（本文）

Power system stability is one of the important issues that must be considered and maintained in order to ascertain power system operation. Problems with associated with dynamic performance were noticed in power systems when synchronous machine were first operated in parallel. The rapid development of power systems generated by increased demand for electric energy initially in industrialized countries and subsequently in emerging countries leads to different technical stability problems in the systems, e.g., stability limitations and voltage stability problems. Environment concern is also one of factors leading to difficulty in construction new power plants and transmission lines, which may cause stability problem in the near future. In addition, by the electric power deregulation, power system in several countries has become deregulated facilities; customer can select their own electricity providers, high competition occurs in electric utility industry. One of several important factors to survive in this competition is cost. One who can reduce cost possibly gains the customer purchase. In order to cut off the cost, power system operation near stability limit is the way mostly taken into account and it is risk that power system may become unstable. Furthermore, power system blackout has become too common occurrence recently; in a span of less than two months, August-September of 2003, 4 blackouts affecting a large number of customers happened in US-Canada, London, Sweden-Denmark, and Italy. Those indicate and reflect the stability problems in the power systems. Therefore, it becomes necessary to enhance and improve power system stability in order to assure the operation of power systems even in more severe conditions and increase the robustness of power systems.

Breaking innovations in power electronics technology and superconductor technology can solve problems in power systems. The power electronics technology enables the manufacture of powerful thyristors and, later, of new elements such as the gate turn-off thyristors （GTO） and insulated gate bipolar transistors （IGBT）. Development based on those semiconductor devices first established high-voltage DC transmission. HVDC technology, in turn, has provided the basis for the development of flexible AC transmission system （FACTS） equipment such as SVC, STATCOM, UPFC, and IPFC. With FACTS devices, active power and reactive power flowing in transmission line can be controlled and power system dynamics can be improved. The superconductor technology also offers solutions to critical problems facing power systems, including the needs for improved power system stability, higher capacity and a “smart grid” in which power flows can be controlled. After discovery of superconducting materials and the great development in power systems, superconducting power apparatus such as superconducting generator （SCG）, transformer, transmission-cable, superVAR dynamic synchronous condenser, superconducting magnetic energy storage （SMES）, and superconducting fault current limiter （SCFCL） can ultimately be developed and realized. Their installations in power system lead to improve power system dynamics.

There are a lot of types of power apparatus developed for power systems. They can be mainly divided into 2 types; non-rotating power apparatus and rotating power apparatus. The non-rotating power apparatus or static type power apparatus is the apparatus without moving parts such as SVC and STATCOM. The rotating power apparatus is the apparatus with moving parts such as synchronous condenser, adjustable speed generator/motor and rotary type frequency converter. Although static type power apparatus are mostly used in power systems and highly contribute to improvement of power system stability, rotating power apparatus is also another choice for the objective and they also has some advantages which can not be realized in the static type power apparatus such as transient overload capability and rotating inertia. It will become attractive and useful to study and research in more detail on rotating power apparatus in order to use them more effectively in power systems for improving power system stability.

In this research, two new types of rotating power apparatus are considered for improving power system stability: rotary type frequency converter and superconducting generator; their characteristics, models and behaviors in power systems are examined; advanced control systems of those rotating power apparatus are designed and proposed for power system stabilization. In addition, installation scheme is also proposed to achieve improvement of power system dynamics. This dissertation is mainly divided into two parts and the explanation in detail of each part is given as follows:

The first part of this dissertation is the topic on the rotary type frequency converter （RFC）. RFC using two sets of adjustable speed generators/motors is expected to be applied at the interconnections between 50Hz and 60 Hz power systems in Japan. RFC can work not only as a power interchanger but also as a power system stabilizer by effectively utilizing energy stored in rotors, so-called rotational energy. This research investigates how the RFC affects power system dynamics and since shaft torsional oscillation and slow response due to mechanical connection are inevitable and may lead to instability in power system, it also presents some countermeasures or control schemes to handle such inherent problems and to improve power system dynamics. Three different control methods （parameter optimization techniques） are employed for the controller designs; eigenvalue sensitivity based parameter optimization technique, energy function based control method, and feedback linearizing control method. The control performances and participations in performance improvement of power system dynamics are examined in the test model system. Each designed control system has its own characteristic and influences power system dynamics differently. RFC with the designed control systems can improve performance of power system dynamics by more effectively utilizing rotational energy. Shaft torsional oscillation can be suppressed well by the designed control systems. The designed control systems are effective to solve the inherent problems of RFC and to improve power system dynamics.

The second part of this dissertation is the topic on superconducting generator （SCG）. SCG with superconducting field winding has many advantages such as small size, light weight, and high generation efficiency. In particular, the property of low synchronous reactances, which is not realized in the conventional generators, is able to improve the power system stability. The high response excitation type SCG has a rotor having thermal radiation shield without damping effect; it can enables excitation power in self-excited operation of the generator to change rapidly enough to affect the conditions of power system. The effect is called “SMES Effect”. In this research, control system designs of SCG with high response excitation in consideration of SMES effect for improving the power system dynamics are proposed. Three different control methods （parameter optimization techniques） are employed for the controller designs; eigenvalue sensitivity based parameter optimization technique, energy function based control method, and feedback linearizing control method. The SMES effect is modeled and put into consideration in the controller designs. Control system designs of reactive power of SMES effect are also proposed. The effectiveness of the designed controllers of SCG with high response excitation are verified in two different test model systems, IEEJ east 10-machine system and west 10-machine system, by showing that they can improve power system stability; however, they contribute to improve the stability differently due to the control concepts. The SMES effect can be utilized effectively to compensate the output variation of SCG, in turn, to improve overall stability. The designed controllers of reactive power of SMES effect contribute to improve voltage regulation. The designed controllers are effective to improve power system dynamics; however, locations of SCGs influence the performances of those controllers.

In addition, installation schemes of SCGs in multi-machine power systems are also discussed, Firstly, examinations of SCG installation in power systems in consideration of dominant mode are done to search for possible locations and some specific parameters. Two new methods for installation schemes of SCG with low response excitation are proposed. The method type 1 employs the observation of global inter-area mode and approximated eigenvalue sensitivity to evaluate installation indices used for the SCG location. Synchronous reactance Xd （Xq） and transient open-circuit time constant T'do are also determined later by eigenvalue sensitivity based parameter optimization technique. The method type 2 employs hierarchical genetic algorithm （HGA） to simultaneously determine SCG location and both parameters. Tuning of control parameters such as gains of AVR and speed governors is also taken into account in both methods. It is verified in the test model system that both methods are effective to improve stability of power system.

RFC and SCG with the designed control systems can improve power system dynamics and appropriate installation schemes of SCGs can also contribute to the improvement of power system dynamics. These results show the effective applications of rotating power apparatus to power system stabilization.

Furthermore, the following research subjects should be further studied.

1）Coordination control between RFC and generators should be studied in order to extend the performance of RFC to improve power system dynamics.

2）Control Systems of Multiple SCGs: In the near future, when SCGs are commercialized, multiple SCGs in power systems can then be realized. How to control multiple of SCGs becomes attractive to study in order to obtain more robust and stable power systems.

3）Installation schemes of SCGs with high response excitation in power systems: SMES effect which is expected to improve power system stability need to be taken into account in selection process of SCG location and some specific parameters.

4）Other types of rotating power apparatus: Applications of other types of rotating power apparatus such as SuperVAR dynamic synchronous condenser for power system stabilization can be studied based on this research.

本論文は、「Applications of New Types of Rotating Power Apparatus with Advanced Control Systems for Power System Stabilization −Rotary Type Frequency Converter and Superconducting Generator-（電力系統安定化のための先端的制御システムを備えた新しい回転型電力機器（回転型周波数変換装置と超電導発電機）の適用）」と題し、8章よりなる。

第1章は「Introduction（序論）」で、電力システムの変遷について述べ、その電力システムへの適用が期待されているパワーエレクトロニクス応用電力機器、及び超電導応用電力機器について紹介するとともに、本研究では、回転型周波数変換装置と超電導発電機を採り上げ、それらに対して電力システムの系統安定度向上等を目的とする先端的な制御系を開発することを述べている。

第2章は「Power System Stability, New Types of Rotating Power Apparatus, and Control Methods（電力系統安定度、新しい回転型電力機器とその制御手法）」と題し、電力系統の運用において考慮すべき系統安定度について説明している。次に、パワーエレクトロニクス技術と超電導技術を適用した新しい回転型電力機器（可変速機、回転型周波数変換装置、超電導発電機）を紹介し、これらの特徴、電力系統導入上の利点について述べている。最後に、系統安定度を向上させるために適用する制御手法とパラメータ最適化手法の概要を紹介している。

第3章は「Rotary Type Frequency Converter（RFC）（回転型周波数変換装置）」と題し、可変速発電電動機を用いた回転型周波数変換装置のモデル化を行っている。先ず、可変速機のモデル、等価回路及び回転型周波数変換装置のモデルと等価回路について述べ、回転型周波数変換装置の特徴である従来の静止型周波数変換装置にない回転体の蓄積エネルギーを有効利用することによって、電力融通のみならず、系統安定化機能も持たせることが可能であることを示している。また、回転型周波数変換装置の機械的な連結の部分の軸ねじれ振動現象の問題について述べている。

第4章は「Control System Design of Rotary Type Frequency Converter（回転型周波数変換装置の制御系設計）」と題し、軸ねじれ振動現象を抑制し、系統安定度を向上するための回転型周波数変換装置の制御系を設計している。固有値感度に基づいたパラメータ最適化手法、エネルギー関数に基づいた制御手法、フィードバック線形化制御手法をそれぞれ適用して制御系を設計し、それぞれの制御系の性能、回転体エネルギーの効果的な利用、系統安定度向上、軸ねじれ振動抑制への貢献をモデル系統において検討し、いずれの制御系でも効果的に抑制されることを確認している。特に、フィードバック線形化制御系が最も効果的であることを示している。

第5章は「Superconducting Generator（超電導発電機）」と題し、小型軽量、高発電効率などの特長を持つ超電導発電機について、構造、特徴及び系統導入上の問題などを述べ、本解析で用いる超電導発電機の等価回路（低速応励磁型と超速応励磁型）を説明している。更に、超速応励磁型超電導発電機に特有で系統安定度向上に期待できる回転子超電導巻線のSMES効果のモデル化について述べている。

第6章は「Control System Designs of Superconducting Generators with High Response Excitation（超速応励磁型超電導発電機の制御系設計）」と題し、系統安定度を向上するためのSMES効果を考慮した超速応励磁型超電導発電機の励磁制御系を設計している。固有値感度に基づいたパラメータ最適化手法、エネルギー関数に基づいた制御手法、フィードバック線形化制御手法をそれぞれ適用し、合わせて無効電力制御系も設計している。この励磁制御系の性能をモデル系統において検討し、SMES効果を効果的に利用して系統安定度を向上させ、電圧調整能力も向上できることを確認している。特に、エネルギー関数に基づいた制御系（SMES効果の無効電力制御機能付き）が、最も効率的であることを示している。

第7章は「Installation Schemes of Superconducting Generators in Power System（超電導発電機の電力系統導入手法）」と題し、系統安定度向上を目的として、電力システムの支配固有値を考慮した低速応励磁型超電導発電機の電力系統導入地点、機器パラメータ決定手法について述べている。本研究では、2つの手法を提案しており、手法1は、グローバルな地域間モードとその近似固有値感度を用いて、導入地点を求めるための評価指標を計算し、同期リアクタンスと過渡開放時定数を固有値感度に基づいたパラメータ最適化手法で求めるもので、複数地点候補を評価することができる。手法2は、階層遺伝的アルゴリズム（HGA）を用いて、同時に超電導発電機の導入地点と超電導発電機のパラメータを求めるもので、一つの解が簡単に得られる利点がある。2つの提案手法をモデル系統において検討し有効性を確認している。

第8章は「Conclusions（結論）」で、各章の結論をまとめ、今後の課題を述べている。

以上を要するに、本論文は、パワーエレクトロニクスの可変速技術を適用した回転型周波数変換装置と超電導技術を適用した超電導発電機の二つの最新回転型電力機器に対して、それぞれの特長を活かした様々な先端的な励磁制御系の設計を行い、さらに超電導発電機の最適な系統導入地点の選定を通じて、これらの機器が本来の機能に加えて電力系統の安定度向上に大きく寄与することをシミュレーションによって明らかにしたもので、電気工学上貢献するところが少なくない。

よって本論文は博士（工学）の学位請求論文として合格と認められる。