Key Technologies Of Safety Risk Preventive Control And Protection For The Main Electric Power Equipment Of The Floating Nuclear Power Plant

At present and in the future,safe,stable and flexible electric power supply is the major demand for marine national defense construction,oil and gas exploitation,and island development.The floating nuclear power plant is the latest technology which is focused on development at home and abroad.It is a small mobile offshore nuclear power plant and supply processing plant which integrates nuclear reactor,shipboard and electric power technologies.It has the advantages of large capacity,flexible mobility,sustainable power supply,energy conservation and environmental protection.It can effectively solve many problems of offshore power supply.For example,the offshore oil field consumes a large amount of power.The diesel power generation mode is mainly adopted.But it causes serious pollution and is restricted by difficult fuel transportation and storage.Restricted by flight safety and land area,wind and solar power generation modes are difficult to apply.In 2016,the National Development and Reform Commission set up the demonstration project of the floating nuclear power plant and listed it as a major national energy science and technology innovation project.The Ministry of Science and Technology has set up a national key research and development program.The first floating nuclear power plant in China is in the process of research and industrial implementation.The floating nuclear power plant technology involves many fields.Among these,the safe power supply of nuclear reactors and the safe operation of nuclear power plants have been included in the key projects.It has the largest generator capacity and the highest voltage class of the shipboard power system.Its main power equipment is dedicated to ships,but the basic structure is similar to the land-based equipment.However,due to the high power supply requirements of nuclear reactors,the harsh marine environment,and the difficulties in maintenance,there are still some electrical safety risks.This paper focuses on the key technologies of safety risk preventive control and protection for the main electric power equipment of the floating nuclear power plant.The electrical safety risk preventive control technology is divided into four links:risk prevention,risk diagnosis,risk suppression and risk blocking.According to this,several key technical problems which have not been solved in the previous risk preventive control and protection measures are studied.In risk prevention,effective no-load closing inrush current suppression of the transformer is an important measure to extend the life cycle of the equipment.The transformer of the plant is closed frequently,and the inrush current can reach several times the rated current,which damage the insulation and structure.The seried small capacity transformer premagnetization technology is more suitable for ship scenes than the phase-time control technology,but its mechanism and application method are not studied.In this paper,the mechanism and influencing factors are expounded.According to the principle of safety and economy,the optimal selection method of the premagnetization transformer parameters and the premagnetization circuit structure is proposed.And a method to determine the optimal premagnetization delay based on the adjacent inrush current energy ratio is proposed.Simulation results show that the proposed method can suppress the peak value of inrush current less than 0.5I_n,and it can cooperate with the differential protection.In risk diagnosis,the key is to solve the problem of stator ground fault degree analysis and fault location.In the marine environment of high corrosion and huge waves,the stator ground fault is most likely to occur in the generator.The fault current causes the arc to burn the winding and the iron core,and then leads to the destructive short circuit fault.The existing protection methods do not reflect the fault current,so they cannot characterize the safety risk degre.The objection and difficulty of the ground fault diagnosis are to determine the ground fault current and fault location to achieve fault suppression and maintenance optimization.As the ground parameters(mainly the distributed capacitance)vary with the generator’s working condition and environment,a real-time grounding capacitance estimating method based on the measured third-harmonic voltage of the generator is proposed.And the accurate calculation of the full ground fault current including fundamental and main harmonics is realized.Since none of the existing fault location methods can realize accurate location of all winding faults.In this paper,an improved online accurate location algorithm based on the generator stator turn-potential distribution characteristic is proposed,and a third harmonic test criterion is used to solve the multi-solution problem.Simulation and dynamic model tests show that the proposed methods can overcome the influence of fault position,transition resistance and operating conditions,which have high accuracy.In risk suppression,the key is to realize arc suppression and adaptive protection of the generator stator ground fault.The stator ground fault arc suppression goals are as follows:avoiding the transient fault developing into a permanent fault,and controlling the permanent fault current below the safe value.The above goals are vital to nuclear power safety.Since the traditional passive arc suppression methods with current compensation mode cannot meet the above requirements,this paper proposes a stator ground fault arc suppression technology based on the principle of fault point voltage compensation.Considering the distribution characteristics of the winding potential,a dual-frequency active regulation arc suppression method is proposed,which realizes effective arc suppression and current limitation of the fundamental and third harmonic fault waves.Furthermore,based on the analysis that this method does not affect the existing protection action and delay characteristics,an adaptive ground fault protection method is proposed based on the fault risk degree and the fault character identification.For the instantaneous fault,the continuous operation is realized after the fault cure.For the instantaneous fault,the stable load transfer and generator tipping are realized after safe current limiting.Simulation and dynamic model tests verify the correctness and effectiveness of the proposed method.In risk blocking,improving the generator circuit breaker(GCB)failure protection is an important measure to improve its reliability.Since the generator of the plant adopts the high-resistance grounding mode and the faults which need to trip the generator are complex,the general circuit breaker failure protection criterion does not apply to GCB.For the failed phase of GCB,this paper studies and illustrates its characteristics that the fundamental voltage phase changes on both sides under the normal load conditions,and the third harmonic voltage remains consistent on both sides under the no-load(light load)conditions.Even taking into account the electric quantity attenuation during the generator flux damping process,the above electric quantity can still effectively reflect the GCB failure.Therefore,this paper proposes a GCB failure protection based on the fundamental and third harmonic voltage phasor difference on both sides of the GCB.Simulation and dynamic test results show that the proposed method has enough sensitivity under different fault conditions and load conditions.At last,the paper summarizes the work,gives the main innovation points and carries on the prospect of the next research.