Graduate School of Science and Engineering Electrical and Electronic Engineering

Power System Analysis Laboratory

Transients and EMC in Infrastructures Such as Power, Telecommunication and Railway Systems
Deterioration Diagnosis of Batteries

Staff

Research Topics

• Circuit and electromagnetic modeling of various components in power, telecommunication and railway systems for transient and EMC simulations
• Circuit modeling of harmonics in power systems
• Development and improvement of computation methods or procedures for lightning electromagnetic pulses and surges
• Development of deterioration diagnoses of various batteries

Research Contents

<1> Introduction
Most of the infrastructures such as feed-water lines, railways, telecommunication lines, power systems and so on are run on electricity. If electricity were shut off, water would not come out, building doors would not open, ATMs would not work, cell-phone stations would not work. Electricity in both terms of “Energy” and “Signal” are indispensable to modern society.
In the Power System Analysis Laboratory, we conduct researches related to the electric energy (electric power) that supports lifelines and infrastructures. Our ultimate goal is to realize stable supply of electric power. Electric power systems must therefore be designed, built, and operated with "safety," "reliability," "efficiency," and "economy". Since electric power systems are very large systems, it is not possible to study phenomena in these systems only experimentally. Thus, theoretical and computational methods are also needed. The main topics of our research are the development of computation methods for electric power systems and the development of numerical models for power equipment. Fig. 1 shows an overview of our research topics.

<2> Modeling
Insulation of electric power systems are threatened by switching over-voltages generated by circuit-breaker operations and lightning over-voltages. It is therefore important to predict over-voltages for economical and optimal insulation design of electric power systems. To this aim, we need very precise numerical analysis models to represent power equipment. Our laboratory develops models of different equipment used in electric power systems.

The primary components of power transmission and distribution systems are overhead power lines and coaxial cables, which are usually represented as distributed-parameter lines. Transient voltage and/or current waves propagating along an overhead power line are affected by the earth constitutive parameters such as conductivity/resistivity, permittivity and permeability. In coaxial cables, since sheath conductors are thin, their propagation characteristics are also functions of frequency. We have developed numerical analysis models that can consider the frequency dependence of these homogeneous lines with high accuracy. We are also developing the models of vertical conical conductor such as a transmission-line tower and a telecommunication tower, which shows both frequency- and direction-dependent characteristics. When developing these models, we conduct theoretical electromagnetic field analysis and develop functional approximation method suitable for representing line models. Then, we conduct measurements using scale models to confirm the validity of the theoretical model. We are also developing devices that measure high voltage impulses economically without contact by applying the induction characteristics of multi-phase distributed-parameter lines.

Substations are built by organically combining an array of equipment such as transformers, bus lines, circuit breakers, and arresters. Models of these components are widely available only in low-frequency domains, but models applicable to surge simulations are limited. We are developing broadband numerical models for surge simulations. Since this kind of equipment has nonlinear voltage-current characteristics, these characteristics must be adequately expressed. The heat generated in an arrester, in which a lightning surge flow, is also computed using discretized Maxwell’s equations and heat equations. For circuit breakers, it is particularly necessary to consider arc characteristics, which are a discharge phenomenon. The model developed by our laboratory can take these characteristics into account with an extremely simple function and has noticeably short calculation times.

Transmission-line towers and substations are grounded, but surge simulations performed in the past have ignored the frequency characteristics of ground resistance. We are developing frequency-dependent models of grounding electrodes.

<3> Transient and steady-state analysis
Simulations with the models described above are conducted in both time domain and frequency domains. Since electric power systems are extremely large, it is desirable that their simulations programs are versatile. Previously, our laboratory developed a power system analysis program in the time domain jointly with the U.S. Department of Energy. This program is called the Electro-Magnetic Transients Program (EMTP) and is still used around the world. We have developed a highly precise numerical forward/inverse Laplace transform method in the analysis of the frequency domain and also have a generalized Frequency-domain Transient phenomenon analysis Program (FTP) that applies it. Power-frequency harmonics are now recognized as a power-quality problem, which is caused by the penetration of inverter-driven equipment and appliances. Harmonic currents that flow through electric power systems can cause damage to the transformer facilities of power consumers. To prevent such accidents, there is an urgent need to estimate harmonics and develop rapid countermeasures. Our laboratory has developed a simple and precise harmonics estimating program, proposed an effective measure to deal with harmonics by making transformer connections multi-phase, and is currently applying it in many electric distribution systems in buildings.

Intelligent buildings have problems with malfunction or breakdown of IoT equipment caused by induction generated by harmonics in electric power distribution systems and by lightning surges. The study of harmonic induction damage focuses on measuring induction voltage and current after damage occurs and has barely considered induction caused by lightning surges, which are transient phenomena. Not surprisingly, countermeasures were generally based on experience, and induction had not been systematically estimated or countermeasures devised. Our laboratory has proposed analysis methods to clarify these power-quality and EMC problems within intelligent buildings which have complex structures.

<4> Railway systems
In railway systems, overhead conductors and on-ground iron rails are used for both power supply and communication lines. In comparison with power systems, attentions have not much paid to over-voltage surges generated in railway systems. We have conducted joint researches with a railway company for developing transient-simulation models of each component of railway system.

<5> Deterioration diagnoses of power equipment and batteries
Insulation breakdown in power equipment is one of the factors causing interruption of stable supply of electricity. If the pre-breakdown condition of equipment were known, it could be possible to replace the equipment before it broke down and caused power supply interruption. In our laboratory, we are developing a method for detecting partial discharges occurring in equipment. To detect partial discharges, we detect partial discharge currents through sound vibration and/or electromagnetic fields coming out from the equipment. Genetic algorithms are also applied to this research. More recently, we have intensively studied deterioration diagnoses of various types of battery in the research project supported by NEDO.

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