Real-time Power System Operation. Energy Management Systems. Introduction

Transcription

1 1 Real-time Power System Operation Energy Management Systems Introduction The time frame for Power Systems Operation varies from a few seconds to a week. To assist with the operation of the system, modern power systems are equipped with two control centers, one for the operation of generation and transmission and one for the distribution system. These control centers are referred to as the Energy Management Systems (EMS) and the Distribution Management Systems (DMS). The EMS may consist of several control center hierarchy. The prime goal of EMS and DMS systems is the secure and economic operation of power systems to ensure uninterrupted, safe flow of power to customers at minimum cost as shown in Figure 1. Another essential responsibility of these systems is Control. To achieve economy, resources have to be used wisely. To ensure the achievement of security and economy the inter-relation given in Figure 2 is used. The control responsibility can be divided to frequency and the line control and other directly exerted controls such as load, generation rejection control. In the following sections an EMS system is described in detail. 2.0 EMS System An overview of an EMS System can be broken down to the following three components: Functionality Hardware overview Software overview

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4 4 2.1 Functionality, Overview The primary function of an EMS is to provide the control supervisor accurate picture of the bulk system operation and also to provide the necessary tools for studying future operating strategies. The following basic activities are performed. A) Data Acquisition B) Data Processing C) Data Display D) Control Aids A. Data Acquisition EMS obtains information on the current operation condition of power system by continually gathering data at key locations. These data such as MW values or circuit breaker positions are transmitted to the System Control Center (SCC) over a telemetry system. The acquisition and telemetering activities are performed cyclically every few seconds which enables control supervisors to have as instantaneous a view of the system as possible. In the case of B.C. Hydro and Ontario Hydro the scanning takes place every 2 seconds. B. Data Processing Upon receipt of power systems data at the Control Center the EMS performs the data processing activity. Security Production and Control related functions. These include: Updating of Power System models The Security Assessment of the current system operating condition. The prediction of future state of the power system to determine if it remains secure The determination of optimal economic means of supplying demand The determination of appropriate control signals C. Data Display The power system data and the results of the analysis are made available to Control Supervisor on various display devices. These display devices include color video monitors, a wall diagram which depicts the power

5 5 system network in simplified form and typers. updated on these devices. The data is automatically D. Control Aids Driven By EMS AGC is a good example of a direct control driven directly by an EMS. The EMS using a schedule input by the control supervisor and pertinent telemetered data, computes and issues raise and lower signals to selected generating plants to automatically control and maintain the system frequency and flows on the tier. 2.2 Hardware EMS systems are normally designed to provide a high degree of availability and reliability. Throughout the system, redundancy/duality is built. The hardware systems is comprised of the following subsystems as shown in Figure 3. Data Acquisition Subsystem Computer Subsystem Man/machine Subsystem Remote Manual Data Entry Auxiliary Power Sub System/Uninterruptible Power Supply In the following sections each of these will be described Data Acquisition Subsystem (DAS) The task of collecting and transmitting data to the control center is performed by DAS. Major power system parameters such as active and reactive power, voltage, frequency, water levels, circuit breakers positions are transmitted to the Control Center to update the data base model of the power system. Two main components of this system are the Remote Terminal Units (RTUs) and the Communication Network Subsystem (CNS). Remote Terminal Units The actual collection of data is performed by RTU s. The field instrumentation, such as transducers or pallet switches in breakers are wired to terminal racks which are themselves connected to RTU s at stations. The analog signals from transducers (e.g. quantitative values such as MW) are presented to the RTU as values between - 5,0, +5 DC which are converted to digital values by analog to digital (A/D) converter

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7 7 within the RTU. Status points such as circuit breaker positions are input to RTU as dry contacts (Figure 4). Communication Network Subsystems (CNS) This Subsystem is responsible for transmission of data from the RTU s to the Control Center. It is designed such that no single contingency fault will result in the loss of communication with an RTU Computer Subsystem The Computer Subsystem provides the following functions. 1. SCADA: data acquisition, alarm processing and man/machine (updating of displayed information on CRT s, Wall diagrams and Typers. 2. Database:Storage and retrieval services 3. Processing of Application Programs: namely security and economyapplications. It has been shown that from 1980 to there has been an increase of 1700 percent in the computer power growth in control centres. This explosive growth will continue to occur and the onus will be on system designers to accommodate this type of growth in a manner that avoids replacing the whole system often. The expansion reported between has been predominantly in the network analysis area. Some of the main requirement of this system is The total processing requirement is much higher for network analysis than for all other parts combined. All areas require a high access rate of the data base The SCADA function must respond to a high interrupt load from external system. It is clear that the diverse requirements of these different areas cannot be satisfied by a single computer or by multiple copies of the same computer. In general everything that can be done to improve the performance of a computer (pipeling, catching, etc.) makes it less effective at processing interrupts. SCADA load characteristically consists of a large number of small quick tasks. The growth in SCADA takes the form of horizontal growth replicating components to add more CRTs and RTU s and growth in

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9 9 network analysis takes the form of vertical growth expanding the size of the computer used for network analysis. To accommodate growth therefore, the SCADA portion should be expanded incrementally as new devices are added and the network analysis computers must be upgraded or replaced to accommodate additional functions and bigger power system models. Because of these different requirements, different architectures have appeared. The main architectures are; Centralized Clustered Distributed 1. Centralized Architecture This system as shown in Figure 5, consists of a pair of mainframe computers and some number of micro processors connected to the main frame. The micro processors perform low level data acquisition and display support functions leaving the bulk of SCADA in the main frame along with application programs. This design addresses the problem of handling a large number of interrupts, the micro processors absorb some of them but does not allow for expansion of SCADA processing capability independent of network analysis. The maintenance is simple. Figure 5: Centralized Architecture

10 10 2. Clustered Architecture This system as shown in Figure 6, consists of a number of similar computer systems used for different tasks and intelligent storage device. This architecture does not handle problems with interrupts. The main limitation of this system is limitation on the intelligent device transaction rate and expandability of SCADA. Figure 6: Clustered Architecture 3. Distributed System This system as depicted in Figure 7, is comprised of two main frames front ended by micro computers in two levels. The first level performs high level SCADA function like alarm processing.

11 11 Figure 7: Distributed System This architecture provides both horizontal and vertical expansion. Horizontal expansion is achieved by adding single 386 processors and CPUs to 386 multiprocessors vertical expansion is provided by extremely wide range of main frames. The prime advantage of this architecture is that it decouples the major parts of an EMS so that they do not compete for resources (alarm processing/network analysis). Finally Table 1 summarizes the characterises of each architecture. System Expandability Architecture Responsiveness Horizontal Vertical Maintainability Centralized Poor Poor Good Good Clustered Poor Fair Poor Good Distributed Good Good Good Fair Table 1: Characteristics of Different Architectures

12 Man/Machine Subsystem The information collected and processed by EMS is made available to control supervisor via Man/Machine Subsystem. The MMS as shown in Figure 8, consists of several consoles, wall diagram, strip chart recorders, typers and hard copy device. 2.3 Software Overview The EMS Software consists of various system software and application software. Here we only discuss application softwares, which are used for monitoring and merging the bulk electricity system. The make up of a typical application program is shown in Figure 9. In summary application programs are classified into three categories of Security Applications, Production and Control Application and Data Logging Security Applications The security application programs fall under 4 major categories. 1. Central Network Modeling 2. Security Assessment 3. Simulation 4. Remedial Action Central Network Modeling The main function of these programs is to maintain an On-line Power System model up-to-date for use by the security application programs as given in Figure 10.

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16 16 This software consists of Network Configuration software State Estimation Network Reduction Program Network Configuration Software This program is responsible for maintaining a single line diagram model of the bulk electricity supply as shown in Figure 11. Estate Estimation This program uses a mathematical technique to compute a constant set of MW, MVar, kv values for the entire power system. This technique is able to produce a set of data that is on the whole more accurate than the individual telemetered measurements. It relies on the redundant metering that is present throughout the BES. Conceptually, the state estimation performs the following steps: Bad data is detected using statistical laws and a replacement value is determined. Network Reduction This is used to reduce the unobservable for security assessment and simulation software Security Assessment This software includes the following programs: Static Security Assessment - Voltage - Thermal Power Dynamic Contingency Assessment - transient stability assessment - using off-line tables - on line calculation - voltage stability limits - using off-line calculated tables - on line calculations Activation of Reserve Simulation

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18 18 This software checks the acceptability of operating reserve distribution for a number of predefined contingencies. Although there may be enough reserve to meet the magnitude requirements, this reserve could be so distributed that the transmission limitations would not afford the reserve to be activated. The steps for reserve activation are: 1. Simulate Generation Contingencies 2. Activate Operating Reserves 3. Check if any limits violated 4. Use an OPF like program to remove violations Line and Transformer Thermal Limit Calculator - weather conditions - maximum design temperature - operating temperatures Simulation Programs These programs provide assistance to Control Supervisor and Outage Scheduler in planning and directing the operation of the system by assessing the security of contemplated control action either on the existing system or an simulated operating conditions (one hour to a week) Remedial Analysis Software These programs are intended to assist the control supervisor in arriving at appropriate remedial control actions to correct for any security violation both existing and simulated operating conditions. The controls for this software could include: generation rescheduling, transferor taps and others Production and Control Applications These programs intend to: Provide assistance to the production scheduler and supervisors in forecasting system demands and scheduling hourly generation from day to day within authorized security constraints minimizing the total costs.

19 19 Automatically regulate the power system frequency and net the line loading Provide information on the economic aspects. Automatically produce reports. The following program categories exist: Forecasting and Scheduling Automatic Generation Control Production Aids Data Logging Forecasting and Scheduling This main function of this program is to forecast the demand and then coordinate the resources and develop hourly schedules for power generation taking into account various constraints. This software normally includes times such as: Load Distribution Prediction This software predicts hourly load at major buses Primary Demand Forecast Forecast model employs weather/load sensitivity coefficients using a forecast model based on a similar hour technique. Generation Scheduler This package is optimization based giving Optimal hourly schedules on all generating units taking into account constraints Automatic Generation Control This program accepts from the supervisor the tie line schedules and computes Control Signals for the regulating plants to man the system frequency and the net schedule interchanges. A program detects and derives instantaneous demand/generation mismatch called ACE (Area Control Error) and produces signals for regulating plants. These control signals take priority over the economic correction signals to retain units at equal incremental cost of operation Production Aids

20 20 These programs produce decisions to produce in house energy or buy from neighboring utilities. Examples of these programs are: - Interconnection Cost Quotation - Production Monitor - Hydraulic Plant Resource Calculator Inter Connection Cost Quotation This program computes the incremental cost of all purchases or sales with foreign power systems. Production Monitor This program monitors parameters to trigger the execution of scheduling/forecasting programs. Hydraulic Plant Resource Calculator This program provides the actual and scheduled hydraulic resource usage Data Logging These programs collect compute or extract data to produce the necessary on-line and historical data summary displays and reports. Examples of the summaries provided are: - Instantaneous primary demand displays - Hourly plant output - Instantaneous unit statistics and limitations