same capacity, thus lowering both plant construction costs and unit investment. 1.3 Operation Mode of the Excitation System The self-excited excitation system adopts a dual-channel redundant fault-tolerant structure. Signal acquisition, adjustment input, calculation, and signal output are all handled by two independent hardware circuits. The two channels are interconnected yet can operate independently. The dual channels operate in parallel in a master-slave configuration, serving as hot-standby for each other. Theoretically, there is no distinction between master and standby channels; the channel powered on first becomes the master channel, and the one powered on later becomes the slave channel. The output pulse of the master channel blocks the output pulse of the slave channel. The channels achieve mutual self-diagnosis, tracking, communication, and switching through software. Furthermore, the excitation system allows for online parameter modification, replacement of faulty components, and real-time communication during operation. Simultaneously, the regulator connects to the DCS system via a serial port. In addition to local operation, excitation parameters can be adjusted through the DCS interface, and the operating parameters and device status information of the excitation system can be uploaded to the DCS system. This excitation system has two operating modes: constant voltage and constant current. The constant voltage mode is automatic, while the constant current mode is manual. These two modes can be dynamically switched without causing fluctuations in terminal voltage or reactive load. The generator operates normally in constant voltage mode. When either of the two automatic excitation regulation circuits fails, the system will automatically switch from "automatic mode" (constant voltage mode) to "manual mode" (constant current mode). After modification, the HaiDian No. 3 generator excitation system uses two sets of nonlinear excitation regulators, A and B, operating in parallel. Signal acquisition, processing, calculation, output, and rectifier output are all handled by two independent hardware circuits. There is no master/slave distinction; if one regulation system fails, the other can meet all system requirements. Both regulation devices in this excitation system operate normally in constant voltage mode, only switching to manual mode during excitation system commissioning. 1.4 Reliability of the Excitation System For DC exciters and three-machine excitation systems, accidents involving rotating parts account for a significant proportion of past excitation system failures. Examples include sparking in DC exciters and loosening and vibration of AC exciter coils. Furthermore, the operation and maintenance of rotating parts require substantial workload. In contrast, the self-excited static excitation system eliminates rotating components such as commutators, bearings, and rotors, greatly simplifying the system structure and wiring. This significantly reduces the workload of operation and maintenance, as well as the potential for accidents, resulting in significantly higher reliability than DC and AC exciter systems. Moreover, the self-excited system employs a redundant structure in its design, allowing for online replacement of faulty components, effectively reducing the probability of downtime and lowering the requirements for operation and maintenance. 1.5 Transient and Steady-State Levels of the Unit Due to the adoption of thyristor electronic technology in the self-excited static excitation system, the system's regulation response speed is further improved. Under small disturbances, the self-excited system can maintain the generator terminal voltage unchanged. For a single-machine infinite bus system, the static stability limit power of the generator is: Pmax=VgVs/Xs——————（1） Where: Vg is the generator terminal voltage; Vs is the system voltage; Xs is the equivalent reactance of the generator and the system. However, the three-machine excitation system only maintains the generator subtransient electromotive force or transient electromotive force Eg' unchanged during the fault process. Its limit power is: P'max=Eq'Vs/（Xs Xd'）——-（2） Where: Eq' is the generator Q-axis transient electromotive force; Xd' is the generator direct-axis transient reactance. According to equations (1) and (2), Pmax is greater than P′max, that is, the static stability limit of the self-excited static excitation system is higher than that of the three-machine excitation system. Under the most unfavorable three-phase short-circuit condition at the generator outlet of the self-excited system, the generator terminal voltage (i.e., the rectified power supply) drops significantly. Even after the fault is quickly cleared, the recovery of the generator terminal voltage still requires a certain amount of time, inevitably reducing the excitation capability of the self-excited system. Therefore, the rectified power supply voltage is calculated as 80% of the generator's rated voltage. Furthermore, since enclosed busbars are used at the generator outlets of large and medium-sized units, the possibility of a three-phase short circuit at the generator terminal is essentially eliminated. Thus, the high excitation multiple, fast voltage response speed, and advanced control model of the self-excited system effectively improve the system's transient stability. Taking a three-phase short circuit at the high-voltage outlet as an example, with the excitation calculated at 2 times, the transient stability of the self-excited excitation system is basically the same as that of a conventional excitation system with an actual time constant Te = 0.35s. If an entire power grid adopts a self-excited excitation system, the transient stability is even better than that of conventional excitation: when a three-phase short circuit occurs, except for the self-excited units closest to the fault point which are affected by voltage drops, the terminal voltage values ​​of the remaining units are relatively high. These rapid adjustment capabilities improve the system's transient stability.