Chapter1
Power System Control:An Overview
This introductory chapter provides a general description of power system control. Fundamental concepts/definitions of power system stability and existing controls are emphasized.The role of power system controls(using automatic processing and human operating)is to prerve system integrity and restore the normal operation subjected to a physical(small or large)disturbance[1].In other words,power sys-tem control means maintaining the desired performance and stabilizing of the sys-tem following a disturbance,such as a short circuit and loss of generation or load.
From the viewpoint of control engineering,a power system is a highly non-linear and large-scale multi-input multi-output(MIMO)dynamical system with numerous variables,protection devices and control loops,with different dynamic respons and characteristics.The term power system control is ud to define the application of control theory and technology,optimization methodologies and expert and intel-ligent systems to improve the performance and functions of power systems during normal and abnormal operations.Power system controls keep the power system in
a cure state and protect it from dangerous phenomena[1,2].
蜂拥而上的意思
1.1A Brief Historical Review
Power system stability and control wasfirst recognized as an important problem in the1920s[3,4].Until recently,most engineering efforts and interests have been concentrated on rotor angle(transient and steady state)stability.For this purpo, many powerful modelling and simulation programs,and various control and protec-tion schemes have been developed.A survey on the basics of power system controls, literature and past achievements is given in[5,6].
Frequency stability problems,related control solutions and long-term dynamic simulation programs have been emphasized in the1970s and1980s follow-ing some major system events[7–10].Uful guidelines were developed by an IEEE working group for enhancing power plant respon during major frequency disturbances[11].
H.Bevrani,Robust Power System Frequency Control,Power Electronics1 and Power Systems,DOI10.1007/978-0-387-84878-51,
c Springer Science+Business Media LLC2009
21Power System Control:An Overview Since the1990s,supplementary control of generator excitation 白色风信子花语
systems,static V AR compensator(SVC)and high voltage direct current(HVDC)converters are increasingly being ud to solve power system oscillation problems[5].There has also been a general interest in the application of power-electronics-bad controllers known asflexible AC transmission system(FACTS)controllers for the damping of system oscillations[12].Following veral power system collaps worldwide [13–15],in the1990s,voltage stability attracted more rearch interests.Powerful analytical tools and synthesis methodologies have been developed.
Since the1980s,veral integrated control design approaches have been de-veloped for power system oscillation damping and voltage regulation[16–19]. Recently,following the development of synchronized phasor measurement units (PMUs),communication channels and digital processing,wide-area power system stabilization and control have become areas of interest[20,21].Attempts to improve data exchange and coordination between the different existing control systems[22], as a wide-area control solution,are considered as an important control trend.
In a modern power system,the generation,transmission and distribution of electric energy can only be met by the u of robust/optimal control methodolo-gies,infrastructure communication and information technology(IT)rvices in the designing of control units and supervisory control and data acquisition system (SCADA)centres.Some important issues for power system control solutions in a n
ew environment are appropriate lines of defence[21],uncertainties consideration and more effective dynamic modelling[23],asssments/predictions and optimal allocations and processing of synchronized devices[24],appropriate visualizations of disturbance evaluations,proper consideration of distributed generation units[25] and robust control design for stabilizing power systems against danger phenom-ena[26].
温酒斩华雄的故事Considerable developments have recently been made on renewable energy sources(RESs)technologies.The increasing penetration of RESs has many tech-nical implications and rais important questions,as to whether the conventional power system control approaches to operate in the new environment are still ade-quate.Recently,there has been a strong interest in the area of RESs and their impacts on power systems dynamics and stability,and possible control solutions[27–31].
1.2Instability Phenomena
The most recent propod definition of power system stability is[32]“the ability of an electric power system,for a given initial operating condition,to regain a state of operating equilibrium after being subjected to a physical disturbance,with most system variables bounded so that practically the entire system remains intact”.
As the electric power industry has evolved over the last century,different forms of instability have emerged as being important during different periods.Similarly, depending on the developments in control theory,power system control technology and computational tools,different control synthes/analys have been developed.
1.2Instability Phenomena3
Fig.1.1Different phenomena that lead to power system instability
Power system control can take different forms and is influenced by the instabilizing phenomena.Conceptually,definitions and classifications are well founded in[32]. As shown in Fig.1.1,important phenomena that lead to power system instability are rotor angle instability,voltage instability and frequency instability.
每日英语Rotor angle instability is the inability of the power system to maintain synchro-nization after being subjected to a disturbance.In ca of transient(large distur-bance)angle instability,a vere disturbance does not allow a generator to deliver its output electricity power into the network.Small signal(steady state)angle insta-bility is the inability of the power system to maintain synchronization under small disturbances.The considered disturbances must be small enough that the assump-tion of system dynamics being linear remains valid for analysis purpos[1,32–34].
The rotor angle instability problem has been fairly well solved by power system stabilizers(PSSs),thyristor exciters,fast fault clearing and other stability controllers and protection actions such as generator tripping.
Voltage instability is the inability of a power system to maintain steady accep-tance voltages at all system’s bus after being subjected to a disturbance from an assumed initial equilibrium point.A system enters a state of voltage instability when a disturbance changes the system’s condition to make a progressive fall or ri of voltages of some bus.Loss of load in an area,tripping of transmission lines and other protected equipments are possible results of voltage instability.
Frequency instability is the inability of a power system to maintain system fre-quency within the speci
fied operating limits.Generally,frequency instability is a result of a significant imbalance between load and generation,and it is associated with poor coordination of control and protection equipment,insufficient generation rerves and inadequacies in equipment respons[35,36].
The size of disturbance,physical nature of the resulting instability,the dynamic structure and the time span are important factors to determine the instability form [1].The above instability classification is mainly bad on dominant initiating phe-nomena.Each instability form does not always occur in its pure form.One may lead to the other,and the distinction may not be clear.
41Power System Control:An Overview
Fig.1.2Progressive power system respon to a rious disturbance
As shown in Fig.1.2,a fault on a critical element(rious disturbance)may influence much of the control loops and the equipments through different channels, andfinally,may affect the power system performance and even stability[1].
诫子书教学设计Therefore,during frequency excursions following a major disturbance,voltage magnitudes and powerflow may change significantly,especially for islanding condi-tions with under-frequency load shedding that unloads the system[3].In real power systems,there is clearly some overlap between the different forms of instability, since as systems fail,more than one form of instability may ultimately emerge[5]. However,distinguishing between different instability forms is important in under-standing the underlying caus of the problem in order to develop appropriate design and operating procedures.
心理讲座1.3Controls Configuration5
Fig.1.3General structure for power system controls
1.3Controls Configuration
Power system controls are of many types including[1,21,37]generation excitation controls,prime mover controls,generator/load tripping,fast fault clearing,high-speed re-closing,dynamic braking,reac
tive power compensation,load–frequency control,current injection,fast pha angle control and HVDC special controls.From the point of view of operations,all controls can be classified into continuous and dis-continuous controls.A general structure for a power system with the main required control loops in a clod-loop scheme is shown in Fig.1.3.
Most of continuous control loops such as prime mover and excitation controls operate directly on generator units and are located at power plants.The continuous controls include generator excitation controls(PSS and automatic voltage regulator (A VR)),prime mover controls,reactive power controls and HVDC controls.All the controls are usually linear,continuously active and u local measurements.
In a power plant,the governor voltage and reactive power output are regulated by excitation control,while energy supply system parameters(temperatures,flows and pressures)and speed regulation are performed by prime mover controls.Au-tomatic generation control balances the total generation and load(plus loss)to reach the nominal system frequency(commonly50or60Hz)and scheduled power interchange with neighbouring systems.
61Power System Control:An Overview The discontinuous controls generally stabilize the system afte
r vere distur-bances and are usually applicable for highly stresd operating conditions.They perform actions such as generator/load tripping,capacitor/reactor switching and other protection plans.The power system controls may be local at power plants and substations,or over a wide area.The kinds of controls usually ensure a post-disturbance equilibrium with sufficient region of attraction[21].Discontinuous controls evolve discrete supplementary controls[38],special stability controls[39] and emergency control/protection schemes[40–42].
播音发声技巧Furthermore,there are many controls and protections systems on transmission and distribution sides,such as switching capacitor/reactors,tap-changing/pha shifting transformers,HVDC controls,synchronous condenrs and static V AR compensators.Despite numerous existing nested control loops that control differ-ent quantities in the system,working in a cure attraction region with a desired performance is the objective of an overall power system control strategy.It means generating and delivering power in an interconnected system is as economical and reliable manner as possible while maintaining the frequency and the voltage within permissible limits.
1.4Controls at Different Operating States
Power system controls attempt to return the system in off-normal operating states to a normal state.
Classifying the power system operating states to normal,alert, emergency,in extremis and restorative is conceptually uful to designing appropri-ate control systems[1,43].In the normal state,all system variables(such as voltage and frequency)are within the normal range.In the alert state,all system variables are still within the acceptable range.However,the system may be ready to move into the emergency state following disturbance.In the emergency state,some sys-tem variables are outside of the acceptable range and the system is ready to fall into the in extremis state.Partial or system wide blackout could occur in the in extremis state.Finally,energizing of the system or its parts and reconnecting/resynchronizing of system parts occurs during the restorative state.
Bad on the above classification,power system controls can be divided into the main two different categories(1)normal/preventive controls,which are applied in the normal and alert states to stay in or return into normal condition and(2) emergency controls,which are applied in emergency or in extremis state to stop the further progress of the failure and return the system to a normal or alert state.
Automatic frequency and voltage controls are part of the normal and the preven-tive controls,while some of the other control schemes such as under-frequency load shedding,under-voltage load shedding and special system protection plans can be considered under emergency controls.
Control command signals for normal/preventive controls usually include active power generation t points,flow controlling reference points(FACTS),voltage t point of generators,SVC,reactor/capacitor switching,etc.Emergency control mea-
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