Fault Judgement Method For Smart Distribution Network Based On Integrity Of The Protected Element Model

Distributed generations(DG)access to distribution networks can help to improve the power supply reliability of distribution network and accelerate the replacement process of traditional fossil energy to new energy sources.Therefore,smart distribution network allows a large number of DGs or micro-grids with various control strategies and capacities to be accessed.This kind of feature not only changes the basic characteristics of the radial structure of the traditional distribution network,but also brings plenty of impacts on the operation and control of the distribution network,especially on traditional relay protection devices.Longitudinal comparison protection does not need simultaneous sampling of electric quantity and has good adaptability to changes in network topology and operation mode.At the same time,it only needs interactive logic information,so it has been widely valued and researched in recent years.The information used in longitudinal comparison protection principle is generally the fault direction,which is completed by the fault directional element.However,due to the complex fault characteristics and control strategies of DGs and other special operating conditions in smart distribution network,traditional fault directional elements can no longer meet the operating conditions of smart distribution network,so the accuracy of the judgment results cannot meet the reliability requirements of the relay protection action in smart distribution network,Therefore,seeking or constructing a fault information quantity which is basically not affected by the DG fault characteristics is a feasible solution to solve this problem.It is also important for improving the power supply reliability of distribution network.In this paper,on the basis of the previous research results,a fault judgement method for smart distribution network is proposed,which is oriented to the protected object model.This method no longer starts with analyzing the fault characteristics of DGs,but take the physical model of the protected object as the breakthrough point.Firstly,an appropriate physical quantity can be defined based on the mathematical constraints of the model.Then,by analyzing the variation law of the defined physical quantity under both the internal and external fault of the model,the integrity of the model can be judged,which can be used as the basic information for longitudinal protection to constitute the corresponding fault judgment method.In this paper,working voltage is taken as the physical quantity to judge whether the model is intact,and the setting principles of the impedance in working voltage method is analyzed in detail,so that the method proposed can adapt to the operating characteristics of smart distribution network.Based on the above work,this paper designs a longitudinal comparison protection scheme using the results obtained from the protective elements at various locations,in order that the protection can be better adapted to the T-connected branch,the change in the equivalent impedance of the line,overload and weak-infeed,the voltage dead zone and other special conditions of smart distribution networks.The simulation analysis shows that the fault diagnosis method proposed in this paper is basically not affected by the fault characteristics of DGs,and each protection can accurately and reliably remove the fault.In particular,the article first proposed a fault judgment method oriented to protected object model.The amplitude and phase of the electrical quantities are constrained by both power supply and physical model of the element.Electrical quantity is produced by the power supply,but its amplitude and distribution must meet the mathematical relationship determined by the physical model.From this point of view,no matter how complicated the fault characteristics of DGs are,it can be regarded as a source of electrical quantity.Regardless of its output characteristics,as long as the physical model of the studied component is determined,then the electrical quantities of the element must satisfy the mathematical relationship determined by the physical model.Therefore,the corresponding fault judgement method can be put forward by analyzing the variation laws of the defined variables in the internal and external faults of the model.Secondly,this paper selects the "working voltage" as the physical quantity to judge"whether the model is intact" and analyzes the setting method of the core variable in working voltage in detail.For the situation that the equivalent impedance angle of the transmission lines may change dynamically in a wide range after different branches or reactive power compensation equipment access,this paper reconducts the setting method of the "setting impedance".On this basis,a fault criterion at a single protection element is put forward,which makes the protection can accurately output the judgement result of the "integrity of the model within the protection range",so as to lay foundation for further comparison and fault removal.Afterwards,this paper designs a longitudinal distribution comparison protection scheme for smart distribution network.Considering that a single protection element may be misjudged due to various reasons and the information carrying by single logic signal is insufficient,this paper optimizes the operation logic of protection elements so that it has a certain fault tolerance.In this paper,the logic data used to determine whether the protection operates or not is generated both by each protection element itself and other protections locates in various places through the longitudinal system,and it's composed of 8 bits.Each bit has a unique physical meaning and range of values.Therefore,reasonable control of the protection action can be achieved by logically comparing the 8-bit logic data.Finally,in this paper,a typical smart distribution network simulation model with multi-branch access is built in PSCAD,and the fault judgment method and longitudinal distribution comparison protection scheme proposed in this paper are simulated.By simulating the phenomena of inductive non-branch line,capacitive non-branch line,T-connected load branch,T-connected power supply branch,heavy load and weak-infeed and voltage dead zone,the reliability of the fault detection method and the accuracy of each protection are verified.