Research On Open Phase Condition Detection Scheme In Nuclear Power Plants

The protection of nuclear power plants is based on conventional islands and nuclear islands,and the design codes and standards differ greatly.Conventional islands are like ordinary power plants,and the protection settings are extremely comprehensive:the more the better.For nuclear islands,the first consideration is that if too many types of protection are set,the probability of malfunctioning of the protection will increase,and the greater the potential nuclear safety hazards will be.Therefore,the protection scheme follows the "subtraction principle",that is,less protection should be configured.The protections generally installed in most nuclear power plants at home and abroad are over-current protection,zero-sequence protection and differential protection.They are not equipped with protection for open-phase faults.At the same time,because nuclear power plants are frequently overhauled,when open phase fault occurs on the high-voltage side of the main transformer or auxiliary transformer under light-load condition,a small change in electrical quantity will make the current-based open-phase protection unable to operate reliably,leaving a big safety threat.A number of field operating experiences have shown that only using methods such as detecting a certain phase current or sequence component unbalance detection for open-phase detection cannot guarantee 100%reliability under all working conditions,especially when the transformer is unloaded or lightly loaded.Under the light-load condition,the electrical quantity changes very weakly,and the current transformer cannot meet the accuracy requirements of the current quantity measurement value,so the existing protection scheme still needs to be perfected and improved.This article analyzes the reasons why it is difficult to detect open-phase faults,and respectively carry out theoretical analysis of magnetic circuits and circuits,as well as open-phase under different locations,different transformer connection groups and core structures,different working conditions,and different loading rates.The fault is simulated in PSCAD/EMTDC,and the phase-failure protection scheme and the switching scheme of the emergency busbar are proposed.Based on the analysis of the magnetic circuit structure of the transformer and the circuit analysis based on the symmetrical component method,the reasons for the difficulty of fault detection after the occurrence of a open phase failure and the characteristics of the electrical quantity change are explained in detail.Based on the specificity of some of these changes and comparing the traditional protection,a new concept of phase failure detection was put forward to provide theoretical support for the subsequent comprehensive protection plan.Using the nuclear power plant data provided by the project partner,the electromagnetic transient simulation platform PSCAD/EMTDC was used to model,and the different types of open-phase faults were simulated and analyzed,and the characteristics of other types of faults were compared in detail,and the unique characteristics were obtained.The simulation results verify the conclusions of the theoretical analysis,and provide data and simulation support for the proposal of the protection scheme.Based on theoretical analysis and simulation analysis,this paper proposes a comprehensive open-phase fault detection method,which considers the influence of working conditions in the logic of the fault detection,and combines the power method,the phase angle difference method,and the sequence component method for logic setting and integration.The above three detection methods have been simulated and verified,showing a good protection effect,and based on this,the switching logic of emergency plant busbars is proposed.The research results of the thesis provide useful development ideas for the improvement of nuclear power plant protection schemes and the safe and reliable operation of nuclear power plants.