Why is a load rejection test conducted?

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The load rejection test is an experiment in which, while the steam turbine generator is connected to the power grid and under load, the main circuit breaker of the generator is suddenly opened to disconnect the generator from the power system. The speed of the unit and the behavior of the various key components of the speed control system during this transition are observed, in order to assess the dynamic stability of the speed control system. The load rejection test should be conducted under the conditions that the speed control system is operating properly, the boiler and electrical equipment are functioning well, and all types of safety valves operate reliably. The load shedding test is generally conducted at three levels: 1/2 of the load, 3/4 of the load, and full load. Only after passing the tests at 1/2 and 3/4 of the rated load can the full-load test be conducted. Purpose of the load rejection test 1: To evaluate the dynamic characteristics of the turbine control system. 2 After load rejection, the maximum speed rise of the turbine must not cause the mechanical emergency shutdown device to trip. 3 After load shedding, the unit’s control system (DEH) enables the unit’s speed to stabilize rapidly at idle speed. Conditions required for the load-shedding test: 1. The main equipment should be operating properly, the operating mechanisms should be flexible, and the main monitoring instruments should be accurate. 2 The oil quality in the oil system is satisfactory, the accumulators are in good condition, and the static tests, static relationships, set logic, and parameters of the DEH control system all meet the requirements. 3. The security system operates reliably; the oiling tests and overspeed tests of the emergency safety devices passed, the remote and manual shutdown devices function properly and reliably, and the tuning performance of the electrical overspeed and OPC protection systems is satisfactory. The tightness tests of the 4 main steam valves and control steam valves passed; the valves showed no sticking, and their closing time met the required specifications. The interlock operation of the 5 extraction check valves and the high-pressure discharge check valve is normal, with rapid and tight closure. The interlock tests of the standby oil pump for the turbine seal oil, the AC lubricating oil pump, and the DC lubricating oil pump were successful; the oil quality was satisfactory, and the vibration levels of the bearings in the turbine and generator were within acceptable limits. 7 The high-pressure heater protection test passed. The standby steam sources for the 8 deaerators and steam seals are ready for operation. The safety valves of the 9 superheaters and reheaters have passed the commissioning and calibration tests. The thermal protection and electrical protection wiring of the 10 boilers is correct, and they are operating normally. All functions of the electronic control unit are operating properly. 11 The plant power supply and emergency power supply are reliable. 12 The main switch and demagnetizing switch of the generator operate normally. The system frequency is maintained within 50±0.2 Hz, with spare capacity available in the system.