Modeling And System Implementation For Digital Simulation Of Protective Relaying

The correct operation of protection relays is crucial to the security and stability of power systems.Traditional relay testing methods cost high and have low flexibility.Moreover,the detailed response process of internal components to a fault can not be observed.Recent years the software based digital simulation technology for relay protection,has been recognized as an useful tool.This new technology could provide with us so many helps,such as physical phenomena of power system under normal and fault conditions,transforming process of CT and PT,dynamic behaviors of different kind of relays as well as analysis to factors impacting relay's operation.So this tool play an very important role in research,development, teaching and training of protective relaying.This paper focuses on solving some key problems related to system modeling, relay modeling and development of simulation system.The main contents are as follows:(1) A new modeling method of current transformer based on "Extreme Pairs" is presented.Two assumptions and three types of magnetizing trajectories for magnetic core modeling based on Preisach theory are summarized.Two factors determining the nature of magnetization trajectory,extreme points experienced and changing trends of magnetic field intensity,are proposed.For the former, memorizing and forgetting property of "extreme pairs" is presented.Magnetic core will memorize the maximum and minimum points experienced by magnetic field intensity(including two kinds of special "extreme points"),and with the time sequence from near to far,these points combine as "extreme pairs".When magnetic field intensity is greater than or equal to(less than or equal to) a maximum point (minimum point) memorized,magnetic core will forget the "extreme pairs" corresponding to the maximum point(minimum point).If only one extreme point is memorized,forgetting phenomenon will not occur.The nearest extreme points in time sequence is the starting point of the general magnetization curve that magnetic core is running on.If none of extreme points is memorized,magnetic core will run on the basic magnetization curve.With this property,a modeling method of magnetic core is proposed and a current transformer model is built using this method. Simulation results testified the accuracy and effectiveness of the model and the modeling method.(2) A new modeling method of Capacitor Voltage Transformer(CVT) based on integral equivalent transformation and superposition theorem is presented. Using trapezoidal integral equivalent transformation,the high-order equivalent circuit of CVT can be transformed into a simple DC circuit just containing DC current sources,DC voltage source and resistors.Then the node-voltage equations of the circuit are established.Simulating the transient characters of CVT just needs to solve these simple linear equations.The decomposition of normal component and fault component based on superposition theorem is used to solve the initial value problem of the modeling algorithm.According to the presented modeling method,a novel correcting method for transient errors of CVT is proposed.Using this method, CVT's primary side voltage can be calculated accurately with the secondary voltage known.Simulation results testified the accuracy and effectiveness of the modeling method and the correcting method.(3) A new oscillation modeling method based on instantaneous frequency is proposed.The existing oscillation modeling methods are all based on changing the "average frequency of 0-t interval",which can not directly reflect actual state of power systems.To solve the problem,this paper proposes a new modeling method based on instantaneous frequency.By fitting the changes of instantaneous frequency under some constraint conditions,the method can accurately simulate the oscillation process of real system without introducing the "average frequency of 0-t interval". With the method,a oscillation model reflecting whole process is built,and two typical cases of oscillation(without/with fault) are simulated using the power system block(PSB) in Matlab.The simulation results show that this model can match well the pre-set curve of frequency during oscillations,and is more suitable for the performance evaluation of relay protection.(4) Aiming to the shortcomings of the relay models available,a multi-purpose .relay simulation platform is developed.The platform has three characteristics.Firstly, a simulation component library for relay protection is developed.The library is comprised of the filter sub-library,the algorithm sub-library,the startup sub-library, the phase selection sub-library,etc.Each sub-library includes the widely used algorithms and component models.Using the components in the library,relay models with different operating principles can be built very easily.Secondly,a friendly GUI is designed for the simulation platform.Resorting to several demonstrating methods such as graphics,waveforms,data forms and vector diagrams etc,power system configuration,parameter setting,operation status, characteristics of fault,principles of relay protection can be displayed vividly. Thirdly,this platform could reflect the physical phenomena of power system in the normal and different fault conditions,the transforming features of CT and VT, especially the whole responding process of relay to faults.(5) Using the relay simulation platform proposed,a dynamic simulation system for microprocessor-based distance protection is designed.The system could simulate the different operating conditions of power system and display the electromagnetic transient process after fault,could reflect the acting and coordinating process of different modules in the distance protection,including startup,Fourier algorithm,phase selection,impedance computing and impedance comparison etc.The simulation system was developed under the same programming environment,so the data- format conversion is not needed.Power system and distance relay is simulated synchronously,thus the closed-loop simulation could be realized more easily.The parameter setting in this system is very flexible,so the various factors affecting the distance relay's operation can be simulated and researched.