Research On New Protective Algorithm And Development Of Digital Relay For Power Transformer

Large power transformer, which acts as a key connecting node and power exchange junction in power system, can influence security and reliability of power system operation and power supplying dependability significantly. At the same time, once power transformer is damaged, it takes long time and is hard to overhaul it, and cause huge economic loss because of its complex structure and high price manufacture cost. Up to now, performance of transformer protective relay is enhanced significantly after many years'efforts; however, the correct operation rate is still low comparatively. Though this situation is caused by many complicate factors, insufficient research on protective principle and technical realization of relay is a key reason for malfunction or failure to operation of relay. On one hand, the normal working flux density of corn for the large transformer is higher, saturation time of corn is lower, and thus the performance of traditional transformer protection based on harmonic restraining principle is deteriorated; on the other hand, the type of transformer structure and connecting type of power system are miscellaneous and operation situation is complicate, thus transformer relay is required to have good adaptability, at the same time, high-performance hardware platform is needed to support it because complex working environments for relay. So, if the research on protective principle for transformer and development of high-performance transformer relay are further deepened, it is of great theoretical and realistic significance to insuring safety of operating transformer and enhancing stable operation level of power system and power supply reliability.First, this paper analyzes and summarizes the research, application and key problems of transformer protection. On the basis of that, this paper focuses on the study on inner fault transient simulation model of transformer, the new protective principle of transformer, and development of general-purpose relay for large transformer.Inner fault simulation of transformer is important basis to the study on protective principle for transformer. There are miscellaneous fault types in transformer. Among them, turn-to-turn faults and phase-to-ground faults are the most main and basic faults. This paper describes a method to establish turn-to-turn fault and phase-ground fault model of transformer for transient simulation with multi-winding transformer model in EMTP simulation software. There are still many technical problems to calculate out inductances of multi-winding. In term of simplifying parameter calculation and easy engineering application, this paper propose a method to establish transformer model, which uses Lest Square Method (LSM) to calculate winding inductances with limited dynamic testing or field recording date.The fault winding can be divided into two parts:normal winding and fault winding based on the differential equations of multi-winding according to the method. The Lest Square Method (LSM) can be adopted to estimate the self-inductance and mutual inductance values of the fault winding and the coupling leakage factor curve can be obtained. According to the curve and the percentage of short turns of transformer, the parameters of transformer winding for simulation can be identified, and the turn-to-turn fault and phase-to-ground fault model of transformer can be established. The dynamic test validates the proposed method in this paper.A new protective principle for transformer based on equivalent circuit provides a new way to enhance the performance of transformer relay. However, it is a key problem to identify the equivalent parameters in application. This paper proposes a new parameter identification algorithm and a convergence criterion for identification result based on statistic method. Various simulations such as transformer energizing, external fault, internal fault, and normal operation testifies that the parameter identification algorithm based on the new transformer model can estimate the winding parameters accurately in various conditions.The traditional differential protection based on harmonic restraining method is seriously influenced by harmonics, the operation time is long, and sensitivity is low, especially during slight turn-to-turn fault in transformer. Aiming to solve this problem, this paper provides a novel protective scheme based on equivalent circuit model equation and equivalent excitation inductance. The criterion of equivalent circuit model equation can provide reliable protection for serious fault while the criterion of equivalent excitation inductance enhances the operation sensitivity. Simulation shows the performance of the protection scheme is not affected by inrush current, and the protection can operate quickly and has high sensitivity. Sampled-value differential protection algorithm (SDPA) has attracted extensive attention because it has such advantages as fast operation speed, being influenced little by inrush current and CT saturation. However, the obvious week of SDPA is difficult to set the relay in application. This paper studies the security and sensitivity of SDPA with mode restraint current, conclude the setting range of SDPA. The conclusion is validated by many simulations.Aiming to accelerate operation speed of unstrained differential protection and enhance the ability of CT anti-saturation, this paper describes a fast unrestrained differential current protection algorithm based on phaselet algorithm. This paper analyzes the relationship between phaselet algorithm using sampling data of one-fourth circle and full circle Fourier algorithm, induces the conjunction array between the two algorithms, analyze magnitude-frequency characteristics of phaselet, and the fast protection criteria is proposed. The static testing and dynamic testing validate feasibility of the proposed algorithm.DSP and ARM parallel work mode is adopted in hardware platform. The platform has abundant resource, is flexible to expand, and can meet the development requirement of general-purpose protective software; Special techniques are considered in design, thus reliability and anti-interference ability of the relay are enhanced. Based on proactive software design method of module protective element and transparent operation logic, development and testing of relay software is easier, higher effective and is good to expand the protective functions to meet different field application requirements. The developed relay has passed the authorized organization testing, Feasibility is validated field application in power system.At the end, this paper summarizes all the paper and looks forward into the further research in future.