Lu Fangcheng

A High-Frequency Circuit Model of a Potential Transformer for the Very Fast Transient Simulation in GIS

The high-frequency circuit model of potential and current transformers (PT/CTs) is indispensable in simulating the voltage stress in secondary circuits through the stray capacitances of the PT/CT particularly caused by the very fast transient overvoltages in gas-insulated subsystems. This paper presents a systematic methodology for establishing the high-frequency (0.1 to 11 MHz) circuit model of PTs based on the scattering parameters. First, the two-port S parameters of the PT are transformed into the Y parameters so that the equivalent ldquopirdquo circuit model can be established. Second, the vector-fitting code is used to approximate the admittances of the equivalent ldquopi rdquo circuit model by rational functions consisting of real as well as complex conjugate poles and residues. At last, a circuit synthesis method for the rational functions is applied to build the circuit model. The proposed method is general and robust. The simulation and measurement results are presented, confirming the validity of the proposed model.

The fault diagnosis method for electrical equipment based on Bayesian network

Bayesian network offers a powerful map framework that can process probabilities inference. It can be used in inference and express of uncertainty knowledge. This paper introduce a new electrical equipment fault diagnosis method based on Bayesian network (BN). For example, power transformer is very important in power system as a electrical equipment. But, its very difficult to diagnose the fault exactly because power transformers complexity configuration. Now, dissolved gas analysis (DGA) is the most effective and convenient method in transformer fault diagnosis. However, the codes of DGA is too absolute, so this paper advances a new transformer fault diagnosis method based on Bayesian network (BN). This method introduces BN method into transformer fault diagnosis and presents a new idea of finding out transformer faults rapidly and exactly. Then, the transformer fault diagnosis model based on Bayesian network and DGA is constructed. Finally, the application examples in the fault diagnosis of transformer are given which shows that this method is effective.

Study on ultrasonic generation mechanism of partial discharge

Partial discharge is important reason to cause insulation deterioration, ultrasonic testing have the advantage of strong immunity, apt to realize on-line monitoring, easy to space location etc., therefore many scholars proceed ultrasonic wave to test partial discharge, also obtained a lot of results. But at present, the study of ultrasonic generation mechanism of partial discharge is not deep enough, Therefore profound study on ultrasonic generation mechanism of partial discharge have the theories guides meaning for ultrasonic method detecting partial discharge. In ultrasonics, oscillation is the major contributor to the formation of ultrasonic waves. While discharging, an impulse force will be exerted on the surrounding material, which is elastic usually, to generate oscillation, i.e. ultrasonic waves. Founded on this thought, resorting to the method of electrical-mechanical-ultrasonics analogy, the dissertation offers the analysis of the sources of electric-field-force-generated ultrasonic waves and heat-stress-generated ones and provides in-depth study on the origins of partial discharge related ultrasonic waves and the relevant model-of-gap-based equivalent circuit for partial discharge-generated ultrasonic waves, Theoretically, the derivation of the relations between ultrasonic signals and the actual discharging magnitude is helpful to make clear some problems of ultrasonic detection. The data of the tests and the experiments has positively demonstrated the validity of the analysis in the paper and confirmed the credibility of the conclusions as well, which, somehow, will be of some value and significance in terms of theory and practice to ultrasonic method of transformer on-line detection.

Influence of power system's harmonic voltage on leakage current of MOA

Surge arresters are important over voltage protection equipment in a substation. It is necessary to monitor the resistive leakage current of MOAs (metal oxide surge arresters) on line so that we can prevent faults from developing in time to avoid economic loss. It is reasonable to adopt the resistive leakage currents basic component, i/sub r/, and thrice component, i/sub 3r/, as metal oxide surge arrester indicators. These indicators can detect the aging phenomenon of a metal oxide surge arrester in time. However, the existence of harmonic voltages in the power system influences the indicator accuracy. We analyze this effect and put forward an improved compensation method to separate the resistive leakage current under the existence of harmonic voltage. Finally, according to these methods, we develop an on-line MOA resistive current monitoring system based on the bus-module.

High frequency circuit model of transformer windings based on frequency responses for VFTO studies

In order to study the effects of VFTO to transformers, the high frequency model for transformer windings must be set up. In this paper, a systematic methodology is presented for establishing the high frequency two-port circuit model of transformer windings under VFTO based on its frequency responses. The pi-equivalent circuit is constructed based on the Y-parameters transformed from the measuring S- parameters. A correction to the rational function of Y approximated by vector fitting (VF) is calculated, which enforces the real part of Y is positive definite. This enforcement can assure the passivity of the whole model, which could bring a stable simulation. The values of the synthesized RLCG elements can be easily calculated form the rational functions. This kind of pi-equivalent circuit model could do stable and exact simulation compared with the experiment results, which shows the validity of the method.

The fault diagnosis method for power transformer based on BN And DGA

Power transformer is very important in power system. Now, dissolved gas analysis (DGA) is the most effective and convenient method in transformer fault diagnosis. This paper advances a new transformer fault diagnosis method based on Bayesian network (BN) and dissolved gas analysis technique. And it constructs the transformer fault diagnosis model based on Bayesian network. Finally, the application examples in the fault diagnosis of transformer are given which shows that this method is effective.

Influence of power system's harmonic voltage and interphase interference to resistive leakage current of MOA

Surge arrester is an important over voltage protection equipment in substation, and the latent faults of MOA can be found by on-line resistive leakage current monitoring. However, the existence of harmonic voltages in the power system will influence the monitoring accuracy. At the same time, because of influence of location and distributing mode of MOA, the voltage of two other phases affect single MOA by interphase interference, and this will surely influence the monitoring accuracy of MOAs resistive leakage current too. In this paper, these influence is analyzed and an improved compensation method to separate the resistive leakage current under the existence of harmonic voltage is described. And the methods to eliminate interphase influence is analyzed.

Fault diagnosis method of capcitive equipment based on grey relational analysis

Capacitive equipments are very important in our power system. So its very necessary to diagnosis the fault exactly. In this paper, according to grey relational analysis, a new capcitive equipment fault diagnosis method is present. It can calculate the grey relational degree between the tanδ and the temperature, humidity and catamination degree. Then get the fault type of capcitive equipment through campare the grey relational degree. This method is very easy using in the site. Finally, the application examples in the fault diagnosis of transformer are given which shows that this method is effective.

UHV test line initialization corona characteristic research based on optical fiber current measurement technology

Transmission line corona characteristic research is one of the key technologies in 1000 kV AC UHV transmission and transformation project in our country. Based on optical fiber current measurement technology, an optical fiber digital UHV corona measurement system is designed in this paper. The system achieves 0.2 level in IEC60044-8 standard, and it is able to measure voltage, current of UHV test line accurately. With this system, corona characteristic of single phase (C phase) conductor is tested, in Wuhan High Voltage Research Institute outdoors test site. The test line is about 200 m long. According to the test data, corona parameters is calculated out, and initialization corona characteristic of the test line is researched. The result shows that, the corona measurement system in this paper is available. With the increase of test voltage, corona loss rises as a exponential function curve approximately. When discharge appears on grading ring and test line, the corona loss and resistance current rise apparently. The corona kneel point is not apparent. With the increase of voltage, equivalent capacitance of test line changes little. According to this situation, it can be considered to adopt voltage resonance voltage loading mode to research corona characteristics.

The Calculation of the Voltage Distribution in Transformer Windings under VFTO Based on FDTD Method

In order to study the voltage distribution in transformer windings under very fast transient overvoltage (VFTO), a model for transformer windings has been developed based on multi-conductor transmission-lines (MTLs) theory and the computing equations of the model based on finite-difference time-domain method (FDTD) is deduced. The vector fitting method is adopted to treat the frequency-dependent parameter by expanding the time-domain form of internal impedance of the transformer windings into a sum of finite exponential functions which are easy for the time-domain convolution. For validation, an actual transformer winding model has been studied. The calculated results are compared with the measured result and a satisfactory result is obtained