APPLICATION OF SCADA
IN POWER TRANSMISSION AND DISTRIBUTION
R. L. Das Anjul
Sr. DGM , Sr. Engineer
Electronics Division, BHEL, Bangalore
1.0 INTRODUCTION
The term SCADA “Supervisory Control and Data Acquisition System” in the most conventional way as applicable to any sector has the bottom line functionality as listed below.
· Shall acquire field data from the various field devices like sensors
· Shall be able to Control various field Actuators by the supervisors
Irrespective of the application area or sector it may be applied, any SCADA system typically has three implementation layers.
· Human Machine Interface
· Communication Interface
· Remote Terminal Units
The interaction between the above mentioned three layers for exchange of data is briefly explained below. The data acquired from the various field devices like sensors etc by the Remote terminal units as digital and Analogue signals will be communicated over various possible and viable communication media to the HMI stations. The HMI station could be a group of computers performing the functions of Operator Station, Engineer station, servers networked on a local area network. Depending on the type of application the servers could be collecting data from the RTUs or PLCs either located in the same location or from RTUs located at geographically distant locations. The data acquired and made available at a centralized location, will be in turn stored & presented by the various standard features of HMI package like Alarms & Events reporting, Logs, Trends, Status Displays etc. which are the requirements, which are the primary requirements for all the SCADA applications.
The specific requirements for these three layers depend on the various sectors where it would be applied. The requirements of HMI systems for various sector or application vary mainly on the type of Communication media interfaces or on the advanced functionalities. The advanced requirements for the HMI packages for power sector demand features like Energy Management Package, interface to numerical relays etc whereas HMI packages for Oil & gas sector calls for features like Leak Detection Systems. These features are making SCADA systems to evolve as intelligent management systems for various Sectors. The requirements for Remote Terminal units from the hardware point of view mainly vary in terms of environment where it has to be applied and possible communication interfaces it can support. The logic implemented in these remote terminal units will be specific to the application based on what has to be monitored or controlled.
DIFFERENCE BETWEEN SCADA AND DCS:
The SCADA systems and Distributed Control Systems (DCS) are similar in terms of the basic functionality of Data Acquisition and Control implemented through a network of computers and Distributed processing units which are called as Remote Terminal units in SCADA application.
DCS is process oriented, and it looks at the controlled process. It presents data to the operators as part of its job. Whereas, SCADA is data-gathering oriented, hence the control centre and operators are its main focus. The various processors in the case of a DCS system will perform a significant amount of closed loop controls whereas closed loop control is not a high priority in a SCADA system
The Distributed Control system will cater to a more confined area where the operator station is normally intimately connected with its I/O (through local wiring, FieldBus or Local area Networks) for collecting the process related data. The SCADA system can collect data on a variety of communication media related to the system catering to larger geographical area.
2.0 POWER PLANT / INDEPENDENT SUBSTATION SCADA
2.1 GENERAL:
The configuration for these two types of SCADA system require a number of Remote Terminal Units networked at the same location to HMI computers on high speed communication network.
The automation systems for Power plants are the following
· Distributed Control System performing the main power plant controls and monitoring functions
· SCADA system which controls the electrical network ( switchyard) connected to the power plant
The Substation SCADA caters to the automation requirements of Independent Substations generally in the transmission sector. The HMI stations and remote terminal units will be located at the substation itself. The typical configuration is similar to the power plant SCADA requirements. The requirements for interconnection to Main power plant DCS system & Remote connection to LDC are generally not there.
The basic configuration for such type of SCADA systems consists of a network of computers performing the functions of Operator Stations, Engineer Stations and Remote Terminal Units. These systems perform the basic functions like listed below.
(a) Monitoring of the substation equipment like isolators, breakers and earth switches
(b) Control of isolators ,breakers etc
Apart from these functions the SCADA systems for such type of applications also have some special interfaces for interfacing to IEDs (Intelligent Electronics Device).
The typical configuration diagram is given below.
2.2 FUNCTIONAL REQUIREMENTS
A. Data Acquisition Function:
The data from the substation equipments are acquired from the remote terminal units as digital and analogue signals. These parameters include:
Analogue Signals like three phase voltages, three phase currents , MW, MVar, Energy etc.
Digital Status like Opened /Closed status of isolators, breakers & earth switches
Alarms like breaker alarms, protection alarms
Accumulators or Pulse Inputs
B. Control Functions:
The operator issues control commands for closing or opening of the breakers and isolators with select-check-execute from the Control Center which are communicated to the Remote Terminal Units
C. Sequential Control
Necessary sequential logic & interlocking as are implemented for breaker or isolator closing/ opening operations.
D. Alarms & Events Management system:
Various alarms like protection alarms or breaker alarms are time tagged by the Remote Terminal units with milliseconds accuracy and sent to the Central Control Center. The alarms have to be annunciated to the operator as audible alarms. Necessary features are provided for Acknowledge/Reset of these alarms. The alarms and other time tagged events are logged in the HMI computers for further viewing and analysis.
E. Communication Requirements:
Power plant switchyard or Independent substations SCADA system require the RTUs located within the same room to communicate with the HMI system on a high speed dual communication network preferably ethernet or RS485 communication link. The architecture implemented for the HMI computers and RTU can be either server type of architecture or a peer to peer communication interface.
The power plant SCADA system generally have necessary communication facility for two way data communication with the Power plant DCS system to achieve control and monitoring of the switchyard from the Power plant Control room. Apart from this, some of the projects require facility for Control and Monitoring of the entire power plant from a remote control center on a PLCC link. The protocol for communication with the remote control center are generally IEC 870-5-101 .
F. Time Synchronization Requirements:
The computers on the LAN and the RTUs are time synchronized to the Master clock. The time synchronization at the RTUs is done using IRIG-B signal.
2.3 requirements for HMI packages
A. GENERAL REQUIREMENTS :
The HMI package should run on the Operator station at the Control centers and shall have the following features.
· Graphical displays depicting the total distribution network
· Supervisory control through suitable password protection from the graphical displays
· Alarm & event message display
· Trend & Bar charts displays
· Printing of reports & Logs
· Engineering/Configuration facility
The above listed requirements for HMI packages exist for all type of SCADA applications
B. Special features of hmi for Power plant & Independent Substation SCADA systems
The special requirements for power plant SCADA systems in the various tender specifications are the following:
· Communication Gateway to Main Power Plant DCS system
The gate way to the Main power plant control room has to perform the following functions:
1) Control & monitoring of the substation shall be possible from the Main power plant control room
2) Collect data from the Main Power plant DCS to forward to the Remote Center
· Communication to HMI station located at the Remote Control Center/LDC via PLCC or dedicated telephone lines
Control and Monitoring of the power plant from the remote control center shall be possible via this link . In some of the projects ,HMI station located at the Remote Control Center shall be able to connect to other third party systems at the remote control center.
· Interface to Numerical Relays from HMI station
The operator shall be able to perform the following tasks from the operator station:
1) View/modify the relay settings ( HMI layer)
2) Viewing/Analysis of Disturbance record collected by the relays ( HMI layer)
The numerical relays available in the market are from three suppliers ABB, Siemens & ALSTOM. The SCADA systems offered by these suppliers offer such integrated features. Similar requirement is being asked in most of the tender specifications.
· Interface to Numerical relay from RTU
· Communication to Trivector Meters
· Energy Management System
The EMS packages are required to implement Load shedding algorithms, Connection /disconnection of capacitor banks etc.
· Synchronization of outgoing lines to the bus from local SCADA as well as from remote control center.
· Integrated Bay control units performing both Control & Protection functions
3.0 DISTRIBUTION SCADA
3.1 GENERAL :
The Distribution SCADA systems deal with the supervisory control and data acquisition systems for power distribution network consisting of Distribution transformers (11 kV/415 V) and substations 33 kV & 66 kV (Sub transmission level) are classified under the category of Distribution SCADA. The typical SCADA system for a Distribution application consists of a number of Remote Terminal Units located in geographically distributed locations and has the possibility of communicating via different communication media like UHF/VHF Radio, Cell phones, Fiber optic, satellite, Public Switched Telephone Network (PSTN ) , Microwave etc to one or more control centers.
Control Center 2
The Master control center (like in a Power plant or Independent Substation SCADA application) consist of a network of computers performing the functions of Operator station, Engineer station etc. In a power plant SCADA application, the remote terminal units will be connected to the HMI computers at the same location. However, in a Distribution SCADA or Pipeline SCADA application, the LAN network will have dedicated servers/Front End processors in redundant configuration which will collect data via their serial ports from the geographically distributed remote terminal units on radio or any other medium of communication listed above. Alternatively the master control center may have Communication Controllers instead of servers for performing this function.
3.2 FUNCTIONAL REQUIREMENTS
3.2.1 Data Acquisition Function :
The data from the various pole top RTUs / Ring main units/ substations/ distribution transformers will be acquired from the remote terminal units. The data is acquired by RTUs as digital and analogue signals. These parameters include :
Analogue Signals like three phase voltages & currents , MW, MVar, Energy etc.
Digital Status like Opened /Closed status of isolators, breakers & earth switches
Alarms like breaker alarms, protection alarms
Accumulators or Pulse Inputs
3.2.2 Control Functions :
The operator should be able to issue control commands for closing or opening of the breakers and isolators (with select-check-execute) from the Control Center. This should be communicated to the Remote Terminal Units.
3.2.3 Sequential Control :
Necessary sequential logics & interlockings as per Operating & control philosophy are required to be implemented for breaker closing/ opening operations.
3.2.4 Alarms & Events Management system:
The various alarms like protection alarms or breaker alarms are to be time tagged by the Remote Terminal units with milliseconds accuracy and sent to the Central Control Center. The alarms are to be annunciated to the operator as audible alarms. Necessary features are to be provided for Acknowledge/Reset of these alarms. The alarms and other time tagged events are to be logged in the HMI computers for further viewing and analysis.
Some of the specifications call for Auto dialing /Auto paging features. This requires facility by which the telephone/pager numbers of the maintenance staff responsible can be configured for Alarms pertaining to the particular areas/zone. This feature is covered in the Trouble Call Management system requirement.
3.2.5 Communication Requirements:
The communication requirements for Distribution SCADA applications are similar to the requirements for pipeline SCADA applications.
The Distribution SCADA applications require Control & monitoring of the various distribution transformers, substations, ring main units etc from either one or more Control Centers. In case of systems where more than one control centers exist, one of the Control center will act as the Master Control Center. The Master Control Center will poll the RTUs over communication media like Radio /Microwave /cell phone / fibre optic etc to collect the data pertaining to the electrical distribution network. The Master Control Center has to update the other Control Centers (one or more varies from project to project) on high speed OFC (Optical Fibre Cable) or on dedicated / leased telephone lines using suitable routers.
All the Control Centers are generally on a WAN link. The status of the Master Control Center will be acquired by the other Center when there is a failure of communication at the present Master Control Center. Apart from this, facility has to be provided in the HMI software for transferring of Master Control Center Status through manual commands.
The Control centers generally have minimum three computers configured as redundant Operator stations (2 nos) and one Engineer station interconnected on a high speed (eg. 100 MBPs) Ethernet LAN. The data collection from the RTUs by the Master Control Centers are done by dedicated redundant Front End Processors or Servers (FEP) which are to be hooked on to LAN along with Operator & Engineer stations. These servers collect data on their serial channels interfaced to the communication equipment ( Radio/Cell phones/Micro wave etc).