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## Graduate School of Science and Engineering

Electrical and Electronic Engineering

### Power System Analysis Laboratory

Where would we be without "air" and "electricity"?

Supporting the lifelines and infrastructure of modern society.

#### Staff

#### Research Topics

- Analysis of phenomena and characteristics in lifelines/infrastructure systems:

Intelligent buildings, wind-power generation, electric railroads, solar-power generation, etc. - Analysis of transient/steady-state phenomena in electrical/electric systems
- Developing numerical analysis methods and modeling methods for transient/steady-state phenomena:

Fourier/Laplace transform, EMTP, numerical electromagnetic field analysis, etc. - Different types of modeling methods, transient/steady-state characteristic research: cars, railway, airplanes, lithium-ion batteries, transformers, electrical appliances, semiconductor elements, measuring instruments, machinery lifetime estimation, etc.

#### Research Contents

<1> Introduction

Virtually most of the lifelines (water lines, railroads, telecommunications, etc.) and infrastructure (buildings, roads, etc.) of modern society run on electricity. If electricity were shut off, water would not run when the water taps were turned on, building doors would not open, ATMs would shut down, cell phones would not recharge, and their base stations would stop transmitting before that anyway. The technologies of "electrical energy as a power source" and "electrical signals for telecommunications and control" are indispensable to modern society.

The Power System Analysis Laboratory conducts research related to the electrical energy (electrical power) that supports lifelines and infrastructure. Its ultimate goal is stable supply of electrical power. Electrical power systems must therefore be designed, built, and operated with "safety," "high reliability," "efficiency," and "economic considerations" in mind. Since electrical power systems are also very large systems, however, it is not possible to fully verify them through experimentation. The main topics of our laboratory are the development of numerical analysis methods for electrical power systems and the development of numerical models for power equipment to be used in this. Fig. 1 shows an overview of our research topics.

Virtually most of the lifelines (water lines, railroads, telecommunications, etc.) and infrastructure (buildings, roads, etc.) of modern society run on electricity. If electricity were shut off, water would not run when the water taps were turned on, building doors would not open, ATMs would shut down, cell phones would not recharge, and their base stations would stop transmitting before that anyway. The technologies of "electrical energy as a power source" and "electrical signals for telecommunications and control" are indispensable to modern society.

The Power System Analysis Laboratory conducts research related to the electrical energy (electrical power) that supports lifelines and infrastructure. Its ultimate goal is stable supply of electrical power. Electrical power systems must therefore be designed, built, and operated with "safety," "high reliability," "efficiency," and "economic considerations" in mind. Since electrical power systems are also very large systems, however, it is not possible to fully verify them through experimentation. The main topics of our laboratory are the development of numerical analysis methods for electrical power systems and the development of numerical models for power equipment to be used in this. Fig. 1 shows an overview of our research topics.

<2> Modeling

Insulation levels in electrical power systems are dictated by the voltage of switching surges generated by breakers and lightning surges caused by lightning. It is therefore important to predict overvoltage precisely when designing economical electrical power systems. To do this, we first need very precise numerical analysis models to represent power equipment. Our laboratory develops models of different equipment used in electrical power systems.

The primary components of transmission and distribution systems are the overhead power lines and cables, which are distributed-parameter lines. In overhead power lines with considerable length, which are affected by earth, wave propagation characteristics have frequency characteristics. In cables, sheaths are thin, so their propagation characteristics also are a function of frequency. Our laboratory has devised numerical analysis models that consider the frequency dependence of these homogeneous lines with great precision. We are also developing a finite-length line model that considers grounding and vertical conductors, as typified by steel towers, which must be treated as non-homogeneous lines. When developing these models, we conduct theoretical electromagnetic field analysis and develop functional approximation method suited to represent line models; we then conduct measurements using scale models to confirm the precision of the analytical model. We are also developing devices that measure high voltages precisely 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, breakers, and arrestors. Models of these are widely available for low-frequency domains, but we are developing the broadband numerical models needed for surge analysis. Since this equipment has nonlinear voltage-current characteristics, these characteristics must be adequately expressed in model development. For 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 notably short calculation times. The model developed by our laboratory was also improved jointly with Mr. Ichiro FUJITA of the Plasma Application Laboratory, and we developed a discharge lamp model that has achieved good results. A paper on this model was presented and awarded a prize at a conference.

Steel towers and substations are grounded, but surge analysis that has been performed in the past has virtually ignored the frequency characteristics of ground resistance. Through theoretical analysis and actual measurements, we are elucidating the transient characteristics of grounding impedance and developing a model.

Insulation levels in electrical power systems are dictated by the voltage of switching surges generated by breakers and lightning surges caused by lightning. It is therefore important to predict overvoltage precisely when designing economical electrical power systems. To do this, we first need very precise numerical analysis models to represent power equipment. Our laboratory develops models of different equipment used in electrical power systems.

The primary components of transmission and distribution systems are the overhead power lines and cables, which are distributed-parameter lines. In overhead power lines with considerable length, which are affected by earth, wave propagation characteristics have frequency characteristics. In cables, sheaths are thin, so their propagation characteristics also are a function of frequency. Our laboratory has devised numerical analysis models that consider the frequency dependence of these homogeneous lines with great precision. We are also developing a finite-length line model that considers grounding and vertical conductors, as typified by steel towers, which must be treated as non-homogeneous lines. When developing these models, we conduct theoretical electromagnetic field analysis and develop functional approximation method suited to represent line models; we then conduct measurements using scale models to confirm the precision of the analytical model. We are also developing devices that measure high voltages precisely 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, breakers, and arrestors. Models of these are widely available for low-frequency domains, but we are developing the broadband numerical models needed for surge analysis. Since this equipment has nonlinear voltage-current characteristics, these characteristics must be adequately expressed in model development. For 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 notably short calculation times. The model developed by our laboratory was also improved jointly with Mr. Ichiro FUJITA of the Plasma Application Laboratory, and we developed a discharge lamp model that has achieved good results. A paper on this model was presented and awarded a prize at a conference.

Steel towers and substations are grounded, but surge analysis that has been performed in the past has virtually ignored the frequency characteristics of ground resistance. Through theoretical analysis and actual measurements, we are elucidating the transient characteristics of grounding impedance and developing a model.

<3> Transient and steady-state analysis

Analysis that uses the models described above is conducted in the time domain and frequency domain. Since electrical power systems are extremely large, their analysis programs must be versatile. Our laboratory is developing a system analysis program in the time domain jointly with the U.S. Department of Energy. This program is called the Electromagnetic Transients Program (EMTP) and is used around the world. We have developed a highly precise numerical forward/inverse Laplace transform method in analysis of the frequency domain and also have a generalized Frequency-domain Transient phenomenon analysis Program (FTP) that applies it. Harmonics have suddenly become a problem in recent years with the spread of equipment driven by power electronics technology. Harmonic currents that flow through electrical power systems can cause damage to the transformer facilities of power consumers and have even been reported to cause bodily harm. 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 electrical distribution systems in buildings.

Intelligent buildings have problems with malfunction or breakdown of IT and other equipment caused by induction generated by harmonics in electrical distribution systems and 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 are generally based on experience, and induction has not been systematically estimated or countermeasures devised. Our laboratory has proposed analysis methods to clarify these EMC problems within intelligent buildings which have complex structures.

Analysis that uses the models described above is conducted in the time domain and frequency domain. Since electrical power systems are extremely large, their analysis programs must be versatile. Our laboratory is developing a system analysis program in the time domain jointly with the U.S. Department of Energy. This program is called the Electromagnetic Transients Program (EMTP) and is used around the world. We have developed a highly precise numerical forward/inverse Laplace transform method in analysis of the frequency domain and also have a generalized Frequency-domain Transient phenomenon analysis Program (FTP) that applies it. Harmonics have suddenly become a problem in recent years with the spread of equipment driven by power electronics technology. Harmonic currents that flow through electrical power systems can cause damage to the transformer facilities of power consumers and have even been reported to cause bodily harm. 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 electrical distribution systems in buildings.

Intelligent buildings have problems with malfunction or breakdown of IT and other equipment caused by induction generated by harmonics in electrical distribution systems and 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 are generally based on experience, and induction has not been systematically estimated or countermeasures devised. Our laboratory has proposed analysis methods to clarify these EMC problems within intelligent buildings which have complex structures.

<4> Analysis of maglev train systems

Maglev trains are being developed with the goal of building a new trunk transportation network because of the limits of the transportation capabilities of the Tokaido Shinkansen. Their power systems are the first systems to make wider use of high-capacity inverter equipment, so one of our topics is the analysis of such new electrical power systems. In addition to this, the routes currently being planned are prone to lightning strikes, so lightning countermeasures are required for these train systems to be highly reliable. Our laboratory is conducting analysis of the characteristics of inverter-driven electrical power systems and of lightning surges while also taking part in the design of a new transportation system.

Maglev trains are being developed with the goal of building a new trunk transportation network because of the limits of the transportation capabilities of the Tokaido Shinkansen. Their power systems are the first systems to make wider use of high-capacity inverter equipment, so one of our topics is the analysis of such new electrical power systems. In addition to this, the routes currently being planned are prone to lightning strikes, so lightning countermeasures are required for these train systems to be highly reliable. Our laboratory is conducting analysis of the characteristics of inverter-driven electrical power systems and of lightning surges while also taking part in the design of a new transportation system.

<5> Machinery lifetime estimation

Insulation breakdown in power equipment is one factor in preventing stable supply of electricity. If it were possible to constantly ascertain the condition of equipment installation and exchange equipment before it broke down, power outages could be further reduced. At our laboratory, we are developing a method of detecting minute partial discharges occurring within equipment and estimating the remaining service life of equipment by processing this information. To detect partial discharges, we measure partial discharge current while also measuring the sound vibration that occurs with it. We combine this measurement data with a model that expresses the high-frequency characteristics of equipment, especially transformers, and apply it to estimating the lifetime of insulators. We also apply information processing methods, such as neural networks, to increase estimating precision.

Insulation breakdown in power equipment is one factor in preventing stable supply of electricity. If it were possible to constantly ascertain the condition of equipment installation and exchange equipment before it broke down, power outages could be further reduced. At our laboratory, we are developing a method of detecting minute partial discharges occurring within equipment and estimating the remaining service life of equipment by processing this information. To detect partial discharges, we measure partial discharge current while also measuring the sound vibration that occurs with it. We combine this measurement data with a model that expresses the high-frequency characteristics of equipment, especially transformers, and apply it to estimating the lifetime of insulators. We also apply information processing methods, such as neural networks, to increase estimating precision.

#### Keywords

- Electric Power Systems
- Electromagnetic Transient Phenomena
- Lightning
- Distributed-constant Lines
- Electro-Magnetic Transients Program (EMTP)

- Computational Electromagnetics
- Electromagnetic Compatibility (EMC)
- Measurement of Transient Phenomena
- Lifetime Estimation
- Power Frequency Harmonics