Inner Mongolia Autonomous Region has abundant coal resources. With the reform and opening up in recent years, the Inner Mongolia power system has seized the favorable opportunity of economic development, vigorously developed the power industry, made full use of its resource advantages, transformed coal transportation into power transmission, and input powerful electrical energy into the North China main grid. At present, due to the change in operation mode, the grid connection between the Inner Mongolia West Power Grid and the North China Power Grid is only connected by a single 500 kV line from Fengzhen Power Plant to Wanquan Substation. If the Fengwan line trips, the Inner Mongolia West Power Grid will have a large amount of active power surplus, and the Inner Mongolia West Power Grid and Fengzhen Power Plant will suffer high-frequency damage after disconnection. Although relevant professionals have conducted necessary analysis and calculations, these are theoretical calculations and there is no real practical operating experience. In order to adapt to the change in this mode and ensure the safe and stable operation of the power grid and our plant, it is of great significance to analyze this mode and take corresponding measures. [b]1 Stability of the Inner Mongolia West Power Grid when the Fengwan line is running alone[/b] The total installed capacity of the Inner Mongolia West Power Grid is 4,279 MW, and the maximum regional load is 1,900 MW. 938 MW is transmitted to the North China Power Grid. Because the North China Power Grid adopted a separate power transmission method for Inner Mongolia and Shanxi in 1998, three interconnecting lines—the Dafeng 220 kV double-circuit line and the Fengtong 500 kV line—weakened the connection between the power grids. If the Fengwan line tripped, the Inner Mongolia power grid would be disconnected from the North China power grid. After disconnection, the Inner Mongolia power grid would have excess active power, and the system frequency would increase (the magnitude of the increase is related to factors such as the amount of excess active power, the system's equivalent inertia time constant, the speed regulation characteristics of the generator speed controller, and the frequency factor of the load). According to relevant analysis and calculation reports, if the transmitted power is 900 MW, after a permanent fault occurs on the Fengwan line, the highest frequency of the Inner Mongolia power grid could reach 51.5 Hz, and the time exceeding 51.5 Hz would be 1.6 s. After adjustment of the load frequency regulation characteristics and the action of the generator speed regulation system, the lowest system frequency would be 50.5 Hz, and the restored frequency would be close to 50.6 Hz. Therefore, the Inner Mongolia power grid installed a high-frequency generator tripping device with a total tripping capacity of 750 MW. The high-frequency generator tripping scheme is shown in Table 1. [img=358,227]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hbdljs-4/13-1.jpg[/img] Analysis shows that the high-frequency generator tripping device can significantly suppress the frequency rise in the Inner Mongolia West power grid after the Fengwan line trips. Because the active power balance in the Inner Mongolia West power grid is very large after the Fengwan line trips (accounting for about 50% of the system load), the frequency rise during the transient process is also relatively large. Even if the high-frequency generator tripping device operates reliably, the harm of high frequency to the Inner Mongolia West power grid still exists. 2. Stability of Fengzhen Power Plant under Single-Line Operation of Fengwan Line [img=321,180]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hbdljs-4/13-2.jpg[/img] 2.1 Operation Mode of Fengzhen Power Plant Fengzhen Power Plant is located at the eastern end of the western Inner Mongolia power grid, with an installed capacity of 6×200 MW. Four units are connected to the 220 kV system of the plant's substation, and two units are connected to the 500 kV substation. The 220 kV system has five outgoing lines, namely Hufeng I, Hufeng II, Fengji, Dafeng I, and Dafeng II (currently out of service); the 500 kV system has three outgoing lines, namely Fengda, Fengwan, and Fengtong (currently out of service). The 220 kV system and the 500 kV system are connected by an in-station interconnection transformer. Under normal operation, Fengzhen Power Plant operates in a "six-to-five" mode, with a maximum output of 900 MW and a minimum output of 600 MW. Power from the Inner Mongolia West Power Grid to the east is transmitted via the Fengwan Line. If the Fengwan Line trips due to a fault, the remaining power will be trapped on the Fengzhen Power Plant busbar. According to the propagation characteristics of frequency, Fengzhen Power Plant will face higher frequency hazards. 2.2 Impact of Fengwan Line Tripping on the Safety and Stability of Steam Turbine Units When the Fengwan Line trips during peak load, the Inner Mongolia West Power Grid has 938 MW of remaining active power. However, even when the high-frequency tripping device operates effectively and reliably, the steam turbine generator unit still has the possibility of overspeeding. According to past experience, when the grid frequency is 50 Hz and the unit is carrying a 200 MW load, due to the slow action of the regulating system, the rotational inertia of the steam turbine generator unit rotor causes the speed to rise sharply, reaching a maximum of 3180 r/min. When the Fengwan line trips, the remaining power will cause the system frequency to rise. When it rises to 51.5 Hz, i.e., the speed reaches 3090 r/min, it enters the high-frequency tripping zone. The high-frequency tripping device activates and trips the generator output switch. Based on this speed, after the turbine generator sheds its load, it continues to rise by 180 r/min or more, and the turbine generator rotor speed will rise to above 3270 r/min. This speed has entered the turbine emergency protection device activation speed range [(8%~10%)nr rated]. At this time, if there is a slight problem with the turbine inlet valve shut-off, it is very likely to cause the turbine to overspeed and damage the entire unit. From this perspective, whether the frequency-based tripping method can use the Fengwan line tripping signal as a criterion to activate the tripping device is safer and more reliable for the turbine is worth exploring. 2.3 Impact of Fengwan Line Tripping on Boiler Safety Operation When the Fengwan line trips, turbine generator units that have not engaged high-frequency switching will experience an increase in speed. This will cause the speed control valves to automatically close, potentially reducing the unit's active power below the minimum stable combustion load. This could lead to boiler safety valve activation or loss of stable combustion at low load, resulting in boiler extinguishing. [b]3 Safety and Stability Measures to be Taken by Fengzhen Power Plant During Fengwan Line Fault Tripping[/b] According to the grid arrangement, the high-frequency switching device of Fengzhen Power Plant is connected to any one of units 1, 2, or 3 that operates at 51.0 Hz, and to any one of units 4, 5, or 6 that operates at 51.5 Hz. 3.1 When the unit fails to operate, if the generator frequency rises to 51.5 Hz, the Fengwan line trip signal is issued, and the generator that was switched on has not tripped despite the operation of the high-frequency tripping device, immediately manually trip the turbine emergency safety valve. The generator will not disconnect; instead, the turbine will run without steam for a short period. Once the grid frequency returns to normal, the turbine will be reconnected. During this period, the turbine exhaust cylinder temperature should be closely monitored. If necessary, other water spray cooling measures should be taken. Simultaneously, the active power output of other units should be reduced to the minimum load specified for normal operation as soon as possible. 3.2 When the device operates reliably, the power supply to the out-of-operation units is affected by the high-frequency generator trip unit's output protection 3BCJ. For units with unit-based wiring in our plant, the result is: the generator unit is disconnected but not demagnetized; the turbine inlet valve is not closed, and steam continues to flow for power. The power generated by the generator is connected to the plant's power supply (normal power supply is 10-13 MW) through the high-voltage plant service transformer at the generator outlet. In this way, the generator is disconnected from the high-frequency grid and its speed is reduced by the turbine's primary frequency regulation capability, becoming an independent single-unit system. Changes in turbine speed and generator outlet voltage affect the operation of auxiliary equipment, and changes in auxiliary equipment operation further affect generator output, entering a vicious cycle. At this time, human intervention is required to ensure stable unit operation. Electrical personnel adjust the generator outlet voltage to the rated 15.57 kV, and turbine personnel adjust the synchronizer to maintain the turbine speed at 3000 r/min. After the grid frequency is restored, the generator unit is synchronized with the system. It is particularly important to strictly prevent generator overpressure and turbine overspeed. If any of these signs are detected, immediately perform manual demagnetization and trip the circuit breaker. After the unit loses its auxiliary power, it should immediately switch to backup power to ensure safe shutdown. This requires strengthening various static tests on the unit's speed control system after major and minor overhauls, especially overspeed tests, to ensure that the overspeed protection reliably operates when the unit speed enters the specified range. This is fundamental to ensuring the safe operation of the turbine unit. The generator should normally operate in automatic excitation mode, and the regulation performance of the automatic excitation regulation system should be reliably guaranteed. On the one hand, it is necessary to strengthen the management of device adjustment tests to ensure its sensitivity is within the acceptable range; on the other hand, advanced regulation devices should be selected to ensure their regulation quality is within the allowable range. Regularly conducting generator load shedding tests is an important means of verifying whether the unit can operate safely and stably after the high-frequency turbine switching device is activated. Currently, only Fengzhen Power Plant in the Mengxi Power Grid has a 200 MW unit, and there is no established experience in load shedding tests. Once conditions are ripe, we will actively carry out this work, and improve and revise various technical measures in a targeted manner to ensure that we can fully adapt to the requirements of this approach in all aspects.