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1 ISSN Available online at International ejournals International Journal of Mathematical Sciences, Technology and Humanities 21 (2011) ENERGY CONTROL CENTER FUNCTIONS FOR POWER SYSTEM 1 1 Department of Electrical & Electronics Engineering and Dean Academics, Hirasugar Institute of Technology, Nidasoshi , Karnataka, India hitdean@rediffmail.com ABSTRACT This paper presents the basic security concept, overview of Energy Control Center (ECC) state of the art, characteristics, functions for power system, Today s Energy Management System (EMS) is for the supervision and control of electric power systems. The responsibility today for supervising and controlling an electric power system is largely centralized. A small number of power system energy control centers each remotely control large number of power stations and substations. Keywords: Energy Control Center (ECC), Energy Management System, SCADA and RTUs. 1. Introduction Energy Control Center (ECC)) is designed and built for system operation. In order to have an efficient power system operation and control, various control centers have to be operated in the hierarchical level. The following Table-1 shows the level decomposition of control centers in the power system. There are 4 types of control centers. i) Local Control Centre ii) Area Load Dispatch Centre iii) State Load Dispatch Centre iv) Regional Control Centre Table-1. Level Decomposition of Control Centers Level System Monitoring & Control First Generating Stations, Substations Local Control Centre Second Sub transmission & Transmission Area Load Dispatch Centre Network Third Transmission System State Load Dispatch Centre Fourth Interconnected Power Systems Regional Control Centre Many functions being done in system control centres for operation planning purposes. These are run in an off-line or via a remote terminal linked to a large computer centre. 1.1 Local Control Centre A number of control functions are performed locally at the power stations and substations. The typical controls carried in the local control centre are,

2 206 Load Monitoring and Control Protection, Circuit Breaker to re-close Voltage Regulation Capacitor Switching Feeder Synchronization Load Shedding Network Restoration and Network Re-configuration etc. 1.2 Area Load Dispatch Centre In this, groups of generating stations or substations control are carried out. This requires system data a network topology for control. The area control center receives information and processes for appropriate control actions. 1.3 State Load Dispatch Centre In this centre, all the information is received from area load dispatch center and local centers. Then minute-to-minute operation of the power system at the state level is carried out. It may have the following functions, System Generation and Load Monitoring and Control for Demand Control. System wide State Monitoring and Control Circuit Breaker Condition Monitoring and Control Load Shedding and Restoration Supervisory Control for Transmission Lines and Equipments. System Alarm Monitoring and Corrective Actions. Planning and Monitoring of Power System Operation 1.4 Regional Control Centre The Regional Load Dispatch Centre may be regarded as a coordinating and monitoring center for state level load dispatch center with covering main objectives: Integrated Operation of State Level Dispatch centre Operation and Maintenance Schedules for maximum capacity Utilization Operation and Maintenance Schedules for Generating Plants Monitor and Control of inter-state Power Transactions Monitor and Control of inter-regional Power Transactions 2.0 Supervisory Control Systems Today s Supervisory control systems normally consist of a computer system and a number of remote terminal units (RTUs), located in the power stations and substations. Communications between the RTUs and the computer usually take place via radio links or via power line carrier equipment. The computer serves the personnel in the control center by presenting information about the current status of the power system and by passing on manually initiated control actions. Automatic control is used for different sub functions, e.g. sequential control and control of the network frequency. Video display units and keyboards are normally used for the man machine communications. In addition, map boards; printer and analog recorders are installed as required. Major supervisory control systems generally consist of several energy control centers, which cooperate in a hierarchical configuration. The basic functions in a supervisory control system consist of Acquisition from the power system of telemetered data, indications and other variables

3 207 Simple arithmetic and logic operations on these variables. e.g. calculations of the variables e.g. calculation of the apparent power P 2 +Q 2.Where P & Q are telemetered values Supervision of acquired and calculated variables with respect to changes and violation of limit values Storage of current variables for sequent use. e.g. in trend curve daily reports Presentation of acquired calculation, stored variables on video display units and other media. These variables may be presented in the form of one line diagram or curves Transmission of commands to RTUs Systems incorporating primarily these basic functions are traditionally called SCADA (Supervisory Control and Data Acquisition) systems. A characteristic feature of these systems is that they treat individual variables, which corresponds to transducers and actuators in the process. This means that the system doesn t need models of the power system as such. A SCADA system can consequently be built up to a large extent from standard software, which is not dependent on the fact that the process being treated involves a power system. Combining the variables available in a SCADA system with a model of the power system makes it possible to introduce functions that handle the power system and its components instead of individual measuring points A detailed network model enables one to calculate all power flows in the network, on the basis of a comparatively small number of telemetered data. A Supervisory Control System, which, in addition to the basic SCADA functions, incorporates functions based on a network model, is traditionally called an Energy Management System (EMS) shown in Fig.1 below. The Fig.2 below represents process oriented database structure. (Physical model and the database model)

4 208 The scope of the functions in an EMS system must always be adapted to the power system and operative organization of the customer. A broad spectrum of computing functions is therefore available. Since most of the building blocks consist of software, it is simple to adapt and upgrade the system as required. In addition to network model calculations, EMS comprises standardized solutions for the following functions Production Control (Automatic Generation Control-AGC) Economic Dispatch Calculation (EDC) Contingency Analysis Operators Load Flow (OFL) Load Forecasting Fig. 3 below gives an overview of the program functions available in the system 3. Overview of the State of the Art In simple terms, the goal of system control centre design is the implementation of security control. Security control requires the proper integration of both automatic and manual control functions, i.e. a total systems approach with the human operator being an integral part of the control system design. Security control requires that all conditions of operation be recognized and that control decisions by the man-computer system must be made not only when the power system is operating normally, but also when it is operating under abnormal conditions. 4. Basic Security Control Concepts The power system may be assumed as being operated under two sets of constraints: load constraints and operating constraints. The load constraints impose the requirement that the load demand must be met by the system. The operating constraints impose maximum or minimum operating limits on system variables and are associated with both steady state and stability limitations. Mathematically, the load constraints can be expressed in the form of the familiar load flow equations. The operating constraints can be expressed in the form of inequalities such as an equipment loadings, bus voltage, phase angle differences, generator real and reactive powers etc. The conditions of operation can than be categorized into three operating states Normal (or Preventive) State Emergency State Restorative State

5 209 A system is in the normal state when the load and operating constraints are satisfied. It is reasonable to assume that in the normal state the power system is in a quasi-steady-state condition. For any given time, the intersection of the load constraints and the operating constraints defines the space of all feasible normal operating states. The power system may be operated any where in this space. A system is in the emergency state when the operating constraints are not completely satisfied. Two types of emergency may be noted. One is when only steady state operating constraints are being violated, e.g. an equipment-loading limit is exceeded or the voltage at a bus is below a given level. The other is when a stability operating constraint is violated and as a result of which the system cannot maintain stability. The first type of emergency may be called Steady State emergency and the second type, dynamic emergency. A system is in the restorative state when the load constraints are not completely satisfied. This means a condition of either a partial or a total system shutdown. In case of a partial shutdown the reduced system may be in an emergency state. This is the start of a cascading situation and, if uncorrected, would lead to a further deterioration of the system. A normal operating point can be classified as being either secure or insecure with reference top an arbitrary set of disturbances or next contingencies. A normal system is said to be secure, i.e. at a secure operating point, if it can undergo any contingency in the next-contingency set without getting in to an emergency condition. On the other hand if there is at least one contingency in the next-contingency set which would bring about an emergency, the normal system would be called insecure. The concept of three operating states breaks up the complex operating problem into three operating sub-problems with different control objects of primary interest and of major impact on the design of system control centers is the control done in the normal state. It is the basically the development and implementation of functions in this area that represent the state of the art in system control centers. Emergency and restorative controls are needed for a complete security control system. 5. General Characteristics of System Control Centers The main features of a modern energy control center are Hierarchical structure consisting of several levels of computer systems. Dual Real-time computers with shared extra memory and peripherals. High speed digital telemetry and data acquisition equipment. System wide instrumentation of electrical quantities and device status. Dynamic wallboard group display. For system operation there were two functions to be implemented i) Area regulation (which we now call automatic generation control) and ii) Economic Dispatch. The totality of real-time features and functions at their centers include the following; Hierarchical Structure consisting of several levels of computer systems. Dual real time processors or multi- processors plus redundant peripherals. High speed digital telemetry and data acquisition equipment. System wide instrumentation of electrical quantities and device status. Colour CRTs with graphics for interactive display. Dynamic wallboard group display. Automatic Generation Control. Economic Dispatch Calculation. Automatic Voltage (VAR) Control. Supervisory Control (breakers, capacitors, transformer taps, generating unit startup and Shutdown. Security Monitoring.

6 210 State Estimation. On-line load flow. Steady State Security Analysis. Optimum Power Flow. Automatic System Trouble Analysis. On-line Short Circuit Analysis. Emergency Control automatic load shedding, generator shedding and line tripping. Automatic Circuit Restoration There is no control system that has all of the functions just enumerated. This is to be expected. Operating problems differ due to different networks and generation resources. Operating philosophy and the structure of operating responsibilities are not the same for all companies. 6. Functions to be Implemented at System Control Centres The functions that are being carried out by the digital computers at the control centers are 6.1 Automatic Generation Control (AGC) The automatic generation control (AGC) function is with very few exceptions the only closed loop control using implemented at system control centers. The sampling time for digital AGC varies from 1 second to 4 seconds. Most control centers send raise and lower signals or MW deviations to the generating units. The basic AGC algorithms, i.e., the calculation of area control error and the assignment of regulation to each unit recognizing the desired base plants, are well known. To apply these algorithms in a system control center requires the addition of modules, which in effect interface with the real time environment. These modules should take care of initializing the AGC function, coordinate all information from other programs which affect AGC, prepare and hand off to the data acquisition subsystem the signals to be sent to the plants and communicate with display subsystem. 6.2 Economic Dispatch Calculation (EDC) Economic Dispatch Calculation is performed every few minutes using the get of coordination equations, which requires that the incremental cost of delivered power from each generating unit to an arbitrary reference point be the same for each unit. The incremental cost of delivered power to a given point from a generating unit is equal to the incremental cost of generated power multiplied by a penalty factor. Traditionally the penalty factors are calculated using transmission loss -constants. The calculation of penalty factors is done on line using a real time optimum power flow. Every time there is a network change or when the system load has changed significantly in magnitude or in relative distribution between areas, the optimum power flow runs automatically and a new set of penalty factors is passed on to the EDC. 6.3 Automatic Voltage/VAR Control (AVC) The AVC regulates the voltage profile and also minimizes losses due to reactive power flow. The control variables are generator reactive powers, transformer taps, shunt capacitors and shunt reactors. The control is a two-step operation. Voltages and VAR flows are checked periodically and when there is any deviation beyond certain tolerances the voltage profile control calculation is initiated. At less frequent intervals the minimum loss calculation and control is executed. 6.4 Security Monitoring (SM) Security monitoring (SM) is the on-line identification and the display of the actual operating conditions of the power system. This one function has made the difference between the

7 211 traditional dispatch center and the modern system control center. SM requires a system wide instrumentation on a greater scale and variety than that required by a center without SM. The types of measurements include; MW and MVAR flows, branch currents, bus voltages, bus MW and MVAR injections, frequencies, energy readings, circuit breaker status or operation counts, manual switch positions, protective relaying operations, transformer tap positions and miscellaneous substation status and alarms. The SM function, in general checks the analog values against limits basically to determine whether the system is close to or at the emergency state. The limit checking also allows some kind of data validation and the rejection of incongruous data, limit checking is done as often as the data is brought in which is usually in the order of every one to a few seconds. The display required for SM entails the use of CRT s and a large number of display formats. The dynamic wall display is also used for SM. Part of the SM function is the online determination of the network topology. In most cases it is sufficient to determine the network configuration. In centers where there is a direct responsibility for transmission switching and safety is a paramount factor, the SM function should include an identification of the electrical status (energized or de-energized) of every physically isolatable segment. 6.5 Static State Estimation (SE) State estimation (SE) may be defined as a mathematical procedure for calculating, from a set of system measurements, a best estimate of the vector of bus voltage magnitude and phase angles of the network. The measurement set is understood to contain an adequate degree and spread of redundancy to allow the statistical correlation and correction of the measurements detect and preferably identify bad data, and yield calculated values for non-telemetered quantities. System control centers with SE in operational use; the measurement set consists of active and reactive line flows, bus injections, and voltage measurements. The SE runs every minute using the last set of measurements which are scanned every second. In addition, SE is started whenever there is a network change. The purpose of the SE is to obtain the vector of bus injections; check for abnormal metering errors. The vector of bus injections is then used by a Newton-Raphson on-line load flow for security analysis and for determining closed-loop corrective control for certain line outages. Bad data detection is based on the value of the sum of the squared residuals, i.e., the performance index, J. After two consecutive failures of the J-test procedures are initiated for bad data identification based on the estimation cycle. If this still fails after a few attempts, a logic procedure is initiated for determining network model errors. SE runs every 15 minutes or on request for major system changes. The results of the SE is used for SM (security Monitoring) and the operator is informed of overloads or other critical conditions. The SE results are stored in a historical file for a (7-day) particular period. This data is available to companies whose lines are represented in the SE model. The bad data detection is based on the performance index J. The measurement set consists of real and reactive power flows, real and reactive bus injections and bus voltages. The purpose of the SE is for security monitoring and for security analysis of the power system. The most important aspect of state estimation is bad data identification. This alone could be a worthwhile justification for including state estimation in a system control center. 6.6 On line Load Flow (OLF) An on line load flow (OLF) is one which is used for real time functions such as security monitoring security analysis and penalty factor calculation, and can also be used for study purpose OLF makes use of real-time data. Security Analysis consists of contingency evaluation and corrective action strategy. OLF may be used for one or both of these functions. The OLF require a vector of bus injections. In the general case, the bus injections are calculated from statistical data obtained on-line and some off-line historical information. The purpose of the OLF or more correctly OPF is to establish the base case for security analysis and for penalty factor calculation. The OLF routine will also be used for contingency evaluation in security analysis.

8 212 The Dc load flow is used as the OLF. This is used for contingency evaluation in security analysis. The on-line load flow is a necessary function for system control centers. 6.7 Steady State Security Analysis (SA) The first function of security analysis (SA) is to determine whether the normal system is secure or insecure. The second function is to determine what corrective action strategy should be taken when the system is insecure. The first function is commonly known as contingency evaluation since, by definition, the security of a system is determined with reference to a set of next contingencies. In present state of-art, only steady state contingency evaluation is done at system control centers. That is the emergency condition that is to be avoided is overloading of equipment or poor bus voltages. The second part of security analysis is Corrective Action Strategy. If the system is insecure, can be made secure? If so, how and at what cost? Conditions improve? Suppose the system is now insecure and there is no way of making it secure, how much load would be shed. 6.8 Optimum Power Flow (OPF) An optimum power flow (OPF) is a steady state solution to an optimization problem where the load flow equations and limits on system variables and on functions of these variables constitute the set of constraints. The OPF is used with the OLF as a subroutine to produce a real time base case for penalty factor calculation and eventually for security analysis. What is needed at a system control center is an OPF which can be used for determining corrective action strategies. 7. Conclusion This paper discusses the basic security concept, level decomposition of energy control centers (ECC), characteristics and its functions for power system. The responsibility today for supervising and controlling an electric power system is largely centralized. The energy management system (EMS) is for the supervision and control of electric power systems. A small number of power system energy control centers each remotely control large number of power stations and substations. References [1] Energy Control Center Functions for Power Systems ASEA Journal Vol.5 PP , [2] Prabha Kundur Power System Stability and Control McGraw Hill 1994.