Large-scale Power System Restoration

The need to securely and fast recover a collapsed bulk power system has become increasingly important. The dissertation investigates difficulties in restoration practices and relative restoration theories. The leader-follower strategy is introduced to hierarchical restoration decision-making for large-scale power systems and an agent-based restoration decision support system is developed to aid dispatching. The practical Shandong Power System has been studied for the application of the decision support system. The results demonstrate that the method and technology are feasible and effective.Main contributions in the dissertation are described as following:1. A leader-follower hierarchical restoration solution for blackout is presented. The train of thought of leader-follower hierarchical restoration is proposed according to the level characteristic of power system dispatching management in China as well as power system restoration characteristics. The black start procedure is defined as six kinds of conditions. The formulation standard and main operating points of black start plan are briefly given. A series of black start schemes are formulated for dispatching units belong to different level in Shandong Power System. The superior dispatching unit coordinates the schemes of subordinate dispatching with responsibility for unification. Based on local black start condition, each of dispatching center, power plant and substation gives detailed sequences of operation and the time limit for state transfer between the six kinds of conditions. It guarantees the black start schemes be applicable under each kind of situation after blackout. Thus, the feasibility of the black start schemes is enhanced.A hierarchical case-based reasoning (HCBR) methodology is proposed for power system restoration decision-making. The structure of hierarchical case database is defined and the practical methods of retrieval, evaluating and learning are presented. In contrast to conventional CBR methods, restoration problem is not represented by single full cases, but organized by collections ofcases at different levels. A flexible combinatorial strategy reduces the case bases scale and makes the HCBR possible to solve a wide range of restoration problems. An HCBR-based decision-making software is developed for the practical Shandong Power System.2. The leader-follower hierarchical decision theory is introduced to power system restoration. A leader-follower hierarchical decision model is proposed for distributed restoration after blackout. This model includes a leader-follower hierarchical restoration decision-making structure and coordination based on commitments and conventions. In the leader-follower hierarchical restoration decision-making structure, each of the leader decision level and the follower decision level has respective state space, state vector, optimal objective and restraints. Each level task is divided into stages by different stage variables. Each stage in upper level is nested with sub-stages of relative sublevel. The uppermost layer of decision-making level is responsible for the global optimization;the decision-making benefit may be calculated from the commitment value of follower level. Each inferior dispatching gives guarantees of its local restoration ability as well as the restoration behavior in form of commitments, and then the higher authority dispatching compiles various local restoration schemes, and forms the entire system restoration plan using proposed optimal method based on the back-tracing algorithm. Through the feasible examination from bottom to top and the goal optimization from top to bottom, the computation burden is reduced. The simulation results show that the proposed decision-making structure fits to large-scale power system hierarchical dispatching and satisfies the coordination of various dispatching demand in different levels.3. A leader-follower hierarchical restoration decision-making system is designed and developed based on multi-agent. The systemic structure of the leader-follower hierarchical restoration decision-making system is designed with agent-oriented technology. The internal function module is constructed for all levels of dispatching agent. Coordination mechanisms for dispatching agents are designed. Dispatching agent acquires the original state of local systemthrough the sensation unit, forms the initial restoration scheme by the decision-making unit, and provides the restoration scheme formatted as commitment to superior dispatching agent. According to the restoration schemes from sublevel, the superior dispatching agent optimizes the global restoration schemes. Each inferior dispatching agent carries on the revision to the restoration schemes of superior dispatching according to the local details, and returns the result to the superior dispatching agent. The interactive processes among different levels may last several times. Finally, both the leader and follower achieve consistently, and make a convention about restoration operation. If an accident happens in the procedure of restoration operation and the prerequisite of the convention is no longer satisfied, the relative dispatching agent breaks the convention and modifies its commitment. From the commitment, the behavior of dispatching may be foreseen. The convention, which is susceptive of the accident in the restoration process, provides the flexibility for cooperation between the leader and followers in the dynamic environment. Based on the coordination mechanisms of multi-agent system for the leader-follower hierarchical decision-making, the restoration decision-making system can not only effectively deal with coordinate dispatching in the distributed decision-making process, but also nimbly deal with the accident in the restoration process.4. The generator field winding overvoltage in black start process is studied. The field current transient characteristics are analyzed when generators energize transmission lines under no-load condition. A transient equivalent circuit of generator is proposed for studying the effect of reverse resistance to the field loop. The procedure of generator energizing a long line under no-load condition is simulated using the electromagnetic transient program EMTDC. The simulation results show that overvoltages may occur in field circuit and happen easier than the phenomena of self-excitation and switching surge overvoltages.