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Tianwan nuclear power station lost the test implementation and optimization scheme of two offsite power sources
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The tests of Units 1 & 2 at Tianwan NPP, involved losing power from two external power supplies during the commissioning phase, it was a significant risk test. The tests on both units were completed successfully at one time.
The test scheme at Tianwan PhaseⅠis different from other NPPs’.
In tests on most NPPs in China, one external power supply would be lost first. During this time, the plant is in a low risk condition, because the reactors automatically shut down. The turbo-generators stop as well as the units are powered by back-up by external sources. Part of the normal operation load relies on the external power supply to keep running.
The whole plant loses its normal AC power supply once the back-up external power is removed, after the units are in stable state.
However, the test scheme at Tianwan PhaseⅠ, two external power supplies are lost at the same time, that means the NPP is in a bad working environment — the reactors shut down and the turbo-generators also stop as they lose the normal operation system. This simulates the real event of the NPP losing two external power supplies, it also checks the dynamic response capacity of the units under a bad working environment.
The details of the test scheme as follows:
The reactor power is 25 percent of rated power, the turbo-generator is connected to the grid and is supplying electricity to it. Before the test, 220KV spare power automatic switching will be locked, simulating over-voltage protection of the generator which causes turbine trip in the test, generators step out, the load used in the plant loses power; then cuts off the power supply to three of the four main pumps. The reactors shut down after a delay of 1.4 seconds. Six diesel generators (including four EDGs and two diesel generators) start and automatically take load.
The aim of the test is to confirm that six diesel generators start and load automatically and establish a steady core cycle, that main pumps coast down, and that turbo-generator sets coast down in losing cooling water and sharply deteriorating vacuum conditions, as well as in related regulator conditions.
- The special test team organized a group to discuss the tests, getting feedback from operation staff, and arranged several simulations.
- The test organization was refined and distributed, and work details were confirmed to individuals. Formulated Contact information was collated, and information was shared between all parties who joined this test.
- All parties performed the test of losing two external power supplies at the stage of commissioning under power on simulation, everyone got familiar with their own tasks during this test.
- Operational staff had two times of oral test of losing two external power supplies at the stage of commissioning under power, they made sure all operational personnel knew all the rules of the test’s operation and were familiar with its progress.
2.3 Preliminary tests and inspection
In order to minimize the risks during each step of the test process, the results of each part were confirmed. The team coordinated with different departments of the power plant to check all equipment which would be involved in this test: step loading of diesel engines, liquid level of the first loop and pressure control, water supply evaporator, pressure control of the second loop and sub-system of turbine generators and so on.
On April 24th 2004, the hot testing stage of losing two external power supplies at the stage of commissioning was completed. On April 1st 2006, the natural cycle test was completed and from May 1st to May 4th 2006 the step loading test of 6 diesel engines was completed. From March to June 2006 the following tests were performed: interlocking and protection of electrical equipments, 6kV switch driving test, UPS AC/DC system check, emergency/regular lighting system check, factory radio and communication systems check, lubricant oil pump start-up test, restarting of the IC system and automatic control cabinet. All the preliminary tests above have verified the reliability of equipment, and a few potential risks and dangers were detected so that we could take corrective and preventive actions in good time.
3. The key design problems found and solved during the test preparation
There are many differences between unit 1 of Tianwan NPP and its reference NPP because it was adopted by full digital control system which is to increase the automation levels. Many of the process control is new also but there are some imperfections between the technology and the preliminary design. Thus, the design was perfected and adjusted with the Sino-Russian cooperation. The problems and the solutions in the test of losing two external power supplies will be introduced as follow.
3.1 Prevention of overload after the auxiliary feed water pump starts
Question 1: when the unit is operating normally, the turbine pump for the LCS system provides power water to the water pipe and then returns to the deaerator, which is also working. The normal lines provide electricity to the valve of the LCS system. If unit losses two external power supplies and steam generator level drops 0.1m (BD21 logic) when the auxiliary feed water system is in hot standby, the auxiliary feed water system will start automatically. The valves of power water and driven water will not close automatically due to the non-reliable power supply, which causes water to return to the deaerator through driven water line and form a circulation loop. It will cause the pressure of the auxiliary pump outlet to below and eventually overload; it thus cannot provide water to steam generator.
Question 2: although Question 1 does not exist, it will happen that the amount of traffic for the auxiliary pump is too great and is cut when the key valve of the steam generator is not closed in time or adjustment valve is switched on due to the huge amount of water through the valve. Pump outlet valves LAH10,20AA102,105 is switched on and the flow capacity of LAH10,20AA102 is the biggest when the auxiliary feed water pump is in standby mode.
Solutions:
1)Formal change of LCS50 system: in order to avoid the back through driven water by LCS50AA601, we should change the connection of LCS50AA601 so that it is closer to the LCS pump outlet, which is in front of LCS50AA601 rather than between LCS50AA601 and LCS50AA102. Besides, the electric valves of power water and driven water are changed to reliable electricity supply and add a protection stop signal when losing two external power supplies.
2)Temporary change of SG auxiliary feed water system: according to the full-cut signals of these five main feed water pumps, the system will send 40 seconds pulse signals to switch off LAB10,20,30,40AA111,211 and LAH10,20AA102 and reduce the forced open signal of LAB10,20,30,40AA212 for BD21 to 20 seconds from 40 seconds. Because this change has been verified in the power-cut test and the auxiliary feed water pump works normally, this temporary change became an official design change after the test.