频监控和控制在基于广域测量系统的电力系统恢复毕业论文外文翻译.doc

外文资料翻译Frequency Monitoring and Control during Power System Restoration Based on Wide Area Measurement SystemFrequency control during power system restoration has not been strongly addressed. Operators are often concerned with the offline sizing of load and generation steps, but, nowadays, the introduction of Wide Area Measurement System (WAMS) makes it possible to monitor the stability of power system online. The constraints of WAMS operation result in some changes in power system frequency control. This paper proposes a novel methodology for frequency control and monitoring during the early steps of power system restoration based on WAMS. Detailed load modeling is achieved based on the static load modeling approach. Power generators modeling is also accomplished utilizing the single machine equivalent of the power system based on PMU measurements. Simulation results of the presented methodology on the 39 bus New England power system clearly show the effectiveness and applicability of the proposed method. The simulation results show that the presented approach has a completely acceptable precision and an outstanding speed with less than 0.05% error. The outstanding speed of the presented approach along with the result precision will result in a great promotion in power system restoration methodologies.1. Introduction The problem of restoring power systems after a complete or partial blackout is as old as the power industry itself. Restoration of a power system after a system blackout is a complex, delicate, and time-consuming problem 1.System restoration after total blackout requires coordination of units, loads and transmission system, and the associated characteristics. Furthermore, various constraints imposed in generating restoration plans must be considered 2.Nowadays, new technologies provide powerful new capabilities in areas such as largescale system analysis, communication and control, data management, artificial intelligence, and allied disciplines. Planning of power system restoration is a combinational problem 2. This issue also involves restrictions and conditions that make it more complicated for operators to judge rendering it. A system to provide answers to this issue with sufficient speed which is appropriate for practical applications has not been developed with such conventional techniques as optimization algorithms.Quick restoration of power system after a blackout is a significant part of system operation. In the early stages of power system restoration, the black start units are of the greatest interest because they will produce power for the auxiliaries of the thermal units without black start capabilities. The black start units are usually those with combustion turbines or hydroelectric units 36.Frequency is an important parameter in power systems, and accurate real-time measured frequency is highly desirable to understand the dynamics of power systems. Throughout the power system restoration, very large steps of power generators loading are prone to result in frequency protection trips and consequently, prolong the whole process of power system restoration.Operators are often concerned with the size of loading steps of power generators but nowadays, the introduction of Wide Area Measurement System （WAMS） which utilizes pharos measurement units （PMUs） leads to online power system monitoring and control. Several major utilities have shown an interest in the synchronous phasor measurement technology application. These include Hydro-Québec, American Electric Power, the New York Power Authority, ´Electricit´e de France （EDF）, and many utilities of the Western Systems Coordinating Council （WSCC）such as Bonneville Power Administration （BPA） and Southern California Edison Company 7, 8.Based on the constraints of WAMS operation, power system frequency control differs from those of the older generations of power system operation and control. The main tasks which can be fully accomplished through WAMS include early recognition of large and small signal instabilities and maximization of load restoration amount 9.It is necessary to make the frequency control system during power system restoration be more effective, which requires the location and the magnitude of all generations and loads. During power system restoration, both generation and load profiles are constantly changing however the conventional approaches for frequency control and protection have only one set point for all scenarios. Offline power generators loading optimization is a common practice for electric power utilities in order to prevent dangerous imbalance between load and generation and strong frequency deviations during power system restorations. Major drawback of conventional approaches for frequency control during power system restoration is that local protection devices do not have a system view, and, therefore, they are not able to take optimized and coordinated actions. Even in the case of frequency control and monitoring, in which the frequency itself is a system index, the actions are taken locally on predefined design rules. Carrying out improper actions during power system restoration, especially in the early stages, will prolong the overall process. Generators loading is one of the most important parameters should be managed considering power system operational constraints, load characteristics, and so forth. Offline scheduling of generators load pickup could not guarantee that the actions will not cause further problems.A necessity exists to develop a frequency control and monitoring approach during power system restoration based on WAMS that can customize frequency control algorithms dynamically in response to any system condition. Proper coordination of power system operation characteristics, especially WAMS characteristics encountering power system frequency control and monitoring, and power system restoration planning is the main objective of the paper.This paper presents a systematic method for power system frequency monitoring and control during power system restoration based on WAMS. The presented approach which consists of detailed load modeling and power generators loading optimization is to prevent dangerous imbalance between the load and generation and strong frequency deviations through power system restorations with a practically acceptable speed and accuracy. Compared with the conventional power system restoration approaches which utilize offline power generators loading optimization, the presented approach optimizes the generators loading just based on the current state of power system which results in a safe, smooth, and quick restoration.Utilization of WAMS provides the operation system prediction of a practically precise load and generation modeling; in order to achieve detailed load model, static load modeling approach is utilized. Power generators modeling is also accomplished utilizing the single machine equivalent of the power system based on PMU measurements. Such a high degree of precision in load and power generation modeling could not be achieved without using WAMS.Proper coordination of load and power generation models leads to a reasonable estimation of the active power imbalance and the steady-state frequency. Using the same model, the amount of load pickup or generation increase required to keep the frequency within the allowable ranges can be calculated. Power generators are studied based on the associated classical models. The generators internal voltage, reactance and rotor phase angle are estimated using PMU measurements. The mechanical power, inertia constant, and damping constant can be obtained by a least square error fitting on swing equations of the multimachine system.The proposed approach for frequency control and monitoring during the early stages of power system restoration considering WAMS approach consists of a couple of steps: preparation of the single machine equivalent model based on PMU measurements and estimation of the active power imbalance and predicting the steady-state frequency. The same approach can be successfully utilized to determine the amount of load pickup or generation increase required to maintain the frequency. The model is highly applicable to assess frequency deviations during power system restoration planning when there is not enough time to use detailed power system frequency control modeling.The proposed approach for power system frequency monitoring and control during power system restoration assists power system operator during the restoration process. Since the restoration should be safe, smooth, and quick, such an approach should be able to quickly and precisely predict frequency oscillations and adjust generators loading so that the operational risks be as minimum as possible. Within the early stages of power system restoration, in which there are a few online power generators, subsystems are prone to be unstable. Improper power system frequency monitoring and control, especially within such stages, will prolong the overall process of power system restoration.Simulation results of the presented methodology on the 39 bus New England power system 10 clearly show the effectiveness and applicability of the proposed method. It is noteworthy that the results show that the presented approach has a completely accepted precision and an outstanding speed. Compared with the well-known power system modeling software packages, power system frequency prediction is accomplished with less than 0.05% error. The outstanding speed of the presented approach along with the result precision will result in a great promotion in power system restoration methodologies.The rest of the paper is organized as follows. Section 2 describes the concepts and basic formulation of frequency monitoring and control. Section 3 presents the result of power system parallel restoration planning based on WAMS capabilities for the New England 39 bus New England power system which is taken into account for further simulations. The proposed frequency monitoring and control approach and detailed load modeling approach are presented in Section 4. This Section also presents and discusses simulation results over the New England 39 bus New England power systems. The conclusion drawn from the study and also the road map for future works are provided in Sections 5 and 6, respectively.2. The Problem of Frequency Monitoring and ControlFrequency stability is a major concern in the operation of power systems. Following severe disturbances, such as insertion of a load block or increase of mechanical power of a large generation station, the average system frequency will change. If the frequency drop is not arrested before the frequency reaches 4748Hz in a 50Hz system, thermal units are tripped to avoid damage from prolonged under frequency operation and this worsens the situation 11. In these situations, it may be necessary to disconnect loads to preserve system integrity. Typical threshold values are 4848.5Hz for a 50-Hz system 11.It is known that the active power mismatch following the addition of mechanical power of a generator or insertion of a load step can be calculated from the initial rate of change of the frequency and the system inertia constant according to (2.1) 12: (2.1)where，andare the active power mismatch, the system inertia constant which is the sum of all generators inertia constants, and the per unit angular velocity, respectively.It is known that the initial rate of frequency change is proportional to the power imbalance and it also depends on the electric power system inertia. Also, the values of the minimum frequency and the new steady-state frequency which are reached during the transient process of generator loading are proportional to the power imbalance and depend on the dynamic properties of power system equipment and loads.The active power mismatch can be used as an indication for the amount of active load that should be restored in order to provide the frequency establishment above the target value, for example, 48.5Hz, or the amount of power generation increase in order to provide the frequency establishment below the target value, for example, 53 Hz. It is noteworthy that power generators are usually equipped with protective relays or other types of protective devices to trip them off line if the frequency either exceeds 106% or drops below 97% 13.The proposed approach for frequency control during power system restoration based on WAMS is based on frequency stability prediction; WAMS capabilities avoid the drawbacks of conventional approaches. Also, decision making about control actions in the proposed approach is based on online measurements through PMUs and WAMS instead of on conservative offline assumptions.In order to maintain the frequency within given bounds, the single machine equivalent model which is derived online is utilized for power system frequency control and monitoring and also to calculate the necessary load pickup or generation increase amount in the early stages of power system restoration. It is noteworthy that the presented approach is completely applicable in the case of generation-rich situations where over-frequency occurs and control actions increase power system loading consequently as well as the cases of under frequency occurrence.The system inertia constant may intensely change at different stages of power system restoration. The presented approach extracts the inertia constants of online generators from WAMS; the loads frequency and voltage sensitivities are estimated online, during the initial stages following a disturbance, and are included in the system model on which the frequency control and monitoring approach is based.3. Power System Parallel Restoration Based on WAMS ConstraintsPower system restoration is the procedure of restoring generators, transmission lines and loads in a minimum time without causing damages to power system equipment and customers. There are different strategies for restoration of a power system. A couple of these strategies is introduced in 1; “build-down” strategy of reenergizing the network before desynchronizing generators, and “build-up” strategy of restoring separated parts, called islands, and then they will be mutually interconnected.In many systems the latter, parallel restoration, is advantageous because of a remarkable reduction in restoration duration and blackout costs. The more the total number of islands rises, the shorter the net duration of power system restoration. This “build-up” strategy leads to a power system restoration process which leads to an observable restoration which satisfies the associated power system operation constraints. Also, based on the constraints of WAMS operation, each island must be fully observable. In this section, an introduction to