Polaris nuclear Network News: Recently, the National Nuclear Safety Administration issued a letter on the issuance of “Fujian Ningde Nuclear Power Plant Unit No. 1” due to power failure led to emergency bus LHB manual shutdown retreat “operational events independent report,” the State Nuclear Safety Letter [2019] No. 56, as follows:
All relevant units:
July 8, 2019, Fujian Ningde Nuclear Power Co., Ltd. to the Bureau notices with Level 1 run event Ningde Nuclear Power Plant Unit 1 occurred: “due to loss of power led to the emergency bus LHB manual shutdown retreat”.Bureau attaches great importance to immediately verify the situation and carry out research, that despite this incident that no actual nuclear safety consequences, but there may be common cause equipment failure, and may have similar problems to other plants.Bureau on July 9-12 experts went to the scene, Fujian Ningde nuclear power plants to carry out an independent investigation of the incident and evaluation.
According to the investigation, the cause of this event for the Essential Service Water System (SEC) superimposed Insulator motor and power switch fuse two equipment failures pump failure; and disposal of power plant incident response is appropriate; Since this event with Unit 1 another night there is an abnormality occurs with common cause failure, power plant according to the international nuclear event Scale and radiation principle, defined as class 1 event level is reasonable.In addition, the incident involving the same batch of material failure of electrical equipment parts, and disposal of events is more complex, in terms of the manufacturing operation and maintenance, operation of the unit operations and other related equipment to be caused by the use of the same batch of great importance to nuclear power plant equipment.
Incident investigation report will now be printed to give your unit, you need to check whether to adopt the same batch or similar device, and do the following work experience feedback.
First, the Fujian Ningde Nuclear Power Company should analyze the reasons SEC pump power supply switch fuse C fuse with failure to normal as soon as possible, and to undertake the necessary corrective action.
Second, the SEC should further develop and pump motor manufacturers to communicate, research to improve the motor structure to ensure accessibility Insulator routine testing and maintenance, and to take effective measures to prevent the insulation performance degradation.
Third, the investigation should be used when the motor Insulator material procurement, manufacturing materials of unsaturated polyester fiber reinforced plastic 4332 (DMC) and Insulator for use in a wet environment, the transformation should be implemented as quickly as possible.
Fourth, should further carding and optimizing preventive maintenance procedures nuclear safety related equipment, by enhancing equipment performance frequently downgraded situation, concerns underlying causes performance degradation of equipment, improve equipment reliability.
Fifth, the unit should be designed in conjunction with further analysis of this anomaly in a nuclear power plant voltage distribution system voltage transformer (PT) resonance of the relevant circumstances, PT resonance study of the mechanism and response measures, the relevant design optimization to improve nuclear power plant, if necessary.
Accessories: main delivery unit list
National Nuclear Security Administration
July 31, 2019
(This piece the public)
Fujian Ningde Nuclear Power Plant Unit No. 1 “due to power failure led to emergency bus LHB manual shutdown retreat” operational events independent report
July 5, 2019, Fujian Ningde Nuclear Power Co., Ltd. (hereinafter referred to Ningde nuclear power) with 0 to National Nuclear Security Administration runs the event oral notice: Fujian Ningde Nuclear Power Plant Unit No. 1 “due to an emergency bus LHB lead to loss of power after a manual shutdown withdraw”.Since then, other anomalies with the event and Unit 1 of the evening appears: “The emergency bus LHA insulated low alarm” there is common cause failure, Ningde nuclear power in formal notice July 8 submission, according to “International Nuclear and Radiation Event Scale Handbook “, the event raised a level, defined as level 1 running events.
Although this incident did not cause actual nuclear safety consequences, but considering the cause of the incident may involve common problems and more power plants, the National Nuclear Security Administration immediately set up a working group of experts organized on July 9-12 visit to the Fujian Ningde Nuclear Power Plant event carried out an independent investigation.Root Cause Analysis report this incident will write separately to investigate the basic situation is as follows.
First, the event procedure
The events can be divided into two events and exceptions.First appearing before dawn July 5 Unit 1 Essential Service Water System (SEC) No. 4 pump motor (1SEC004MO) into the junction box shorted to ground overlay motor switch fuse fails to fuse, resulting in emergency AC bus (1LHB) loss trip electrical events.Second Unit No. 1 5 July SEC system appears evening pump motor 3 (1SEC003MO) into the junction box ground fault (Common Cause Failure), and the pressure distribution system resonance occurs after cutting in turn leads to the pump voltage transformer abnormal damage.
The first incident occurred before, Unit 1 is in a power running state.
Short circuits after 5 seconds, the junction box A / C, B / C phase; 41,1SEC004MO junction box C phase ground fault occurs, resulting in Unbalance: Event origination point of time is 04 am: 05.
After 14 ms, 1SEC004MO in switch A and B phases fuses have blown, but the fuse is not normally fuse phase C, followed by bus-phase shortcircuit end of the fuse, the short-circuit current of 46kA peak.
After 269 ms, 1LHB normal bus in switch overcurrent tripping due.1LHQ emergency diesel generators start, but the bus short-circuit fault, the diesel engine can not access, 1LHB loss of power bus.Master operator perform procedures according to SOP accident alarm card.
04: 06: 08, SEC system automatically switches to run 1SEC003PO.
Thereafter, the unit according to SOP procedures, to NS / RRA (RRA intermediate duplex operating mode shutdown operation) retreat mode, and a manual trip to 05:09, scene to carry out maintenance work 1LHB.
18: 39,1LHB end overhaul to restore power transmission.
The second exception occurs before starting, Unit 1 in NS / SG (normal shutdown cooling evaporator) mode.
Abnormal point night departure time 22: 19: ground fault occurs 35,1SEC003MO junction box C phase, single-phase ground master trigger alarms and the insulating low-.
22:43, the master boot 1SEC001PO, outage 1SEC003PO, single-phase ground fault alarm disappears, but 1LGA / 1LGB / 1LHA bus voltage decreased to about 6.0kV, and began to slight fluctuations.
23:00, into the line or bus master trigger PT failure / fault alarm switchboard, but 1LGA / 1LGB / 1LHA bus voltage is restored to 6.4kV.The inspection revealed 1LGA power strip smells burned, two master start the fire and the fire brigade intervention teams.Subsequent isolation switchboard maintenance, damage is detected voltage transformer.
6 July 23: 18,1LGA bus complete overhaul power transmission.
At 00:36 on July 7, the main control unit confirmed to meet the state criteria SOP quit, quit SOP accident procedures.
July 8th 03: No. 50, 1 unit of critical re.
Second, the scene situation
After the incident, the two failures of the motor Ningde nuclear replacement or repair of the embodiment; fault bus voltage transformers and switches for the overall replacement.
After the initial analysis that low insulation capability of fault motor Insulator, Ningde nuclear power experience feedback in a timely manner, for Power Plant Units 3, 4 each pumping a pump motor according to the SEC inspection, testing Insulator performance.
Ningde nuclear power plant Insulator material of investigation where the whole pump motor SEC, SEC for all pump motors Insulator embodiment uses old material to replace, and before replacing antifouling composite coating with an insulating (PRTV) for spraying.
For events SEC pump motor failure to extend the emergency bus, the Ningde nuclear power plant will be zero-sequence SEC pump motor is protected by an alarm to tripping, improve bus safety margin and switch.
Third, the main focus of investigation
Compliance (a) the environmental conditions and equipment design requirements
This event matching motor pump SEC sourced from Jiamusi Electric Co., Ltd. (hereinafter referred to Jiamusi Electric).The apparatus maintenance manual operation of the motor, environmental conditions during the operation of the motor: pump room temperature: -5 ℃ ~ 45 ℃; external pressure: 970.6mbar ~ 1037.1 mbar; maximum relative humidity: 100%; mass air: Air salt; Waterproof: Spray; Dust: large.
SEC pump house plant non-sealed inner space of the apparatus operating environmental conditions, primarily the need to control the temperature.Ningde Nuclear Power Plant Unit 1 SEC DWS pump room is provided with a ventilation system, may be greater than the temperature of the air pump room at 35 ℃ start ventilate.Unit operation in accordance with DWS system startup and the main signal determination pump room temperature alarm.When this event occurs, DWS system is not an alarm is triggered, the abnormal state of the environment does not appear.Investigation team said: pumping station site environment meets the SEC pump motor running environmental design requirements.
(Ii) the SEC pump motor failure origination member (Insulator) relevant circumstances
According to the event-related alarms, and data analysis equipment overhaul case, the SEC Insulator pump motor fault originating member, the two motors Insulator there are different degrees of burn discharge case (see Figure 1), the scene analysis the reason is that insulator insulation creepage lead to decline.
Insulator Jiamusi also provided by the above-described motor, unsaturated polyester fiber reinforced plastic 4332 (DMC) is cast.The “glass fiber reinforced unsaturated polyester molding compound industry standard” (JB / T 7770-1995), the material does not meet the above performance “Insulator Jiamusi technical conditions in the motor 1 October 2009 release (OEE. 058.001-2009) “enterprise standard requirements -” 4.Absorbent material 4 Insulator (23 ± 1 ℃) ≤20mg, 24h immersion insulation resistance ≥1.0 × 107MΩ “.
Now the investigation team maintenance records of the motor, and then the identification found after the preventive maintenance, the insulation of the motor, the absorption ratio, polarization index does not satisfy the requirements of multiple occurrences acceptance criteria.When this occurs, power plants generally use by blowing compressed air motor stator bore, and again measuring the electrical insulation, absorption and polarization index until it is determined to meet the requirements.Investigation team believes the preventive maintenance records showed that in wet conditions, the pump motor Insulator insulation properties of SEC declined.
In addition, according to a letter provided by the survey plants, Yangjiang Nuclear Power Co., Ltd. Jiamusi Electric had sent a letter asking the manufacturers of condensation problems SEC Insulator pump motor, the Jiamusi Electric said in a reply letter, “in 2012, good power Insulator material for improved optimization of the research in order to further improve the withstand voltage performance Insulator, together with suppliers to improve Insulator.Now is the use of nuclear power motors Insulator better electrical insulation properties of resin-based active complex DX [1969] Material cast from.”
Unit 3 and 4 of Fujian Ningde Nuclear Power Plant SEC pump motor is also sourced from Jiamusi Electric, but because of the late procurement, Insulator has been using new materials.Now according to the investigation of preventive maintenance records, insulation performance qualification tests where the pump motor SEC Units 3,4 1,2 superior maintenance unit.
In summary, the investigation team said: older Insulator material Jiamusi Motor Manufacturing SEC pump motor used in the design failed to meet the relevant requirements, prone to insulation degradation under SEC pumping station environment, resulting in two SEC pump motors have common cause failure.Deep-seated reasons Ningde nuclear power in the previous SEC preventive maintenance work the pump motor, the failure to identify further insulation frequent degradation, failure is a contributing factor to the above.
(C) 1SEC004PO pump motor with the power switch is not normally blown fuses C
Unit 1 Ningde nuclear power plant design, even if the pump motor SEC phase fault occurs, the overcurrent protection fuse 1LHB802 motor switch integrated security and overload protection device and a short-circuit fault can be isolated.
In this event, the A-phase and B-phase fuses normal fuse, but fuses abnormal phase C (C phase along the fuse arc marks on the surface, the insulating layer destruction, form a conductive path, see FIG. 2).Phase C is not normally fuse is blown, and arc discharge occurred, is the main reason for the failure to extend the emergency bus.
Investigation Group that: the electrical protection system, the role of which is to prevent the spread of the fuse in the event of failure, and is generally considered to be highly reliable protective device, it should be possible to confirm the design features.This event, the failure mode fuses are rare, there is need for further analysis to confirm the cause, and implement the appropriate investigation and corrective action.
(D) a motor provided selection and zero sequence overcurrent protection alarm
Zero sequence protection SEC pump motor Fujian Ningde nuclear power plant there are two settings, one for the police action, and second, the action in the trip.The actual operation of power plants, SEC pump motor integrated protection devices typically zero sequence protection investment alarm bit.In addition to zero-sequence protection, comprehensive security device further provided with overcurrent and overload protection tripping, the two protective corresponding power plant is described in document FSAR.
After the incident, the Ningde nuclear power implementation of corrective actions, SEC zero sequence of the pump motor is protected by a vote cast tripping the alarm changed to improve bus safety margin and switch.Investigation team, and communicate with designers through research and analysis that Unit 1 of the Fujian Ningde nuclear power plant emergency power distribution system voltage is ungrounded system, the pump motor when the SEC single-phase ground failure, the design should allow SEC pump with short-term trouble-free operation, in order to ensure the overall function of the system, and to stay out of the operator troubleshooting time.
Investigation team said: Given the current SEC insulation Insulator pump motor and the power supply switch fuse plant conditions, the SEC pump motor is protected by a vote of zero sequence alarm trip instead cast as a temporary corrective action is acceptable, but it should be hidden in related equipment after the elimination of the zero sequence protection and restoration as soon as possible to vote alarm, ensure the overall security of the system.
The resonance of the (five) voltage transformer occurs
In this “emergency bus LHA insulating low alarm” exception, Unit No. 1 No. 3 SEC system the pump motor with single-phase ground C occurs, then with fault-phase operation for about 24 minutes, after which the operator cut the pump failure.However, since the instantaneous phase voltage switching mutation, medium voltage distribution system grounding point high side to ground capacitor discharge circuit is formed and the LGA busbar voltage transformer (PT), leading to a rapid saturation of PT, PT inductance decrease caused at this time and to ground the phenomenon of resonance capacitor, and eventually lead to damage PT.
Investigation team said: accident analysis showed that Unit 1 medium voltage distribution system neutral point ungrounded way, there is a certain relationship with the occurrence of resonance PT.The relevant units should be combined with abnormal, further analysis of the mechanism of resonance to study countermeasures, if necessary, to optimize the design-related improvements.
Fourth, the cause of the incident analysis
Investigation team through staff interviews, data review and site visit, that the main reason for this event is the SEC pump motor Insulator material deterioration of insulation performance in wet conditions, which occurred in single-phase ground and cause phase fault; important contributing reason is that SEC C-phase power supply switch of the pump is not normally blown fuses and arcing occurs.Superposition of these two reasons, resulting in a small room air pump switch SEC insulation deterioration and ultimately lead to phase short LHB bus and emergency power loss, so that the electrical pump motor failure SEC extended to the upstream bus.
Abnormality occurs in the night, another occurs due to co-operation of the SEC pump is switched after the fault, using a voltage between the transformer and the resonant capacitance to ground of the voltage distribution system ungrounded configuration occurs, and resulting in damage to the voltage transformers.
After the event processing, power plant operators to enter the SOP accident procedures according to LHB power failure alarm card in a timely manner to the NS / RRA retreat mode, and reduced to P10 in the following units of power implemented a manual shutdown.Verification of the accident investigation team to perform recording procedures, the relevant personnel to operate the order requirements.
BRIEF 1 SEC discharge pump motor burning situation Insulator
Figure 2 C phase fuse and damage the surface of the insulating layer where arcing
annex:
Send a list of the main unit
Ministry of Ecology Environment in North China Nuclear and Radiation Safety Supervision Station
Ecological Environment East China Nuclear and Radiation Safety Supervision Station
Eco-Environment, South China Nuclear and Radiation Safety Supervision Station
Ecological Environment Northeast Nuclear and Radiation Safety Supervision Station
Ecological Environment Department of Nuclear and Radiation Safety Center
Beijing Nuclear Safety Evaluation Center
Productivity Promotion Center in the machine
Suzhou Nuclear Safety Center
World Nuclear Industry Group Co., Ltd.
World China Guangdong Nuclear Power Group Co., Ltd.
State Power Investment Group Co., Ltd.
World Huaneng Group Co., Ltd.
World Nuclear Power Co., Ltd.
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World Nuclear Power Institute
Shanghai Nuclear Engineering Research and Design Institute
INET of Tsinghua University
Institute of Nuclear Power Operation
World Nuclear Power Engineering Co., Ltd.
China Guangdong Nuclear Power Engineering Co., Ltd.
State Nuclear Power Engineering Co., Ltd.