(本文)

In recent years, Distributed Generation (DG) has attracted more and more attention from both distribution utilities and electricity users. The advantages of DG are of both engineering and economic view points. The advantageous applications of DG can be summarized as follows: backup generation, voltage regulation, loss reduction, grid expansion postponement, environmental concerns, peak load service, rural and remote application, combined heat and power generation, and financial and trading purposes. This dissertation concentrates on the issue of backup generation and voltage regulation.

As for the first part, DG can be applied as backup generation when the main supply from the upstream side is interrupted. Without the main electricity source, isolated system is formed and obtains electricity supply from the DG located in the isolated area. As backup generation, DG is supposed to improve the system reliability if the related concerns are deliberately considered and the solutions are strictly implemented. However, protection miscoordination, especially recloser-fuse miscoordination, can have a considerable adverse impact on the reliability of typical radial distribution systems. The installation of DG can interfere with the existing recloser-fuse coordination by changes in fault current, and cause improper sequential operation of the protection system. IEEE standard 1457-2003 requires DG to be disconnected during system abnormality, including the case that DG itself operates abnormally, so that the fault current from DG cannot interfere with the existing protection coordination. Although the risk of DG continuing to operate (i.e. failing to be disconnected) and contribute to the fault current is low, it is not zero. Furthermore, it cannot be guaranteed that all DG sources will be disconnected faster than the operation of system protective devices, which usually operate within only a few cycles. Besides, disconnection is not always preferable, especially when the DG penetration is very high. In such cases, the protection systems may be interfered with and even lose their coordination, e.g. the coordination between recloser and fuse. The details of the mentioned facts are explained, studied, and resolved in this research.

Accordingly, the first part of this dissertation pays attention to the impact and contribution of DG to system reliability. To understand the impact on system reliability and the prevention of reliability degradation, the evaluation and comparison of system reliability are carried out for outage cost and load point indices. The two load point indices are SAIFI (System Average Interruption Frequency Index) and SAIDI (System Average Interruption Duration Index). In regards to the energy index, ENS (Energy Not Supplied) is evaluated. In addition, the prevention of reliability degradation due recloser-fuse miscoordination is proposed. The investigation and the application of the proposed methods have been implemented in a typical distribution and RBTS Bus 2 test systems.

As for the second part, DG can be used to regulate or support the system voltage profile if the installation has been well studied and carried out in an appropriate direction. With the varieties of dispersed locations, operating modes, and installed capacities, DG, in contrast, can be properly designed and controlled to help system voltages be maintained within their standard levels. For example, DG with constant voltage has the advantage of voltage support at remote areas. In addition, DG with capacitive power factor can intentionally help increase the voltage profile at low voltage areas. This kind of DG is over-excited synchronous generators. Likewise, DG with inductive power factor can help decrease the voltage profile at high voltage areas. This latter kind of DG is under-excited synchronous generators and induction generators. However, as is known, the installation of DG, especially ones that derive energy from renewable resources, can bring about the voltage fluctuation and violation to distribution systems. For instance, the installation of DG may raise the voltage profile until the upper voltage limit is exceeded, especially when voltage regulator and/or capacitor bank are operating. These problems are derived mainly from the counter flow and reactive power change. On the other hand, the installation of DG may cause the voltage profile to be lower than the lower limit. This is possible when DG is shut down immediately and the existing voltage regulation cannot respond in time. The types and operating modes of DG also play an important role for the reactive power injection or consumption that is substantially related to the voltage variation. The details of the mentioned facts are explained, studied, and resolved in this research.

The second part of this research utilizes the varieties of locations, modes, and capacities to balance and maintain the system voltage profile in conjunction with the existing voltage regulation. As reinforcement for the prevention of voltage fluctuation, the uncertainty of renewable energy sources, e.g. solar and wind, that is considered the main factor of voltage fluctuation is modeled by Probabilistic Load Flow (PLF). Afterwards, this uncertainty is integrated into the optimization process whose objective function is to regulate the voltage profile to prevent the voltage violation and momentary interruptions. The effectiveness of PLF is not only for the reduction of voltage violation, but also for other applications such as the maximization of DG in a distribution system. The investigation and the application of the proposed methods have been implemented in IEEE-34 Bus test system.

本論文は、「Impact and Contribution of Distributed Generation to System Reliability and System Voltages in Distribution Systems(配電系統における分散型電源導入の供給信頼性度および系統電圧に対する影響)」と題し、8章よりなる。

第1章は「Introduction(序論)」で、系統連系された分散型電源(DG)が配電系統に与える影響について述べ、本論文で扱うDGの定義、種類を紹介するとともに、DGと一般の発電機の比較について述べている。

第2章は「Principles for Impact and Contribution of DG to System Reliability(分散型電源の系統供給信頼性へ与える影響と貢献の原理)」と題し、配電系統のリクローザ(再閉路遮断器)とヒューズの保護協調の不具合が、DGを連系する際に、配電系統の供給信頼度上、重大な問題となることを述べている。また、DGのバックアップ発電としてのメリット、そのために必要な配電系統の保護協調について述べ、保護協調の不具合の例について、リクローザとヒューズを流れる事故電流の解析を紹介している。

第3章は「Calculation for Impact and Contribution of DG to System Reliability(分散型電源の系統供給信頼性へ与える影響と貢献の計算手法)」と題し、先ず、リクローザとヒューズの保護協調の不具合の制約条件を、DGの電圧、配電用変電所事故電流、保護協調の設定パラメータの関数として求め、これら保護装置の運転範囲の制約条件とともに、多数台のDGが連系された場合にも適用できるように拡張している。次に、供給信頼性の計算の基礎となる事故電流計算法と保護装置の整定計算法について述べている。そして、供給信頼性の複数の指標を、解析的手法に基づいて高速に計算を行う方法を示し、リクローザとヒューズの保護協調の不具合を回避して信頼性が下がることを防ぎ、配電系統に連系するDGの容量を最大化するための最適化手法を提案している。

第4章は「Numerical Simulation for Impact and Contribution of DG to System Reliability(分散型電源の系統供給信頼性へ与える影響と貢献の数値シミュレーション)」と題し、モデル系統に対して前章で述べた計算手法を用いて得られた数値シミュレーション結果を示している。まず、リクローザとヒューズの保護協調を考慮して得られたDGの最大連系容量を計算し、その容量以内であれば、DGの配電系統への連系は供給信頼度を向上することができることを、前章で示した供給信頼性指標より明らかにしている。また、配電系統が放射状構成ではなく、ループ構成をしている場合でも、このようなDGの連系は供給信頼性向上に効果のあることを示している。

第5章は「Principles for Impact and Contribution of DG to System Voltages(分散型電源の系統電圧へ与える影響と貢献の原理)」と題し、まず、配電系統ではDGの出力が変動することによって系統電圧が変動することを示している。次に、この系統電圧の変動の理由を述べ、最後に、自然エネルギーを利用するDGの出力の不確実性を確率密度関数で表現することを提案している。

第6章は「Calculation for Impact and Contribution of DG to System Voltages(分散型電源の系統電圧へ与える影響と貢献の計算手法)」と題し、まず、自然エネルギーを利用するDGの出力の不確実性に起因する系統電圧、送電線潮流の変動を計算する確率潮流計算手法について述べ、次に、この電圧変動、送電線潮流変動を用いて、系統電圧を基準以内に維持しつつ制御可能なDGの出力を最大化する手法と配電系統の電圧変動偏差を最小化するDGの最適出力を求める手法を提案している。最後に、電圧変動によって生じる一時的な供給支障(停電)の最大頻度指標(MAIFE)の計算方法を説明している。

第7章は「Numerical Simulation for Impact and Contribution of DG to System Voltages(分散型電源の系統電圧へ与える影響と貢献の数値シミュレーション)」と題し、モデル系統に対して前章で述べた二つの手法を用いて得られた数値シミュレーション結果を示している。その計算結果から、提案した二つの手法が有効であることを確認している。また、電圧変動による一時的な供給支障の最大頻度指標の計算結果より、自然エネルギーを用いたDGの出力の不確実性を考慮して制御可能なDGの出力最適化を行うことで、配電系統の供給信頼性が改善されることを明らかにしている。

第8章は「Conclusions(結論)」で、各章の結論をまとめている。

以上を要するに、本論文は、配電系統に連系された分散型電源が供給信頼性と系統電圧分布に与える影響を、配電系統の保護協調や自然エネルギー利用分散型電源の出力変動を考慮して確率的に評価し、分散型電源の連系容量の限界を算出する手法を提案し、制御可能な分散型電源の連系により配電系統の電圧品質と供給信頼性が向上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。