Perfection of thermal protection in Zhejiao C Powe

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Improvement of thermal protection in Shajiao C Power Plant

the three 660 MW units in Shajiao C power plant in Guangdong Province have a high degree of automation and many protections, which play a good role in ensuring the safe operation of the units, but a series of malfunctions have also occurred. For this reason, the factory has improved some of the protection against misoperation and misoperation

1 improvement of cooling water protection system

on March 24, 1999, when the chemical personnel sampled the cooling water of the generator stator and measured the conductivity, they opened the water drain valve at the bottom of the water level gauge of the stator cooling water tank of unit 3 according to the Convention, and discharged water continuously for 2 ~ 3 minutes. Due to the inexperience of the operators and unstable operation, the water level of the water tank fluctuated, the low water level alarm occurred, and the a cooling water pump tripped. Before the tripping signal of low and low water level in the water tank was reset, the interlock could not start the B cooling water pump, resulting in the interruption of the cooling water of the generator stator and the rapid rise of the generator winding temperature. When the generator winding temperature rises to 85 ℃, the unit still does not trip and is forced to switch off and shut down manually

Jinan new era assay instrument Co., Ltd. specializes in developing software for various experimental machines. Figure 1 is the logic diagram of stator cooling water protection. When the water level in the water tank is too low, trip the stator cooling water pump, and then jump the steam turbine generator set after 5 s

Figure 1 generator stator cooling water protection logic diagram

in actual operation, when the water level signal ll of the water tank is low, immediately trip two stator cooling water pumps. If the water level low-low signal is reset and disconnected within 5S at this time, both stator cooling water pumps have been tripped, but the command relay axr of tripping cooling water pump 030.02 and axr 030.03 power loss reset due to ll disconnection, time relay tmr 030.01 will stop timing, tmr 030.01 is not timed to 5 s, so the command to trip the unit will not be issued. As a result, there is a dangerous working condition that all stator cooling water pumps have been shut down and the unit continues to operate. It can be seen that the tripping command of the cooling water pump can not accurately reflect the tripped state of the pump, nor can it truly reflect whether the stator cooling water is interrupted. In order to completely eliminate the protection rejection and prevent maloperation to the greatest extent, three flow switches are installed in the stator cooling water system. Each switch sends two sets of stator cooling water flow low low signals to two different tripping channels. After two out of three logic operation in each channel, the command of tripping the unit is issued to protect the safety of the generator unit

2 modify the DCS software and interlocking cabinet hardware

on August 10, 1999, during the regular AC and DC sealing oil pump linkage test of unit 1, the sealing oil/hydrogen differential pressure fluctuated, and the CRT in the central control room showed that the sealing oil/hydrogen differential pressure was 0.051 MPa, while the exciter side sent a low-low signal of 0.02 MPa, resulting in protection maloperation and unit tripping. In order to prevent maloperation of seal oil/hydrogen differential pressure low-low protection during linkage test, the self shutdown conditions of main seal oil pump pp041 with low outlet pressure of 025psl and AC emergency seal oil pump pp042 with low outlet pressure of 026psl were cancelled in DCS software and interlocking cabinet hardware. When the outlet pressure of the sealing oil pump is low, the alarm will send a signal of linkage with another AC or DC emergency sealing oil pump, while maintaining its own operation, which will more effectively prevent the further reduction of the sealing oil pressure and improve the safety of the steam turbine generator unit

3 improve the boiler full water protection

add three high-high signals of drum water level. After taking two out of three as the originator of MFT, it is responsible for reviewing the overall deployment and important planning conditions to promote the development of new material industry, as shown in Figure 2, which makes up for the omissions in design and improves the boiler full water protection, so as to prevent steam turbine water impact accidents and ensure unit safety

Figure 2 boiler full water protection logic diagram

4 cancel the loss of primary air trip protection for all coal mills

in the past, all coal mills tripped due to excessive reliance on primary air/furnace differential pressure low single signal, resulting in multiple protection malfunctions. And because the original measurement reference point is shared by multiple instruments, perforation or air leakage of any instrument capsule will affect the pressure of the reference point, seriously interfere with each other, and cause extremely unstable signals. For this reason, the protection of losing the primary air trip of all coal mills (but still retaining its alarm function) is cancelled, the protection of tripping a single coal mill with the air volume of each coal mill less than 60% is increased, and multi-point measurement is adopted to disperse the danger and reduce the protection maloperation

5 improvement of B induced draft fan control

on July 18, 2000, the bearing temperature at the drive end of induced draft fan a of unit 3 was high, and the operator found that the lubricating oil of the fan had deteriorated through on-site inspection, so the induced draft fan a was stopped for oil change. When starting after oil change, induced draft fan a tripped due to high vibration. Start induced draft fan a again, and a few seconds later, induced draft fan a trips due to high vibration. Affected by it, the forced draft fan and induced draft fan on side B are switched to manual control, and the operator returns to automatic after checking that they are normal. Then, the guide vane of induced draft fan on side B was rapidly reduced, the furnace pressure was high, MFT acted, and the unit tripped. After inspection, the thermal control personnel found that the control logic of induced draft fan on side B was unreasonable. Combined with the accident analysis of high fan vibration and unsuccessful fan trip runback for many times, corresponding measures are taken:

(1) improve the grounding system and eliminate external electromagnetic interference such as lightning. High and low-pass filters and band-pass filters are added to filter the harmonic of fan speed and fan blade resonance, so as to effectively prevent the maloperation of high and high vibration protection during variable load air volume regulation. Add a set of vibration measurement system and modify the protection logic (see Figure 3). Only when the two systems detect the vibration high high signal lasting for 10 s at the same time, the tripping command will be issued

Figure 3 fan protection logic diagram

(2) cancel the tripping protection of high fan bearing temperature and increase the alarm of high fan bearing temperature. The neutral point grounding temperature measurement of e-graduation thermocouple is changed to PT100 thermal resistance insulation temperature measurement, which eliminates the threat of MV signal vulnerable to interference and protection maloperation to the safe operation of the fan

(3) corrected the logic error of the software that the fan stop and trip signals are not separated, realized the fan linkage, and improved the fan runback function

(4) change the feedback potentiometer of the fan blade actuator into an inductive feedback device to prevent the MFT signal caused by the imbalance between the feedback signal and the control signal caused by the potentiometer fault, which leads to the regulation system closing the moving blade by mistake and the furnace pressure protection maloperation

6 improvement of feedwater heating protection

on November 11, 1999, the operating personnel withdrew the 3 and 4 low-pressure heaters of unit 1 from operation according to the requirements of the maintenance work ticket. 14: The water level of 22, 3 and 4 low-pressure heaters is high, and the signal of opening the bypass valve of 3 and 4 low-pressure heaters is sent. Because the signal was wrongly connected to the terminal of closing the bypass electric valve of 3 and 4 low-pressure heaters during design and installation, the bypass electric valve of 3 and 4 low-pressure heaters was forcibly closed, and the water supply of deaerator was interrupted. 14: 32. When the drum water level is low, MFT protection acts and the unit trips. The cause was found out and corrected in time. In order to further improve the feedwater heating protection and reduce the misoperation of the water level protection of the high-pressure and low-pressure heaters, the logic of fully opening the emergency drain valve with the high-pressure linkage of the liquid level of the high-pressure and low-pressure heaters is added, a high water level switch is installed, and the easily jammed pontoon type liquid level gauge is replaced with 1151 differential pressure water level gauge, eliminating the influence of high-temperature air flow

7 improvement of feed water pump protection

on March 3, 2000, during the startup of unit 1, the bearing of feed water pump B leaked oil and was forced to shut down and out of standby. At 00:30, when the load was increased to 230 MW, the high temperature of the balance water of feed pump C tripped, and the linkage feed pump a started. A feedwater pump tripped due to the false low signal of inlet/outlet differential pressure due to the falling off of the sampling pipe of feedwater flow signal after dozens of seconds of operation. Three feed water pumps were all stopped, the boiler water was cut off, the drum water level fell rapidly, and the drum water level was low and low after manual tripping and shutdown. The tripping accidents caused by previous feed pump failures are comprehensively analyzed, and the transformation is carried out pertinently

(1) the channel resistance of the balance water temperature measurement card was replaced, and the measurement range was changed from 0 ~ 200 ℃ to 0 ~ 300 ℃, so that the trip alarm value of 220 ℃ fell to 2/3 of the range, avoiding protection rejection and improving the stability and accuracy of the measurement signal; Add an entek-ird temperature monitoring card and two balance water temperature measurement points te010b and te010c. The three balance water temperature measurement signals are sent to three measurement cards respectively. The card output contact sends out the instruction of tripping the feed water pump after two out of three logic operation (see Figure 4). In this way, the danger is dispersed and the misoperation is effectively reduced

Figure 4 feed pump protection logic diagram

(2) in order to completely eliminate the false signal when measuring the speed of the feed pump axially and avoid the fault that the feed pump is easy to trip when the feed water regulation is unstable, the speed measurement method is changed to radial, which eliminates the influence of shaft coupling when the load changes

(3) the price is subject to change. It is considered to increase the full stop signal of three feed pumps as the trigger condition of MFT, and improve the boiler water cut-off protection together with the existing low drum water level, poor boiler water circulation and reheat protection

8 improvement of control system

on October 15, 1998, UPS power supply failed. In the process of instantaneous switching of UPS power supply, all circulating water pumps of each unit tripped, resulting in unit tripping and external accident power failure of the whole plant. After many simulation experiments and analysis, the control system loses power instantly, and the configuration data of its analog input card is also lost. In the process of power on recovery, the host starts to work normally after the power on self-test, scans the data in the i/o image table in a specific time sequence, and executes the program. Because the initialization configuration of the analog input card after power on is completed by the btw command in the host execution program, the data collected before the analog input card is configured well and works normally are incorrect. The bad value of circulating water pump motor winding temperature higher than 130 ℃ is wrongly collected, and the circulating water pump is tripped by mistake after the program is executed. After fully studying the hardware and software of the programmable controller and analyzing the ladder logic diagram, using the feedback information of the self-test of the programmable controller, take the i/o frame fault signal as the locking condition of the high temperature tripping signal of the circulating water pump motor winding, and lock the high temperature signal of the circulating water pump motor winding within 5 s after the frame fault or i/o card is powered on (see Figure 5 for the development history of Jinan assaying), so as to prevent the protection malfunction caused by the high temperature signal of the circulating water pump motor winding. Since then, the pump tripping accident caused by instantaneous power loss or voltage instability has been eliminated

Figure 5 circulating water pump protection logic diagram

after several years of improvement, the unreasonable design of thermal protection system logic has been corrected, the installation position of field instruments and equipment has been transformed, the canopy and air filter have been installed, and the sampling pipes and instrument air pipes of important instruments have been replaced with stainless steel pipes. The reliability of primary elements and protection logic has been greatly improved, and the number of unit accident trips has been significantly reduced. (Su Lechun)

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