0 Introduction In the existing generating units of the Inner Mongolia West Power Grid, the self-excited excitation system at the generator terminal 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 self-excited excitation control system at the generator terminal 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-generator 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-generator 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 length, and improves shaft 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 channel 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 or 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 operation and maintenance workload and potential hazards, 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 downtime probability and lowering operation and maintenance requirements. 1.5 Transient and Steady-State Levels 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 constant. For a single-machine infinite bus system, the static stability limit power of the generator is: P'max = 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' constant 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 level 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 level is even better than that of conventional excitation: when a three-phase short circuit occurs, except for the self-excited units closer 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. 2. Recommendations 2.1 Overvoltage Issues in the Excitation System For high-power, high-voltage thyristor rectifier bridges, commutation overvoltages are unavoidable during the switching and commutation process of the thyristor elements. According to relevant literature, these commutation overvoltages can sometimes reach as high as 4500V, significantly impacting the thyristor elements and generator rotor coils. Especially for the rotor coils, this overvoltage affects not only the insulation to ground but also the inter-turn insulation, as it is a traveling wave for the coils. Simultaneously, due to parasitic capacitance between the excitation transformer windings, the connection or disconnection of the generator's excitation transformer power supply, and atmospheric overvoltages, overvoltages can all be generated in the transformer. Therefore, generators using self-parallel excitation systems must, on the one hand, pay special attention to the design and testing of rotor-to-ground insulation and inter-turn insulation strength, improving insulation levels and test voltages; the withstand voltage to ground should be at least 4500V or higher. On the other hand, appropriate measures must be taken to limit overvoltages. Simply using a combination of fuses, nonlinear resistors, and SCRs (thyristors) will not provide reliable protection for the generator windings. Therefore, for our plant's 2*200MW units, the following measures are recommended for the excitation circuit: a. Add an isolation shielding layer between the primary and secondary windings. b. Connect a ground capacitor to the secondary winding. c. Since there is a surge arrester on the primary side, it is recommended to install a reliable overvoltage absorption device on the low-voltage side. 2.2 System Stability Issues Haidian is located at the end of the Mengxi Power Grid, in Wuhai City, Inner Mongolia. In recent years, with the rapid economic development of Wuhai, according to relevant data calculations, due to the relatively lagging power source construction, coupled with the grid structure and the tree-like power supply structure of the regional power grid, the system where Haidian's four units are located is prone to low-frequency oscillations. For the sake of safe operation of the power grid, the demand for PSS (Power Supply System) functionality in the Mengxi Power Grid is becoming increasingly urgent. The excitation systems of the two 100MW units in Phase I of the Haidian Power Plant were replaced with GEC-1 microcomputer nonlinear optimal excitation controllers developed by Tsinghua University during major overhauls in March 1995 and May 1996, respectively. Since commissioning, they have adopted NEC control mode (Nonlinear Excitation Control). The two newly commissioned 2*200MW units in Phase II use GES-3223 generator terminal transformer self-parallel excitation systems manufactured by Dongfang Electric Machinery Factory. Since commissioning, they have adopted PID control mode (Proportional, Integral, Differential). The excitation regulators of all four units are equipped with PSS (Power System Stabilizer) additional functions at the factory, and the functions of PSS can be easily implemented through software modifications. At present, according to data analysis, for single-unit infinite power systems, the PID PSS excitation control method is more adaptable than the NEC PSS excitation control method and has a stronger ability to maintain the generator output voltage. Therefore, it is recommended that the PID PSS control scheme be adopted for all four units in Phase I and Phase II. In this way, when the system experiences low-frequency oscillations, the double lead-lag function in the PSS device can increase the positive damping of the system, solve the system oscillation problem, and improve the dynamic stability of the system. 2.3 Rectifier Cooling Fan Issues The three GZL-4A rectifier cabinet fans of the Phase I 2*100MW units are designed with dual power supply, both indirectly drawn from a single 380/220V busbar. Normally, one power supply operates while the other is on standby. If both power supplies fail, the AC side switch will automatically trip after 15 seconds, causing generator demagnetization. Several similar incidents have occurred since commissioning. To prevent similar occurrences, the following methods are recommended: a. Draw the power supply for the two fans from two independent busbars; b. After a dual power supply failure, the AC side power switch of the three rectifier cabinets should not trip with a delay. Instead, send a signal to the centralized control system to appropriately reduce active and reactive power output within a specified time and contact relevant personnel for timely handling. If the issue cannot be resolved within the specified time, it is recommended to manually disconnect the units from the grid.