Abstract: By comparing the command redundancy schemes of the MEH-ⅢA system, including switching using solid-state relays on the AO card and LC5 card terminal board, and switching using two individual relay contacts on the DEH system (3-8) DO card terminal board, the long-term safe operation of the MEH system is ensured. Keywords: MEH-ⅢA, LC, command redundancy CLC Number: TP273 Document Code: B Overview The MEH-щA boiler feedwater pump turbine digital electro-hydraulic control system, produced by Xinhua Power Plant Control Engineering Co., Ltd., can accept commands from the coordinated control system, perform closed-loop speed control of the small turbine, has convenient and flexible configuration functions, and can be networked with other control systems in the power plant for information sharing. The steam driving the turbine comes from two steam sources: one is the boiler high-pressure steam source, and the other is the main turbine extraction steam source. Each source is equipped with one main steam valve and one regulating steam valve. When the main turbine is below 25% load, high-pressure steam is used for steam supply, and the steam flow is controlled by the high-pressure regulating valve (HPGV). When the main turbine load reaches 25%-40%, steam is supplied by both the high-pressure steam source and the extraction steam source, mainly controlled by the high-pressure regulating valve, while the low-pressure regulating valve is basically fully open. Above 40% load, extraction steam is used for steam supply, and the turbine speed is controlled by the low-pressure regulating valve. The MEH-щA control system has three operating modes: boiler automatic, speed automatic, and manual. Switching between these modes is achieved through the MEH-шA control panel and software. The main functions of the MEH-шA control system include automatic control of boiler feedwater flow, automatic control of feedwater pump turbine speed, regulating valve position control, normal operation and monitoring, overspeed protection and testing, networking with DEH-шA, operating MEH-шA from the DEH-шA operator station, and performing fault tracing and printing. 1. MEH-ⅢA System Software Configuration The MEH-ⅢA software mainly includes an engineer station software package and an operator station software package. The operator station software runs on the MS-WINDOWS multitasking operating system and includes software modules for communication drivers, display, printing, operation processing, and recording. The engineer station software also runs on the MS-WINDOWS system and includes offline graphics generation software, report generation tools, and configuration software. 1.1 Small Unit Startup 1) Press "Open Speed-Close Valve" on the MEH panel below. 2) After the speed-close valve is fully open, the "Open Speed-Close Valve" light will illuminate, and the "Trigger" light will go out. Press the "Soft Manual Operation" key; the light will illuminate. Slowly operate the "Valve Position Increase" key to open the low-pressure regulating valve "LP". 3) Press "Increase Rate," input 200 r/min, and increase the speed to 600 r/min to warm up the unit. 4) After warming up, when the unit speed is above 600 r/min, press "Automatic Speed," and the light will illuminate. 5) Press the "Target Value" key to set the target speed to 2800 r/min. 6) Press "Increase Rate," and set the increase rate to 300 r/min. 7) When the speed reaches 2800 r/min, open the steam-driven feedwater pump outlet valve. After receiving the "Boiler Start-up Allowed" signal from CCS, press "Boiler Automatic," and the indicator light will illuminate. 1.2 Turbine Shutdown: Reduce the turbine speed, transfer the load to the electric pump, press the "Trigger" button on the MEH screen, all turbine doors will close, and the speed will drop to zero. Due to frequent LC card failures in Xinhua Company's MEH system, the turbine speed could not be adjusted. At our factory's request, Xinhua Company researched and implemented an LC card redundancy configuration scheme. AO cards are used. The AO card has 8 outputs. When each individual output fails, the output remains unchanged, as shown in Figure 2. The AO card provided by the manufacturer was installed in slot 10. The low-profile door command was simultaneously sent to the AO card and the dedicated LC card (hereinafter referred to as LC9) in slot 9, which originally controlled the low-profile door. The electro-hydraulic converter was switched via a solid-state relay on the LC card terminal board in slot 5. However, the switching test failed to achieve seamless switching, exhibiting significant signal fluctuations. The manufacturer reported to headquarters, concluding that the AO card and LC card could not switch between each other, rendering the solution unfeasible and rejecting it. 2. Switching using a solid-state relay on the LC5 card terminal board Xinhua Company considered a new solution, preparing to test another ordinary LC card (located in slot 5, referred to as LC5) that connects to the CCS. The channel on the LC5 card used for small machine speed display in the CCS was used as the output channel. The low-profile door command was simultaneously sent to LC9 and LC5. The electro-hydraulic converter was switched via a solid-state relay on the LC5 card terminal board. The switching test was successful. By removing the LC5 card, it was discovered that the signal controlling the electro-hydraulic converter via the LC9 card in normal mode was lost. Verification revealed that the solid-state relay on the LC5 card terminal board had a normally open contact. Inserting the LC5 card caused this normally open contact to close, hindering the normal operation of the LC9 card and facilitating LC5 card replacement, posing a significant equipment hazard. The manufacturer rejected this solution. 3. Switching via two individual relay contacts on the DEH system (3-8) DO card terminal board The third solution's design concept is to switch the control commands of the LC5 and LC9 electro-hydraulic converter via two individual relay contacts on the DEH system (3-8) DO card terminal board. In normal mode, the LC9 controls the low-profile door with a normally closed contact, while in abnormal mode, the LC5 controls the low-profile door with a normally open contact, as shown in Figure 4. [align=center] Figure 4 Relay switching on the DEH system (3-8) DO card[/align] Through testing, it was verified that during "redundancy removal" and "redundancy activation" operations, there was a 3.3% probability that the low-profile door would completely close. Under normal operation of the LC9 card controlling the electro-hydraulic converter, inserting or removing the DEH(3-8)DO card will not affect the LC9 card's control of the low-profile door opening. To ensure rapid handling in case of failure, a related alarm function has been added. If the LC9 card controlling the low-profile door fails under normal operation, the "M" yellow indicator will flash on the CRT screen. After 3 seconds, the redundant LC5 card will be automatically switched to the active position. The low-profile door will have a 2mm travel fluctuation. The "Remove Redundancy" green button will automatically switch to the "Activate Redundancy" red button. Operators can also select the "Remove Redundancy" and "Activate Redundancy" buttons in the soft manual operation mode and control the low-profile door by adding or removing commands. When the LC5 card fails, the "E" yellow indicator will flash. Do not click the "Activate Redundancy" button. When the DEH(3-8)DO card fails, the "H" yellow indicator will flash. At the same time, the relay will lose power and the contacts will be in the "Remove Redundancy" mode. The "Remove Redundancy" and "Activate Redundancy" operations cannot be performed in the hard manual operation mode. The faulty LC9 card can only be replaced under MEH manual operation. Replacing the faulty LC9 card in MEH boiler automatic mode will risk causing the CCS to require a speed reduction of approximately 200 rpm. Since the channel on the LC5 card used for sending signals to the CCS for turbine speed display is occupied, a backup channel in the DEH system should be identified for output to the CCS for turbine speed display. The AO card provided by the manufacturer for redundancy configuration has switching issues and cannot be used; they have already taken it. To avoid any connection with the DEH system, we suggest that the manufacturer independently complete the redundancy configuration of the MEH systems for Units #3 and #4, and then use an LC card to replace the signal output from the DEH system to the CCS for turbine speed display. This can also serve as a temporary backup in case the LC9 card fails. We have already sent a fax requesting the manufacturer to resolve the 3.3% probability of valve closure. 4. After the MEH upgrade and improvement, a DPU fault test was conducted, and the results are shown in Table 1. The MEH-ⅢA self-test status is shown in Table 2. Conclusion : After the redundant configuration of the LC card of the MEH system of Unit #4 was implemented, the test results were basically satisfactory. However, during the switching of redundant cards, there was a 3.3% probability that the low-profile door would close. The main reasons are: 1. The two DO output operation cycles in the logic are not synchronized, causing one relay to act earlier and the other relay to act later, resulting in asynchronous switching of the two relays. 2. The two relays themselves have different action times.