Supervisory Systems

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Supervisory SystemsSupervisory Systems

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Supervisory Systems: Introduction

Purpose:

• To provide the user with the capability to exercise control over a specific device.

• To confirm its performance in accordance with the directed action.

• The commonly used name is SCADA.

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Supervisory Systems in Power Systems

Objective:

• To provide system operators with efficient information and control capabilities.

• To operate the power system in a safe, secure and economic manner.

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Typical Supervisory System

Modem

Modem

Us erInterfac e

AnalogO utput

D igitalO utput

In telligentElec tronic

D evic es

Us erInterfac e

AnalogInput

D igitalInput

Communications

M as terS tation

Rem oteT erm inal

Units

Fig.1 Typical Supervisory System

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Types of Supervisory Systems

1. SCADA (Supervisor Control and Data Acquisition)

2. SCADA/AGC

3. EMS (Energy Management System)

4. DMS (Distribution Management System)

5. Load Management System (LMS)

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1. SCADA

• A SCADA system, strictly speaking, is limited to performance of traditional function such as the gathering of data and performance of control functions.

• A small amount of record keeping and other data reporting functions are usually included.

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2. SCADA / AGC

• A SCADA and automatic generation control (AGC) system is similar to a simple a SCADA system except some limited generation control capabilities are included.

• These capabilities include the ability to calculate area control error, monitor frequency and tie lines and perform a limited economic dispatch of a few generating units.

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3. EMS (Energy Management System)

• The energy management system (EMS) incorporates all the features of SCADA systems in terms of gathering data and performing control.

• It typically will include a fair amount of computing power and probably extensive online data storage.

• The user interface (UI) may be very complex, including the use of full graphic CRT’s, dynamic map boards and many recording charts.

• Software frequently includes compute-intensive programs for contingency analysis, security functions, scheduling, load flows and optimal power flows.

• Because the EMS is often considered the heart of the utility’s main control center, it probably includes extensive capabilities for record keeping and data exchange.

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4. DMS (Distribution Management System)

The distribution management system (DMS) is the next logical step after a utility has implemented an EMS. Initially it was viewed as a means to monitor distribution feeder loads and to control the distribution portion of the substation.

Technology has lowered the cost of control and supervision of devices located throughout the feeder lines to the point where it is now becoming routine.

The DMS frequently includes topology analysis and load

flow programs that allow rapid identification of problems and restoration of service.

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5. Load Management System (LMS)

1. The intent of a load management (LM) system is to control peak demand and provide other economic benefits without major inconvenience to the customer.

2. An LM system can be stand-alone or it can be integrated into an EMS or DMS.

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Network Application in EMS

Typical ApplicationsTypical Applications[1] State Estimation[2] Security Analysis[3] Power/Load Flow[4] Optimal Power Flow

Unlike other EMS applications, they depend on power system network models. They may execute in EMS real-time and study environments

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Real Time and Study Environment

In the real time environmentreal time environment network applications execute periodically with virtually no operator interaction.

They draw attention to actual or potential problems and provide operating data enabling a corresponding course of action.

In the study environmentstudy environment they execute on demand to facilitate operator analysis of different operating scenarios associated with past, present or future power system conditions.

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A. Real Time EnvironmentSecurityAnalysis

Optimal PowerFlow

StateEstimation

SCADA

ActiveControl

ReactiveControl

PowerSystem

Fig. 2 Real Time Environment

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Real Time Environment (Contd.)

State Estimation, Security Analysis and Optimal Power Flow applications provide the real time means of developing controls for operating power systems securely.

To achieve this objective, they execute sequentially, validating the condition of the power system.

Fig.2 illustrates the real time environment.

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Real Time Environment (Contd.)• Execution of the network applications may be initiated

several ways. • Commonly they execute continually at periodic rates

that range from 5 to 20 minutes. • Operators can set the optimum rates within limits

governed by EMS computing power.• State estimation is the first application to execute.

State Estimation uses telemetered, scheduled and manually entered data to determine the network topology and state of the power system.

• Network topology refers to the connectivity of the power system components and is determined from status information collected by SCADA.

• Once the state of the power system is calculated security analysis may be execute, followed by optimal power flow.

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• Security Analysis determines the potential impact of contingencies.

• OPF recommends the control that may be used to correct actual violations and/or prevent or correct contingency violations.

• In Fig.2 it is assumed that the controls recommended by OPF are implemented automatically in closed loop fashion via active control, reactive control and SCADA.

• SCADA sends EMS commands controlling circuit breakers, transformers, phase shifters and generators via remote terminal units.

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• Typically active control includes automatic generation control and economic dispatch.

• Acting through SCADA, AGC/ED performs generation control for very few seconds to meet load and interchange schedules economically.

• OPF, however, passes generating unit limits at the slower periodicity of the network applications.

• Reactive control provides an OPF interface that serves the potential need for coordinating reactive power controls such as generator voltage schedules; transformer taps and shunts capacitor/reactor banks.

• Though the controls may pass from OPF to SCADA directly reactive control also allows for more frequent power system feedback to be taken into account.

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B. Study Environment

Studycase

Optimalpower flow

SecurityAnalysis

Save case

Changes Save case

ApplyContingency

Fig.3 Study Environment

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Study Environment• The network applications commonly used for studies

are Power flow, OPF and security analysis.

• Fig.3 illustrates the study environment where OPF includes the Power flow applications.

• Execution of each study application is controlled by the operator in an environment that is separate from the real time environment.

• Typically, the study environment allows several operators to make concurrent studies on a noninterference basis and is supported by the ability to store and retrieve input/output results through a library of save cases.

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Study Environment (Contd.)• Once satisfied that the study case is correct the

operator normally execute power flow or OPF and reviews the results.

• The operator can save the results and execute Security Analysis to determine the potential impact of contingencies.

• After reviewing the output from security analysis, the operator may choose to apply a particular contingency such as the one that has the severest impact to the study case.

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Network Topology Processing

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S(2 )K (2 ) S(3 )

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Topological Classification

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Computer Control of Power SystemsN ational c ontro l

operatorN ational c ontro l

c om puter

G rid c ontro larea c om puter

Controldes k

Area c ontro lc om puter

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pow er output

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c ontro l ofauxiliar iesplant

c onditions

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des ired boundaryline f low s

governors et point

T m in & 30 m inins truc tions

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des ired boundary f low s ,m ac hine s ync tim es

area equivalentc irc uit; effec tivec os ts

Experimental automatic area control

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No Function Description

1 Automatic Generation Control (AGC)

The function of the AGC is to allocate the generation among the system generator units in real-time to meet the system load.

2 Economic Dispatch Calculation (EDC)

This program allocates generation among the available units so as to minimize the cost of supplying the system load.

3 Unit Commitment. The Unit Commitment function is to establish the minimal cost operating policy over a specified time period, (usually a day to one week) within a set of specified constraints.

4 System Load Forecast To forecast then system load, usually for the next one day to one week, taking account of historical as well as weather load models.

5 State Estimator The State Estimator is a mathematical procedure for producing the best estimate of the status of a network, from a set of measurements. The result of the State estimation provides the base from which the Contingency Analysis program is run.

6 Power Flow The Power Flow Program provides the capability for operators and operation planners to study the effects on the power network under postulated conditions.

7 Short Circuit Analysis This application program performs the calculation of three phase and/or single phase to ground short circuit currents and the associated voltage profile.

8 Bus Load forecast Provides short term forecasting of the loads on feeders and stations.

9 Transient Stability Simulates and analyses the dynamic response of an interconnected system for several seconds following a disturbance.

10 Contingency Analysis This application program automatically assesses the impact of selected outages on the real-time power system and alerts the user to rating violations on affected pieces of equipment.

11 Dispatcher Training Simulator

This equipment provides the engineers with familiarization with the operations in a power control system.

Documents

Supervisory Systems